A Simulation of the Separate Climate Effects of Middle-Atmospheric and Tropospheric CO2 Doubling.

The separate climate effects of middle-atmospheric and tropospheric CO₂ doubling have been simulated and analyzed with the ECHAM middle-atmosphere climate model. To this end, the CO₂ concentration has been separately doubled in the middle-atmosphere, the troposphere, and the entire atmosphere, and the results have been compared to a control run. During NH winter, the simulated uniformly doubled CO₂ climate shows an increase of the stratospheric residual circulation, a small warming in the Arctic lower stratosphere, a weakening of the zonal winds in the Arctic middle-atmosphere, an increase of the NH midlatitude tropospheric westerlies, and a poleward shift of the SH tropospheric westerlies. The uniformly doubled CO₂ response in most regions is approximately equal to the sum of the separate responses to tropospheric and middle-atmospheric CO₂ doubling. The increase of the stratospheric residual circulation can be attributed for about two-thirds to the tropospheric CO₂ doubling and one-third to the middle-atmospheric CO₂ doubling. This increase contributes to the Arctic lower-stratospheric warming and, through the thermal wind relationship, to the weakening of the Arctic middle-atmospheric zonal wind. The increase of the tropospheric NH midlatitude westerlies can be attributed mainly to the middle-atmospheric CO₂ doubling, indicating the crucial importance of the middle-atmospheric CO₂ doubling for the tropospheric climate change. Results from an additional experiment show that the CO₂ doubling above 10 hPa, which is above the top of many current GCMs, also causes significant changes in the tropospheric climate. [ABSTRACT FROM AUTHOR]