Pre-monsoon potential energy budget and prospects of ensuing southwest monsoon over India

Abstract

The tropical atmosphere is generally presumed to be of negligible capacity to store available potential energy. This being so, the question arises what is the reason for failure of tropical monsoon ? Whether hemispheric interactions help to sustain monsoon or the tropical extratropical interactions sustain the activity of monsoon ? In the present attempt, the contributions of southern hemisphere or extra-tropics towards the m1lintenance of southwest monsoon is not studied. However, the effects of deficit potential energy distribution during pre-monsoon (especially April) on the seasonal rainfall over various parts of India are studied for about 12 years. The study reveals the following features, (1) The potential energy storage during premonsoon (April) over various parts of India is not the same every year: There are slight variations. Even these variations affect the rainfall pattern during ensuing southwest monsoon, (2) The lower and upper tropospheric potential energe storages play different roles. The higher storages in the lower troposphere (1000-500 mb) is more useful whereas higher storage in the upper troposphere is not helpful. The higher ratio of P.E. of 1000-500 mb and 500-100 mb layers and the higher potential energy in the layer 1000-500 mb and are both favourable for best monsoon over any sub-division. When both are not higher, the rainfall percentage is nearly reduced to normal whereas the deficient rain occurs when both are lower, (3) The optimum potential energy in the layer 1000-500 mb for normal rain cannot be accurately determined. Similarly the optimum ratio P.E. .1000-500 mb/P.E. 500-.100 mb cannot be exactly worked out.

However, the deviations of these parameters from the lowest observed values indicate on two sets of charts lows, heights, troughs or ridges. The lows or troughs in both correspond to deficit rain. The low in one and the high in the other indicate nearly normal rain. The highs in both indicate excess rain. The study of these charts for 12 years thus sugges that if we can evaluate the absolute minimum values of these parameters, (P.E. 1000-500 mb) and (P.E. 1000-500 mb)/(P.E. 500-100 mb), the prediction of seasonal rainfall departures over various parts would be possible.