With data from 12 coupled models in the 4th Assessment Report of Intergovernmental Panel on Climate Change (IPCC-AR4), climate under year 2000 greenhouse gas (GHG) + aerosol forcing was compared with climate under pre-industrial conditions. In the tropical Pacific, the warming in the mean sea surface temperatures (SST) was found to have an El Nino-like pattern, in agreement with the observed SST data (1900-1999). The models showed that both the equatorial zonal overturning circulation and the meridional overturning circulation weakened under increased GHG forcing.

For the El Nino-Southern Oscillation (ENSO), the asymmetry in the SST anomalies between El Nino and La Nina was found to be enhanced under increased GHG, for both the ensemble model data and the observed data. Furthermore, the enhanced GHG simulations showed that the asymmetry between El Nino and La Nina increased in the zonal wind stress anomalies, the equatorial undercurrent anomalies and the meridional overturning circulation anomalies.