The angular momentum of the Earth-atmosphere system is conserved to good approximation because external torques are small. Variations in the angular momentum of the atmosphere must therefore be accompanied by and directly connected with variations in the angular momentum of the Earth and hence with variations in the length of day (lod). The atmospheric angular momentum budget simulated in the Canadian Climate Centre general circulation model is investigated in order to assess the fidelity of the model to the atmosphere but, more importantly, to investigate the nature of the simulated interaction between the atmosphere and the underlying surface and the implications for variations in the length of day. These interaction terms are difficult to obtain based on observations. The general circulation model successfully reproduces many of the observed aspects of the angular momentum budget, including the partition of the angular momentum between hemispheres, the dominance of the seasonal cycle and, as well as may be determined, the climatological distribution of the mountain torque and stress torque interaction terms. As to the simulated interaction between atmosphere and underlying surface on shorter time scales, it is found that on time scales of days the mountain and stress torque terms contribute to the rate of change of angular momentum (and by implication the lod) in the ratio of 60--40%, while at weekly and longer time scales the stress torque dominates by increasing amounts. The notable contemporaneity of changes in the length of day and of atmospheric angular momentum suggests that much of the interaction occurs directly with the solid Earth and that the oceans, despite their large area, play a lesser role. This is verified in the simulation where the sum of the stress and mountain torque over land contributes of the order of 85--90% to the variation in angular momentum for time scales of days to months.