Duration of El Niño and La Niña events : mechanisms and multiyear predictability
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El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability and affects global weather patterns via atmospheric teleconnections. Both warm (El Niño) and cold (La Niña) phases of the ENSO usually begin in boreal spring-summer and peak near the end of the calendar year. About two-thirds of El Niño and half of La Niña events terminate in the following year, but other events persist for another year or longer, prolonging the climate impacts. These long-lasting ENSO events, however, are not predicted in the current operational ENSO forecasts, which are generally limited to 12 months. This study investigates the mechanisms controlling the diverse duration of ENSO events and its multiyear predictability. Analyses of observational data and the Community Earth System Model, version 1 (CESM1), show that the leading factor controlling the event duration is the onset timing for El Niño and the amplitude of preceding warm event for La Niña. El Niño events developing in boreal spring-summer usually terminate after the peak due to early arrival of delayed negative oceanic feedback and fast adjustments of the tropical Atlantic and Indian Oceans to the tropical Pacific warming, whereas those developing after summer tend to last for an additional year. La Niña events preceded by a strong warm event usually persist into the second year because of large initial discharge of the equatorial oceanic heat content and delayed interbasin adjustments. Variability external to ENSO dynamics also contributes to the diversity of event duration. Motivated by previous modeling studies that show potential predictability of multiyear La Niña events preceded by a strong warm event, the predictability of El Niño duration is tested through idealized CESM1 experiments. The model successfully predicts the duration of El Niño events when initialized with oceanic conditions in the onset months. The predictability is attributed to the timing of delayed negative oceanic feedback and interbasin adjustments, in agreement with the diagnostic analyses. To further explore the predictability of ENSO event duration in the real world, we conduct multiyear ensemble CESM1 forecasts initialized with observed oceanic conditions on March 1, June 1, and November 1 of each year during 1954–2015. The CESM1 shows high prediction skills of event duration with lead times ranging from 6 to 25 months. The predictability of event duration arises from initial thermocline depth anomalies in the equatorial Pacific, as well as sea surface temperature anomalies within and outside the tropical Pacific. The forecast error growth of event duration, on the other hand, originates mainly from atmospheric variability over the North Pacific in boreal winter. The high predictability of event duration indicates the potential for extending the operational ENSO forecasts for up to two years.