Asymmetry of Atmospheric Circulation Anomalies over the Western North Pacific between El Niño and La Niña*

The asymmetry of the western North Pacific (WNP) low-level atmospheric circulation anomalies between the El Niño and La Niña mature winter is examined. An anomalous WNP cyclone (WNPC) center during La Niña tends to shift westward relative to an anomalous WNP anticyclone (WNPAC) center during El Niño. Two factors may contribute to this asymmetric response. The first factor is the longitudinal shifting of El Niño and La Niña anomalous heating. The composite negative precipitation anomaly center during La Niña is located farther to the west of the composite positive precipitation anomaly center during El Niño. The westward shift of the heating may further push the WNPC westward relative to the position of the WNPAC. The second factor is the amplitude asymmetry of sea surface temperature anomalies (SSTAs) in the WNP, namely, the amplitude of local cold SSTA during El Niño is greater than that of warm SSTA during La Niña. The asymmetry of SSTA is originated from the asymmetric SSTA tendencies during the ENSO developing summer. Although both precipitation and surface wind anomalies are approximately symmetric, the surface latent heat flux anomalies are highly asymmetric over the key WNP region, where the climate mean zonal wind speed is small. Both the anomalous westerly during El Niño and the anomalous easterly during La Niña in the region lead to an enhanced surface evaporation, strengthening (weakening) the enhancement of the cold (warm) SSTA in situ during El Niño (La Niña). The asymmetry of the SSTA in the WNP is further amplified due to anomalous wind differences between El Niño and La Niña in their mature winter. Atmospheric general circulation model experiments demonstrate that both factors contribute to the asymmetry between the WNPAC and WNPC. The asymmetric circulation in the WNP contributes to the asymmetry of temporal evolutions between El Niño and La Niña.