The El Nino-Southern Oscillation in the second Hadley Centre coupled model and its response to greenhouse warming

This paper describes El Nino-Southern Oscillation (ENSO) interannual variability simulated in the second Hadley Centre coupled model under "control" and "greenhouse warming" scenarios. The model produces a very reasonable simulation of ENSO in the control experiment-reproducing the amplitude, spectral characteristics, and phase locking to the annual cycle that are observed in nature. The mechanism for the model ENSO is shown to he a mixed SST-ocean dynamics mode that can he interpreted in terms of the "ocean recharge paradigm" of Jin. In experiments with increased levels of greenhouse gases, no statistically significant changes in ENSO are seen until these levels approach four times preindustrial values. In these experiments, the model ENSO has an approximately 20% larger amplitude, a frequency that is approximately double that of the current ENSO (implying more frequent El Ninos and La Ninas), and phase locks to the annual cycle at a different time of year. It is shown that the increase in the vertical gradient of temperature in the thermocline region, associated with the model's response to increased greenhouse gases, is responsible for the increase in the amplitude of ENSO, while the increase in meridional temperature gradients on either side of the equator, again associated with the models response to increasing greenhouse gases, is responsible for the increased frequency of ENSO events.