Summer Circulation and Water Masses along the West Australian Coast

Transcription

1 Summer Circulation and Water Masses along the West Australian Coast Lai Mun Woo, B.Eng. (Hons.) This thesis is presented in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of Western Australia, School of Water Research Submitted June, 2005

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3 All merit in my research is dedicated to my beloved family: Doris, Chong Wah, Lai Yee, Lynn, Ben and Ernest; to my dharma teacher and true friend, Jue Ru; and to the hundreds of thousands who lost their lives to the Indian Ocean this past summer. 3

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5 As I wend to the shores I know not, As I list to the dirge, the voices of men and women wreck d, As I inhale the impalpable breezes that set in upon me, As the ocean so mysterious rolls toward me closer and closer, I too but signify at the utmost a little wash d-up drift, A few sands and dead leaves to gather, Gather, and merge myself as part of the sands and drift. WALT WHITMAN, AS I EBB D WITH THE OCEAN OF LIFE (1860) 5

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7 Contents LIST OF FIGURES...11 LIST OF TABLES...17 ACKNOWLEDGEMENTS...18 PREFACE...21 ABSTRACT...23 CHAPTER ONE: INTRODUCTION The Gascoyne- a Significant Marine Environment Motivation Objective...30 CHAPTER TWO: LITERATURE REVIEW Local Setting of the Gascoyne Climate Bathymetry Prominent Features of Ocean Circulation Conventional Eastern Ocean Boundary Currents Leeuwin Current Observational Studies Modelling Studies Overview Leeuwin Undercurrent Coastal Equatorward Current Hydrographical Structure Conclusion

8 CHAPTER THREE: SUMMER SURFACE CIRCULATION ALONG THE GASCOYNE CONTINENTAL SHELF, WESTERN AUSTRALIA...51 Abstract Introduction Methodology Results and Discussion Topographic Controls The Leeuwin Current The Capes Current The Ningaloo Current Shark Bay Outflow Conclusions...85 CHAPTER FOUR: HYDROGRAPHY AND WATER MASSES OFF THE WEST AUSTRALIAN COAST...87 Abstract Introduction Data Collection Results and Discussion Water Masses Tropical Surface Water (TSW) Salinity Minimum South Indian Central Water (SICW) Salinity Maximum Subantarctic Mode Water (SAMW) Oxygen Maximum Antarctic Intermediate Water (AAIW) Salinity Minimum Northwest Indian Intermediate (NWII) Water Oxygen Minimum Shallow Oxygen Minimum Surface and Sub-surface Current Systems Conclusions

9 CHAPTER FIVE: DYNAMICS OF THE NINGALOO CURRENT OFF POINT CLOATES, WESTERN AUSTRALIA Abstract Introduction Methodology Numerical Model Field Data Results and Discussion Conclusions CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS Field Study Numerical Modelling Study Recommendations for Future Work REFERENCES

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11 List of figures Figure 1: Location map of the study area flanking the Gascoyne, Western Australia 28 Figure 2.1: Generalised profile across the continental margin showing the relationships between the provinces (adapted from Anikouchine and Sternberg, 1981). 33 Figure 2.2: Bathymetric sections showing the shape of the seabed off the Gascoyne coast. 34 Figure 2.3: Generalised current patterns in a typical ocean basin showing the major circulation cells and the influencing wind systems (after Davis Jr., 1986). 35 Figure 2.4: A common map of the surface circulation of the world (after Apel, 1987). The eastern boundary current off the Australian continent is frequently erroneously depicted as flowing equatorward. 36 Figure 2.5: Annual average temperatures show downwelling along (a) Western Australia, in contrast to typical coastal upwelling along (b) California and (c) the west coast of South Africa (after Godfrey and Ridgway, 1985). 37 Figure 2.6: Schematic chart of large-scale circulation in the Indian Ocean (after Pearce and Cresswell, 1985). 41 Figure 2.7: SeaWifs chlorophyll concentration image for 1 st Nov 1997, showing an inshore (northward) current swinging anti-clockwise at Point Cloates (Pearce, 1998). 44 Figure 2.8: T/S diagram showing a vertical profile of water masses found at a station ( S, E) offshore Shark Bay. (Depths along T/S curve are in metres. Isopleths of constant density are in σ t. Cruise S05/86 station data retrieved from CSIRO on-line database.) 45 Figure 2.9: T/S diagram envelopes showing different water masses in the surface 40m of coastal Gascoyne waters. (Isopleths of constant density are in σ t. Cruise FR01/96 station data retrieved from CSIRO on-line database.) 48 Figure 3.1: Location map of the study area including the RV Franklin cruise track through the Gascoyne continental shelf. Location of the CTD stations are shown as unfilled circles. 52 Figure 3.2: Bathymetry off the Gascoyne continental shelf and offshore regions. Changes in the continental shelf and slopes of selected transects (B, E, G, I 11

12 and J see Figure 3.1) are shown to a maximum depth of 1000m with the location of the 200m isobath. 57 Figure 3.3: Wind data (arrows point with direction of wind flow) collected on board the vessel and together wind-roses from land based stations: Learmonth, Shark Bay and Abrolhos Island for November Figure 3.4: T/S diagram for 100m surface layer of water, from the coast to the 1000m isobath, including all 11 transects. Larger filled circles indicate LC at its strongest poleward flow on each transect. 60 Figure 3.5: Surface sea temperature (SST) image together with surface current vectors measured from the ship-borne ADCP. 61 Figure 3.6: Surface salinity distribution obtained from thermosalinograph data. 63 Figure 3.7: Transect D: cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 64 Figure 3.8: Transect J cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 65 Figure 3.9: (a) Alongshore velocities along the line of maximum LC poleward flow. Shaded region indicates poleward flow >0.3 ms -1. (b) Surface alongshore surface velocity across individual transects. 67 Figure 3.10: (a) Salinity and (b) temperature properties along the length of the Leeuwin Current; and (c) geostrophic flow relative to 300db across the 1000m isobath. Unshaded areas indicate flow towards the Leeuwin Current (ie eastward flow). 68 Figure 3.11: (a) Anticyclonic eddy at Transect D, transporting warm, low saline, tropical water. (b) Clockwise eddy at Transect I, anti-clockwise eddy at Transect J; both transporting cooler, saline water from offshore. 70 Figure 3.12: Transect A cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 72 Figure 3.13 (a) Schematic diagram depicting detail of Ningaloo Current flow near Point Cloates. (b) Shipboard ADCP data showing surface current flow. 75 Figure 3.14: Transect E cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 76 Figure 3.15: Transect F cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 78 Figure 3.16: Transect G cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow

13 Figure 3.17: TS-diagram for surface waters at Transect G, showing the presence of a pronounced 35.2 water as well as Leeuwin Current water. 80 Figure 3.18 (a) T/S diagram at Transect H shows the presence of 35.2 water at 6 coastal stations. (b) Salinity profiles show the position of the 35.2 water to be throughout the water column at the shallowest coastal station, and at the deeper parts of 5 subsequent stations offshore. 82 Figure 3.19: Transect I cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 83 Figure 3.20: Schematic of the general surface circulation pattern of the major currents along the Gascoyne continental shelf. Isobaths drawn at 100m increments. 84 Figure 4.1: Location map of the research area including the positions of CTD transect lines performed during voyages SS09/2003 and FR10/00, and CTD stations (over 1000m-isobath) taken from voyages FR87/03 and FR87/ Figure 4.2: Temperature-salinity (with σ T contours) and temperature-oxygen diagrams exhibit interleaving positions of property extrema. 92 Figure 4.3: Three-dimensional blocks of ocean depicting the cross-shelf (across Transect J, the southernmost transect made by FR10/00) and along-shelf (along 1000 m-isobath) distribution of (a) salinity extrema, and (b) dissolved oxygen extrema. 93 Figure 4.4: Major water masses observed at the 1000 m-isobath along the Western Australian shelf. Asterisks on the surface indicate CTD stations positions. This chart combines data from voyage FR10/00 ( S) and voyage SS09/2003 ( S). 94 Figure 4.5: Sparse CTD data indicate the major water masses observed at the 1000 m-isobath in SAMW was less ventilated in 1987 than in 2000/3 (Figure 4.4). Asterisks on the surface indicate CTD stations positions. 98 Figure 4.6: Differences between observations from 1987 (Figure 4.5) and 2000/3 (Figure 4.4) of (a) dissolved oxygen, and (b) salinity. Shaded areas indicate values were greater in 1987 than in 2000/3. 99 Figure 4.7: 1000 m-isobath cross-sections of (a) dissolved oxygen, and (b) chlorophyll. The dashed line in both charts traces the core of the band of shallow oxygen minimum water. 102 Figure 4.8: Schematic diagram illustrating the general flow patterns at the continental margin

14 Figure 4.9: Transect I cross-section of ADCP alongshore velocities (ms -1 ) shows equatorward LU and coastal current, and a poleward LC. The trace lines show the positions of the LC and LU as numerically modelled by Meuleners et al. (2005). 105 Figure 4.10: Transect I cross-section of geostrophic flow (ms -1 ) relative to the surface shows the LU flowing equatorward at a depth of 400 m. 106 Figure 4.11: Cross-section of dissolved oxygen levels for Transect I shows the presence of a > 252 microm/l core at 400 m depth. 106 Figure 4.12: Transect I cross-sections of (a) salinity, and (b) temperature. Circles along the surface indicate CTD stations positions. 108 Figure 4.13: Geopotential anomaly in m 2 s -2 plotted versus latitude from CTD stations recorded along the 1000 m-isobath with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 996 m. 109 Figure 4.14: Geopotential anomaly in m 2 s -2 plotted versus longitude from CTD stations recorded along Transect E with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 730 m. 110 Figure 4.15: Relationship among density, geostrophic velocity, and the slope of the interface between layers, as given by Margule s equation. (Adapted from Knauss, 1997.) 111 Figure 5.1: Locality map showing the position of the research area that was used as the numerical-modelling domain in this study. 116 Figure 5.2: Arrows indicate the general surface circulation pattern observed on a Sea-Surface Temperature (SST) satellite image from Ningaloo, 18 th November Isobaths are 200m and 1000m. 119 Figure 5.3: A 3-dimensional bathymetry chart detailing the shelf structure in the vicinity of the Point Cloates 120 Figure 5.4a: CZCS satellite image from March 1980 showing no evidence of a NC recirculation event in surface chlorophyll patterns south of the promontory at Point Cloates. Surface wind speeds were low. 125 Figure 5.4b: (i) CZCS image from September 1980, and (ii) SeaWiFS image from November Both show an anticlockwise re-circulation feature in the surface chlorophyll south of Point Cloates, but no coastal current proceeding north along the peninsula s edge

15 Figure 5.4c: (i) CZCS image from May 1980 showing surface chlorophyll levels, and (ii) AVHRR image from January 1991 showing sea-surface temperature. Both display anticlockwise re-circulation features south of Point Cloates, as well as the NC along the coast on both sides of the promontory. 126 Figure 5.5a: Model run R2. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 2m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). 128 Figure 5.5b: Model run R3. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 3m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). 129 Figure 5.5c: Model run R4. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 4m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). 130 Figure 5.5d: Model run R5. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 5m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). 131 Figure 5.6: Location of four transect lines across the numerical modelling domain. Volume transports were calculated for the region coastward of the 70m isobath. 132 Figure 5.7: Relationship between northward NC volume transport and windforcing velocity, at four locations, i.e S (a-a ), S (b-b ), S (c-c ) and S (d-d ). The NC volume transport recorded in field data taken close to the location of c-c is also shown. 132 Figure 5.8: The fate of NC water travelling northward through the model domain. 133 Figure 5.9: NC distribution chart showing the percentage volume retained or lost through the re-circulation event alone, under different southerly wind velocities

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17 List of tables Table 1: Evidence of coastal equatorward currents along the West Australian coast in summer. 43 Table 4: The different characteristics of each of the water masses found in the 1km-deep water column defined. This table combines data from voyage FR10/00 ( S) and voyage SS09/2003 ( S). Table 5.1: Summary of general constants used in HAMSOM modelling work. 123 Table 5.2: Forcing combinations for each of the five runs

18 Acknowledgements The following people are gratefully acknowledged for their contributions toward the successful completion of this work: My supervisor, Professor Charitha Pattiaratchi, for his continued support and guidance. It has been a privilege working with, and learning from this gracious and learned gentleman. Dr William Schroeder, for his constructive comments and reviews, and for the enthusiasm that he injected into the research each time that he came to visit. The Captain, crew and scientific support staff of the RV Franklin and FRV Southern Surveyor, for the successful execution of the voyages FR10/00 and SS 09/2003. The RV Franklin shipboard scientific party: Christine Hanson, Tony Koslow, Elisabeth Nahas, Peter Thompson, Anya Waite, for their assistance and constructive discussions. David Griffin (CSIRO Marine Research) for supplying real-time satellite imagery for use in Chapters Three and Five. Michael Meuleners for generously making output-data from his numerical modelling available for use in Chapter Four, and for helping with data retrieval from Geoscience Australia. CSIRO data centre for providing historical shipboard data used in Chapter Four. British Ocean Data Centre, National Geophysical Data Center, and the Australian Geological Survey Organisation for having ocean bathymetry data available. Fellow residents of CWR room 2.14: Guy, Nicola, Matt, Kathy, Alexis, Claire and Ralf, for providing stimulating conversation and contributing to a supportive, productive working environment. 18

19 My parents, Chong Wah and Doris, for their personal sacrifices in support of my academic success, and for helping me get through difficult times of injury and poverty. My sisters, Lynn and Lai Yee, for their love and constancy. Jue Ru Shifu, Jue Ying Shifu and fellow postulants at the Australia Buddhist Bliss Culture Mission, for shouldering some of my monastic duties so I had the chance to finish my last paper (Chapter Five). Nel, for accompanying me to the office in the small hours of morning to complete my numerical modelling for Chapter Five. This work could not have been undertaken without the support of the University of Western Australia through a University Postgraduate Scholarship, Centre for Water Research Adhoc Scholarship, and a Higher Education Contribution Scheme Exemption Scholarship. The University of Western Australia and the Department of Environmental Engineering provided financial assistance for my trip to San Diego, California, USA, to present my scientific findings at the Oceans 03 conference. 19

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21 Preface The body of scientific research in this thesis is presented as three separate chapters, all of which are self-contained works. Chapter Three has been submitted to Continental Shelf Research under the title of Summer surface circulation along the Gascoyne Continental Shelf, Western Australia (Centre for Water Research reference ED 1898 MW). Chapter Four is to be submitted to Ocean Dynamics under the title of Hydrography and water masses off the West Australian Coast (Centre for Water Research reference ED 1899 MW). Chapter Five has been submitted to Marine and Freshwater Research under the title of Dynamics of the Ningaloo Current off Point Cloates, Western Australia (Centre for Water Research reference ED 1900 MW). To maintain completeness of each chapter, a small amount of repetition in the background and description of the study site has been unavoidable. The content of this thesis is the author s own work. Specific acknowledgements have been made in the front of this thesis. Prof. Charitha Pattiaratchi is listed as the joint author of each of the scientific journal papers produced from this study, in recognition of the useful reviews and discussions that have come of the collaborative studentsupervisor relationship. 21

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23 Abstract The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21 o and 28 o S. This study presents CTD and ADCP data together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the continental margin, and the hydrography present in the upper 1km of ocean, between latitudes 21 and 35 S. It also discusses the outcome of a numerical modelling study that examined the physical factors contributing to a bifurcation event persistently observed in satellite imagery at Point Cloates. The region comprises a complex system of four surface water types and current systems. The Leeuwin Current dominated the surface flow, transporting lower salinity, warmer water poleward along the shelf-break, and causing downwelling. Its signature aged from a warm (24.7 o C), lower salinity (34.6) water in the north to a cooler (21.9 o C), more saline (35.2) water in the south, as a result of 2-4Sv geostrophic inflow of offshore waters. The structure and strength of the current altered with changing bottom topographies. The Ningaloo Current flowed along the northernmost inner coast of the Gascoyne shelf, carrying upwelled water and re-circulated Leeuwin Current water from the south. Bifurcation of the Ningaloo Current was seen south of the coastal promontory at Point Cloates. Numerical modelling demonstrated a combination of southerly winds and coastal and bottom topography off Point Cloates to be responsible for the recirculation, and indicated that the strength of southerly winds affect recirculation. Hypersaline Shark Bay outflow influenced shelf waters at the Bay s mouth and to the south of the Bay. The Capes Current, a wind-driven current from south of the study region was identified as a cooler, more saline water mass flowing northward. Results of the hydrography study show five different water masses present in the upper-ocean. Their orientations were affected by the geopotential gradient driven Leeuwin Current/Undercurrent system at the continental margin. The Leeuwin Undercurrent was found at the shelf-slope, carrying (>252 μm/l) Subantarctic Mode Water at a depth of 400m. 23

