Western Pacific interannual variability associated with the El Niño‐Southern Oscillation

Abstract

Observations of sea surface temperature (SST), sea level pressure (SLP), surface wind, and outgoing longwave radiation (OLR) show that the El Niño‐Southern Oscillation (ENSO) displays western Pacific anomaly patterns in addition to eastern Pacific anomaly patterns. During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off‐equatorial western Pacific and high OLR anomalies in the off‐equatorial far western Pacific. Also, while the zonal wind anomalies over the equatorial central Pacific are westerly, those over the equatorial far western Pacific are easterly. The nearly out‐of‐phase behavior between the eastern and western tropical Pacific is also observed during the cold phase of ENSO, but with anomalies of opposite sign. These western Pacific interannual anomaly patterns are robust features of ENSO, independent of data sets. It is argued that equatorial easterly (westerly) wind anomalies over the far western Pacific during the warm (cold) phase of ENSO are initiated by off‐equatorial western Pacific cold (warm) SST anomalies, and that these winds are important for the evolution of ENSO. An atmosphere model is employed to demonstrate that small off‐equatorial western Pacific cold (warm) SST anomalies (compared to those in the east) are sufficient to produce equatorial easterly (westerly) wind anomalies as observed over the far western Pacific. The coupled ocean‐atmosphere model of Zebiak and Cane is then modified to investigate the evolution of the western Pacific interannual anomaly patterns in a coupled ocean‐atmosphere system, by including a meridional structure to the subsurface temperature parameterization in the western Pacific. The modified model produces both western and eastern Pacific interannual anomaly patterns.