Oceanography and circulation pattern of the Zeewijk Channel, Houtman Abrolhos Islands, Western Australia.

Transcription

1 Oceanography and circulation pattern of the Zeewijk Channel, Houtman Abrolhos Islands, Western Australia. MICHAEL MASLIN This dissertation is submitted as partial requirement for the degree of Bachelor of Engineering (Applied Ocean Science) Supervisor: Prof. Charitha Pattiaratchi

2

3

4

5 i ABSTRACT The Houtman Abrolhos Islands are a significant contributor to Australia s western rock lobster industry and they are also a major tourist attraction for Geraldton and the central west coast. Understanding the physical and biological characteristics of the region has been critical for well-developed management in the past, and is essential for a future that may involve aquaculture developments within this Fish Habitat Protection Area. Oceanographic processes on a range of scales affect the channels between island groups, channels which comprise a large portion of the Abrolhos functionality as both a fishery and worthwhile tourist destination. Understanding the circulation patterns, with this thesis particularly interested in the Zeewijk Channel, can give valuable and practical insights that may be used in future assessment of the island chain. For the spring period in 2002 and 2003 wind, wave and climate data has been reviewed and correlates with a strong influence on surface dynamics in the channel. An ADCP measured surface and bottom currents during this time frame with current strengths and direction varying greatly between surface and bottom. Sustained periods of strong westerly currents at the surface indicate that water expulsion towards the continental shelf slope from the channel is significant and linked to the local wind pattern. Bottom currents exhibit a strongly cyclical trend related to the local tidal regime. Bathymetric, salinity and temperature information from the CSIRO vessel Southern Surveyor in 2003 aided in assessing the hydrodynamic properties of the channel as relatively well mixed. It also depicted the Leeuwin Current s influence over the continental shelf slope during the testing period and the likely influence on channel circulation indirectly associated with its presence. The dynamics of the Zeewijk Channel currents give a high degree of support to the aquaculture trial proposed for its confines due to the water exchange that they represent at the surface. However further investigation into vertical particle transport is required since the re-circulation of particles at the bottom may present an environmental risk. Seasonal and inter-annual variability has also influenced the outcome of this thesis greatly.

6 ii

7 iii ACKNOWLEDGEMENTS This thesis has been produced with the general support and assistance of a number of people. First and foremost, I would like to thank my supervisor, Professor Charitha Pattiaratchi for his help and guidance throughout the year. His patience, time and assistance are greatly appreciated. I would like to thank the Australian Hydrographic Service, the Bureau of Meteorology, DPI and the CSIRO for contributing the data to make this study possible. It was not necessarily meant for me initially, but when I approached all parties there assistance was prompt and friendly, and the information crucial. To all the students at the Centre for Water Research, thanks for the most enjoyable and challenging year of university I have ever had. You were great company through the long hours spent in the lab and the occasional tavern visits. Cheers to the masters of MATLAB and Excel, you know who you are and the invaluable aid you gave me on the computer front is appreciated. A special and sincere thank you must go out to Team Ocean. I look forward to the consultancy we open in a decades time, and the fulfilment of the power and potential that is. Team Ocean. To my mother and father, a huge thanks for putting up with me all year. Without your support and direction I might not have made it here (I would have run out of petrol). To all my friends that told me to work like a dog this year, thanks for the inspiration. Michael Maslin October 2005 A man may write at any time, if he will set himself doggedly to it. -- Samuel Johnson

8 iv

9 Contents v TABLE OF CONTENTS ABSTRACT... I ACKNOWLEDGEMENTS... III TABLE OF CONTENTS... V LIST OF FIGURES...VII LIST OF TABLES... VIII 1. INTRODUCTION BACKGROUND PHYSICAL SETTING The Houtman Abrolhos Islands Bathymetry Zeewijk Channel Bathymetry Geomorphology Climate Air Temperature Rainfall Wind BIOLOGICAL SETTING Biodiversity Seabird population Fisheries Western Rock Lobster Southern Saucer Scallop Aquaculture Proposal OCEANOGRAPHIC SETTING The Leeuwin Current The Capes Current El Niño/Southern Oscillation Physical Parameters Sea Temperature Salinity Stratification and mixing Hydrodynamic Processes Baroclinic Circulation Barotropic forcing Wind-driven circulation Coriolis and Rossby Number Ekman Veering Atmospheric Pressure Changes Continental Shelf Waves Waves Currents Shelf Currents Tides METHODOLOGY DATA COMPILATION THE SOUTHERN SURVEYOR CTD ADCP WIND ANALYSIS DIGITISATION OF BATHYMETRY RESULTS CTD Surface Temperature Water Column Temperature Zeewijk Channel Surface Salinity Water Column Salinity...46

10 Contents vi Zeewijk Channel Sea water Density Middle Channel vs. Zeewijk Channel ADCP Current Roses Current Variability Particle Progression Particle Excursion Spectral Analysis WIND Spring Spring WAVES Spring EDDY FORMATION Spring DISCUSSION CONCLUSIONS RECOMMENDATIONS FOR FUTURE WORK...77 REFERENCES...79

11 Contents vii LIST OF FIGURES Figure 1.1: the Houtman Abrolhos Islands from space. Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Centre...3 Figure 2.1: the Houtman Abrolhos Islands. The extensive reef system can be seen in green relative to the small island areas illustrated in yellow (Penn (ed)1999)...5 Figure 2.2: focus on the Zeewijk Channel bathymetry looking in the along-channel direction. Half moon reef is represented by the dark red section on the middle right of the figure and the Easter Island reef system is on the left. The image was created using the digitisation method described in the methodology....7 Figure 2.3: typical winter synoptic weather chart for Australia in 2003 (B.O.M. 2005)....8 Figure 2.4: typical summer synoptic weather charts for Australia in 2003 (B.O.M. 2005)....9 Figure 2.5: map of the sea-cage site in the Zeewijk channel (Diver & Prince 2003)...14 Figure 2.6: The progression of cooler, Capes Current water northwards can be seen in this satellite image. Also of note are the systems of eddies off the continental shelf that are comprised of Leeuwin Current water that has entrained the Capes water from the shallower regions (Pearce & Pattiaratchi 1999) Figure 2.7: Southern Oscillation Index for the beginning of 2000 until mid Negative SOI for extended periods will mean El Niño conditions, positive means La Niña (B.O.M. 2005) Figure 2.8: Sea surface temperatures as measured by mercury thermometer on monthly visits to the puerulus collectors; estimated accuracy 0.3 C. The period of data analysis is from 1970 to 1976 and then 1984 to Sampling is ongoing (CSIRO 2005) Figure 2.9: Monthly mean salinities from the CSIRO hydrographic station near Geraldton. Salinity at the surface and at a depth of 40m is illustrated (adapted from Sukumaran 1997). 21 Figure 2.10: the processes driving stratification and mixing of a water column...21 Figure 2.11: Ekman transport in the Northern Hemisphere. The wind-induced surface current turns 45 to the direction of the wind (Pattiaratchi 2005)...27 Figure 2.12: wind and current directions according to the recognised naming conventions...33 Figure 3.1: Southern Surveyor transects and approximate sampling station locations through the Zeewijk and Middle Channels, October 27 th and 28 th, 2003 (Australian Hydrographic Service, 2005) Figure 3.2: the ADCP used in the Zeewijk Channel and its deployment location Figure 3.3: digitised bathymetry of the Zeewijk Channel. Depth is in metres on the z-axis, latitude and longitude are on the y and x-axis respectively. Data courtesy of the Australian Hydrographic Service (2005)...41 Figure 4.1: the change in surface water temperature from the continental shelf slope, into the Zeewijk Channel. A noticeable temperature decrease is observed after 20km where the testing entered waters beyond the 60m contour line of the continental shelf...43 Figure 4.2: temperature-depth profiles of stations 1 to 5 on the Zeewijk transect, 27/10/ Figure 4.3: temperature contour plot relative to the depth of the continental shelf...44 Figure 4.4: temperature-depth profiles of stations 6 to 9, and a microscopic view of Figure 4.2, to compare the temperatures of the inner and outer stations in the surface 100m on the Zeewijk Channel transect, 27/10/ Figure 4.5: the change in surface water salinity from the continental shelf slope, into the Zeewijk Channel. A noticeable salinity increase is observed after 20km where the testing entered waters beyond the 60m contour line of the continental shelf. The same annotations as the surface temperature plot apply...46 Figure 4.6: salinity-depth profiles of stations 1 to 5 on the Zeewijk transect, 27/10/ Figure 4.7: salinity contour plot relative to the depth of the continental shelf Figure 4.8: salinity-depth profiles of stations 6 to 9, and a microscopic view of Figure 4.6, to compare the temperatures of the inner and outer stations in the surface 100m on the Zeewijk Channel transect, 27/10/

12 Contents viii Figure 4.9: TS diagrams to compare the water column properties of the Zeewijk and Middle Channels. The lower figures represent the water column properties of the channel stations, that is, stations 6 to 9. Station 1 has been omitted from all figures to aid interpretation due to the scale of its depth and properties. Station 9 of the Middle Channel is located at approximately (35.61, 19.2), and is hard to view as it is almost a point Figure 4.10: Temperature-salinity diagram highlighting the currents present in the water column on the Gascoyne continental shelf slope, particularly out at the 1000m contour Figure 4.11: current rose of surface currents in the Zeewijk Channel during spring Figure 4.12: current rose of bottom currents in the Zeewijk Channel during spring Figure 4.13: surface current variability in the Zeewijk Channel for the cross- and along-shore directions during spring Figure 4.14: bottom current variability in the Zeewijk Channel for the cross- and along-shore directions during spring Figure 4.15: plot of the path travelled by a particle under the influence of the mean daily surface currents in spring The current path is elliptical in the along-channel direction Figure 4.16: progressive plot of the path travelled by a particle under the influence of the mean daily bottom currents in spring There is a strong north-west to south-easterly current trend...59 Figure 4.17: the distances travelled by a particle at the surface under the daily mean current conditions for each day of the testing period...60 Figure 4.18: the distances travelled by a particle near the bottom under the daily mean current conditions for each day of the testing period...61 Figure 4.19: Spectral density versus frequency for surface currents in the Zeewijk Channel during spring Figure 4.20: Spectral density versus frequency for bottom currents in the Zeewijk Channel during spring Figure 4.21: wind influence on surface currents for 16 days of the testing period, between 1/9/03 and 17/9/ Figure 4.22: wind influence on surface currents for 10 days of the testing period, between 28/10/03 and 6/11/ Figure 4.23: current rose of winds recorded at the North Island recording station 1/10/02 and 31/12/ Figure 4.24: depicts the wave height, period and direction of waves in the Zeewijk Channel for the period 12/10/02 to 16/11/ Figure 4.25: sea level anomaly, related geostrophic velocity and sea surface temperature for post-october 6 th 2003 (CSIRO 2005) Figure 4.26: sea level anomaly, related geostrophic velocity and sea surface temperature for post-november 9 th 2003 (CSIRO 2005) LIST OF TABLES Table 2.1: tidal constituents for the Pelsaert Island Group 4. Adapted from the Australian National Tide Tables (2002) Table 3.1: exact locations, and the associated water depth, of the sampling stations for CTD testing on the Zeewijk and Middle Channel transects Table 4.1: temperature, salinity and related density of surface water inside the Zeewijk Channel and above the continental shelf slope...49