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25 Chapter One: Introduction Out in space, if you gazed upon our world, you would see a glistening blue planet. But how much do we know of ourselves, of our own world, this Planet Oceanus? With the sheer vastness of the oceans making up most of our planet, the task of oceanographic exploration and research is a massive undertaking, seemingly unending, stretching back several millennia to a time when ancient mariners built crude vessels and bravely ventured out into the great unknown of the seas. Today, with the aid of modern technologies, such as satellites, acoustic profilers, computerised water-property sensors, and a host of other scientific instruments aboard ships and in research stations on land as well as in space, the tradition continues, as oceanographers carry forward the quest to unravel the mysteries of the still largely unstudied oceans. After all, in many ways the ocean is not unlike our mind. Its surface is filled with constant activity, waves of ups and downs, and various influences from a diversity of external phenomena. But in its deepest recesses, it is quite still and old; carrying with it relics and imprints from encounters long ago. And all of it is a continuous and connected whole, which in turn exerts its effects on the external world. How do we know anything, if we do not even look within and know our own minds? The same goes for our Planet Oceanus. This study carries on in the spirit of the ancient mariners. Hopefully, looking within our oceans, we may develop knowledge and understanding, to replace myth and ignorance, so that we may be able to take care of our world, our lives and each other with greater wisdom. The goriest tale of Australian maritime history illustrates to us the need to understand our coastal seas. During this tragedy which took place in 1629, the Batavia was shipwrecked near the isolated Abrolhos shoals, off the Western Australian coast (Saville-Kent, 1897). In desperation, the Captain and some of his officers set off in a skiff to fetch supplies from the mainland. However, to their dismay, they were overwhelmed by an unexpected coastal current that pulled them so far northeast that they finally decided to proceed to Indonesia to find a vessel that would bring them back. In the three months that followed, unspeakable atrocities ensued among the abandoned 25

26 shipwrecked sailors. There were murders, madness, cannibalism and a bloodthirsty mutiny, which left almost everyone dead. Since those early days, the Western Australian population has rapidly grown into a modern society of a couple of million people, most of whom live their lives on the fringes of the sea. But nonetheless, much of the West Australian coast remains largely unknown and unstudied. Consequently, as the population continues to uncover more and more uses for the ocean (e.g. transport, aquaculture, marine parks), and to discover valuable resources to be extracted from it (e.g. oil and gas mining, fish harvests), the lack of appreciation for the governing marine processes has put both marine as well as coastal inhabitants into a needlessly precarious position. Thus this study is an effort to address this problem, and to develop a good understanding of the oceanic processes of the Western Australian coastal region. 26

27 1.1 The Gascoyne- a Significant Marine Environment Along the central coastline of Western Australia is the Gascoyne region (Figure 1) a region possessing marine environments of remarkable ecological, scientific and commercial significance. Ningaloo Reef stretches 260km along Gascoyne s northernmost coast (Figure 1). It is the only extensive fringing coral reef on an eastern ocean boundary (Taylor and Pearce, 1999). At only 1-6 km from shore, it is also the only extensive reef found so close to a continental landmass (Hearn et al., 1986). The reef hosts a profusion of 250 species of coral, 520 species of tropical fish, and significant populations of dugongs, humpback whales, shore birds and turtles (Preen et al., 1997; WATC, 1998). One week after the full moon during March and April each year, mass spawning of the corals occurs in a spectacular three-day event (CALM, 1998). This is followed soon after by the arrival of Whale Sharks (Rhiniodon typus) (Taylor, 1996). From mid-march to mid-may each year, visitors from all over the world converge at Ningaloo Reef to swim alongside these majestic creatures. Recent years have seen the development of a healthy tourist and recreational industry in the Ningaloo area. In 1987, under the management of the Western Australian Department of Conservation and Land Management, the Ningaloo Marine Park was established (CALM, 1998). Situated south of the Ningaloo Marine Park (Figure 1) is a large (14,000km 2 ), semienclosed hypersaline coastal embayment called Shark Bay (Burling, 1998). In recognition of its abundance of unique flora and fauna, Shark Bay was gazetted as World Heritage in 1991 (GTA, 2000). An outstanding feature of the Bay is its scientifically important seagrass banks. Shark Bay has the largest area of seagrass, and the largest number of seagrass-species (12 species; 9 per m 2 in some places) recorded to date (CALM, 1998). A myriad of marine life exists in Shark Bay: there are green and loggerhead turtles, manta rays, whales, several shark species, a secure herd of 16,000 dugongs, as well as the Monkey Mia dolphins that have become internationally known for their penchant for interacting with humans (Preen et al., 1997; WATC, 1998). On the western reaches of Shark Bay is Hamelin Pool, where underwater towers of rocklike Stromatolites provide evidence of the earliest life forms that colonised the earth some 3.5 billion years ago (Playford, 1979). 27

28 In addition to its natural beauty and rich diversity of marine life, the Gascoyne marine region also exhibits considerable commercial potential. Already, there are nurseries for penaeid prawns (Penaeus esculentus), saucer scallops (Amusium balloti), western rock lobsters (Panulirus cygnus), pink snapper (Pagrus auratus), spot-tail and blacktip sharks (Carcharhinus sorrah and C. tilstoni), anchovies, as well as aquacultures of pearl oysters (based primarily on P. maxima) and freshwater aquarium fish. Moreover, the aquaculture industry is set to expand further, with a Gascoyne Region Aquaculture Development Plan having been put forward, and pilot projects (e.g. edible oysters, giant clams, beta-carotene production) having been trialled successfully (Fisheries WA, 2000). Figure 1: Location map of the study area flanking the Gascoyne, Western Australia 28

29 1.2 Motivation Considering the commercial, scientific and ecological importance inherent in the Gascoyne marine environment (as discussed in section 1.1), it is imperative that the dynamics of the ocean in which it exists be fully understood, as this knowledge would lead: 1) to a better understanding of the marine ecosystems 1, 2) to better management of the wild fisheries (e.g. Phillips et al., 1978, Lenanton et al., 1991), 3) to an understanding of the factors influencing rainfall (Weaver, 1990), and 4) to defining the ocean circulation for sea safety, environmental protection and aquaculture. Furthermore, in view of the oil industry s interest in Cape Range Peninsula, an understanding of current mechanisms at Ningaloo would be essential in any oil spill contingency planning (Taylor and Pearce, 1999). However, due to complexity of the summer ocean dynamics (section 2.2) and to lack of data, the understanding of oceanic processes in the region remains extremely vague. This project is an attempt to rectify this lack of knowledge. 1 Ocean conditions have been implicated to influence: coral reefs (Hatcher, 1991), seagrass beds (Walker, 1991), tropical organisms (Hutchins, 1991), and seabird distributions (Dunlop and Wooller, 1986; Wooller et al., 1991); as well as the presence of migratory filter feeders e.g. whale sharks (Taylor and Pearce, 1999). 29

30 1.3 Objective The principal objective of this study is to investigate the physical processes on the Gascoyne continental shelf. Specifically, the objectives are to quantify the following: 1. The summer circulation along the Gascoyne continental shelf. In particular, to determine: shelf surface currents and their driving forces, the absence/presence of coastal upwelling, and the structure of the currents over varying continental shelf widths. 2. The hydrography of the upper ocean. In particular, to describe: water masses, currents in deeper waters, and their driving forces, the effect of ocean currents on water masses at the continental margin. 3. The interaction between the northward coastal current and the southward Leeuwin Current at the coastal promontory at Point Cloates. In particular, to investigate: the anti-cyclonic re-circulation pattern identified from satellite imagery immediately south of Point Cloates, and physical processes (eg. wind speed and direction) that contribute to development of the re-circulation pattern. 30

31 Chapter Two: Literature Review 2.1 Local Setting of the Gascoyne Climate The Gascoyne region is situated on the Tropic of Capricorn, in the north west of Western Australia. The region encompasses both tropical and temperate climatic features: the northern part is arid and tropical, while the southern part tends towards a more temperate, Mediterranean climate. Climatic conditions in the Exmouth region (Figure 1) are dominated by tropical cyclones, most of which occur during the summer months between January and March. The climate is characterised by hot temperatures and low rainfall from November to March. The majority of the rainfall occurs as a result of cyclonic activity. Rainfall is highly variable but averages 278 mm per year. The mean daily maximum temperatures are highest in January (38 0 C) and lowest in July (24 0 C). South of Exmouth, the Carnarvon region (Figure 1) has a more moderate climate. Mean daily maximum temperatures are at their highest in February and lowest in July, ranging 32 0 C 22 0 C. In contrast to the northern part of the region, rainfall occurs mainly in winter and averages 226 mm per year. The Shark Bay area (Figure 1) has a dry, warm Mediterranean climate characterised by hot, dry summers and mild winters. The mean daily maximum temperatures here are similar to those of Carnarvon. South-easterly winds are dominant over the Gascoyne coastal region through much of the year. During winter (July), moderate southerly winds (3ms -1 ) occur near the shelf edge. These winds strengthen from July through November, and remain strong through summer (January-March), often blowing for several consecutive days at over 7 ms -1. Then in May, a weaker, more variable winter wind pattern is again re-established (Godfrey and Ridgway, 1985; Hearn et al., 1986; Taylor and Pearce, 1999). 31

32 2.1.2 Bathymetry Profiles across continental margins are commonly categorised into different provinces (Davis, 1986). The continental shelf may be seen as a shallow, gently sloping section extending immediately seaward from the coast. This gentle province comes into contact with a steep one (i.e. continental slope), followed by a gentler gradient at the continental rise (Figure 2.1). Guided by this definition, the continental margin that flanks the Gascoyne shall be examined 2 and described. The bathymetry off the Gascoyne coastline exhibits a range of continental shelf shapes. As seen in Figure 2.2, the continental shelf 3 north of Point Cloates is extremely narrow (17km at Cape Range, 6km at Point Billie). It descends in a cliff-like manner into the 5km-deep Cuvier Abyssal Plain, without any visible slope-steepening at the 200misobath to indicate the shelf break. South of Point Cloates (Figure 2.2), the coastline veers sharply eastward away from the 200m-contour, making space for a gentler, much wider shelf (e.g. 38km at Coral Bay). Southwards, the shelf remains wide (i.e. 62km at Gnaraloo, 139km at Carnarvon, 84km at Dirk Hartog), whilst the shelf break becomes an increasingly pronounced feature. As seen from Figure 2.2, the shelf break at Coral Bay and Gnaraloo is distinguishable by a noticeable change in gradient, past the 200misobath. And further south, at Carnarvon and Dirk Hartog, the break exists very distinctly as the edge of a step-like structure. Shark Bay, a large (14,000km 2 ) semi-enclosed embayment, is located at the southern section of Gascoyne (Figure 1). The Bay is shallow (average depth 10m) and hypersaline, with a bottom that declines seaward. The coastline at Shark Bay is broken at three places (Figure 2.2), namely at: 1) Geographe Channel- 35km wide, 35m deep, 2) Naturaliste Channel- 25km wide, 40m deep, and 3) South Passage- 2km wide, 6m deep. These channels (especially the former larger two) serve as the only flux paths between the Bay and the coastal shelf waters (Burling, 1998). 2 The author has processed shelf shapes through interpolation of a 5x5-minute gridded matrix of regional bathymetry retrieved from 3 Continental shelf at Gascoyne is taken to be the seabed shoreward of the 200m-isobath. 32

33 Figure 2.1: Generalised profile across the continental margin showing the relationships between the provinces (adapted from Anikouchine and Sternberg, 1981). 33

34 Figure 2.2: Bathymetric sections showing the shape of the seabed off the Gascoyne coast. Figure 2.2: Bathymetric sections showing the shape of the seabed off the Gascoyne coast. 34

35 2.2 Prominent Features of Ocean Circulation Conventional Eastern Ocean Boundary Currents Off the subtropical west coasts of continents in the Atlantic and Pacific Oceans, the dominant currents (e.g. Canary, Benguela, Peru and California Currents) are typically equatorward currents forming the eastern limb of subtropical gyres (Figure 2.3) (Church et al., 1989; Cresswell and Peterson, 1993; Pearce, 1991; Smith et al., 1991). Generally, these eastern boundary currents are recognised as steady surface flows of slow (<10cms -1 ), broad (~1000km), cool waters, driven by equatorward wind drifts from subtropical anti-cyclonic wind fields (Andrews, 1977; Allen, 1980; Huyer, 1990). Figure 2.3: Generalised current patterns in a typical ocean basin showing the major circulation cells and the influencing wind systems (after Davis Jr., 1986). In addition to being the dominant driving force of the eastern boundary currents, the prevailing equatorward winds at the Atlantic and Pacific Oceans also drive offshore surface drift, forcing persistent upwelling of cold nutrient-rich water to the surface at the coast (Allen, 1980; Huyer, 1990). Consequently, a concomitant high rate of primary production results there (Lenanton et al., 1991; Pearce et al., 1996). 35

36 Our study-area off the coast of Western Australia is located on the eastern boundary of the Indian Ocean. The geography, topography as well as predominantly equatorward direction of local winds in the region make it appear analogous to the other eastern boundary current regions. Accordingly, the surface circulation off Western Australia is routinely depicted to be a cool equatorward-flowing West Australian Current, forming the eastern limb of an anti-cyclonic gyre similar to those of the other subtropical oceans (Figure 2.4). However, in reality this is a misleading representation. Not only has there been no evidence of a regular equatorward current within 1000km of the Western Australian coast (Wyrtki, 1962; Hamon, 1965, 1972), the surface current that actually flows there has been found to be warm and flowing in the opposite direction (Cresswell and Golding, 1980), creating conditions more favourable to downwelling than upwelling (Figure 2.5). This current has been named the Leeuwin Current in honour of Leeuwin - the first Batavia-bound Dutch vessel to explore the waters off the southwestern coast of Australia (Cresswell and Golding, 1980). Figure 2.4: A common map of the surface circulation of the world (after Apel, 1987). The eastern boundary current off the Australian continent is frequently erroneously depicted as flowing equatorward. 36

37 Figure 2.5: Annual average temperatures show downwelling along (a) Western Australia, in contrast to typical coastal upwelling along (b) California and (c) the west coast of South Africa (after Godfrey and Ridgway, 1985) Leeuwin Current The Leeuwin Current has been the subject of extensive observational and modelling studies. This section provides a comprehensive overview of these, followed by a summary of the nature of the Leeuwin Current, as learnt from these studies Observational Studies The earliest documentation of a poleward flowing current along the West Australian coast was made by Saville-Kent (1897), who whilst studying marine fauna of the Abrolhos Islands ( S), discovered an anomalously warm current of water transporting tropical species to the region. Dakin (1919) analysed temperature data from the same area, and noticed that the current was warm and most defined in winter. From drift bottles and salinity measurements, Rochford (1969b) ascertained that the current consisted of low salinity water that extended south of Rottnest Island (32 0 S) in winter. In summer, flow reversal occurred. Holloway and Nye (1985) found a similar seasonal pattern, with maximum flows occurring along the southern portion of the Northwest Shelf (22 0 S) in February-June. Kitani (1977) observed transportation of low salinity water to 32 0 S in November 1975, thus showing that although the current appeared to have a seasonal nature (being most pronounced in winter), its occurrence was not confined to that period. 37

