Decadal amplitude modulation of two types of ENSO and its relationship with the mean state

In this study, we classified two types of El Niño-Southern Oscillation (ENSO) events within the decadal ENSO amplitude modulation cycle using a long-term coupled general circulation model simulation. We defined two climate states-strong and weak ENSO amplitude periods-and separated the characteristics of ENSO that occurred in both periods. There are two major features in the characteristics of ENSO: the first is the asymmetric spatial structure between El Niño and La Niña events; the second is that the El Niño-La Niña asymmetry is reversed during strong and weak ENSO amplitude periods. El Niño events during strong (weak) ENSO amplitude periods resemble the Eastern Pacific (Central Pacific) El Niño in terms of the spatial distribution of sea surface temperature anomalies (SSTA) and physical characteristics based on heat budget analysis. The spatial pattern of the thermocline depth anomaly for strong (weak) El Niño is identical to that for weak (strong) La Niña, but for an opposite sign and slightly different amplitude. The accumulated residuals of these asymmetric anomalies dominated by an east-west contrast structure could feed into the tropical Pacific mean state. Moreover, the residual pattern associated with El Niño-La Niña asymmetry resembles the first principal component analysis (PCA) mode of tropical Pacific decadal variability, indicating that the accumulated residuals could generate the change in climate state. Thus, the intensified ENSO amplitude yields the warm residuals due to strong El Niño and weak La Niña over the eastern tropical Pacific. This linear relationship between ENSO and the mean state is strong during the mature phases of decadal oscillation, but it is weak during the transition phases. Furthermore, the second PCA mode of tropical Pacific decadal variability plays an important role in changing the phase of the first mode. Consequently, the feedback between ENSO and the mean state is positive feedback to amplify the first PCA mode, whereas the second PCA mode is a negative feedback to lead the phase change of the first PCA mode due to their lead-lag relationship. These features could be regarded as evidence that the decadal change in properties of ENSO could be generated by the nonlinear interaction between ENSO and the mean state on a decadal-to-interdecadal time scale.