13 ix

14 x

15 Introduction 1 1. Introduction As part of a large Dutch East India Company fleet carrying considerable wealth and trade goods, the Zealand ship Zeewijk left Holland in November of 1726 under strict orders to follow a prescribed course across the Indian Ocean and into safe anchorage at Batavia, Java. Against these orders and into disaster, its ambitious captain took a heading to the east-northeast, with the intent to explore the relatively undiscovered Eendracht, known today as Western Australia. At 7:30pm on the 9 th of June 1727 and less than 70km from the Western Australian coastline, the ship crashed into the northern edge of Half Moon Reef at the Houtman Abrolhos Islands. The lookout had perilously mistaken the line of breakers as the moons reflection on the sea surface. Unbeknownst to the crew, they had come agonisingly close to sailing smoothly through a safe channel fractionally to the north, and into Eendracht s coastal waters (Ingleman-Sundberg 1976). Unlike the ill-fated vessel Batavia, also shipwrecked at the Abrolhos Islands and famous for its tales of mutiny, murder and cannibalism, the crew of the Zeewijk faced shipwreck and stranding with firm resolve against the isolation of the Abrolhos and fierce surf on the offending reef during salvage. Eventually they constructed a sloopy capable of carrying the survivors to Java. Their journey lasted over a month and when they finally reached Batavia in April of 1728, only 82 of the original 208 strong party that had set out from the Netherlands remained. The channel that had offered tantalising access to the Abrolhos Islands and in turn Western Australia s coastal waters, is named in recognition of their plight (Ingleman- Sundberg 1976). The Zeewijk Channel is one of three breaks in the Houtman Abrolhos archipelago that enables transition between the continental shelf slope and coastal shelf waters. Situated between the Pelsaert and Easter Group Islands, it also provides access to the lagoons and islands of these systems. The local current pattern is viewed as a major cause of the geological evolution of the island chain. Research has shown it to be strongly affected by mesoscale features such as the Leeuwin Current on the continental shelf slope side, as well as more direct and localised parameters such as wind and the tidal regime. It is important to understand these processes and the nature of the channel s circulation as it has important ramifications for the biota of the region and anthropogenic exponents of its fisheries.

16 Introduction 2 The Abrolhos Island s have been classified a Fish Habitat Protection Area by Fisheries Western Australia, such is the ecological value of the region. Juxtaposed against this classification is the significant proportion of the states Western Rock Lobster catch, nearly 15% at an estimated value of $50 million, taken in the immediate vicinity. The Abrolhos role in lobster recruitment for the rest of the state is still being investigated but early studies have indicated its importance to the industry state-wide, as a lobster puerulus breeding ground with significant oceanographic parameters distributing the puerulus over vast distances. Diversification of the local fishery includes a proposed yellow fin tuna aquaculture project to be established in the Zeewijk Channel. An understanding of the channel s currents and therefore the water circulation pattern could aid future environmental impact assessments in the context of an aquaculture farm in the area. The choice of aquaculture sites on the basis of the circulation or water exchange regime of the area makes sound environmental reasoning considering the potential for benthic habitat damage associated with these endeavours. This thesis aims to examine the oceanography and circulation pattern of the Zeewijk Channel, specifically in terms of the spring conditions in 2002 and The mesoscale oceanography of the Gascoyne continental shelf and shelf slope has also been examined, to assess the extent of its influence on the Zeewijk Channel circulation pattern. A background consisting of the Abrolhos Islands physical, biological and oceanographic characteristics has been established through critical review of previous studies of the region (chapter 2). The procedures employed to obtain useful and varied sets of data are outlined in the methodology (chapter 3). A successful collation of data was paramount to achieving the aims of this dissertation. Analysis and synthesis of this data gives insight into the pattern of circulation in the Zeewijk Channel and is displayed in the results and discussion (chapters 4 and 5). These chapters also speculate on the mesoscale processes that may be driving circulation in the channel.

17 Introduction 3 Figure 1.1: the Houtman Abrolhos Islands from space. Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Centre.

18 Background 4 2. Background 2.1. Physical Setting The natural characteristics of the Zeewijk Channel, and greater Houtman Abrolhos Islands, will be outlined in this chapter with particular consideration of the meteorological and oceanic processes that shape them. The physical and biological characteristics of the region are closely related to these processes The Houtman Abrolhos Islands An archipelago of 122 small islands and a considerable reef system, the Houtman Abrolhos Islands are located approximately 70 km west of Geraldton, Western Australia, and 10 km inside the 200 metre depth contour of the nearby continental shelf. At latitude to 29 S, and longitude to E, it comprises the southern most coral reef complex in the Indian Ocean with a general area of around 1100km 2 to the 50m depth contour, and a low tide island area of approximately 18.5 km 2. The Houtman Abrolhos can be divided geographically into three main groups of islands and each group is roughly triangular in shape. The Pelsaert (Southern) Group is separated from the Easter Group by the Zeewijk Channel and the Middle Channel divides the Easter and North Island-Wallabi Groups. The channels are each approximately 40 m deep and there is an extensive submerged reef platform connecting North Island to the rest of the Wallabi Group.

19 Background 5 Figure 2.1: the Houtman Abrolhos Islands. The extensive reef system can be seen in green relative to the small island areas illustrated in yellow (Penn (ed)1999).

20 Background Bathymetry The Gascoyne continental shelf is located along the northern central coastline of Western Australia between latitudes 21 S and 29 S. Off Geraldton, the shelf is relatively flat and shallow with a slope of approximately 1:750 (Pearce 1997). The Islands are encompassed by a 50m isobar, despite the isobar extending less than half the distance offshore to the immediate south of the Pelsaert Group and similarly to the north. The continental shelf slope steepens drastically approximately 10km west of the Abrolhos, and this shelf break can be seen in the relatively short distance over which water depth alters from 100m to 500m plus, in Figure Zeewijk Channel Bathymetry This dissertation is particularly interested in the Zeewijk Channel; therefore its bathymetry should be examined more closely. The channel is approximately 35m deep when entering from the landward side but it is characterised by a drop in depth of around 20m close to its western opening. This depth intrusion goes far enough to be in line with Gun Island to the south and Wooded Island to the north, and is inside the fringing reef line. There is a Pleistocene ridge nearly 12km offshore from the step feature, characterised by a narrow opening or break approximately 1.5km in width on its northern extremity. The channel length can be estimate as 30km from the ridge feature through to Snapper Bank, shown as a mound at the top of the bathymetric map. The width of the western opening is approximately 6km, although channel width increases significantly moving eastwards through the channel. The along-channel path is orientated from south-west to north-east, in other words it is tilted approximately 42 from an east/west orientation.

21 Background 7 (m) Figure 2.2: focus on the Zeewijk Channel bathymetry looking in the along-channel direction. Half moon reef is represented by the dark red section on the middle right of the figure and the Easter Island reef system is on the left. The image was created using the digitisation method described in the methodology.

22 Background Geomorphology Early surveys conducted on the Houtman Abrolhos Islands found the islands to be built of Pleistocene coralline limestone topped with aggregated coral rubble and sand (Wilson 1978). The more central islands found in the three groups are platform islands that can be aged to the last Interglacial period, whereas the windward and leeward reefs are Holocene in age(collins et al. 1996). The various reef structures in the Abrolhos include platform, fringing, shore platform and lagoon patch reefs in both the windward and leeward directions but all with varying degrees of coral growth (Wilson 1978). The Islands are atoll-like due to their high abundance of corals with relatively low species diversity, and yet there geomorphology classes them an archipelago (Sukumaran 1997) Climate The Abrolhos Islands share the meteorological and climatic characteristics of the nearby midwest coast of Australia. The region is subtropical with hot dry summers and mild to cool winters (B.O.M. 2005). Its weather patterns rely heavily on the eastward progression of high pressure cells from the Indian Ocean, and the North-South movement of the subtropical anticyclonic wind-belt (CSIRO 2005). Typical synoptic weather patterns that influence the Western Australian coastline in summer and winter are illustrated in Figure 2.3 and Figure 2.4. Figure 2.3: typical winter synoptic weather chart for Australia in 2003 (B.O.M. 2005).

23 Background 9 Figure 2.4: typical summer synoptic weather charts for Australia in 2003 (B.O.M. 2005) Air Temperature Monthly mean air temperatures at the Abrolhos vary from a low of 17.7 C in August to a high of 23.5 C in February. The minimum temperature recorded between 1990 and 1995 was 10.7 C and the maximum temperature recorded was 37.7 C. Both the monthly means and minimum and maximum temperatures in the Abrolhos are ameliorated by the ocean when compared to the inshore area at Geraldton (Fisheries 2000). Consider how the difference in monthly mean air temperature differs by more than 10 C at Geraldton and yet there is only a difference of 6 C at the islands. Air temperature is appreciably warmer at the islands during winter months compared to Geraldton (Pearce 1997) Rainfall The Abrolhos Islands archipelago has relatively low rainfall, with an average of 89 rain days per year producing mean annual precipitation of 461mm. Rainfall is largely seasonal, and nearly 100mm of the annual precipitation occurs in June. This seasonality is further emphasised since 86% of the regions entire rainfall occurs in the six months between April and September (Pearce 1997).

24 Background Wind Localised wind conditions are an important source of energy for water circulation and mixing. The Houtman Abrolhos Islands wind pattern can change considerably over diurnal and seasonal timescales. In the summer months, the stronger winds result from a combination of the synoptic situation and strong sea breezes (Pattiaratchi 1993). In May, a weaker, more variable winter wind pattern is re-established (Pearce 1997). The prevailing winds in summer are moderately strong south-east to south-westerlies that can reach velocities in excess of 6ms -1 for 75% of their duration, with only short periods of calm (Wells 1997). In winter months winds are more variable from the south-west to north-east, however storm events are more frequent and often of a higher intensity producing the highest velocity winds experienced by the region. Three types of storms occur in the Abrolhos. Firstly the rare and infrequent tropical cyclone, they usually reach this far south once in three years but they are potentially very destructive with wind speeds over 30ms -1. Secondly there are summer squalls, usually between December and April with wind speeds between 25 and 30 m/s that can occur from any direction. Finally winter gales resulting from the passage of east bound storms south of the Abrolhos may cause wind gusts and speeds of up to 35m/s (Sukumaran 1997). The diurnal heating of land and water creates what is known as a sea breeze affect, where cool seaward air flows onshore to replace hot dry air that has been heated over land during the day and risen (Pattiaratchi & Imberger 1991). The Western Australian coastline experiences a sea breeze, locally termed the Fremantle Doctor, an order of magnitude stronger than that found at the majority of locations in the world whom experience the same phenomena (Pattiaratchi 1993). The meteorological station on North Island in the Houtman Abrolhos documents wind data for the region and has been used to collect the relevant data for this report.

