El Nino phase-dependent high-frequency variability in Western Equatorial Pacific

The intensities of high-frequency (HF) variability with period less than 90 days at different phases of El Nino 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 Nino 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 Nino. The essential role of large-scale environmental conditions in modulating the HF variability during the two episodes is further confirmed by modeling experiments.