Atmospheric and Oceanic Excitation of the Free Core Nutation Estimated from Recent Geophysical Models

We study the excitation of the observed free core nutation (FCN) signal by diurnal variation of the atmospheric and oceanic angular momenta (AAM, OAM), using AAM time series based on three different atmospheric reanalysis models, NCEP-NCAR, ERA-40 and ERA-interim, combined with OAM data from the ocean model OMCT. Comparison with the VLBI nutation data shows that the excitation based on the ERA-interim model is not reliable at the diurnal retrograde frequencies. Our spectral analysis of other excitation data sets confirms the earlier conclusion that the mass term of excitation contains sufficient power to explain the observed amplitude of the FCN oscillation. The coherence analysis of geodetic and geophysical excitations is not conclusive, presumably because the signal-to-noise ratio in AAM and OAM data is too low in vicinity of the FCN frequency.