The Global S $_1$ Tide in Earth’s Nutation

Surveys in Geophysics, Feb 2016

Diurnal S$_1$ tidal oscillations in the coupled atmosphere–ocean system induce small perturbations of Earth’s prograde annual nutation, but matching geophysical model estimates of this Sun-synchronous rotation signal with the observed effect in geodetic Very Long Baseline Interferometry (VLBI) data has thus far been elusive. The present study assesses the problem from a geophysical model perspective, using four modern-day atmospheric assimilation systems and a consistently forced barotropic ocean model that dissipates its energy excess in the global abyssal ocean through a parameterized tidal conversion scheme. The use of contemporary meteorological data does, however, not guarantee accurate nutation estimates per se; two of the probed datasets produce atmosphere–ocean-driven S$_1$ terms that deviate by more than 30 $\upmu$as (microarcseconds) from the VLBI-observed harmonic of $-16.2+i113.4$ $\upmu$as. Partial deficiencies of these models in the diurnal band are also borne out by a validation of the air pressure tide against barometric in situ estimates as well as comparisons of simulated sea surface elevations with a global network of S$_1$ tide gauge determinations. Credence is lent to the global S$_1$ tide derived from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) and the operational model of the European Centre for Medium-Range Weather Forecasts (ECMWF). When averaged over a temporal range of 2004 to 2013, their nutation contributions are estimated to be $-8.0+i106.0$ $\upmu$as (MERRA) and $-9.4+i121.8$ $\upmu$as (ECMWF operational), thus being virtually equivalent with the VLBI estimate. This remarkably close agreement will likely aid forthcoming nutation theories in their unambiguous a priori account of Earth’s prograde annual celestial motion.

This is a preview of a remote PDF: https://link.springer.com/content/pdf/10.1007%2Fs10712-016-9365-3.pdf

Michael Schindelegger, David Einšpigel, David Salstein, Johannes Böhm. The Global S $_1$ Tide in Earth’s Nutation, Surveys in Geophysics, 2016, 643-680, DOI: 10.1007/s10712-016-9365-3