Predictability of El Niño Duration in a Coupled General Circulation Model

Previous studies on the predictability and prediction of El Niño have focused on the onset of events. The prediction of the duration of El Niño events, however, has been largely ignored, even though the persistence of events could exacerbate the climate impacts. Diagnostic analysis of both observational data and an 1800-year control simulation of the Community Earth System Model, version 1 (CESM1) suggests that most important factor that affects the El Niño duration is the onset timing. El Niño events that onset early tend to terminate quickly after the mature phase because of early arrival of delayed negative oceanic feedback and fast adjustments of the tropical Atlantic and Indian Oceans to the equatorial Pacific sea surface temperature (SST) anomalies. To test the predictability of El Niño duration based on the onset timing, a set of perfect model experiments are conducted using the CESM1. We select two El Niño events that onset in April and September from the control simulation and conduct 20-member ensemble forecasts for each event by initializing the model with the same oceanic condition in the onset month but with slightly different atmospheric conditions. In agreement with the diagnostic analysis, the majority of El Niño events in the April-initialized forecasts terminate after the mature phase, whereas more than half of El Niño events in the September-initialized forecasts persist into the following year and reintensify in boreal winter. The predictability of the Niño-3.4 SST index, measured by the relative magnitude of the ensemble mean and spread, remains high in fall-winter of the second year for both sets of forecasts, indicating the presence of deterministic processes. The error growth of the forecasts, on the other hand, is related to the processes independent of the El Niño state, particularly coupled ocean-atmosphere variability over the tropical Indian and Atlantic Oceans, as well as stochastic atmospheric variability over the extratropical North Pacific.