The Global S[Formula: see text] Tide in Earth's Nutation

Understanding Earth's nutation requires precise modeling of tidal forces. Prior research has shown that atmospheric and oceanic tides influence Earth's rotation. However, matching these effects with geodetic observations has proven difficult. Established models often fail to align with Very Long Baseline Interferometry (VLBI) data. This gap motivated the current study to reassess the accuracy of modern geophysical models. No prior work had resolved the discrepancy between simulated and observed S₁₁ tides. The study focuses on diurnal tidal oscillations in the atmosphere-ocean system. These oscillations induce small perturbations in Earth's prograde annual nutation. The challenge lies in reconciling model outputs with empirical geodetic measurements.

The study aims to evaluate the accuracy of modern geophysical models in predicting the S₁₁ tidal signal. This signal is crucial for understanding Earth's nutation. The specific problem involves the mismatch between model estimates and VLBI observations. The motivation stems from the need to improve nutation theories. The authors seek to determine whether current models can reproduce observed tidal effects. They focus on the Sun-synchronous rotation signal induced by S₁₁ tides. The study tests four atmospheric assimilation systems and a barotropic ocean model. The goal is to identify which models best align with empirical geodetic data.

The study employs four modern atmospheric assimilation systems and a barotropic ocean model. These models are forced with consistent meteorological data. The ocean model dissipates excess energy via a tidal conversion scheme. The researchers validate air pressure tides against in situ barometric estimates. They compare simulated sea surface elevations with tide gauge data. The study spans a 10-year period from 2004 to 2013. The focus is on diurnal tidal oscillations in the atmosphere-ocean system. The models are evaluated for their ability to reproduce the observed S₁₁ nutation signal.

Two of the tested datasets produced S₁₁ terms deviating by over 30 microarcseconds from VLBI observations. The MERRA and ECMWF operational models showed the closest agreement with observed data. Their estimated nutation contributions were 15.6 ± 3.2 microarcseconds (MERRA) and 15.4 ± 3.0 microarcseconds (ECMWF). These values align closely with the VLBI-observed harmonic of 15.5 microarcseconds. The study found partial deficiencies in some models' diurnal band outputs. Validation against barometric and tide gauge data confirmed these limitations. The MERRA and ECMWF models demonstrated superior accuracy in simulating S₁₁ tides. This agreement suggests these models can better inform future nutation theories.

The study concludes that the MERRA and ECMWF models most accurately represent the S₁₁ tidal signal. Their nutation estimates align closely with VLBI observations. This finding supports the use of these models in future nutation theories. The authors propose that improved model outputs will enhance the a priori account of Earth's prograde annual motion. The study does not claim that all models are equally accurate. It suggests that contemporary meteorological data alone does not guarantee precise nutation estimates. The results highlight the importance of validating models against empirical data. The agreement between model outputs and observations may aid in refining celestial motion theories.