Weakening of nonlinear ENSO under global warming

This is the pre-peer reviewed version of the following article: Kohyama, T., D. L. Hartmann, and D. S. Battisti (2018), Weakening of nonlinear ENSO under global warming, Geophys. Res. Lett., in press., which has been published in final form at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL079085 Abstract: The ENSO amplitude response to global warming is examined in two global climate models with realistic nonlinearity of the El Niño Southern Oscillation (ENSO). GFDL-ESM2M and 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 subsurface 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, so the real world may terminate strong, nonlinear El Niños sooner than model-based projections.