Effect of long-period ocean tides on the Earth's polar motion

The second-degree zonal tide raising potential is symmetric about the polar axis and hence can excite the Earth's polar motion only through its action upon non-axisymmetric features of the Earth such as the oceans. In the long-period tidal band, spectral peaks occur at the fortnightly tidal frequencies in power spectra of polar motion excitation observations from which atmospheric effects have been removed. Different observed polar motion excitation series are studied, and differ ent methods of removing atmospheric effects from the polar motion excitation observations are explored in order to assess the robustness of the resulting empirical models for the observed effect of long-period ocean tides on polar motion excitation. At fortnightly frequencies, the various observed empirical models are found to agree with each other to within about 1 sigma. However, improved observations of the effect of fortnightly ocean tides on polar motion excitation are required since the observed effects at the Mf and Mf' tidal frequencies are not consistent with the expectation that the oceans should have the same relative response to the tidal potential at these two nearby tidal frequencies. The observations are then compared with predictions of three ocean tide models: two purely hydrodynamic models and one estimated from Topex/Poseidon altimetric sea surface height measurements. At the fortnightly tidal frequencies, the three ocean tide models predict polar motion excitation amplitudes that differ from each other and from the observations by factors as large as 2, and phases that differ by more than 100 degrees. This illustrates the need for improved models for the effect of long-period ocean tides on polar motion excitation. (C) 1998 Elsevier Science Ltd. All rights reserved.