Atmospheric contributions to nutations and implications for the estimation of deep Earth's properties from nutation observations
Abstract
We propose a new estimation of the atmospheric contributions to Earth's nutations based on three reanalyses of atmospheric global circulation models (GCM), namely the two reanalyses of the National Center for Environmental Prediction (NCEP) and the ERA-40 reanalysis of the European Center for Medium-Range Weather Forecasts (ECMWF). We estimate the complex amplitudes of the periodic terms in the atmospheric forcing and convolve them with a transfer function for a three-layers Earth with an anelastic mantle and dissipative couplings at the fluid core boundaries. Unlike previous estimations based on operational GCMs, the results we obtain here from the three reanalysis GCMs are in good agreement, which makes them more reliable. From a joint inversion of the three atmospheric models on their common time span (from 1979 to 2002.3), we estimate the atmospheric contributions to nutations to be -38.2 ± 0.4 μas in-phase (ip) and 65.1 ± 0.4 μas out-of-phase (op) on the prograde annual term (S1), -64 ± 5 μas ip and 29 ± 5 μas op on the retrograde annual term (ψ1), and -11.3 ± 0.3 μas ip and 41.5 ± 0.3 μas op on the prograde semi-annual term (P1). As the atmospheric contributions to nutation vary in time, we also compute their time-variability on the time span from 1979 to 2010. In particular, we show that the contribution to ψ1 has a very large time variability but that these variations are well determined by the atmospheric models that we use. Finally, we explore the implications of the atmospheric contribution to ψ1 on the estimation of Earth's deep interior properties from nutation observations. We show that this contribution is too small to affect significantly the estimation of these properties.
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