Submesoscale instabilities and the forward energy cascade in seamount wakes

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Patterns of energy dissipation and of mixing of heat, freshwater, dissolved gases and other tracers affect their transport in the global ocean. These patterns also affect upward motions (upwelling) in the deep branches of the deep-ocean motions. Seamounts can be sites of prominent mixing of waters with distinct buoyancy and of energy dissipation in the global ocean, but recent results suggest that the amount of energy that goes to mixing and dissipation depends heavily on processes that develop at scales between hundreds and thousands of meters (submesoscale instabilities). These processes may be critical in constraining the role of seamounts in ocean energetics and motion. The proposed research will investigate the dynamics of flows past seamounts with high-resolution mathematical simulations, to examine the role seamounts play in the global vertical motions, that is, the overturning circulation. General circulation models currently approximate only a limited subset of these processes and do not correctly represent the effects of the transfer of energy from thousands of meters to meters. This project will investigate the problem in a wide range of realistic configurations to examine how the patterns of mixing and energy dissipation change, depending on environmental and seamount characteristics. This knowledge will inform future approximations that can eventually include these small-scale impacts in global and climate simulations. The numerical tools will be readily available in the open-source community package 'Oceananigans,' which promotes reproducibility of results and provides these tools to other projects and other researchers. The project supports the postdoctoral work of an early career researcher. Submesoscale instabilities extract energy from the balanced flow field and generate a forward cascade to turbulent mixing. The energy pathways for this forward cascade in turbulent wakes are not currently known, nor is the parameter dependence of the energy flux and the nature of the secondary instabilities that mediate the transition to turbulence. The work proposed here will address three potentially transformative questions: Q1: What processes control the forward energy cascade in submesoscale wakes? Q2: How do submesoscale instabilities impact the integrated (along topography and in the wake) dissipation and mixing for flow encountering seamounts? Q3: How well are the energetics of the submesoscale-mediated energy cascade around seamounts represented in General Circulation Models? These questions will be addressed in an idealized numerical domain with turbulence-resolving nonhydrostatic Large-Eddy Simulations (LES) using Oceananigans, which is an open-source, flexible fluid dynamics solver that uses a finite volume discretization based on that of MITgcm. Two options for LES turbulence closures are available, with which the project PIs will investigate the subgrid-scale model dependence. The LES approach differs from Reynolds-Averaged approaches (RANS - which most other recent work on related topics has used) in that LES allows the relevant scales of the energy to be resolved, including the vital role of centimeter-scale turbulence in the forward energy cascade. A small number of complementary RANS simulations will also be performed to evaluate how accurately turbulence closures represent topographic wake turbulence using the GCM CROCO, which is based on a formulation of the Regional Oceanic Modeling System (ROMS). CROCO uses a topography-following sigma- coordinate with a higher order pressure gradient force scheme to minimize errors at steep topographic features and has a variety of available BBL turbulence closures, making it well-suited for investigations of flow-topography interaction problems. 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.