Decadal amplitude modulation of two types of ENSO and its relationship with the mean state Jung Choi • Soon-Il An • Sang-Wook Yeh Received: 14 February 2011 / Accepted: 3 September 2011 Ó Springer-Verlag 2011 Abstract In this study, we classified two types of El Nin ˜o–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 Nin ˜o and La Nin ˜a events; the second is that the El Nin ˜o–La Nin ˜a asymmetry is reversed during strong and weak ENSO amplitude periods. El Nin ˜o events during strong (weak) ENSO amplitude periods resemble the Eastern Pacific (Central Pacific) El Nin ˜o in terms of the spatial distribution of sea surface temperature anomalies (SSTA) and physical char- acteristics based on heat budget analysis. The spatial pat- tern of the thermocline depth anomaly for strong (weak) El Nin ˜o is identical to that for weak (strong) La Nin ˜a, 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 Nin ˜o–La Nin ˜a 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 Nin ˜ o and weak La Nin ˜a 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 sec- ond PCA mode is a negative feedback to lead the phase change of the first PCA mode due to their lead-lag rela- tionship. These features could be regarded as evidence that the decadal change in properties of ENSO could be gen- erated by the nonlinear interaction between ENSO and the mean state on a decadal-to-interdecadal time scale. Keywords ENSO Á Central Pacific El Nino Á Amplitude modulation Á El Nin ˜o–La Nin ˜a asymmetry 1 Introduction The El Nin ˜o–Southern Oscillation (ENSO) is modulated on a decadal-to-interdecadal time scale in terms of its ampli- tude, frequency, and other characteristics (Trenberth and Hurrell 1994; Wang and Wang 1996; An and Wang 2000; Wang and An 2001; McPhaden and Zhang 2002; Fl} ugel et al. 2004; Imada and Kimoto 2009; An 2009). In par- ticular, the amplitude of ENSO underwent changes on time scales of 10–20 years (Gu and Philander 1997; Torrence and Webster 1998; Sun and Yu 2009), and these decadal amplitude modulations of ENSO were accompanied by the nonlinearity of ENSO represented especially by the El Nin ˜o–La Nin ˜a asymmetry (Timmermann 2003; Rodgers J. Choi Á S.-I. An (&) Department of Atmospheric Sciences, Global Environmental Laboratory, Yonsei University, Seoul 120-742, Korea e-mail: sian@yonsei.ac.kr S.-W. Yeh Department of Environmental Marine Science, Hanyang University, Ansan, Korea 123 Clim Dyn DOI 10.1007/s00382-011-1186-y