Salinity variation in the southeastern Arabian Sea: A revisit

Transcription

1 Indian Journal of Geo-Marine Science Vol. 44(9), September 2014, pp Salinity variation in the southeastern Arabian Sea: A revisit P V Hareeesh Kumar Naval Physical and Oceanographic Laboratory, Kochi , India [ hari2745@gmail.com] Received 6 February 2013; revised 11 February 2013 Salinity variability in the southeastern Arabian Sea (SEAS) is discussed utilizing the monthly hydrographic data collected within a 2 o x2 o grid (8.5 o N to 10.5 o N) between August 2007 and May In the climatology, the absolute salinity values were low and fine scale features were absent when compared with the ground truth measurements. Arabian Sea Watermass was present as a subsurface maximum throughout the year between 50 and 100 m depths. Bay of Bengal Watermass occupied the top 10 m in the offshore and 50 m near the coast in January. Between October and January, this water diluted the surface layers of the coastal regions by ~1.5 psu and the offshore regions by ~2 psu. A noticeable observation during August and September was the presence of a low salinity plume (<34.5 psu) that extended upto a depth of 10m and sandwiched between saline waters on both side. The curling of currents around a cyclonic gyre that formed in the SEAS during the southwest monsoon brings fresh water from the equator and near coastal regions into the offshore, thereby created the low salinity plume. Replacement of southward currents west of 75 o E in September by the eddy circulation diluted the surface layers in those regions. [Key words: Salinity, watermass, low salinity plume, southeastern Arabian Sea] Introduction The Arabian Sea (AS) is a basin in the north Indian Ocean that undergoes significant changes in response to the seasonal reversal of winds, leading to the reversal of circulation pattern (Fig. 1). In the southeastern Arabian Sea (SEAS), during the summer monsoon season, i.e. June-September, the winds are from northwest and the West India Coastal Current (WICC) flow towards equator 1,2. On the other hand, during winter, the winds are from northeast and the WICC flow towards north. Moreover, the northeasterly winds Northeast Monsoon Current (NMC) that transports water from Bay of Bengal (BoB) into the AS 3. A fraction of the NMC splits around the Laccadive High centered at 10 o N, 70 o E, and move northeastward along the Indian coast as the northward WICC. Fig. 1 - Surface circulation in the southeastern Arabian Sea based on ship drifts data. The box indicates the study region in the SEAS from where hydrographic data were collected during August May 2008.

2 HAREESH KUMAR: SALINITY VARIATION IN THE SOUTHEASTERN ARABIAN SEA 1651 The changes in the winds and circulation pattern produce large variability in the physical and dynamical characteristics of the surface layers of the SEAS. Here, most of the hydrographic studies were confined to limited regions or specific months The salinity data from coarsely sampled hydrographic sections helps to study the basin-scale processes. But these data are insufficient to describe and document the local processes that are highly time dependent especially in a region like SEAS. It was assumed that processes viz., runoffs from rivers like Periyar, Muvatupuzha; seasonally reversing currents; intrusion from BoB; coastal upwelling etc. produces spectacular changes in the spatial and temporal variability of salinity in these regions. Many studies have indicated that the noninclusion of proper salinity degrade the performance of various models 22,23. In addition, the model experiments with and without salinity highlighted the role of salinity on various tropical ocean dynamics. Studies by Rao and Sivakumar 24 and Hareesh Kumar et al. 21 suggested a definite link between salinity stratification and development of a warm pool in the SEAS. Utilizing the 3-D circulation model POM, Hareesh Kumar et al. 21 showed the importance of in situ salinity in the evolution of Arabian Sea mini warm pool in SEAS. The study also shows that the combination of net heat flux, temperature and salinity in the nearsurface layer better explained the dynamics of the warm pool (rather than net het flux and temperature alone). In spite of this, many of the numerical models uses salinity relaxed to the sea surface salinity from climatology The main objective of this paper is to investigate and document the salinity and watermass characteristics in the SEAS utilizing the monthly hydrographic data collected during August 2007-May The study also examines the discrepancies in salinity between the field measurements and climatology. The possible reasons for the presence of a low salinity plume in the SEAS during the summer monsoon season are also provided. Materials and Methods In the SEAS, INS Sagardhwani carried out physical oceanography surveys within a 2 o x2 o grid between 30 m and ~2000 m depth contours (Fig. 2). Surveys were carried out along 8.5 o N, 9 o N, 9.5 o N, 10 o N, 10.5 o N with stations at 5 NM intervals between 30 m and 200 m depth contours and at 15 NM intervals offshore beyond 200m depth contour (55 stations). From all the stations, vertical profiles of temperature and salinity were collected upto a maximum depth of 500 m. Constraints on the availability of ship restricted the surveys to be carried out continuously only for only eight months during , i.e. August 2007, September 2007, October 2007, November 2007, January 2008, March 2008, April 2008 and May Such continuous measurements of the hydrographic properties are sparse in the Arabian Sea. Later, to fill the missing gaps, the same transects were covered during February 2009, June 2009, July 2009 and December In each month, the vessel took four days to complete one survey within the specified 2x2 degree grid. Data collected from these missions were thoroughly checked for its quality following the criteria defined in the National Oceanographic Data Center (NDOC) before its acceptance. Sea level anomaly (SLA) data for the period (http.//las.aviso.oceanobs. com) were also used to describe the meso-scale features in this region. Ducet and Le Traon 28 suggested that the blended SLA of TOPEX-Poseidon and ERS-1/2 satellites provided high accurate sea level measurements with good spatial resolution. We have also used the gridded surface currents based on ships drift data to understand the prevailing surface circulation pattern in the SEAS. Fig. 2 - Study region. Blue dots indicate locations from where monthly temperature and salinity data is collected. Stations are separated by 5NM between 30 and 200m depth contour and by 15 NM beyond the 200m depth contours. Temperature and salinity data were collected using Mini CTD systems (accuracy: temperature o C, salinity PSU, pressure +0.02% of 1000m). Results T-S characteristics To summarize the temperature-salinity characteristics in the near-surface layers, T-S diagrams are prepared based on the data from 2x2 grids (Fig. 3). Prominent watermasses identified are the Arabian Sea Watermass (ASW) and Bay of Bengal Watermass (BBW), which agrees with earlier studies 3,6. ASW is observed throughout the

