Abstract
Understanding the role of mesoscale eddies in the ocean is fundamental to giving insight on the factors that control the strength of the ocean circulation. We present results of an analysis of a high-resolution numerical simulation of the global ocean which was performed on the Earth Simulator of Japan. In particular we perform the analysis of energetics in density space. Such an approach clearly demonstrates the role of layer thickness form drag (residual effects of hydrostatic pressure perturbations) which is hidden in the classical analysis of the energetics of flows.
We determine the global distribution of energy conversion done by layer-thickness form drag for the first time in oceanic studies. Our study provides direct evidence to verify some basic characteristics of layer-thickness form drag which have been often assumed or speculated in previous theoretical studies. Our results justify most of the previous assumptions and speculations, which are those associated with (i) the presence of an oceanic energy cycle explaining the relationship between layer thickness form drag and wind forcing, (ii) the way layer-thickness form drag removes the energy of vertically-sheared geostrophic currents, and (iii) the reason why the work of layer thickness form drag nearly balances the work of eddy-induced overturning circulation in each vertical column. However the result of our analysis disagrees with a speculation in previous studies that the layer-thickness form drag in the Southern Ocean is the agent to transfer downward the wind-induced momentum near the sea sea surface to bottom layers.
The improved understanding of layer-thickness form drag has now led to inclusion of the parameterization of layer-thickness form drag in coarse resolution ocean general circulation models.
Although the thickness-weighted mean formulation used in the present analysis is not much known in oceanic studies, this formulation is analogous to the Favre filtering which is used in the community of LES.
Related Links
- http://iprc.soest.hawaii.edu/people/aiki.html - the first author's home page