Asymmetry Between Positive and Negative Phases of the Pacific Meridional Mode: A Contributor to ENSO Transition Complexity

The Pacific Meridional Mode (PMM) plays a critical role in affecting El Nino-Southern Oscillation (ENSO). This study examines the phase asymmetry of PMM events triggered by tropical and extratropical forcings, namely successive and stochastic events, respectively. It is shown that successive events exhibit negative asymmetry due to stronger trigger in the negative phase, while stochastic events display positive asymmetry due to stronger growth in the positive phase. The opposite phase asymmetry of two types of events respectively results in more frequent persistent La Nina events than El Nino events and more frequent episodic El Nino events than La Nina events, which increase ENSO transition complexity. This research provides a comprehensive understanding of PMM asymmetry and reconciles conflicting perspectives from previous studies. Additionally, the newly proposed contribution of positively asymmetric stochastic PMM events to more frequent episodic El Nino events in this study may enhance our comprehension of ENSO transition complexity.