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 (FCN) and some forced nutations are evaluated numerically from the global pressure field provided by the European Center for Medium Range Weather 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 torques from the atmosphere as well as the elastic deformational effects induced by atmospheric loading. The pressure torque is computed from the surface pressure field acting on the Earth's topography and the antagonist gravitational torque is also dependent on this pressure field but acting on the gravitational equipotential surface (assuming vertical hydrostatic equilibrium between density and pressure). The response of the oceans to pressure excitation is approximated by the static ocean 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 nutations. 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 the values from a previous calculation by Dehant et al, [Dehant, V., Bizouard, Ch., Hinderer, J., Legros, H., Lefftz, M., 1996, On atmospheric pressure perturbation on precession and nutations, Phys, Earth Planet. Interiors 96, 25-39.] based on the 20-yr-old S-1 pressure field from Haurwitz and Cowley [Haurwitz, B., Cowley, A.D., 1973. The diurnal and 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 gravitational nutations of lunisolar origin. We also estimate the excitation power available from the atmospheric pressure, gravitational torques and elastic surface loading to explain the mean observed FCN amplitude as derived from VLBI (Very Long Baseline Interferometry) observations. It is suggested that the atmosphere is a good candidate for randomly exciting this free rotational mode. (C) 1998 Published by Elsevier Science B.V. All rights reserved.