Zonally symmetric vs asymmetric North Pacific Ocean sea surface temperature influence on Indian summer monsoon through modulation of upper tropospheric circulation

Abstract

This study asks the following question: what role the zonal distribution of sea surface temperature (SST) of North Pacific Ocean (NPAC) plays in the general circulation and vertical stability of the atmosphere during boreal summer season. Results here show that while zonally symmetric SST warming of NPAC shifts the upper tropospheric jet northward, zonally asymmetric warming and cooling alter the phase of mid latitude Rossby waves in east-west direction. These changes in circulation modify the vertical static stability of the atmosphere and precipitation, particularly in the Indian region. In particular, warmer SSTs in the western NPAC reduce the stability over Indian region leading to an enhanced monsoon precipitation even when El Nino-Southern Oscillation (ENSO) is not in favorable phase. In conjunction with an ENSO forcing, however, this phenomenon can augment or suppress its impact on Indian summer monsoon. This result is used to derive an index that explains more than 80% of the interannual variance of Indian summer monsoon (ISM) precipitation in June-July. Since June-July contributes about 50% to the total June-September precipitation in this region, this discovery increases the potential predictability of ISM. Finally, using historical simulations from the Coupled Model Intercomparison Project (CMIP5), we show that SST anomalies over NPAC strongly influence the strength of the relationship between ENSO and Indian summer monsoon. The mechanism proposed in this study can be used to diagnose the skill of general circulation models.