25 Background Biological Setting Western Australia is home to the only reef system to thrive in an eastern boundary region on the globe. The most notable component of this reef system is the Ningaloo Reef in the states north, but it is considered to stretch as far south as Cape Leeuwin. The system south of North West Cape is recognized as having great significance as a zone of bio-geographic overlap, with the oceanographic properties of the continental shelf a driving factor Biodiversity The Houtman Abrolhos is home to a variety of biota whose species have migrated from tropical and temperate regions of the Western Australian coastline, as well as a small number of endemic species of flora and fauna. Research has shown that tropical species, corals and fish in particular, dominate the area (Wilson 1978). It has been speculated that the abundance of tropical fish is a result of the pole ward flowing Leeuwin current bringing water from the north (Pearce 1997). The occurrence of temperate species is believed to be a product of the Capes current and its progression northwards along the inner-continental shelf (Pearce & Pattiaratchi 1999). These species most notably consist of macro algae and seagrasses (Brearley 1997) Seabird population The Houtman Abrolhos Islands is an important nesting and breeding ground for a variety of seabirds. It supports the largest breeding colonies in Western Australia of a number of bird species including the White-faced Storm Petrel (Pelagodroma marina) and the Whitebreasted Sea Eagle (Haliaeetus leucogaster) (A.I.T.F. 1988). It is also home to the rare Redtailed Tropic Bird (Paethon Rubricauda) that is close to extinction and has endemic species such as the Painted Button-quail (Turnix varia) in residence (A.I.T.F. 1988) Fisheries Commercial fishing is the primary industry in the Abrolhos Islands. The three major commercial fisheries operating at the Abrolhos include the Western Rock Lobster, Southern Saucer Scallop and a selection of finfish. The finfish catch will not be addressed in detail as it has less bearing on the Zeewijk Channel, but it targets species such as pink snapper (Pagras auratus) and coral trout (Plectropomus leopardus) using hook and line (Fisheries 2000).

26 Background Western Rock Lobster One of the most important resident species is the Western Rock Lobster, Panulirus Cygnus. In the last decade it has become Australia s most valuable single species fishery, with a seasonal gross value of production between $300 and $350 million, and around 15% of the seasonal catch is produced in the Houtman Abrolhos region despite a shortened season (Chubb & Barker 2005). The Abrolhos Islands fishery is restricted to the period 15 March to 30 June, unlike the rest of the coast that is fished from as early as November through to 30 June. The impact of the reef system and its circulation patterns on the lobster industry is not easy to quantify but studies have established its importance as a nursery for young lobster or puerulus (Phillips et al. 1991). In fact the Abrolhos importance to the Western Rock Lobster industry goes beyond its catch strength, as 50 to 80% of the total larvae for the state may be produced here (Chubb & Barker 2005). The ability of the rock lobsters to move off and onshore is affected by localised sea conditions and the oceanographic properties of the region. After hatching in summer near the edge of the continental shelf, millions of larvae are transported to the order of hundreds of kilometres from the West Australian coastline, via surface currents including the Leeuwin and Capes Currents. They float free in the Indian Ocean for several months but eventually return onshore. Artificial seaweed is monitored monthly on the full moon so that the puerulus content of the seaweed can be measured. The annual indices are then used to predict the strength of the early season catch in 4 years time (CSIRO 2005). The catch consists of newly-moulted lobsters or whites heading off-shore to breed but the index can alternatively predict the late season catch for three years time of mature coastal lobsters, or reds that have resumed residence on the reef system (Caputi 1995). Studies have shown that annual fluctuations in the strength of the Leeuwin Current, and subsequently the ENSO or El Nino effect, directly influence the yearly catch of the Western Rock Lobster which means that the interaction between shelf break and Abrolhos water bodies takes on additional, in particular financial, importance (Caputi 1995).

27 Background Southern Saucer Scallop The most prominent commercial species of scallop in Western Australia the Southern saucer scallop, Amusium balloti, is found in harvestable quantities in the benthic habitats of the Middle and Zeewijk Channels. This fishery was declared limited-entry at the Abrolhos in 1986 and 16 licensed vessels currently fish it (Penn (ed)1999). The scallop season runs between April and June, therefore it coincides with the Abrolhos Island Rock Lobster Fishery, and the aim is to take the maximum yield available as the scallops have finished spawning by the time the season opens (Penn (ed)1999). In 2003, a particularly good year for the fishery with high scallop abundance, there were standardised trawl hours recorded during an extended season for an estimated catch value of $19.6 million (Chubb & Barker 2005). This contrasts with the 2002 season when only 912 trawl hours were recorded, but highlights that a significant amount of bottom trawling can occur annually and that it is an important factor when determining management of the channels. While the relationship is not completely understood, the Leeuwin Current is believed to affect scallop abundance and therefore catch sizes in a similar manner to that in which it affects the Western Rock Lobster (Chubb & Barker 2005) Aquaculture Proposal The Zeewijk Channel has been speculated upon as a possible aquaculture site in an attempt to further diversify fisheries in the Abrolhos region. The Aquaculture Plan for the Houtman Abrolhos Islands (Fisheries 2000), outlines the relevant concerns regarding all types of likely aquaculture ventures from ocean-ranching, for example stocking and harvesting scallops on defined areas of the sea-floor, to the tuna farming now prevalent in South Australia. A proposal has gone as far as approval by the Environmental Protection Authority, that entails capturing via purse seine yellow fin tuna, Thunnus albacares, in waters between Geraldton and Exmouth before transporting and culturing them in sea-cages in the Zeewijk Channel (Diver & Prince 2003). This process would be closely scrutinised by the Australian Fisheries Management Authority. Details of the proposal can be found in the relevant EPA report (2003) or in the work done by Diver and Prince for Latitude Fisheries Pty Ltd.

28 Background 14 An initial biomass of up to 200 tonne of tuna is speculated to be caught and held in eight 40 metre diameter sea-cages for up to 7 months whilst supplementary feed increases their biomass. A 30 hectare area would be chosen to house these cages within the greater licensed area illustrated in Figure 2.5. The cages that will be used to hold the tuna are similar to those used in the South Australian blue fin tuna industry. Key environmental issues raised both in the EPA report and after public consultation include: the potential for impact on the Abrolhos Island bird populations the potential for impact on benthic habitat and water quality the disease risks associated with using bait fish and feeding methods Figure 2.5: map of the sea-cage site in the Zeewijk channel (Diver & Prince 2003) The circulation pattern of the Zeewijk Channel will directly affect two of these concerns, and indirectly affect the third. Disease risks, water quality issues and concerns for the benthic habitat can be assuage by confirmation that the Zeewijk Channel has sufficient water exchange with the areas beyond its confines. Conversely, analysis indicating minimal water exchange with outside sources or poor circulation would establish sound reasoning for reassessment of the aquaculture proposal.

29 Background Oceanographic Setting This chapter outlines the dominant forces and physical processes affecting the Zeewijk Channel, and the greater Houtman Abrolhos Islands. It begins with an in depth review of the Leeuwin Current and associated features, due to this currents significance in terms of the regional oceanography The Leeuwin Current The Leeuwin Current s biological and physical oceanographic influence on Western Australia stretches from the North West Cape to as far south as the Great Australian Bight. It is an anomalous eastern boundary current as it flow s pole ward, rather than equator-ward like other eastern boundary currents, and it travels against the prevailing south westerly winds with a core velocity of up to 1ms -1 (Cresswell et al. 1989). The current originates from the tropics and in particular the Pacific Ocean, where water is pushed westward by the south-east trade winds and channelled via the Indonesian through-flow into the Eastern Indian Ocean. The Coriolis force and geostrophic factors push the current onto the Western Australian continental shelf and force it pole ward. During its passage down the shelf it is typically 50km wide and also relatively shallow, at approximately 250m deep (Smith et al. 1991). There is a northwards moving counter current, the Leeuwin Under-current, recognised but not investigated in detail, flowing below this 250m deep surface layer (Pattiaratchi 2005). It is generally considered that the pole ward movement of the current is driven by a steric height gradient that runs south from the North West Shelf as a result of temperature contrasts from north to south (Smith et al. 1991). The steric height gradient causes geostrophic flow of water eastwards in the Indian Ocean which is then blocked by the Western Australian coast and causes the pole ward momentum. A relatively deep mixed layer at the surface of up to 50m, means that the momentum induced by equator ward wind is distributed over a relatively deep layer, and hence the associated wind stress is too weak to overcome the steric height forcing and the Leeuwin Current travels south (Smith et al. 1991).

30 Background 16 Research has found the Leeuwin Current to be stronger in winter then in summer months and this seasonal variability is coupled with a strong inter-annual variability linked to the southern oscillation index. El Niño events pre-empt a weakened Leeuwin Current and La Niña events enhance it (CSIRO 2005). It is characterised by warmer water of low salinity and depleted nutrient levels, in comparison to the more temperate waters surrounding it (Pearce 1997). Studies of the currents seasonal variability indicate it has volume transport of 5-7 Sv in winter (Smith et al. 1991) and 1.4 Sv in summer (Pearce & Griffiths 1991). This seasonal change in intensity has been attributed to the wind stress variability of localised regions. Previous studies, including (Holloway 1995), indicate that there are systems of eddies, meanders and alongshore jets associated with the Leeuwin Current as well as sections of a single current core and these features can often be deduced from satellite imagery (Pattiaratchi & Buchan 1991). Its impact on up welling and down welling of water along the coastline is debatable due to the significant biological productivity of the regions it traverses which would typically be attributed to high nutrient levels caused by localised up welling in other eastern boundary locations. This biological productivity is not considered to be a direct response to the low nutrient levels of the Leeuwin Current, but rather a by-product requiring further investigation but possibly linked to the Capes Current (Cresswell et al. 1989). The extent of the Leeuwin Currents intrusion into shallower shelf waters is an ongoing research topic and is relevant in this dissertation with regards to its interaction with the waters past the 50m depth contour surrounding the Abrolhos Islands The Capes Current The dynamics of the Capes Current have been described by (Gersbach 1999). Essentially the southerly wind stress overcomes the alongshore pressure gradient forcing surface water to flow equator ward inside the 50m contour of the continental shelf (Pearce & Pattiaratchi 1999). This surface layer move tends to move offshore due to the Coriolis force, which in turn results in up welling of colder bottom water onto the continental shelf. The Leeuwin Current interacts with the Capes Current through edification and mixing and will often migrate further offshore under the Capes Current s influence (Gersbach 1999).

31 Background 17 (Pearce & Pattiaratchi 1999) demonstrated the seasonality of the Capes Current using satellite images covering the period 1987 to The cool northwards flow started in October and decayed significantly by April the following year as a result of the seasonal strengthening of the northwards wind stress. The Capes Current is to a large degree displaced by the relatively strong pole ward flowing Leeuwin Current in winter months (Pearce & Pattiaratchi 1999). Figure 2.6: The progression of cooler, Capes Current water northwards can be seen in this satellite image. Also of note are the systems of eddies off the continental shelf that are comprised of Leeuwin Current water that has entrained the Capes water from the shallower regions (Pearce & Pattiaratchi 1999).

32 Background El Niño/Southern Oscillation One of the most important factors affecting Australia s climate is the El Niño/Southern Oscillation (ENSO) effect. This phenomenon, derived from coupled processes in the ocean and atmosphere, occurs predominantly in the equatorial Pacific region although its influence can be observed over most of the globe (CSIRO 2005). It has a largely irregular period of between 3 and 7 years, although there are important longer period fluctuations as well. A commonly accepted measure of the strength of ENSO is the Southern Oscillation Index (SOI), which is the normalized sea-level pressure difference between Darwin and Tahiti (B.O.M. 2005). The SOI shown graphically in Figure 2.7, shows we were coming out of a strong El Niño event from 2002 to mid-2003 and that the SOI was nearly negative 3 in October of 2003 when the CTD field testing occurred. A weighted average of approximately 0 SOI could be calculated for the 3 month period in which ADCP testing took place in the Zeewijk Channel. Due to time lag effects, the relatively large El Niño event in the year and a half leading up to the field work is likely to be of greater importance than the SOI during the testing period (B.O.M. 2005). Figure 2.7: Southern Oscillation Index for the beginning of 2000 until mid Negative SOI for extended periods will mean El Niño conditions, positive means La Niña (B.O.M. 2005).

