The intensities of high-frequency (HF) variability with period less than 90 days at different phases of El Niño events were investigated through observational data analysis. A large asymmetry in the HF variability intensity between the developing phase and decaying phase (i.e., pre-peak stage versus post-peak stage) of eastern Pacific (EP) El Niño is revealed, while the amplitude and spatial pattern of the sea surface temperature anomaly during these two stages are almost same. The diagnosis shows that the asymmetry is significant not only on intraseasonal time scale (20–90 days) but also on synoptic time scale (less than 20 days). The anatomy analysis further unveils that the asymmetric synoptic variability between the two episodes arises from the asymmetric intensities of the equatorial Rossby and mixed Rossby gravity (MRG) waves. We suggest that the stronger vertical easterly wind shear in the pre-peak stage than that in the post-peak stage plays a vital role in causing the stronger synoptic equatorial Rossby and MRG waves in the pre-peak stage. Meanwhile, the drier atmosphere and more descending motion in the post-peak stage contribute to the weakened intraseasonal and synoptic variabilities in that stage. The aforementioned weakened easterly wind shear, drier atmosphere and more descending motion in the post-peak stage can be traced back to the occurrence of the anomalous anticyclone circulation over the western North Pacific since the decaying phase of El Niño. The essential role of large-scale environmental conditions in modulating the HF variability during the two episodes is further confirmed by modeling experiments.

通过观测数据分析，研究了厄尔尼诺事件不同阶段的小于90天的高频（HF）变异强度。揭示了东太平洋（厄尔尼诺）厄尔尼诺现象的发展阶段和衰减阶段（即峰前阶段与峰后阶段）之间的HF变异强度存在很大的不对称性，而海面温度的振幅和空间格局这两个阶段的异常几乎相同。诊断表明，不对称性不仅在季节内时间尺度（20-90天）上很重要，而且在天气时间尺度上（少于20天）也很重要。解剖学分析进一步揭示，两个事件之间的不对称天气变化是由赤道Rossby波和混合Rossby重力波（MRG）的不对称强度引起的。我们认为，在峰前阶段比在峰后阶段更强的垂直东风切变在导致峰前阶段更强的天气赤道Rossby波和MRG波起着至关重要的作用。同时，峰后阶段的大气干燥和更多的下降运动导致了该阶段季节内和天气变化的减弱。峰后阶段上述减弱的东风切变，较干燥的大气层和更多的下降运动可以追溯到自厄尔尼诺现象以来，北太平洋西部反气旋环流异常。建模实验进一步证实了大规模环境条件在两次发作期间调节HF变异性中的重要作用。