Diurnal angular momentum budget of the atmosphere and its consequences forEarth's nutation

The three-dimensional diurnal atmospheric angular momentum (AAM) budget is investigated, and its geographical features and meteorological dynamics are explored. It is shown that the diurnal atmospheric torques are associated with mass effect (ellipsoidal torque for the equatorial component and mount ain torque for the axial component). It is shown that there is an important difference between the torque and angular momentum approaches on diurnal t imescales, with the AAM time derivative generally exceeding the calculated torques. For the retrograde diurnal band, however, which is of interest for Earth nutation, the torque approach predicts much larger AAM changes than those observed. The AAM budget equation is also investigated theoretically, in order to give condition on the torques and AAM in order to verify the e quation. Alternatively to the condition proposed by Bell, [1994], a conditi on on the local torque is shown sufficient to verify the AAM budget equatio n in the retrograde band, that is, the local torque have to balance the ell ipsoidal effect. This condition is shown to be verified qualitatively. Howe ver, when separating the AAM budget equation into prograde and retrograde p art, this last part become very sensitive to the error on the computation o f the local torque. This explains why the torque approach always give one o rder of magnitude larger than the AAM approach at the retrograde diurnal fr equencies.