Sverdrup and nonlinear dynamics of the Pacific equatorial currents

Abstract

The Sverdrup circulation in the tropical Pacific is constructed from satellite scatterometer winds, compared with measured ocean currents, and diagnosed in an ocean GCM. Previous depictions of the Sverdrup circulation near the equator have shown only weak vertically integrated flows; here it is shown that the actual transports are not weak. This discrepancy could be due either to inaccuracies in the wind forcing or to Sverdrup dynamics being too simple in this region. Scatterometer winds show a strip of positive curl along the SST front north of the equator in the eastern Pacific that is due to wind speed changes induced by the front. Including that additional element of curl forcing greatly improves the realism of the Sverdrup representation, but the magnitudes of the equatorial transport are still too small by a factor of about 2. Although the nonlinear (advective and friction) terms are small in the model momentum balance, they are O(1) in the vorticity balance, especially because their meridional derivatives are large near the equator. Examining the effect of the nonlinear terms through the vorticity balance shows that advection acts to intensify the mean currents of the tropical Pacific, including both the Equatorial Undercurrent and the westward off-equatorial South Equatorial Current. However, the principal non-linearity is due to the acceleration and deceleration of the Equatorial Undercurrent, not to meridional convergence as has been previously argued.