Unravelling the Linkages between the Intraseasonal Variability of the West Pacific Subtropical High and Indian Summer Monsoon Rainfall

Abstract

The present study explores the intraseasonal variability of the west Pacific subtropical high (WPSH) and its relation with Indian summer monsoon rainfall (ISMR) based on the IMD rainfall data and NCEP-NCAR reanalysis data sets for the 1950–2021 period. The longitudinal position of the western edge of WPSH around 20° N is about 139.3° E in June, which gradually extends eastward up to 151° E by September end. The zonal movement in the western edge of WPSH exhibits a 30–60 day periodicity, which is prominent in July -August months during WPSH expansion. In contrast, the western edge of WPSH shows a periodicity of about 10–25 days, which is dominant from mid-June to early September. These two periodicities are significant at a 90% confidence level. As compared to the climatology, the WPSH shifted about 11° (10°) westward (eastward) along with an intensification (weakening) at the center of WPSH during expansion (contraction) cases. The surplus (deficit) rainfall occurred over entire India (east central India) during the WPSH expansion. In WPSH contraction, surplus (deficit) rainfall was noticed over the east-central and northern India (southern peninsular and northwest India). The sea surface temperature (SST) anomalies during expansion (contraction) follows the Modoki type of La Niña (El Niño) patterns over the central Pacific Ocean. During WPSH expansion, an intense mid-tropospheric updraft, abundance of atmospheric moisture along with its convergence over the ISM regions are observed and these are conducive to above normal rainfall.