Sensitivity of the CCM1 hydrologic cycle to CO2


  • J. O. Roads,

  • S. Marshall,

  • R. Oglesby,

  • S.-C. Chen


To better understand the global hydrologic cycle, climatological simulations with the National Center for Atmospheric Research global atmospheric general circulation model (GCM) or community climate model (CCM1), coupled to a mixed layer model, are compared to available global hydrologic observations. We find that CCM1 simulates, reasonably well, the large-scale precipitable water, moisture flux convergence, precipitation, and evaporation potentials. Regional discrepancies are noticeable, though, especially in the eastern tropical Pacific. Smaller-scale moisture flux convergence, precipitation, and evaporation also compare less well since effects of resolution can be discerned between low-resolution simulations and higher-resolution global observations. Sensitivity experiments to changed CO2 show that the CCM1 hydrologic response is similar to other GCMs. Precipitable water, evaporation, and precipitation increase while snow decreases with increasing CO2. Soil moisture changes are seasonally and regionally dependent. In middle latitudes, soil moisture increases during the winter and decreases during the summer with increasing CO2. Changes in moisture transport between land and ocean are also seasonally dependent. Increased CO2 induces increased moisture divergence over the central equatorial Pacific during December, January, February (DJF). This water is transported to the rest of the northern hemisphere land and ocean where precipitation is much greater than evaporation over the climatologically cold land surface. During June, July, August (JJA), water is preferentially converged over the western Pacific Ocean.