Changes in ENSO Stability Under a Warmer Climate in CGCMs

In this study, a Bjerknes stability (BJ) index, proposed by Jin et al. (2007), is adopted to assess the overall stability of El Niño and Southern Oscillation (ENSO) in state-of-the-art coupled GCMs. The BJ index depicts the dependence of the growth rate of the leading coupled ENSO-like mode on the damping processes and the three dynamical positive feedbacks, specifically zonal advective feedback, thermocline feedback, and Ekman upwelling feedback. Each feedback depends on the model mean states and several other physical parameters, which assess the sensitivity of an atmospheric response to an ENSO SST anomaly and of an oceanic response to an anomalous surface wind. The 12 coupled models from the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) modeling groups are used for validating and estimating the BJ index. Through a comparison of the BJ index derived from the first 100-year monthly data for preindustrial experiments (control runs) and from the first 100-year after stabilization for CO2 increase experiments (CO2 runs), we investigate changes in dynamical feedbacks and atmospheric and oceanic responses associated with ENSO under a future warmer climate, and their impacts on ENSO stability. The models show a wide range of ENSO stability behavior from groups with relatively strongly damped ENSO modes to groups with relatively strongly growing ENSO modes. It is shown that models with a relatively large (small) ENSO amplitude exhibit a relatively large (small) BJ index, which is consistent with theoretical studies. The correlation of the above relation for the control runs and CO2 runs is 0.88 and 0.87, respectively. The diversity of the ENSO stability is attributed to different mean states and the different sensitivities of oceanic and atmospheric responses to a forcing from model to model. However, inconsistency between the models in terms of changes in the model sensitivity of the responses to a forcing and model mean states under a warmer climate makes it difficult to conclude the influence of greenhouse warming on the stability of the ENSO