NONTIDAL OCEANIC EXCITATION OF NUTATION AND DIURNAL/SEMIDIURNAL POLAR MOTION REVISITED

Abstract

A new 7.5-yearlong time series of hourly estimates of nontidal ocean angular momentum, representing signals forced by 6-hourly surface atmospheric pressure and winds including those forced by the atmospheric tides, is used to assess the influence of a dynamic ocean on nutation and on diurnal and semidiurnal polar motions. Using available atmospheric angular momentum data, we estimate the total excitation by the dynamically coupled atmosphere-ocean system and compare it to that of the atmosphere alone. In case of the retrograde diurnal excitation, significant contributions are found for the retrograde annual, prograde annual, and prograde semiannual nutations and for the constant offset of the celestial pole. For the prograde annual nutation corresponding to the retrograde S1 component of excitation, the atmosphere-ocean model does not improve the agreement with VLBI observations beyond that obtained using the atmosphere model alone. We also considered separately an irregular contribution to nutation, which can excite both the free core nutation signal and other broadband variability. The irregular component of the atmosphere-ocean model is found to be significantly correlated with that derived from the atmosphere model. For the prograde diurnal component of excitation, which contributes to polar motion, the only nonnegligible effect (~9 microarc seconds (muas)) is associated with the prograde S1 harmonic. The only significant component of the semidiurnal excitation found is the elliptical oscillation S2 in polar motion with major semiaxis of 9 muas, but this estimate is uncertain because the 6-hourly atmospheric fields used to force the ocean do not properly resolve semidiurnal signals. In any case, this contribution is much smaller than that from the S2 ocean gravitational tide.