38 Through the course of time, further evidence of the Leeuwin Current was uncovered through a host of different observations: e.g. ship drift observations (Nederlandsch Meteologisch Institut, 1949), time series water property data (Rochford, 1969b), historical bathythermograph data (Gentilli, 1972), research vessel surveys (Kitani, 1977; Godfrey and Ridgway, 1985) and biological data sets (Wood, 1954; Colborn, 1975; Krey and Babenerd, 1976; Markina, 1976). These established the current to be a surface flow of warm, low salinity, nutrient depleted tropical water, beginning as a broad (400km) and shallow (50m) stream at North West Cape, tapering (<100km wide) and deepening (<300m) as it moved poleward along the continental slope (Church et al., 1989; Smith et al., 1991; Pattiaratchi et al., 1998). The current transported 7 Sv of water in midwinter (Smith et al., 1991), and 1.4 Sv in summer (Pearce, 1991). This seasonal change in intensity was attributed to regional wind stress variability, i.e. the current flows weakly against maximum southerly (opposing) winds in October-March, and strongly against weaker southerly winds in April-March (Godfrey and Ridgway, 1985). The current was also often associated with coastal downwelling. Deployment of satellite-tracked drifting buoys in added a new dimension to data collected of circulation patterns off the Western Australian coast. In charting buoy tracks, Cresswell and Golding (1979) observed the existence of mesoscale eddies on the western side of the Leeuwin Current. Also, buoys from the eddies accelerated on entry into the current and decelerated on exit; thus providing evidence of a high-speed core current (clocking 170cms -1 from buoy positions) that was clearly defined on the continental shelf break. The mid 70s also saw the introduction of both the infrared and Advanced Very High Resolution Radiometer (AVHRR) imagery. The high spatial resolution (1km) and temperature discrimination (<0.1 0 C) provided by these satellite techniques showed a large wedge of warm water in Northeast Indian Ocean funnel into a narrow current near North West Cape, and then move south along the shelf and slope (Pearce and Cresswell, 1985). Past Cape Leeuwin, it turned eastward to spread into the Great Australian Bight (Legeckis and Cresswell, 1981). The eastward continuation into the Great Australian Bight had previously been inferred from temperature data (Colburn, 1975) and plankton data (Markina, 1976). 38

39 Between September 1986 and August 1987, very extensive current meter measurements of the Leeuwin Current were made as part of the Leeuwin Current Interdisciplinary Experiment (LUCIE). These measurements revealed that the current was strongest in February August and weakest in September February (Boland et al., 1988). Smith et al. (1991) reasoned that this seasonal variation in current strength resulted from variations in wind-stress rather than in alongshore pressure gradient, since the latter had little seasonal dependence. The LUCIE measurements also reflected a poleward acceleration of the Leeuwin Current, as well as the presence of an equatorward undercurrent (section 2.2.3) beneath it (Boland et al., 1988) Modelling Studies Thompson and Veronis (1983) were the first to model the Leeuwin Current. Their work suggested that winter winds on the Northwest Shelf could generate a poleward current. This theory was debunked by current meter observations (Holloway and Nye, 1985), and also rejected by Thompson (1984). Thompson (1987) proposed instead that an alongshore steric height gradient was the primary forcing mechanism, with winter deepening of the mixed layer offsetting the effects of equatorward wind stress. Godfrey and Ridgway (1985), who quantified contributions of alongshore pressure gradient and equatorward wind-stress, supported this. Godfrey and Ridgway (1985) also hypothesised that the large steric height gradient was the result of flow from Pacific Ocean through Indonesian Archipelago into the Northeast Indian Ocean. This agreed well with Gentilli s (1972) previous suggestion that a winter throughflow isolated in Northeast Indian Ocean during summer could be a source for the Leeuwin Current. Later, support for this theory was revealed in satellite imagery showing a large wedge-shaped mass of warm water off Northwest Australia funnelling into a poleward current (Pearce and Cresswell, 1985). The idea of an Indonesian throughflow causing the steric height gradient (Godfrey and Ridgway, 1985) was rejected by McCreary et al. (1986), who postulated that the cause was in fact a thermohaline gradient. Their model showed poleward surface flow as well as an equatorward undercurrent comparable in strength to observations. Subsequently, Kundu and McCreary (1986) modelled the throughflow alone. Production of a weak 39

40 poleward flow led them to conclude that the throughflow was a secondary forcing mechanism. Weaver and Middleton (1989) investigated contributions from both an alongshore density gradient and warmer fresher waters from the Northwest Shelf. Although lacking in mesoscale variability, their model presented a realistic Leeuwin Current. They thus concluded that the Leeuwin Current was driven by an alongshore density gradient and strengthened by the Northwest Shelf waters Overview In a summary, the observational and modelling studies have shown the Leeuwin Current (Figure 2.6) to: be produced by a pressure gradient that overwhelms the opposing equatorward wind stress (this is generally agreed upon, despite dispute over the generation mechanism of the pressure gradient itself); have a warm, low salinity, tropical source on the Northwest Shelf, possibly originating in the Pacific Ocean; begin broad (400km) and shallow (50m) at North West Cape, narrowing (<100km), deepening (<300m) and accelerating (to 1-1.5ms -1 ) poleward, whilst being augmented by geostrophic inflow from the west; flow poleward along the continental shelf break, down the western coast, pivoting at Cape Leeuwin to continue eastward into the Great Australian Bight; transport 7 Sv in midwinter and 1.4 Sv in summer; flow weakly against maximum southerly (opposing) winds in October-March (summer), and strongly against weaker southerly winds in April-September (winter); be associated with downwelling on the coastward side, cyclonic and anticyclonic eddies on the seaward side and a cooler, more saline equatorward undercurrent (Leeuwin Undercurrent) beneath it. 40

41 Figure 2.6: Schematic chart of large-scale circulation in the Indian Ocean (after Pearce and Cresswell, 1985). 41

42 2.2.3 Leeuwin Undercurrent Very little is understood of the Leeuwin Undercurrent (LU). Off North West Cape and Shark Bay, Thompson (1984) reported the existence of an undercurrent beneath the Leeuwin Current at depths of m. He observed the undercurrent to transport 5 Sv of high salinity (>35.8), oxygen-rich, nutrient-depleted South Indian Central Water (SICW) at a rate of 32-40cms -1 northward, and then offshore. An equatorward undercurrent was also apparent in steric height charts at 500db/3000db (Wyrtki, 1971) and at 450db/1300db (Godfrey and Ridgway, 1985), as well as in current meter data (at m) from the LUCIE experiment (Smith et al., 1991). Thompson and Cresswell (1983) reasoned that the source of the undercurrent was likely to be cool, high salinity, high oxygen water from the surface of the South Indian Ocean. Driven by an equatorward geopotential gradient located at the depth of the undercurrent (Thompson, 1984), the water advected northward and downward underneath the Leeuwin Current (Thompson and Cresswell, 1983) Coastal Equatorward Current Saville-Kent (1897) provided the earliest recorded observation of a distinct northward coastal current up the western coast of Australia by recounting how sailors from the shipwrecked Batavia (near the Abrolhos shoals) were unable to get to the mainland because of prevailing currents that carried them too far northeast. Since then, other studies have provided evidence of high-salinity currents, driven northward along much of the Western Australian coastline by local summer southerly winds (Table 1). Of particular interest to our study, is the northward coastal current indicated at Ningaloo during summer, i.e. the Ningaloo Current (Taylor and Pearce, 1999). Evidence of the Ningaloo Current first came about from aerial whale shark surveys made during and from current plume observations made from boats made during (Taylor and Pearce, 1999). When the northward current was present along reef front, a definite line became evident on the water, separating the calmer coastal waters from the rougher Leeuwin Current pushing southward (against prevailing southerly winds) some 2km offshore. These preliminary observations showed the northward cool coastal 42

43 current to be present at Ningaloo between March and early April each year. Subsequent confirmatory data were derived from satellite imagery ( ) showing the Ningaloo Current to be predominant from September through April (Taylor and Pearce, 1999). Table 1: Evidence of coastal equatorward currents along the West Australian coast in summer. Location Latitude Data Type Reference Cape Leeuwin Cape Naturaliste 34 0 S Satellite imagery, current meter, (Gersbach et al., 1999; Pearce and Pattiaratchi, 1999) hydrological data Perth 32 0 S Current meter (Cresswell and Golding, 1980) Rottnest Island 32 0 S Drift bottle movements, (Rochford, 1969b) hydrological data Fremantle S Drifting buoys (Cresswell and Golding, 1980) Abrolhos Islands Abrolhos Islands 29 0 S Current meter (Cresswell et al., 1989; Pearce, 1997) Geraldton 28 0 S Current meter (Cresswell et al., 1985) Carnarvon 25 0 S Current meter (Smith et al., 1991) Ningaloo Reef 23 0 S Aerial surveys, boat (Taylor and Pearce, 1999) observations, satellite imagery Ningaloo Reef 23 0 S Current meter (Smith et al., 1991) In the southern reef where the shelf is wider (Figure 2.2), the Ningaloo Current flows broadly (up to 3km wide), and is easily seen in satellite images. However, north of Point Cloates where the seabed is cliff-like, the current is often less than 2km wide too narrow at times to be discernible in satellite imagery (Taylor and Pearce, 1999). The Ningaloo Current is thought to be driven by southerly winds (Taylor and Pearce, 1999), much like the more southern coastal currents (e.g. Pearce and Pattiaratchi, 1999). Considering records of cold water anomalies at Ningaloo coast (Simpson and Masini, 1986), Taylor and Pearce (1999) postulated that coastal upwelling might occur at Ningaloo. However, convincing evidence for this has yet to be found (Taylor and Pearce, 1999). 43

44 A recurring feature revealed by satellite imagery is an anti-cyclonic circulation pattern located immediately south of Point Cloates (Figure 2.7). Although the actual dynamics of this feature is still poorly understood, a Platform Transmitter Terminal (PTT) (which fortuitously detached from a whale shark, effectively becoming a current drogue) has indicated it to be a region of some degree of cross-shelf exchange/re-circulation (Taylor and Pearce, 1999). Taylor and Pearce (1999) speculated that the presence of the Ningaloo Current and the circulatory movement of water could have major implications for the regional ecosystem. It had previously been accepted that mass coral spawning at Ningaloo during March and April each year brought about a significant export of protein out of the reef via the Leeuwin Current (Simpson, 1985). But Taylor and Pearce (1999) hypothesised that if a circulatory movement retained the planktonic biomass within the Ningaloo ecosystem, it would play an important role in the survival of the reef. Also it might explain the presence of a very active food chain (possibly linked to the concurrent appearance of filter-feeding whale sharks) at that time of year. Figure 2.7: SeaWifs chlorophyll concentration image for 1 st Nov 1997, showing an inshore (northward) current swinging anti-clockwise at Point Cloates (Pearce, 1998). 44

45 2.3 Hydrographical Structure Generally, a m thick mixed layer lies on the surface of the Indian Ocean. Beneath this, the ocean temperature falls rapidly through a thermocline, dropping to 5 0 C at 1000m depth. Water properties within this topmost kilometre of ocean (commonly known as the upper-ocean ) are intricately layered so that a number of water masses make up the upper Indian Ocean off the Gascoyne. These water masses can be identified through examination of relationships on T/S diagrams, which are graphs showing the relationship between temperature and salinity as observed together at, for example, (a) various depths in a water column, or (b) various sampling stations across a stretch of ocean. Figure 2.8: T/S diagram showing a vertical profile of water masses found at a station ( S, E) offshore Shark Bay. (Depths along T/S curve are in metres. Isopleths of constant density are in σ t. Cruise S05/86 station data retrieved from CSIRO on-line database.) 45

46 Offshore the Gascoyne, where water depth exceeds 1 km, the vertical water column T/S diagram (e.g. Figure 2.8) typically shows the presence of i) surface tropical water, ii) subtropical salinity maximum core, iii) South Indian Ocean Central Water (SICW) and iv) Antarctic Intermediate Water (AAIW). i) Surface tropical water - low salinity (<35.00), high temperature ( C) surface water. Generally, the low salinity character of these surface waters derive from an influx of low salinity Pacific Ocean water through the Indonesian Archipelago, as well as from an excess of rainfall over evaporation in the NE quadrant of the Indian Ocean. This water mass is also associated with low nutrient (near zero) and high dissolved oxygen (near saturation ml/l) concentrations. ii) SICW (South Indian Ocean Central Water) - high salinity (>35.60), C waters, typically found drifting northwards on σ t. This high salinity layer has been called southern subtropical surface water (Muromtsev, 1959), tropical surface waters (Ivanenkov and Gubin, 1960) and subtropical surface water (Wyrtki, 1973). Depth of the salinity maximum core decreases with latitude. Rochford (1969a) observed it at a depth of 300m around 10 0 S, and at less than 100m at 33 0 S. And in-between those latitudes (at 18 0 S), Warren (1981) found the core at an intermediate depth of 250m. The salinity maximum can be traced back to the sea surface at latitudes S where high salinity water is found across the breath of the Indian Ocean (Wyrtki, 1971). On the sea surface at these latitudes, an excess of evaporation over precipitation forms the high salinity water (Baumgartner and Reichel, 1975). This then extends northward below the surface water until S (Church et al., 1989), where it abuts against the low salinity water flowing westward from the Indonesian Archipelago in the South Equatorial Current (Sharma, 1972). iii) SAMW (Subantarctic Mode Water) - low salinity (<35.00), C water, drifting northwards on σ t surface. This water is associated with high dissolved oxygen concentrations (>5.0ml/L) and is believed to originate by Subtropical Convergence at latitudes S (Sverdrup, Johnson and Fleming, 1942; Muromtsev, 1959). Reviewing names used for this water, Warren (1981) found that by subtropical subsurface water (a C layer), Muromtsev (1959) had indicated the upper part of the same water mass. Later, in identifying the high oxygen properties of this mass, 46

47 Ivanenkov and Gubin (1960) extended the name subtropical subsurface water to include a layer virtually synonymous with the Indian Ocean Central Water. Wyrtki (1973) adumbrated a different explanation for the formation of the water mass, suggesting that the oxygen maximum was in fact a relic of deep vertical convection rather than Subtropical Convergence. Corroborating with this theory, Colborn (1975) and McCarthy (1977) provided evidence of well-mixed layers extending m from the sea surface at S latitudes in late winter, and subsequently named the layer Subantarctic Mode Water. Retaining this name, Toole and Warren (1993) also explained that a shallow ( m) oxygen maximum in their 18 0 S section formed during winter cooling, by deep convective overturning in the zone between the Subtropical Convergence and the Subantarctic Front. iv) AAIW (Antarctic Intermediate Water) - low salinity (<34.6), C water, drifting northward on σ t surface. This layer extends northward from the Antarctic Polar Front to latitudes of S. It is associated with an oxygen minimum, and is thought to flow more slowly than the overlying oxygen maximum layer (Warren, 1981). v) NWII (North West Indian Intermediate) high salinity (34.6), C water within σ t. Although the core of this water mass was beyond the depth range of the station shown in Figure 2.9, influence by this water type can nonetheless be seen. A water mass of similar description has been reported by Rochford (1961), Newell (1974), Webster et al. (1979), Warren (1981) and Toole and Warren (1993) in other parts of the Indian Ocean. It originates from the Red Sea (Rochford, 1969a). 47

48 Figure 2.9: T/S diagram envelopes showing different water masses in the surface 40m of coastal Gascoyne waters. (Isopleths of constant density are in σ t. Cruise FR01/96 station data retrieved from CSIRO on-line database.) The T/S diagram of a 40m-deep section of surface waters found along a CTD transect at Ningaloo, shows the T/S signatures of three different water masses (Figure 2.9). The Leeuwin Current (described in Section 2.2.2), a comparatively warm, low salinity water mass, separates the cool, coastal Ningaloo Current (described in Section 2.2.4) from the offshore afore-described surface tropical water. 48

49 2.4 Conclusion This chapter has outlined some important aspects of the Gascoyne marine region, including the regional climate, bathymetry and general hydrological structure. However, it can be seen that there is a good deal of uncertainty with regard to (i) the summer coastal circulation, eg. the Ningaloo Current and Leeuwin Undercurrent, (ii) details of the three-dimensional water masses along the West Australian continental margin, and (iii) the re-circulation pattern south of Point Cloates. What is understood of these has either been inferred by comparisons with seemingly analogous regions along the West Australian coast, or derived from speculations on very limited data. In order to establish a real understanding of the oceanic system in the region, direct studies have been made to address the following questions: Summer surface circulation along the continental shelf What shelf currents are present? What are the driving forces of the shelf currents? Is the Ningaloo Current wind driven? Is there upwelling associated with the Ningaloo Current? How does the structure of the currents change as they progress along the shelf? Subsurface circulation and hydrography Which are the major water masses along the West Australian coastline? What are their attributes and alongshore structures? Is the Leeuwin Undercurrent present? What is its driving force? How do processes at the coastal margin affect the cross-shore distribution of water masses? Re-circulation pattern south of Point Cloates identified from satellite imagery What physical processes contribute to the development of the pattern there? Can this pattern result from a combination of wind, local topography and the Leeuwin Current? If so, what combination? 49

