Tropical and Subtropical Pacific Sources of the Asymmetric El Niño‐La Niña Decay and Their Future Changes

El Niños (EN) are known to decay more rapidly, while La Niñas (LN) tend to decay more slowly. Observational analyses and coupled model experiments are conducted to show that sea surface temperature (SST) anomalies over the subtropical northeastern Pacific (SNEP) and equatorial western Pacific (EWP) are key factors to determine the decay pace of the EN and LN and their asymmetry. In the present climate the LN produces larger cold SST anomalies over the regions than the warm SST anomalies produced by the EN. The magnitude difference over the SNEP and EWP helps to slow down the LN decay via subtropical footprinting and tropical thermocline variation mechanisms, respectively. Coupled Model Intercomparison Project Phase 6 models project the magnitude differences of SNEP SST anomalies between EN and LN to reduce in the future warming world, causing the asymmetric EN‐LN decay to weaken. Plain Language Summary There are obvious asymmetric features in mature winter and decaying summer between the warm (El Niño [EN]) and cold phases (La Niña [LN]) of El Niño‐Southern Oscillation (ENSO). Cold sea surface temperature (SST) anomalies over the subtropical northeastern Pacific (SNEP) and equatorial western Pacific (EWP) in winter are larger in amplitude during LN than EN. This asymmetric feature in winter enables the LN to delay slower and last longer than the EN in the following summer, which is another asymmetric feature between these two phases of the ENSO. The large cold anomalies over the SNEP can spread to the equatorial Pacific to slow down the LN decay through subtropical ocean‐atmosphere coupling processes. The large cold anomalies over the EWP also contribute to the slow decay of LN but mainly through tropical ocean‐atmosphere coupling processes. Climate models project that the future global warming may reduce the magnitude differences of the SNEP SST anomalies between EN and LN and cause a rapid decline of LN in a warmer world. Key Points The slow La Niña and rapid El Niño decays are caused by differences in their anomalies over the subtropical and tropical Pacific The subtropical (tropical) anomalies activate a seasonal footprinting (thermocline variation) mechanism to affect the decay pace The asymmetric decay is projected to decrease in the future warming world due to the reduction of the subtropical anomaly difference