Study of the Physical Oceanographic Properties of the Persian Gulf, Strait of Hormuz and Gulf of Oman Based on PG-GOOS CTD Measurements

Transcription

1 Journal of the Persian Gulf (Marine Science)/Vol. 5/No. 18/December 2014/12/37-48 Study of the Physical Oceanographic Properties of the Persian Gulf, Strait of Hormuz and Gulf of Oman Based on PG-GOOS CTD Measurements Azizpour, Jafar * ; Chegini, Vahid; Khosravi, Maziar; Einali, Abbas Iranian National Institute for Oceanography and Atmospheric Science, Tehran, IR Iran Received: June Accepted: November Journal of the Persian Gulf. All rights reserved. Abstract The present study covers the results from the CTD observations of the PG-GOOS cruises. CTD profiles were collected in eight cruises at 169 stations, from fall 2012 to the late summer Spatial and temporal distributions of the temperature, salinity and density were investigated. In summer, water column at deep stations was strongly stratified and at shallow stations water column was moderately well-mixed. The coincidence of surface heating resulted in the strength of summer stratification. Increasing in temperature and particularly salinity near the bottom in southern stations of the Strait of Hormuz indicated trace of the Persian Gulf Water outflow. Strong and weak thermocline layers are formed in summer ( 12 ) and winter in deep stations ( 6 ), respectively. T-S diagrams at deep part of the Strait of Hormuz showed two water masses i.e. the Indian Ocean Surface Water (IOSW) inflow to the Persian Gulf and Persian Gulf outflow dense water. The movement of scatter plots along temperature and salinity axes indicated the influence of the seasonal variations of the Indian Ocean Surface Water (IOSW). The results of this project are the most historically magnificent oceanographic survey of the Persian Gulf and Gulf of Oman with wide range of applications. Keywords: Persian Gulf, Strait of Hormuz, Gulf of Oman, PG-GOOS, Physical Oceanography, Hydrography. 1. Introduction I.R. Iran is situated along 5700 kilometer of coastlines of the Persian Gulf, the Strait of Hormuz, and the Gulf of Oman in the South as well as the Caspian Sea in the North. Oceanographic and marine studies, bearing in mind the geographical, political, and economic situation of the I.R. of Iran, depend on collecting information and data. Without doubt, the oceanic region including the Persian Gulf, the Strait * azizpour@inio.ac.ir of Hormuz, and the Gulf of Oman is one of the most political and economical waterways in the world (Fig. 1). The Persian Gulf (a shallow semi-enclosed basin) is a very important piece of world s ocean water because of its rich gas and oil sources. The Straits of Hormuz which is only 56 km wide at its narrowest point, connects the Persian Gulf with the Gulf of Oman and the northwestern part of the Indian Ocean. Despite of limited and sparse investigations in the coastal waters to the best of our knowledge, a few basin-wide oceanographic surveys have been 37

2 Azizpour et al. / Study of the Physical Oceanographic c Properties of the Persian Gulf, Strait done in this region (Reynolds, 1993). Regional and local oceanographic dataa collected via vessels, satellites or other means are sources of information to develop appropriate models for objective-oriented goals in marine ecosystems. Regional countries have investigated their coastal waters which in the in the Persian Gulf, the scope of the data is limited to temporal and spatial coverage and only a few basin-wide survey results are published. Temperature-salinity data was published by Emery (1956) from the German ship Meteor expedition covering the 1948 summer cruise. Later, Brewer and Dyrssen (1984) reported a wintertime survey of the Atlantis from Woods Hole Oceanographic Institution. Reynolds (1993) reported the first comprehensive set of CTD measurements, current meter mooring data, buoy tracking, and observation of many meteorological dataa in the Persian Gulf. The survey continued for about 4 months, coveringg the period of February to June Water propertiess measurement did nott cover the shallow shelf of the southernn Persian Gulf. Swift and Bower (2003) explained e the aspects of the water properties in thee Persian Gulf by using available hydrographic dataa from January to August. Moreover, the Regional Organization for the Protection of the Marine Environment (ROPME) also conducted several basin-wide surveys in the Persian Gulf during summer of 2000 and 2001 and winter of Fig. 1: Map of the Persian Gulf, the Strait of Hormuz and the Gulf of Oman, CTD casts. Boxes show the eight cruise study area. In Box 2, cruise was done inn 2 seasons (2 times) and in Box 3 cruise repeated 3 times. 38

