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Atmospheric and oceanic forcing in polar motion and length of day

Atmospheric and oceanic forcing in polar motion and length of day. DANOF / UMR 8630 Observatoire de Paris 61 avenue de l’Observatoire 75014 Paris - France E-Mail : Christian.Bizouard@obspm.fr. C. Bizouard, S. Lambert. Atmospheric and oceanic effects on Earth rotation.

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Atmospheric and oceanic forcing in polar motion and length of day

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  1. Atmospheric and oceanic forcing in polar motion and length of day DANOF / UMR 8630Observatoire de Paris 61 avenue de l’Observatoire 75014 Paris - France E-Mail : Christian.Bizouard@obspm.fr C. Bizouard, S. Lambert

  2. Atmospheric and oceanic effects on Earth rotation • Recent studies [Brzezinski and Nastula (2000), Gross (2000)] have shown that the combined forcing of atmosphere and oceans better matches the observed polar motion (PM), especially the Chandler term. • Whereas Atmospheric Angular Momentum (AAM) data are provided routinely since 1979, Oceanic Angular momentum (OAM) series are still sparse. Their production has been initiated in 1998. • This study is a validation as well as an extension of former works • Length of day (LOD) is considered • Correlation between geodetic and geophysical excitation functions is fully analysed • Effects on seasonal components of PM and LOD are estimated.

  3. Data • AAM IBO time series from 1968.0 to 2000.0, NCEP/NCAR Reanalysis project, sampled at 0.25 day • OAM time series from 1985.0 to 1996.3 by Ponte (1998), sampled at 5 days • Polar motion / UT1-TAI from 1958.0 to 2000.0, IERS combined series (C04), sampled at 1day

  4. Preparation of data • Tidal terms (9.12, 9.13, 13.63, 0.52, 13.66, 27.56 days) removed from polar motion • The effect of zonal tides is removed from UT1 • We derived AAM, OAM and geodetic excitation functions sampled at 5 days and spanning 1985.0-1996.3 • 1 year, 1/2 year and 1/3 year terms and trend are then estimated in excitation functions and removed

  5. Seasonal terms in equatorial excitation functions

  6. Seasonal terms in excitation functions (II) Seasonal terms in equatorial excitation functions

  7. Seasonal terms in axial excitation functions OAM series does not improve significantly the budget

  8. x y z Observed AAM-IB 0.384 0.557 0.755 Observed AAM-IB+OAM 0.526 0.636 0.755 Global Correlation of excitation functions • Global correlation of geophysical excitation functions with geodetic one over 1985-1996 (seasonal terms included)

  9. Coherence analysis : equatorial excitation • Coherence is increa-sed by adding oceanicexcitation • Better coherence for prograde component • The increase concerns the frequencyband [-20 cpy,20 cpy], especially around +1 cpy • The power of geophysical excitation is too large for frequency larger than 10 cpy

  10. AAM+OAM : 33% of the frequency components have a phase/C04 in the interval [-20°,20°] • AAM : 27% of the frequency components have a phase/C04 in the interval [-20°,20°]

  11. Coherence analysis : axial excitation (I)

  12. Coherence analysis of axial excitations : Ratio of amplitudes

  13. FFT around the Chandler frequency By assuming a quality factor Q=170 for the Chandler term fc=0.843 cpy (Wilson and Vicente, 1980) and a white noise excitation (constant PSD), 99% of the excitation comes from the band [fc+Df/2,fc-Df/2], with Df=0.32 cpy, that is [0.685 cpy,1.001 cpy].

  14. Power at the Chandler frequency

  15. Coherence around the Chandler frequency • Mean coherence C04-AAM over the span [fc+Df/2,fc-Df/2], at the Chandler frequency : 0.16 ; at 0.843 cpy : 0.85 • Mean coherence C04-AAM+OAM over the span [fc+Df/2,fc-Df/2] at the Chandler frequency (0.843 cpy) ; at 0.843 cpy : 0.79

  16. Phases of excitations at the Chandler frequency • Phase of the combined atmospheric-oceanic excitation with respect to the geodetic one at the Chandler frequency : 26° • This means that OAM+AAM precedes the geodetic excitation by about 31 days (similar results reported by Brzezinski and Nastula)

  17. Conclusion • Seasonal terms in polar motion excitation and LOD : • Equatorial component : Taking into account oceans together with the atmosphere explains almost all the observed geodetic excitation at seasonal periods (1 year, 0.5 year, 1.3 year) except for semi-annual amplitude • The non tidal oceanic excitation is negligible for the axial component • Coherence between geophysical and geodetic excitations : • Correlation is improved significantly by adding OAM to AAM, for frequency spanning [-10 cpy,10 cpy] • Below 36 days (10 cycle / year) too much power in geophysical excitations (AAM as well AAM + OAM) • Chandler term : • adding OAM to AAM explains by about 80% of the power of the Chandler excitation, but still remains a phase shift of 1 month • AAM and OAM are more or less in phase at 0.845 cpy and contribute constructively to Chandler term

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