We perform Large-eddy simulations (LES) to explore how the variability of turbulent heat transport affects mixed layer properties during an event of summer Monsoon Intra-seasonal Oscillations (MISO) in the Bay of Bengal. Unlike most past LES studies, these simulations use realistic air-sea fluxes collected during the summer 2018 MISO-BOB field campaign to examine the roles of wind-driven, convective and Langmuir turbulence on the vertical heat transport. Wind acceleration is accompanied with heavy rainfall and substantial surface cooling during the active period of the event while, during the break period, the wind relaxes and the solar heat flux intensifies. The variability of turbulent heat transport in the mixed layer is examined with two different background conditions: a barrier layer (BL) with and without a thermal inversion layer (TIL). The mixed layer depth (MLD), sea surface temperature (SST) and sea surface salinity (SSS) are modulated by the air-sea fluxes and the thermal inversion layer at multiple timescales. The MLD deepens the fastest during the near-inertial oscillations induced by a wind burst during the active phase. Strong solar heat flux during the break phase creates a diurnal warm layer (DWL) which traps the solar heat flux near the surface. Episodes of strong precipitation causes the MLD to shoal and the SSS to decrease for a few hours. Comparison of simulations with and without Langmuir turbulence illustrates the importance of Langmuir dynamics in modulating mixed layer properties. Simulations with 1-D mixing models show a wide disparity in the evolution of MLD, SST and SSS.

Publications:

1. Pham, H. T., S. Sarkar, L. Johnson, B. Fox-Kemper, P. P. Sullivan, and L. Qing, “Multi-scale variability of turbulent mixing during a Monsoon Intraseasonal Oscillation in the Bay of Bengal: an LES study,” Journal of Geophysical Research: Oceans, 128, e2022JCO18959 (2023).

2. Johnson, L., B. Fox-Kemper, Q. Li, H. T. Pham, and S. Sarkar, “A finite-time ensemble method for mixed layer model comparison,” J. Phys. Oceanogr., 53, 2211-2230 (2023).