Rainfall distribution over India during the monsoon months in the absence of depressions and cyclonic storms

Abstract

It is well known that monsoon depressions and cyclonic storms play an important role in the distribution of rainfall over India. Naturally, their absence from the Indian area during any month of the monsoon season is considered to be responsible for causing deficient rainfall and consequent drought conditions. It is, therefore, of interest to know as to what extent the absence of these synoptic systems cause deficient rainfall during the Individual monsoon months. This aspect has been examined by computing the average rainfall of the different meteorological sub-divisions of the Indian plains north of Lat. 15°N for those monsoon months which were free from depressions and cyclonic storms.

There have been in all 25, 13, 7 and 4 occasions of June, July, August and September respectively, when no depressions or cyclonic storms moved through the Indian land area during the period 1891-1980. This study has shown that (i) of the 4 monsoon months, the September month is worst affected by the absence of these disturbances and (ii) in the case of other monsoon months of June, and August, rainfall can be abnormally deficient depending upon the presence of any of the following meteorological  situations :

            (a) Late arrival and subsequent weak monsoon con lions over the country,

            (b) The axis of monsoon trough remains shifted to the north of its normal position, and

            (c) Non-occurrence of low pressure areas (or land lows) across the country.

It has been seen that rainfall over the country can be normal or even above normal inspite of the absence of these disturbances if the meteorological situations like the above are not there.

This study has, therefore, shown that the absence of monsoon depressions and cyclonic storms are not the main factors which cause deficient rainfall and consequent drought conditions in this country.