Eastward surface jets in the central equatorial Pacific, November 1991–March 1992

Abstract

The central equatorial Pacific typically has westward surface and near-surface flow, interrupted by periods of eastward flow. The processes that may have been responsible for two periods of eastward flow during the 1991-92 El Nino between October 1991 and March 1992 are explored here, based on realistic hindcasts from a high-vertical resolution ocean general circulation model. The model balances indicate that the November-December 1991 surface trapped jet is fully nonlinear, and both wind stress and horizontal pressure gradient forcing are significant. The January-February 1992 jet is less strongly nonlinear. Both jets are strongly affected by local and remote westerly wind event forcing. Linear dynamics are not consistent with the model balances. We note that the mid-November to early December 1991 jet plausibly created the vertical salinity gradient that was observed in late December 1991. Although this salinity gradient can contribute to the maintenance of a vertically sheared eastward flow, it does not appear to have been fundamental in the creation of the two jets observed during this period. Accurate sampling of this region during periods of high variability requires frequent measurements. Local dynamics arrays with high vertical resolution current profiles, from the surface to beneath the Equatorial Undercurrent, are critical to the advancement of our understanding of equatorial dynamics, to evaluating the skill of our numerical models, and to accurate simulation and state estimation for this fascinating area of the ocean.