Variability of West Pacific subtropical high and its potential importance to the Indian summer monsoon rainfall

The variability of West Pacific subtropical high (WPSH) during the peak monsoon (July-August) season for the period 1951-2015 and its association with Indian summer monsoon rainfall (ISMR) is investigated based on various observational data sets. In the present study, we explored the long term variability of WPSH based on location index (zonal displacement of western edge) and intensity index (intensity values at the centre of WPSH). We observed significant interannual variability in zonal displacement of western flank at around 20 degrees N, and this variability is coherent with the variability in intensity at the centre of WPSH. The significant long term trend in location index is about 1.4 degrees per decade towards east, which is also consistent with weakening trend (-2.6 gpm per decade) in intensity index. Moreover, both the indices show contrasting trends prior to and post 1976 periods. The variability of location and intensity indices shows significant relationship with ISMR by modulating the low level circulations over the Indo-Pacific region. The present study revealed that the correlations of WPSH indices with ISMR are significant at 95% confidence level. The spatial distribution of rainfall over India during WPSH expansion years depicts the significant enhancement in southern peninsula and north-central India while a significant reduction in east central India; however, contrasting rainfall patterns are observed during WPSH contraction years. In general, changes in large-scale circulation during the westward (eastward) shift and intensification (weakening) of WPSH favour (unfavour) monsoon rainfall. The southern peninsula and northcentral India (east central India) experience surplus (deficit) rainfall during WPSH expansion due to modulation of moisture supply from the Arabian Sea, Bay of Bengal and west Pacific Ocean. Additionally, we noticed upper (lower) level divergence (convergence) and lower (upper) level convergence (divergence) during the expansion (contraction) years over the south Asian monsoon region.

Automated summary

The study investigates the variability of the West Pacific subtropical high (WPSH) during the peak monsoon season and its association with Indian summer monsoon rainfall (ISMR) based on observational data sets. The results show significant interannual variability in the zonal displacement of the WPSH western flank and intensity at the center, with a long-term trend towards weakening. The WPSH indices exhibit significant correlations with ISMR, affecting rainfall distribution over India.