The severe drought over Tamil Nadu during the retreating monsoon period of 1968 and its associations with anomalies in the upper level flow patterns over the northern hemisphere

Abstract

The mean contours and isotherms over the Indian sub-continent at the 850.mband 300-mb levels and the moisture- content of the air over Tamil Nadu in November 1968 (a month of severe drought in that State) and in November 1966 (a month of fairly good retreating monsoon rainfall over the same area) have been discussed. The daily 700 and 500-mb charts for the Indian sub-continent, the 300 and 200-mb charts for Asia and Europe and the monthly mean 700-mb charts for the northern hemisphere during the same two months have also been studied. It has been shown that, during the drought month, the Subtropical Jet Stream System (STJS) was not only in unusually low latitudes over India but it was also markedly stronger than usual. This abnormality in the STJS was directly or indirectly associated with large-scale anomalies in the upper level flow patterns in the atmosphere over the middle and higher latitudes in the northern hemisphere. The genesis of these anomalies has been traced back step by step as far west as the United States of America.

The anomalous fluctuations of the STJS were indirectly responsible for the cyclonic storms in the Bay of Bengal being 'steered away' from Tamil Nadu and were thus also partly responsible for the drought conditions over Tamil Nadu.

The observed progressive trend of equatorward movement of the STJS along with the polar front jet and a deep cyclonic vortex in the subpolar latitudes in November 1968 has been discussed in relation to the progressive increase in the rainfall deficiency over Tamil Nadu during the Same period. It has been suggested that the parameters utilised in the present study could be tried out in medium range forecasting of rainfall over Tamil Nadu during the retreating monsoon period.