Identification of the atmospheric water sources and pathways responsible for the East Asian summer monsoon rainfall

Abstract

The East Asian Summer Monsoon (EASM) rainfall provides water security and socio-economic benefit for over 20% of the global population. However, the sources of this rainfall and how it is carried to the East Asian landmass is still uncertain. To address this, atmospheric water sources and pathways associated with the East Asian summer rainfall are identified and quantified in the present study using atmospheric water trajectories, calculated with a novel Lagrangian framework. Evaporated water from the East Asian landmass is found to be the major contributor to East Asian rainfall, amounting to local recycling. The results further indicated that the South Indian Ocean is a major non-local source for rainfall over southern East Asia during June to August. The role of the South Indian Ocean as a source of atmospheric water is one of the major findings of the study and would help in better understanding and predicting the East Asian summer rainfall. Evaporated waters from the Pacific Ocean (particularly the far-west Pacific Ocean) dominate the non-local contribution to precipitation over northern East Asia during June to September, and over southern East Asian rainfall during September. The spatial structure of the East Asian rainfall is reported to be determined by the atmospheric waters that are evaporated and transported from the non-local sources. The role of the North Indian Ocean and the South Asian landmass as a source of water for East Asian precipitation is minimal and restricted to southern East Asia. The cross-equatorial Somali jet and equatorial trade winds associated with the western North Pacific Subtropical High are important pathways for East Asian precipitation sourced over the South Indian Ocean and the Pacific Ocean respectively. In contrast, minor roles are attributed to the Bay of Bengal as a source, and mid-latitude westerlies as a transport pathway, for East Asian precipitation.