33 Background Physical Parameters Studies of water column properties such as temperature and salinity at the Abrolhos in the past have primarily been concerned with water near the continental shelf, and its exchange with landward sources. These studies have been endorsed by Fisheries Western Australia, to fill knowledge gaps concerning the Western Rock Lobster s puerulus stage and to further scientific knowledge associated with continental shelf features such as the Leeuwin Current and/or its interaction with coastal water such as the Capes Current as outlined earlier Sea Temperature According to research by the (CSIRO 2005), there is a seasonally-reversing temperature gradient across the Western Australian continental shelf. In summer, shallow near-coastal waters warm because of heat input from the sun and the atmosphere, hence water temperature fall slightly with increasing distance offshore. Conversely in winter, coastal waters cool rapidly because of heat loss to the atmosphere, and at the same time the Leeuwin Current is maintaining warm conditions offshore, so there can be a large increase in surface temperature (up to 4 C) between the coast and the edge of the continental shelf. The Abrolhos is significant as its relatively shallow bathymetry, and close proximity to the continental shelf break, means it is effectively fronting this temperature gradient. Sea surface temperatures have been recorded at the Fisheries Western Australia puerulus collection site in the Abrolhos Islands since 1970, though with gaps in the period of testing. The site is about 2 km northwest of Rat Island (28 45 S) and the Easter Group which makes it nearly 80km from the WA shoreline and hence near the edge of the continental shelf. The site was chosen to determine the influence of the southward-flowing Leeuwin Current and shows the seasonality of sea surface temperatures. Summer sea temperatures can be as high as 26 C between February and April and the lowest values recorded where approximately 18 C. Sea temperatures below 18 C for short periods of time were found to occur during instances of prolonged easterly winds (Wilson 1978).

34 Background 20 Figure 2.8: Sea surface temperatures as measured by mercury thermometer on monthly visits to the puerulus collectors; estimated accuracy 0.3 C. The period of data analysis is from 1970 to 1976 and then 1984 to Sampling is ongoing (CSIRO 2005) Salinity (Pearce 1997) determined two main influences on surface and sub-surface salinity across the continental shelf region near the Abrolhos. High salinity coastal waters pushing westwards and the relatively low salinity Leeuwin Current water transported from the north. The extent of their interaction depends on the relative strengths of the currents as prescribed by their seasonal and inter-annual variability (Pearce 1997). The CSIRO source that provided sea temperature data near Rat Island was not available for salinity, therefore monthly mean surface salinities from the Geraldton hydrographic station up to 1997 can be found in Sukumaran (1997). They illustrate that higher mean salinity is experienced by the region in summer, with little difference through the 40m water column. Salinity can decrease by over 0.5ppt into the winter period, and through this drop the surface and near-bottom salinity levels follow each other closely. Salinity steadily increases through spring and into summer as the prevailing southerly winds increase in intensity. Sukumaran (1997) speculates that the resumption of the northerly wind stress in summer forces the relatively low salinity water back offshore with Leeuwin Current water, and that relatively high salinity water replaces via up welling and northwards transport.

35 Background 21 Salinities at Geraldton Hydrographic Station Salinity (ppt) Surface Depth- 40m Month (Jan-Dec) Figure 2.9: Monthly mean salinities from the CSIRO hydrographic station near Geraldton. Salinity at the surface and at a depth of 40m is illustrated (adapted from Sukumaran 1997) Stratification and mixing Stratification in a water column is effectively the input of buoyancy to a system as a result of driving mechanisms such as radiative heating, evaporation and/or freshwater inputs (Pattiaratchi 2005). De-stratification requires the input of energy. In the case of the Zeewijk Channel this energy is contributed by wave motion and tides, however the dominant input is wind. Figure 2.10: the processes driving stratification and mixing of a water column.

36 Background 22 The stronger the pycnocline or change in density through the water column, the greater the amount of energy required to breach the layer and in turn allow a significant degree of mixing to occur. The source of this energy is turbulence which is generated both at the sea-surface via wind stress and mesoscale current fields, and at the seabed due to currents that are often tidally driven. Pearce (1997) found thermal stratification to be limited at the Abrolhos Rat Island testing site and deduced that this property was likely for the majority of the Abrolhos region. The waters are currently considered well-mixed, with relatively high temperature and salinity throughout the Abrolhos water columns in summer (Fisheries 2000). Evaporation and subsequently evaporative cooling effects, can contribute to the mixing process and these temperature and salinity conditions (Pattiaratchi 2005). The lower temperatures and salinity presiding in winter can be attributed to higher levels of precipitation during the winter period. The easing and increased variability of the prevailing southerly winds can also be a factor towards decreased salinity levels in winter, as the Leeuwin Current can more easily flood the shelf waters under this less debilitating wind regime (Pearce 1997)

37 Background Hydrodynamic Processes This section will outline the dynamic processes occurring at the surface, and through the water column, likely to have contributed to the aforementioned water properties. They are particular important when considered as the driving mechanisms of the Zeewijk Channel circulation pattern Baroclinic Circulation Horizontal gradients in density are an important mechanism in driving ocean currents. Variations in density are directly related to variations in temperature, salinity and pressure (Mellor 1996). In situ sea water density can be calculated from the known values of these parameters using mathematical programs such as MATLAB s Sea Water toolbox. A density difference of as little as 0.01kgm -3 between two locations can have important dynamic consequences (Pattiaratchi 2005). The velocity, U, of a density driven surface current between two points can be estimated using the following equation, U = g h ρ K m 4 z ρ x Where, h is the average water depth between the two positions ρ m is the mean density K z = 0.01m 2 s -1 g is the acceleration due to gravity = 9.81ms -2 and ρ x is the difference in density over the distance between the two positions. Horizontal density gradients contribute to vertical variations in horizontal pressure gradients. Consequently, horizontal currents may also vary in the vertical and the result is baroclinic flows. For example, a mass of low salinity water will be induced to flow towards a region of high salinity water that is effectively sinking relative to the less dense water. After the two masses have entered each others density fields then the resultant salinity of the system will eventually be some median value between the original salinities due to circulation and mixing. A subsequent result of baroclinic forcing is therefore a change in the density field of the system (Pattiaratchi 2005).

38 Background 24 On the relatively shallow landward side of the Abrolhos Islands diurnal variations in heating and cooling, and alternatively freshwater inputs, may also drive thermohaline circulation within continental shelf waters. (Fahrner & Pattiaratchi 1994) illustrated this type of current formation in the shallow waters of Geographe Bay, south-west Australia. A density gradient resulting from freshwater inputs at the near shore, for example from the Spalding River in the regional context, could have a similar affect Barotropic forcing Barotropic forces are external forces such as wind stress, tides, atmospheric pressure variations or the Coriolis force driving the circulation of a water body (Mellor 1996). The main barotropic forces acting on the Zeewijk Channel would be the wind stress and windrelated waves, Coriolis and tides. The movement of water due to geostrophic anomalies over the continental shelf slope may also be a significant mode of barotropic forcing, depending on how near to the Abrolhos they occur. Barotropic forcings are characterised by little or no change in the density field of the water body after it has been transported (Pattiaratchi 2005). Put simply there is no change in the waters temperature or salinity after the forcing. This is one reason why cold water that has been entrained off-shore by eddy formations can often still be seen inside the eddy as a spiral via satellite imagery Wind-driven circulation Wind blowing over the sea surface creates friction between the moving air mass and the sea surface that induces currents (Pond & Pickard 1983). The speed and direction of wind induced currents are controlled by factors such as the momentum of the wind, inertia of the water surface and water column, pressure gradients due to wind induced motion, the Coriolis force and bottom friction. The wind stress τ, when measured as the horizontal force per unit area, is given for the sea surface by, 2 τ = C D ρ A U 10

39 Background 25 Where, C D is the drag coefficient (dimensionless) U is the wind speed 10m above sea level and ρ A is the density of air. Water at the surface of the ocean is driven at approximately 3% of the wind speed at 10m (Fahrner & Pattiaratchi 1994). Below the surface, current speeds decrease with depth as the stress is transferred from layer to layer (Pattiaratchi & Imberger 1991). (Pugh 1987), neglected the affect of the Coriolis force and assumed that the water column had constant density to give a velocity profile through the water column of, U Z u * z = U 0 ln κ z 0 Where, Κ is the von Karman co-efficient (~0.4) U 0 is the surface current speed (~3% of U 0 ) u * is the wind friction velocity = (τ/ρ A ) 0.5 and z 0 is the roughness coefficient, usually between m and m. (Csanady 1973), described how this forcing induces currents in lakes. In shallow water the direction of the wind induces a current in the same direction at the lakes surface. From the above equation we know that this will decrease with depth, but in fact it may decrease to the degree where the current reverses, relative to the surface direction, at the bottom. For shallower water the dominant force balance is between surface wind stress and bottom friction. For deeper water, current may flow in opposition to the wind, proportional to depth, as the force balance is between the pressure gradient force and the bottom friction. So if we have a basin whose bathymetry consists of shallow to deep water, we would generate a topographic gyre. (Fischer et al. 1979), elaborated on this further. An assumption is made that the wind is inducing a uniform stress everywhere on the basin of water s surface. The line of action of the wind stress will be through the centroid of the water surface. The centre of mass of water in the basin is towards the deeper end, since there is obviously a greater mass on that side. A torque is induced since the line of action of the wind stress is to the shallow side of the centre of mass, and the water mass will rotate in the windward direction at the shallow region and against the wind at the deep region (Hunter & Hearn 1987).

40 Background Coriolis and Rossby Number The Coriolis force is an inertial oscillation created by the Earth s rotation that forces currents to the left of their original momentum in the Southern Hemisphere (Pugh 1987). The Coriolis parameter is defined as, f = 2Ωsinφ Where, Ω is the Earth s angular velocity, Ω = and φ is the latitude of the water body being affected. 5 rad / s The Zeewijk Channel is between 28.5 and 29 S, therefore the Coriolis parameter of the 5 1 channel at mean latitude of S is approximately s. The Rossby Number will indicate how significantly the Coriolis force will affect the barotropic flow of a given water body at its designated latitude. As described in (Fischer et al. 1979), the Rossby Number is the ratio of the period of rotation to the time of advection. It can be calculated from the formula, R = U fl where, U is the characteristic velocity scale L is the characteristic length scale of the water body and f is the Coriolis parameter. If the Rossby Number is significantly larger than one, then the fluids momentum would be sufficient to most likely overcome the effects of the Coriolis force. A Rossby Number less than one would mean that the water body could be expected to exhibit some degree of rotational transport (Fischer et al. 1979), anticlockwise in the Zeewijk Channel s case. Calculating the Rossby Number for the Zeewijk Channel will determine if the Coriolis force is an important factor in its circulation pattern.

