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

A two-gyre circulation system consisting of an anticyclonic gyre (ACG) in t he 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 inten se southwestward flow in the central Bay. The circulation pattern character izes the transition from the pre-monsoon (spring season) ACG to the southwe st 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 loc ated below the surface mixed layer, in the oxycline and nutricline, and abo ve the seasonal thermocline/pycnocline, which coincides with the deep stabi lity 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, respect ively. The magnitude of the chlorophyll maximum (CMAX) is nearly twice as h igh in the CG zone as in the ACG zone. The depths of both DCM and DSM, howe ver, 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 chlo rophyll a (both DCM and CMAX) in the Bay of Bengal. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.