Multiyear ENSO Dynamics as Revealed in Observations, Climate Model Simulations, and the Linear Recharge Oscillator

El Nino-Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multiyear ENSO. However, the dynamical processes are not well understood. This study aims to elucidate the unified mechanisms of multiyear ENSO using observations, phase 6 of the Coupled Model Intercomparison Project (CMIP6) models, and the theoretical linear recharge oscillator (RO) model. We found that multiyear El Nino and La Nina events are roughly symmetric except for cases of multiyear La Nina following strong El Nino. The composite multiyear ENSO reveals that anomalous ocean heat content (OHC) in the equatorial Pacific persists beyond the first peak, stimulating another event. This prolonged OHC anomaly is caused by meridional Ekman heat transport counteracting geostrophic transport-induced recharge-discharge process that otherwise acts to change the OHC anomaly. A meridionally wide pattern of sea surface temperature anomalies observed during multiyear ENSO is responsible for the Ekman heat transport and multiple factors such as decadal variability, subtropical processes, and ENSO diversity modulate the ENSO meridional structure. CMIP6 multimodel ensemble shows a significant correlation between the ENSO meridional width and the occurrence ratio of multiyear ENSO, supporting the aforementioned mechanism. A multiyear ENSO-like oscillation was simulated using the linear RO model that incorporates a seasonally varying Bjerknes growth rate and a weak recharge efficiency representing the effect of Ekman transport. When the recharge efficiency parameter was estimated using reanalysis data based on geostrophic transport alone, a multiyear ENSO rarely occurred, confirming the importance of Ekman transport in retarding the recharge-discharge process.

Automated summary

This study aims to uncover the unified mechanisms of multiyear El Nino-Southern Oscillation (ENSO) using observations, CMIP6 models, and the RO model. The study finds that multiyear El Nino and La Nina events are roughly symmetric, with the exception of multiyear La Nina events following strong El Nino events. Anomalous ocean heat content (OHC) in the equatorial Pacific persists beyond the first peak, stimulating another ENSO event. The meridionally wide pattern of sea surface temperature anomalies observed during multiyear ENSO is responsible for the Ekman heat transport, and multiple factors modulate the ENSO meridional structure.