Lagrangian modeling of Aquarius surface salinity in the Bay of Bengal

Abstract:

Monsoonal freshwater runoff into the Bay of Bengal makes it one of the freshest oceans and results in a large spatial gradient in surface salinity (8-10 psu over 8-10 degrees longitude). Our aim is to understand dispersal and mixing of freshwater in the Bay of Bengal, its underlying mechanisms, and the spatial and temporal patterns of variability using Aquarius salinity data. Lateral stirring by the mesoscale flow field is a dominant mechanism for freshwater dispersal in this region. We track the satellite-observed salinity along water parcel trajectories calculated from satellite-derived surface velocity fields. By diagnosing the change in salinity along water parcel trajectories, we disentangle the effects of mesoscale lateral advection from other processes affecting the salinity, including evaporation-precipitation, vertical mixing, and smaller scale lateral dispersal. We attempt to relate the along-trajectory rate of change to flow characteristics such as strain rate, as well as to surface fluxes. Though Aquarius salinity measurements are limited in their spatial resolution, we find satellite along-track lateral salinity gradients to be representative of ship-based measurements at spatial scales greater than 9 km. However, the lack of reliable salinity and velocity fields near the coast makes it difficult to capture the largest salinity gradients. Our results show that during the early monsoon, water entering the Bay from the Arabian Sea at 6 degrees N freshens at a rate of about 0.1 psu/day. During the late monsoon, when freshwater is observed moving north to south along the western boundary of the Bay, other processes cause that fresh water mass to increase in salinity at a rate of about 0.4 psu/day. We provide an estimate for the rate at which unresolved processes dissipate lateral salinity gradients generated by the stirring of fresh and salty water.