Asymmetric Efficacies Between Warm and Cold Pacific Meridional Modes in Inducing ENSO

This study investigates boreal spring events of Pacific Meridional Mode (PMM) from 1950 to 2022, revealing that cold PMM is more effective in triggering subsequent La Ni & ntilde;a compared to warm PMM's induction of following El Ni & ntilde;o. This asymmetry stems from the varying origins and sub-efficacies of PMM groups. The cold PMM is primarily initiated by pre-existing La Ni & ntilde;a, while the warm PMM is comparably activated by pre-existing El Ni & ntilde;o and internal atmospheric dynamics. PMMs initiated by pre-existing El Ni & ntilde;o or La Ni & ntilde;a play a crucial role in determining the efficacies of PMMs in triggering subsequent El Ni & ntilde;o-Southern Oscillation (ENSO). The strong discharge of pre-existing El Ni & ntilde;o hampers warm PMM's induction of subsequent El Ni & ntilde;o, whereas weak recharge from pre-existing La Ni & ntilde;a enhances the efficacy of cold PMM in inducing subsequent La Ni & ntilde;a. Comprehending not only the PMM phase but also its origin is crucial for ENSO research and prediction. This study investigated the efficacies of warm and cold Pacific Meridional Mode (PMM) events in triggering El Ni & ntilde;o and La Ni & ntilde;a events from 1950 to 2022. Contrary to previous beliefs, the research concludes that cold PMM are more adept at inducing La Ni & ntilde;a. The varying efficacies are linked to the fact that cold PMM are primarily initiated by preceding La Ni & ntilde;a occurrences, while warm PMM are comparably activated by the subtropical atmospheric internal dynamics and previous El Ni & ntilde;o events. Due to the weaker ocean heat content recharge associated with pre-existing La Ni & ntilde;a compared to the discharge associated with pre-existing El Ni & ntilde;o, La Ni & ntilde;a-induced cold PMM encounters less competition from tropical discharge-recharge processes in inducing a subsequent La Ni & ntilde;a. In contrast, El Ni & ntilde;o-activated warm PMM faces stronger competition in inducing an El Ni & ntilde;o. Consequently, the distinct origins of cold and warm PMM phases, along with their competition with tropical discharge-recharge processes, contribute to their respective efficacies in inducing El Ni & ntilde;o and La Ni & ntilde;a. The cold phase of Pacific Meridional Mode (PMM) has a higher efficacy in inducing following La Ni & ntilde;a than warm PMM in inducing El Ni & ntilde;o Disparate efficacies arise from distinct origins of the two PMMs and their varied competition with tropical discharge-recharge processes Cold/warm PMM, induced by a previous La Ni & ntilde;a/El Ni & ntilde;o, encounters weak/strong competition from recharge/discharge in triggering El Ni & ntilde;o-Southern Oscillation