Multiple Influences on Equatorial Atlantic Variability

The equatorial Atlantic hosts a zonal mode of variability that is associated with signif-icant precipitation anomalies over tropical South America and Africa. Warm and cold events are referred to as Atlantic Niños and Niñas, respectively, due to their apparent similarity with El Niño-Southern Oscillation (ENSO) events. Like ENSO, the Atlantic zonal mode is thought to involve a positive feedback mechanism, in which weakening of the equatorial easterlies, thermocline deepening, and SST warming mutually rein-force each other. In the present study we use NCEP reanalysis, a high-resolution OGCM hindcast, and a coupled GCM to re-examine the dynamics governing Atlantic Ni-ños. We find that equatorial wind stress forcing and ENSO-like dynamics can explain some of the observed warm events but not all of them. In particular there are warm events that occur despite easterly surface wind anomalies in the preceding months. This is due to sub-surface warm anomalies at approximately 5N that are advected toward the equator. The sub-surface warming is linked to downwelling wind stress curl anomalies north of the equator. These, in turn, are related to SST anomalies in the northern tropical Atlantic (NTA), which induce weakening of the trade winds in the northern tropical Atlantic. Our results suggest an indirect pathway through which NTA SST anomalies can influence the zonal mode of variability.