Abstract The persistence of multi-year La Niña events (MLa Niñas) drives catastrophic climate anomalies worldwide, yet their initiation and maintenance mechanisms remain poorly understood. The conventional recharge/discharge theory effectively explains MLa Niñas preceded by strong El Niño events (SE-MLa Niñas), but fails to account for MLa Niñas occurring without preceding strong El Niño influences (NSE-MLa Niñas). Here we show, through integrated observational analyses and climate model simulations, that the Pacific meridional mode (PMM) acts as a key driver for the initiation and maintenance of NSE-MLa Niñas. Unlike SE-MLa Niñas, which rely on El Niño-induced oceanic memory, NSE-MLa Niñas are sustained by the PMM-related subtropical air–sea coupling processes, notably through an intensified wind–evaporation–sea surface temperature feedback mechanism as quantitatively diagnosed in this study. Future projections indicate that under global warming scenarios, the frequency of NSE-MLa Niñas may increase more relative to SE-MLa Niñas, possibly due to the enhanced PMM variability. These findings deepen our understanding of ENSO dynamics by uncovering subtropical processes as critical drivers of persistent La Niña conditions, with profound implications for advancing ENSO prediction systems.
Shi et al. (Tue,) studied this question.