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

This paper describes El Nino-Southern Oscillation (ENSO) interannual variab ility simulated in the second Hadley Centre coupled model under "control" a nd "greenhouse warming" scenarios. The model produces a very reasonable sim ulation of ENSO in the control experiment-reproducing the amplitude, spectr al 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-oce an dynamics mode that can he interpreted in terms of the "ocean recharge pa radigm" 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 approxim ately 20% larger amplitude, a frequency that is approximately double that o f the current ENSO (implying more frequent El Ninos and La Ninas), and phas e locks to the annual cycle at a different time of year. It is shown that t he increase in the vertical gradient of temperature in the thermocline regi on, associated with the model's response to increased greenhouse gases, is responsible for the increase in the amplitude of ENSO, while the increase i n meridional temperature gradients on either side of the equator, again ass ociated with the models response to increasing greenhouse gases, is respons ible for the increased frequency of ENSO events.