Surface Mixing by Geostrophic Flows in the Bay of Bengal

Abstract: Mixing in the Bay of Bengal, driven by altimetry derived daily geostrophic surface currents, is studied on subseasonal timescales. Hovm{\"o}ller and wavenumber-frequency diagrams with power spectra confirm the multiscale nature of the flow. Advection of bands immediately brings out the chaotic nature of mixing in the Bay via repeated straining and filamentation of the tracer field. A principal finding is that mixing is local, i.e., of the scale of the eddies, and does not span the entire basin. Indeed, Finite Time Lyapunov Exponent (FTLE), Relative Dispersion (RD) and Finite Size Lyapunov Exponents (FSLE) maps in all seasons are patchy with minima scattered through the interior of the Bay. Non-uniform stirring of the Bay is reflected in long tailed histograms of FTLEs, that become more stretched for longer time intervals. Quantitatively, advection for a week shows the mean FTLE lies near 0.15-0.16 $day^{-1}$, while extremes reach almost 0.5 $day^{-1}$. Averaged over the Bay, RD initially grows exponentially, this is followed by a power-law at scales between approximately 100 and 250 $km$, which finally transitions to an eddy-diffusive regime. These findings are confirmed by FSLEs; in addition, below 250 $km$, a scale dependent diffusion coefficient is extracted, while above 250 $km$, eddy-diffusivities range from $6 \times 10^3$ - $10^4$ $m^2/s$. Finally, with satellite salinity data, these Lagrangian tools are used in the analysis a single post-monsoonal fresh water mixing event. Here, FTLEs and FSLEs allow the identification of transport barriers, and elucidate how eddies help preserve the identity of fresh water.