Warm and Cold Episodes in Western Pacific Warm Pool and Their Linkage With ENSO Asymmetry and Diversity

Abstract

AbstractThis study investigates the role of temperature anomalies over the western Pacific warm pool (WPWP) in the asymmetry and diversity of El Niño‐Southern Oscillation. Based on observations, we select 12 cold and 13 warm WPWP episodes averaged over July to October. Using composite analyses, we reveal that the occurrence probability of El Niño events is much larger following the cold episodes than that of La Niña events related to warm episodes. The main reason for the different frequencies is that the westerlies associated with cold episodes are stronger and located eastwards than the easterlies for warm episodes, exerting more effective influences on the sea surface temperature in central‐eastern equatorial Pacific through Bjerknes feedback. As for intensity, the El Niño events tend to be stronger when the cold anomalies over WPWP are stronger, evidenced by both observations and Coupled Model Intercomparison Project Phase 6 simulations. The warming rate from July to October for El Niño events following cold episodes is three times greater than that of other historical events, denoting a prominent contribution from WPWP to the quick development of El Niño. Furthermore, we highlight the implication of WPWP on El Niño diversity with a greater probability of EP‐type El Niño under a colder WPWP state and successive westerly wind bursts. As such, the cold state of WPWP can provide an effective modification for the earlier operational prediction on El Niño intensity and flavors. On the contrary, the warm WPWP state may not be useful for future La Niña alerts.