Past studies have suggested that increasing atmospheric CO2 will lead to a significant reduction of soil moisture during summer in the extratropics. These studies showed an increase in wintertime rainfall over most mid-latitude continental regions when CO2 is doubled, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer. We find that the above processes are important in simulated summer drying in a newly developed climate model. In addition to these thermodynamic processes, we find that changes in atmospheric circulation play an important role in regional hydroclimatic changes. Additional experiments show that the atmospheric circulation changes are forced by the CO2-induced warming of the ocean, particularly the tropical ocean. These results highlight the importance of sea surface temperature changes for regional hydroclimatic changes.