Seasonal cycle of the hydrography and circulation of the Bay of benga

Abstract

The Bay of Bengal is the smaller of the two semi-enclosed basins of the North Indian Ocean. Due to the impact of monsoons, the Bay exhibits strong seasonal changes in the circulation and in salinity of its surface layer. We have analyzed available climatological data on hydrography, air-sea fluxes, ship-drifts, etc., to construct a comprehensive picture of the seasonal cycle of the hydrography and circulation of the Bay.

The special features of the thermohaline structure of the Bay of Bengal are the following. In the upper 100 m, variation in salinity dominates the vertical stratification. Below this depth, temperature has the dominant role. Most of the variability in the T–S characteristics of the Bay is confined to the upper 100 m. The vertical distribution of temperature and salinity in the upper Bay can be classified into four types:

\begin{enumerate}[label=(\alph*)] \item The classical mixed layer, which is well mixed in temperature and salinity. \item A salt-mixed layer, which is about three times shallower than the thermally mixed layer. \item A layer which is isothermal and has a well-defined salinity gradient. \item A layer in which a well-defined salinity gradient exists and has a subsurface temperature maximum. \end{enumerate}

These profiles are observed in different regions during different seasons. The continuously decreasing salinity profile (type c) is maintained by influx of freshwater from precipitation and river discharge. The subsurface temperature maximum (type d) is caused by the winter cooling of the Bay. This cooling is restricted only to a layer near the surface.

Analysis of ship-drifts and dynamic computations show the presence of an anticyclonic gyre in the Bay during February–May. This gyre has a poleward boundary current as its western arm. A strong narrow southward coastal current flows along the east coast of India during October–December. There is a northward coastal current in the southern Bay (south of 15°N) during the southwest monsoon season. North of 15°N, there exists a southward current close to the coast but not attached to it. The anticyclonic gyre and the poleward western boundary current can be explained in terms of the Sverdrup theory. It is necessary to bring in other mechanisms such as local coastal wind forcing and freshwater discharge by rivers to explain the southward coastal current during the northeast monsoon and the northward current during the southwest monsoon.

The Bay of Bengal is a region of net annual heat gain and freshwater gain from the atmosphere and rivers. Based on a two-layer box model, we propose that a part of the heat gain is transferred downward in order to maintain the heat balance. The salt balance may be achieved by horizontal transport in the surface layer alone.