Physical oceanography of the Bay of Bengal and Andaman Sea

Physical oceanography of the Bay of Bengal and Andaman Sea is reviewed for the first time. All available information for over 50 years is consolidated in this review. To begin with, information on peripheral or related aspects of climate of the hinterland, such as pressure, rainfall, storms, winds, sunshine etc., are presented. The Bay of Bengal and Andaman Sea are divided into two regions, viz., the coastal belt and the opean ocean areas, on the basis of large differences in water properties. Considerable variation in salinity and density exists all over the coastal areas due to abundant freshwater discharges, especially during the rainy months. The coastal circulation is mainly driven by river runoff and wind thrust and is influenced by branches of the Equatorial Current system. Strong wind-driven upwelling and sinking are seen along the eastern Indian coast. The surface circulation system in the Bay of Bengal and Andaman Sea can be divided into three gyres, viz., (a) the northwestern gyre, (b) the large gyre in the southern Bay of Bengal and (c) the Andaman gyre. Gyres are noted also at 500 m and 1000 m. The net water exchanges [Evaporation (E) + Precipitation (P) + Runoff (R)] at surface, for the whole area north of 5 degrees N are -11.5 cm and + 63.7 cm during the winter monsoon and summer monsoon, respectively. The net water transports across 6 degrees N between surface and 1250 m, are found to be -79 x 10 sup(12) m sup(3) and + 129 x 10 sup(12) m sup(3) for winter monsoon and summer monsoon periods, respectively. High salinity water masses from the Red Sea, Persian Gulf and eastern and southern Arabian Sea flow into the Bay of Bengal and play an important role in its water structure. A strong gradient of surface temperature is observed during winter (25.5 to 28.5 degrees C) from the head of the Bay to 5 degrees N; similarly for salinity (21.0 to 34.5 ppt) during summer. Variation in mixed layer thickness is greater during summer (about 25 to 100 m) and less during the winter (60 to 100 m). Abyssal water flows and properties are least studied in the Bay of Bengal