IMPACT OF INCREASED CO2 ON SIMULATED ENSO-LIKE PHENOMENA

The impact of a CO2-induced global warming on ENSO-like fluctuations in a global coupled ocean-atmosphere GCM is analyzed using two multi-century experiments. In the 4xCO2 experiment, CO2 increases by a factor of four in the first 140 years and then remains constant at 4xCO2 for another 360 years; in the control experiment, CO2 remains constant at 1xCO2 for 1000 years. The standard deviation of tropical Pacific SST fluctuations (7-degrees-N-7-degrees-S, 173-degrees-E-120-degrees-W; 2 to 15 year timescales) is 24% lower in the 4xCO2 experiment than in the control experiment; for the model's Southern Oscillation Index, a 19% decrease occurs, whereas for central tropical Pacific rainfall, a 3% increase occurs. An important feature of the control simulation is the internally generated modulation of variability on a multi-century timescale, which is comparable in magnitude to the changes occurring with 4xCO2. We conclude that despite an order 5 K warming of the tropical Pacific, and order 50% increase in time-mean atmospheric water vapor under 4xCO2 conditions, ENSO-like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude.