Oceanic and soil moisture contributions to seasonal polar motion

Oceanic contributions to the annual and semi-annual wobble of polar motion have been evaluated by Wünsch (Wünsch, J., 2000. Oceanic influence on the annual polar motion. J. Geodynamics 30, 389–399) using three different ocean circulation models: (a) the Parallel Ocean Climate Model (Semtner, A.J., Chervin, R.M., 1992. Ocean circulation from a global eddyresolving model. J. Geophys. Res. 97, C4, 5493–5550; (b) the model used by Ponte et al. (Ponte, R.M., Stammer, D., Marshall, J., 1998. Oceanic signals in observed motions of the Earthʹs pole of rotation. Nature 391, 476–479); (c) the Hamburg Ocean Model for Circulation and Tides used by Thomas and Sündermann [Thomas, M., Sündermann, J., 1998. Zur simultanen Modellierung von allgemeiner Zirkulation und Gezeiten im Ozean und Auswirkungen auf bestimmte Erdrotations parameter. In: Freeden, W. (Ed.), Progress in Geodetic Science. Aachen, pp. 144–151]. That result is extended by considering oceanic refinements as well as time variable soil moisture and snow load. Five soil moisture models were used. The snow load according to Chao et al. (Chao, B.F., OʹConnor, W.P., Chang, A.T.C., Hall, D.K.,Foster, J.L., 1987. Snow-load effect on the Earthʹs rotation andgravitational field 1979-1985. J. Geophys. Res. 92, 9415–9422) was added to this in an attempt to close the balance. The NCEP/NCAR reanalysis atmosphere+Ponte et al. (1998) ocean+Chao and OʹConnor (Chao, B. F., OʹConnor, W. P., 1988. Global surface-water-inducedseasonal variations in the Earthʹs rotation and gravitationalfield. Geophys. J. 94, 263–270) (rain+snow) nearly close the annual balance of polar motion excitation, i.e. it is very close to the geodetic excitation functioncomputed from the time series of the IERS (International Earth Rotation Service). The other models of soil moisture differ in theirpolar motion contribution from the Chao and OʹConnor (1988) estimates. Further improvements in this work await results from the space gravity missions CHAMP, GRACE and GOCE.