50 Consequently, three scientific papers have resulted. These are presented sequentially in the following chapters. The papers are written as self-contained works, each focusing on the study of a different domain of the Gascoyne coastal ocean. Firstly, a detailed examination of the complex circulation of surface waters (top 300m) is presented. This is followed by a description of the hydrography and subsurface circulation of the deeper ocean, down to a depth of 1km. Finally, a numerical modelling study to further explain the recurring re-circulation pattern observed in the surface waters at Point Cloates is discussed. 50

51 Chapter Three: Summer Surface Circulation along the Gascoyne Continental Shelf, Western Australia Abstract The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21 o and 28 o S. This paper presents CTD and ADCP data collected in November 2000 together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the Gascoyne continental shelf and slope. It is shown that the region comprises of a complex system of currents that are influenced by offshore eddies, wind stress, varying shelf-widths, coastal topography and outflow from the hypersaline Shark Bay. Four different water types and current systems were identified from the field measurements. The Leeuwin Current is the major current flowing through the region. It transports lower salinity, warmer water along the 200 m isobath, poleward. The signature of the Leeuwin Current gradually transformed from a warm (24.7 o C), lower salinity (34.6) water in the north to a cooler (21.9 o C), more saline (35.2) water in the south resulting from geostrophic inflow of offshore waters. The width and depth of the current also changed continuously responding to the changing bottom topography and the orientation of the coastline: in the northern section under the influence of the narrow shelf and steep slope, the current was strong (~0.75 ms -1 ) and extended deeper into the water column. In contrast, the current decelerated (to ~ ms -1 ) when flowing past the wider continental shelf offshore of Shark Bay and then accelerated along the southern section along the steep continental slope. Downwelling events were persistently associated with the current. The Ningaloo Current was confined to the northern Gascoyne shelf within 35 km of the coast. Although upwelling was detected along the northern section of the study region, adjacent to the Ningaloo coral reef, water properties suggest a re-circulation of Leeuwin Current water from the south. Changes in the shelf width at Point Cloates have a significant influence on the Ningaloo Current resulting in bifurcation of the northward current. The higher salinity outflow from Shark Bay influences the continental shelf region immediately offshore of the main entrances to the Bay through the mixing of the higher salinity outflow water with the shelf waters. 51

52 The Capes Current, a wind-driven current originating to the south of the study region was identified as a cooler, more saline water mass flowing northward. Figure 3.1: Location map of the study area including the RV Franklin cruise track through the Gascoyne continental shelf. Location of the CTD stations are shown as unfilled circles. 52

53 3.1 Introduction Eastern boundary current systems generally consist of cooler (associated with coastal upwelling) equatorward currents. However, the exception to this general pattern occurs along the eastern boundary of the Indian Ocean, where the Leeuwin Current (LC) flows poleward along the Western Australian coastline (Church et al., 1989). Observational and modelling studies undertaken over the past two decades have shown the LC to be generated by a meridional pressure gradient that overwhelms the opposing equatorward wind stress (Thompson, 1984, 1987; Godfrey and Ridgway, 1985; Weaver and Middleton, 1989; Batteen and Rutherford, 1990; Pattiaratchi and Buchan, 1991). The LC transports warmer, lower salinity, low nutrient water southwards, from a tropical source off northwestern Australia. It flows broad (400km) and shallow (50m) at the northern section of the study region, gradually narrowing (~100 km) and deepening (~300m) as it accelerates (1-1.5 ms -1 ) poleward along the shelf break, whilst being augmented by geostrophic inflow from the west (Hamon, 1965; Andrews, 1977; Church et al., 1989; Smith et al., 1991). The strength of the current varies seasonally with a volume transport of 5-7 Sv in winter (Smith et al., 1991; Feng et al., 2003) and 1.4 Sv in summer (Pearce, 1991). This seasonal change in intensity has been attributed to regional wind stress variability, i.e. the current flows weakly against maximum southerly (opposing) winds in October-March, and is stronger from April to September in the absence of strong prevailing winds (Godfrey and Ridgway, 1985). As a result of onshore flow resulting from geostrophic balance, the current is also associated with coastal downwelling (Smith et al., 1991). The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21 o and 28 o S (Figure 3.1). The region encompasses both tropical and temperate climatic features; the northern part is arid and tropical, while the southern part tends towards a more temperate, Mediterranean climate. Southeasterly winds prevail over the coastal region throughout much of the year. During winter (July), moderate southerly winds (mean monthly mean of 3 ms -1 ) occur near the shelf edge. These winds strengthen from July through November, and remain strong during summer (January-March), often blowing for several consecutive days in excess of 7 ms -1. In the summer months, the stronger winds result from a combination of the synoptic situation and strong sea breezes (Pattiaratchi et al., 1997) In May, a weaker, more variable winter wind pattern is re-established (Godfrey and Ridgway, 1985; Taylor and Pearce, 1999). 53

54 Inshore of the LC, evidence of higher salinity, wind-driven currents has been recorded at various locations along the Western Australian coastline (Rochford, 1969; Cresswell and Golding, 1980; Cresswell et al., 1989; Smith et al., 1991; Pearce, 1997; Gersbach et al., 1999; Pearce and Pattiaratchi, 1999). The Capes Current (CC) along part of the coastline north of 34 o S (Figure 3.1) is well established around November when winds in the region become predominantly southerly due to the strong sea breezes (Pattiaratchi et al., 1997) and continue until about March when the sea breezes weaken. The dynamics of the CC have been described by Gersbach et al. (1999). Here, the southerly wind stress overcomes the alongshore pressure gradient which results in the surface layers moving offshore, colder water upwelling onto the continental shelf, and the LC to migrate offshore. The Capes Current is generally located inshore of the 50 m contour. Gersbach et al. (1999) have demonstrated that inshore of 50 m contour, the wind stress dominates over the alongshore pressure gradient (see also Thomson, 1987). Of particular interest to the present study is the northward flowing Ningaloo Current (NC) (Taylor and Pearce, 1999) postulated to be driven by southerly winds, similar to the Capes Current to the south (e.g. Pearce and Pattiaratchi, 1999; Gersbach et al., 1999). Considering records of cold water anomalies along the Ningaloo coast (Simpson and Masini, 1986; Taylor and Pearce, 1999; Wilson et al., 2002), it has been suggested that coastal upwelling occurs offshore of the Ningaloo coral reef. However, detailed observational data to confirm this upwelling are unavailable. A recurring feature of interest, revealed by satellite imagery, was an anti-cyclonic circulation pattern located immediately to the south of Point Cloates (Figure 3.1). Although the actual dynamics of this feature are still not clearly understood, a Platform Transmitter Terminal (PTT) (which inadvertently detached from a whale shark and became a current drogue) indicated it to be a region of some degree of cross-shelf exchange/re-circulation (Taylor and Pearce, 1999). Through analysis of satellite imagery and numerical modeling results, strength of the re-circulation has been related to the local wind speed (see Chapter Five). It has been speculated (Taylor and Pearce, 1999) that the presence of the NC and the circulatory movement of water may have major implications for the regional ecosystem. 54

55 Shark Bay, a large (14,000km 2 ) semi-enclosed embayment, is located along the southern section of the Gascoyne coastline (Figure 3.1). The Bay is shallow (average depth 10m) and hypersaline, with a bottom that declines seaward. The Bay is open to the ocean at three locations: (1) Geographe Channel - 35 km wide, 35 m deep; (2) Naturaliste Channel - 25 km wide, 40 m deep; and (3) South Passage - 2 km wide, 6 m deep. These channels, particularly the two largest, serve as the only flux paths between the Bay and the continental shelf waters (Burling et al., 2003; Nahas et al., 2005). Shark Bay receives minimal terrestrial runoff and experiences higher levels of evaporation than rainfall. Thus, the salinities experienced inside Shark Bay are consistently above oceanic levels and the innermost reaches regularly exceed 60 (Logan and Cebulski, 1970) In this paper, we use CTD and ADCP data, collected in early austral summer (November, 2000), together with meteorological data and satellite imagery to describe the structure of the upper ocean current system, to a depth of 300 m, between Shark Bay and North West Cape (Figure 3.1). For a description of the deeper waters, the reader is referred to Chapter Four. The extensive data set revealed the dynamics of the different surface current systems (the Leeuwin, Capes and Ningaloo Currents) on the Gascoyne continental shelf during the summer. The physical oceanographic controls on primary productivity of the region are presented in Hanson et al. (2005), while numerical simulation of the mean flow properties of the LC system is presented in Meuleners et al. (2005). 55

56 3.2 Methodology Data were collected aboard the RV Franklin, a 55 m Australian National Research Facility vessel, between 13 and 27 November 2000, at the beginning of the Austral summer. Instrumentation used on board included a Neil-Brown Conductivity- Temperature-Depth (CTD) profiler with 24x 5L-bottle Niskin rosette for calibration and water sampling; a 150-kHz RDI Acoustic Doppler Current Profiler (ADCP) linked to the Global Positioning System (GPS); meteorological sensors and a near surface thermosalinograph. Eleven standard transects were conducted across the continental shelf and upper slope between 21 0 S and 28 0 S (see Figure 3.1). At each transect, 5-13 CTD stations were occupied depending on the shelf width. The transects generally extended from the coast (30 m isobath) to the 1000 m contour, with stations positioned over the isobaths of 30 m, 100 m, 150 m, 200 m, 250 m, 300 m, 500 m, 750 m, and 1000 m, and at intermediate intervals between isobaths if they were widely spaced, especially along the continental shelf region < 100 m depth. Surface salinity, temperature and water depth were monitored continuously. In this thesis, salinity is expressed according to the practical salinity scale and thus has no units. Wind data during the voyage were recorded using the underway meteorological station on board. In addition, half-hourly wind data were obtained from three land-based meteorological stations at Learmonth, Shark Bay and the Abrolhos (North) Island (Figure 3.3). During the cruise, concurrent Advanced Very High Resolution Radiometer (AVHRR) and Sea-viewing Wide Field-of-view Sensor (SeaWifs) data were made available through a collaborative arrangement with David Griffin of the CSIRO Division of Marine Research (see also Griffin et al., 2001). 56

57 Figure 3.2: Bathymetry off the Gascoyne continental shelf and offshore regions. Changes in the continental shelf and slopes of selected transects (B, E, G, I and J see Figure 3.1) are shown to a maximum depth of 1000m with the location of the 200m isobath. 57

58 Figure 3.3: Wind data (arrows point with direction of wind flow) collected on board the vessel and together wind-roses from land based stations: Learmonth, Shark Bay and Abrolhos Island for November

59 3.3 Results and Discussion The extensive data set provides detailed insight into the dynamics of the coastal circulation regime on the Gascoyne continental shelf and upper slope during the early summer of It includes the Leeuwin, Capes and Ningaloo Currents as well as the outflow from Shark Bay. Figure 3.4 shows the T/S diagram constructed from CTD data taken from the 100 m surface layer across all 11 transects, from the coast out to 1000 m isobath. Notably, signatures of Shark Bay and Capes Current waters were both conspicuously separate from that of the LC. In contrast, the NC comprised re-circulated Leeuwin water, so its signature and that for the LC overlapped. The signature for the LC itself appeared elongated due to its ageing as it progressed along the coast. The LC changed its T/S characteristics markedly through the study area, and also its velocity structure through acceleration, deceleration and directional changes Topographic Controls Within the study region, the continental shelf and slope change remarkably and exhibit a range of continental shelf shapes (Figure 3.2, see also James et al., 1999). North of the coastal promontory at Point Cloates (Figure 3.1), the continental shelf 4 is extremely narrow (< 10 km). It descends in a cliff-like manner into the 5 km deep Cuvier Abyssal Plain without any visible slope-steepening at the 200 m isobath to indicate the shelf break (Figure 3.2). South of Point Cloates, the coastline veers sharply eastward away from the 200 m contour, resulting in a broader shelf (~ 40 km at Transect E). At Shark Bay (Figure 3.1), the shelf width is at a maximum (~85 km offshore Naturaliste Channel), and the shelf break becomes an increasingly pronounced feature. To the south of Shark Bay, the shelf width decreases with a distinct shelf break and the continental slope steepens (Figure 3.2). These changes in topography largely influence the circulation patterns within the region. 4 Continental shelf along the Gascoyne shelf is taken to be the seabed shoreward of the 200m-isobath. 59

60 Figure 3.4: T/S diagram for 100m surface layer of water, from the coast to the 1000m isobath, including all 11 transects. Larger filled circles indicate LC at its strongest poleward flow on each transect. 60

61 Figure 3.5: Surface sea temperature (SST) image together with surface current vectors measured from the ship-borne ADCP. 61

62 3.3.2 The Leeuwin Current The continental shelf-break circulation off Western Australia is dominated by the LC and was clearly identified in the field data collected during the study. The satellite derived SST indicated a large raft of warm water (> 24 o C) present in the north (Figure 3.5), reflecting the source waters of the LC. The surface ADCP data collected along the cruise-path indicated that the strongest southward LC flow, i.e. the core, was located along the 200 m isobath (Figure 3.5). As the location of the 200 m isobath changed, the location and the flow direction of the core also changed (see below). The surface temperature distribution showed higher temperature water in the north of the study area and a general cooling towards the south. For example, at the northern Transect A, the SST was o C whilst to the south of Shark Bay (Transects I and J), the SST was o C (Figure 3.5). The presence of several meanders and eddies entraining the warmer LC water offshore can also be identified from the SST imagery (Figure 3.5). The surface salinity distribution also indicates changes in salinity as the LC flows southwards. In the northern region, the surface salinity is 34.6 increasing to 35.2 at the southern end of the study region (Figure 3.6). The cross-shelf CTD and ADCP transects provide detailed information on the crosssectional structure of the LC. For example, Transect D (Figure 3.7) demonstrates that the LC can exist as a distinct core that has lower salinity (< 35 for this transect), is warmer (> 22 o C for this transect) than the surrounding water and flows strongly poleward (up to 0.45 ms -1 ), transporting 1.67 Sv. Temperature and salinity isopleths beneath the LC appear depressed, suggesting downwelling. Transect J (Figure 3.8) displays similar features of a relatively warm, low salinity core, except that the temperature and salinity values of the LC are now > C and < respectively, with the maximum current up to 0.75 ms -1 transporting 2.23 Sv. 62

63 Figure 3.6: Surface salinity distribution obtained from thermosalinograph data. 63

64 Figure 3.7: Transect D: cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 64

65 Figure 3.8: Transect J cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 65

66 As the LC flows southward along the Gascoyne shelf, it traverses over a region where the width of the continental shelf varies greatly; from an extremely narrow cliff-like structure in the north, to a very wide and gradual slope extending from Shark Bay, to a shelf which narrows with a steeper slope towards the south (Figure 3.2). Consequently, the structure and strength of the LC also changed (Figure 3.9). Examining the alongshore surface flows in ADCP records taken across the 11 transects (Figure 3.9b), the LC was observed to be narrow and strong in the north, flowing in a south-westerly direction (Transect D), broadening and decelerating as it rounded the wide section of the shelf flowing southwards (between Transects F and DH) and then rapidly narrowing and accelerating to the south of Shark Bay, along a steepening shelf aligned south-south-east (Transects I through J). Thus, at its narrowest, its surface width measured only 36 km (Transect J) with a maximum surface current of 0.6 ms -1, while at its broadest its width was in excess of 100 km with the maximum surface current 0.3 ms -1. Examining the water column beneath the maximum LC surface poleward flow, it was also seen (by tracking the -0.3 ms -1 contour) that where the LC decelerated and broadened (between Transects F and DH), it also decreased in depth (Figure 3.9a). Between Transects A to E, the -0.3 ms -1 contour was located at ~150 m. Its depth decreased to < 50 m as the current flowed past Shark Bay and then increased again south of Shark Bay to ~200 m (Figure 3.9a). Through the extraction of CTD data collected at the station closest to the strongest poleward flow, a CTD pseudo-transect line was constructed, slicing through the length of the LC core (Figure 3.10). These results indicate that the LC core became cooler and more saline as the current progressed southward. Examining the CTD data, taken from successive transects, it was impossible to define the LC presence merely by tracking water of a fixed T/S signature, because as the LC moves southwards, the T/S signature is continuously modified through mixing. The Leeuwin Current T/S signature, beginning at the top left corner of the T/S diagram (i.e. higher temperature and lower salinity), gradually moved towards the bottom right corner with lower temperature and increased salinity (Figure 3.4) with a corresponding increase in density. Hence, through the course of its passage within the study area, the core temperature and salinity of the LC reduced by ~3 o C, and increased ~0.6, respectively (see Figure 3.4). 66

