A Near-Annual Pacific Ocean Basin Mode

Some fairly regular and nearly annual variability in the equatorial Pacific after the major 1997/98 El Niño event is studied. Sea level, sea surface temperature (SST), and surface wind anomalies of this variability are tied together in a way similar to the slow cycles of El Niño–Southern Oscillations (ENSO). Despite a slightly longer-than-annual time scale, similar variability was also found in the datasets prior to 1997/98. This fastcoupled mode is superimposed on the slow 3–5-yr ENSO cycles in the tropical Pacific. It contributed to the occurrences of some minor El Niño and La Niña events. Zonal currents associated with the equatorial waves play a dominating role in generating SST anomalies of this fast mode through anomalous zonal advection. It is suggested that this fast mode may be best understood as a coupled Pacific Ocean basin (POB) mode. The existence of this fast mode, which appears independent of the slow ENSO mode, has important implications in understanding and predicting the tropical Pacific SST anomaly.