3 Journal of the Persian Gulf (Marine Science)/Vol.5/No.18/December 2014/12/37-48 The Iranian National Institute for Oceanography and Atmospheric Science (INIOAS) performed the Persian Gulf and Gulf of Oman Oceanographic Study (PG-GOOS) project in an interdisciplinary format. This project is the longest Oceanographic field operation in the Persian Gulf and Gulf of Oman that commenced on November 5, 2012 and was continued for one year (Table 1). As far as we are aware of, there is no previous survey to cover the area during four seasons. Table 1. List of Observational Cruises for Oceanographic data collection Cruises Date Number of Stations Location PG-GOOS /11/ East Part of PG PG-GOOS /12/ West Part of PG PG-GOOS 3 27/1-1/ Hormuz Strait PG-GOOS /2/ Gulf of Oman PG-GOOS 5 2-4/3/ From Hormuz Strait to Jask PG-GOOS 6 2-6/5/ Hormuz Strait PG-GOOS /7/ Hormuz Strait PG-GOOS /8/ East Part of PG 2. Materials and Methods 2.1. Study Area The study area is located in the Iranian part of the Persian Gulf (PG), Strait of Hormuz, and the Gulf of Oman (Fig. 1). The stations were selected based on the bathymetric contours and other disciplines to cover some specific depth within the mention area. The Persian Gulf is about 990 km long and has a maximum width of 370 km between Iran and U. A. E. The average depth of the Persian Gulf is 36m and it occupies a surface area of about 239, 000 km 2 (Emery, 1956). The Persian Gulf is located between latitudes N that is exposed to arid, sub-tropical climate (boundary of the tropical and mid-latitude weather systems) due to southern deserts, which surrounded it. The seasonal shifting of the tropical and mid-latitude systems leads to seasonal changes in the meteorological conditions (Reynolds, 1993). The evaporation rate is variable and fluctuates between 1.4 and 5.0 m/yr (Privett, 1959; Meshal and Hassan, 1986; Ross and Stoffers, 1978; Johns et al., 2003). The total run off to 39 the Persian Gulf is reported about 35.3 to 110 Km 3 /yr (Chao, et al., 1992; Reynolds, 1993).The most significant weather phenomenon in the Persian Gulf is northwesterly Shamal wind, which occurs during the year (Perrone, 1981). Winds in the Gulf of Oman are influenced by the Indian Ocean monsoon system, reversed seasonally between northwest southeast in winter and summer, respectively (Reynolds, 1993) Field Measurements Vertical profiles of the conductivity, temperature and pressure were measured at 169 stations that covered the Persian Gulf, Strait of Hormuz and off Chabahar Bay in the Gulf of Oman (Fig. 1), using a multi-parameter CTD probe, at 8 multidisciplinary cruises from fall 2012 to the late summer 2013 (Table 1). CTD casts were performed using an Ocean Seven 316, Idronaut, mounted on a 12-bottle Rosette equipped with the ph, chlorophyll-a, turbidity and dissolved oxygen sensors. The accuracy of the temperature and conductivity sensors were C and ms cm with a resolution of C and ms cm, respectively (Idronaut, 2002). Before the cruise, the CTDs probe were calibrated, cross checked and also set to the timed data acquisition mode with a 1 second time step. The CTD lowered into the water columns with a constant speed of 1 ms during the different casts. To avoid turbulence caused by the rosette package on the up casts, the present study employed those down casts data obtained through different CTD casts. Finally, all vertical profiles were low-pass filtered with a cut-off length of 5 m, and ordered of 4 to suppress the high-frequency noises, and also to avoid aliasing errors (Wieczorek et al., 2008). 3. Results 3.1. Spatial and Temporal Variations of the Temperature, Salinity and Potential Density The locations of measurements during the first

