The northeastern arm of the Indian Ocean, located between peninsular India and Burma. It covers about 2,200,000 sq. km and is bordered on the north by the Ganges and Brahmaputra River deltas, on the east by the Burmese peninsula and the Andaman and Nicobar Islands, on the west by India proper and Ceylon, and on the south by the Indian Ocean proper. The average depth is around 3000 m with maximum depths reaching over 400 m in the southern parts.

Major circulation features are the East Indian Current, a northward current flowing along the Indian shelf from January through October, and the East Indian Winter Jet, a southwestward flowing current that replaces during the remainder of the year. This current reversal is due to the seasonal change from the Northeast to the Southwest Monsoon and the concomitant wind forcing. General clockwise and conterclockwise circulation gyres are seen throughout the Bay accompanying, respectively, the Current and the Winter Jet, although the situation becomes a bit more complicated during the transition periods.

The monsoonal wind variations and the resulting circulations also serve to induce upwelling near the coasts during the spring (with the northward current) and the piling up of surface water along the coasts during the late fall and early winter (with the southward currents). Thus the isopycnals tilt upwards and downwards towards the shore during, respectively, the spring and late fall. The annual mean SST for the region is above 28. 5° C. , although upwelling can reduce this to 25-27° C during the spring. The salinities are kept lower than normal oceanic values (especially in the western parts) by extensive monsoonal river runoff.