67 Figure 3.9: (a) Alongshore velocities along the line of maximum LC poleward flow. Shaded region indicates poleward flow >0.3 ms -1. (b) Surface alongshore surface velocity across individual transects. 67

68 Figure 3.10: (a) Salinity and (b) temperature properties along the length of the Leeuwin Current; and (c) geostrophic flow relative to 300db across the 1000m isobath. Unshaded areas indicate flow towards the Leeuwin Current (ie eastward flow). 68

69 Evaporation and atmospheric cooling most probably contributed to the changes in water properties. However, the major reason for the LC ageing is the entrainment and mixing with the offshore waters. The LC is driven by an alongshore geopotential gradient coupled with an eastward geostrophic flow from the central Indian Ocean (Smith et al., 1991). Using the CTD data collected during the present study, the geostrophic inflow, relative to the 300db level across the 1000 m and 500 m isobaths, was estimated to be 4 Sv and 2 Sv, respectively, with the highest inflow of water between Transects F and I (see Figure 3.10c). These values are similar to previous estimates of the geostrophic inflow predicted using field data (between Sv from Smith et al., 1991) and numerical modelling (2.2 Sv by Meuleners et al., 2005). The source waters of the inflow are characterised by cooler, more saline water originating from Indian Ocean Central Water. Meuleners et al. (2005) estimated that, within the study region, up to 40% of the total flow of the LC can be derived from the geostrophic inflow and thus the entrainment of the relatively cooler, more saline water into the LC is the dominant mechanism for the LC ageing. Eddies and meanders are features of the LC (e.g. Pearce and Griffiths, 1991) and the present study documented oceanic eddies on three transects. At Transect D, a large anticyclonic eddy was observed (see Figure 3.11a) and resulted in the water at the offshore end of the transect to flow equatorward at speeds of up to 0.65 ms -1 (Figure 3.5). The eddy consisted of warmer, lower salinity water, similar to the younger LC water found further north. Eddies were also observed at the ends of Transects I and J, spinning clockwise and anti-clockwise, respectively (Figure 3.11b). In addition to entraining LC into the direction of their flow (seaward and coastward, respectively), these eddies also introduced cooler (20.9 o C), more saline surface waters (35.4) into the Leeuwin Current region. 69

70 (a) (b) Figure 3.11: (a) Anticyclonic eddy at Transect D, transporting warm, low saline, tropical water. (b) Clockwise eddy at Transect I, anti-clockwise eddy at Transect J; both transporting cooler, saline water from offshore. 70

71 3.3.3 The Capes Current The structure of the continental shelf circulation along the southwest coast of Australia during the summer months has shown the existence of a cooler northward current, the Capes Current (CC), with the southward LC, in general, located further offshore (Gersbach et al., 1999; Pearce and Pattiaratchi, 1999). Pearce and Pattiaratchi (1999) observed the CC moving equatorward along the southern WA coast from Cape Leeuwin (34 o S) to north of Perth (32 o S), and suggested that the CC may extend as north as the Houtman Abrolhos Islands (29 o S). A cooler, higher salinity signature found at the coast on Transect J provides a likely indication that the CC had reached a little further north (28 o S). Its properties can be identified on the T/S diagram (Figure 3.4) as a band of water with salinity between and and temperature between 21.0 o C and 21.4 o C. The cooler water can also be identified in the satellite SST image between latitudes 30.5 o S and 27 o S (Figure 3.5) and is also associated with higher salinity (Figure 3.6). The cross-sectional properties of the current can be seen in Figure 3.8. Here, the continental shelf water, inshore of the LC, indicates a northward current carrying cooler, more saline water northward at a maximum alongshore velocity of 0.15 ms -1 or 0.2 Sv. There is no evidence of upwelling along this transect. However, as the local winds were southerly, it is likely that the cooler, more saline water resulted from both upwelling and advection into the study region from the south. Along this transect enhanced biological productivity was observed (Hanson et al., 2005) and is further evidence of upwelling. 71

72 Figure 3.12: Transect A cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 72

73 3.3.4 The Ningaloo Current The Ningaloo Current, defined as a northward flowing current on the continental shelf (Taylor and Pearce, 1999), was observed at the northernmost region of the study area, between 21 o S and S. As is typical of the coastal currents found along the coastline further south of our study area, the NC at Transect A was also confined to within 35 km of the coast, inshore of the LC (Figure 3.9b) (with exceptions where coastal offshoots at Transects B and C occurred during the study). CTD and ADCP data indicated that the coastal flow at Transect A comprised of colder (< 23 o C) saline water (34.92) when compared to offshore waters. This surface water mass, with a depth of 50 m, was moving northward with the prevailing wind at a maximum surface alongshore velocity of 0.35 ms -1 (Figure 3.12). The upward-sloping isohalines and isotherms beneath the NC are indicative of coastal upwelling (Figure 3.12). Hanson et al. (2005) found that the inshore region of Transect A contained higher nutrient concentrations which were reflected in higher phytoplankton biomass and maximum regional primary production rate of 1310 mg C m -2 d -1. This confirms the observation of coastal upwelling along this transect with the source waters for upwelling originating from depths of ~100m (Figure 3.12; see also Hanson et al., 2005). Elsewhere in the NC system, complexities to the flow are observed, most notably as a result of interactions with a coastal promontory and its associated bathymetry, as well as with the LC, which flows close to the coastline of the Exmouth peninsula. The seaward extension of the coastal promontory at Point Cloates effectively blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north (see Figures 3.1 and 3.2). The reduction in the cross-sectional area, to the 50 m contour, between to the south and to the north of the promontory is ~80%. Satellite SST imagery and ADCP measurements showed that on the southern side of Point Cloates, the NC flowed northward along the coast (Figures 3.5 and 3.7). On approaching the promontory at Point Cloates, it turned anti-clockwise with the curvature of the promontory, moving westward across the shelf at 0.2 ms -1 onto the shelf-break (see Figures 3.5 and 3.13). This resulted in relatively cool, higher salinity (mean values: 22.7 o C, 35.0) NC water being present over the inner shelf at Transect D (Figure 3.7). On reaching the seaward margin of the promontory, part of the NC continued northward 73

74 towards North West Cape, inshore of the 200m contour and the Leeuwin Current, whilst the rest of the current flowed poleward along with the LC (Figure 3.13). The dynamics of this recirculation pattern using satellite imagery and numerical modelling is described in Chapter Five. In the satellite SST imagery, the arm of the NC proceeding northward along the edge of the peninsula was seen as a narrow strip of cooler water along the coast (Figures 3.5 and 3.13). This flow, which is also reflected in the CTD data, transported water with different TS characteristics to the local LC flowing offshore. However, its T/S signature bore close resemblance to that of the older LC found further south at Point Cloates. This indicates re-circulation of the older LC northward within the NC. Whilst moving northward along the constricted shelf off the peninsula, the NC interacted with the LC (which flowed in the opposite direction at the shelf-break very close to the coast) and created two coastal offshoots along Transects B and C, extending beyond the 1000 m contour (Figures 3.5 and 3.13). These offshoots, with surface currents (cross-shore) up to 0.35 ms -1 flowed perpendicular to the coast (and thus the Leeuwin Current) transporting colder, high productive water offshore (Hanson, et al. 2005). These two offshoots also had an influence on the geostrophic flow at the 1000 m contour which reflects the offshore movement of water (Figure 3.10). The arm of the NC that did not progress northward of the promontory was swept southward along the LC against the wind. This produced the poleward NC observed in Transect E (Figure 3.14). Here, although all the coastal water is moving southward, T/S analysis clearly revealed that apart from LC water, there was the cooler, more saline NC water present near the coast. Notably, the NC water detected both here and at the previous transect (i.e. at Transects D and E) bore close resemblance to the aged LC water found in the next transect south (Transect F). It is likely that the water from the LC at Transect F had been re-circulated northward up the coast as the NC. This receives some support from the finding that the surface waters at the coastal boundary of the LC were seen turning towards the coast and flowing northward. A northward flowing coastal current was not identified at Transect E. It is possible that it had traveled undetected to Transect D as a narrow coastal current beyond the coastal extent of the transect. Alternatively, it may have resulted from previous (wind induced) flowreversals in the coastal waters. 74

75 (a) (b) Figure 3.13 (a) Schematic diagram depicting detail of Ningaloo Current flow near Point Cloates. (b) Shipboard ADCP data showing surface current flow. 75

76 Figure 3.14: Transect E cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 76

77 3.3.5 Shark Bay Outflow Shark Bay is a semi-enclosed coastal embayment with open, deeper waters to the north and two shallower Gulfs to the south, bounded by three large islands to the west (Figure 3.1). The Bay is open to the ocean through two main channels; Geographe and Naturaliste Channels and a smaller channel, South Passage (Figure 3.1). Nahas et al. (2005) demonstrated the existence of a two layer flow at the Geographe and Naturaliste Channels with the less dense oceanic water (lower salinity) flowing into the Bay at the surface and the higher density Bay water (higher salinity) exiting the Bay near the sea bed. James et al. (1999) postulated the presence of a southward flowing plume of higher salinity water on the continental shelf near the seabed. Data collected during this study confirm these findings. The higher salinity of the outflow is clearly identified from the T/S diagram as a distinct surface water mass on the continental shelf, with a temperature range between 21.2 o C and 22.9 o C and salinity up to 36.1 (Figure 3.4). At Transect F, which extended offshore from Geographe Channel (Figure 3.1), higher salinity water (> 35.3) can be seen to flow from Geographe Channel out onto the shelf, with the highest salinity water (36.0) present near the seabed (Figure 3.15). Similarly, Transects G, H, DH and I, located to the south of Shark Bay, all contained higher salinity water (up to 35.4) near to the seabed at coastal stations up to 30 km offshore. A similar southward distribution of higher salinity waters on the continental shelf offshore Shark Bay was reported by James et al. (1999). This indicates that the higher salinity water after exiting Shark Bay flows southward due to the influence of the Coriolis force. 77

78 Figure 3.15: Transect F cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 78

79 Figure 3.16: Transect G cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 79

80 From close examination of the surface velocity map (Figure 3.5) and transect crosssections (Figure 3.16), it can be seen that at Transect G, whilst the LC continued to flow poleward at the shelf-break, the current along the shelf was also flowing poleward. CTD measurements show this coastal current to transport water of a distinctive 35.2 salinity signature (Figure 3.17), well mixed in the vertical water column at the coastal CTD stations landward of the 100m-isobath. This well mixed 35.2 water was subsequently detected on the coastal side of the LC, through all the southern transects of the study area (i.e. Transects G to J, Figure 3.1), hence existing as common shelf water. The water did not result from mixing between the CC and LC, since its position at 25 o S could not be explained; the CC did not extend far enough north, and the common shelf water was flowing poleward. This coastal water would not have resulted from local upwelling as the poleward current does not reflect upwelling Temperature ( o C) salinity Figure 3.17: TS-diagram for surface waters at Transect G, showing the presence of a pronounced 35.2 water as well as Leeuwin Current water. 80

81 The most plausible explanation emerged from ship-based meteorological data, which showed the wind streamed eastward on the shelf at Transect G, and then poleward on approaching the land (Figure 3.3). Moreover, meteorological data from the land based stations at both Shark Bay and the Abrolhos (North) Island showed that the northerly wind persisted for 2 consecutive days. This wind pattern would have been conducive to the development of a wind-driven southward coastal current, which in turn may have promoted mixing between the Shark Bay outflow and the coastal edge of the LC, forming the 35.2 water on the shelf. Transect H was performed a week after Transect G was completed. In the time that elapsed in between, the winds had swung back around to a southerly direction, and it could be seen from salinity profiles (Figure 3.18) that the vertically mixed structure of the coastal water column had begun to collapse. Here, although the 35.2 water was still present on the shelf at the 6 coastal stations (furthest station being at 135m-isobath), apart from the well-mixed water column at the nearest station, the upper 40 m of the other 5 coastal stations clearly exhibited more influence by LC water. Transect I extended seaward from South Passage, the southernmost and smallest opening out of Shark Bay (see Figure 3.1). Here, inshore of the LC, evidence of the common shelf water was also seen at the coast (Figure 3.19). However, salinity of the water here was 0.02 higher than in the previous transects, due to the proximity to the hypersaline bay water through South Passage. At Transect J, at the southern extent of the research area, the common shelf water was still present, albeit not immediately adjacent to the coast, since this was now where the CC flowed (Figure 3.8). The 35.2 water was detected at the stations in the coastal edge of the LC. It is likely that the coastal water had been swept along in the LC and taken offshore with it. It should be noted, however, that because sustained northerly winds are generally not a regular occurrence at the Gascoyne in summer, specifically 35.2 common shelf water might not be a frequently encountered phenomenon. A schematic of the generalised surface flow regime, during the summer, based on the field measurements presented here is shown on Figure The main features of the currents and associated water masses defined in this study are presented. The Leeuwin Current flowed from north to south and was located along the 200 m isobath. Inshore of the Leeuwin Current, three different currents and water types were identified: (1) the Capes Current flowing northwards advecting cooler, higher salinity water from the 81

82 south; (2) Shark Bay outflow, warmer higher salinity water exiting from Shark Bay and flowing southward; and, (3) the Ningaloo flowing northward, re-circulating Leeuwin Current water as well as a contribution from upwelling of colder water. In addition to the demarcation of the water types from the temperature/salinity characteristics, each of these current systems has a unique biological identity with different primary production regimes and phytoplankton species composition (Hanson et al., 2005). Figure 3.18 (a) T/S diagram at Transect H shows the presence of 35.2 water at 6 coastal stations. (b) Salinity profiles show the position of the 35.2 water to be throughout the water column at the shallowest coastal station, and at the deeper parts of 5 subsequent stations offshore. 82

83 Figure 3.19: Transect I cross-sections (to 150m) of (a) salinity, (b) temperature, and (c) alongshore velocities. Shaded region indicates poleward flow. 83

84 Figure 3.20: Schematic of the general surface circulation pattern of the major currents along the Gascoyne continental shelf. Isobaths drawn at 100m increments. 84

85 3.4 Conclusions The coastal circulation in the Gascoyne region, during the summer, comprises of a complex system of currents and their interaction with oceanic eddies, wind patterns, varying shelf-widths, a coastal promontory and a hypersaline bay. The Leeuwin Current (LC) is the major current flowing along the Gascoyne continental shelf. It transported lower salinity, warmer water along the 200 m isobath, poleward. However, because the current is continually ageing as it moves southward, cooling and gaining salt, it could not be traced as a river of water with fixed properties. The signature at its core (i.e. maximum poleward flow) is gradually transformed from a warm ( C), lower salinity (34.6) water in the north to a cooler ( C), more saline (35.2) water in the south. Although evaporation and atmospheric cooling are contributing factors to these changes, inflow of offshore waters due to geostrophic balance is the main process that controls this transformation. The LC also featured several meanders, eddies and offshoots. On its seaward side, offshore eddies were observed interacting with the current; and on the landward side, bathymetry and coastal topography also had an influence. Where unimpeded by seaward jets or outflows from the bay, the LC flowed strongly at ms -1. In contrast, it slowed to about ms -1 when flowing past the wider continental shelf offshore of Shark Bay. Downwelling events were persistently associated with the LC. The Ningaloo Current (NC) flowed along the northernmost coast of the Gascoyne within 35 km from the coast. Although upwelling was detected, its water properties were also clearly suggestive of re-circulation of LC water from more southern locations. A recurring anticlockwise flow pattern, detected south of the promontory of Point Cloates, would have caused such a re-circulation. Some indication of possible recirculation was also observed just north of Shark Bay at Transect F. The impact of such a system on the coastal ecosystem may be significant, as it would imply that coastal substances swept away by the LC could be re-circulated and returned up the coast in the NC. Shark Bay influenced the coastal system mainly through Geographe and Naturaliste Channels. While the most hypersaline water was found on the seabed within 30 km 85

