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El Nino is frequently followed by La Nina, but the opposite case rarely happens. Here we explore a mechanism for such an asymmetrical transition and its future changes. Internally, the asymmetrical response of upper ocean waves against surface wind stress anomaly exerts a primary cause of El Nino-Southern Oscillation (ENSO) transition asymmetry. Externally, the asymmetrical capacitor effects of both Indian and Atlantic Oceans play some roles in driving the ENSO transition asymmetry via the interbasin interactions. The historical runs of Coupled Model Intercomparison Project Phase 5 show that the intermodel transition asymmetry is significantly correlated with the intermodel asymmetry in ocean wave response to surface wind forcing but not with that in the interbasin interactions. In addition, the El Nino-to-La Nina transition tendency was weaker in moderate global warming scenario runs (Representative Concentration Pathway 4.5) while slightly enhanced in strong warming scenario runs (Representative Concentration Pathway 8.5). Similar changes also appeared in the asymmetrical response of ocean waves against the surface wind forcing.

Plain Language Summary El Nino and its opposite phase, La Nina, are known as an oscillatory phenomenon, and so sometime in the past La Nina was considered as a mirror image of El Nino. However, El Nino and La Nina are not symmetric. In particular, El Nino is frequently followed by La Nina in the following year, while the opposite case rarely happens. Here we showed that the stronger response of ocean waves against surface wind during El Nino than during La Nina plays a primary cause of a dominant El Nino-to-La Nina transition tendency. In addition to such internal Pacific process, externally the interbasin interactions with both Indian and Atlantic Oceans also play some roles in driving the transition asymmetry. However, the twentieth century climate simulation obtained from the Coupled Model Intercomparison Project shows that the model-to-model difference in transition asymmetry is significantly related to that in ocean wave response to surface wind forcing but not with that in the interbasin interactions. In addition, the dominant El Nino-to-La Nina transition tendency appeared in twentieth century climate simulation and observation data become weaker in the climate model simulations under moderate global warming scenario while slightly enhanced in those under strong warming scenario.