Eddy-freshwater Interaction using Regional Ocean Modeling System in the Bay of Bengal

TL;DR

This study uses high-resolution regional ocean modeling to analyze how mesoscale eddies trap and homogenize freshwater from rivers in the Bay of Bengal, revealing complex dynamics of salinity and energy budgets over a sub-monthly timescale.

Contribution

It provides the first detailed simulation of eddy-freshwater interactions in the Bay of Bengal, highlighting the roles of vertical advection, instabilities, and energy budgets in freshwater evolution.

Findings

Eddies trap and homogenize river water within weeks.

Vertical advection and instabilities drive salinity and energy changes.

Reanalysis data confirm the occurrence of trapping events across years.

Abstract

Eddy-freshwater interaction is studied in the north Bay of Bengal (BoB) with a high-resolution simulation using the Regional Ocean Modeling System. Following observations, the model simulates the trapping and homogenization of river water by a cyclonic mesoscale eddy on a sub-monthly time scale from October-November of 2015. As fresh river water is trapped in the eddy, it is characterized by strong vertical and lateral gradients in salinity. Within a few weeks, these gradients relax along with the progressive homogenization of freshwater within the eddy. A mixed layer salinity budget shows the importance of ageostrophic vertical advection in addition to lateral advection during the evolution of salinity within the eddy. An analysis of the eddy kinetic energy (EKE) budget in the upper ocean indicates the development of barotropic and baroclinic instabilities. The vertical profiles of EKE conversion terms reveal that the surface freshwater was involved in the evolution of baroclinic instability within the mixed layer. In addition, an eddy available potential energy (EPE) budget shows that the entrainment of the river water raises the EPE, which is due to an increase in lateral salinity gradients across the eddy during the trapping event. Subsequently, the salinity homogenization leads to a decrease in the EPE, and its rate of decay is modulated by a correlation between surface buoyancy fluxes and density anomalies. Finally, reanalysis data show similar trapping and homogenization events across multiple years, highlighting the importance of this mechanism of subseasonal freshwater evolution in the BoB.