  Freshwater from rivers influences Indian summer monsoon rainfall and regional
tropical cyclones by shallowing the upper layer and warming the subsurface
ocean in the Bay of Bengal. Here, we use in situ and satellite data with
reanalysis products to showcase how river water can experience a significant
increase in salinity on subseasonal timescales. This involves the trapping and
homogenization of freshwater by a cyclonic eddy in the Bay. Specifically, in
October 2015, river water is shown to enter a particularly long-lived eddy
along with its attracting manifolds within a period of two weeks. The eddy
itself is quite unique in that it lasted for 16 months in the Bay where average
lifespans are of the order of 2-3 months. This low salinity water results in
the formation of a highly stratified surface layer. In fact, when freshest, the
eddy has the highest sea-level anomalies, spins fastest, and supports strong
lateral gradients in salinity. Subsequently, observations reveal progressive
homogenization of salinity and relaxation of sea-level anomalies and salinity
gradients 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 on the order of timescale of a month. This pathway
is distinct from vertical mixing and likely to be important in the eddy-rich
Bay of Bengal.
