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 degrees 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 degrees E, contributing the westward SSLCC.
This study explores the crucial role of intra-seasonal dynamics in driving the westward south Sri Lanka coastal current (SSLCC) during summer. Using reanalysis data and a simple linear, continuously stratified (LCS) ocean model, the research reveals the combined effects of wind forcing from three sources of southern Sri Lanka coast, equatorial Indian Ocean, and southern Bay of Bengal (BoB), contributing 53%, 30%, and 17%, respectively. These three forcings exhibit distinct physical processes: local wind forcing directly affects the westward SSLCC through the local cyclonic wind stress curl, while wind forcing from equatorial and southern BoB promotes the westward SSLCC through the first-second baroclinic mode and first baroclinic mode of the upwelling Rossby waves, respectively. This study provides valuable insights into the mechanisms of the SSLCC variability and deepens our understanding of coastal circulation dynamics.
The wind forcing from southern Sri Lanka (53%), equatorial (30%), and Bay of Bengal (BoB, 17%) is responsible for the westward south Sri Lanka coastal current (SSLCC) The local wind forcing induces the cyclonic wind stress curl along the south coast of Sri Lanka, directly facilitating the westward SSLCC The equatorial (BoB) forcing affects the westward SSLCC via the westward propagation of upwelling Rossby wave in the first-second (first) baroclinic modes