Lunisolar torque on the atmosphere and Earthʹs rotation

The atmospheric effect on Earthʹs rotation is classically computed from atmospheric angular momentum (AAM) series by considering that the whole AAM variations are transmitted to the solid Earth. In fact, such a procedure is an approximation because it ignores that the atmosphere exchanges also angular momentum with the moon and the sun. The computation of the lunisolar gravitational torque on the atmosphere is the core of the present study. It is derived analytically and then estimated from pressure data over the period 1975–2000. The mean surface pressure is the main contributor, at the level of 1017 N m for the equatorial component, 1016 N m for the axial one, whereas the thermally driven pressure variations provide about 10% of the variance. This torque plays a significant role in the budget of the atmospheric angular momentum, but does not account for the inconsistency between the present series of the atmospheric angular momentum and the interaction torque between the solid Earth and the atmosphere. From the tidal waves of this torque we derive accurately the contribution of the atmosphere to the lunisolar precession–nutation of the Earth (0.31 marcsec/year for the precession in longitude, 46 microarcseconds for the nutation in 18.6 years).