Fortnightly Earth rotation, ocean tides and mantle anelasticity

Abstract

SUMMARY This study of the fortnightly Mf tide comprises three main topics: (1) a new determination of the fortnightly component of polar motion and length of day (LOD) from a multidecade time-series of observed space-geodetic data; (2) the use of the polar motion determination as one constraint in the development of a hydrodynamic ocean model of the Mf tide and (3) the use of these results to place new constraints on mantle anelasticity at the Mf tidal period. Our model of the Mf ocean tide assimilates more than 14 years of altimeter data from the Topex/Poseidon and Jason-1 satellites. Because the Mf altimetric signal-to-noise ratio is very small, it is critical that altimeter data not be overweighted. The polar motion data, plus tidegauge data and independent altimeter data, give useful additional information, with only the polar motion putting constraints on tidal current velocities. The resulting ocean-tide model, plus the dominant elastic body tide, leaves a small residual in observed LOD caused by mantle anelasticity. The inferred effective tidal Q of the anelastic body tide is 90 and is in line with a ω α frequency dependence with α in the range 0.2–0.3.