A combination mode of the annual cycle and the El Niño/Southern Oscillation

The El Niño/Southern Oscillation (ENSO) is characterized by two main states: El Niño events defined by positive sea surface temperature anomalies in the eastern tropical Pacific Ocean and La Niña events marked by cooler surface temperatures in the same region. ENSO is broadly considered to be an oscillatory instability of the coupled ocean–atmosphere system in the tropical Pacific1–6 that shows a tight interaction with the seasonal cycle. El Niño events typically peak in the boreal winter, but the mechanism governing this phase synchronization7 is unclear. Here we show, using observational data and climate model experiments, that the nonlinear atmospheric response to combined seasonal and inter-annual sea surface temperature changes gives rise to a near-annual combination climate mode with periods of 10 and 15 months. Specifically, we find that the associated southward shift of westerly wind anomalies during boreal winter and spring triggers the termination8 of large El Niño events. We conclude that combination mode dynamics and related shifts in western tropical Pacific rainfall patterns occur most prominently during strong El Niño events. Present ENSO theories, such as the recharge oscillator paradigm9, although very successful in explaining some observational features of ENSO, do not account for the interaction between interannual and seasonal timescales. In particular, they do not provide any insight into why strong El Niño events peak towards the end of the calendar year (in December–January–February: DJF) and terminate in the subsequent months. Previous extensions to these theories that rely on nonlinear concepts10–13, such as subharmonic frequency locking and frequency entrainment, nonlinear resonance, the quasi-periodic transition to chaos or parametric excitation7,14,15, capture some aspects of ENSO/annual cycle interactions. None of these extended dynamical systems’ concepts, however, describes the observational finding16 that a weakening and southward shift of westerly wind anomalies on the Equator accompanies the termination of strong El Niño events. Numerous modelling studies8,17–20 have confirmed this observational evidence, thus supporting the notion of strong annual cycle/ENSO interactions originating in the tropical western Pacific. Here, we set out to provide a simple unifying dynamical framework to understand various aspects of ENSO, such as the physics of seasonally paced El Niño transitions, spectral characteristics and ENSO’s hydroclimatic impacts. The seasonal weakening and southward shift of westerly wind anomalies that contributes to the transition between El Niño