Wave-topography interaction and impact on oceanic kinetic energy distribution

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

The impact of variable topography on Rossby waves and mesoscale eddies is a basic question in geophysical fluid dynamics, and topography can alter the vertical structure and phase speed of Rossby waves and nonlinear eddies. The vertical structure of eddies in turn impacts the vertical structure of ocean eddy kinetic energy (EKE) and hence horizontal stirring. Therefore, understanding the impact of topography on the propagation (phase speed) and vertical structure of geostrophic motions is of global significance. Specifically, eddy phase speed has been shown to modulate eddy fluxes of heat and salt, so understanding the phase speed and structure of eddies is directly relevant to the development of improved eddy parameterizations for coarse-resolution ocean models. Also, satellite altimetry provides accurate, global measurements of mesoscale EKE at the ocean surface, but no way to extrapolate to depth. An improved understanding of the vertical structure of oceanic eddies is necessary to estimate sub-surface EKE from readily available surface observations. The research will investigate the impact of variable topography with scales of 10-100km on geostrophically balanced motions, e.g. Rossby waves and nonlinear mesoscale eddies. The code for computing the Rossby wave modes with topography will be made publicly available, along with the algorithm for projecting surface EKE estimates to depth. This project will support an early-career researcher in his first independent study. The researcher is a Pacific Science Center Science Communication Fellow, and regularly participates in "Meet a Scientist" outreach programs. The proposed work will complement this ongoing outreach efforts. Bottom topography with wavelengths on the order of 10-100km is nearly ubiquitous in the ocean, appearing on the flanks of mid-ocean ridges and near seamounts. Previous idealized theoretical and modeling studies have shown that topography on these scales can impact the vertical structure and propagation speed of linear Rossby waves and eddies. The impact of topography in more realistic cases, including nonlinear dynamics in a stratified ocean with realistic topography, remains to be examined. Further, the dependence on important parameters, including topographic amplitude, topographic wavelength, bottom friction, and stratification, has not been fully explored. Finally, the topographically altered vertical structure of nonlinear eddies may have an important impact on the turbulent energy cascade and barotropization. This is an analytical and modeling study to systematically explore these effects. Specific goals are to (1) determine whether previous linear solutions with one-dimensional topography carry over to the nonlinear regime with two-dimensional topography in a quasigeostrophic (QG) numerical model, (2) examine the vertical structure of eddies in historical current-meter records and an ocean general circulation model (OGCM) to determine the extent to which topography has an observable impact on mesoscale eddy vertical structure, and (3) study the impact of topography on the turbulent energy cascade and energy dissipation, through its effect on the vertical structure of currents.