Parameterizing Eddy Heat Flux in a 1/10◦ Ocean Simulation: A Wavenumber Perspective (original) (raw)

The skill of an eddy heat flux parameterization based on the downgradient diffusion of temperature is assessed as a function of wavenumber in the Southern Ocean of the 1/10 degree Parallel Ocean Program. Coherence analysis suggests that the meridional eddy heat flux, v T , is coherent with its diffusive parameterization, ∂ y T , on lengthscales larger than 50 • while the divergence of the eddy heat flux, ∇ • u T , is coherent with the Laplacian of mean temperature, ∇ 2 T , on scales less than 10 •. Eddy heat fluxes can be partitioned into rotational and divergent components; only the divergent component influences the heat budget. The curl and divergence of the eddy heat flux provide measures of the rotational and divergent components. At the scales at which most of the eddy heat flux energy occurs the rotational component accounts for more than 95% of the total flux, increasing to more than 99% with increasing lengthscale. As a result, the significant coherence of v T with ∂ y T at large scales is due to the rotational component, and the coherence between ∇ • u T and ∇ 2 T is low since it is associated with very small and noisy features. Diffusivities of about 500 m 2 s −1 were found within the ACC, with highest values of up to 2000 m 2 s −1 in the Aghulas Retroflection area close to the surface. Diffusivities estimated from the eddy heat flux showed a high wavenumber dependence and were as high as 5000 m 2 s −1 , reflecting the presence of the rotational component. The rotational component can be approximated by assuming it to consist of geostrophic flow along temperature contours. However, this rotational component only accounts for a fraction of the total rotational component, leaving a residual that is still more rotational than divergent.