86 seaward of the Channels, the surface water mixed with the LC, forming a distinctive vertical salinity profile of This mixing was likely encouraged by persistent northerly winds at the start of the voyage. The 35.2 coastal water was then swept poleward and out seaward along with the LC. Because sustained northerly winds are generally not a regular occurrence at the Gascoyne in summer, specifically the 35.2 coastal water may not be a recurring phenomenon. A cool, relatively saline coastal current flowed equatorward along the southernmost end of the research area. This was the northern continuation of the Capes Current (CC), a wind-driven current originating from further south. 86

87 Chapter Four: Hydrography and water masses off the West Australian Coast Abstract The water mass characteristics of the eastern Indian Ocean margin, between latitudes 21 and 35 S adjacent to the coastline of Western Australia are described using field measurements. Results indicated the presence of five different water masses, in the upper 1 km of the ocean, as interleaving layers of salinity and dissolved oxygen concentrations. These included: (i) lower salinity tropical surface water (TSW); (ii) higher salinity South Indian Central Water (SICW); (iii) higher dissolved oxygen Subantarctic Mode Water (SAMW); (iv) lower salinity Antarctic Intermediate water (AAIW); and, (v) lower oxygen North West Indian Intermediate (NWII) water. Through comparison of the present data (collected in 2000 and 2003) with historical data (1987), the inter- annual variability of the tropical surface water and subantarctic Mode Water were identified and were linked to ENSO events. Within the study region, the circulation pattern can be described as the Leeuwin Current system consisting of three major currents: the Leeuwin Current (LC); Leeuwin Undercurrent (LU) and shelf current systems consisting of the Capes and Ningaloo Currents. Within the northern region of the study area, geopotential gradients were found to drive both the Leeuwin Current (LC) and Leeuwin Undercurrent (LU) with a (negative) sea surface slope of 4x10-7 driving the LC poleward, whilst a (positive) slope of 1x10-7 beneath the LC was found to be the driving force of the LU equatorward. It was also found, from cross-shelf geopotential anomalies, that the surface layer (in the upper 300 m) sloped seaward at a gradient of 1.7x10-6, whilst the subsurface layer (between depths of 300 and 730 m) sloped coastward at a gradient of 6.3x10-7. This arrangement of geopotential slopes, together with the positions of the LU relative to the LC, indicated a dynamical relationship between the LU and the LC. The data indicated that the core of the LU to be located at a depth of 400 m, transporting SAMW water equatorward. The water mass distribution at the continental margin was influenced by the presence of the Leeuwin Current and Leeuwin Undercurrent at the continental shelf break and slope, 87

88 respectively. Downwelling caused by the LC resulted in the surface and subsurface water mass (SICW) and the upper edge of the SAMW to slope downward toward the shelf break, whilst subsurface upwelling beneath the LU moved AAIW and the bottom edge of the SAMW upward. Figure 4.1: Location map of the research area including the positions of CTD transect lines performed during voyages SS09/2003 and FR10/00, and CTD stations (over 1000m-isobath) taken from voyages FR87/03 and FR87/04. 88

89 4.1 Introduction The eastern Indian Ocean region, between latitudes 21 and 28 S, flanking the northcentral Gascoyne coastline of Western Australia (Figure 4.1) has received little focus. Due an absence of data collected from the local waters, the hydrography of the region has been inferred from transoceanic sections performed across other parts of the South Indian Ocean. These include: (a) meridional sections along 110 E (Rochford, 1969a); (b) zonal sections along 18 S (Warren, 1981; Field, 1997); and, (c) zonal sections along 32 S (Wyrtki, 1971; Toole and Warren, 1993). Within the study region, while the accepted classical model of water masses for the upper 1 km of ocean consists of South Indian Central Water (SICW) and Antarctic Intermediate Water (AAIW) (Pinet, 1992), data from other studies (see a-c above) have revealed the presence of additional water masses including lower salinity tropical surface water, Subantarctic Mode Water (SAMW), and North West Indian Intermediate (NWII) water. Along the eastern boundary margin of the Indian Ocean the circulation pattern is described as the Leeuwin Current system, a system of three currents: the Leeuwin Current; Leeuwin Undercurrent and shelf current systems consisting of the Capes and Ningaloo Currents (Chapter Three). The Leeuwin Current (LC is an anomalous eastern boundary current that carries warm relatively fresh tropical waters poleward along the continental shelf break) (see Section 3.3.2). Extensive observational and modelling studies have shown that the LC results from an alongshore geopotential gradient that overwhelms the opposing equatorward wind stress (Thompson, 1984, 1987; Godfrey and Ridgway, 1985; Weaver and Middleton, 1989; Batteen and Rutherford, 1990; Pattiaratchi and Buchan, 1991). The LC flows weakly (1.4 Sv) against maximum southerly (opposing) winds in October March (summer) (Pearce, 1991) and strongly (7 Sv) against weaker southerly winds in April September (winter) (Smith et al., 1991). It is persistently associated with downwelling (see Chapter Three). The Leeuwin Undercurrent (LU) flows northwards beneath the LC at depths of m transporting 5 Sv of higher salinity (> 35.8) oxygen-rich nutrient-depleted water at a rate of ms -1 northward (Thompson, 1984). An equatorward undercurrent was also apparent in steric height data at 500db/3000db (Wyrtki, 1971) and 450db/1300db (Godfrey and Ridgway, 1985), as well as in current meter data (at m) from the Leeuwin Current Interdisciplinary Experiment (LUCIE) (Smith et al., 1991). The LU 89

90 has been postulated to be driven by an equatorward geopotential gradient located at the depth of the Undercurrent (Thompson, 1984), and its water to be advected northward underneath the LC transporting cooler, higher salinity, oxygen rich water from the surface of the South Indian Ocean (Thompson and Cresswell, 1983). Inshore of the LC, higher salinity wind-driven currents have been recorded along various locations on the Western Australian coastline (Rochford, 1969b; Cresswell and Golding, 1980; Cresswell et al., 1985; Cresswell et al., 1989; Smith et al., 1991; Pearce, 1997; Gersbach et al., 1999; Pearce and Pattiaratchi, 1999; also see Chapter Three). Generally, the northward coastal currents are strongest in summer when the wind pattern is predominantly southerly and has the greatest contribution from sea breezes (Pattiaratchi et al., 1997). In order to obtain a direct understanding of the ocean off the Gascoyne region of Western Australia, a research cruise was conducted at the beginning of the austral summer of In this chapter, we use CTD and ADCP data to describe the hydrography of the upper ocean and to investigate the coastal circulation processes. 4.2 Data Collection The data reported here were collected during the austral summer (between 13 and 27 November 2000) on eleven standard transects across the continental shelf between 21 S and 28 S (see Figure 4.1). Instrumentation deployed onboard RV Franklin (a 55 m Australian National Research Facility vessel) included a Neil-Brown Conductivity- Temperature-Depth (CTD) recorder with a 24 x 5L-bottle Niskin rosette for calibration and water sampling, a 150-kHz RDI Acoustic Doppler Current Profiler (ADCP) linked to the Global Positioning System (GPS), a Turner Designs Fluorometer, meteorological sensors, and a near surface thermosalinograph. CTD data was also similarly obtained from FRV Southern Surveyor voyage SS09/2003 southward to S. This voyage was completed over the October/November months of the austral summer of

91 Additional historical CTD data 5 obtained during the January March period of the 1987 austral summer were retrieved from RV Franklin voyages FR87/03 and FR87/04 for comparison purposes. 4.3 Results and Discussion Five different water mass types were detected in the upper Indian Ocean along the West Australian coast (see table 4) and they correspond with accepted classical water masses of the Indian Ocean (Wyrtki, 1971; Warren, 1981). These were observed in the vertical distribution of salinity and dissolved oxygen as interleaving layers of salinity and dissolved oxygen. In terms of increasing depth these water masses were: (i) lower salinity tropical surface water (TSW) (ii) higher salinity South Indian Central Water (SICW) (iii) higher oxygen Subantarctic Mode Water (SAMW) (iv) lower salinity Antarctic Intermediate Water (AAIW) (v) lower oxygen North West Indian Intermediate water (NWII) Table 4: The different characteristics of each of the water masses found in the 1km-deep water column defined. This table combines data from voyage FR10/00 ( S) and voyage SS09/2003 ( S). Water Mass Temperature Range Salinity Range Dissolved Oxygen Range Tropical surface water (TSW) C μm/l South Indian Central Water (SICW) Subantarctic Mode Water (SAMW) Antarctic Intermediate Water (AAIW) C μm/l C μm/l C μm/l North West Indian Intermediate water (NWII) C ~ μm/l 5 CSIRO online database: 91

92 Comparing the temperature-salinity diagram and a temperature-oxygen diagram obtained from measurements from the northern study region, it is evident that the salinity and dissolved oxygen distributions have an inverse relationship (Figure 4.2). This is highlighted when examined through a three-dimensional view. A north-south cross section along the 1000 m-isobath line, parallel to the shore is shown on Figure 4.3. The salinity distribution along the cross-section indicates a salinity-maximum and two minima (Figure 4.3a) whilst the dissolved oxygen distribution also indicates an oxygenmaximum and two minima, but located at different depths to the depths of the salinity maximum and minima (Figure 4.3b). Figure 4.2: Temperature-salinity (with σ T contours) and temperature-oxygen diagrams exhibit interleaving positions of property extrema. 92

93 (a) (b) Figure 4.3: Three-dimensional blocks of ocean depicting the cross-shelf (across Transect J, the southernmost transect made by FR10/00) and along-shelf (along 1000 m-isobath) distribution of (a) salinity extrema, and (b) dissolved oxygen extrema. 93

94 Figure 4.4: Major water masses observed at the 1000 m-isobath along the Western Australian shelf. Asterisks on the surface indicate CTD stations positions. This chart combines data from voyage FR10/00 ( S) and voyage SS09/2003 ( S). The location of each the five water masses and their relative position relative to each other can be identified for the whole length of the coastline from North West Cape (21 o S) to Point D Entrecasteaux (35 o S) (see Figure 4.1 inset) using both salinity and oxygen (Figure 4.4). In the following sections, the characteristics of each of the water masses are discussed in detail. 94

95 4.3.1 Water Masses Tropical Surface Water (TSW) Salinity Minimum A layer of lower salinity (< 35.1) warmer (> 22 C) tropical water was found in the surface water in the northern region and corresponded with the temperature/salinity characteristics of the Leeuwin Current water. This water mass is derived from the Australasian Mediterranean Water (AAMW), a tropical water mass with origins in the Pacific Ocean Central Water and formed during transit through the Indonesian archipelago (Tomczak and Godfrey, 1994). Field data revealed that this surface water mass was associated with lower nutrient (near zero) and higher dissolved oxygen concentrations. (The reader is referred to Chapter Three for detailed discussions on the different coastal surface water types and their dynamics). At the North West Cape (21 o S), the northern extent of the study region, this water mass extends to 180m (Figure 4.4) with the surface salinity < The depth of the water mass decreases southwards with the passage of the Leeuwin Current and at ~26 o S its salinity signature (< 35.1) disappears. This is due to the dynamics of the Leeuwin Current. The Leeuwin Current is driven by an alongshore geopotential gradient and entrainment of cooler more saline South Indian Central Water (see below) from offshore due to geostrophic inflow is a feature of the Leeuwin Current (Chapter Three) South Indian Central Water (SICW) Salinity Maximum South Indian Central Water (SICW) is identified here as a salinity maximum layer ( ). Along the 1000m bathymetric contour, ADCP data revealed its core moving northward along 26.8 σ T, with a maximum speed of 0.3 ms -1. However, near the shelf break this same water mass is part of the Leeuwin Current flowing southwards (Chapter Three). Here, ADCP data indicated that the Leeuwin Current extends up to 300m water depth which is the total depth of this water mass (Figure 4.4). SICW had a temperature range of C and was associated with weak minima of dissolved nitrate, silica, and phosphate. It was found at the surface south of 29.0 S and the depth of the salinity maximum increased northward: from the surface at 29.0 S to 245 m at 21.5 S. In the 95

96 northern latitudes of the study region, the water mass subducted underneath the tropical surface water derived from Australasian Mediterranean Water (AAMW). The observation of surface salinity maximum is in agreement with Wyrtki (1971) who found higher salinity water was found across the breath of the surface Indian Ocean at latitude range S. At these latitudes, an excess of evaporation over precipitation forms the higher salinity water at the sea surface (Baumgartner and Reichel, 1975). This is then subducted below the surface water (Karstensen and Tomczak, 1997), extending northward until S (Church et al., 1989) where it meets the lower salinity Australasian Mediterranean Water (AAMW), flowing westward from the Indonesian Archipelago in the South Equatorial Current (Sharma, 1972; Tomczak and Godfrey, 1994). In addition to being termed South Indian Central Water (Webster et al., 1979; Rochford, 1969a) and Indian Central Water (Karstensen and Tomczak, 1997), this high salinity band has also been referred to as southern subtropical surface water (Muromtsev, 1959), tropical surface waters (Ivanenkov and Gubin, 1960) and subtropical surface water (Wyrtki, 1973) Subantarctic Mode Water (SAMW) Oxygen Maximum Beneath the South Indian Central Water (SICW), a water mass with high dissolved oxygen concentrations of µm/l can be identified as Subantarctic Mode Water (SAMW), whose core occurred at m. The data revealed that SAMW consisted of water with temperature range of C and salinity range of Its σ T value ranged between 28.9 and SAMW is formed by deep winter convection at S in the zone between the Subtropical Convergence and the Subantarctic Front to the south of Australia (Wyrtki, 1973; Colborn, 1975; McCartney, 1977; Toole and Warren, 1993; Karstensen and Tomczak, 1997). It is postulated that the SAMW formed to the south of Australia is transported westward by the Flinders Current (Middleton and Cirano, 2002) and is the source waters for the Leeuwin Undercurrent transporting water northward along the WA coast (see below). 96

97 As SAMW is formed by deep convection rather than subduction, newly formed SAMW penetrates to a greater depth than the newly subducted SICW (thus, is comparatively better ventilated) and then moves northward from its formation region. Due to its high oxygen content, the SAMW plays an important role in ventilating the lower thermocline of the southern hemisphere subtropical gyres (McCartney, 1982). SAWM also corresponds to the Indian Ocean Central Water (ICW) defined by Sverdrup et al. (1942). SAMW and ICW often have similar temperatures and salinities; consequently SAMW has been thought to contribute to the depth range of ICW (Karstensen and Tomczak, 1997). According to Karstensen and Tomczak (1997), the source characteristics of SAMW differ from region to region depending on prevailing atmospheric conditions during its formation. Evidence of the SAMW make-up changing with prevailing atmospheric conditions (as mentioned above) is found by comparing our data, which was collected during a non-el Niño-Southern Oscillation (ENSO) years (2000 and 2003), with data from 1987, an ENSO, year. The CTD data from RV Franklin voyages FR03/87 and FR04/87 (Figure 4.1), a composite image of water masses following the 1000 m-isobath along the West Australian coastline was constructed (Figure 4.5). Although relatively low resolution compared to Figure 4.4 due to scarcity of data, the overall picture from 1987 clearly shows the change in SAMW; its dissolved oxygen signature was 5 10 µm/l higher in 2000 than it had been in 1987 (Figure 4.6a). Additionally, the lower boundary of the SICW overlapped with the top of the SAMW (Figure 4.5), the interaction resulted in lower salinity at that interface (Figure 4.6b). Overall, the positions of all the major water masses remained relatively unchanged between the years, and there were no changes to the core-defining signatures of any other subsurface water mass. Understandably, because atmospheric conditions have direct influence on the water, changes were seen in the surface water, i.e. the surface salinity minimum was less intense, and surface water with as low salinity did not appear to extend as far south in 1987 as in The Leeuwin Current would have been a contributing factor to this observation since it transports more lower-salinity water southward during ENSO years than non-enso years (Cresswell et al., 1989; Pattiaratchi and Buchan, 1991). 97

