Response of the atmospheric angular momentum and the length of the day to the surface temperature increase for an aqua planet model

We examine the effect of the surface temperature increase on the atmospheric angular momentum to study relations between the decadal variations in the atmospheric angular momentum and those of the rotation of the solid earth. We use an atmospheric general circulation model forced by simple surface temperature distributions (an aqua planet model). We vary the temperature distribution in two ways and investigate the responses. In one set of calculations, the surface temperature is increased uniformly globally. In the other set of calculations, the surface temperature is increased locally near the equator. We have found that a globally uniform warming results in an increase of the atmospheric angular momentum. This is because the tropopause becomes higher and the depth of tropospheric westerly shear increases as the surface temperature increases. The altitude of the jet (maximum of the westerlies) increases, while its latitude does not change. These dependencies can be explained by the dynamics of the Hadley circulation. We have also found that a warming localized near the equator results in an westerly jet built up around the equatorial tropopause and the increase in the angular momentum. The results of the two cases investigated here show that the effect of warming on the atmospheric angular momentum is too small to explain the decadal variations of the length of the day. Furthermore, they show that the sense of the change is opposite to that proposed by Lambeck and Cazenave [1976], which is based on observational data of the lower atmosphere.