On the circulation in the Bay of Bengal during northern spring inter-monsoon (March-April 1987)

Abstract

Temperature and salinity data collected from the Bay of Bengal 22 March-28 April 1987 (Northern spring inter-monsoon) identified the seasonal anticyclonic gyre (ACG) at 16 degrees N, 86 degrees E characterized by warm (greater than 24 degrees C) and low-salinity (less than 34.2 PSU) water at 125 m depth. The western edge of the ACG was demarcated by a narrow, intense and meandering northward flow, termed the Western Bay of Bengal Current (WBBC) of the spring inter-monsoon period. Two cyclonic eddies (CE1 and CE2) were observed to the left of the WBBC; CE1 at 14 degrees N:82.5 degrees E and CE2 at 18.5 degrees N:87.5 degrees E. These eddies were characterised by low temperature (approx. 16.5 degrees C for CE1 and 14 degrees C for CE2) and relatively high-salinity water (greater than 34.85 PSU) at their cores. The velocity of WBBC increased from 0.40 ms u-1 at 12 degrees N to 0.70 ms u-1 at 17.5 degrees N where it left the coast and turned eastward. The mean northward transport of the WBBC in the upper 200 m was 12 Sv (1 Sv=10 u6 m u3 s u-1). These circulation features (the ACG, WBBC and cyclonic eddies) are well depicted in the maps of sea-surface height (SSH) topography derived from the residual SSHs of GEOSAT altimeter for the period 12 February-28 April 1987. The results show that the WBBC flowed against the northeasterly winds during February and that the current sets up almost three months ahead of the wind reversal. The ACG and in turn the WBBC intensified with the wind reversal in April when the wind stress curl attained a negative maximum. The SSH maps further indicate that formation of the ACG started in February and reached its maximum intensity during April with the coalescence of two anticyclonic cells