The Bay of Bengal (BoB) is a semi-enclosed basin lying between 6-22°N and 80-100°E, connected to the equatorial Indian Ocean to the south. The Ganga-Brahmaputra-Meghna (GBM), Irrawaddy, Godavari, and Mahanadi river systems are the major sources of freshwater to the BoB (Papa et al., 2012). The climatological annual mean discharge from the GBM and Irrawaddy, the largest of these rivers, is ∼8.7 × 10 4 m 3 s −1 and 3.4 × 10 4 m 3 s −1 ; about 70% of discharge comes in the summer monsoon season June-September (Chaitanya et al., 2014;Dai & Trenberth, 2002;Papa et al., 2012). Near-surface salinity has a well-defined seasonal cycle deduced from climatological datasets and measurements along shipping lanes (Rao & Sivakumar, 2003) as well as interannual variability (Pant et al., 2015). The pathways for the export of river water are along both the western and eastern boundary of the Bay. The western pathway is quantified in hydrographic surveys off of India, and Sri Lanka (Lee et al., 2016;Shetye et al., 1996); but the eastern route is not quantified due to a lack of measurements. Some of this freshwater retains its identity for at least 2-3 seasons while being

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Mixing of passive tracers in the Bay of Bengal, driven by altimetry derived daily geostrophic surface currents, is studied on subseasonal timescales. To begin with, Hovmöller plots, wavenumber-frequency diagrams and power spectra confirm the multiscale nature of the flow. Advection of latitudinal and longitudinal 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. Further, FTLE, FSLE and RD maps show that the Bay experiences a seasonal cycle wherein rapid stirring progressively moves from the northern to southern Bay during pre and post monsoonal periods, respectively. The 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 powerlaw at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. These findings are confirmed by FSLEs; in addition, 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 × 10 3 -10 4 m 2 /s. Finally, in concert with satellite salinity data, these Lagrangian tools are used to analyse a single post-monsoonal fresh water mixing event. Here, while stirring the salinity field at large scales, FTLEs and FSLEs allow the identification of transport barriers, and elucidate how individual eddies help preserve the identity of fresh water.

<p><em>Stirring of passive tracers on the surface of the Arabian Sea is studied from a long-term data set comprising AVISO surface geostrophic and Globcurrent on a sub-seasonal and interannual time scale. To begin with, we elucidate the spatiotemporal characterization of surface flow and sea-surface salinity. We then use lagrangian metrics such as Finite-Time Lyapunov exponents (Finite-Size Lyapunov exponents) to characterize the chaotic mixing and its probability density function on a sub-monthly timescale over the basin. Further, the M-function is used to identify and showcase the lagrangian barrier associated with mesoscale eddies and its role in transporting freshwater from the Bay of Bengal to the Arabian Sea during the northeast-monsoon period (November to February).</em></p>

Freshwater from rivers is trapped in a long-lived cyclonic eddy in the north Bay of Bengal during the postmonsoon season.• Observations show a substantial increase in sea surface salinity of trapped freshwater 10 on the timescale of a month. 11• Lagrangian tracer advection elucidates horizontal mixing across the eddy boundary.