Abstract
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.
Highlights
The upper ocean experiences large spatio-temporal variability compared to the rest of the ocean and it forms an important region for understanding both short-term and longterm changes including climate change
To understand the processes affecting the mixed layer variability we examined the monthly mean climatology of SST, sea surface salinity (SSS), incoming short wave radiation, net heat flux (NHF), wind speed (WS), momentum flux and the fresh water flux in tandem with mixed layer depth (MLD)
For brevity we have presented the monthly mean climatology of all the above parameters except MLD in the Appendix
Summary
The upper ocean experiences large spatio-temporal variability compared to the rest of the ocean and it forms an important region for understanding both short-term and longterm changes including climate change. The intense mixing in the upper ocean by heat, momentum and freshwater flux results in the formation of a homogeneous layer with nearly uniform properties known as the mixed layer. It is this layer that couples the underlying ocean to the atmosphere through the transfer of mass and energy. Within a given geographical region, such as the tropics, the structure and variability of mixed layer largely depends on the regional oceanographic characteristics and atmospheric forcing
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