Atmospheric and oceanic contributions to Chandler wobble excitation determined by wavelet filtering

摘要

Fluctuations of Earth rotation are associated with the redistribution and motion of mass elements in the Earth system. On seasonal to interannual timescales, the largest effects are due to mass redistributions within atmosphere and oceans. In order to study the Earth's reaction on geophysical excitations, the dynamic Earth system model DyMEG has been developed. It is based on the balance of angular momentum in the Earth system. The model is forced by consistent atmospheric and oceanic angular momenta from reanalysis data and a global ocean circulation model. As rotational deformations of the Earth are regarded, forced variations of Earth rotation due to atmospheric and oceanic excitations influence the free polar motion (Chandler wobble) of DyMEG. The comparison of the numerical model results with geodetic observations reveals a good agreement in both the annual and the Chandler frequency band. In order to support the presumption that the excitation energy of atmospheric and oceanic angular momenta within a spectral band around the Chandler frequency is sufficient to reproduce the observed Chandler wobble, the excitation series are band-pass filtered. For consistent filtering and signal analysis, wavelet techniques based on the Morlet function are applied. When DyMEG is forced with the filtered excitations, the resulting polar motion resembles the actually observed Chandler oscillation, which is determined from geodetic observations applying the same filtering method. Between 1980 and 2002 the correlation coefficient between the unconstrained model result and the observed Chandler wobble amounts to 0.99 and the root-mean-square difference is 16 milliarc seconds.