On ENSO Theory
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 talk provides a review on the recent progress in ENSO theory based on two classes of relatively simple models: (i) the Zebiak-Cane (ZC) type models of intermediate complexity and (ii) conceptual low-order models reducible from the ZC-type models. Two co-existing linear dynamical modes in the ZC-type models are reminiscent of the different observed ENSO types (i.e., Eastern and Central Pacific ENSO). Moreover, the leading mode of ENSO variability – in reanalysis data, the ZC-type models, and comprehensive climate models – can be reduced into the simplest possible coupled oscillator known as the Recharge Oscillator (RO). By incorporating seasonality, nonlinearity, and multi-scale processes into the RO framework, improved understanding can be gained on how key physical processes determine ENSO’s basic properties, such as its amplitude, periodicity, phase-locking, asymmetry, as well as its nonlinear rectification onto the mean state. As these key physical processes can be easily computed from both model and reanalysis data, the RO framework can also be used to assess the simulation and prediction of ENSO in climate models.