Revisiting the Excitation of Free Core Nutation

Earth possesses a Poincare mode called Free Core Nutation (FCN) due to the misalignment of the rotation axes of the mantle and fluid outer core. FCN is the primary signal in the observations of Celestial Pole Offsets (CPO) and maintained by geophysical mechanisms that are yet to be understood. Earlier studies suggested an origin in Atmospheric Angular Momentum (AAM)-and to a lesser degree Oceanic Angular Momentum (OAM)-but discrepancies between these geophysical excitations and the geodetic (CPO-based) excitation were too large to reach definite conclusions. Here we use newly calculated, 3-hourly AAM and OAM series for the 1994-2022 period, in conjunction with the latest CPO series from the International Earth Rotation and Reference Systems Service (IERS 20 C04 series), to demonstrate a markedly lower power ratio (similar to 4.6) of geophysical over geodetic excitation at the FCN frequency compared to previous works (ratio similar to 10). Among all excitation sources, the AAM pressure term exhibits the highest coherence (0.56) and correlation (0.48) with the geodetic excitation, whereas the coherence with OAM is smaller by a factor of 3. Similar analyses using existing angular momentum series give comparable, albeit smaller coherence and correlation results. We attribute the relevant AAM pressure term signal to Northern Hemispheric landmasses and further show consistent temporal variations in the amplitude of geophysical and geodetic excitations around the FCN band. Our results thus corroborate evidence for large-scale atmospheric mass redistribution to be the main cause of continuous FCN excitation. Plain Language Summary We revisit the excitation of Free Core Nutation (FCN) by generating new effective Atmospheric and Oceanic Angular Momentum (AAM and OAM, respectively) time series since 1994 up to the end of 2022. We follow the deconvolution approach and compute the geophysical excitation based on complex demodulation at retrograde diurnal frequency band, as well as geodetic excitation from a digital filter based on broad-band Liouville equation. We show that geodetic and geophysical excitations exhibit consistent patterns of temporal variation, having significant coherence and correlation with largest contribution from the pressure term of AAM. Comparison between our new AAM and OAM series with those of European Center for Medium-Range Weather Forecasts show that our series are more correlated to the geodetic excitation. These findings confirm the role of atmosphere on the excitation of FCN and can be used to more accurately model and predict the FCN variations, thereby enhancing the nutation theory.