Excitation of nutation by the global radiational S1 tide

Abstract:

Cyclic mass redistributions in the atmosphere and oceans related to the global radiational S₁ tide elicit seasonal perturbations of Earth’s nutation at a level of 0.1 mas (milliarcseconds). The present study provides an up-to-date assessment of these excitation effects on the basis of 10-year surface and isobaric level data from three, previously unavailable global atmospheric reanalysis systems. We retrieve numerical values of in- and out-of-phase nutation corrections for seasonally modulated S₁ variations and indicate how model improvements, specifically in terms of the representation of tidal oscillations, lead to different estimates with respect to earlier reanalyses. Motion term signals in nutation display a close agreement across all probed datasets, whereas larger disparities remain among mass term excitation estimates due to their dependency on small-scale diurnal surface pressure oscillations. A simple time-stepping model for barotropic ocean dynamics, based on the shallow water equations and driven by air pressure tide climatologies, represents an appropriate means to determine global S₁ estimates of sea level heights and currents that are consistent with the respective forcing fields from each reanalysis. We address the intricacies of constructing such a model and compare our preliminary oceanic angular momentum solutions to those from more established hydrodynamic forward integrations. The combined influence of the S₁ tide on Earth’s nutation, associated with both atmosphere and ocean dynamics, is found to yield a rough agreement with observations from geodetic VLBI (Very Long Baseline Interferometry) measurements.