Dynamics of the Summer Counter-Wind Current Along South Sri Lanka Coast: 2. Relative Contributions of Local and Remote Forcing on the Intra-Seasonal Timescale

Abstract

It is well known that the intra-seasonal component is critical for the occurrence of the westward south Sri Lanka coastal current (SSLCC) during summer. In this study, the relative contributions and physical processes that determine the westward SSLCC during summer on the intra-seasonal timescale are quantified using the reanalysis data and a simple linear, continuously stratified (LCS) ocean model. A comprehensive analysis of the westward SSLCC events 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 boreal summer intra-seasonal oscillation (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.