Decisive role of nonlinear temperature advection in El Nino and La Nina amplitude asymmetry

The asymmetry of El Nino and La Nina in amplitude is a distinct feature of the observed El Nino-Southern Oscillation (ENSO). In this paper, we investigate the theoretical ENSO asymmetry by applying conditional nonlinear optimal perturbation in a theoretical model and an intermediate ENSO model. It is shown that the nonlinear temperature advection (NTA) plays an important role in ENSO asymmetry. By investigating the different types of nonlinearities in the Zebiak-Cane model, we identify clearly the origin of ENSO asymmetry and further emphasize the decisive role of the NTA. The NTA enhances the El Nino amplitude but has little effect on La Nina, resulting in the asymmetry of ENSO in amplitude. We also demonstrate that the stronger the El Nino event is, the larger the nonlinear effect related to the NTA is and the more significant the ENSO asymmetry is. This finding may explain why the strong ENSO events after the 1976 climate shift are of significant asymmetry.