WHOI PO

Title: Surface stirring in the stratified Bay of Bengal

Abstract: 

Stirring of passive tracers in the Bay of Bengal driven by altimetry-derived daily geostrophic surface currents is studied on sub-seasonal timescales. To begin with, Hovm{\"o}ller plots, wavenumber-frequency diagrams, and power spectra confirm the multiscale nature of the flow. 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. Indeed, Finite Time Lyapunov Exponent (FTLE) and Finite Size Lyapunov Exponent (FSLE) maps in all the seasons are patchy, with minima scattered through the interior of the Bay. Further, these maps bring out a seasonal cycle wherein rapid stirring progressively moves from the northern to the southern Bay during pre- and post-monsoonal periods. The non-uniform stirring of the Bay is reflected in long-tailed probability density functions of FTLEs that become more stretched for longer time intervals. Quantitatively, advection for a week shows the mean FTLE lies between 0.13$\pm$0.07 day$^{-1}$, while extremes reach almost 0.6 day$^{-1}$. Averaged over the Bay, Relative dispersion grows exponentially, followed by a power-law at scales between 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 represent surface stirring in the Bay properly.