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

The central equatorial Pacific typically has westward surface and near-surf ace flow, interrupted by periods of eastward flow. The processes that may h ave 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 reali stic hindcasts from a high-vertical resolution ocean general circulation mo del. The model balances indicate that the November-December 1991 surface tr apped jet is fully nonlinear, and both wind stress and horizontal pressure gradient forcing are significant. The January-February 1992 jet is less str ongly nonlinear. Both jets are strongly affected by local and remote wester ly wind event forcing. Linear dynamics are not consistent with the model ba lances. We note that the mid-November to early December 1991 jet plausibly created the vertical salinity gradient that was observed in late December 1 991. Although this salinity gradient can contribute to the maintenance of a vertically sheared eastward flow, it does not appear to have been fundamen tal in the creation of the two jets observed during this period. Accurate s ampling 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 crit ical to the advancement of our understanding of equatorial dynamics, to eva luating the skill of our numerical models, and to accurate simulation and s tate estimation for this fascinating area of the ocean.