3 1652 INDIAN J. MAR. SCI., VOL 44, NO.9, SEPTEMBER 2014 year in the t ranges of and T-S range of C and psu. On the other hand, the BBW is observed during winter and disappears with the onset of summer monsoon. Temperature, salinity and t of BBW are o C, psu and less than 22 respectively. Presence of BBW in the shelf regions of the southwest coast of India are the result of their advection from south during winter 3. Signatures of this watermass are also observed in April with a modified temperature. nearshore region, especially in the continental shelf, the BBW occupies the 50 m depth and thereby makes the water column isohaline upto this depth. Fig. 3 - Monthly T-S diagrams based on ground truth measurements. The variability is further examined considering the composite profiles of salinity during monsoon (August 2007), winter (January 2008) and premonsoon (May 2008) seasons within the 2x2 degree grid (Fig. 4). Salinity profiles show large variance in all the three seasons. The variability is mostly due to the presence of different types of watermasses, river runoff, horizontal and vertical advective processes. During the monsoon season (Fig. 4a), concentrations of profiles in two salinity bands are evident in the surface layers; one centered at 34.5 psu and other in excess of 35 psu. This observation is more pronounced towards north. Low salinity values correspond to the nearshore region, where there is significant river runoff during the monsoon season 17. In August, the freshwater (<34.5 psu) is noticed upto a depth of ~10 m in the nearshore region. In all the profiles, a layer of salinity maximum is present in the upper 50 m, but slightly diffused compared to other seasons. Subsurface core gradually sinks to deeper depth towards offshore. North of 9.5 o N isohaline layers upto 60 m depth with salinity in excess of 35.5 psu associated with the ASW is very distinct in the offshore. In January (Fig. 4b), low salinity water (<34.5 psu) is evident upto 50 m along 8.5 o N and upto 30 m along 10.5 o N. Below, the core of ASW is present. In the surface layers, variability in salinity increases upto 9 o N and decreases thereafter. Presence of BBW in the surface layers and ASW at subsurface levels results in strong salinity gradient (~0.03 psu/m) in the upper layers during winter. In the Fig. 4 - Vertical distribution of salinity in the upper 150m water column along 8.5 o N, 9 o N, 9.5 o N, 10 o N and 10.5 o N during (a) August 2007, (b) January 08 and (c) May The vertical extension of BBW is more along 8.5 o N (upto 50 m) whereas it is ~25 m along 10.5 o N. In the surface layers, minimum spread (1 psu) is noticed along 8.5 o N, as the BBW occupies the entire surface layers. Similarly, along 10.5 o N also, the spread is found to be less than 1 psu. Increased spread in the surface layers between these two latitudes are due to the north-south and east-west variation in the watermass characteristics (Fig. 6). Below BBW, the ASW is present with its core at 80m along 8.5 o N and ~50 m along 10.5 o N. In May (Fig. 4c), the shape of the profiles changes completely compared to January. In the surface layers, salinity increases and its spread reduces significantly compared to winter. Here, the high stratified surface layer is replaces with a layer of uniform salinity (<35.5 psu). Below this layer, salinity increase upto ~75 m, where the ASW is present. Moreover, there is also an increase in the surface salinity towards north. In nutshell, along all transects, two bunches of profiles concentration having distinct characteristics, representing the coastal and deep waters are clearly seen in all the three seasons. To quantify the monthly salinity variation within the 2x2 degree grid, the difference between maximum and minimum salinity ( S) at different depth levels for each month is computed (Fig. 5).

