The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. Despite the contrasting chlorophyll and primary productivity pattern, sediment trap data shows that annual fluxes of organic carbon reach comparable rates in both the basins. The traditional mechanisms of nutrient supply to the upper ocean waters cannot account for this. We propose eddy pumping as a possible mechanism of vertical transfer of nutrients across the halocline to the oligotrophic euphotic zone during summer monsoon when upper ocean is highly stratified. This would induce rapid biological uptake and in turn significantly increase biological production. In the northern Bay, riverine input acts as an additional source of nutrients and augments the subsurface nutrient injection to the euphotic zone by eddy pumping. Notwithstanding this, the lower than expected primary production in the north suggests the possible role of riverine sediment in limiting the sunlight for photosynthesis.

The signature of cold-core eddies and their role in altering the biological productivity of the Bay of Bengal was examined using two recent sets of hydrographic data collected along the central and western Bay of Bengal during fall (14 September -12 October, 2002) and spring (12 April -7 May, 2003) intermonsoons under the Bay of Bengal Process Studies (BOBPS) programme. Based on the thermohaline structure and the satellite-derived sea-level anomaly maps 9 cyclonic eddies were identified. Out of this, 4 cyclonic eddies -2 each along the central Bay and along the western boundary -occurred during fall intermonsoon 2002, while 5 occurred -3 along the central Bay and 2 along the western boundary -during spring intermonsoon. The eddy depressed the temperature, which varied from 3 o C to 7 o C at 120 m depth. Maximum depression of temperature was associated with spring-time eddies in the northern Bay, where subsurface stability was low. The reduced water column stability in spring leads to greater eddypumping, thereby cooling the water to a greater extent. However, the cyclonic eddies were unable to break the stratification of the top 20m layer, thereby curtailing their effects below this depth during both seasons. Eddypumping not only cooled the water column but also enhanced the nutrient concentrations. This in turn increased the biological productivity of the Bay to 1½-2 times. In addition, the subsurface chlorophyll maximum (SCM), which is generally located between 40 and 70 m in fall and 60 and 90 m in spring intermonsoons, shallowed under the influence of the eddies and also enhanced the chlorophyll concentration in the SCM to more than double. Thus, eddy-pumping of nutrients controls the biological productivity of the Bay of Bengal during both the seasons. In the fall intermonsoon, however, the riverine input of nutrients and sediments in the northern Bay also plays a role in altering the biological productivity. This has an overall implication to the basin-wide new production and export flux and, at least partly, resolves the reason for the comparable annual fluxes of organic carbon

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Situated in similar latitudes and subjected to similar atmospheric forcing, the tropical basins of the Arabian Sea looses fresh water due to excess evaporation over precipitation while Bay of Bengal receives freshwater via excess rain and river run off. The hydrological imbalance thus created on an annual scale will have to be balanced by the inter‐basin exchange. In winter this happens through the intrusion of Bay of Bengal waters into the Arabian Sea, when the southward flowing East India Coastal Current carrying low salinity waters from the northern Bay feeds into the West India Coastal Current flowing north along the shelf in the Arabian Sea. Advection of nutrients by this intrusion triggers enhanced levels of chlorophyll near the southern part of the western shelf of India and may play a role in altering the biogeochemistry of this intense hypoxic region.

Abstract. The mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the oceanic primary productivity. We examined the basin-scale processes controlling the seasonal variability of mixed layer depth in the Bay of Bengal and its association with chlorophyll using a suite of in situ as well as remote sensing data. A coupling between mixed layer depth and chlorophyll was seen during spring intermonsoon and summer monsoon, but for different reasons. In spring intermonsoon the temperature-dominated stratification and associated shallow mixed layer makes the upper waters of the Bay of Bengal nutrient depleted and oligotrophic. In summer, although the salinity-dominated stratification in the northern Bay of Bengal shallows the mixed layer, the nutrient input from adjoining rivers enhance the surface chlorophyll. This enhancement is confined only to the surface layer and with increase in depth, the chlorophyll biomass decreases rapidly due to reduction in sunlight by suspended sediment. In the south, advection of high salinity waters from the Arabian Sea and westward propagating Rossby waves from the eastern Bay of Bengal led to the formation of deep mixed layer. In contrast, in the Indo-Sri Lanka region, the shallow mixed layer and nutrient enrichment driven by upwelling and Ekman pumping resulted in chlorophyll enhancement. The mismatch between the nitrate and chlorophyll indicated the inadequacy of present data to fully unravel its coupling to mixed layer processes.