Understanding ENSO Complexity as a Result of ENSO Modes–Annual Cycle Interaction

In the observations and model simulations, ENSO manifests as two flavors differing in their spatial patterns, periods and triggering mechanisms. Many attempts have been made to explain such pattern diversity and frequency complexity. Here, using a modified Zebiak-Cane model, we propose a new nonlinear theory for ENSO complexity based on two co-existent intrinsic ENSO modes: a quasi-quadrennial (QQ) eastern Pacific (EP) mode and a quasi-biennial (QB) central Pacific (CP) mode. The two theoretical modes are consistent with the observed ENSO flavors. We find that the nonlinear ENSO bimodal interactions are essential to ENSO complexity in this model. With stochastic wind stress forcing, neither one of the two modes can cause ENSO complexity alone, no matter if annual cycle (AC) is included. When there is nonlinear bimodal interaction, diverse ENSO patterns appear and the spectral energy peaks can be found in both of the QQ and QB bands, as in the observations. In these cases, the dominances of ENSO spatial patterns and periods depend on the instability of the most unstable mode. However, AC can modulate the ENSO bimodal interaction: it enhances the EP mode but suppresses the CP mode, so the changes in the intensity of AC leads to ENSO regime shift, as in some Palaeo-climate reconstructions of ENSO. In plain language, The interactions between the two ENSO modes and between ENSO modes and annual cycle provide a “hybridization” theory to understand ENSO complexity. In plain language, the two ENSO modes are like two genes, and their interaction leads to ENSO pattern diversity and frequency complexity, which is also similar to the genic recombination that causes species diversity.