A Hierarchy of Models for ENSO Diversity in Past, Present, and Future

The existence of two distinct ENSO flavors versus an ENSO continuum remains an open question. Here, we employ a hierarchy of models to explore the existence of ENSO flavors or regimes under different forcing scenarios. First, we use a nonlinear version of the recharge-discharge model, which focuses only on ENSO diversity associated with the strength of the warming and not the pattern. We show that the nonlinearity in the Bjerknes feedback associated with the activation of deep convection in the cold eastern Pacific is sufficient to produce a bimodal distribution of peak ENSO SST anomalies, similar to the observed. Second, we use a modified version of the Zebiak-Cane model which simulates two ENSO modes with SSTa patterns reminiscent of ENSO flavors, and show that changes in the cold tongue annual cycle lead to changes in the relative frequency of the two modes and, consequently, in the distribution of peak ENSO SSTa. We then employ metrics of the distribution of ENSO peak SSTa (strength and patterns) to analyze Global Climate Models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We show that the shape of the pdf of ENSO SSTa patterns can serve as metric of accurate balance of ENSO feedbacks. Further, we find that in models that exhibit strong ENSO diversity, the projected tropical Pacific sea surface temperature warming pattern is closely linked to the projected ENSO response, highlighting that ENSO simulation biases may lead to potentially biased projections in long-term SST and precipitation trends, with great significance for regional climate adaptation strategies.