41 Background 27 Assume the characteristic velocity scale, U, to be the average of the absolute values of the along-channel currents recorded by the ADCP in Spring 2003, giving a value of 0.32ms -1. The characteristic length scale of the channel has been determined from the latitudes and longitudes of stations 7 and 9 on the Zeewijk Channel transect using the method of (Kirvan 1997). This calculation gave a length scale estimate of 26.7km. An approximate value of the Rossby Number for this system will therefore be 0.17, well below one and an indication that the Coriolis force may influence the circulation pattern. It is important to note however, that the physical barriers that impede currents to the north and south of the along-channel current may also negate this influence, and restrict rotational flow physically Ekman Veering Ekman veering is the next progression in the process of a barotropic surface current being pushed to the left by the Coriolis force in the southern hemisphere. As the currents influence extends deeper down the water column, the lower layers are deflected slightly more to the left than the ones above and a distinct spiralling pattern develops (Pattiaratchi 2005). This Ekman Spiral affect will usually occur in deeper offshore waters well away from the coast and where the bottom friction will not affect the direction of the currents. In the southern hemisphere the theoretical deflection of surface currents is 45 to the left of the direction of the barotropic forcing, for example the wind stress, and this deflection increases with depth but simultaneously reduces in terms of current strength (Pond & Pickard 1983). Figure 2.11: Ekman transport in the Northern Hemisphere. The wind-induced surface current turns 45 to the direction of the wind (Pattiaratchi 2005).

42 Background 28 The depth of the Ekman layer,d E, where water is moving in the opposite direction to the original surface current due to the progressive rotation down the water column, can be calculated with (Pugh 1987), D E A = π 2 ρf Z where, f is the Coriolis parameter ρ is the density of sea water, ρ = 1025kgm 3 and A Z is the vertical eddy coefficient, assume A Z 1 1 = 40kgm s The depth of the potential Ekman layer under the conditions stated for the Zeewijk Channel is 104.9m. Since the channel ranges from 30 to 60m in depth, a full Ekman spiral will not occur due to the influence of the Coriolis force. This does not mean that Ekman transport will not be significant through the water column, just that total reversal of the current direction from surface to bottom is unlikely and it will be impeded also by bottom friction. Trends of opposing current directions at the surface and bottom will need to be considered in terms of other forcing mechanisms Atmospheric Pressure Changes The inverse barometric effect occurs where the difference in atmospheric pressure between two points has driven a change in water level also between those points (Pond & Pickard 1983). Static sea level response to air pressure dictates that a decrease in air pressure of 1hPa will translate to a rise in sea level of approximately 1cm. This can be described mathematically by, Δη = ΔP atm where Δ η is the sea level change in centimetres, and Δ Patm the pressure change in hectapascals.

43 Background 29 Pressure systems are not stationary in nature however, and since they will typically be moving at some speed C A, the dynamic sea level response can be calculated using the static response to the system, gravity ( g ) and water column depth ( h ), dynamic sealevel response = static sealevel response 2 C A 1 gh Sea level changes due to meteorological influences such as atmospheric pressure and wind stress should be considered in addition to localised tidal and wave effects. Sea surface velocities produced by these meteorological conditions are usually relatively small (Mellor 1996), but it is important to remember that they can have devastating effects in terms of storm surge levels and coastal flooding, especially on low lying regions relative to mean sea level such as the Abrolhos Islands Continental Shelf Waves Continental shelf waves are the result of storm events passing over a coastal boundary and forcing a set-up of coastal water due to variations in atmospheric pressure and the action of associated wind stress (Gill & Schuman 1974). This stored energy will then propagate as waves in the alongshore direction on the continental shelf (Robinson 1964). During the Western Australian summer, these long period waves usually originate from the northwest due to the incidence of cyclones in this season. Continental shelf waves have relatively large periods of 10 to 20 days and can travel at up to 6ms -1 for distances of 1000 to 2000km alongshore (Chua 2002). They have a time lag down the coast, and would be expected to attenuate considerably away from the associated weather system (Gill & Schuman 1974). Continental shelf waves have the potential to influence the surrounding water levels of the Abrolhos and subsequently the passage of water through the Zeewijk Channel. If this influence is occurring with some significance then it can be identified in spectral analysis of the circulation pattern.

44 Background 30 Continental Shelf Seiche Seiches are standing rather than progressive waves and are important in closed and semienclosed basins, bays, marginal seas and Gulfs. Seiches usually develop when a strong prevailing wind blows over a basin, or when there is an imbalance in barometric pressure at opposing ends of a basin (Ingmanson & Wallace 1985). A standing wave is characterised by nodes and anti-nodes. Nodes are stationary points on the wave and anti-nodes are the maximum and minimum heights of the water. The physics of a standing wave involves the wave entering the basin, or conversely being generated, reflecting from the basin end and returning in exactly the opposite formation that it arrived. This process creates a situation where the water oscillates back and forth with nodes of no movement positioned between the anti-nodes of maximum and minimum wave height. Seiches are not known for causing the damage and erosion to shorelines that progressive waves are recognised for. Rather their importance is related to enclosed bays and harbours, where seiche periods close to that of the local swell and natural forcing processes can disrupt moorings and harbour functionality (Drake et al. 1978). A seiche exists in the semi-enclosed basin formed by the coastline near Geraldton (the closed end), and approximately the 100m contour line of the continental shelf. The period of this seiche can be calculated using the following equation, T = 4L gh where, T is the tidal period in s L is the approximate length of the basin, L = 70km g is the acceleration due to gravity, g = 9.81 ms-2 h is the average depth of the water column, h = 35m The period of the continental shelf seiche near Geraldton is approximately 4.2 hours using the assumed values. Its significance in terms of the circulation pattern of the Zeewijk Channel will be determined through spectral analysis.

45 Background Waves Waves possess kinetic energy in the form of the orbital motion of the particles it displaces. They have potential energy through the displacement of water above sea level (Ingmanson & Wallace 1985). Wind is the major cause of waves although submarine earthquakes, submarine landslides, submarine volcanic eruptions, landslides into the sea, ships and tidal forces are also causes of waves. Wave period is the time for one wave to pass a specific point (wave frequency is the inverse of this), wave amplitude is the height of the wave above or below sea level and wavelength is the distance between equal points on adjacent waves (Mellor 1996). Wave conditions depend on a number of factors including fetch length, that is the area over which the wind blows, the duration of the wave generating wind, wind speed, bathymetry and the distance from the area of fetch. Wave velocities increase with increasing duration, fetch length and wind speed however they decrease as the waves distance from the fetch area increases ((Pattiaratchi 2005). Sea waves are choppy waves with short periods that form in the vicinity of storms or due to localised wind affects. Swells are waves that are present on calm days, away from the winds that generated them and these have longer periods and a smoother appearance. In deep water, swells can travel thousands of kilometres away from a storm system without imparting significant energy, moving more rapidly than waves with shorter wavelengths. Waves can further be classified as shallow-water waves and deep-water waves according to the relation of their wavelength to water depth. Wave heights in the open ocean near the Abrolhos Islands average approximately 2m. However 10% of wave heights exceed 4m and these can be even greater during storms (Fisheries 2000). In fact the Abrolhos has a reputation among sea farers in the region for its large brutal seas during the winter months. The greatest wave energy is experienced on the southwest facing reef margins and wave heights are substantially lower in the island groups of the archipelago as a result of significant dampening by the shallow reefs and islands. Refracted swell and wind waves will still enter the Zeewijk channel and a primarily westward swell may pass relatively unimpeded along its course.

46 Background Currents The circulation features of currents near the continental shelf were tested in (Cresswell et al. 1989). These studies used current meters to record the strength and direction of surface currents and then a comparison was made of the results with atmospheric and coastal tide gauge data. They found that on a seasonal scale, the time frame was March to August and therefore this was a winter assessment, the continental shelf currents near the Abrolhos had directional persistence or means of ten days and that they were predominantly southward at approximately 0.2ms -1. In instances when this prevailing direction did not occur, the current had a heavy northerly component at 0.1ms -1. Alongshore currents had variability of up to 0.5ms -1, and had strong correlation to local sea level changes as well as the alongshore wind pattern recorded at Geraldton after a phase lag was incorporated. Currents are also generated by large scale wind fields that are associated with the passage of weather systems and changes in mean sea levels. Southerly wind events in summer produce sea level troughs at latitudes close to the Abrolhos and generate northward current pulses along the Western Australian coastline (Pattiaratchi 2005). Conversely frontal systems, and associated low pressure passing over Cape Leeuwin, produce sea level crests to the north and southerly currents are pulsed down the coast (Pattiaratchi 2005). Analysis of cross-shore currents near the Abrolhos Island s have indicated that they occur for approximately 33% of the year, with magnitudes less than 0.2 ms -1 (Pearce 1997). Crossshore currents take on particular importance when considering larvae or puerulus transport on and off the continental shelf. They can also be analysed when considering pollutant fluxes, for example nutrient exchange in the context of aquaculture cages within the Zeewijk Channel. In this instance cross-shore currents must be responsible for channel flushing due to the physical barriers, reefs and islands, in the alongshore directions. The circulation pattern of the Houtman Abrolhos is considered responsible, to a large degree, for the triangular evolution of the Pelsaert and Easter Group Islands (Wells 1997).

47 Background 33 Terminology regarding current direction must be clarified to avoid confusion during analysis of results. Currents heading towards the south are generally referred to as southerly currents. This is a direct contradiction to wind direction terminology but if we consider that a northerly wind creates a southerly wind stress at the surface, then the natural progression will be a current of heading south and excluding other oceanographic factors. Northerly Wind Southerly Current Figure 2.12: wind and current directions according to the recognised naming conventions. In this project it was more appropriate to describe currents as being cross-channel and alongchannel, rather than along-shore and cross-shore respectively. Cross-channel currents refer to currents with an approximately north-south orientation in the channel (to be defined more clearly in the methodology), which would translate to alongshore currents on the nearby Western Australian coastline. Along-channel currents effectively move through the channel and would be termed cross-shore if incident on the nearby coast Shelf Currents We have earlier recognised the presence and influence of the Leeuwin Current on the continental shelf waters of Western Australia. It may be helpful to understand the typical shelf currents found at other eastern boundary regions around the globe. Currents such as the California, Benguela and Canary Currents follow the dominant wind pattern equator-ward and are a result of subtropical gyre systems in their respective oceans (Lass & Mohrholz 2005).

