Modes of climate variability as simulated by a coupled general circulationmodel. Part I: ENSO-like climate variability and its low-frequency modulation

The El Nino-Southern Oscillation (ENSO) is investigated in a multicentury i ntegration conducted with the coupled general circulation model (CGCM) ECHA M3/LSG. The quasiperiodic interannual oscillations of the simulated equator ial Pacific climate system are due to subsurface temperature anomaly propag ation and a positive atmosphere-ocean feedback. The gravest internal wave m odes contribute to the generation of these anomalies. The simulated ENSO ha s a characteristic period of 5-8 years. Due to the coarse resolution of the ocean model the ENSO amplitude is underestimated by a factor of three as c ompared to observations. The model ENSO is associated with the typical atmo spheric teleconnection patterns. Using wavelet statistics two characteristi c interdecadal modulations of the ENSO variance are identified. The origins of a 22 and 35 y ENSO modulation as well as the characteristic ENSO respon se to greenhouse warming simulated by our model are discussed.