UC San Diego

Mixing from turbulence is key to the distribution of oxygen, salt, and heat in the ocean. Climate models which do not appropriately represent this mixing cannot accurately interpret present or future climate. Topographic features with steep slope on the ocean bottom are sites of significant energy conversion from the oscillating tide to internal waves. Such sites can also host intense turbulence and reportedly are the primary source of deep ocean mixing. In this research, We investigate the internal wave dynamics, and magnitude and spatial distribution of turbulence at realistic topographies as well as isolated model obstacles using three-dimensional, high-resolution numerical simulations to develop physical parameterizations of conversion and dissipation rates in the near-field. Direct Numerical Simulations (DNS) and Large Eddy Simulation (LES) are performed on model ridges and realistic ocean topographies to investigate the effect of topographic and flow properties such as Reynolds number (Re), excursion number (Ex) and criticality (e) on internal wave fields, turbulence mechanisms and energy budget terms. These simulations close the energy budget, match with observations and illustrate significant local energy loss generated from mechanisms including Lee waves breaking during flow reversal, downslope jets, critical slope boundary layer, internal wave beams, off-slope lee-wave breaking, and valley flows.\

The physical scales of processes driving mixing during the internal waves generation in the ocean spans several orders of magnitude from the outgoing low-mode internal tide (vertical scale of order 1 km, horizontal of order several tens of km, time of order hours) to the nonlinear formation of higher wavenumber modes to, finally, turbulence events with spatial scale of order meters and time scale of order minutes. This range of scales poses a severe constraint on realistic simulations. I am involved in development of a comprehensive multiscale tool with a novel hierarchical approach that combines Large Eddy Simulation (LES) at small scales with the Stratified Ocean Model with Adaptive Refinement (SOMAR) for the large scales. These simulations are used to assess the accuracy of inferred estimates of turbulent dissipation using density overturn-based methods. This method is commonly used by oceanographer due to the complexity and cost of direct microstructure measurements. Result of this work have shown bias in the magnitude of dissipation at locations with high convectively-driven turbulence. To address this, we have introduced an alternative density overturn-based model for such situation.