98 Figure 4.5: Sparse CTD data indicate the major water masses observed at the 1000 m-isobath in SAMW was less ventilated in 1987 than in 2000/3 (Figure 4.4). Asterisks on the surface indicate CTD stations positions. 98

99 Dissolved oxygen (microm/l) depth m latitude S (a) Salinity depth m latitude S (b) 0 Figure 4.6: Differences between observations from 1987 (Figure 4.5) and 2000/3 (Figure 4.4) of (a) dissolved oxygen, and (b) salinity. Shaded areas indicate values were greater in 1987 than in 2000/3. 99

100 Antarctic Intermediate Water (AAIW) Salinity Minimum Below the SAMW, a salinity minimum ( ) was observed, indicating the presence of Antarctic Intermediate Water (AAIW) along the coast. The water was cold (4.5 8 C) and the position of its core became shallower northward (core depth of 875 m at 27.5 S and 520 m at 21.5 S). Its σ T values spanned It has been reported the AAIW extends northward from the Antarctic Polar Front to latitudes S, and is thought to flow more slowly than the oxygen maximum layer above it (Warren, 1981) Northwest Indian Intermediate (NWII) Water Oxygen Minimum An oxygen minimum signature of < 110 µm/l in the northern region ( S) indicated the presence of Northwest Indian Intermediate (NWII) water immediately beneath the AAIW. Occupying depths of m, with σ T values of , its orientation implied southward deepening. As such, it is possible it extends further south into the deeper ocean. The temperature of the NWII water was recorded at less than 5 C and its salinity ranged NWII water was associated with maxima of dissolved nitrate, silica, and phosphate. A similar water mass of Red Sea origin (Rochford, 1964) was observed by Rochford (1961), Newell (1974), Webster et al. (1979), Warren (1981), and Toole and Warren (1993) in other regions of the Indian Ocean. The low oxygen values are the result of insitu consumption of dissolved oxygen in water that has not been in contact with the atmosphere for a long time, presumably due to much slower overall horizontal flow at such depths (Warren, 1981). 100

101 Shallow Oxygen Minimum Although not associated with a particular water mass, an oxygen minimum layer was observed occurring beneath the surface layer throughout the study region (Figure 4.4). It was associated with maxima in nitrate, silica, and phosphate concentrations. Its core followed the 26.1σ T level, reaching depths of m. The intensity of the oxygen minimum increased northward from 204 µm/l at 27.5 S to 175 µm/l at 21.5 S. A similar oxygen minimum layer was reported by Rochford (1967, 1969a), Webster et al. (1979), Warren (1981), and Church et al. (1989). Rochford (1969a) also found a salinity minimum associated with the oxygen minimum layer and Rochford (1969a) postulated that as the oxygen minimum strengthened southward, he concluded that the layer had been formed by a southward advection of lower salinity, lower oxygen tropical water. It is likely that the southward motion recorded by Rochford (1969a) in closer to the shelf edge was actually the result of the Leeuwin Current s southward motion, which dominated the flow at the shelf break and slope (Section 3.3.2). In contrast to Rochford (1969a), the data collected during this study does not indicate that the shallow oxygen minimum layer was associated with a lower salinity layer. In fact, the oxygen minimum layer was associated with higher salinity south of 23 S and lower salinity to the north. This is because the depth of the shallow oxygen minimum s core remained fairly constant at approximately 160 m throughout the study region (Figure 4.4). Depending on the latitude at which observations were made, the oxygen minimum layer was present in two different water masses. To the north it existed within the surface tropical water whilst to the south it was associated with the higher salinity SICW sometimes within its core (i.e. at S). The data indicate that the shallow oxygen minimum layer was closely related to a deep chlorophyll maximum (DCM), centred at a depth approximately 80 m, found throughout the study area. The shallow oxygen minimum layer was located directly beneath the DCM at each station (Figure 4.7). This finding is important when coupled with the detection of increased level of nitrate, silica, and phosphate concentration, as well as an increasing minimum northward, as this provides evidence to reject Rochford s (1969a) hypothesis. Warren (1981) proposed that the water could be northward flowing, with an intensifying oxygen minimum occurring when sinking detritus gradually decayed and released nutrients. Photosynthetic organisms of the DCM 101

102 220 live at depths with an optimal balance of radiance from the surface and nutrients from below (Hanson et al., 2005). Hence, the depth of the oxygen minimum layer forms beneath the DCM regardless of the type of water mass present dissolved oxygen (microm/l) 230 depth m oxygen min line (a) depth m fluorometer units oxygen min line latitude 0 E (b) Figure 4.7: 1000 m-isobath cross-sections of (a) dissolved oxygen, and (b) chlorophyll. The dashed line in both charts traces the core of the band of shallow oxygen minimum water. 102

103 4.3.2 Surface and Sub-surface Current Systems Along the West Australian continental shelf margin, there are three main current systems which are collectively defined here as the Leeuwin Current System (Figure 4.8): (1) a wind-driven equatorward coastal surface currents, the Capes and Ningaloo Currents, on the continental shelf present mainly during the summer months under strong southerly wind stress; (2) Leeuwin Current (LC), a poleward surface current generally located along the shelf break; (3) Leeuwin Undercurrent (LU), an equatorward subsurface undercurrent located on the continental slope. The dynamics of the Capes and Ningaloo Currents as well as the Leeuwin Current have been discussed in detail in Chapter Three. Here, we examine the dynamics of the Leeuwin Undercurrent, which has received very little attention in the literature. These three current systems can be identified clearly from a cross-shore ADCP transect obtained along Transect I (Figures 4.1 and 4.9). Due to the depth limitation of the ADCP (to approximately 300 m), only the upper portion of the LU can be identified; nonetheless, the pattern was indicative of an undercurrent at greater depth. The pattern observed in the ADCP data was closely correlated with the numerical modelling results of the LU performed by Meuleners et al. (2005) which indicated the core of the LU to be centred at a depth of 400 m (Figure 4.9). 103

104 Figure 4.8: Schematic diagram illustrating the general flow patterns at the continental margin. 104

105 The depth of the LU core can be confirmed by estimate of the geostrophic flow obtained from CTD data (Figure 4.10). The LU is closely associated with the Subantarctic Mode Water (SAMW) identified in section Along Transect I, the longest cross-shore CTD transect (Figure 4.1), a complete cross-section of the LU core can be identified from the dissolved oxygen distribution: the core of the current consisted of a dissolved oxygen maxima (252 μm/l) centred at a depth of approximately 400 m (Figure 4.11). Although the LU may contain some SICW in the upper layers the data presented here clearly shows that the LU consists of SAMW water and not a tongue of higher salinity SICW water as reported by Thompson (1984) (Section 4.1). Figure 4.9: Transect I cross-section of ADCP alongshore velocities (ms -1 ) shows equatorward LU and coastal current, and a poleward LC. The trace lines show the positions of the LC and LU as numerically modelled by Meuleners et al. (2005). 105

106 depth m longitude 0 E Figure 4.10: Transect I cross-section of geostrophic flow (ms -1 ) relative to the surface shows the LU flowing equatorward at a depth of 400 m. Figure 4.11: Cross-section of dissolved oxygen levels for Transect I shows the presence of a > 252 microm/l core at 400 m depth. 106

107 The Leeuwin Current is driven by an alongshore geopotential gradient (Thompson, 1984; Godfrey and Ridgway, 1985; Smith et al., 1991) which through geostrophy results in onshore flow and downwelling. This is clearly seen as downward-sloping isotherms and isohalines at the shelf break (Figure 4.12) and results in the surface water together with the SICW and the upper edge of the SAMW to be depressed downward approaching the shelf break (Figure 4.3). Since the LU flows in the opposite direction, the opposite occurs, resulting in concomitant upwelling. This is seen as upward-sloping isotherms and isohalines beneath 400 m (Figure 4.12). Hence, the AAIW and NWII water are drawn upward approaching the shelf slope, as is the bottom edge of the SAMW (Figure 4.3). The alongshore geopotential gradients estimated from the data from the present study along the 1000 m isobath (Figure 4.13) indicate a negative sea surface slope of 4 x10-7 (from north to south) at the surface 0 to 300db level which is the driving force of the Leeuwin Current (see above). In the 300 to 1000 db layer the slope was reversed with a value 1 x10-7 which is the driving force of the LU equatorward. These values are comparable with previous studies: Thompson (1984) found that the surface slope to be 3.6 x 10-7 whilst Smith et al. (1991) reported it be 2.6 x Similarly, for the Leeuwin Undercurrent, sub-surface slopes of 0.4 x 10-7 and 0.2 x 10-7 were reported by Thompson (1984) and Smith et al., respectively. These results confirm that the alongshore geopotential gradient remains almost constant through both seasonal and inter-annual time-scales (Godfrey and Ridgeway, 1985). The cross-shelf geopotential anomalies indicate (Figure 4.14) thicker surface layer coastward and a thicker subsurface layer seaward. We have limited the calculations for the subsurface layer to a maximum depth of 730 m in order to have sufficient data points to plot a line with. To counter the scarcity of data points, calculations were repeated for Transects E, F, G, and H, and average values for geopotential slope were subsequently obtained (The other transects were excluded as their data proved unsuitable for calculations due to interference from jets and eddies, or from insufficient measurement depths). It was found that the surface layer (6 300 m depth) sloped seaward at a gradient of 1.7x10-6, and the subsurface layer ( m depth) sloped coastward at a gradient of 6.3x10-7. In geostrophic balance, this arrangement of geopotential slopes contributed to the poleward flow in the surface layer and the equatorward flow in the subsurface layer. 107

108 Figure 4.12: Transect I cross-sections of (a) salinity, and (b) temperature. Circles along the surface indicate CTD stations positions. 108

109 10 geopotential anomaly m 2 s D 6/ D 300/ latitude 0 S Figure 4.13: Geopotential anomaly in m 2 s -2 plotted versus latitude from CTD stations recorded along the 1000 m-isobath with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 996 m. 109

110 geopotential anomaly m 2 s D 6/ D 300/ longitude 0 E Figure 4.14: Geopotential anomaly in m 2 s -2 plotted versus longitude from CTD stations recorded along Transect E with straight-line fits. (a) Calculated between depths 6 and 300 m, and (b) between depths 300 and 730 m. 110

111 Due to the Coriolis force, the LC remains adjacent to the coast but the dynamics which control the LU, causing it to be adjacent to the continental slope is unclear. Thomson (1984) suggested that the presence of the continental slope allows the constraint of the earth s rotation to be broken, although no theoretical explanation was presented. Based on the geopotential gradients discussed above, however, we suggest a mechanism based on Margule s equation (Figure 4.15). At the shelf edge, the LC has an onshore component of flow which results in the depression of the lower layer, particularly beneath the LC. The cross-shore gradient of the subsurface layer is thus increased, causing an associated LU flow close to the continental slope. This implies that anywhere along the continental slope wherever the LC flow results in downwelling, an undercurrent is induced to flow closely beneath it. Along the southern shelf of the Australian continent, the Flinders Current, an undercurrent along the continental slope has also been identified beneath the eastward flowing LC. Here, the Flinders Current is flowing westward (Middleton and Cirano, 2001). Similar to the LU, the Flinders Current is also found along the shelf slope and centred at a depth of 400 m (Middleton and Cirano, 2001). It is most likely that the Leeuwin Undercurrent is a continuation of the Flinders Current transporting subantarctic mode water (SAMW) from its generation region northward. ρ 1 i 2 = f g ρ2v2 ρ1v ρ2 ρ1 1 ρ 2 = flow out of paper = flow into paper Figure 4.15: Relationship among density, geostrophic velocity, and the slope of the interface between layers, as given by Margule s equation. (Adapted from Knauss, 1997.) 111

112 4.4 Conclusions Along the coast of Western Australia, five different water masses were identified as interleaving layers of salinity and dissolved oxygen extrema. From the surface these water masses were: (i) low salinity tropical surface waters; (iii) high salinity SICW; (iv) low oxygen SAMW; (v) low salinity AAIW; and, (vi) low oxygen NWII water. The shallow oxygen minimum layer was formed as a result of sinking detritus from a gradually decaying DCM layer. This was confirmed in the detection of depleted oxygen levels that intensified northward with the direction of flow, raised nutrient levels, lack of a fixed salinity signature, and the persistent presence of a DCM immediately above the shallow oxygen minimum band. With regard to changes in the water masses due to prevailing atmospheric conditions, it was found that the extrema for both the oxygen minimum of the SAMW and the salinity minimum of the surface waters were weaker during the 1987 ENSO year than during the anti-enso years of 2000/3, while attributes of the other subsurface water masses remained relatively unchanged. With regard to changes in the water masses upon approaching the West Australian continental margin, it was found that coastal currents drew the coastward edges of the water masses upward or downward depending on their location in the water column. The LC flowing poleward at the surface along the shelf break caused water to downwell. Consequently, beneath it (to a depth of 350 m), the surface water, SICW, and the top part of the SAMW were depressed. The core of the LU carrying SAMW equatorward flowed at 400 m, resulting in upwelling, and causing the uplifting of NWII water, AAIW, and the bottom part of SAMW. Geopotential gradients were found to drive both the LC and LU. In the study area, a -4x10-7 slope drove the LC poleward, while a 1x10-7 slope drove the LU equatorward. Cross-shelf geopotential anomalies show a 1.7x10-6 seaward gradient for the surface layer (depth m) and a 6.3x10-7 coastward gradient for the subsurface layer (depth m). This orientation of geopotential slopes allows the direction of both currents, and the position of the LC adjacent to the coast, to be explained by the Coriolis effect; this cannot, however, explain the position of the LU at the shelf slope. 112

113 Invoking Margule s equation, an explanation for the latter is proposed: when the added surface mass at the coast (due to the LC flow) reduces the thickness of the lower layer, the resulting increase in the subsurface layer s cross-shore gradient produces a stronger LU positioned close to the shelf slope. 113

114 114

115 Chapter Five: Dynamics of the Ningaloo Current off Point Cloates, Western Australia Abstract The Ningaloo Current (NC) is a wind driven, northward flowing current present during the summer months along the continental shelf between the latitudes of 22º and 24ºS off the coastline of Western Australia. The southward flowing Leeuwin Current is located further offshore and flows along the continental shelf break and slope, transporting warm relatively fresh tropical water poleward. A recurrent feature, frequently observed in satellite images (both thermal and ocean colour), is an anti-clockwise circulation located offshore Point Cloates. Here, the seaward extension of the coastal promontory blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north. The reduction in the cross-sectional area, coast to the 50 m contour, between southward and northward of the promontory is ~80%. Here, a numerical model study is undertaken to simulate processes leading to the development of the recirculation feature offshore Point Cloates. The numerical model output reproduced the recirculation feature and indicated that a combination of southerly winds and coastal and bottom topography off Point Cloates is responsible for the recirculation. The results also demonstrated that stronger southerly winds generated a higher volume transport in the NC and that the recirculation feature was dependent on the wind speed, with stronger winds decreasing the relative strength of the recirculation. 115

116 Figure 5.1: Locality map showing the position of the research area that was used as the numericalmodelling domain in this study. 116

