The decadal mean ocean circulation and Sverdrup balance

Abstract

Elementary Sverdrup balance is tested in the context of the time-average of a 16-year duration time-varying ocean circulation estimate employing the great majority of global-scale data available between 1992 and 2007. The time-average circulation exhibits all of the conventional major features as depicted both through its absolute surface topography and vertically integrated transport stream function. Important small-scale features of the time average only become apparent, however, in the time-average vertical velocity, whether near the surface or in the abyss. In testing Sverdrup balance, the requirement is made that there should be a mid-water column depth where the magnitude of the vertical velocity is less than 10[superscript -8]m/s (about 0.3 m/year displacement). The requirement is not met in the Southern Ocean or high northern latitudes. Over much of the subtropical and lower latitude ocean, Sverdrup balance appears to provide a quantitatively useful estimate of the meridional transport (about 40% of the oceanic area). Application to computing the zonal component, by integration from the eastern boundary is, however, precluded in many places by failure of the local balances close to the coasts. Failure of Sverdrup balance at high northern latitudes is consistent with the expected much longer time to achieve dynamic equilibrium there, and the action of other forces, and has important consequences for ongoing ocean monitoring efforts.