Variability of Near-Surface Circulation and Sea Surface Salinity Observed from Lagrangian Drifters in the Northern Bay of Bengal During the Waning 2015 Southwest Monsoon

A dedicated drifter experiment was conducted in the northern Bay of Bengal during the 2015 waning southwest monsoon. To sample a variety of spatiotemporal scales, a total of 36 salinity drifters and 10 standard drifters were deployed in a tight array across a freshwater front. The salinity drifters carried for the first time a revised sensor algorithm, and its performance during the 2015 field experiment is very encouraging for future efforts. Most of the drifters were quickly entrained in a mesoscale feature centered at about 16.5 degrees N, 89 degrees E and stayed close together during the first month of observations. While the eddy was associated with rather homogeneous temperature and salinity characteristics, much larger variability was found outside of it toward the coastline, and some of the observed salinity patches had amplitudes in excess of 1.5 psu. To particularly quantify the smaller spatiotemporal scales, an autocorrelation analysis of the drifter salinities for the first two deployment days was performed, indicating not only spatial scales of less than 5 km but also temporal variations of the order of a few hours. The hydrographic measurements were complemented by first estimates of kinematic properties from the drifter clusters, however, more work is needed to link the different observed characteristics.