4 HAREESH KUMAR: SALINITY VARIATION IN THE SOUTHEASTERN ARABIAN SEA 1653 S shows large variability in the upper 60 m (>1 psu) and decrease towards deeper levels (0.5 psu below 60 m). During winter, S in excess of 2 psu is noticed in the upper 60 m, whereas during the monsoon season, the S confines to the top 15 m. The minimum S (<1 psu) is ntoiced during March- April in the upper 25 m water column. Fig. 5 - Monthly variaiton of salinity ( S) in the 2x2 o grid. Spatial and temporal variability of salinity in the SEAS The monthly vertical sections of salinity in the upper 200 m water column presented in Figure 6 shows low salinity waters (<35 psu) in the upper 15 m of the SEAS between the coast and 75 o E during the summer monsoon season and continues till October. In August, the ASW reaches to the surface west of this longitude and the surface salinity (S o ) enhances to 35.5 psu, which agrees with Prasanna Kumar and Prasad 29. Studies 13,14,17 have suggested that the increased rate of evaporation and vertical mixing of surface waters with the subsurface high salinity water under intense monsoonal forcing 1 have contributed to the increase in S o. In November, a low salinity plume (<33 psu) appears in the nearshore regions along 8.5 o N and freshen the surface layers by ~1 psu compared to October. Lowering of salinity in this region is marked by an increase in the sea level to 5 cm (Fig. 11). The transition of sea level from negative to positive in November is due to the arrival of downwelling Kelvin wave 30,31. There is evidence from Figure 1 that a circulation exists around the southern tip of India from BoB to the AS. Studies 32,33 have indicated that the downwelling Kelvin wave force the equatorward EICC in the western BoB and the EICC brings BBW that forms in the northern BoB into the AS. In January, the minimum salinity of 32.5 psu hugs the southwest coast. Associated with the westward shifting of positive sea level (Fig. 11), freshening is observed in the surface layers of those regions (35.25 psu in November to psu in January at 75 o E). In January, the vertical extent of BBW is 50 m near the coast and 10 m in the offshore. By this time, the EICC reverses its direction to northward and stops the supply of BBW into the AS (Fig. 1) resulting in an increase in S o thereafter. The BBW also spread northward and dilutes the coastal regions by ~1.5 psu between October (34 psu) and January (32.5 psu). In the SEAS, the signature of BBW is evident upto May, but west of 75.5 o E. However, its temperature and salinity gets modified (>30 o C, ~34.5 psu, Fig. 3), but maintains the same t. Hareesh Kumar et al. 21 also reported BBW in May 2000 at the core of Arabian Sea Mini Warm pool. During the summer monsoon season of 2007, there is no signature of this watermass. To have an insight into the salinity modification due to the BBW, the vertical profiles of salinity between October 2007 and April 2008 are presented in Figure 7. During winter, the WICC flow northward along the west coast of India (Fig. 1) as a continuation of the southward EICC along the east coast of India that carries low salinity BBW along with it. The figure shows comparatively saline waters in the entire water column in October, especially along 8.5 o N and 9.5 o N. From November, salinity reduces till January (<33 psu) as the BBW enters the AS. The minimum of 32.5 psu is observed in the nearshore regions along 8.5 o N. The intrusion of BBW reduces the salinity by ~1.5 to 2 psu upto the bottom. Once the supply of BBW into the AS stops (Fig. 1), salinity increases in the water column. This cycle of salinity variability in the entire water column of the southwest coast of India during winter is typical in the SEAS. Another observation from Figure 6 is the subsurface salinity maximum associated with the ASW. The watermass is present throughout the year with its core below the surface mixed layer. In August, the ASW is noticed between 20 and 50 m east of 75 o E, whereas it surfaces west of this longitude. The decrease in core salinity towards the coast might be due to the increased vertical mixing between northward flowing undercurrent and the southward flowing surface current observed during this period 16. With the entry of BBW in November, the spreading of

