Upper ocean stratification and circulation in the northern Bay of Bengal during southwest monsoon of 1991

Abstract

During the southwest monsoon (July) of 1991 a large plume (300 x 250 km sup(2)) of warm (greater than 29 degrees C) and less saline (less than 29 PSU) water is noticed in the top 30 m to the east of 87.5 degrees E in the northern Bay of Bengal. To the west of 87.5 degrees E cold and saline waters are noticed in the upper 50 m, with a distinct thermohaline front along 87.5 degrees E. The thermohaline fields show the influence of wind forcing west and south of the front, but up to the base of the freshwater plume east of the front. Stable stratification and anticyclonic circulation associated with the freshwater plume restrict the entrainment of cold waters to the base of the stratified layer. In contrast, cyclonic circulation dominates in the plume area with reference to 500 db. The OGCM simulations driven by NCEP daily winds and COADS monthly winds for July 1991 agree well with the observations outside the plume area. The former simulation also identify a strong equatorward coastal current where there were no direct observations during the study period. The offshore location and thinning of the plume follow the Ekman dynamics in agreement with the numerical solutions of river plume.