The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Hydrographic data collected during March-April 1991 show the presence of a poleward current along the westem boundary of the Bay of Bengal north of about 10 o N carrying warmer waters of southem origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 x 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available elimatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the westem boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the westem boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the cuff of wind stress, which has a well-defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January-May. The pattem of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress cuff over the bay is the sole mechanism to force the gyre. A feature common to the major basins of the world oceans is the semipermanent anticyclonic subtropical gyre (STG), located approximately between latitudes of 10 o and 40 o , with a slow equatorward drift over most of its area and a rapid poleward flow, the western boundary current (WBC), along its western edge. Though there are many aspects of the equatorward drift and the poleward WBC that need further study [see, for example, Huang, 1991 ], there is general agreement that to a good approximation, (1) the equatorward drift is a consequence of the Sverdrup flow driven by the curl of wind stress over the basin, and (2) the poleward WBC arises to compensate for the equatorward drift. The North Indian Ocean is unique because of the absence of such a semipermanent STG. This peculiarity in the behavior of the North Indian Ocean is linked to the wind field. The low-level atmospheric circulation over the other basins is dominated throughout the year by the presence of (1) the subtropical high-pressure belt approximately at the latitude of 30 o , (2) the high-latitude "low," and (3) the narrow belt of low pressure, the Intertropical Convergence Zone (ITCZ) close to the equator. The atmospheric conditions over the North Indian Ocean during the southwest monsoon (approximately May-September) are in striking contrast to the pattern of sea level pressure described above. During this season the ITCZ over the Indian Ocean fluctuates