5 1654 INDIAN J. MAR. SCI., VOL 44, NO.9, SEPTEMBER 2014 Fig. 6 - Monthly vertical section of salinity along different transects. Contour interval is 0.25 psu. Yellow colour represents psu. Fig. 7 - Evolution of salinity in the coastal region between October 2007 and April Fig. 8a - Monthly vertical section of salinity along 8.5 o N and 10.5 o N transects based on 1x1 degree average. Contour interval: 0.25 psu. Yellow colour represents psu. ASW towards south as a core in the coastal regions of SEAS between 60 and 80 m is very conspicuous. In November, the ASW is noticed ~60 m near the coast and there is a change in the slope of the core of ASW. The core (35.75 psu) further deepens to 80 m in January, while it is noticed at shallower depth (20 m) west of 74.5 o E. Retreat of BBW from the SEAS and the onset of upwelling change the sign of the slope of ASW core in May (60 m) compared to January. Thereafter, the core shoals towards the coast due to upwelling.

6 HAREESH KUMAR: SALINITY VARIATION IN THE SOUTHEASTERN ARABIAN SEA 1655 To highlight the importance of ground truth measurements, the salinity sections along 8.5 o N and 10.5 o N (Fig. 8a) and the T-S diagrams (Fig. 8b) based on the commonly using 1x1 degree grids averaged climatology are presented. Vertical sections of climatology (Fig. 8a) reproduces most of the features observed in the area, viz. appearance of BBW during winter and its spreading, variation in the ASW core, etc. But, the main deviation from the ground truth measurements (Fig. 6) is the absence of fine scale features and the slightly low salinity. For example, the low salinity plume observed during the monsoon season, which will be described later, is absent in the climatology. In January (Fig. 8b), the minimum salinity of BBW is ~ 33 psu in the climatology whereas it is less than 32 psu in the measurements. Similarly, at the core of ASW, the salinity is around psu in the climatology, whereas it exceeds 36 psu in the ground truth measurements. There is also marked difference in the depth of the core of ASW and its thickness between the climatology (~110 m) and measurements (~75 m). Similar observations are seen in other months also. The nearshore freshening during the monsoon season due to river runoff is also less marked in the climatology. Climatology is very good for obtaining the gross features but one should be very careful while utilizing this data to infer small-scale oceanic variability. Fig. 8b TS diagrams based on monthly climatology. Discussion The most striking observation in the ground truth measurement is the presence of a band of low salinity water (<34.5 psu) in August 2007 (Fig. 6, 9a), with core of psu. Similar feature is observed in July 2009 also. The low salinity plume sandwiches between slightly higher salinity waters (>34.5 psu) on both side and extends upto a depth of 10 m. In August 1993, Stramma et al. 34 also reported leakage of BBW into the Indian coast at 8 o N. Similar observations were made by Schott et al. 35, where they observed a narrow band of westward flow south of Sri Lanka in July. Along 8.5 o N, the plume is present between o E and o E having a width of 100 km and its width reduce to 48 km (75.24 o E and o E) along 9 o N. It further reduces to 28 km (75.25 o E and 75.5 o E) along 10 o N, while the plume is present east of o E along 10.5 o N. At the core, there is an increase in salinity from 8.5 o N (33.75 psu) to 9.5 o N (34 psu), whereas it is psu centered at 10 o N. The plume continues to present in the SEAS in September 2007 also but south of 9.5 o N. However, the core salinity increases by ~1 psu compared to August, i.e in August to psu in September. On the western side of the plume, the surface salinity drops by ~3.5 psu. Temperature, salinity and sigma-t characteristics of the low salinity plume are 26-27; ; 22, which coincides with the Indian Equatorial Water 3,6,36. In the climatology (Fig. 8, 9b), the low salinity plume is absent and hence one should be very careful while utilizing the climatology to infer small-scale oceanic variability. To explain the probable reasons for the low salinity plume during the monsoon season, the surface temperature (SST) from the ground truth measurements for the same period (Fig. 10) and the SLA data between June 2007 and December 2007 overlie with estimated geostrophic currents are presented (Fig. 11). The SST during August shows a band of comparatively cold water (27 o C) within the low salinity plume. The band of cold water is sandwiched between comparatively warmer water (27.5 o C) on either side. The SLA corresponding to June 2007 (Fig. 11) indicates a sea level low in the coastal region with southward geostrophic currents. The sea level low shifts towards offshore in July and a weak cyclonic circulation is evident in the SEAS centered at o E, 8 o N and come very prominent by August. The southern limit of the gyre is ~5 o N. The present study also shows opposite flows along the crossshelf transect, with northeastward geostrophic currents on the eastern arm and southward currents on the western arm of the cyclonic gyre. The low salinity plume (Fig. 6, 9a) is noticed on the eastern arm of the gyre, where the currents are northeastward. The curling of the currents around the cyclonic gyre brings fresh water from the equatorial region into the SEAS and creates the low salinity plume in August. It is also possible that as the currents curl around this cyclonic gyre, it recirculate the low salinity, cold water (27 o C) present in the coastal region. However, the present observation does not support the findings of Stramma et al. 33, as there is no current from BoB into the AS is observed in August.

