Excitation of nutation by the global radiational S1 tide

Monday, 15 December 2014: 11:50 AM
Michael Schindelegger1, David A Salstein2, David Einspigel3 and Johannes Boehm1, (1)Vienna University of Technology, Vienna, Austria, (2)AER Inc, Lexington, MA, United States, (3)Dublin Institute for Advanced Studies, School of Cosmic Physics, Dublin, Ireland
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.