Dynamics of the Summer Counter‐Wind Current Along South Sri Lanka Coast 1: The Dominant Role of Intra‐Seasonal Variability.

The spatiotemporal characteristics of the south Sri Lanka coastal current (SSLCC) during summer are examined in this study. Climatologically, the SSLCC flows eastward as a part of the southwest monsoon current during summer. However, westward SSLCC occurred lasting more than 20 days in the summer of 2013, 2016, 2017, and 2018 based on reanalysis data, implying significant interannual variability of the SSLCC. The analysis on the summer extreme westward SSLCC indicates that the intra‐seasonal wind associated with atmospheric boreal summer intra‐seasonal oscillation (BSISO) is the main factor leading to the westward SSLCC. Firstly, the northward propagation of the BSISO induces the westerly wind anomaly and positive wind stress curl anomaly along the south and east coast of Sri Lanka, which induces the westward SSLCC. Secondly, driven by equatorial Indian Ocean intra‐seasonal wind, the low sea level anomaly associated with upwelling Rossby wave reflected from the west coast of Sumatra Island propagates westward. This propagation significantly influences the cyclonic circulation off the south coast of Sri Lanka, thus facilitating the occurrence of the westward SSLCC. Plain Language Summary: Climatologically, the south Sri Lanka coastal current (SSLCC) transports high‐salinity water from the Arabian Sea to the Bay of Bengal as it flows eastward during summer. However, extreme westward SSLCC occurred in the summer of 2013, 2016, 2017, and 2018, lasting roughly 20 days and affecting the salinity balance of the Northern Indian Ocean. Our study aims at investigating the characteristics of the SSLCC and explores the mechanism of the summer westward SSLCC. The findings suggest that the westward SSLCC is primarily regulated by the intra‐seasonal component (30–105 days). The regulation of this intra‐seasonal signal involves a combination of factors, including a positive wind stress curl anomaly along the south and east coast of Sri Lanka and the presence of upwelling Rossby waves along the 5°N section. Key Points: The south Sri Lanka coastal current (SSLCC) exhibits significant intra‐seasonal, semi‐annual, and annual componentsIntra‐seasonal signal associated with boreal summer intra‐seasonal oscillation dominates the emergence of the summer westward SSLCCThe synergistic effect of the cyclonic wind stress curl anomaly on the southern Sri Lanka coast and upwelling Rossby waves reflected from Sumatra Island facilitates the summer westward SSLCC [ABSTRACT FROM AUTHOR]