Variability of the Indian summer monsoon in relation to oceanic heat budget over the Indian seas

Abstract

The influence of the surface marine meteorological parameters and air-sea fluxes of heat and moisture over the Indian seas (Bay of Bengal, Arabian Sea and equatorial Indian Ocean) on the interannual variability of the Indian summer monsoon has been studied in detail. The data base for this study, which consists of mean monthly marine meteorological fields for a period of 30 years (1950–1979), forms a part of the Comprehensive Ocean Atmospheric Data Set (COADS) analysed on a regular 1° latitude/longitude resolution. Using the surface marine meteorological fields, different components of the oceanic heat budget were computed. The basic meteorological fields, as well as the components of the oceanic heat budget, over the Indian seas were examined for the contrasting years of the summer monsoon (large excess rainfall: flood monsoon and large deficient rainfall: drought monsoon). To ascertain which regions were statistically significant in terms of heat budget over the Indian seas, the difference fields between two extreme categories of monsoon were subjected to Student's t-test. The study revealed that in the month of May preceding a flood monsoon, there were stronger surface winds and more abundant cloud cover over most parts of the Arabian Sea compared with a corresponding period prior to a drought season over India. Thus, as a result of enhanced evaporation and reduction of incoming solar radiation during the pre-monsoon month of May, the difference fields of the oceanic heat budget between the extreme years (flood/drought) illustrate a statistically significant zone of heat loss over the entire equatorial Arabian Sea. The analysis during the month of May provides a useful qualitative indication of the subsequent mean monsoon activity over the Indian sub-continent. With the commencement of the monsoon (June–August) and its northeastward progress over the Arabian Sea, all the difference fields of oceanic heat budget maintain the same signatures as found during the pre-monsoon month of May. However, the magnitude of the differences shows an overall reduction, with a shift of statistically significant zones from the equatorial region to the northern and eastern sectors of the Arabian Sea.