Atmospheric excitation of the Earth’s annual wobble

Global meteorological analyses from the European Centre for Medium Range Weather Forecasts are employed to compute the atmospheric excitation ß of the polar motion for the 9-year period of 1980-1988. Both the matter component V(matter) and the motion component V(motion) are com-puted, the former with and without the oceanic inverted barometer (lB) effect. It is found that V(motion) contributes significantly to the total excitation V overall and nonnegligibly to the annual signal in V, or the annual wobble excitation, in particular. Our results for the annual wobble excitation, in terms of the prograde component V + and the retrograde component V- for January 1, are V +--(16.8 milliarc seconds (mas),-93 ø) and V- = (15.6 mas,-98 ø) with lB, and V + = (17.3 mas,-101 ø) and V- = (28.1 mas,-112 ø) without lB. These results are within the (rather large) range of previous estimates. The lB effect has a small impact on V+, whereas its impact on V- is considerable. The (better determined) prograde components V + are then compared with that observed from the Lageos satellite laser ranging data: (17.3 mas,-61ø). Although the amplitudes are nearly equal, large phase discrepancies exist between the atmospheric and this observed value. The resolution of this discrepancy awaits a better knowledge of the seasonal angular momentum budget of the Earth's surface fluid elements. 1