MOMS: a Minimal Ocean Mixing System

Despite occurring on centimeter and smaller scales, turbulent stratified mixing in the ocean affects the circulation at basin scales. Much work has been devoted to understanding how to express the efficiency of the process in terms of local turbulent parameters. Recent work though suggests that the physics of stratified mixing cannot be abstracted from the larger context in which it occurs. In this project, laboratory experiments will be conducted in a tank large enough, and suitably forced, that it may approximate more faithfully the conditions observed in the ocean, a Minimal Ocean Mixing System (MOMS). The laboratory measurements will provide a way to test theoretical and practical ways to represent mixing in computer models. Stratified turbulent mixing in a similar environment is also present in the atmosphere, and this research will therefore have a spill-over effect in atmospheric sciences. The present measurements and numerical simulations will also constitute a new dataset which can be used as a training set for Artificial Intelligence algorithms reduced order models and data-assimilation techniques which require a substantial knowledge of the flow for their validation. Finally, reducing uncertainties of irreversible mixing using the present research will improve the predictive power of general circulation models at climate scales. The PI will make every effort to advertise the postdoc position broadly so to reach candidates that traditionally have been marginalized. The PI will also work with the local chapter of SCIREN, an organization that connects STEM researchers with local teachers to improve the quality and scientific literacy of youths.

It is clear that the recent work surveyed shows a shift in the focus from a purely local characterization of stratified mixing (e.g., the quest for parameterizations of mixing efficiency based on the buoyancy Reynolds number) to a view that sees mixing as the end product of a dynamic that starts at much larger scales and is (potentially) much more intertwined than previously recognized. Experimentally, this requires rethinking the problem of stratified mixing to include a more natural way to force the problem, and a domain sufficiently large to accommodate both the anisotropic and the isotropic part of the spectrum, a MOMS. The dataset collected in these experiments and numerical simulations will therefore, constitute a novel way to test and validate theoretical arguments to model the energy exchanges in stratified turbulent flows across the anisotropic / isotropic divide. In addition, this research aims to provide a link between vertically/horizontally towed instruments and planar / volume-resolved measurements. This project also intends to establish a link between near-ocean conditions combining theoretical with technical challenges at unprecedented scales for which the UNC Joint Fluids lab is designed. In addition, the Craya-Herring framework combined with the structure functions methods will provide the state-of-the-art theoretical foundations to analyze the role of the turbulence where intermittency remains a key open question to analyze and interpret field measurements, since mixing events are rarely observed nor statically converged in field measurements of strongly stratified turbulence.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.