The El-Nino/Southern Oscillation is examined in two of the Hadley Centre coupled climate models in both control simulations, relevant to pre-industrial times, and in simulations with increased levels of greenhouse gases and other radiatively active substances.
HADCM2 - the second Hadley Centre Climate Model - has a relatively coarse resolution ocean model and uses flux correction to prevent the mean climate of then model drifting. However, the model control simulation agrees well with the observations in terms of both the amplitude and spectral characteristics of the NINO3 SST index. In HADCM2 experiments with increased levels of greenhouse gases, no changes in ENSO variability are seen until the levels reach at least four times pre-industrial values. In these simulations, ENSO increases in amplitude by approximately 30% and changes from having a broad 2-8 year spectral peak to having a sharp peak in the 2-3 year band, implying more frequent El-Nino and La-Nina events. Also, a phase-shift occurs in which the timing of ENSO events changes from northern winter to late northern summer/early autumn.
HADCM3 - the third Hadley Centre Climate Model - uses finer resolution in the ocean model, has no flux correction term and has a stable climate. The control simulation of this model however, has and ENSO which is too strong and which has a much more well defined spectral peak compared to the broad band peak in the observations. The HADCM3 simulations with increased levels of greenhouse gases show no significant changes in ENSO variability with respect to the control, in contrast to the HADCM2 results.