What Controls the Duration of El Niño and La Niña Events?

Interannual warming and cooling of the equatorial Pacific associated with the El Niño-Southern Oscillation (ENSO) phenomenon affect weather patterns worldwide. Previous studies showed that El Niños tend to terminate after maturing near the end of the calendar year while La Niñas often persist through the following year, exacerbating their climatic impacts. The duration of individual El Niño and La Niña events, however, varies greatly from this average evolution. To understand dynamical processes important for the duration of El Niño and La Niña events, we analyze a suite of observational data, as well as two long control simulations of the Community Earth System Model, version 1 (CESM1) and Community Climate System Model, version 4. Observational and model analyses both show that El Niño and La Niña events lasting only one year tend to onset earlier following weaker events of opposite phase, compared to multi-year events. The earlier onset of events leads not only to earlier arrival of delayed negative oceanic feedback but also to faster adjustments of the tropical Atlantic and Indian Oceans to the equatorial Pacific sea surface temperature (SST) anomalies. The faster SST adjustments in the adjacent oceans decrease anomalous interbasin SST gradient towards the equatorial Pacific and weaken surface wind anomalies over the western equatorial Pacific in early boreal spring following the peak of El Niño and La Nina, leading to event termination. The relation between onset timing and event duration is particularly strong for El Niño. Perfect model experiments using CESM1 support this finding and show that the duration of El Niño is highly predictable when the model is initialized at the onset of El Niño events.