A boreal summer warming in the subsurface of the Bay of Bengal and its dynamics

A pronounced subsurface warming occurs in the southern Bay of Bengal (BOB) during the summer monsoon (from July to September), as revealed by the data from Array for Real-time Geostrophic Oceanography (Argo). This subsurface warming can be attributed to the deepened thermocline, which plays a crucial role in regulating the ocean subsurface temperature. The variations of thermocline in the southern BOB are primarily influenced by local and remote forcing, with local forcing dominating and remote forcing being a secondary contributor. To better understand the individual effects of each forcing mechanism, a 1.5-layer reduced gravity model is employed in this study. The model shows that the equatorial zonal wind stress reaches its first peak in May, inciting downwelling Rossby waves from the eastern boundary of BOB. These equatorial waveguides rapidly deepen the thermocline of the entire southern BOB in June. From July to September, the southwest monsoon intensifies and prevails, forcing the annual Rossby wave and local Ekman pumping that sustains the thermocline in the southwestern BOB. Simultaneously, the remote forcing causes negative effect by shoaling the thermocline in the eastern region, thus accentuating the observed warming trend in the west. This study discusses the complex air-sea interaction in the BOB, contributing to a deeper understanding of the upper thermal structure within BOB. In addition, the findings provide valuable insights to enhance weather forecasting capabilities in the surrounding countries.