EVIDENCE FOR EXCITATION OF POLAR MOTION BY FORTNIGHTLY OCEAN TIDES

The second-degree zonal tide raising potential, which is responsible f or tidal changes in the Earth's rotation rate and length-of-day, is sy mmetric about the polar axis and hence can excite the Earth's polar mo tion only through its action upon nonaxisymmetric features of the Eart h such as the oceans. Ocean tidal excitation of polar motion in the di urnal and semidiurnal tidal bands has been previously detected and exa mined. Here, the detection of ocean tidal excitation of polar motion i n the long period tidal band, specifically at the Mf' (13.63-day) and Mf (13.66-day) tidal frequencies, is reported. Spectra of the SPACE94 polar motion excitation series exhibit peaks at the prograde and retro grade fortnightly tidal periods. After removing effects of atmospheric wind and pressure changes, an empirical model for the effect of the f ortnightly ocean tides upon polar motion excitation is obtained by lea st-squares fitting periodic terms at the Mf and Mf' tidal frequencies to the residual polar motion excitation series. The resulting empirica l model is then compared with the predictions of two hydrodynamic ocea n tide models.