Kelvin-Helmholtz turbulence: how much mixing does it generate in the Equatorial Oceans?

Oregon State University, Corvallis OR

Investigators

Abstract

Kelvin-Helmholtz (KH) instability is the simplest form of shear instability, and is arguably the most common mechanism for mixing in the oceans. Despite its ubiquity, much remains unknown about KH instability and the turbulence it generates, including the fundamental question of what determines the amount of mixing accomplished. Two constraints limit our ability to answer this question. First, much of what is known comes from lab experiments and direct numerical simulations (DNS) at scales smaller than are typical of the ocean. Second, observations are sparse due to the high resolution of the required measurements. In recent years both of these constraints have been relaxed. Petascale computing now offers the opportunity to execute DNS at greatly increased Reynolds number, while the introduction of long-term, moored measurements via temperature dissipation pods provides extended records that both identify KH turbulence and quantify the resultant mixing. Overall, this project will improve our understanding of an important class of ocean mixing processes, deliver new parameterizations for use in observational data analysis and, ultimately, climate forecasts, and complement ongoing NSF-funded studies of mixing in stratified shear flows. An existing Ocean DNS code will be upgraded and distributed for community use, and a postdoctoral scientist schooled in turbulence theory and numerical simulation will be trained in observational techniques and data analysis and provided with the opportunity to take part in seagoing research. The main objective of this project is to document the mixing properties of turbulent Kelvin-Helmholtz billows, leading to parameterizations of the net turbulent fluxes. New DNS will be used in combination with existing and ongoing measurements to learn how turbulence statistics relevant to ocean mixing change as the Reynolds number is increased, and to predict the energy released in a mixing event, the dissipation rate, and the effective diffusivity. The results will be used to create a multi-decade record of mixing in the equatorial Pacific, and will serve as a model for application to the Atlantic and to all near-critical stratified shear flows.