Oceanography

Water masses in the Bay of Bengal

The Bay of Bengal is in the north-eastern corner of the Indian Ocean and is enclosed on the northern, western and eastern sides. The southern boundary extends from the south end of Sri Lanka to the north of Sumatra and is open to the Central Indian Ocean. The northern Indian Ocean wind system, together with the Himalaya Mountains causes the world’s highest rainfall, which drains into the Bay of Bengal.

The surface water mass characteristics of the bay exhibit wide variations from one region to another due to differences in fresh water inflow and heat exchange. The Bay of Bengal surface water masses are broadly divided into: (1) the North Dilute water mass and (2) the Southern Bay of Bengal water mass (LaFond, 1958). The North Dilute water mass is formed during October-Novemebr in the northern end of the bay by the addition of fresh water.

Based on an analysis of the physical oceanographic data collected in the southern and central Bay of Bengal during the transition period between winters and summer seasons, two water masses in the upper 100m has been identified. The first water mass found in the upper 50m in the Eastern Bay of Bengal and the Andaman Sea which is characterized with salinity less than 33.0 is named as the Eastern Dilute Water of Indo –Pacific origin. This water is considered to form by the admixture of the run of water from the Burmese coast with the (less saline) waters of Pacific origin entering the south eastern bay through the Straits of Malacca. The second water mass which is encountered in the upper 100 m of the Southern Bay and the western and middle parts of the Central Bay with the salinity range 33.0-34.0. Southern Bay of Bengal water reported by LaFond(1958). A high salinity water mass traced in a specified region in the central part of the bay below 300m depth is assumed to be the Persian Gulf Water, probably overlying the Red sea water. The major subsurface water mass with the temperature range 4-16°C and salinity range 34.8-35  is found to be present in all parts of the Southern and Central Bay below 100-150m depth.

Varadachari et.al.(1968) have identified two water masses in the upper 1000m of the Bay of Bengal with distinct salinity and characteristics. The upper water mass has the characteristics of Persian Gulf water and the lower one, that of Red Sea water. Both the water masses appear to enter the Bay south of 9°N latitude. Two isolated high salinity pockets have been identified as the remnants of the Arabian Sea High Salinity Water mass. The salinity maxima found in the depth range of 200-900 m have been considered to be due to the penetration of the isohaline layer, which forms (at the boundary) when Persian Gulf water mass and Red Sea water mass come in contact in the Arabian sea. Varkey and Sastry (1992) utilizing the historical hydrographic data collected during several cruises, have confirmed the presence of the following tropospheric water masses in the Bay of Bengal: (i) Bay of Bengal Low Salinity Water around 20.8, (2) Arabian Sea High Salinity Water around 23.92 (3) Water Mass around 23.94 (4) Persian Gulf Water around 26.81 (5) Red Sea Water around 27.19 (6) Equatorial Surface Water around 22.5.

Deep water masses in the Bay of Bengal are charactized by temperatures as low as 1.1°C or even lower. As indicated by Sewel (1932), the main course of the Antarctic Bottom drift from the South Polar region through the Indian ocean to the north appear to be three main tongues of bottom water, of which the third can be traced into the Bay of Bengal. In the region of the Bay of Bengal, the major portion of the bottom drift is considered to undergo a further division into two subsidiary streams. One of these streams is likely to pass northwards between Carpenter’s ridge on the west and the Andaman-Nicobar ridge on the east through a deep channel, where water of a temperature as low as 1.0°C can be traced on the bottom as far north as latitude 12°N. One portion of the other current branch is likely to flow northwards into the Bay of Bengal along the east coast of India, Where water of a temperature of only 1°C has been found at a depth of about 2000m as for north as latitude 9°N. Therefore the deep and bottom water masses present in the Bay of Bengal can be considered as the water masses that are derived from the south polar region, similar to those present in the Arabian Sea at deeper levels.