Variability of mixed layer turbulence in the Bay of Bengal during a MISO event: an LES study

Hieu T Pham, University of California San Diego, Mechanical and Aerospace Engineering, La Jolla, CA, United States, Sutanu Sarkar, Univ California at San Diego, La Jolla, CA, United States, Leah Johnson, Brown University, Providence, RI, United States, Qing Li, Los Alamos National Laboratory, Los Alamos, NM, United States, Baylor Fox-Kemper, Brown University, Earth, Environmental and Planetary Sciences, Providence, RI, United States and Peter P Sullivan, National Center for Atmospheric Research, Mesoscale Microscale Meteorology, Boulder, CO, United States
During an event of Monsoon Intra-seasonal Oscillations (MISO) in the Bay of Bengal, the vertical heat transport in the mixed layer is influenced by the strong modulations of rainfall, wind stress and surface heat fluxes. The heat transport is also affected by the variability of mixed layer depth (MLD) in the Bay, e.g., a shallower MLD due to the large freshwater influx in the northern part of the Bay. Large-eddy simulations (LES) are performed to contrast the heat budget in two different configurations of the mixed layer: (1) a deep mixed layer representative of the southern part of the Bay, and (2) a shallow mixed layer capped by a barrier layer with thermal inversion, a condition that is typically observed in the northern part of the Bay. Both model configurations are forced by the observational data collected during an MISO event which includes periods of weak and strong wind, episodic rain and diurnally varying heat fluxes. The effect of Langmuir turbulence is also included in the LES model. Under the condition of steady wind and strong warming flux, a diurnal warm layer with enhanced shear and stratification develops during morning periods in both cases. The enhanced shear descends toward the bottom of the mixed layer during the day and causes a burst of turbulence in the late afternoon. Enhanced shear and turbulent mixing are also observed after a period of rainfall. In the case of the shallow mixed layer, when the wind burst is sufficiently strong, the mixed layer penetrates through the barrier layer and exposes the warmer water to the surface. Turbulence and the associated transport of heat across the ocean boundary layer will be contrasted quantitatively between the two configurations.