Influence Of Intraseasonal Oscillation On The Asymmetric Decays Of El Ni?o And La Ni?a

In this paper,observations and reanalysis data from 1979 to 2016 are used to analysis the influence of intraseasonal oscillation over the western Pacific on the asymmetric decays of El Ni?o and La Ni?a.The main conclusions are as follows:The synthesis analysis of Ni?o3 index display that the decay speed of El Ni?o is significantly greater than the decay speed of La Ni?a.The composite Ni?o3 index of El Ni?o crosses the zero line and turns into negative valuesquickly.However,during the La Ni?a decay phase the negative SSTA is much slower.The averaged decay rate of El Ni?o Ni?o3 index is 0.24?/month,while that of La Ni?a is only 0.15?/month,indicating an evident asymmetry in decay speed between El Ni?o and La Ni?a events.To explore the physical mechanism responsible for this asymmetric decay speed,the asymmetric features of atmospheric circulation over the tropical Western Pacific(WP)in El Ni?o and La Ni?a mature-to-decay phases are analyzed.It is found that the zonal wind anomaly in the equatorial WP has significant asymmetry.By calculating the variances of the interannual and intraseasonal components of the outgoing longwave radiation(OLR)as well as the 850 hPa zonal wind anomalies,it is found that the interannual variances of OLR and 850 hPa zonal wind anomalies over the tropical WP during El Ni?o(La Ni?a)mature-to-decay phase are much stronger(weaker)than the intraseasonal variances.It is argued that the enhanced(weakened)intraseasonal oscillation during El Ni?o(La Ni?a)tends to favor a stronger(weaker)interannual variation of the atmospheric wind,resulting in asymmetric tropical WP zonal wind anomalies between El Ni?o and La Ni?a decay phases.The analysis of SSTA anomaly field and circulation field in the equatorial Pacific region shows that this difference in intraseasonal oscillations are due to the distribution of SSTA anomalies in the equatorial Pacific region during El Ni?o and La Ni?a.The negative(positive)SSTAs in the equatorial WP and positive(negative)SSTAs in the equatorial central and eastern Pacific during El Ni?o(La Ni?a)weaken(strength)the latitudinal gradient of SST and produce an anti-Walker(Walker)circulation anomaly.The weakening(strengthing)of the original Walker circulation weakens(strength)the updraft in the equatorial WP region and inhibits(actives)the development of convective activities in the region.Results of numerical experiments demonstrate that the asymmetric zonal wind stress anomalies during El Ni?o and La Ni?a decay phase can lead to asymmetric decay speed of SSTA in the central-eastern equatorial Pacific by stimulating equatorial Kelvin waves.The largest negative anomaly over the Ni?o3 region caused by the zonal wind stress anomalies during El Ni?o can reach three times of the positive Ni?o3 SSTA anomalies during La Ni?a,indicating that the stronger zonal wind stress anomalies over the tropical WP play important role on the faster decay speed during El Ni?o.