A comparison of soil-moisture sensitivity in two global climate models

Two global spectral general circulation climate models coupled to simple slab-ocean mixed layers are compared for the climates simulated with present-day (control) and increased atmospheric carbon dioxide (CO₂). The models were developed at the National Center for Atmospheric Research (NCAR) and the Geophysical Fluid Dynamics Laboratory (GFDL). For increased CO₂ both models simulate more snowmelt in winter and early spring and less in late spring, more runoff in early spring and less in late spring, and increased precipitation, evaporation, and latent-heat flux, particularly in spring. However, the models differ in several key respects for the increased CO₂ compared to the control. In spite of a qualitatively similar annual cycle of soil moisture, the NCAR model simulates soil-moisture amounts much less than saturation, while the GFDL model shows soil moisture near saturation in winter and spring. Therefore, the runoff is significantly less in the NCAR model than in the GFDL model. Accordingly, the increased winter and spring precipitation with increased CO₂ (similar in both models) mostly runs off in the GFDL model, but is retained in the soil-moisture "buckets' at most gridpoints in the NCAR model. The increased springtime evaporation rate (also similar in both models) results in a soil-moisture deficit in the GFDL model almost as soon as the warm season begins, while in the NCAR model the excess soil moisture retained in the "buckets' during winter and spring must first be evaporated before a soil-moisture deficit can occur. This delays or even prohibits a "summer drying' which is noted in the GFDL model. -from Authors