Variability of Summer Monsoon Rainfall in India on Inter-Annual and Decadal Time Scales

Indian Summer Monsoon Rainfall (ISMR) exhibits a prominent inter-annual variability known as troposphere biennial oscillation. A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature (SST) anomalies over the tropical Indian Ocean and cold SST anomalies over the western Pacific Ocean. These anomalies persist until the following monsoon, which yields normal or excessive rainfall. Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since 1841, when rainfall measurements began in India. Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase. In both of these variabilities, anomalies in monsoon heat source, such as deep convection, and middle latitude westerlies of the upper troposphere over south Asia have prominent roles.