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The amplitude response of the El Nino-Southern Oscillation (ENSO) to global warming is examined in two global climate models with realistic ENSO nonlinearity. Geophysical Fluid Dynamics Laboratory Earth System Model Version 2M (GFDL-ESM2M) and Model for Interdisciplinary Research on Climate version 5 (MIROC5) are the two models that exhibit realistic ENSO nonlinearity. With quadrupled atmospheric carbon dioxide, the ENSO amplitude of GFDL-ESM2M decreases by about 40%, whereas that of MIROC5 remains almost constant. Because GFDL-ESM2M exhibits stronger climatological thermal stratification than MIROC5, greenhouse gas forcing increases the upper ocean stability and causes the thermocline to be less sensitive to wind perturbations. The stiffer thermocline inhibits the nonlinear variations of sea surface temperature so that the ENSO amplitude substantially weakens. Idealized nonlinear recharge oscillator model experiments further support climatological thermal stratification as a determinant of the warming response. Observations exhibit stronger thermal stratification than both models, which suggests that the real world may terminate strong, nonlinear El Ninos sooner than model-based projections.

Plain Language Summary The El Nino-Southern Oscillation (ENSO) is the most prominent mode of interannual natural variability that influences global-mean temperature, severe weather, tropical cyclone activity, marine ecosystems, and food production worldwide. Potential future changes in ENSO under global warming have been disputed, however, due to difficulty in reproducing the observed ENSO features by global climate models. ENSO nonlinearity (i.e., El Nino events tend to be stronger than La Nina events in the real world) is one of those difficult features. Here we show that ENSO nonlinearity, though not reproduced well by most state-of-the-art climate models, could play a major role in determining the ENSO response. We examine two climate models that successfully reproduce realistic ENSO nonlinearity to discuss potentially important physical processes. Our results support a notion that strong El Nino events may be terminated in the near future.