Preceding winter La Niña reduces Indian summer monsoon rainfall
Leaving out the strong El Nino Southern Oscillation (ENSO) years, our understanding in the interannual variation of the Indian summer monsoon rainfall (ISMR) stands poor for the rest. This study quantifies the role of ENSO in the preceding winter on ISMR with a particular emphasis on ENSO-neutral summer and La Nina winter. Results show that, unlike the simultaneous ENSO-ISMR relationship, La Nina of previous winter reduces mean rainfall over the country by about 4% even during ENSO neutral summer. Moreover, when ENSO changes phase from La Nina in winter to El Nino in summer, ISMR is anomalously lower than during persisting El Nino years (−14.5% and −5.3%, respectively), increasing the probability of severe drought. This suppression effect of La Nina of the preceding winter on summer monsoon precipitation over India is mostly experienced in its western and southern parts. Principal component analysis of the zonal propagation of surface pressure anomalies from winter to summer along Northern Hemisphere subtropics decomposes interannual variations of seasonally persisting anomalies from zonal propagations. The dominant modes are associated with the seasonal transition of the ENSO phase, and are well correlated with date of onset and seasonal mean rainfall of monsoon over India. These results improve our understanding of the interannual variations of ISMR and could be used for diagnostics of general circulation models.