Dynamics of Summer Monsoon Currents around Sri Lanka

TL;DR

This study uses a high-resolution ocean model to analyze the complex dynamics of the summer monsoon current around Sri Lanka, highlighting the roles of eddies, Rossby waves, and wind forcing in its evolution.

Contribution

It provides a detailed classification of the SMC's stages and elucidates the mechanisms driving its variability, including eddy activity and wave interactions.

Findings

Eddy activity is significant in the SMC region, involving barotropic and baroclinic instabilities.

Rossby waves and eddies energize and accelerate the SMC through eddy-potential-vorticity flux.

Baroclinic instability releases available potential energy, leading to SMC meanders.

Abstract

$ $From June--September, the summer monsoon current (SMC) flows eastward south of Sri Lanka and bends northeastward to form a swift jet that enters the Bay of Bengal (BoB). As such, it is a crucial part of the water exchange between the Arabian Sea (AS) and BoB. The processes that determine the evolution, intensification and meandering of the SMC are only partly understood. They involve both local and remote forcing by the wind, as well as interactions with westward-propagating Rossby waves and eddies. In this study, we investigate these processes using an Indian-Ocean general circulation model (MOM4p1) that is capable of simulating the SMC realistcally. Because eddies and meanders are smoothed out in the climatology, our analyses focus on a single year of 2009, a period when a strong anticyclonic bend in the SMC was observed. An eddy-kinetic-energy budget analysis shows the region to be a zone of significant eddy activity, where both barotropic and baroclinic instabilities are active. Based on the analysis, we classify the evolution of SMC into stages of onset, intensification, anticyclonic bend, anticyclonic vortex formation, meandering and termination. In addition, analysis of eddy-potential-vorticity flux and eddy-enstrophy decay reveal when, where, and how the eddies tend to drive the mean flow. Rossby waves and westward-propagating eddies arriving from the east energize the SMC in June and accelerate the mean flow through an up-gradient eddy-potential-vorticity flux. At the same time, local winds also strengthen the flow, by increasing its mean, near-surface, kinetic energy and raising isopycnals, the latter building up available potential energy (APE). The baroclinic instability that takes place in late--July and early--August releases APE, thereby generating the SMC meanders.