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 Niño‐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 Niña events than El Niño events and more frequent episodic El Niño events than La Niña 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 Niño events in this study may enhance our comprehension of ENSO transition complexity. Plain Language Summary In this study, we investigated the influences of the Pacific Meridional Mode (PMM), an important modulator of El Niño‐Southern Oscillation (ENSO), on the diverse transition preference of ENSO events (ENSO transition complexity). We first divided PMM events into two types that are respectively triggered by tropical and extratropical forcing, and named them as successive and stochastic PMM events. Successive PMM events tend to enhance persistency of ENSO events, whereas stochastic PMM events incline to initiate ENSO events from a neutral state. We found that the successive PMM events are negatively asymmetric because the triggering effect of tropical forcing is stronger in the negative phase. This negative asymmetry leads to longer persistency of La Niña events than El Niño events. On the contrary, the stochastic PMM events are positively asymmetric because the growth rate of PMM is stronger in the positive phase. This positive phase asymmetry of stochastic PMM events results in more frequent episodic El Niño events than La Niña events. This study is the first to show the contribution of positively asymmetric stochastic PMM events to higher frequency of episodic El Niño events, which may supplement our understanding of ENSO transition complexity. Key Points Pacific Meridional Mode (PMM) events can be divided into tropical‐forced successive events and extratropical‐forced stochastic events Successive P