Is there any frequency dependent time lag between atmospheric and geodetic excitation functions ? Wiesław Kosek 1 , Waldemar Popiński 2 , Harald Schuh 3 , Michael Schmidt 4 1. Space Research Centre, Polish Academy of Sciences, Warsaw, Poland, e-mail: kosek@cbk.waw.pl 2. Department of Standards, Central Statistical Office, Warsaw, Poland, e-mail: w.popinski@stat.gov.pl 3. Institute of Geodesy and Geophysics, University of Technology, Wien, Austria, e-mail: hschuh@luna.tuwien.ac.at 4. Deutsches Geodätisches Forschungsinstitut (DGFI), München, Germany, e-mail: schmidt@dgfi.badw.de ABSTRACT. The purpose ot these investigations is to find frequency dependent time lags between complex-valued polar motion and its atmospheric excitation using the wavelet and Fourier transform techniques. The wavelet transform with Morlet analysing function (MWT) (Chui 1992), harmonic wavelet transform (HWT) (Newland 1998) and the Fourier transform band pass filter (FTBPF) (Popiński and Kosek 1995) techniques are applied. All these methods enable changing the frequency resolution of the coherence, cross-covariance and time lag functions. These functions are computed from the wavelet transform coefficients representing two time series in time-frequency domain (Popiński and Kosek 1994) or from the outputs of the FTBPF. The frequency dependent time lags computed for oscillations with periods ranging from 3 to 250 days correspond to maxima of the modules of cross-covariance functions between the polar motion and atmospheric excitation functions. The statistical errors of the computed coherence and time lag functions were determined in the Monte-Carlo experiment using white noise data. The coherences are significant for all short period oscillations. Significant time lags were obtained for oscillations with periods of about -50, -85 and 115 days. 1. DATA SETS Atmospheric excitation functions: ib p w ib p w + + + + 2 , 1 χ χ - equatorial components of the effective atmospheric angular momentum (EAAM) reanalysis data in 1958.0-2002.2 from the U.S. NCEP/NCAR, the top of the model is 10 hPa (Barnes et al. 1983; Salstein et al. 1986; Kalnay et al. 1996; Salstein and Rosen 1997; AER 2002), Geodetic excitation functions: , - computed from the IERSC04 1 ψ 2 ψ y x, pole coordinates data in 1962- 2002 (IERS 2002) using the time domain Wilson and Haubrich (1976) deconvolution formula (Chandler period equal to 1 days , quality factor Q ). 435 / = c F 100 = c