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Processes controlling the Mixed layer and Barrier layer thickness and its impact on the surface chlorophyll concentration in the Bay of Bengal

Abstract: The oceanic mixed layer, the interface between the ocean and the atmosphere, is an important region of the upper ocean which is well mixed and constantly exchanging mass and energy. The Oceanography of the Bay of Bengal, in general, and the information on the formation and variability of mixed layer and barrier layer in particular is inadequate due to the lack of in-situ Oceanographic data. The availability of Argo temperature and salinity data in the recent years had provided a new opportunity to address this. Hence, the present study investigated spatial variability of the mixed layer, barrier layer, thermal inversion and its seasonal cycle. Also the physical processes were responsible for the observed variability and its relation with surface chlorophyll concentration.

The mixed-layer depth (MLD) clearly showed a distinct north, central and south regime. Fresh water flux, E-(P+R) caused haline stratification, during fall, raising the water stability (up to 9× 10-5 m-1) towards north near the Irrawaddy river mouth that lead to a decrease in MLD (< 30 m) in the northern Bay. The northern Bay, however, a thin mixed layer was observed throughout the year without showing any seasonality. During winter, maximum net heat loss in the northern Bay caused temperature inversion, which in combination with negative wind stress curl lead to deep ILD and thick BL (50-70 m). The winter monsoon generally showed comparatively deep MLD (~50 m) along the central Bay within 8°N-16°N latitude due to decreased stability of the water column (up to 4× 10-5 m-1). A thin isothermal layer (< 45 m) and a shallow MLD were seen during spring (20 m) and fall monsoon (20-40 m). This resulted from primary and secondary heating associated with increased incoming solar radiation as seen from high SST (~ 29.2°C) and weaker winds (4-6 m s -1) during this period. The high wind-mixing (7-10 m s-1) along the western Bay during summer and in the central Bay during winter lead to increased MLD. In summer, a prominent patch of positive wind curl observed in the south coast of Sri Lanka (> 36 x10-8 Pascal m-1) associated with high patch of chlorophyll concentration (0.3- 1 mg m-3) along an upwelling zone. The positive wind stress curls which developed in May, peaked in June and collapsed by September. Deep MLD and BLT perennially seen along the south-eastern corner were due to the convergence and associated down welling.

The ILD and MLD started shoaling from March and attained minimum in April-May when the BLT (0-20 m) also showed the minimum. The amplitude of the seasonal cycle of barrier layer was about 65 m with thickest barrier layer (60-70 m) in the north during December-March that peaked in January. Thin barrier layer (10 m) was seen in the west of the central Bay during April-June and shallowest in May. The chlorophyll distribution (0.08 to 0.2 mg/m3 away from coast) in the upper ocean during spring monsoon (low) was strongly coupled with the MLD (shallow) variability, whereas during summer river runoff, high saline Arabian sea water, positive wind curl and cold-core eddies in tandem strongly regulated the chlorophyll specially along the Sri Lankan coast and head Bay. Negative relation between chlorophyll and PAR implied that PAR was not the limiting factor for chlorophyll in the Bay of Bengal. MLD was positively correlated with salinity, sigma-t and also wind speed, whereas it was negatively correlated with SST, PAR and also DMI. In winter the chlorophyll concentrations were high along the western boundary that must be the effect of EICC (East India Coastal Current) implying the role of circulation in controlling the chlorophyll concentration in the upper layer in comparison to the mixed layer and barrier layer.

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