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Abstract

The amplitude of El Niño/Southern Oscillation (ENSO) is investigated in 40 historical and RCP8.5 runs of the CESM1-CAM5-BGC large ensemble (CESM-LE), which simulates ENSO asymmetry quite realistically. Within the 40 historical runs the ENSO amplitude varies quite substantially. The difference between runs with high and low ENSO amplitude results mainly from the amplitude of Eastern Pacific (EP) El Niños, while the amplitude of Central Pacific (CP) El Niños is comparable between the runs. Further, the asymmetry between El Niño and La Niña events is strongly related to the ENSO amplitude, as strong EP El Niño events are found to dominate both ENSO amplitude and asymmetry. In the RCP8.5 projections the ENSO amplitude increases under global warming, which is related to an increase in the frequency of strong EP El Niño events. Further, also the spread in ENSO amplitude change between the individual members can be explained by their different frequency changes of strong EP El Niños. More frequent strong EP El Niños are related to a projected El Niño-like mean state change, which lowers the convective threshold over the eastern equatorial Pacific and strengthens the wind-SST feedback. A comparison with 360 years-long CESM1 experiments with a -4.0 K colder and +3.7 K warmer mean state indicates that the present-day ENSO may be in a transition zone between a CP El Niño dominated and an EP El Niño dominated ENSO state and that ENSO may lock-in into the EP El Niño dominated state under global warming.