Arabian Sea High Salinity Water Research Articles

Enhanced biological production in the southeastern Arabian Sea during spring intermonsoon

Hydrographic observations in the southeastern Arabian Sea (SEAS) have identified a warm (>30°C) and stratified water mass (stability >5 × 10−5 m−1) along the near shore area between 10°N and 15°N during spring intermonsoon. This water mass was relatively low saline (34.2) and nitrate-rich (0.5 µM), favoring moderate primary production (6.7 mg C m−3 d−1). Since the mixing of the Arabian Sea and the Bay of Bengal waters is an important process during this period, the enhanced primary production in the SEAS is attributed to the entrainment of unconsumed nitrate left over during the previous season from the northern Arabian Sea. The season was further characterized by the presence of a deep chlorophyll a maximum (0.5 mg.m−3) in the outer shelf below (>50 m) the subducted Arabian Sea High Saline Waters, which was photosynthetically less active (<1.5 mg C m−3 d−1) due to light limitation.

Seasonal Variability of the Observed Barrier Layer in the Arabian Sea*

Abstract The formation mechanisms of the barrier layer (BL) and its seasonal variability in the Arabian Sea (AS) are studied using a comprehensive dataset of temperature and salinity profiles from Argo and other archives for the AS. Relatively thick BL of 20–60 m with large spatial extent is found in the central-southwestern AS (CSWAS), the convergence zone of the monsoon wind, during the peak summer monsoon (July–August) and in the southeastern AS (SEAS) and northeastern AS (NEAS) during the winter (January–February). Although the BL in the SEAS has been reported before, the observed thick BL in the central-southwestern AS during the peak summer monsoon and in the northeastern AS during late winter are the new findings of this study. The seasonal variability of BL thickness (BLT) is closely related to the processes that occur during summer and winter monsoons. During both seasons, the Ekman processes and the distribution of low-salinity waters in the surface layer show a dominant influence on the observed BLT distributions. In addition, Kelvin and Rossby waves also modulate the observed BL thickness in the AS. The relatively low salinity surface water overlying the Arabian Sea high-salinity water (ASHSW) provides an ideal ground for strong haline stratification in the CSWAS (during summer monsoon) and in NEAS (during winter monsoon). During summer, northward advection of equatorial low-salinity water by the Somali Current and the offshore advection of low-salinity water from the upwelling region facilitate the salinity stratification that is necessary to develop the observed BL in the CSWAS. In the SEAS, during winter, the winter monsoon current (WMC) carries less saline water over relatively high salinity ambient water to form the observed BL there. The winter West India Coastal Current (WICC) transports the low-salinity water from the SEAS to the NEAS, where it lies over the subducted ASHSW leading to strong haline stratification. Ekman pumping together with the downwelling Kelvin wave in the NEAS deepen the thermocline to cause the observed thick BL in the NEAS.

Hydrographic characterization of southeast Arabian Sea during the wane of southwest monsoon and spring intermonsoon

Seasonal variation of the hydrography along the southeast Arabian Sea is described using data collected onboard FORV Sagar Sampada in September--October 2003 (later phase of Southwest monsoon, SWM) and March--April 2004 (Spring inter monsoon, SIM). During the later phase of the SWM, upwelling was in the withdrawal phase and the frontal structure was clearer in the northern sections (13 and 15 degrees N lat) indicating strong upwelling in the area. The driving force of upwelling is identified as the combination of alongshore wind stress and remote forcing with a latitudinal variability. Although a more prominent upwelling was found in the north, a maximum surface Chlorophyll-a was found in the south (10 degrees N). During the SIM, the area was characterized by oligotrophic water with relatively high Sea Surface Temperature (>29 degrees C) and low salinity (33.8 to 35.4). During March, the surface hydrography was found to be controlled mainly by the intrusion of low-saline waters from the south, while during September by the high saline water from the north. The presence of various water masses [Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), Red Sea Water (RSW)] and their seasonal variations in the region is discussed and their decreasing influence towards the south is noted during both periods of observation. During the SWM, the dynamic topography showed the equator-ward flow of the West India Coastal Current (WICC) at the surface and a pole-ward coastal under current at sub-thermocline depth. During the SIM, surface circulation revealed the WICC flowing pole-ward north of 13 degrees N, but equator-ward flow in the south, with a clockwise circulation around the Lakshadweep High.