4 36.6 Azizpour et al. / Study of the Physical Oceanographic Properties of the Persian Gulf, Strait and eighth cruises in the east part of the Persian Gulf are shown in Box 2, Figure 1. In November, surface temperature and salinity; changed from 26.8 to 28.5 C and 36.2 to 39.5 psu, respectively. Alternatively, in August, temperature varied from 29.0 to 34.1 C, and salinity changed from 36.9 to 39.1 psu (Figures 2I, II). From the Strait of Hormuz to the Persian Gulf, both temperature and salinity increased in two seasons and core of the warm and salty water located between E. Cold and low saline water of the Gulf of Oman went through at the Iranian Part of the Persian Gulf and developed in the Persian Gulf. The length of the mentioned inflow development depended on the wind speed (Reynolds, 1993). I NOVEMBER Temperature (Surface Layer) Latitude (degree) N IRAN Latitude (degree) Salinity (Surface Layer) Longitude (degree) II AUGUST Temperature (Surface Layer) Latitude (degree) N IRAN Latitude (degree) Salinity (Surface Layer) Longitude (degree) Fig. 2: Horizontal distribution of surface temperature and salinity for; I: first and II: eighth cruise in east part of the Persian Gulf

5 Journal of the Persian Gulf (Marine Science)/Vol.5/No.18/December 2014/12/37-48 The surface inflow of Indian Ocean Surface Waters (IOSW) in winter was warmer than the Persian Gulf waters (Fig. 2), and as it advected into the Persian Gulf, the IOSW underwent the surface cooling (longitude ~ 55 E) and IOSW overlaid the Persian Gulf surface cold water (Fig. 6 II). Density variations (not shown here) showed penetrations of the low saline IOSW in the western part of the Strait of Hormuz and in the east part of the Persian Gulf that, expansions of IOSW were changed seasonally, i.e. it was expanded deeper in the Persian Gulf during summer in contrast to winter. On the other hand, density differences were affected by the low saline IOSW in the cold seasons, while both salinity and temperature affected the density changes in the winter time. Density increased from the Strait of Hormuz to the head of the Persian Gulf all year round, which was confirmed through Figures of the CTD casts in the first and final cruises and was in agreement with previous studies (e.g. Reynolds, 1993; Yao, 2008). Comparing of salinity contours (Figures 2 I, II) showed that the Persian Gulf surface waters were more saline in November and near the Strait of Hormuz in August. In summer, the salinity of the IOSW layer increased to ~ 37.9 psu, and the surface temperature raised to abou 30 C. However, the November cruise showed that the salinity and the temperature of IOSW were 37 psu and lower than 27.5 C, respectively. In November, the length of mixing layer expanded from the surface to ~ 50 m (Figure 3a) and whole water column was well mixed near the coastal zone and also near the islands, while in August (Figure 3b) its length decreased to ~ 20 m. Under mixed layer, weak thermocline ( t~6 layer appeared and expanded to ~ 70 m in the early fall. Fig. 3: Vertical profiles of temperature, salinity and sigma-t for some selected deep stations in a: November, b: August. 41

