Observed southwest monsoon rainfall changes in Sri Lanka and possible mechanisms

• Published in: Modeling earth systems and environment 2022-09, Vol.8 (3), p.4165-4175
• Main Authors: Shelton, Sherly, Pushpawela, Buddhi
• Format: Article
• Language: English
• Subjects: Chemistry and Earth Sciences; Computer Science; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Ecosystems; Environment; Math. Appl. in Environmental Science; Mathematical Applications in the Physical Sciences; Original Article; Physics; Statistics for Engineering
• ISSN: 2363-6203; 2363-6211
• DOI: 10.1007/s40808-021-01346-7

Southwest monsoon (SWM) is one of the major monsoon systems that brings significant rainfall over western and southwestern parts of Sri Lanka. Therefore, this study investigates the SWM rainfall trend for 1980–2013 using monthly rainfall data from 19 meteorological stations and an associated mechanism by analyzing moisture transport and related atmospheric variables. The influence of the Indian Ocean sea surface temperature (SST) on SWM rainfall is also evaluated. The results show a statistically non-significant decreasing trend of SWM rainfall in most stations, while the regional average rainfall in the wet, intermediate and dry climate zones decreased by –21.3, –19.3, and –5.3 mm for the 34 years, respectively. It is further evidenced by the decreasing trend of precipitable water content, especially over the southwest, south, and eastern parts of Sri Lanka. The observed decreasing rainfall is attributed to the decreasing trends of total moisture transport over the study domain (–7.14 kg m −1  s −1 ), resulting from the weakening of low-level westerlies at 850 hPa and decreasing specific humidity at 850 hPa in the Arabian sea (AB), Central Indian Ocean (CI, and Bay of Bengal (BB) regions. This study found that SST in CI, AB, and BB regions are increasing by 0.48°C, 0.31°C, and 0.32°C for the study period. As a results, the weakening of the SST gradients between AB and BB can deteriorate the monsoon circulation untimely, which weakens SWM over Sri Lanka.