The role of local topography and sea surface temperature on summer monsoon precipitation over Bangladesh and northeast India

Bangladesh receives most of its precipitation from June to September in the form of rainfall as a part of the Asian summer monsoon system. Bangladesh is a relatively flat region, surrounded by the southern Himalayas and Meghalaya Plateau in the north, Arakan Mountains in the east, and the Bay of Bengal (BOB) in the south. Although several studies have investigated the mechanisms that drive the Asian monsoon precipitation, very few studies have focused on the monsoon precipitation in Bangladesh. This study investigated the influence of the topography of the surrounding regions and sea surface temperature on the summer monsoon precipitation of Bangladesh and the surrounding regions. Using observed data, we showed that moisture convergence near the mountains contributes to the precipitation of Bangladesh, whereas the BOB acts as a source of moisture. A strong low-level jet carries the moisture inland as the land–sea thermal contrast intensifies the wind circulation during the summer. Three differently forced simulations of the Euro-Mediterranean Centre on Climate Change coupled climate model (CMCC CM2) were analysed to investigate the influence of the surrounding region's topography and sea surface temperature on the summer monsoon precipitation. The low-resolution simulation showed no spatial variability of precipitation and dry bias due to the overly smooth topographical representation of mountains. The high-resolution coupled simulation, with a better representation of topography, improved the moisture convergence at the foothills and precipitation bias. The high-resolution prescribed sea surface temperature further improved the precipitation bias by intensifying the low-level jet that transports moisture over Bangladesh.