Wavelet-based wavenumber spectral estimate of eddy kinetic energy: Application to the North Atlantic

Abstract

An ensemble of eddy-rich North Atlantic simulations is analyzed, providing estimates of kinetic energy wavenumber spectra and spectral budget. A wavelet transform technique is used to estimate a localized `pseudo-Fourier' spectrum, permitting comparisons to be made between spectra at different locations in a highly inhomogeneous and anisotropic environment. We find evidence of a Gulf Stream imprint on the near Gulf Stream eddy field appearing as enhanced levels of energy in the North-South direction relative to the East-West direction. Surprisingly, this signature holds into the quiescent interior. We detect forward cascades of energy and enstrophy but find no clear evidence of upscale energy cascades in the separated Gulf Stream region. The spectral slopes inferred from our analysis are significantly steeper than expected from quasi-geostrophic theory, but roughly in line with a primitive equation extension of the enstrophy inertial-range theory. Lastly, we propose that the spectral shapes are somewhat universal throughout our domain, over a broad wavenumber range. Deviations from this structure occur at high wavenumbers in locations characterized by strong surface fronts. A summary conclusion is that expectations built on quasi-geostrophy are at best only weakly supported in primitive equations.

Imprint of chaos on the ocean energy cycle from an eddying North Atlantic ensemble

“…analyzing ocean ensembles with higher spatial resolution (currently under production; Uchida et al 2023b, their Supplementary Material) to better resolve the effect of eddy dynamics on the energy cycle. In the context of climate, our framework is extendable straightforwardly to fully-coupled climate ensemble simulations (e.g.…”

“…One of the difficulties in quantifying the impact of mesoscale eddies lies in the identification of the eddies themselves (Wunsch 1981), which exist in a soup of anisotropic and inhomogeneous flows (Uchida et al 2021c(Uchida et al , 2023b. For practical reasons, eddies have often been defined via a Reynold's decomposition about a spatial and/or temporal coarse graining (e.g.…”