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. The relative contributions and physical processes of different intra-seasonal variations that determine the summer westward SSLCC are quantified using the reanalysis data and a simple linear, continuously stratified (LCS) ocean model. It reveals that the wind forcing from south Sri Lanka coast, equatorial Indian Ocean, and southern Bay of Bengal (BoB) is responsible for the westward SSLCC on the intra-seasonal timescale, contributing 53%, 30%, and 17%, respectively. The local wind forcing along the southern Sri Lanka is linked to the BSISO. Specifically, the negative phase of the BSISO, characterized by active atmospheric convection, induces cyclonic wind stress curl along the south coast of Sri Lanka, and directly promoting the westward SSLCC. The equatorial forcing plays a secondary role in the westward SSLCC via mixing behavior of the first and second baroclinic modes upwelling Rossby waves (0.64 m s⁻¹), propagating westward along the 5°N section as the cyclonic vortices. On the contrast, influenced by the modulation of the BSISO signal, the forcing from the southern BoB excites the rapid westward propagation of the first baroclinic mode upwelling Rossby waves (0.85 m s⁻¹) near 90°E, contributing the westward SSLCC.
 

Counter-Wind Current, Sri Lanaka, Intra-seasonal oscillation, Rossby waves,