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Stratified and Stirred: Monsoon Freshwater in the Bay of Bengal

Gualtiero Spiro Jaeger, Ph.D., 2019
Amala Mahadevan, Advisor

Submesoscale ocean dynamics can play a critical role in setting the ocean’s surface boundary layer thickness and density stratification, and contribute to tracer dispersal. These dynamics are investigated in the Bay of Bengal with hydrographic observations and numerical simulations, motivated by the upper ocean’s potentially coupled interactions with Monsoon winds and convection. We analyze passive tracer distributions (spice ≡ density-compensated temperature (𝑇) and salinity (𝑆) anomalies) based on over 9000 vertical profiles of 𝑇 and 𝑆 measured along ∼4800 km of ship tracks. Observations in the surface mixed layer reveal ∼1 km scale-selective  -𝑆 compensation reducing cross-front density gradients by ∼50%. This is caused by surface cooling over restratified fronts slumped by submesoscale instabilities. In the stratified interior, a spectral analysis of horizontal spice variance statistics reveals that at scales <10 km, in the shallowest pycnocline, passive tracer power spectra are redder (𝑘−3) than predicted by quasi-geostrophic or frontogenetic theories. Complimentary observations reveal patterns of spice, stratification, and sometimes shear, coherent over at least 30 km with multiple, parallel, ∼10 m thin layers crossing isopycnals. We suggest that both submesoscale sheared eddies, and thin near-inertial waves, form such layers, potentially enhancing horizontal mixing by shear dispersion.