Modeling ocean-induced rapid Earth rotation variations: an update

We revisit the problem of modeling the ocean’s contribution to rapid, non-tidal Earth rotation variations at periods of 2–120 days. Estimates of oceanic angular momentum (OAM, 2007–2011) are drawn from a suite of established circulation models and new numerical simulations, whose finest configuration is on a "Image missing"$$^\circ $$ ∘ grid. We show that the OAM product by the Earth System Modeling Group at GeoForschungsZentrum Potsdam has spurious short period variance in its equatorial motion terms, rendering the series a poor choice for describing oceanic signals in polar motion on time scales of less than $$\sim $$ ∼ 2 weeks. Accounting for OAM in rotation budgets from other models typically reduces the variance of atmosphere-corrected geodetic excitation by $$\sim $$ ∼ 54% for deconvolved polar motion and by $$\sim $$ ∼ 60% for length-of-day. Use of OAM from the "Image missing"$$^\circ $$ ∘ model does provide for an additional reduction in residual variance such that the combined oceanic–atmospheric effect explains as much as 84% of the polar motion excitation at periods < 120 days. Employing statistical analysis and bottom pressure changes from daily Gravity Recovery and Climate Experiment solutions, we highlight the tendency of ocean models run at a 1$$^\circ $$ ∘ grid spacing to misrepresent topographically constrained dynamics in some deep basins of the Southern Ocean, which has adverse effects on OAM estimates taken along the 90$$^\circ $$ ∘ meridian. Higher model resolution thus emerges as a sensible target for improving the oceanic component in broader efforts of Earth system modeling for geodetic purposes.

Modeling rapid Earth rotation variations – where are we going next?

&lt;p&gt;Non-tidal Earth rotation variations at intraseasonal periods are almost exclusively driven by mass redistributions within the atmosphere and ocean. Our capacity to model these signals has advanced over the past decades, but differences between the observed and modeled portions of the planetary angular momentum budget are still as large as 1&amp;#8211;5 cm when expressed as axis displacement at the Earth's surface. A likely source for these significant errors is the ocean, poorly sampled with observations and thus not amenable to the sequential data assimilation machinery developed for the atmosphere. Moreover, the recent delineation of basin-wide ocean mass exchanges associated with the Madden-Julian Oscillation (MJO) in a high-resolution baroclinic model emphasizes that a revisit of standard forward modeling choices (e.g., grid spacings of &amp;#8764;100 km) may be in order to better describe rapid, large-scale oceanic mass motions. In this contribution, I provide a brief overview of recent progress in the field and suggest that dynamically consistent model-data syntheses, as practiced by the consortium on Estimating the Circulation and Climate of the Ocean (ECCO), are a viable route to mitigate deficiencies in present oceanic angular momentum (OAM) series on intraseasonal (but also longer) time scales. As ocean state estimates continue to be refined by the central ECCO production, I assess the benefits of higher model resolution with OAM series from an eddy-permitting (1/6&amp;#176;) forward simulation, descending from the current ECCO release in its discrete setup. The resulting mass and motion terms indeed provide smaller Earth rotation residuals than other available OAM estimates, possibly due to the model resolving important topographic interactions and the dynamic response to MJO in the 30&amp;#8211;80-day band. However, these improvements come at disproportionally large computational costs, and iteratively fitting an eddy-permitting general circulation model to oceanographic observations may still be prohibitive in the near future. Instead, efforts should be devoted to extending the present coarser-resolution ECCO framework to new data constraints and shorter adjustment intervals. Of particular interest in the context of Earth rotation are non-standard daily GRACE gravity field solutions, which contain realistic information on oceanic mass-field variability below the nominal GRACE Nyquist period of 60 days.&lt;/p&gt;

Earth rotation variations observed by VLBI and the Wettzell “G” ring laser during the CONT17 campaign

The VLBI (Very Long Baseline Interferometry) technique can provide the full set of parameters needed for the transformation between celestial and terrestrial reference frames with high accuracy. Yet it has some limitations regarding temporal resolution and continuity, and the accuracy of the resulting Earth Orientation Parameters (EOP) varies depending on the network geometry. In this work we explore the benefit of combining VLBI observations with the measurements of the large ring laser gyroscope “G” in Wettzell for deriving highly resolved ERP (Earth Rotation Parameters, i.e. polar motion and universal time variations, δUT1). We examine the observations collected by two simultaneously operating VLBI networks during the 15 d of the CONT17 campaign. These two networks, of 14 globally distributed telescopes each, were designed for the estimation of Earth rotation variations, for which reason the resulting hourly ERP are appointed as benchmark in this investigation. To evaluate the advantage of a VLBI and ring laser combined solution, we create degraded versions of the original networks, containing only six stations. The ERP derived from those sparse networks and from the VLBI sparse plus ring laser solutions are then compared in terms of differences to the reference values. It should certainly be considered that these are relative numbers, since they are also determined by the number and selection of the stations remaining in the sparse networks. The root mean square of the difference to the benchmark is reduced by 24 % in case of δUT1 from one network. The polar motion yp component from the same network moves 14 % closer to the reference value due to the inclusion of the ring laser data. The impact on xp and on all ERP from the other network ranges between 2 % and 9 %. The research again confirms the feasibility and also the potential gain of a combined evaluation of VLBI and ring laser observations, but the full capacity of such a sensor fusion will emerge once the ring laser gyroscopes reach a level of accuracy similar to VLBI.