Mechanisms of barrier layer formation and erosion from in situ observations in the Bay of Bengal

During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8N, 89E from 4-14 July, 2016. These observations captured events of barrier layer (BL) erosion and re-formation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10-20 m; a BL below of 30-40 m thickness with similar temperature but higher salinity; a high salinity core layer, associated with Summer Monsoon Current. Each of these three layers was in relative motion to the others, leading to regions of high shear at the interfaces. However, haline stratification overcame the destabilising influence of the shear regions, and preserved the three-layer structure. A salinity budget using in situ observations suggested that during the BL erosioni, high salinity surface waters (34.5 PSU) with weak stratification were advected to the time series location and replaced the three-layer structure with a deep ML (~60 m). Weakened stratification at the time series location also allowed atmospheric wind forcing to penetrate deeper. Turbulent kinetic energy dissipation rate and eddy diffusivity showed elevated values above 10-7 W kg-1 and 10-4 m2 s-1, respectively, in the upper 60 m. Later, the surface salinity decreased again (33.8 PSU) through horizontal advection, stratification became stronger and elevated mixing rates were confined to the upper 20 m, and the BL reformed. A 1-D model analysis suggests that in the study region, advection of temperature-salinity characteristics is essential for the maintenance of the BL and to the extent to which mixing penetrates the water column.