Influence of Intraseasonal Oscillation on the Asymmetric Decays of El Niño and La Niña

Warm and cold phases of El Niño–Southern Oscillation (ENSO) exhibit a significant asymmetry in their decay speed. To explore the physical mechanism responsible for this asymmetric decay speed, the asymmetric features of anomalous sea surface temperature (SST) and atmospheric circulation over the tropical Western Pacific (WP) in El Niño and La Niña mature-to-decay phases are analyzed. It is found that the interannual standard deviations of outgoing longwave radiation and 850 hPa zonal wind anomalies over the equatorial WP during El Niño (La Niña) mature-to-decay phases are much stronger (weaker) than the intraseasonal standard deviations. It seems that the weakened (enhanced) intraseasonal oscillation during El Niño (La Niña) tends to favor a stronger (weaker) interannual variation of the atmospheric wind, resulting in asymmetric equatorial WP zonal wind anomalies in El Niño and La Niña decay phases. Numerical experiments demonstrate that such asymmetric zonal wind stress anomalies during El Niño and La Niña decay phases can lead to an asymmetric decay speed of SST anomalies in the central-eastern equatorial Pacific through stimulating different equatorial Kelvin waves. The largest negative anomaly over the Niño3 region caused by the zonal wind stress anomalies during El Niño can be threefold greater than the positive Niño3 SSTA anomalies during La Niña, indicating that the stronger zonal wind stress anomalies over the equatorial WP play an important role in the faster decay speed during El Niño.