Has modulation of Indian summer monsoon rainfall by sea surface temperature of the equatorial Pacific Ocean, weakened in recent years?

Abstract

In this paper, the east–west tropical ‘Walker circulation’ and its linear association with sea surface temperature (SST) of the Nino 3 region and Indian summer monsoon rainfall (ISMR) have been investigated. ‘Walker circulation’/‘reverse Walker circulation’ is primarily forced by SSTs of the equatorial Pacific Ocean. In this study, velocity potential field of 0.21 sigma level over the tropics was considered as proxy of the zonal tropical circulation (‘Walker circulation’/‘reverse Walker circulation’). Principal component analysis of the monsoon season tropical velocity potential data of 0.21 sigma level for the two periods 1951–1980 and 1981–2010, was done separately. We find that earlier, two different patterns of the velocity potential field, forced by probably two distinct modes of El Nino episodes, were associated with the ISMR. These two El Nino episodes, respectively, correspond to the strong El Nino events, where in warming was extended up to the date line (primarily zonal) and the moderate El Nino events in which, warming having north south extension, was limited to the eastern Pacific Ocean only. However, in recent years, only the first pattern of the velocity potential field, induced by the strong El Nino events (warming extending up to the date line), was correlated with the ISMR. Further, in the later period (1981–2010), velocity potential field at 0.21 sigma level over the tropical Pacific and Indian Oceans, which appeared to be primarily driven by SST anomalies of the equatorial Pacific Ocean in the first period, was found to be significantly correlated with the extra tropical circulation anomalies also. Therefore, modulation of the ISMR through velocity potential field over the tropical Indian and Pacific Oceans, in the later period, may have additional significant impact of the extra-tropical circulation anomalies. This might have led weak correlation between the ISMR and SSTs of the Nino 3 region, which is actually being observed in recent years.