Understanding the ENSO–CO2 Link Using Stabilized Climate Simulations

The influence of atmospheric CO2 concentration on the El Niño–Southern Oscillation (ENSO) is explored using 800-yr integrations of the NCAR Community Climate System Model, version 3.5 (CCSM3.5), with CO2 stabilized at the a.d. 1850, 1990, and 2050 levels. Model mean state changes with increased CO2 include preferential SST warming in the eastern equatorial Pacific, a weakening of the equatorial trade winds, increased vertical ocean stratification, and a reduction in the atmospheric Hadley and oceanic subtropical overturning circulations. The annual cycle of SST strengthens with CO2, likely related to unstable air–sea interactions triggered by an increased Northern Hemisphere land–sea temperature contrast. The mean trade wind structure changes asymmetrically about the equator, with increased convergence in the Northern Hemisphere and divergence in the Southern Hemisphere leading to corresponding deepening and shoaling of the thermocline. The proportion of eastern versus central Pacific–type El Niño events increases with CO2, but the significance of the changes is relatively low; ENSO amplitude also increases with CO2, although the change is insignificant at periods longer than 4 yr. The 2–4-yr ENSO response shows an enhancement in equatorial Kelvin wave variability, suggesting that stochastic triggering of El Niño events may be favored with higher CO2. However, the seasonal cycle–ENSO interaction is also modified by the asymmetric climatological changes, and forcing by the Southern Hemisphere becomes more important with higher CO2. Finally, higher-resolution CCSM4 control simulations show that ENSO weakens with CO2 given a sufficiently long integration time. The cause for the difference in ENSO climate sensitivity is not immediately obvious but may potentially be related to changes in westerly wind bursts or other sources of high-frequency wind stress variability.