Drivers of Past and Predicted Changes of Rainfall in and Around Mainland Southeast Asia

Abstract Observational rain gauge/satellite and reanalysis datasets since the 1950s are evaluated for trends in mean and extreme rainfall over Mainland Southeast Asia (MSEA). Rain gauge data indicate strong increases exceeding 50% in both annual mean precipitation and various extreme precipitation indices over Vietnam and the northwestern part of the peninsula since 1979. Increasing precipitation in MSEA is associated with increased monsoon intensity in southeast Asia and a northward shift of the monsoon activity centre towards MSEA over 1979-2018. Re-analysis data reveal warming-driven increases in evaporation over the seas adjacent to MSEA, including the main oceanic source regions feeding precipitation over the peninsula - the Arabian Sea, the Bay of Bengal, and the South China Sea. Furthermore, moisture budget analysis based on the ERA5 re-analysis also showed increasing oceanic moisture transport along the typical winter and summer moisture pathways towards the MSEA. However, the majority of the increased moisture from these oceanic sources ends up as summer precipitation over the oceanic regions adjacent to the MSEA. In contrast, the ERA5 data do reveal pronounced increases in winter precipitation over the MSEA, in accordance with rain-gauge data, that are associated with increased transport of moisture that originated from the western tropical Pacific and the South China Sea. Long-term amplification of the regional hydrological cycle is further investigated, through analysis of CMIP5 coupled climate models in historical and RCP4.5/8.5 21st century scenario simulations. The CMIP5 ensemble mean shows robust wide-spread trends in wet season precipitation with increasing frequency and intensity of extreme precipitation events over the MSEA, following strong increases in evaporation in the oceanic moisture sources.