Weakly Penetrative Mixing in the Surface Layer of the Bay of Bengal

Abstract:

The first microstructure measurements under the ASIRI program were conducted in Bay of Bengal (BoB) during November 2014 in a region bounded by 16.95-16.25 N and 86.7- 87.0 E. Measurements were taken during the research cruise of r/v Roger Revelle using the VMP profiler down to ~ 140 m to infer the kinetic energy dissipation rate eps, temperature T, salinity S, and potential density anomaly sigT.

Very strong stratification was observed just below a thin (less than 15-20 m deep) vertically homogeneous turbulent surface layer (SL), which effectively decoupled the former from the underlying thermohalocline. The upper boundary of the near-surface pycnocline was at z ~ 10 - 11 m and its lower boundary at z ~ 15 m. Under moderate (11 - 12 m/s) winds, the dissipation in the SL gradually decreased from the sea surface down to 10^-6 – 10^-8 W/kg at depths z ~ 10 - 15 m. Below z ~ 15 m, the dissipation rate sharply dropped to ~10^-9 W/kg, remaining approximately constant downward. The horizontal/temporal differences of T, S and sigT in the middle of SL were as high as dT ~ 0.25 C, dS ~ 0.4 psu, and dsigT ~ 0.22, respectively, over a distance of ~ 2 km.

On the next day, under higher winds (~ 16 - 18 m/s), the SL deepened only slightly, being nominally decoupled from the pycnocline. The intensification of wind stress from ~ 0.16 to ~ 0.51 N/m2, led to an increase of eps to 10^-6 – 10^-7 W/kg across the entire mixing layer. The enhanced turbulence not only produced vertical mixing, but also initiated horizontal stirring, thus dramatically reducing thermohaline differences in the mixed SL down to dT ~ 0.017C, dS ~ 0.02 psu, and dsigT ~ 0.008, respectively. However, very strong stratification in the pycnocline suppressed the wind-induced vertical mixing, which did not penetrate below z ~ 22 - 25 m. No internal sources of turbulence were evident in the water interior, suggesting that under mild or even relatively strong but short-sustained winds small-scale dynamics of the SL and that of the pycnocline are effectively detached from each other. Internal wave radiation and their breaking below the pycnocline appear to be damped by local dynamics. Near-surface turbulence of this genre, which produces weakly-penetrative vertical mixing but effective horizontal stirring could be a characteristic feature of the BoB dynamics in the regions away from strong local frontal zones.