Moisture variability over the Indo-Pacific region and its influence on the Indian summer monsoon rainfall

Abstract

The Indo-Pacific Ocean (i.e. region between 30E and 150E) has been experiencing a spread warming since the 1950s. At the same time the large-scale summer monsoon rainfall over India and the moisture over the East Africa/Arabian Sea are both decreasing. In this study we intend to investigate how the decrease of moisture over the East Africa/Arabian Sea is related with the Indo-Pacific Ocean warming and how this could affect the variability of the Indian summer monsoon rainfall. We performed the analysis for the period 1951-2012 based on observed precipitation, sea surface temperature and atmospheric reanalysis products and we verified the robustness of the result by comparing different datasets. The decreasing trend of moisture over the East Africa/Arabian Sea coincides with an increasing trend of moisture over the western Pacific region. This is accompanied by the strengthening (weakening) of the upward motion over the western Pacific (East Africa/Arabian Sea) that, consequently, contributes in strengthening the western Pacific-Indian Ocean Walker circulation. Associated with it, the low-level westerlies are weakening over the peninsular India, thus contributing to the reduction of moisture transport towards India. Therefore, rainfall has decreased over the Western Ghats and central-east India. Differently from previous decades, since 2003 moisture over the East Africa/Arabian Sea started to increase and this is accompanied by the strengthening of convection due to increased warming of sea surface temperature over the western Arabian Sea. Despite this moisture increase over the Arabian sea, we found that moisture transport is still weakening over the Indian landmass in the very recent decade and still contributing to the decreased precipitation over the northeast India and southern part of the Western Ghats.