Secular variability of ENSO events in a 1000-year climatic simulation

The CSIRO Mark 2 coupled global climatic model has been used to generate 1000 years of simulated climatic variability corresponding to present climatic conditions. A small climatic drift was noted during the simulation, and all results presented are based on detrended conditions. The emphasis here is on El Nino/Southern Oscillation (ENSO) events and their secular variability during the simulation. A number of features of the simulated ENSO climatology are presented and compared with observations. These demonstrate that the model reproduced the major characteristics of observed ENSO variability. A variety of analyses is given to illustrate secular variability of ENSO-related climate. Thus, while the simulated Southern Oscillation is shown to be a robust feature of the model climatology, decadal and oscillatory time variations occurred. Time-smoothing of the Southern Oscillation Index revealed underlying multi-decadal episodes, with associated climatic anomalies. Examination of 'strong' ENSO events highlighted an asymmetry between El Nino and La Nina events, similar to that observed, with clear secular variability in the occurrence rate of La Nina events. Persistent (multi-annual) ENSO events, such as occurred in the 1990s, were found to exist within the simulation, suggesting that these may be attributable to natural climatic variability. An investigation of the temporal variation of the anomalous depth of the 20 °C isotherm in the Pacific Ocean, revealed secular variability, which, if predictable, could identify decadal-length climatic trends. Other analyses involved the behaviour of the Pacific North American Oscillation and its secular variability. Distinct climatic impacts over North America were identified with this variability. Importantly, the simulation identified a number of features of ENSO-related climatic variability which have been poorly, if at all, documented in observations to date.