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

The El Nino/Southern Oscillation (ENSO) is characterized by two main states: El Nino events defined by positive sea surface temperature anomalies in the eastern tropical Pacific Ocean and La Nina 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 Pacific(1-6) that shows a tight interaction with the seasonal cycle. El Nino events typically peak in the boreal winter, but the mechanism governing this phase synchronization(7) 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 termination(8) of large El Nino events. We conclude that combination mode dynamics and related shifts in western tropical Pacific rainfall patterns occur most prominently during strong El Nino events.