Stirring and mixing driven by mesoscale eddies in the stratified Bay of Bengal

Abstract

The stirring of passive tracers driven by altimetry-derived daily surface geostrophic currents is studied on subseasonal timescales in the Bay of Bengal. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e., of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre-and post-monsoonal seasons. The spatially non-uniform stirring at the surface of the Bay is reflected in long-tailed probability density functions of Finite-Time Lyapunov Exponents (FTLEs), which become more stretched for longer time intervals. Quantitatively, advection for a week shows that mean FTLEs lie between 0.13$\pm$0.07 day$^{-1}$, while extremes reach almost 0.6 day$^{-1}$. Averaged over the Bay, relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale-dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from $6 \!\times \!10^3$ $\!-\!$ $1.6\times 10^4$ m$^2$s$^{-1}$ in different regions of the Bay. These estimates provide a useful guide for resolution-dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay.\\

A particularly important tracer field in the Bay is the sea surface salinity; indeed, freshwater from rivers influences Indian summer monsoon rainfall and tropical cyclones by stratifying the upper layer and warming the subsurface ocean in the Bay of Bengal. We use {\it in situ} and satellite data with reanalysis to showcase how river water experiences a significant increase in salinity on sub-seasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Using a specific example from 2015, river water is shown to enter an eddy along its attracting manifolds within a period of two weeks. This leads to the formation of a highly stratified subsurface layer within the eddy. When freshest, the eddy has the largest sea-level anomaly, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal a progressive increase in salinity inside the eddy within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity.\\

The eddy-freshwater interaction, or adjustment, is then studied using a high-resolution Regional Ocean Modeling System. Apart from lateral advection, a mixed layer salinity budget shows the importance of ageostrophic vertical advection during the evolution of salinity within the eddy. An analysis of the depth-integrated eddy kinetic energy indicates the development of both barotropic and baroclinic instabilities. The vertical profile associated with these conversion terms reveals that the surface freshwater was likely involved in developing baroclinic terms in the mixed layer. In addition, an eddy available potential energy (EPE) budget suggests that the entrainment of the river water raises the EPE, which is reflected in the development of gradients in salinity within the eddy. The EPE is lowered with homogenization, signifying irreversible mixing. Further, EPE rates are modulated by the correlation of buoyancy fluxes with density anomalies, which involves lateral advection of freshwater associated with surface cooling and local, regional rainfall. Finally, the adjustment of this freshwater eddy triggers submesoscale dynamics that appear to be an integral part of salinity homogenization. The observation and reanalysis data also showcase the presence of these events across different years, thus bringing out the broader impact of mixing freshwater into high salinity ambient water by eddies in the Bay. This pathway is distinct from vertical diffusive mixing and is likely to be important for the evolution of salinity in the Bay of Bengal.