Effect of vertical stability and circulation on the depth of the chlorophyll maximum in the Bay of Bengal during May–June, 1996

Abstract

A two-gyre circulation system consisting of an Anticyclonic Gyre (ACG) in the northwestern Bay of Bengal and a Cyclonic Gyre (CG) west of the Andaman Islands is shown by the hydrographic data collected during May–June, prior to the southwest monsoon of 1996. This circulation system leads to an intense southwestward flow in the central Bay. The circulation pattern characterizes the transition from the pre-monsoon (spring season) ACG to the southwest monsoon CG. A Deep Chlorophyll Maximum (DCM) is found in the depth range 30–50 m in the coastal region and 50–100 m in the open sea. The DCM is located below the surface mixed layer, in the oxycline and nutricline and above the seasonal thermocline/pycnocline, which coincides with the Deep Stability Maximum (DSM). In the ACG, the DCM is located at 100 m when DSM is at 140 m deep, whereas in the CG, the DCM and DSM rise to 50 and 80 m, respectively. The magnitude of the Chlorophyll Maximum (CMAX) is nearly twice as high in the CG zone as in the ACG zone. The depths of both DCM and DSM, however, are twice as large in the ACG zone as in the CG zone. This shows that the meso-scale circulation patterns affect the spatial distribution of chlorophyll a (both DCM and CMAX) in the Bay of Bengal.