El Nino- or La Nina-like climate change?

The potential for the mean climate of the tropical Pacific to shift to more El Nino-like conditions as a result of human induced climate change is subject to a considerable degree of uncertainty. The complexity of the feedback processes, the wide range of responses of different atmosphere-ocean global circulation models (AOGCMs) and difficulties with model simulation of present day El Nino southern oscillation (ENSO), all complicate the picture. By examining the components of the climate-change response that projects onto the model pattern of ENSO variability in 20 AOGCMs submitted to the coupled model inter-comparison project (CMIP), it is shown that large-scale coupled atmosphere-ocean feedbacks associated with the present day ENSO also operate on longer climate-change time scales. By linking the realism of the simulation of present day ENSO variability in the models to their patterns of future mean El Nino-like or La Nina-like climate change, it is found that those models that have the largest ENSO-like climate change also have the poorest simulation of ENSO variability. The most likely scenario (p=0.59) in a model-skill-weighted histogram of CMIP models is for no trend towards either mean El Nino-like or La Nino-like conditions. However, there remains a small probability (p=0.16) for a change to El Nino-like conditions of the order of one standard El Nino per century in the 1% per year CO2 increase scenario.