Untangling El Niño-La Niña Asymmetries Using a Nonlinear Coupled Dynamic Index

The linear recharge oscillator model for the El Niño–Southern Oscillation (ENSO) was expanded to a nonlinear model, thus allowing identification of a nonlinear dynamic ENSO index. This index was applied for a dynamic examination of the El Niño-La Niña asymmetry. Here, the nonlinear physical processes including the nonlinear dynamical heating were implemented into the linear recharge oscillator model in the form of quadratic nonlinearity. This nonlinear recharge oscillator model revealed that nonlinear (linear) physical processes play a critical role in the long-term ENSO skewness (amplitude) changes quantitatively. Particularly, the large part of the long-term ENSO skewness change could be explained by the quadratic nonlinearity associated with the nonlinear ocean dynamical heating, which is closely related to change in mean thermocline. This research provides a unified framework to understand better the past-present-future ENSO changes by applying the interdecadal change dynamics on a low hierarchical level.