Energetics of the global ocean: The role of layer-thickness form drag

Understanding the role of mesoscale eddies in the global ocean is fundamental to gaining insight into the factors that control the strength of the circulation. This paper presents results of an analysis of a high-resolution numerical simulation. In particular, the authors perform an 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. For the first time in oceanic studies, the global distribution of layer-thickness form drag is determined. This study provides direct evidence to verify some basic characteristics of layer-thickness form drag that have often been assumed or speculated about in previous theoretical studies. The results justify most of the previous assumptions and speculations, including those associated with (i) the presence of an oceanic energy cycle explaining the relationship between layer-thickness form drag and wind forcing, (ii) the manner in which 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 the analysis disagrees with speculation in previous studies that the layer-thickness form drag in the Antarctic Circumpolar Current is the agent that transfers the wind-induced momentum near the sea surface downward to the bottom layers. The authors present a new interpretation: the layer-thickness form drag reduces (and thereby cancels) the vertical shear resulting from the eddy-induced overturning circulation (rather than the vertical shear resulting from the surface wind stress). This interpretation is consistent with the results of the energy analysis conducted in this study.