Projected Future Changes in Extreme Precipitation Events over the Southeast Asian Monsoon Zone Using CMIP6 HighResMIP Simulations

Monsoon precipitation provides water supply for large populations and agricultural productions, but extreme precipitation associated with the monsoon can have devastating socioeconomic impacts. This study investigates the projected changes in mean and extreme precipitation events over the Southeast Asian monsoon zone and examines the physical drivers of the changes using CMIP6 HighResMIP simulations of the current (1995–2014) and future (2030–2049) climates. We used 7 HighResMIP atmospheric model simulations with prescribed SST (atmosphere-only) and versions of the same model coupled to a dynamic ocean model (coupled). These models are further categorized based on their horizontal resolutions as high-resolution (~ 25 km), intermediate-resolution (~ 50 km), and low-resolution (~ 100 km) and the ensemble of all the above 7 models (Ensemble). Simulations of the current climate are compared with different observational precipitation products (e.g., CHIRPS, GPMV6, TRMM3B42, & ERA5 reanalysis). The atmosphere-only simulations are found to realistically capture the observed mean, annual cycle, and intraseasonal variability of precipitation. However, the coupled models show a dry bias compared to observations and the atmosphere-only simulations. The model bias is more prominent during the summer monsoon season (JJAS) over the land and the post-monsoon season (Oct - Nov) over the Bay of Bengal. Dry biases in the coupled simulations are attributed to the drier lower levels of the atmosphere in the coupled simulations compared to the atmosphere-only simulations. Rainfall PDFs also suggest that the atmosphere-only and coupled ensemble simulations have too frequent light precipitation (<10 mm day-1) and too little heavy rainfall (> 10 mm day^-1). The higher-resolution models perform better than the intermediate and low-resolution models. The intensity and the frequency of the extreme precipitation events (i.e., 95 and 99 percentiles) are weaker in both the atmosphere-only and coupled simulations compared to the ERA5 reanalysis. Mean precipitation does not change significantly in the future in any of the simulations except the high-resolution models. Extreme precipitation events are projected to increase over the foothills of the Himalayan region, but no significant changes are found over the areas with the largest precipitation (e.g., west coast, central India, and north Bay of Bengal). Noticeably, the very extreme precipitation (99-percentiles) is projected to increase in both intensity and frequency along the foothills of the Himalayas in the future.