Positive Response of ENSO Amplitude to Higher CO2 Forcing is Linked to The Asymmetry Between El Niño and La Niña

El Niño –Southern Oscillation (ENSO), a natural mode of variability of the climate system, has world-wide consequences on weather patterns, ecosystems, agriculture, and public health. Whether this natural mode of variability would assume a larger amplitude in response to a higher CO2 concentration in the atmosphere bears special interest to climate scientists and the public at large. Averaged across all the climate models collected by CMIP5—the models that we consider as the-state-of-the-art modes, the answer to the question seems to be “NO”, contrasting a theoretical prediction from an analytical, but nonlinear box model that links the very existence of ENSO to the intensity of radiative heating. Here we present evidence to suggest that the average result across all the models from CMIP5 is inappropriate to answer this question. In particular, we note that CMIP5 models on average significantly underestimate the asymmetry between the two phases of ENSO, an observed feature of ENSO, and is also simulated by the nonlinear box model. We further find that the number of models that have a consistent positive response of ENSO amplitude to higher CO2 forcing under two different levels of CO2 increase is comparable to the number of models that consistently project an insignificant response of ENSO amplitude to CO2 increases. ENSO simulated in the historical runs of the models that predict a positive response in ENSO amplitude are found to be more comparable to the observations in amplitude and asymmetry. The results add support to the prediction that a higher CO2 forcing would yield higher ENSO amplitude. The results also highlight a key area for further improvements in climate models for a greater convergence in their predictions of ENSO response to CO2 increases--the asymmetry between El Niño and La Niña.