Effect of the ocean tide on the Earth nutation: An updated assessment

Abstract

Ocean tides perturb the nutation of the Earth rotation axis to a level of 2 mas, an order of magnitude that is 20 times that of the uncertainty of nutation estimations. Ocean tidal effects on nutation represent about 5% of the effects caused by the non-rigidity of the Earth. Therefore, it is of fundamental importance for the development of the nutation theory of a real Earth, in particular for determining the frequencies of the normal modes due to the presence of the fluid outer core and solid inner core. The last published estimates date back to the 2000s. They were obtained by (Mathews et al., 2002) in the frame of the building of the non-rigid Earth nutation model “MHB2000”, which was adopted as the standard in astronomy and geodesy by IAU in 2000. This study aims at reassessing those effects by considering more recent ocean tidal models, namely FES 2014, TPXO8 and EOT11. It extends the computation of the diurnal and semi-diurnal ocean tidal contributions on polar motion and UT1 recently done by (Bizouard and Cheng, 2024) using the principal waves of the ocean tidal angular momentum derived from the models mentioned before. We take into account the recent development of Earth rotation theory, especially the frequency dependence of the transfer functions in sub-diurnal bands. We find not only significant corrections to the terms evaluated by MHB2000 – up to 100 $\mu$as –, but also significant contributions for circular terms with periods of $+9.13$ days, $+13.63$ days, $+121.75$ days, $-27.55$ days, and $-9.3$ years. The modelled $-27.55$ day term with an amplitude of 60 $\mu$as is clearly reflected in the celestial pole offsets (CPO) with the same phase and one third of the predicted amplitude. Moreover, the 16 largest in-phase or out-phase perturbations produced by ocean tides, which were not taken into account in the reference model MHB2000, are found in the CPO time series with a systematically smaller amplitude. These results strongly suggest that the current nutation model needs to be revised.