Pathways and Effects of the Indonesian Throughflow Water in the Indian Ocean Using Particle Trajectory and Tracers in an OGCM

Abstract The 3D pathways of the Indonesian Throughflow (ITF) in the Indian Ocean are identified using an OGCM, with a combined set of tools: 1) Lagrangian particle trajectories, 2) passive tracers, and 3) active tracers (temperature and salinity). Each of these tools has its own advantages and limitations to represent the watermass pathways. The Lagrangian particles, without horizontal and vertical mixing, suggest that at the entrance region the surface ITF subducts along the northwestern coast of Australia and then travels across the Indian Ocean along the thermocline depths. The subsurface ITF more directly departs westward and crosses the Indian Ocean. Using the passive tracers, which are mixed vertically under convection as well as horizontally due to diffusion, the ITF is shown to undergo vigorous mixing as soon as it enters the Indian Ocean and modifies its upper temperature–salinity (T–S) characteristics. Thus, the surface and subsurface ITF watermasses lose their identities. Upon reaching the western boundary, the ITF reroutes into three distinct depth ranges, owing to the seasonal reversal of the Somali region: route 1—across the Indian Ocean just to the south of the equator (200–300 m); route 2—across the Indian Ocean to the north of the equator (100–200 m); and route 3—upwells in the Somali region and spreads all over the surface of the northern Indian Ocean. The seasonality of the Somali Current is crucial to spread the ITF along route 3 during the summer monsoon (April–October) and route 2 during the winter monsoon (November–March). The basinwide spreading is responsible for a long residence time of the ITF in the Indian Ocean to be at least 20 yr. The effects of the ITF on the temperature and salinity are mainly accompanied with the major pathways. However, indirect effects are visible in a few spots; that is, the warm and saline feature is produced in the subsurface off the southwestern coast of Australia around 30°S caused by the eastward surface current, which is under the thermal wind relationship owing to the warm and fresh ITF component. This component also enhances vertical convection and warms the surface around 40°S. The Arabian Sea high salinity water is produced extensively with the effects of the Somali upwelling, which is originally strengthened by the fresh and warm ITF.

On the circulation of water masses across the Mascarene Plateau in the South Indian Ocean

The South Equatorial Current (SEC) is the major westward current in the South Indian Ocean. It crosses the Mascarene Plateau, an extensive range of banks and islands, near 60°E, but how this occurs has until now been unclear. Here, we present the results of a recent survey during June–July 2002 using a suite of modern instrumentation, and provide a detailed examination of this process, and the water masses involved. Upstream from the Plateau the SEC carries 50–55 Sv (1 Sv=10 6 m 3/s) westwards between 10 and 16°S. As it approaches the Plateau, 25 Sv of this is constricted to pass over a narrow sill (about which we provide new information) between the Saya De Malha and Nazareth Banks at 12–13° S. This then forms a northern core to the SEC between 10 and 14°S downstream from the Plateau (25 Sv). The remainder of the inflow passes either around the northern edge of the Saya De Malha Bank (8–9°S) or between Mauritius and the Cargados Carajos Bank (18–20°S). The former may retroflect to flow eastwards near 8°S, joining the South Equatorial Counter Current (SECC), whereas the latter, strengthened near Mauritius by further flows from the south, forms a southern core to the SEC downstream from the Plateau (20–25 Sv between 17 and 20°S). The overall effect of the Plateau is to split the SEC into two cores. On reaching Madagascar, these cores may then form the Northeast and Southeast Madagascar Currents. The SEC also forms a sharp boundary between upper and intermediate level water masses. Subtropical Surface Water (STSW), Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are present on the southern side of the SEC, whereas Arabian Sea High Salinity Water (ASHSW) and Red Sea Water (RSW) are found on its northern side. As they approach the Plateau, the STSW and SAMW are partially drawn northwards, and Tropical Surface Water (TSW) is drawn southwards, in order to flow across the sill near 12–13°S. At deeper levels, North Indian Deep Water (NIDW) passes southwards below the SEC on the western side of the Plateau, and while there is no indication of North Atlantic Deep Water, Antarctic Bottom Water (AABW) is present west of the Plateau. Finally, there is evidence of significant mixing in the upper and intermediate waters (the TSW, STSW, SAMW and RSW) as they pass across the sill at 12–13°S, and also in the deeper waters on the eastern side of the Plateau above the rough bottom topography of the Central Indian Ridge.

Hydrography of the eastern Arabian Sea during summer monsoon 2002

Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.

