INFLUENCE OF ATMOSPHERIC-PRESSURE ON THE FREE CORE NUTATION, PRECESSION AND SOME FORCED NUTATIONAL MOTIONS OF THE EARTH

The atmospheric pressure effects on the Earth's Free Core Nutation (FC N) and some forced nutations are evaluated numerically from the global pressure field provided by the European Center for Medium Range Weath er Forecast (ECMWF) on the Earth's solid surface using a 12-year long pressure data set sampled every 6 h on a(1.125 degrees x 1.125 degrees ) grid. Our model incorporates both the pressure and gravitational tor ques from the atmosphere as well as the elastic deformational effects induced by atmospheric loading. The pressure torque is computed from t he surface pressure field acting on the Earth's topography and the ant agonist gravitational torque is also dependent on this pressure field but acting on the gravitational equipotential surface (assuming vertic al hydrostatic equilibrium between density and pressure). The response of the oceans to pressure excitation is approximated by the static oc ean model which is different from the classical non-inverted barometer (NIBO) and the inverted barometer (IBO) hypotheses and depends on the degree of the spherical harmonic decomposition of the pressure field. The most efficient term in perturbing the nutations is the S-1 solar barometric tide of thermal origin which induces a contribution to the prograde annual nutation of gravitational origin. Seasonal modulations of S-1 also appear clearly which cause perturbations to other nutatio ns. We show that the contributions to the nutation values are ranging from a few tenths of a milliarcsecond up to the milliarcsecond for the annual prograde term and therefore are close to the lower bounds of t he values from a previous calculation by Dehant et al, [Dehant, V., Bi zouard, Ch., Hinderer, J., Legros, H., Lefftz, M., 1996, On atmospheri c pressure perturbation on precession and nutations, Phys, Earth Plane t. Interiors 96, 25-39.] based on the 20-yr-old S-1 pressure field fro m Haurwitz and Cowley [Haurwitz, B., Cowley, A.D., 1973. The diurnal a nd semidiurnal barometric oscillations, global distribution and annual variation. Pageoph. 102, 193-222.] and speculations on its amplitude modulation. These effects are therefore not negligible and this study points out the importance of atmospheric pressure corrections on the g ravitational nutations of lunisolar origin. We also estimate the excit ation power available from the atmospheric pressure, gravitational tor ques and elastic surface loading to explain the mean observed FCN ampl itude as derived from VLBI (Very Long Baseline Interferometry) observa tions. It is suggested that the atmosphere is a good candidate for ran domly exciting this free rotational mode. (C) 1998 Published by Elsevi er Science B.V. All rights reserved.