6 Azizpour et al. / Study of the Physical Oceanographic c Properties of the Persian Gulf, Strait Moreover, during middle of summer strong thermocline ( t~12 was even generated between 20 and 60 m. Naturally, bellow the thermocline layer, the temperature, salinity and sigma-t did not change, and the data showed the same results and consequently water column was well mixed. Overall variation ranges of the temperature, salinity and density were, , psu and Kg m in November and also were , psu and Kg m in August, respectively. Contours of the spatial variations of the temperature, salinity and density, along the transect II (Box 1 in Fig. 1) of the PG-GOOS second cruise are shown in Figure 4. There are two weak temperature fronts between stations 2 and 5. Vertical patterns of temperature were uniform between stations 11 and 14. Distribution of the density was similarr to that of salinity owing to t winter density changes initiated primarily by change in salinity. High-density water existed in lower layer between the stations 2 and 8 and particularly bottom part of the station s 5. It sounds that in offshore, the main reason for changes of potential density was salinity changes while in near shore, bothh salinity and temperature changes affected potential density alterations (Figs 4I, II). Density (salinity) front between stations 11 and 8 appeared to bend towards station 8 which it extension to the sea surface could not be confirmed (only expands to t ~ 20 m, Figure 4I). Tongue of Saline and warmm water expanded in upper layer from the offshore stations to coastal zone and in contrast, low-salinitbe observed in the lower layer (Fig. and low-temperature water tongue t could 4II). Fig. 4: Vertical distributionn of temperature, salinity, and potential density in cross shelf (I) and along out line of observations (II) of second Cruise (PG-GOOS II), see Figure 1 left side for positions 42

7 Journal of the Persian Gulf (Marine Science)/Vol.5/Noo.18/December 2014/12/37-48 Figures 5I, II show, vertical sections of temperature, salinity and density along and cross the Hormuz strait axis (Box 4 Fig. 1, cruises 5). Temperature, salinity and density changed from 22.5 to 24.0 C 36.4 too 39.3 psu and to 27.8Kg m, respectively. From m ~ 90 m to bed, a considerable increase of salinity ( ) and temperaturee ( C) indicated to outflow of the Persian Gulf Water (PGW). Vertical variations of both temperature and salinity suggested strong stratification between stations 6-11 bellow 40 m (Fig. 5I) and moderate mixed layer between stations 3 and 6. These conditions observed in the cross section transect (Fig. 5II) ) except for the lower layer (bellow 90 m) ) that fairly was well mixed. Fig. 5: Vertical sections of temperature (upper), salinity (middle) and densityy (lower) along strait axis (I) between Sts. 3, 6, 9 and 11, and cross strait axis (II) between Sts. 7, 8 and 9. Positions of Sts. shownn on Box 4, Figure 1 43

8 Azizpour et al. / Study of the Physical Oceanographic c Properties of the Persian Gulf, Strait Figures 6I, II, III, and IV show vertical distribution of the temperature, the salinity and the density in the Strait of Hormuz (Box 3, Fig. 1, cruise 3). At path I, water column was strongly stratified and only a weak front could be find bellow 60 m between Stations 21 and 22. The cold cores weree traceable in ~ 50 m depth at Stations 17 and 24, and these cores were PGW lying bellow Indian Ocean surface warm water (Fig. 6 II). Between two mentioned cores, waterr column was homogenous. Some part of other cold core was tangible between stations 9 and 10, which it sank and make a temperature front bellow 50 m between stations 13 and 6. The Persian Gulf salty water sank in deep part of stations between 24 and 191 and it laid d bellow the Indian I Ocean dens water. When slightly less dense of the Indiann Ocean Water (the compare with the Persian Gulf outflow dense waters) w and the Persian Gulf outflow dense waters gett together, the Indian Ocean waters ascended (Fig. 6II, between stations 17 and 19) and overlaid the Persian Gulf outflow waters. Density variations conformed salinity variations completely. At path III the temperature and the salinity variations in water column were ~1, and 0.3 psu, respectively. Water column was w stronglyy stratified. At path IV, bellow 50m depth, d the temperaturee increased remarkably between stations 8 and 9. In this layer, the salinity increased to 38.6 psu. Fig. 6-1: Vertical sections of temperature (upper), salinity (middle) and density (lower) west part of Hormuz strait inn cross strait axis (I), along straitt axis (II), in middle part of strait cross strait axis (III) and in east part of strait cross strait axis (IV). Positions of Sts. shown on Box 3, Figure 1. 44