Water mass properties and transports in the Arabian Sea from Argo observations

The information acquired from Argo floats such as temperature and salinity profiles is used to study water mass properties in the Arabian Sea from 2002 to 2004. An examination of water mass structure at different locations reveals the presence of high salinity water of marginal seas in the Arabian Sea. During the southwest monsoon season, the impact of the early onset of southwesterlies is noticed in the upper ocean temperature and salinity structure over the Western Arabian Sea (WAS) during 2002. Surface density variations are found to be more during the southwest monsoon season due to strong wind forcing. Argo temperature and salinity profiles showed that the winter cooling and the formation of Arabian Sea High Salinity Water (ASHSW) over the Northern Arabian Sea (NAS) began during the second half of November within the upper 100 m depth. In the NAS, the Persian Gulf Water (PGW) salinity is above 36, as PGW moves towards the south along isopycnal layer of 26.6σθ (σθ is potential density) salinity decreases. It is observed that the PGW high salinity water is not continuously prominent over the WAS in 2002 and in 2003. In the WAS the 27.2σθ isopycnal layer depth, corresponding to Red Sea Water (RSW), did not exactly follow the pattern of isotherms as is seen in the northern and eastern Arabian Sea. The variability related to RSW salinity is due to the underwater currents. The present study also confirms that RSW is prominent in the southeast Arabian Sea at the potential density of 27.2 with a maximum in summer monsoon compared to other seasons. The observed peak in the salinity at 27.2 density level during the spring intermonsoon is due to the influence of winter time spreading of RSW to the south of Socotra in 2002. Westward movement of Argo floats in the region east of Socotra during the winter is evident in both the observations and model studies. Water mass properties change when they move away from their source region due to the consistent horizontal advection. The changes in the water mass properties along the Argo float trajectory are confirmed by comparing with the climatological mean monthly values from the World Ocean Atlas 2001 data set.

Salinity variability in the Arabian Sea

Argo floats deployed in the Arabian Sea provided an opportunity to look at the temporal variability of the core‐depth of Arabian Sea High Salinity Water mass (ASHSW) at three locations in the north, central and southern Arabian Sea. These three locations show distinctly different variability patterns. At the northern location we see a prominent semi‐annual cycle, whereas at the central location an annual cycle dominates. Comparison with TOPEX/JASON sea level data shows that this difference can be attributed to the influence of Rossby waves at the central location. In the southern Arabian Sea the variability in core depth is also dominated by an annual‐mode, but there are 2 high frequency components near the Madden‐Julian Oscillation frequencies. Thus the salinity distribution in the Arabian Sea apparently is influenced by both remote forcing and ocean atmospheric coupling.

A numerical study of the seasonal variability of Arabian Sea high‐salinity water

Using a level 2 three‐dimensional turbulent closure model, the physical processes affecting the upper‐ocean salinity budget in the Arabian Sea have been studied with particular focus on the seasonal variability of the Arabian Sea High‐Salinity Water (ASHSW). The volume integrated salt budget terms for four subdomains in the Arabian Sea (south of 10.5°N, north of 10.5°N, east of 62.5°E, and west of 62.5°E) shows a clear balance between the horizontal advection and evaporative freshwater loss in the annual mean. The horizontal advection dominates the seasonal cycle in salinity. The seasonal balance is clear, when the terms of the salt budget are integrated between 10.5°N and 2.5°N and east of 62.5°E. In these subdomains the two major episodes of horizontal advection during winter and summer are both associated with the seasonal reversal of the Arabian Sea circulation. Model estimation of the upper‐ocean salinity budget terms integrated between the surface and 100 m for the winter and summer monsoons agree with a qualitative description of the ASHSW variability. After formation in the northern Arabian Sea during the winter monsoon, the ASHSW spreads predominantly southward along the eastern boundary during the summer monsoon. Advection causes salinity to increase by 1.0 kg m−2 day−1 along these regions. At this time, the poleward advection of low‐salinity water from the south (and upwelling regions) by the Somali current causes the salinity along the western Arabian Sea to decrease by 0.6 kg m−2 day−1. During the winter monsoon, westward advection of the fresher Bay of Bengal water by the North Equatorial current (NEC) causes a salinity decrease by ∼1.0 kg m−2 day−1 in the region south of 10.5°N. The combined effect of precipitation and evaporation tends to increase the salinity in the Arabian Sea, and horizontal advection is found to be important for maintaining the observed seasonal cycle.

The Wintertime Water Mass Formation in the Northern Arabian Sea: A Model Study

Abstract Using a level 2 one-dimensional turbulent closure model, the evolution of the Arabian Sea High Salinity Water mass (ASHSW) has been studied. Model-derived heat fluxes, sea surface temperature (SST), and mixed layer depths (MLD) are found to be in reasonable agreement with the observations. Winter cooling is characterized by a net heat loss of 40 W m−2 from the ocean, while there is a mild heat loss in summer. Wind-driven turbulent mixing is considerably stronger than buoyancy forcing during the summer monsoon and explains the mechanism for governing deep MLD and cool SST. During winter, a large latent heat release by the ocean due to prevailing dry air from the north, together with reduced solar radiation and increased longwave radiation due to lower cloudiness, decreases the net heat flux considerably. Negative buoyancy flux plays a major role in the formation of ASHSW during winter, while kinetic energy-driven vertical mixing is relatively weaker than during the summer monsoon. A sensitivity te...