48 Background 34 The mean longshore current for all three of these eastern boundary currents is characterized by a two layer structure. The wind-driven surface current is observed in the upper 30m of the water column. It is subject to variability but is generally equator-ward with the prevailing wind, and the pole-ward current is relatively constant below 30m in the water column. The measured mean velocity of the Benguela under-current is approximately 0.04ms -1 according to (Lass & Mohrholz 2005). This highlights the anomalous nature of the current system found along the central Western Australian coastline Tides Tides are periodic movements of the ocean which are directly related in amplitude and phase to some periodic geophysical force. Gravitational forcing, due to the simultaneously occurring moon-earth revolution, earth-sun revolution and declination effect of the moon, is the dominant influence on tidal regimes. Meteorological forces can also contribute to smaller tidal movements within each solar day. The periodic nature of tides means that variations can be represented by a number, n, of harmonic terms in the following equation, (Pattiaratchi 2005) u p ( t) = H n cos( ω nt g n ) where u p is the surface level, Hn is an amplitude, gn is a phase lag from the equilibrium tide at Greenwich and ω n is an angular speed (Pugh 1987). Consequently with T n as the tidal period the angular speed is given by, 2π ωn = T n

49 Background 35 The nature of a tide at any location in the world can be determined using the form factor equation and the dominant tidal constituent s specific to that location. In the Abrolhos Islands the dominant diurnal constituents are K 1 and O 1, and the dominant semi-diurnal constituents are M 2 and S 2. Their respective values are shown in Table 2.1 with respect to the Pelsaert Island Group as found in the Australian National Tide Tables (2002). The form factor, F, equation is expressed as, F = K M O + S 1 2 A form factor greater than 3 indicates that diurnal tides dominate the system whilst a value less than 0.25 will mean semi-diurnal tides dominate. Between these two values the system is mixed, however it is a sliding scale and the system may be mixed but predominantly diurnal or semi-diurnal (Pattiaratchi 2005). Using the tidal constituents for 2002, the calculated form factor is 2.3. The Abrolhos tidal regime can therefore be classified as mixed, although the diurnal tidal components dominate. This is expected since Pearce (1997) observed that the tidal range at the Abrolhos had a strong correlation to that of Geraldton, where the tidal range is close to 0.6m and it has a form factor of 2.4 (Australian National Tide Tables 2002). Table 2.1: tidal constituents for the Pelsaert Island Group 4. Adapted from the Australian National Tide Tables (2002). Pelsaert Island (28.97 S E) LAT (m) Constituents M 2 S 2 K 1 O H - Amplitude (m) g - Phase ( )

50 Methodology Methodology The methods used to collect, source and analyse oceanographic data to describe the circulation pattern of the Zeewijk Channel are outlined in this chapter Data Compilation This thesis presented the opportunity to analyse past data collected by independent sources for the purpose of understanding many facets of a regions properties and dynamics. This is in contrast to a methodology that would implement field testing as a direct influence of the formulation of a question or concept. Inherent problems emerge in the demonstrated approach, in particular the lack of a means to rectify methodology that is not ideal for answering the thesis question. Gaps in field reporting caused by equipment malfunction and spatial or temporal necessities cannot therefore be re-assessed. The assembled wind and current data included spring and Summer time periods for 2002 and Testing of water column properties by a CSIRO research vessel occurred in spring of 2003 and wave data was recorded in spring There were data shortages due to equipment malfunction in the 2003 wind, and 2002 current, recordings. Due to the importance of a full current data set when considering circulation patterns, and the opportunity to relate this pattern with known water properties, the 2003 data has been analysed. Deficiencies in the wind pattern for the identical time frame exist however. The general faults in the 2002 data contributed to it not being used for comparison purposes, although the 2002 wind and wave data can be useful as a description of the seasonal characteristics and prevailing conditions.

51 Methodology The Southern Surveyor The CSIRO research vessel, Southern Surveyor, traversed the Western Australian coastline from September to November of 2003 between latitudes 27 and 36 south. Its mission was to test the oceanographic properties of this vast expanse of water and in particular to assess the interaction of coastal and offshore waters across the continental shelf during this time frame. The ship managed to track two separate paths through the Houtman Abrolhos Islands that offer reasonable substance for analysis in this dissertation. The Southern Surveyor wound its way through the Middle Channel on the 27 th of October after previous testing of waters to the north. A nine station transect was recorded as the vessel travelled east to west. It headed out beyond the 1000m depth contour of the continental shelf before turning back eastward to traverse a route through the Zeewijk Channel and recorded a further 9 testing stations along this transect and into coastal waters. The positions of the sampling stations reflect the interests of the CSIRO regarding the properties of water specifically on the continental shelf slope, and the exact positions with the respective water depth are recorded in Table 3.1. Table 3.1: exact locations, and the associated water depth, of the sampling stations for CTD testing on the Zeewijk and Middle Channel transects. Zeewijk Channel Middle Channel Station Latitude Longitude Depth (m) Latitude Longitude Depth (m) The stations are not ideal for an accurate assessment of the properties within the Zeewijk Channel, but still offer the opportunity for surmisal of the basic channel properties. Comparisons of the water s properties between the two transects can also be made.

52 Methodology Figure 3.1: Southern Surveyor transects and approximate sampling station locations through the Zeewijk and Middle Channels, October 27th and 28th, 2003 (Australian Hydrographic Service, 2005). The sampling station locations can be classified into two groups. Shelf stations, 1 to 5, and channel stations, which must therefore include stations 6 to 9 and these classifications apply for both of the transects. The two groups of stations are separated by the 100m contour and should enable the ability to contrast continental shelf water and that associated with the Abrolhos Island Channels. Examining the groups separately at the appropriate stages will prevent results from being obscured by effects such as scaling also CTD The Conductivity, Temperature and Depth probe is a multi-parameter instrument which measures and records conductivity, temperature and pressure as it is lowered through the water column. This enables the analysis of salinity, temperature and depth and often a host of other parameters. The CTD probe contains three internal sensors (one each for temperature, conductivity and pressure), around which water can flow for measurement. The signals from the three sensors are then transmitted to the surface to be recorded in a data logger. Across the CSIRO vessels transects deployment of the CTD occurred at the stations illustrated in Figure 3.1, with subsequent sampling at approximately 2 metre intervals down the water column and beginning near the surface.

53 Methodology 39 Supplementing the water column testing was the pumping of surface water through an onboard analyser to provide surface values of temperature and salinity. This data was recorded at far more regular intervals than the prevalence of testing stations to highlight surface water properties with greater accuracy. Recorded also was the water column depth at each of these surface sampling positions, which can be used to give a bathymetry across the continental shelf and through each of the channels relative to the alignment of the transect. The surface water analysis begins from the coordinates (28.93 S, E) and ends at (28.28 S, E). The water column depth range across the transect, in terms of where the surface sampling occurred, is approximately 850m to 40m ADCP The Acoustic Doppler Current Profiler (ADCP) is a eulerian, or fixed position device, used to measure the current speeds and directions of the water moving through the Zeewijk Channel. It was deployed north of Gun Island in the Zeewijk Channel at coordinate (28.8 S E) for two periods of record, firstly from October to December in 2002, and secondly from September 1 st through to November 28 th in Figure 3.2: the ADCP used in the Zeewijk Channel and its deployment location.

54 Methodology 40 The profiler illustrated in Figure 3.2 is a khz upward looking instrument that was fixed to the bottom in approximately 33m of water. The ADCP has a blanking distance of 2m, therefore no data was recorded within 2m of the seabed and bin placements, were at 1m intervals through the water column. Acoustic beams are emitted from a transducer and then scattered by small particles and zooplankton moving with the currents. The reflected beams are measured by a sensor on the instrument. The bins used for current analysis were located close to the sea bed and also near the top of the water column. Current roses will be used to illustrate the prevailing current directions through the Zeewijk Channel for the 88 day period of testing, at the surface and near the sea bed. To highlight current variability, the data has been converted into cross-channel and along-channel components. Channel orientation is predominantly east to west but from observation of the topography, a tilt of approximately 42 towards the north has been factored into the calculations. Cross channel is therefore across the y-axis, north-westerly being positive and south-easterly negative, with these directions characterised by the barriers at their extremities. Along channel effectively means through the channel, therefore north-easterly currents are positive and south-westerly currents are negative. MATLAB (MathsWorks Inc.), programming has been used to create these figures after manipulation of the data sets in Microsoft Excel Wind Analysis Wind data recorded at the Bureau of Meteorology s North Island testing station (28 18 S, E) was obtained for the periods of testing. Significant gaps exist in the spring 2003 wind data as a result equipment malfunction. The spring 2002 data offers a complete period of record therefore it has been analysed as a reference. Wind speed and direction values are actually an averaged figure over the 10 minutes prior to the hourly observation time. Wind speeds are in kilometres per hour, and although wind direction could be output as either degrees or points of the compass, degrees were chosen with values rounded to the nearest 10 degrees. Feather plots, again created in MATLAB, have been used to illustrate the incidence of wind-induced currents. The wind data has been converted into its component form and re-orientated so that the feather plots point in the direction the wind is heading, rather than where it has come from to assist interpreting of wind stress on currents. Wind roses demonstrate the prevailing seasonal conditions.

55 Methodology Digitisation of Bathymetry Since the resolution of bathymetry for the region was at 1000m from available sources, enquiries were made through the DPI and Australian Navy to determine if finer resolution data existed. This was eventually obtained from the Australian Hydrographic Service. The data was stored as a column matrix consisting of longitude, latitude and the depth recorded at each of these coordinates by the testing vessel. This data was then loaded into MATLAB and the following script used to create the digitised bathymetry: >>surf(xi,yi,zi) >>shading flat >>caxis([-150 0]) >>colorbar( vert ) Note that area s where the testing vessel was physically unable to proceed encompasses both shallow reefs and actual islands and this default (zero) surface representation is plotted as dark red on the bathymetric map. The colour bar depicts a 0 to 150m depth scale, so that the channel bathymetry is clearly shown. Figure 3.3: digitised bathymetry of the Zeewijk Channel. Depth is in metres on the z-axis, latitude and longitude are on the y and x-axis respectively. Data courtesy of the Australian Hydrographic Service (2005).

56 Results Results Field results from the respective surveys will be used to assess the oceanographic properties of the Zeewijk Channel and to determine a general circulation pattern for the period of testing. The dominant forcing mechanisms driving these currents will be identified in the synthesis of results CTD The conductivity, temperature and depth analysis will identify if discrete water bodies are present at the surface, and progressing through the water column of the two transects tested. The temperature and salinity properties of the water columns should indicate the degree of mixing occurring in the channels relative to that outside of it. The surface temperature and salinity recordings are for the Zeewijk Channel transect. Middle Channel data was not obtained for analysis. The annotations to the surface temperature and surface salinity plots indicate the relative water depth to the surface water properties at significant locations along the transect. The depth of the water column at the first recorded sample is 850m and the depth of the last, 41m Surface Temperature The temperature of surface water decreased considerably, after the Southern Surveyor entered waters inside the 60m contour. The difference in temperature is nearly 0.5 C between the shelf break water and continental shelf water. This trend suggests the presence of a relatively warm mass of surface water above the continental shelf slope near the Abrolhos at the time of testing.

57 Results 43 Temperature (C) Surface Water Temperature Change Across Transect Annotation Distance of Transect (km) Depth (m) Figure 4.1: the change in surface water temperature from the continental shelf slope, into the Zeewijk Channel. A noticeable temperature decrease is observed after 20km where the testing entered waters beyond the 60m contour line of the continental shelf Water Column Temperature There is a steady decrease in temperature with depth for all of the shelf stations. Station 1, the deepest and western-most station, is colder than the other stations at the same depths through the water column until after 350m. Stations 2 to 5 have very similar temperature profiles beginning at surface temperatures very close to 19.7 C for each station, and decreasing at a slightly faster rate after the 100 to 150m mark. A temperature profile relative to the continental shelf contour is illustrated also, although the scaling affect of the relative length scale is not conducive to accurate visualisation of the temperature profile above the 200m contour. The characteristic increase in temperature with depth of the offshore stations is evident however.

58 Results 44 Temperature (C) Depth (m) Station 1 Station 2 Station 3 Statoin 4 Station 5 Station 6 Staton 7 Station 8 Station Figure 4.2: temperature-depth profiles of stations 1 to 5 on the Zeewijk transect, 27/10/03. Figure 4.3: temperature contour plot relative to the depth of the continental shelf.