9 Journal of the Persian Gulf (Marine Science)/Vol.5/Noo.18/December 2014/12/37-48 Fig. 6-2: Vertical sections of temperature (upper), salinity (middle) and density (lower) west part of Hormuz strait in cross straitt axis (I), along strait axis (II), in middle part of strait cross strait axis (III)( and in east part of strait cross strait axis (IV).. Positions of Sts. shown on Box 3, Figure T-S Diagrams other hand, due to possibility of high rate of evaporation in summer time, deeperr inflow of the Figure 7 shows T-S diagrams for the Persian Gulf, Strait of Hormuz and the Gulf of Oman of PG-GOOS cruises. Figures 7A and F illustrate T-S diagrams for the eastern section of the Persian Gulf during winter and summer, respectively (Box 2 in i Fig. 1). At the western section of the Strait of Hormuz variations of the temperature, salinity and potential density in winter and summer time were, 4, 3.5 C ( , ), 2.0, 0.5 psu (37-39, ) and 3.0 and 1.0 Kg m ( , ), respectively. Onn the IOSW into the Persian Gulf was resulted. Due to strong thermocline layer inn deeper stations (out of the strait), water column c was strongly stratified. This was evident fromm heating att the sea surface s and in agreement with previous observationss (Alessi et al., 1999; Bower et al., 2000; Reynolds, 1993). Moreover, movement off scatter plots along temperature and salinity axes was w an evidence of seasonal variations in IOSW penetration (Figs 7AA and F). Figures 7C and D show two layers in the Strait of Hormuz. At Iranian 45

10 Azizpour et al. / Study of the Physical Oceanographic c Properties of the Persian Gulf, Strait side of strait, water column was homogeneous in shallow stations. In deep stations at the Strait of Hormuz, water column was consisting of two layers, e.g. surface layer with ~ constant salinity about 36.5 psu.. In this layer, water temperaturee decreased to ~ 22 2 (Fig. 7D). At lower layer, the temperature was rather constant, but salinity increased about 3 psu. Fig. 7: T-S diagrams: (A) first cruise of PG-GOOS (East part of PG, winter time), (B) secondd cruise of PG-GOOS and (F) Eights cruise of PG-GOOS (East part of PG, summer time), see Figure 1 for locations. (West part p of PG), (C) East part of Hormuz Strait to Jask, (D) Strait of Hormuz, (E) Gulf of Oman, off Chabahar Bay, 46

