Circulation and water masses of the Arabian Sea
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.
Available from: Jong-Mi Lee
- "On the T–S diagram, it seems that salinity is slightly elevated at this density layer (Fig. 2), but no particular water mass has been documented for this layer of the Arabian Sea. Water masses that are known to be ventilated from the northern Arabian Sea are either too shallow (e.g., ASHSW at r t = $24 kg m À3 and NASHSW at r t = $25 kg m À3 ) (Kumar and Prasad, 1999; Banse and Postel, 2009) or too deep (e.g., PGW at r t = $26.5 kg m À3 , RSW r t = $27.1 kg m À3 ) (Shetye et al., 1994; Prasad et al., 2001), and hence we cannot identify the origin of the maximal 206 Pb/ 207 Pb and 208 Pb/ 207 Pb. "
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ABSTRACT: Pb and Pb isotope ratios in the modern ocean have been altered significantly by anthropogenic Pb inputs over the past century. Most studies of anthropogenic Pb in the ocean have focused on the North Atlantic and North Pacific Oceans, and the impact of anthropogenic Pb inputs to the Indian Ocean and processes controlling the distribution of Pb in the Indian Ocean are poorly known. This study presents the Pb and Pb isotopic composition (206 Pb/ 207 Pb, 208 Pb/ 207 Pb) of 11 deep stations from the Indian Ocean Japanese GEOTRACES cruise (KH-09-5), from the Bay of Bengal and Arabian Sea to the Southern Ocean (62°S). The Pb isotope ratios of the Indian Ocean range 1.140–1.190 for 206 Pb/ 207 Pb and 2.417–2.468 for 208 Pb/ 207 Pb, with lower ratios appearing in the shallow waters of the northern Indian Ocean and higher ratios appearing in the deep layers of the Southern Ocean. This result agrees with a previous study on Pb concentrations (Echegoyen et al., 2014) showing that the Indian Ocean, particularly its northern part, is largely perturbed by anthropogenic Pb inputs. 206 Pb/ 207 Pb and 208 Pb/ 207 Pb of the Indian sector Southern Ocean are still lower than natural Pb, showing this region was also affected by anthropogenic Pb. Anomalously low or high 206 Pb/ 207 Pb and 208 Pb/ 207 Pb were observed in the thermocline and shallow waters of the southern Indian Ocean and the Arabian Sea, which are ascribed to water mass distribution (e.g., Subantarctic Mode Water) and evolving Pb isotope ratios of this region as dominant anthropogenic Pb sources change. 206 Pb/ 207 Pb and 208 Pb/ 207 Pb in the Bay of Bengal are higher than those in the Arabian Sea, which might be the result of the anthropogenic Pb inputs from different provenance or seawater exchanging Pb isotopes with natural particles derived from rivers and/or sediments at the basin boundaries.
Available from: Arga C Anil
- "Seasonal variation in the atmospheric forcings in the Arabian Sea causes remarkable changes in physical environment such as upwelling during south west monsoon and convective mixing during winter (Banse, 1968; Banse and McClain, 1986; Shetye et al., 1994; Madhupratap et al., 1996; Brock et al., 1993). December– February, the northern Arabian Sea is cooled due to reduced incoming solar radiation as well as enhanced evaporation under the influence of the dry northeast trade winds from continental origin (Banse, 1968; Shetye et al., 1994) causes density of upper ocean to increase, initiating convective mixing (Banse and McClain, 1986; Madhupratap et al., 1996) and ventilates the upper part of the permanent pycnocline. This convective mixing and ventilation of the subsurface water together with irradiance exerts ecological pressure on different phytoplankton enforcing changes in food web dynamics. "
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ABSTRACT: Physical forcing can replenish nutrients within the mixed layer by convective mixing or via upwelling. Conventional wisdom holds this enrichment fuels phytoplankton growth, for example ventilation of subsurface water during winter monsoon is known to enhance primary productivity in the northern Arabian Sea. One important numerically dominant phytoplankton known to have ecological niche in the ocean is Prochlorococcus. In the Arabian Sea, they occur in oligotrophic surface water and below the oxycline representing two different light and biogeochemical regimes. Here we show convective mixing in the northern Arabian Sea inhibits Prochlorococcus growth owing to change in physical environment. Pigment observations carried out during early and peak winter monsoon revealed contrasting picoplankton distribution. Divinyl chlorophyll a (a marker for Prochlorococcus) which was the most abundant picoplankton pigment during early winter monsoon was not detected with the onset of winter convection covarying with high nutrients in the surface water. We propose two possible mechanisms for such sudden disappearance which involves changes in light and biogeochemical regimes. This physico-chemical control could be critical for their existence but not limited to and can play an important role in regions experiencing such phenomenon. We also highlight the linkages between Prochlorococcus succession and basin scale dynamcis from the Arabian Sea which hitherto remains poorly understood.
Available from: Andreas Lückge
- "The monsoon climate is generally defined as the seasonal reversal of the prevailing surface winds and accompanied precipitation, driven by the migrating low surface pressure belt of the Intertropical Convergence Zone (ITCZ) and atmospheric pressure over Central Asia (Wyrtki, 1971). In the Arabian Sea region, strong and moisture-laden southwesterly winds prevail during summer, when the low-pressure zone of the Hadley circulation is on the northward position over continental Asia and northern Arabia (Shetye et al., 1994). The reversed mode during winter drives dry northeasterly winds of lower velocity toward the low-pressure zone above the open ocean. "
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ABSTRACT: The Indian monsoon system is an important climate feature of the northern Indian Ocean. Small variations of the wind and precipitation patterns have fundamental influence on the societal, agricultural, and economic development of India and its neighboring countries. To understand current trends, sensitivity to forcing, or natural variation, records beyond the instrumental period are needed. However, high-resolution archives of past winter monsoon variability are scarce. One potential archive of such records are marine sediments deposited on the continental slope in the NE Arabian Sea, an area where present-day conditions are dominated by the winter monsoon. In this region, winter monsoon conditions lead to distinctive changes in surface water properties, affecting marine plankton communities that are deposited in the sediment. Using planktic foraminifera as a sensitive and well-preserved plankton group, we first characterize the response of their species distribution on environmental gradients from a dataset of surface sediment samples in the tropical and sub-tropical Indian Ocean. Transfer functions for quantitative paleoenvironmental reconstructions were applied to a decadal-scale record of assemblage counts from the Pakistan Margin spanning the last 2000 years. The reconstructed temperature record reveals an intensification of winter monsoon intensity near the year 100 CE. Prior to this transition, winter temperatures were >1.5°C warmer than today. Conditions similar to the present seem to have established after 450 CE, interrupted by a singular event near 950 CE with warmer temperatures and accordingly weak winter monsoon. Frequency analysis revealed significant 75-, 40-, and 37-year cycles, which are known from decadal- to centennial-scale resolution records of Indian summer monsoon variability and interpreted as solar irradiance forcing. Our first independent record of Indian winter monsoon activity confirms that winter and summer monsoons were modulated on the same frequency bands and thus indicates that both monsoon systems are likely controlled by the same driving force.
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