The excitation of free core nutation: a reappraisal

Free Core Nutation (FCN) is a rotational normal mode of the Earth arising from the misalignment of the rotation vectors of the mantle and the fluid outer core. There has long been suggestive evidence that this mode is excited against dissipation by variations in Atmospheric and Oceanic Angular Momentum (AAM and OAM, respectively), but efforts to reconcile model-based AAM and OAM estimates with the FCN in geodetic data (the Celestial Pole Offsets, CPO) have remained futile. In particular, prior assessments suggest that the power of geophysical excitation from AAM/OAM data exceeds the power of geodetic excitation at the FCN frequency by a factor of 10 or more. Here we reassess the geophysical excitation of FCN using 3-hourly AAM and OAM series from two latest-generation atmospheric reanalyses and consistently forced ocean forward models, called MERRA-2. We focus on the pressure terms and transform them to the celestial frame by complex demodulation. In addition, we use the latest CPO series of the International Earth Rotation and Reference Systems Service (IERS 20 C04) and convert it to geodetic excitation using a digital filter based on the broad-band Liouville equation. By filtering the geodetic and geophysical excitation series near the FCN frequency band (approximately –440 to –420 days in the celestial frame) we show that the agreement between these two series is improved relative to previous studies, such that the ratio of the power spectrum of geophysical to geodetic excitations is ~4.6. Moreover, we find clear similarities between observed and modeled (excitation) time series in the FCN band. Among the different drivers, the impact of AAM is at least 3 times larger than that of OAM. Our results represent progress in finding the cause of continuous FCN excitation, which is most probably the variability of atmospheric pressure over continental landmasses.