On foreshadowing southwest monsoon rainfall over India with mid-tropospheric circulation anomaly of April

Abstract

In recent years there have been attempts to relate the distribution of rainfall of Indian southwest monsoon with antecedent upper air factors near the scene of activity for the purpose of long range forecasting" Several upper air factors arc critically examined in this context to make a judicious selection. It is found that the total monsoon rainfall over the country is significantly correlated with the latitudinal position of the sub-tropical ridge on the mean circulation chart of April at 500 mb level. A regression equation is developed with a sample data for a period of 21 years (1950-1970). On testing the regression equation for an earlier eleven-year period (1939-1949) and a later five-year period (1971-1975), it is observed that the average deviation between computed and actual southwest monsoon seasonal rainfall distribution turns out to be 6 per cent, which is quite encouraging.

Years, when the sub-tropical ridge at 500 mb in April is poleward of the normal position are followed by good monsoon rainfall distribution while years when the ridge is equatorward of the normal position are followed by poor southwest monsoon rainfall. The study specially brings out that it may be possible with this technique to obtain an indication, in early May, about the probability of occurrence of a highly deficient southwest monsoon season. Such an advance indication of a probable extreme sub-normal rainfall year would be of vital economic significance in the execution of such weather dependent projects as water resources planning, power distribution adjustment, alternative food production programmes etc.