Ch.6 Simple ENSO Models

Abstract

The fundamental dynamical mechanisms of the El Nino-Southern Oscillation (ENSO) phenomenon have been extensively studied since Bjerknes envisioned ocean-atmosphere interaction in the equatorial Pacific as its main cause. This chapter provides a review of the recent progress in ENSO theory based on two classes of relatively simple models: (i) the Cane-Zebiak (CZ) type models of intermediate complexity and (ii) conceptual low-order models reducible from the CZ-type models. The leading mode of ENSO variability, in reanalysis data, the CZ-type models, and comprehensive climate models, can be reduced into the simplest possible coupled oscillator known as the recharge oscillator (RO). Incorporating seasonality, nonlinearity, and multiscale processes into the RO framework, allows for basic understanding of how key physical processes determine ENSO's properties, such as its amplitude, periodicity, phase-locking, asymmetry, and nonlinear rectification onto the mean state. As these key physical processes can be easily quantified in both model and reanalysis data, the RO framework can be used to assess the simulations and projections of ENSO in climate models.