117 5.1 Introduction The Leeuwin and Ningaloo Currents dominate the summer continental shelf dynamics between 22º and 24ºS (Figure 5.1) off the coastline of north-central Western Australia. The Leeuwin Current (LC) is stronger and flows along the continental shelf break and slope, transporting warm relatively fresh tropical water poleward (Smith et al., 1991). Over the past two decades, extensive observational and modelling studies have revealed that LC is generated by a meridional pressure gradient that overwhelms the opposing equatorward wind stress (Thompson, 1984, 1987; Godfrey & Ridgway, 1985; Weaver & Middleton, 1989; Batteen & Rutherford, 1990; Pattiaratchi & Buchan, 1991). The seasonal change in the LC is generally attributed to regional wind stress variability: in summer the LC is weaker (~1.4 Sv) as it flows against maximum southerly (opposing) winds and flows strongly (~7 Sv) in winter in the absence of strong southerly winds (Godfrey & Ridgway, 1985; Pearce, 1991). The northward flowing Ningaloo Current (NC) is located along the inner-shelf between the LC and the coast. It is driven by the strong southerly wind stress (Taylor & Pearce, 1999) similar to the Capes Current along the Southwestern Australian coastline (Pearce & Pattiaratchi, 1999; Gersbach et al., 1999). The early evidence for the presence of the Ningaloo Current was obtained from aerial whale shark surveys undertaken in and from surface current plume observations from boats during (Taylor & Pearce, 1999). These preliminary observations showed the NC moving northward along the Ningaloo reef front, forming a distinct line in the water, separating the coastal waters from the Leeuwin Current flowing southward (against prevailing southerly winds) some 2 km offshore. Subsequently, satellite imagery ( ) revealed that the NC was predominant at Ningaloo from September through April, although along the coastal segment north of Point Cloates where the shelf is very narrow, the constricted (2 km wide) current was often indiscernible in satellite imagery (Taylor & Pearce, 1999). The structure of the continental shelf circulation during the summer months, along the southwest coast of Australia, has been addressed in several recent studies using limited field data and satellite imagery (Cresswell and Peterson, 1993; Pearce and Pattiaratchi, 1997; Pearce and Pattiaratchi, 1999; Gersbach et al., 1999). All of these studies have 117

118 shown the existence of a cooler northward current on the continental shelf with the southward Leeuwin Current, in general, located further offshore. The Capes Current, generally located along the southern part of the west coast appears to be well established around November when winds in the region become predominantly southerly due to the strong sea breezes (Pattiaratchi et al., 1997) and continues until about March when the sea breezes weaken. The dynamics of the Capes Current are such that the southerly wind stress overcomes the alongshore pressure gradient resulting in the surface layers moving offshore, colder water upwelling onto the continental shelf, and the Leeuwin Current to migrate offshore (Gersbach et al., 1999). Numerical model results have shown that a wind speed of 8 ms -1 is sufficient to overcome the alongshore pressure gradient on the inner continental shelf with the northward flowing Capes Current generally limited to depth less than 50 m (Gersbach, 1999). The dynamics of the Ningaloo Current is similar i.e. a wind driven current, generally limited to depth <50m. Based on records of cold water anomalies at Ningaloo coast recorded by Simpson & Masini (1986), Taylor & Pearce (1999) suggested the possibility of coastal upwelling at Ningaloo. This has now been confirmed by recent field data which indicated that the NC, which has similar water characteristics to the Leeuwin Current, is also associated with upwelling and high primary productivity with distinct phytoplankton species (Hanson et al., 2005; also see Chapter Three). A recurring feature, revealed by satellite imagery (Taylor & Pearce, 1999) and field measurements (Chapter Three), was an anti-cyclonic circulation pattern located immediately to the south of Point Cloates. Here, the Ningaloo Current appears to move across the shelf, and then flow southward parallel to the coast and the LC (Figure 5.2). Taylor and Pearce (1999) intimated that the cross-shelf exchange/re-circulation is important to the dispersal of coral larvae and the retention of planktonic biomass within the Ningaloo ecosystem. This recirculation pattern is the focus of this study, which aims to examine the processes that may be responsible for the formation of this recirculation feature through the application of a numerical model. The study area (Figure 5.1) is situated on the Tropic of Capricorn in the northwest of Western Australia. The winds are predominantly southeasterly throughout much of the year. In July (austral winter), the wind blows moderately, approximately 3 ms -1 near the shelf edge. It subsequently strengthens from July through November, and remains strong through summer (January March), frequently maintaining a velocity over 7 ms

119 for several consecutive days. In May, a weaker more variable winter wind pattern is again re-established (Godfrey & Ridgway, 1985; Hearn et al., 1986; Taylor & Pearce, 1999). Figure 5.2: Arrows indicate the general surface circulation pattern observed on a Sea-Surface Temperature (SST) satellite image from Ningaloo, 18 th November Isobaths are 200m and 1000m. Topography of the study region is non-uniform. In the northern section (Figure 5.1), between Point Cloates and North West Cape, the shelf (200 m contour) runs parallel to a straight coast along the edge of the peninsula. Here, the shelf is narrow (about 10 km wide) and extremely steep. However, south of the peninsula, immediately beyond the foreland at Point Cloates, the coast veers sharply eastward, allowing space for a gentler much wider shelf. In this study, we focus on the area at Point Cloates where the transition in bathymetry occurs most abruptly. From the detail provided in 1 km x 1 km bathymetry data (Figure 5.3), it is apparent that north of the coastal promontory the shelf descends into the abyss (Cuvier Abyssal Plain see Figure 5.1) without any distinct change in slope gradient to indicate a shelf break. Whereas southward immediately beyond the foreland of the peninsula, the bathymetry is quite transformed, with the inner shelf (shore to 20 m-isobath) exhibiting a more gradual slope, and a terrace appearing at a depth of 65 m. The seaward extension of the coastal promontory at Point Cloates effectively blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north (see Figure 5.3). The reduction in the cross- 119

120 sectional area, to the 50 m contour, to the south and to the north of the promontory is ~80% and therefore would have a major influence on the dynamics of the Ningaloo Current in this region. Satellite imagery (Figure 5.2) revealed that at Point Cloates, the wind-driven coastal current (Ningaloo Current) moved offshore in response to the westward extension of the coastline, subsequently interacting with the southward-flowing Leeuwin Current and recirculating in an anticlockwise direction. With this observation in mind, we applied a numerical model to determine if wind, topography and the Leeuwin Current could combine to simulate a wind-driven NC and the associated recirculation patterns observed in shipboard data and satellite imagery (of sea-surface temperature and surface chlorophyll distribution) recorded south of Point Cloates. The numerical modelling was undertaken using HAMburg Shelf Ocean Model (HAMSOM), which was initially developed by Backhaus (1985) for the North Sea shelf area. It is shown that the recirculating pattern is controlled by the topographic feature at Point Cloates and the prevailing winds. Figure 5.3: A 3-dimensional bathymetry chart detailing the shelf structure in the vicinity of the Point Cloates 120

121 5.2 Methodology Numerical Model HAMSOM is a three-dimensional, semi-implicit, finite-difference primitive equation model developed by Backhaus (1985). It is governed by equations of i) mass conservation, ii) momentum conservation in x- and y-directions, iii) hydrostatic equilibrium, iv) conservation of temperature and v) conservation of salinity. (i) Mass conservation: = z w y v x u (ii) Momentum conservation in x-direction: = z u Av z y u A y x u A x x P fv z u w y u v x u u t u H H ρ 1 Momentum conservation in y-direction: = z v Av z y v A y x v A x y P fu z v w y v v x v u t v H H ρ 1 (iii) Hydrostatic equilibrium: = 0 + g z P ρ (iv) Conservation of temperature: + + = z T K z y T K y x T K x z T w y T v x T u t T v H H 121

122 (v) Conservation of salinity: S t S + u x S + v y S + w z = K x H S + K x y H S + K y z v S z where u(x,y,z,t) = velocity component in the x-direction v(x,y,z,t) = velocity component in the y-direction w(x,y,z,t) = velocity component in the z-direction f = Coriolis parameter P( x,y,z,t) = pressure ρ(x,y,z,t) A H A v g T(x,y,z,t) S(x,y,z,t) K H K v = density = horizontal eddy viscosity coefficient for momentum = vertical eddy viscosity coefficient for momentum = gravitational acceleration = temperature = salinity = horizontal eddy viscosity coefficient for mass = vertical eddy viscosity coefficient for mass HAMSOM has been successfully applied to coastal and estuarine systems worldwide (Backhaus, 1985; Stronach et al., 1993; Pattiaratchi et al., 1996; Ranasinghe and Pattiaratchi, 1998; Burling et al., 2003; Nahas et al., 2003). The model uses horizontal layers of variable thickness with fixed permeable interfaces so that governing equations are vertically integrated over the thickness of each layer to allow the horizontal velocity components to become depth mean transports over the specified depth range. Implicit algorithms enter an approximation for external gravity waves and vertical shear stress to each space co-ordinate. In this study a kinematic boundary condition is applied at the surface while a non-linear bottom stress condition is used at the bottom. At the lateral boundaries, a no-slip condition was specified whilst at the open boundaries a clamped boundary condition (fixed sea surface elevation in time) was used with the open boundaries located some distance away from the region of interest (Figure 5.1). 122

123 The model domain included an internal 1 km x 1 km grid flanking the Ningaloo coast. Bathymetry for this region ( S, E) was obtained from Geoscience Australia (GA). The domain boundaries were then extended in 2 km grid intervals northward to 20.5 S, southward to 25.5 S and westward to E. Boundaries were thus moved far from the domain of interest for the purpose of minimising the boundary-effects upon the flow in the study area. Horizontal layer thicknesses of 20, 50, 100 and 200 m, were prescribed. A spatially and temporally invariant alongshore geopotential gradient was applied in the north south direction to simulate the presence of the Leeuwin Current. The magnitude of the gradient, equivalent to a sea surface gradient of 2 x10-7 was based on the field data reported by Smith et al. (1991) and was successfully utilised by Pattiaratchi & Backhaus (1992) for studies along the Perth Metropolitan coastline. A summary of parameters used in the model is presented in Table 5.1. Table 5.1: Summary of general constants used in HAMSOM modelling work. Constant Value Description C D 2.5 x 10-3 bottom drag coefficient H max 200 m maximum depth of numerical domain Δt 60 s time step F 8.2 x 10-5 s -1 Coriolis parameter G 9.81 ms -1 acceleration of gravity 123

124 Five numerical runs, each using the same bathymetry data and elevation gradient, were performed, only with a different wind intensity blowing constantly from the south for two simulated days (Table 5.2). Table 5.2: Forcing combinations for each of the five runs. Run Elevation Gradient Wind Forcing ms -1 R1 2x10-7 gδt 0 R2 2x10-7 gδt 2 R3 2x10-7 gδt 3 R4 2x10-7 gδt 4 R5 2x10-7 gδt Field Data The results of the numerical modelling were compared to field data collected between 13 and 27 November 2000 (during the austral summer) onboard the RV Franklin (Chapter Three). Instrumentation used onboard included a 150-kHz RDI Acoustic Doppler Current Profiler (ADCP) linked to the Global Positioning System (GPS) and a Neil-Brown Conductivity-Temperature-Depth (CTD) recorder with a 24 x 5 L-bottle Niskin rosette for calibration and water sampling. Winds during the voyage were continuously logged by an underway meteorological station onboard, and monitored at half-hourly intervals from three land-based meteorological stations at Learmonth, Shark Bay and the Abrolhos (North) Island. Additionally, Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Advanced Very High Resolution Radiometer (AVHRR) and Coastal Zone Color Scanner (CZCS) satellite images were used to identify instances of the re-circulation occurring at Point Cloates, and to examine the surface structure of these events when they occurred. 124

125 5.3 Results and Discussion Sea-surface temperature (SST) and surface-chlorophyll satellite imagery showed that throughout the year there were changes to the structure of the coastal processes at Point Cloates. There were many instances when there appeared to be no evidence of the Ningaloo Current flowing along the Ningaloo coast, let alone the NC re-circulation pattern (Figure 5.4a). Occasionally, the NC was clearly seen flowing along the coast toward Point Cloates, sweeping into its familiar anticlockwise pattern (Figure 5.4b); and yet on other occasions the arm of the NC continued northward away from Point Cloates, flowing broadly enough to be seen via satellite (Figure 5.4c). Through numerical modelling undertaken using HAMSOM, it was found that recirculation at Point Cloates may result from the combined effects of a southerly wind, a poleward LC (through the introduction of a surface elevation gradient) and local bathymetry. Furthermore, altering the intensity of the southerly wind simulated the differences in flow patterns during the re-circulation (as seen in satellite images Figure 5.4). Figure 5.4a: CZCS satellite image from March 1980 showing no evidence of a NC recirculation event in surface chlorophyll patterns south of the promontory at Point Cloates. Surface wind speeds were low. 125

126 (i) (ii) Figure 5.4b: (i) CZCS image from September 1980, and (ii) SeaWiFS image from November Both show an anticlockwise re-circulation feature in the surface chlorophyll south of Point Cloates, but no coastal current proceeding north along the peninsula s edge. (i) (ii) Figure 5.4c: (i) CZCS image from May 1980 showing surface chlorophyll levels, and (ii) AVHRR image from January 1991 showing sea-surface temperature. Both display anticlockwise recirculation features south of Point Cloates, as well as the NC along the coast on both sides of the promontory. 126

127 As stated in Table 5.2, the initial simulation R1 was performed with surface elevation forcing alone. As expected, the results indicated a poleward surface flow (not shown), which represented the LC. Subsequently, in simulation R2, the introduction of a constant 2 ms -1 southerly wind clearly produced the NC in the surface 20 m adjacent to the shoreline (mostly within the 20 m isobath) south of Point Cloates (Figure 5.5a). The water then followed the curvature of the coastline westward at the promontory and extended out across the shelf terrace (Figure 5.3). The movement of the NC as it proceeded northward along the steep edge of the peninsula was not evident. When the wind-forcing was increased to 3 ms -1 in simulation R3, the NC broadened and was apparent along the narrow shelf north of Point Cloates (Figure 5.5b). While the NCflow strengthened within the surface 20 m layer all along the coast, it was now also detected in the next 50 m-layer underneath. Moreover, there appeared to be a small amount of upwelling, with the surface waters moving offshore and the deeper waters moving onshore. Meanwhile, the cross-shelf motion of the NC at Point Cloates continued to occur (Figure 5.5b). Increasing the wind-speed to 4 ms -1 in R4 caused the events observed in R3 to become more pronounced (Figure 5.5c). The surface NC broadened to the 65 m isobath. Additionally, a circular shape began to form over the width of the shelf-terrace, enabling a cross-shelf exchange south of the promontory at Point Cloates. This was evident in the m and m layers. Finally, strengthening the wind-forcing to 5 ms -1 in run R5 generated the strongest NC flow up the coast to Point Cloates and beyond; as well as the clearest anticlockwise sweep of the NC in the m and m layers at the foreland. 127

128 70-170m depth 20-70m depth 0-20m depth Figure 5.5a: Model run R2. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 2ms -1 southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5.3). Figure 5-5a: Model run R2. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 2m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3) latitude N Scale: Scale: Scale: m/s 0.05m/s 0.05m/s longitude E

129 70-170m depth 20-70m depth 0-20m depth Figure 5.5b: Model run R3. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 3ms -1 southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5.3). Figure 5-5b: Model run R3. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 3m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3). latitude N Scale: Scale: -65 Scale: m/s 0.05m/s 0.05m/s longitude E

130 70-170m depth 20-70m depth 0-20m depth Figure 5.5c: Model run R4. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 4ms -1 southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5.3). Figure 5-5c: Model run R4. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 4m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3) latitude N Scale: Scale: -65 Scale: m/s 0.05m/s 0.05m/s longitude E

131 70-170m depth 20-70m depth 0-20m depth latitude N Figure 5.5d: Model run R5. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 5ms -1 southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5.3). Figure 5-5d: Model run R5. Simulated (depth mean) flow velocities resulting from 2 days forcing by a constant 5m/s southerly wind and a surface elevation gradient. Arrows point in direction of flow. Isobaths at 20m, 65m,110m and 200m (see Figure 5-3) Scale: Scale: Scale: m/s 0.05m/s 0.05m/s longitude E

132 In order to examine the spatial distribution of the NC circulation and its response to wind speed, data was extracted from each simulation to constitute vertical transects at four latitudes across the model domain (Figure 5.6). Initially, volume transport of NC, generated by different wind speeds, was calculated for each transect. The results showed that, at each location along the coast, the NC strengthened with increased wind-forcing (Figure 5.7). Figure 5.6: Location of four transect lines across the numerical modelling domain. Volume transports were calculated for the region coastward of the 70m isobath volume transport (Sv) a-a' b-b' c-c' d-d' transect C (field data) Poly. (c-c') wind velocity (m/s) Figure 5.7: Relationship between northward NC volume transport and wind-forcing velocity, at four locations, i.e S (a-a ), S (b-b ), S (c-c ) and S (d-d ). The NC volume transport recorded in field data taken close to the location of c-c is also shown. 132