7 1656 INDIAN J. MAR. SCI., VOL 44, NO.9, SEPTEMBER 2014 Fig. 9 - Monthly distribution of surface salinity based on (a) ground truth measurements from the 2x2 degree grid and (b) climatology. Fig Monthly distribution of surface temperature in the 2x2 degree grid. In September, the cyclonic gyre splits into two gyres having dimensions of ~350 km each. The observational track, i.e. 8.5 o to 10.5 o N, lies well within the core of northern gyre. Between 75 o and 76 o E, where the low salinity plume is observed in September, the currents flow north-eastward upto 10 o N. Moreover, in August, the south-westward current present west of 73 o E in the study region brings comparatively saline water from north. By September, this region comes under the influence of a cyclonic gyre. The replacement of the southerly currents by the cyclonic gyre and the re-circulation of the low salinity water within the gyre cause considerable reduction in the salinity west of 75 o E, as noticed in the ground truth measurements.

8 HAREESH KUMAR: SALINITY VARIATION IN THE SOUTHEASTERN ARABIAN SEA 1657 Fig Sea surface height anomaly for the period June to December Red line indicates the observational tracks along 8.5 o N and 10.5 o N. Conclusion Monthly physical oceanography survey was carried out within a 2 o x2 o grid in the SEAS onboard the research vessel INS Sagardhwani. Hydrographic data were collected along 8.5 o N, 9 o N, 9.5 o N, 10 o N, 10.5 o N with stations at 5 NM between 30 m and 200 m depth contours and at 15 NM offshore beyond the 200m depth contour. Ground truth measurements indicate maximum salinity variability at the surface layer winter and summer monsoon season (~3.5 psu) and its decrease with depth (~0.5 psu below 150 m depth). Comparison of the ground truth measurements with the climatology shows that climatology reproduces the features observed in the area. However, salinity is comparatively low and the fine scale features in the ground truth measurements are absent in the climatology. Therefore, one should be careful while evaluating the model results with climatology. T-S analysis indicates the prominence of the ASW and BBW in the study region. ASW is present as a subsurface maximum between 50 and 80m depth levels throughout the year and its core depth varies under the influence of coastal upwelling and the arrival of BBW. In the year 2007, BBW enter the SEAS in November and disappear with the onset of summer monsoon. BBW occupies the upper 10 m in the offshore region and upto 50 m near the coast. BBW dilutes the surface layers of the coastal and offshore regions by ~1.5 psu (34 psu in October to 32.5 psu in January along 8.5 o N) and ~2 psu (35.75 psu in October to psu in January at 75 o E) respectively. Increase in salinity after January was due to the reversal of EICC that stopped the supply of low salinity water from BoB. The noticeable observation in the salinity field is the presence of a band of low salinity water (<34.5 psu) sandwiches between waters with salinity more than 34.5 psu on both side during August and September This low salinity plume extended upto depth of 10 m. During August 2007, a cyclonic gyre is observed in the SEAS, with northeastward currents on its eastern arm and southward currents on its western arm. Curling of the currents around the cyclonic gyre brings fresh water from the equatorial region into the SEAS and creates the low salinity plume in August. Replacement of the southerly currents west of 75 o E in September by the eddy circulation causes considerable dilution in the surface layers in those regions. Acknowledgements Author is thankful to Director, NPOL for providing the necessary encouragement to carry out this work. Thanks are also to the Commanding officer and other staffs of INS Sagardhwani, for successfully carrying out the various cruises. References 1. Hastenrath, S. & Lamb, P.J., Climatic Atlas of the Indian Ocean (University of Wisconsin Press, Madison, Wisconsin), 1979, 97 pp. 2. Cutler, A.N. & Swallow, J.C., Surface currents of the Indian Ocean (to 25 S, 100 E): Compiled from historical data archived by Meteorological Office, Bracknell, U.K., Rep. 187, 1984, 8 pp., 36 charts. 3. Darbyshire, M., The surface waters of the coast of Kerala, southwest India, Deep Sea Res., 14 (1967) Banse, K., On upwelling and bottom-trawling off the southwest coast of India, J. Mar. Biol. Ass. India, 1 (1959)

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