The Hindu Business Line : Bay of Bengal, Arabian Sea surface temperatures vary

DIFFERENCE in the structure and magnitude of prevailing winds has been found responsible for keeping the mean temperature of the top 50 metres of the Bay of Bengal warmer than that of the Arabian Sea during the summer monsoon.

The genesis of this phenomenon has been traced to the asymmetry of the wind field in the north Indian Ocean, according to findings by Mr S.S.C. Shenoi, Mr D. Shankar and Mr S.R. Shetye of the Physical Oceanography Division of the National Institute of Oceanography.

Arabian Sea and the Bay of Bengal are similar in several aspects such as their location on the same latitude band, semi-enclosed nature, opening on the southern side and exposure to changing monsoon winds etc. Both receive similar amounts of solar radiation at the top of the troposphere. In spite of this, there are striking dissimilarities between the two.

First, the winds over the two basins are different especially during the summer monsoon as a consequence of the `low-level jet' that occurs over the Arabian Sea.

Second, the precipitation exceeds evaporation in the Bay while the evaporation exceeds precipitation in the Arabian Sea. In addition, the Bay receives an annual runoff of 1.5x1012 m3 from rivers flowing into it.

Therefore, the surface layer in the Bay is much fresher than that in the Arabian Sea; the average salinity of the top 50 m in the Arabian Sea exceeds that in the Bay by more than a couple of notches. The large inflow of freshwater from precipitation and runoff results in strong near-surface stratification in the Bay.

These differences extend to the evolution of sea surface temperature (SST) in the two basins. Prior to the onset of the summer monsoon, during April-May, the north of Indian Ocean becomes the warmest area among the world oceans. Soon after the onset of the monsoon in June, the winds strengthen and SST decreases.

The Arabian Sea cools rapidly, but SST in the Bay remains higher than 28 deg C, the threshold for deep convection in the atmosphere over tropical oceans.

This difference in SST is reflected in the convective activity in the atmosphere, with the convection over the Bay being perhaps the largest in the tropics during summer. The number of low pressure systems (LPSs) that form there by far exceed those in the rest. These LPSs move westward over India bring rainfall to central and northern India.

Analysis of the heat budget of the Arabian Sea and Bay of Bengal show that in the Arabian Sea, the dominant oceanic processes are directly influenced by the winds and they cause overturning.

They assume significance primarily during the summer monsoon, when they remove heat from the `control volume'. Diffusion, the only oceanic process not directly influenced by the winds, is important throughout the year.

In the Bay, these wind-forced processes have a minor impact on the heat budget; diffusion overwhelms other oceanic processes, the scientists say.

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