59 Results Zeewijk Channel The channel stations profiles indicate two separate bodies of water according to temperature with depth. Stations 6 and 7, situated west of the step feature in the Zeewijk Channel recorded higher temperature with depth than the landward stations. Stations 8 and 9 also demonstrated relatively constant temperature with depth after the initial few metres. Temperature (C) Temperature (C) Station 2 Station 3 Depth (m) Station 6 Staton 7 Station 8 Station Station 4 Station 5 Station Figure 4.4: temperature-depth profiles of stations 6 to 9, and a microscopic view of Figure 4.2, to compare the temperatures of the inner and outer stations in the surface 100m on the Zeewijk Channel transect, 27/10/03. Comparing the channel and shelf stations over the same depth range indicates that stations 2 to 7 have similar temperature profiles including a shared surface temperature of approximately 19.7 C and a moderate thermocline between the 5 and 10m interval. Station 1 has cooler water in the upper 100m as evidenced earlier, but it shares this property, including a surface temperature of approximately C with stations 8 and 9 from inside the channel. Stations 8 and 9 have almost uniform temperature through the relatively short depth of their water columns however.

60 Results Surface Salinity The salinity of surface water increased after the Southern Surveyor entered waters inside the 60m shelf contour. The salinity increase was approximately 0.05ppt. The region of testing above the continental shelf slope has low salinity, relative to the adjacent water inside the 60m contour. Surface Water Salinity Across Transect Salinity (ppt) Distance (km) Figure 4.5: the change in surface water salinity from the continental shelf slope, into the Zeewijk Channel. A noticeable salinity increase is observed after 20km where the testing entered waters beyond the 60m contour line of the continental shelf. The same annotations as the surface temperature plot apply Water Column Salinity Salinity initially increased with depth by approximately 0.2ppt for the first 200 to 250m of the water column before decreasing fairly rapidly below this mark. Station 1 has the highest salinity level at the surface at 35.68ppt. Station 1 s salinity increases with depth at a similar rate to the other stations but it is the first station to reverse this trend and have decreasing salinity beginning from approximately the 100m mark. The contour plot for salinity depicts the increasing salinity in the first 200 metres of the water column as a bubble of lower salinity water positioned on the upper shelf break.

61 Results 47 Salinity (ppt) Depth (m) Station 1 Station 2 Station 3 Station 4 Station Figure 4.6: salinity-depth profiles of stations 1 to 5 on the Zeewijk transect, 27/10/03. Figure 4.7: salinity contour plot relative to the depth of the continental shelf.

62 Results Zeewijk Channel The channel stations profiles indicate very little difference in salinity levels between the four stations, especially in the first 40m of the water column. Stations 6 and 7 have increasing salinity with depth. The increase is only slight but is particularly evident in the first 10m and below the 40m mark of the water column. Stations 8 and 9 had relatively uniform salinity with depth. Salinity (ppt) Salinity (ppt) Depth (m) Station 7 Station 8 Station 9 Station 6 Depth (m) Station 1 Station 2 Station 3 Station 4 Station Figure 4.8: salinity-depth profiles of stations 6 to 9, and a microscopic view of Figure 4.6, to compare the temperatures of the inner and outer stations in the surface 100m on the Zeewijk Channel transect, 27/10/03. Comparing the shelf and channel stations for the first 100m of the water columns highlights that Station 1 is markedly more saline at the surface, by nearly 0.1ppt, than all other stations. Stations 8 and 9 are distinct due to their uniform salinity between 35.6 and 35.62ppt. Stations 6 and 7 tend towards the trend exhibited by the intermediate stations.

63 Results Sea water Density An estimate of the density difference between the 2 discrete surface water masses illustrated in the surface temperature and salinity plots is necessary to determine if currents in the channel are being driven by baroclinic forces. The discrete water bodies can be classified as being either offshore of the 60m depth contour and therefore over the shelf slope (in context the 60m contour line is near the Pleistocene ridge formation), or inshore of the 60m contour and therefore within the Zeewijk Channel. The temperature, salinity and related density of the surface water is described in Table 4.1. Table 4.1: temperature, salinity and related density of surface water inside the Zeewijk Channel and above the continental shelf slope. Temperature ( C) Salinity (ppt) Density (kgm-3) Shelf Slope Channel The calculation of density was made using MATLAB s Sea Water tool box. Since assessment of the surface water density was required, pressure was assumed to be atmospheric and temperature and salinity were inputted in the units shown in Table 4.1. The density difference between the two water masses is 0.1kgm -3. According to (Pattiaratchi 2005) this is a sufficient density difference to impact on the hydrodynamics of the area. An estimate can be made of the baroclinic velocity induced by these different sea surface properties. Using the equation and constants outlined in Chapter , and stations 4 and 8 as the geographic estimates of the water body positions, gives an estimate of U, the surface velocity, as being approximately ms -1 under these conditions.

64 Results Middle Channel vs. Zeewijk Channel The Middle and Zeewijk Channels can be compared in terms of water column properties to determine if these properties are localised or typical of the greater system. TS Diagram Zeewijk Channel TS Diagram Middle Channel Station 2 20 Station 2 Temperature (C) Station 3 Station 4 Station 5 Station 6 Station 7 Station 8 Station 9 Temperature (C) Station 3 Station 4 Station 5 Station 6 Station 7 Station 8 Station 9 Salinity (ppt) Salinity (ppt) TS Diagram Zeewijk Channel TS Diagram Middle Channel Temperature (C) Station 6 Station 7 Station 8 Station 9 Temperature (C) Station 6 Station 7 Station 8 Station Salinity (ppt) Salinity (ppt) Figure 4.9: TS diagrams to compare the water column properties of the Zeewijk and Middle Channels. The lower figures represent the water column properties of the channel stations, that is, stations 6 to 9. Station 1 has been omitted from all figures to aid interpretation due to the scale of its depth and properties. Station 9 of the Middle Channel is located at approximately (35.61, 19.2), and is hard to view as it is almost a point. There is close correlation between the trends in water properties of the two channels despite the significant differences in depths between stations 2 to 5. The Middle Channel stations are slightly warmer, surface temperatures are over 20 C for the intermediate stations, and salinity is also lower by approximately 0.05ppt in the shelf slope region. The correlation is strongest in terms of temperature decreasing with depth, and salinity increasing for a period and then decreasing after the 200m mark, for the shelf stations on both transects.

65 Results 51 The uniformity of properties throughout the water column s of stations 8 and 9, is shared by the Middle and Zeewijk Channels. This is evidenced in the TS diagram by the properties profiles represented as points, or very small lines, rather than curves. Station 6, situated just inside the 100m contour on both transects, has the same TS profile of decreasing temperature and increasing salinity with depth for both channels. Station 9 has very similar water column properties on both transects, with approximate temperature of 19.2 C and salinity 35.61ppt, for their entire depth s of less than 40m. The Zeewijk Channel T-S diagram with station 1 included is notable for its visualisation of the three major current systems off the continental shelf. The Leeuwin Current, Leeuwin Undercurrent and the presence of Antarctic Intermediate water can be identified from the T-S diagram in Figure 4.10, distinguished by the gradient changes in the figure. This is a typical T-S diagram for the region in 1000m of water (Pattiaratchi 2005). TS Diagram Temperature (C) Salinity (ppt) Figure 4.10: Temperature-salinity diagram highlighting the currents present in the water column on the Gascoyne continental shelf slope, particularly out at the 1000m contour.

66 Results 52

67 Results ADCP The ADCP data was recorded in the Zeewijk Channel from the 1 st of September through to the 28 th of November in 2003 at the location described in the methodology. This spring period of 2003 yielded a complete data set from the ADCP and has been selected over the incomplete 2002 data set. It has been analysed for the surface and bottom currents in terms of the prevailing current directions and current variability. Spectrum analysis will be employed to determine the factors influencing current variability Current Roses Current roses will demonstrate the net, or prevailing, current directions at the surface and bottom of the Zeewijk Channel during spring of Using the speed and direction recorded by the ADCP, the roses illustrate the percentage incidence of currents. Surface The dominant surface current direction is westerly, contributing to over a third of the surface current pattern. A significant west to south-westerly component exists as well which means that net transport at the surface is offshore. The influence of barriers in the form of Island groups and reefs to the north and south is evident from the east to west dominance of the current rose. Bottom The bottom currents are more evenly distributed compared to the surface currents, with orientation between north-westerly and south-easterly. There is a higher incidence of crosschannel currents at the bottom, especially towards the north, and it should also be noted that peak bottom current speeds are approximately half the magnitude of those at the surface.

68 Results 54 N 40 % NW 30 % NE 20 % 10 % W E SW SE Speed scale in cm/s S Figure 4.11: current rose of surface currents in the Zeewijk Channel during spring N 40 % NW 30 % NE 20 % 10 % W E SW SE Speed scale in cm/s S Figure 4.12: current rose of bottom currents in the Zeewijk Channel during spring 2003.

69 Results Current Variability Analysis of the cross- and along-shore variability will determine periodic features of the current pattern. In relating the current variability to the current rose results, the along-shore and cross-shore currents are to be viewed as the respective components of the actual current direction at any point in the time series. For example stages of positive cross-shore current that correlate to negative along-shore currents at the same time, are actually periods of westerly current flow. Surface Surface current variability is characterised by a high incidence of ocean-ward or westerly current direction. The major alternative current direction is easterly. The current fluctuations vary from a relatively large 5 day oscillation to daily changes in current strength. The 5 day oscillation is interrupted by an extended period of westerly currents, lasting 10 to 15 days and with diurnal fluctuations within the time frame. It returns to the 5 day oscillation pattern at the end of this phenomenon and persists until the end of the period of record. Along-channel Surface Currents Cross-channel Surface Currents Current Speed (cm/s) Day Figure 4.13: surface current variability in the Zeewijk Channel for the cross- and along-shore directions during spring 2003.

70 Results 56 The maximum current speed at the surface is approximately 0.7ms -1 in the cross-channel direction. When combined with a relatively high but negative along-channel component the resulting vector is of westerly orientation and also has magnitude greater than 0.7ms -1. Bottom The short, sharp oscillations in the along- and cross-shore bottom currents illustrate a high incidence of diurnal factors influencing the bottom current direction and magnitude. Maximum magnitudes are nearly five times less at the bottom compared to the surface, and this is true in both current directions. Only one incidence of bottom currents exceeding 0.15ms -1 can be seen and this occurred as a negative (south-easterly) in the cross-channel direction. Since the maximum along-channel current correlates with this component the maximum bottom current speed reached is approximately 0.22ms -1 and in an easterly direction. An oscillation is also evident for an approximately 10 to 15 day period in both the along- and cross-channel directions. 20 Along-channel Bottom Currents Cross-channel Bottom Currents Current Speed (cm/s) Day Figure 4.14: bottom current variability in the Zeewijk Channel for the cross- and along-shore directions during spring 2003.

71 Results 57 The cross-channel currents demonstrate a shift from predominantly north-westerly (positive) to predominantly south-easterly (negative) over the 88 days of testing. The along-channel currents do not share this trend and are relatively constant in their oscillation between positive and negative values. The resulting bottom current is relatively balanced between north-westerly and south-easterly. However we now know that the contribution of the northerly component was primarily in the first half of the testing period, and the southerly component was therefore predominantly in the second half of the testing period.

