El Nino-Southern Oscillation (ENSO) is an inter-annual coupled ocean-atmospheric phenomenon that influences global weather pattern. In the ocean, the El Nino event begins with the eastward propagation of the warm pool which is built up on the western side of the Pacific basin for several years by the trade wind. In the ENSO event in 2014-2016, a warm pool with a record high sea surface temperature anomoly is captured by the TAO mooring arrays as it propagates eastward along the equator. The mooring data shows sudden changes in temperature, salinity, and currents in the upper ocean surface over a short period of a few months. The variability is diverse including the effects of eastward propagating Kelvin waves and westward propagating Tropical Instability Waves (TIW). In this project, we use combination of observational data collected at the TAO moorings and high-resolution Large-eddy Simulations (LES) to quantify the variability of flow conditions in the upper ocean surface layer during the ENSO event with a focus on turbulence physics. The turbulent heat flux in the upper ocean layer is quantified in the models to illustrate the role of deep-cycle turbulence as the ocean responses to ENSO dynamics.

Some preliminary studies that I have contributed to this project are:

(1) Comparative analysis of late-afternoon and night-time bursts of DC turbulence. The DC turbulence is often found to occur in multiple bursts. The first burst is triggered by a descending shear layer induced by surface wind stress and stabilizing surface heat flux in the late afternoon. In contrast, the subsequent bursts occur when the surface heat flux is destabilizing and the wind shear in the mixed layer is significantly weaker. We find that the convective turbulence while significantly weaker than wind-driven shear turbulence still can enhance the shear rate in the marginally unstable layer. As a result, local shear instability develops to give rise to the bursts of DC turbulence during night-time.

(2) Parametric study of DC turbulence: A series of LES are performed to examine how the turbulent mixing varies with the background shear and wind stress. The DC turbulence becomes stronger with increasing shear and wind stress. As the shear and wind stress increase, the lifespan of the bursts shortens, e.g. a larger number of bursts occurs over night-time period.

(3) Mixing rate of Kelvin-Helmholtz (KH) shear instability in a uniformly stratified ambience at high Reynolds number: Since the DC turbulence is found to be initiated by local shear instabilities, I performed DNS to quantify turbulent mixing due to a KH instability. The instability induces a mixing efficiency that depends on background Richardson number (e.g. stratification). For a Richardson number between 0.08 and 0.16, the mixing effciency is approximately 0.32 which is higher than the value of 1/6 typically used in GCMs. The efficiency decreases for smaller Richardson numbers. For larger Richardson number, the KH instability does not develop. 

Publications:

1. VanDine, A., H .T. Pham, and S. Sarkar, "Investigation of LES models for a stratied shear layer, "Computers and Fluids, 198, 104405 (2020).

2. VanDine, A., H. T. Pham, and S. Sarkar, "Turbulent shear layers in a uniformly stratified background: DNS at high Reynolds number," J. Fluid Mech., 916, A42 (2021).

3. W. D. Smyth, S. J. Warner, J. N. Moum, Pham, H. T., and S. Sarkar, "What controls the Deep Cycle? Proxies for equatorial turbulence," J. Phys. Oceanogr., 51, 2291-2302 (2021).

4. Pham, H. T., and S. Sarkar, “A comparative study of turbulent stratified shear layers: Effect of density gradient distribution,” Environmental Fluid Mechanics (2022).

5.   Pham, H. T., S. Sarkar, W. D. Smyth, J. N. Moum, and S. J. Warner, “Deep-cycle turbulence in the upper Pacific Equatorial Ocean: Characterization by LES and heat flux parameterization,” J. Phys. Oceanogr., , 54, 577-599 (2024).

6. Schmitt, M., H. T. Pham, K. Klingbeil, S. Sarkar, and L. Umlauf, “Diurnal Warm Layers in the ocean: Energetics, non-dimensional scaling and parameterization,” J. Phys. Oceanogr., 54, 1037-1055 (2024)