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The impact of a CO 2 ‐induced global warming on ENSO‐like fluctuations in a global coupled ocean‐atmosphere GCM is analyzed using two multi‐century experiments. In the 4×CO 2 experiment, CO 2 increases by a factor of four in the first 140 years and then remains constant at 4×CO 2 for another 360 years; in the control experiment, CO 2 remains constant at 1×CO 2 for 1000 years. The standard deviation of tropical Pacific SST fluctuations (7°N–7°S, 173°E–120°W; 2 to 15 year timescales) is 24% lower in the 4×CO 2 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 4×CO 2 . We conclude that despite an order 5 K warming of the tropical Pacific, and order 50% increase in time‐mean atmospheric water vapor under 4×CO 2 conditions, ENSO‐like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude.
Knutson et al. (Sat,) studied this question.