Effects of nonlinear terms during ENSO developing and decaying phases

The features of sea surface temperature (SST) anomaly skewness in the equatorial Pacific are investigated based on the SST dataset from the Hadley Center and the Simple Ocean Data Assimilation (SODA 2.2.4) dataset from the University of Maryland. Impacts of nonlinear terms on the developing and decaying speeds of different types of ENSO events are further studied through analyzing the heat budget of mix-layer sea temperature. The results show that the asymmetry of ENSO amplitude is mainly attributed to the EP El Niño, and the SST anomaly amplitude associated with the CP El Niño displays a quasi-symmetric feature with that of La Niña. Nonlinear terms exhibit positive effects during the developing phases of EP El Niño and La Niña, which tend to strengthen the EP El Niño amplitude but suppress the La Niña amplitude. However, the influence of nonlinear terms on the CP El Niño is relatively smaller compared to that on other types of ENSO. The overall effects of the nonlinear terms largely lead to the formation of the ENSO amplitude asymmetry. During the ENSO decaying phases, nonlinear terms remain positive values during La Niña but with a much weaker intensity. In addition, nonlinear terms accelerate and decelerate the EP and CP El Niño's decay, respectively. This may be responsible for the faster decaying speed during the EP El Niño events compared to that during the CP El Niño events.