Ocean-atmosphere interaction on a seasonal scale over north Indian Ocean and Indian monsoon rainfall and cyclone tracks -A preliminary study

Abstract

A preliminary study of synoptic sea surface temperature (SST) measurements made by ships plying over the north Indian Ocean has revealed the existence of an interesting Ocean- atmosphere interaction. Large scale monsoon failures over India during 1965 and 1966 caused increase in SST in the Arabian Sea and Bay of Bengal. The warm seas are the likely cause of a persist anti-cyclone which formed in the upper troposphere over the Indian seas immediately after the 1966 monsoon and persisted there till the monsoon of 1967 which was a normal monsoon. Following this monsoon SST over the Arabian Sea registered a fall. Conditions again became favourable for monsoon failure. Intensity of upwelling over the coastal waters of Somalia and Arabia (which is apparently regulated by tile strength of the monsoon) is a factor with major role in this interaction. These large-scale changes are found to affect the cyclone tracks also. This mechanism could, therefore, be the cause or the three year oscillation in sub-tropical westerlies/tropical easterlies over south Asia, Indian monsoon rainfall and tracks of the post-monsoon severe cyclones of the Bay of Bengal observed during the decade 1965 to 1974 and reported by Joseph (1975, 1976).

While in the sub-Saharan areas of Africa (like Sahel) there was large rainfall deficit during every year from 1966 to 1974 (with the year 1972 having the largest deficit), in India the generally bad monsoon decade 1965 to 1974 was broken by a normal monsoon once every three years, i.e., 1967, 1970 and 1973. In the past also this area of Africa had similar long runs of dry spells (Kraus 1977), but an examination of Indian rainfall of 100 years has shown that no interval between successive good monsoon years has exceeded 4 years. This difference in African and Indian rainfall series must be due to the newly found ocean-atmosphere interaction, occurring over the Arabian Sea und Bay of Bengal.