Admittance of the Earth Rotational Response to Zonal Tide Potential

Abstract

This study is devoted to the determination of the admittance parameters describing the Earth rotational response to the components of the zonal tide potential. First, in order to better grasp the physical content of those admittance coefficients, we revisit the theoretical description of the length of day (LOD) changes at sub-decadal time scale, where forcing is dominated by zonal tides and hydro-atmospheric mass transports. This theoretical reminder specifies the rheological coefficients permitting to apply the hydro-atmospheric corrections to isolate the tidal part of the LOD. Then, the admittances are determined from the LOD series corrected from hydro-atmospheric contributions at the frequencies of the dominant zonal tidal terms between 7 and 365 days. In contrast of the former kindred studies, we both address the discrepancy of the results brought by various EOP series and the hydro-atmospheric corrections on the LOD. Our study forwards the complementary corrections brought by the ocean, the land water and sea level changes. Below 32 days, removing the atmospheric-oceanic excitation from LOD allows to much better constraint the admittance complex coefficients κ than applying the atmospheric correction only: the discrepancy with respect to modeled values is reduced up to 70%, and the frequency dependence of the imaginary part brought by the ocean dynamical response is confirmed. A systematic effect with respect to the values modeled by Ray and Erofeeva (2014), https://doi.org/10.1002/2013jb010830 has been detected and hints a defect of this model. Moreover, the role of land water and associated sea level variation is notable at the semi-annual period.