The Effect of Greenhouse SSTs on ENSO Simulations with an AGCM

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Abstract
The potential for changes to the El Niño–Southern Oscillation (ENSO) phenomenon under enhanced greenhouse conditions is addressed in a series of atmospheric general circulation model (AGCM) experiments with both 1×CO2 and 2×CO2 radiative forcing and prescribed sea surface temperatures. In the first series of experiments, climate anomalies associated with the 1987 “warm” event SSTs are simulated for present-day (1×CO2) conditions. The results reveal realistic large-scale perturbations to mean sea level pressure, rainfall, evaporation, and low-level and upper-level winds compared to simulations using climatological SSTs. In the second series of experiments, the same warm anomalies are superimposed on the equilibrium SSTs calculated during the course of 2×CO2 equilibrium experiments. The resultant SSTs provide an analog warm event for enhanced greenhouse conditions. The simulated climate anomalies for this greenhouse event are similar to those of the present-day event, but, although some changes are found in the western Pacific region, the results reveal little evidence of any large-scale intensification associated with warmer equilibrium SSTs. The results suggest that increased static stability may dampen the effects of any nonlinear increase in evaporation and that any changes in the behavior of ENSO due to predicted temperature increases in the Tropics may not be significant.

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