Warm and cold phases of El Niño-Southern Oscillation (ENSO) exhibit a significant asymmetry in their decay speed. Generally, El Niño tends to turn into a La Niña event in the following June-July after its mature phase; however, the negative sea surface temperature anomaly (SSTAs) associated with La Niña events can persist for more than one year after peaking, resulting in a longer duration than that of El Niño.
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Source: Phys.org