Hydrography and circulation of the bay of Bengal during withdrawal phase of the southwest Monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18° N and the other at 90° E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18° N / 87° E and as high as 34.79 at 8° N / 90° E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90° E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18° N / 87° E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18° N transect with velocity slightly in excess of 30 cm s −1. Similar high velocity (> 40 cm s −1) pockets were also noticed to extend up to 30 m depths in the southern region of the 90° E transect. However, the currents below 250 m were weak and in general < 5 cm s −1. The net geostrophic volume transports were found to be of the order of 1.5 × 10 6 m 3 s −1 towards the north and of 6 × 10 6 m 3 s −1 towards west across the 18° N and 90° E transects respectively. The surface circulation patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12° N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data.

Formation and spreading of Arabian Sea high‐salinity water mass

The formation and seasonal spreading of the Arabian Sea High‐Salinity Water (ASHSW) mass were studied based on the monthly mean climatology of temperature and salinity in the Arabian Sea, north of the equator and west of 80°E, on a 2° × 2° grid. The ASHSW forms in the northern Arabian Sea during winter and spreads southward along a 24 sigma‐t surface against the prevailing weak zonal currents. The eastern extent of the core is limited by the strong northward coastal current flowing along the west coast of India. During the southwest monsoon the northern part of the core shoals under the influence of the Findlater Jet, while the southern part deepens. Throughout the year the southward extent of the ASHSW is inhibited by the equatorial currents. The atmospheric forcing that leads to the formation of ASHSW was delineated using the monthly mean climatology of heat and freshwater fluxes. Monsoon winds dominate all the flux fields during summer (June‐September), while latent heat release during the relative calm of the winter (November‐February) monsoon, driven by cool, dry continental air from the north, results in an increased density of the surface layer. Thus excess evaporation over precipitation and turbulent heat loss exceeding the radiative heat gain cool the surface waters of the northern Arabian Sea during winter and drive convective formation of ASHSW.

Short term variability of hydrography and currents in relation to stability, sound speed and mixing characteristics off Bombay, west coast of India

Analysed fields of hydrography revealed dominant influences by Arabian Sea High Salinity Water (ASHSW), strong at a deep continental slope site and weak at a shallow shelf site off Bombay, during January 1986. The deep site was characterized by the temporal variability of a weakly stable surface mixed/sonic layer over a steep thermocline/pycnocline, coincident with the ASHSW intrusions around a core depth of 50 m. Warmer, isothermal and weakly stable waters at the shallow site exhibited similar periodicity in temporal variability with nearbottom high saline intrusions. Amplitudes of perturbations were reduced to half of those at the shallow site. The dominant influence on density by salinity was above 75 m at the deep site, while the same was reduced to a subsurface layer (30–50 m) at the shallow site. The spatial variability suggested a general offshore decrease/increase in temperature/density, modified by salinity front and advective fields near the shelf edge. At the shallow site, the surface layer responded more to the moderately strong wind stress, resulting in a faster residual southwesterly flow, while weak residual flows resulted from the dominant semidiurnal rotations in the middle layers. Faster residual flow existed near the bottom in the absence of tidal rotations. The middle layers were characterized by maximum dynamic stability. Maximum values of vertical eddy coefficients of viscosity and thermal diffusivity were observed in the surface layers.

Oceanography of the Arabian Sea during the southwest monsoon season-Part III : Salinity

The distribution of salinity in the Arabian Sea during the southwest monsoon season is presented through several vertical sections and spatial distribution charts. The vertical salinity structure has been found to be quite complex and shows several maxima and minima upto depths of about 1000 m. An analysis of the water mass characteristics shows that the Arabian Sea high salinity water forms in the northeastern Arabian Sea as a result of excessive evaporation over precipitation and is located between 300 and 500 el/t steric levels. The Persian Gulf and Red Sea Waters have been identified by their salinity maxima occurring above and below the 100 cl/t surface respectively. In addition, it has been found that, in the depth range of 0-1000 m, the Antarctic intermediate water, subtropical subsurface water and the south equatorial water penetrate into the Arabian Sea at different levels and intermingling of these with the Red Sea, Persian Gulf and Arabian Sea high salinity waters give rise to the observed complexities in the vertical salinity structure. A comparison of the curves of potential temperature versus depth, salinity and oxyty in the Somali and Arabian basins with those in the circumpolar regions has shown that the deep and bottom water mass in these basins at depth exceeding 2500 m is of circumpolar origin.