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 we show that the actual transports are not weak, and investigate whether the discrepancy is due 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 air-sea interaction. Including that additional element of curl forcing greatly improves the realism of the Sverdrup representation, however the magnitudes of the equatorial transport are still too small by a factor of about two. While the nonlinear (advective and friction) terms are virtually negligible in the model momentum balance, they are order (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 eastward jet and the westward off-equatorial South Equatorial Current.
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Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
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