Laboratory and Numerical Experiments on Ocean-scale Turbulent Stratified Mixing

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

This project will use laboratory experiments and numerical modelling techniques to identify the mechanism by which waters in the surface of the ocean are mixed with heavier deeper waters through a process that is called overturning. Water in the world oceans is arranged in nearly-horizontal layers of density increasing with depth. Because of the difference in density, transfer of material between layers (i.e., lifting of denser water and sinking of lighter water) requires an external source of energy and the whole process is called mixing. There is considerable interest in understanding mixing in stratified bodies of water. This process is slow; it is estimated that it takes about 1,000 years to completely overturn the ocean. Yet, since the deep ocean holds large stores of heat and carbon dioxide, the overturning process is important as it regulates climate on the planet over millennial and longer time scales, and thus of great societal interest. Stratified mixing requires a source of energy (stirring), and the aim of this project is to characterize the efficiency of the process. To understand the concept of efficiency, it is convenient to think by analogy to an automobile. The gas burned in the engine releases a certain amount of energy, but only a fraction of it goes into moving the drivetrain. The rest is lost as heat. Likewise, only a fraction of the energy provided by the stirring mechanism goes into doing work against gravity, the rest being dissipated as frictional heat. Direct observations of ocean mixing with increasingly high resolution using, for example, acoustic sensors and fast temperature sensors, have shown surprising structures in shear-driven turbulence that suggest mixing at ocean scales may behave differently than small-scale laboratory measurements have suggested. This project addresses these issues through the study of turbulent mixing in carefully controlled laboratory experiments at scales relevant to oceanic flows and the use of numerical simulations. The experimental component is conducted in the UNC Joint Fluid Lab tank, a 36m long facility in which stratified shear flows with high buoyancy Reynolds number can be generated. This ensures a clear separation between the Kolmogorov and Ozmidov scales. The experiments are designed to establish the behavior of the flux Richardson number (a measure of efficiency), in the limit of high buoyancy Reynolds number and finite gradient Richardson number. The data acquired in the lab are used to train, via data-assimilation, a suite of increasingly complex numerical models, with the final goal of creating tools that can be used to diagnose efficiency from ocean data and to improve the parameterization of stratified mixing in global models.