Circulation and water masses of the Arabian Sea

Journal of Earth System Science (Impact Factor: 0.7). 103(2):107-123. DOI: 10.1007/BF02839532

ABSTRACT The dynamics and thermodynamics of the surface layer of the Arabian Sea, north of about 10N, are dominated by the monsoon-related
annual cycle of air-sea fluxes of momentum and heat. The currents in open-sea regime of this layer can be largely accounted
for by Ekman drift and the thermal field is dominated by local heat fluxes. The geostrophic currents in open-sea subsurface
regime also show a seasonal cycle and there is some evidence that signatures of this cycle appear as deep as 1000 m. The forcing
due to Ekman suction is an important mechanism for the geostrophic currents in the central and western parts of the Sea. Recent
studies suggest that the eastern part is strongly influenced by the Rossby waves radiated by the Kelvin waves propagating
along the west coast of India.

The circulation in the coastal region off Oman is driven mainly by local winds and there is no remotely driven western boundary
current. Local wind-driving is also important to the coastal circulation off western India during the southwest monsoon but
not during the northeast monsoon when a strong (approximately 7 × 106m3/sec) current moves poleward against weak winds. This current is driven by a pressure gradient which forms along this coast
during the northeast monsoon due to either thermohaline-forcing or due to the arrival of Kelvin waves from the Bay of Bengal.

The present speculation about flow of bottom water (deeper than about 3500 m) in the Arabian Sea is that it moves northward
and upwells into the layer of North Indian Deep Water (approximately 1500–3500m). It is further speculated that the flow in
this layer consists of a poleward western boundary current and a weak equatorward flow in the interior. It is not known if
there is an annual cycle associated with the deep and the bottom water circulation.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An unusual phytoplankton bloom dominated by unidentified green coloured spherical algal cells (∼5 μm diameter) and dinoflagellates (Heterocapsa, Scripsiella and Gymnodinium) was encountered along the coast of Goa, India during 27 and 29 January, 2005. Pigment analysis was carried out using both fluo-rometric and HPLC methods. Seawater samples collected from various depths within the intense bloom area showed high concentrations of Chl a (up to 106 mg m −3) associated with low bacterial produc-tion (0.31 to 0.52 mg C m −3 h −1) and mesozooplankton biomass (0.03 ml m −3). Pigment analyses of the seawater samples were done using HPLC detected marker pigments corresponding to prasinophytes, dinoflagellates and diatoms. Chlorophyll b (36–56%) followed by peridinin (15–30%), prasinoxanthin (11–17%) and fucoxanthin (7–15%) were the major diagnostic pigments while pigments of cryptophytes and cyanobacteria including alloxanthin and zeaxanthin formed <10%. Although microscopic analysis indicated a decline in the bloom, pheaophytin concentrations in the water column measured by both techniques were very low, presumably due to fast recycling and/or settling rate. The unique composition of the bloom and its probable causes are discussed in this paper.
    Journal of Earth System Science 12/2011; 120(6):1145-1154. · 0.70 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 on occasions during the two time series. These ephemeral barrier layers were caused by in 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. Department of Science and Technology, Department of Ocean Development
    Journal of Earth System Science 01/2005; · 0.70 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Results of measurements of 14C in the upper 1500 m water column in 10 stations of the Arabian Sea are presented. The study carried out during 1994-1995, which includes reoccupation of three Geochemical Ocean Sections Study (GEOSECS) stations (1977-1978), provides an opportunity to assess the temporal variations in the distribution and inventory of bomb radiocarbon over a period of ~2 decades. The Delta14C values of surface waters (5 m) range between 31 and 580/00, ~200/00 less than those measured during 1978 at the three GEOSECS stations. The higher Delta14C values measured during the present (as well as GEOSECS) study occur in the southern Arabian Sea, which can be attributed to the influx of low-salinity and high-Delta14C waters from the Bay of Bengal. The Delta14C in the Persian Gulf Water (PGW) and the Red Sea Water (RSW) are in the range of -34 to -590/00 and -89 to -1130/00, respectively, and do not show any significant differences from those measured during the GEOSECS. The bomb 14C inventories at the three GEOSECS stations during 1977 and 1995 show that at station 417 in the central Arabian Sea it has increased by ~30%, whereas at the other two stations, 416 and 418 in the north and south central Arabian Sea, it is nearly the same. In general, the bomb 14C inventory in the region ranges between 3.5 and 7.7×109atomscm-2. The CO2 air-sea exchange rates, derived from the bomb 14C inventory, yield values of 7-16 molm-2 yr-1. These exchange rates coupled with reported pCO2 measurements in surface Arabian Sea waters yield CO2 evasion rates in the range of 50-180 Tg-C yr-1 for the Arabian Sea.
    Journal of Geophysical Research 01/2000; 105(C6):14273-14282. · 3.17 Impact Factor