Excitation of Nutation as Deduced from Results of the Recent Atmospheric Reanalysis Project

Nearly diurnal variations in the equatorial components of the atmospheric angular momentum are expected to excite minor but well measurable nutational motions of the earth’s pole. These motions include the free core nutation (FCN) of variable amplitude between 100 and 500 microarcseconds (µas), as well as a contribution to the amplitudes of some important constituents of the lunisolar nutation. We investigate this problem using a 29-years long homogeneous series of the 4-times daily EAM (effective angular momentum) estimates based on results of the common U.S. NCEP/NCAR reanalysis project. The most important atmospheric contributions are found for the following nutation constituents: prograde annual (77 µas), retrograde annual (53 µas), prograde semiannual (45 µas), and for the constant offset of the pole (dψ sin e0 = -86 µas, de = 77 µas); among them only the prograde semiannual component is driven mostly by the wind term of the EAM function while in all other cases the pressure term is dominating. Comparison with the VLBI corrections to the IAU 1980 nutation model and taking into account the ocean tide contribution, yields a good agreement for the prograde annual and semiannual nutations, that is our estimation receives an important observational confirmation. We also investigated time variability of the atmospheric contribution to the nutation amplitudes by performing the sliding window least squares analysis of both the atmospheric excitation and VLBI nutation data.