Mechanisms linking multi-year La Niña with preceding strong El Niño

El Nino-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Nina often persists for more than two years, called "multi-year La Nina". Observational records show that multi-year La Nina tends to accompany strong El Nino in the preceding year, but their physical linkage remains unclear. Here we show using reanalysis data that a strong El Nino excites atmospheric conditions that favor the generation of multi-year La Nina in subsequent years. Easterly wind anomalies along the northern off-equatorial Pacific during the decay phase of the strong El Nino are found crucial as they act to discharge ocean heat content (OHC) via an anomalous northward Ekman transport. The negative OHC anomaly is large enough to be restored by a single La Nina and, therefore, causes another La Nina to occur in the second year. Furthermore, analyses of the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models support the abovementioned mechanisms and indicate that the occurrence frequencies of multi-year La Nina and strong El Nino are highly correlated.