11 Journal of the Persian Gulf (Marine Science)/Vol.5/No.18/December 2014/12/ Discussion References The CTD observations in the PG-GOOS cruises were carried out to elucidate the distribution of temperature, salinity and water column stratification. Penetrations of the IOSW depend on evaporation rate and northwest wind speed (Reynolds, 1993). Results of salinity contours showed that influence of the IOSW in the summer was wide spreading and extended to the north head of the Persian Gulf. While in the winter, influences of IOSW were limited due to force of northwesterly wind. Density contours of second cruise (not shown here) revealed one of dense water source in the northern end of the Persian Gulf, which was reported in earlier studies (see e.g. Swift and Bower, 2003; Yao, 2008; Yao and Johns, 2010). Mixed layer thickness increased in winter time and reached to ~ 50 m. Due to surface heating in summer time, mixed layer thickness got to 20 m in ultimate conditions. Alternatively, thickness of thermocline layer increased to ~ 40 m in hot season and in cold time, it was relatively absent in the Persian Gulf. There was a reasonable relationship between thermocline and halocline layer in water column. The temperature and the salinity vertical section contours in eastern section of the Strait of Hormuz revealed that inflow water volume in August was much greater than in November, which could be because of high evaporation rate and low river inflow in summer. In majority of stations, when temperature decreased with depth sharply, salinity increased sharply, except in cases with intrusion of different water mass. Temporal and spatial distributions of water masses, were different in the study area. In the Persian Gulf region, whole water column was mixed in winter and consequently there was only one water mass while with changing season to the summer, two water masses were demonstrable, especially in deep stations. In the Strait of Hormuz, there were two water masses throughout the year. Finally, three water masses is common in deep stations in the Gulf of Oman. Alessi, C. A., H. D. Hunt and A. S. Bower, Hydrographic data from the US Naval Oceanographic Office: Persian Gulf, Southern Red Sea, and Arabian Sea , Woods Hole Oceanog. Inst. Tech. Rep., WHOI-99-02, 74P. Bower, A. S., Hunt, H. D., and Price, J. F., Character and dynamics of the Red Sea and Persian Gulf outflows. Journal of Geophysical Research: Oceans ( ), 105(C3): Brewer, P. G. and Dyrssen, D., Chemical Oceanography of the Persian Gulf, Progress in Oceanography, 14: Chao, S. Y., Kao, T. W., and Al-Hajri, K. R., A numerical investigation of circulation in the Persian Gulf. Journal of Geophysical Research: Oceans ( ), 97(C7): Emery, K. O., Sediments and water of Persian Gulf. American Association of Petroleum Geologists Bulletin, 40(10): Idronaut, Ocean seven 316/319 CTD multiparameter probes operation s manual, Idronaut, Brugherio, 145P. Johns, W. E., Yao, F., Olson, D. B., Josey, S. A., Grist, J. P. and Smeed, D. A., Observations of seasonal exchange through the Straits of Hormuz and the inferred heat and freshwater budgets of the Persian Gulf. Journal of Geophysical Research: Oceans ( ), 108(C12): Meshal, A. H., and Hassan, H. M., Evaporation from the coastal water of the central part of the Gulf. Arab Journal of Scientific Research, 4(2): Najafi, H. S., Modelling tides in the Persian Gulf using dynamic nesting, PhD Thesis, University of Adelaide, Adelaide, South Australia, 136P. Perrone, T. J., Winter Shamal in the Persian Gulf. Naval Environmental Prediction Research Facility, Monterey, California, 180P. Privett, D. W., Monthly charts of evaporation 47

12 Azizpour et al. / Study of the Physical Oceanographic Properties of the Persian Gulf, Strait from the N. Indian Ocean (including the Red Sea and the Persian Gulf). Quarterly Journal of the Royal Meteorological Society, 85(366): Reynolds, R. M., Physical oceanography of the Gulf, Strait of Hormuz and the Gulf of Oman- Results from the Mt. Mitchell Expedition. Marine Pollution Bulletin, 27: Ross, D. A., and Stoffers, P., General data on bottom sediments including concentration of various elements and hydrocarbons in the Persian Gulf and Gulf of Oman. Woods Hole Oceanographic Institution Technical Report 78-39, 107P. Swift, S.A., Bower, A.S., Formation and circulation of dense water in the Persian Gulf. Journal of Geophysical Research: Oceans ( ) 108(C1): Wieczorek, G., Hagen, E. and Umlauf, L., Eastern Gotland Basin case study of thermal variability in the wake of deep water intrusions. Journal of Marine Systems 74: S65-S79 Yao, F., Water mass formation and circulation in the Persian Gulf and water exchange with the Indian Ocean, PhD Thesis, University of Miami, Coral Gables, Florida, 144P. Yao, F., and Johns, W. E., A HYCOM modeling study of the Persian Gulf: 2. Formation and export of Persian Gulf Water. Journal of Geophysical Research: Oceans ( ), 115(C11): Azizpour et al. / Study of the Physical Oceanographic Properties of the Persian Gulf, Strait Journal of the Persian Gulf (Marine Science)/Vol.5/No.18/December 2014/12/37-48 Journal of the Persian Gulf (Marine Science)/Vol. 5/No. 18/December 2014/12/