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 4×CO₂ experiment, CO₂ increases by a factor of four in the first 140 years and then remains constant at 4×CO₂ for another 360 years; in the control experiment, CO₂ remains constant at 1×CO₂ 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×CO₂ 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×CO₂. 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×CO₂ conditions, ENSO‐like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude. This paper is not subject to U.S. copyright. Published in 1994 by the American Geophysical Union.