Impact of convection over the equatorial through on Indian summer monsoon activity

Abstract

The existence or nonexistence of break and active cycle of rainfall over the Indian subcontinent during the mid-monsoon months of July and August decides the success and failure of Indian monsoon rainfall. Effect of active or break cycle, convective systems and convection over the equatorial trough (ET) on Indian summer monsoon rainfall (ISMR) were studied. An increased number of convective systems in the west Pacific showed high negative correlation with rainfall over equatorial trough regions. During active monsoon years, cyclones are focused between 10° - 25° N and 110° - 140° E whereas the cyclones were widely distributed during the break monsoon years. The co-occurrence of active or break spells and convective activity have considerable influence on ISMR. During an active spell, enhanced convection was observed over North Indian Ocean (NIO) while during break spell it was over ET regions. Strong, widespread and short duration cyclonic activity over North West Pacific (NWP) resulted in a break spell in ISMR. The relative influence of El-Niño Southern oscillation and Indian Ocean Dipole (IOD) events on ISMR have been further studied for the period 1950-2010. In order to look into the role of El-Niño on the monsoon, the SST data over five regions (REG-I to REG-IV) were studied. SSTs in the central Pacific (Niño 3.4; REG- II) and eastern Pacific (Niño 3; REG- III) regions were more correlated with ISMR during the observation period. Decadal analysis showed that the temperature of central Pacific region indicated rapid cooling in the recent decades and increased the temperature gradient between western and eastern Pacific Ocean. This cooling in the central Pacific could create La-Niña type conditions. During El-Niño (La-Niña) years, the weakened (strengthened) Walker circulation influence the summer monsoon rainfall over the Indian subcontinent. The newly created Walker circulation index is a promising tool to examine the strengthening (weakening) of the Walker circulation to predict the Indian summer monsoon in future research.