Understanding uncertainties in the response of ENSO to greenhouse warming

The response of ENSO to increases in greenhouse gases is found to be markedly different in climate change simulations using two different versions of the Hadley Centre Coupled Models (HadCM2 and HadCM3). Using HadCM2, Collins [2000] found an increase in the amplitude, an increase in the frequency, and a shift in the phase of ENSO at 4xCO(2). In HadCM3 it is shown that there are no significant changes in ENSO statistics at 4xCO(2). Differences in the two models ENSO response are traced to differences in the response of the mean meridional gradients of temperature either side of the equator. These differences in the mean pattern of climate change are brought about by subtle changes in the physical parametrisation schemes, rather than the main differences between HadCM2 and HadCM3 which are enhanced horizontal ocean resolution and the removal of flux adjustments. Thus the response of ENSO variability to increasing greenhouse gases can depend in an unexpected and quite non-linear way to subtle changes in the models representation of sub grid-scale processes.