Turbulence and Instabilities in the Southeastern Bay of Bengal

Abstract:

In 2019, VMP microstructure measurements were taken onboard the R/V Sally Ride in the southeastern Bay of Bengal (BoB) at 8^oN, 89^oE. Here we present preliminary results of these measurements taken along 4 mini-sections (8-11 km each, 86 profiles total). The upper layer stratification in the central southeastern BoB did not show sharp density jump below the surface mixed layer (SML ~ 50 m deep) that is usually associated with a barrier layer, and thus a relatively uniform pycnocline (N² ~ 3.3x10^-4 s^-2) was not decoupled from the SML as it was observed in the northern and western BoB. The cumulative probability distribution of the dissipation rate CDF(ε) in the SML is lognormal. In the pycnocline, CDF(ε) is well approximated by the Burr distribution but with a substantially larger median value 4.3x10^-9 W/kg compared to (3-8)x10^-10 W/kg observed in the northern BoB. Absence of a barrier layer in the southeastern BoB is expected to facilitate vertical exchange between SML and pycnocline, enhancing turbulence in stratified environment. Variations of shear instabilities in SML and pycnocline during a semidiurnal cycle were evaluated using statistics of the gradient Richardson number Ri. The CDF(Ri) pertinent to the pycnocline did not change much during the observational period, while in the SML CDF(Ri) showed a variability associated with diurnal heat flux at the sea surface. The probability of subcritical Ri (< 0.25) for SML was about 50% in the beginning of the night, increasing to ~ 70 % in the morning hours due to nocturnal mixing and decreasing again to ~ 45% when weak re-stratification of SML appear due to daytime heating. The time-dependent shear of drift currents is also a factor in the variation of shear-induced instability in SML.