72 Results Particle Progression The particle progression plots were created in MATLAB to illustrate the progressive motion of a particle at the surface or bottom for the duration of ADCP recording. For clarity in the resulting plot, the mean of the 48 current vectors that contribute to each 24 hour period was calculated. The plot is therefore a representation of the progressive travel of a particle using the daily mean current vectors for the 88 day period of record. A standard Cartesian plane has been used, with north/south and east/west orientation of the axis. Surface The important feature of the surface particle progression plot is that the current pattern follows the calculated 42 degree orientation of the channel in terms of the hypothetical movement of a particle past this point. There is generally a north-east to south-west direction of currents at the surface. This significantly elliptical circulation of surface currents in the along-channel direction, as specified in the ADCP methodology, is evident in Figure 4.15Error! Reference source not found.. 40 Particle Progression Plot - Surface South-North (km) West-East(km) Figure 4.15: plot of the path travelled by a particle under the influence of the mean daily surface currents in spring The current path is elliptical in the along-channel direction. A particle is unlikely to travel more than 90km either side of the ADCP position due to the mean daily currents before a direction change would force it to return channel-ward. The implications of these particle progressions on the circulation pattern are explored in the discussion.

73 Results 59 Bottom The bottom progressive vector plot suggests tendencies towards north-west and south-east migration of particles due to bottom currents. This may be linked to the channel bathymetry and the gap in the ridge at the northern end of the ocean-ward channel opening. The magnitude of particle movement at the bottom is significantly smaller than the magnitude of particle movement at the surface. Particle Progression Plot - Bottom 10 5 South-North (km) West-East (km) Figure 4.16: progressive plot of the path travelled by a particle under the influence of the mean daily bottom currents in spring There is a strong north-west to south-easterly current trend. The range of the particle circulation is approximately 30km in comparison to the surface range that is over 150km. Since the length scale of the channel is approximately 30km also, it appears that bottom circulation is largely confined to the channel region according to this result.

74 Results Particle Excursion The particle excursion plots were also created in MATLAB, and illustrate the possible distance a particle could travel at the surface or bottom in a day using the daily mean current vectors for the period of record. A standard Cartesian plane has again been used, with northsouth and east-west orientation of the axis. Surface As expected, the surface particle excursion plot follows the 42 degree orientation of the channel that was illustrated in the surface particle progression analysis. The particle excursions are therefore typically in the east to north-east and west to south-west directions. The maximum particle excursion in one day was nearly 50km and it occurred for both directions. This plot provides a better representation then the progressive figure, of the range a particle could be expected to travel through the Zeewijk Channel in one day. 20 Particle Excursion Plot - Surface South-North (km) West-East (km) Figure 4.17: the distances travelled by a particle at the surface under the daily mean current conditions for each day of the testing period.

75 Results 61 Bottom The bottom particle excursion plot depicts the tendency towards north-west and south to south-east excursions. The maximum distanced travelled by a particle in one day is approximately 8km. Particle excursion near the bottom is over 5 times less than the maximum particle excursions at the surface. The excursion range of nearly 16km, 8km in either direction, is further evidence that bottom circulation is highly channel-centric. 8 Particle Excursion Plot - Bottom South-North (km) West-East (km) Figure 4.18: the distances travelled by a particle near the bottom under the daily mean current conditions for each day of the testing period.

76 Results Spectral Analysis The spectral density analysis illustrates the spectrum of energy in the system for the time frame of testing. Energy fluxes that correspond to barotropic forcing mechanisms are distinguished by the frequency, and therefore period, at which these processes are known to oscillate. The energy spectrum for periods greater than approximately 20 days is inherently large as a result of the time scale of the field data, around 90 days, and this makes it statistically weak as a result. The energy in the system is nearly 10 2 times greater at the surface than at the bottom. Also, the along-shore component has greater energy input to the system at the surface than its related cross-shore component. This trend is not depicted at the bottom. Following the colour convention used in current variability, the red line represents along-channel currents and the blue represents cross-channel currents for the surface and bottom graphs. Surface By far the most significant proportion of the surface current s energy spectrum correlates to periods between 5 and 15 days. As mentioned earlier this is expected, but the spectral density of these longer period frequencies is still a significant factor for understanding of the channels circulation as they are associated with mesoscale oceanography and climatology. There are definite spikes in spectral density at 24 hours and to a lesser degree close to 12 hours, associated with the tidal components of the channel. The 2.2 days or 52 hours frequency represents a greater energy input than the tidal component, and the implications of this will be discussed later. The last significant spike in spectral density is at the frequency for a period of 4 hours. This value corresponds to the period of the continental shelf seiche for the region. Bottom The feature of spectral analysis of the bottom currents is the greater significance in the relative energy of the tidal components. Evidence of this includes the 5 day frequency containing less spectral density than the diurnal tidal regime. There is still some 10 to 15 day controlling factor that can be associated with the majority of energy found in the system. This can be linked to the spring/neap tidal cycle, further evidence that the tidal regime is far more important for bottom circulation relative to its importance at the surface. The 52 hour frequency also has less significance for the bottom circulation. The continental shelf seiche exhibits a small degree of influence on bottom currents also.

77 Results 63 Figure 4.19: Spectral density versus frequency for surface currents in the Zeewijk Channel during spring Figure 4.20: Spectral density versus frequency for bottom currents in the Zeewijk Channel during spring 2003.

78 Results Wind Wind data for the periods 1/9/03 to 16/9/03, and 28/10/03 to 6/11/03, has been used for analysis as these periods represented the largest sections of complete recording by the North Island station. It has been plotted relative to the surface currents at the same time to depict the incidence of wind-induced currents in the system. The spring 2002 data will be represented as a wind rose to illustrate the prevailing wind conditions Spring 2003 The wind data from 1/9/03 to 16/9/03 shows a correlation with the direction of surface currents. The feather plot illustrates a particularly strong change in wind pattern. Fluctuating north-west to north-easterlies interrupt the incidence of southerly winds and induce a southeasterly surface current in the channel for a short time. The return to prevailing wind conditions negates the southerly wind stress and the surface current pattern returns to what seems to be the prevailing oscillatory conditions, including the significant westerly direction. The strongest winds experienced in this period of record were the north-westerlies. They reached 19ms -1 approximately at there peak. Along-channel Surface Currents Cross-channel Surface Currents Current Speed (cm/s) Speed (m/s) and Direction Day Wind for days 243 to 260 Figure 4.21: wind influence on surface currents for 16 days of the testing period, between 1/9/03 and 17/9/03.

79 Results 65 The second section of spring wind data, 28/10/03 to 6/11/03, is shorter than the first by 6 days and consists of a steady stream of easterly to southerly winds. In fact, the wind swings from east to south repeatedly and the period of each swing is in the order of 1 to 2 days. Wind strength increases as its orientation becomes increasingly southerly also. Along-channel Surface Currents Cross-channel Surface Currents Current Speed (cm/s) Speed (m/s) and Direction Day Wind for days 301 to 310 Figure 4.22: wind influence on surface currents for 10 days of the testing period, between 28/10/03 and 6/11/03. The surface currents represent a nine day period of largely westerly flow that has periodic oscillations in magnitude. These oscillations correlate to the east-south variations in wind speed and direction, although other diurnal processes including the tidal regime cannot be discounted in terms of importance. The strongest southerly wind peaked at approximately 13ms -1, and correlates with a period of strong westerly surface current at the end of day 303. The strongest current during this time frame occurred during day 305 under the influence of moderate southerly winds, and reached an estimated 0.70ms -1.

80 Results Spring 2002 The wind rose of percentage wind incidence and magnitude for spring of 2002 highlights the prevailing southerly wind pattern in the region during this season. The wind rose depicts data from the 1 st of October to the 31 st of December. This is slightly longer than the data sets analysed for spring 2003, however the rose is still useful as a representation of the seasonal wind pattern. Over 70% of the recorded wind directions had strong southerly components. Figure 4.23: current rose of winds recorded at the North Island recording station 1/10/02 and 31/12/02.

81 Results Waves Wave data was recorded at the ADCP location north of Gun Island for the period 12/10/02 to 16/11/02. Wave heights are relatively small due to the sheltering aspect of Half moon Reef and the Pelsaert Island Group Spring 2002 Wave heights appear periodic with peaks in magnitude at approximately 5 day intervals. The maximum and minimum wave heights experienced at the testing location were 1.25 and 0.3m respectively. Between days 306 and 317 there was relatively constant wave height between 0.5 and 1 metre with diurnal fluctuations evident over this time frame. The period of recorded waves ranges between 10 and 20 seconds for the majority of the testing record. However there are 3 distinct drops in period. They are at days 298, where its rise from the low of nearly 3 seconds correlates with a rise in wave height to the maximum of 1.25m, and at days 316 and 318 which could be associated with the apparatus failure that caused the loss of data at this time. Wave direction stays in the range between approximately 270 and 330 for the majority of the testing period which correlates to a predominantly west to south-westerly wave direction. Waves follow the same naming conventions as winds therefore they are basically travelling into the western entrance of the channel after being generated in the Indian Ocean. The apparent direction change at day 318 may have been induced by a period of calm or the occurrence of very strong easterly winds forcing offshore wind waves at the sea surface for a short period of time.

82 Results 68 Figure 4.24: depicts the wave height, period and direction of waves in the Zeewijk Channel for the period 12/10/02 to 16/11/02.

83 Results Eddy Formation Using plots of sea level anomaly obtained from the CSIRO Marine Division (2005), eddy formation near the Abrolhos Islands during the period of testing can be examined. There are at least two instances in the spring of 2003 where the geostrophic anomaly induced eddy formations that impacted upon water circulation around the archipelago. They are both the result of a mound of higher sea level to the south of the Pelsaert Group Spring 2003 The first eddy formation identified is from the 6 th of October. The geostrophic anomaly represents a difference in sea level of approximately 0.3m. The anti-clockwise motion of the eddy is illustrated by the directional markers. They also quantify the geostrophic velocity outside the Zeewijk Channel as between 0.1 and 0.3ms -1 (some magnification of the figure may be required for clarity) with an associated westerly or offshore heading. The sea surface temperature is approximately 20 C at the centre of the eddy and slightly cooler at the position identified as in front of the Zeewijk Channel opening. The southern side of the eddy is forcing relatively cold water towards the Western Australian coastline. The northern side is entraining coastal water from the Abrolhos landward side, through the channels due to the bathymetry of the archipelago, and offshore. The second eddy formation is on the 9 th of November. This time the geostrophic anomaly is slightly greater, with a sea level difference closer to 0.4m. In turn the geostrophic velocity of the resulting current pattern is also greater, between 0.2 and 0.4ms -1, and the direction of flow in front of the Zeewijk Channel is south and westerly. The circulatory influence and presence of this eddy is clearer than the eddy on October 6 th, its size and influence is easily discernible from the image of sea surface temperature. There is transport of water northwards inside the 100m contour of the continental shelf from as far south as Cape Leeuwin, although the geostrophic velocity of this transport fluctuates in intensity between 0 and 0.3ms -1. The November 9 th eddy pushes offshore water into the northward coastal stream at its southern margin, effectively strengthening the northward geostrophic velocity that had weakened approximately 200km north of Perth. The eddy entrains coastal water offshore at its northern end and since this is aligned with the Abrolhos Islands, the water is brought through the channels.

84 Results 70 Figure 4.25: sea level anomaly, related geostrophic velocity and sea surface temperature for post-october 6 th 2003 (CSIRO 2005). Figure 4.26: sea level anomaly, related geostrophic velocity and sea surface temperature for post- November 9 th 2003 (CSIRO 2005).