Role of distinct flavours of IOD events on Indian summer monsoon

Abstract

The summer monsoon contributes to about 75 % of mean annual rainfall over the various meteorological subdivisions of India. The role of ocean–atmosphere phenomena such as Indian Ocean Dipole (IOD) and El Nino–Southern Oscillation (ENSO) on the Indian monsoon activity is intriguing. The impacts of ENSO, IOD and Equatorial Indian Ocean Oscillation on monsoon are distinct. The ENSO (IOD) in general affects the monsoon negatively (positively). The present study aims to understand the role of different types of IOD such as early IOD (EIOD), normal IOD and prolonged IOD (PIOD) on Indian Summer Monsoon Rainfall (ISMR). We find that an EIOD, which peaks in the mid-monsoon months (July and August), plays a significant role like PIOD in enhancing ISMR even though it has a medium Dipole Mode Index amplitude value compared to other IODs. During an EIOD, the combined effect of excess evaporation from Arabian Sea and the stronger cross-equatorial flow leads to the enhanced monsoon activity. In addition, there is a substantial decrease in the number of break spells during EIOD years.