M. D. Thomas, A. M. deBoer, H. L. Johnson and D. P. Stevens, Journal of Physical Oceanography, in press.
Sverdrup balance underlies much of the theory of ocean circulation and provides a potential tool for describing the interior ocean transport from only the wind stress. Using both a model state estimate and an eddy permitting coupled climate model we here assess to what extent and over what spatial and temporal scales Sverdrup balance describes the meridional transport. We find that Sverdrup balance holds to first order in the interior subtropical ocean when considered at spatial scales greater than approximately 5 degrees. Outside of the subtropics, in western boundary currents and at short spatial scales, significant departures occur due to failures in both the assumptions that there is a level of no motion at some depth and that the vorticity equation is linear. Despite the ocean transport adjustment occurring on time scales consistent with the basin crossing times for Rossby waves, as predicted by theory, Sverdrup balance gives a useful measure of the subtropical circulation after only a few years. This is because the inter-annual transport variability is small compared to the mean transports. The vorticity input to the deep ocean by the interaction between deep currents and topography is found to be very large in both models. These deep transports, however, are separated from upper layer transports that are in Sverdrup balance when considered over large scales.
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