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SCORE

Simulating Oceanic Contributions to Earth Rotation

 

Funding Agency

FWF - Der Wissenschaftsfonds

Duration (TU Wien)

01.04.2017 – 31.01.2018

Duration (Bonn University)

01.02.2018 – 31.03.2021

Principal Investigator

Prof. Dr. techn. Michael Schindelegger

Contributing Scientists

M.Sc. Alexander Harker (PhD Student)

M.Sc. Daniel Kotzian (Research Assisstant)

Dr. Makan Karegar


Project Description

Ocean waters are flowing in complex patterns affected by winds, bottom topography, and the gravitational attraction from heavenly bodies. The movement of masses associated with these processes have long been recognized as important source for changes in Earth’s gravity field and small yet measurable irregularities in our planet’s rotation. Knowledge of these signals is an essential element in various scientific and practical endeavors, such as navigation, the construction of a global coordinate system for surveying activities, and gravity field analysis with the dual-satellite mission GRACE (Gravity Recovery and Climate Experiment). In this project, we explored relatively fast ocean dynamics, on time scales of a fraction of a day out to several months, and the resultant changes in Earth’s rotation, Earth’s figure (the sea surface in this case), and ocean bottom pressure. The methodological challenge consisted in the dedicated use and development of numerical ocean models to solve the known equations of fluid motion.


Key Results

  • Daily GRACE gravity field solutions provide a realistic description of ocean bottom pressure variability with periods as short as ~4 days. Numerical models with horizontal grid spacing of ≤30 km and energy losses concentrated over undulating seafloor are most commensurate with GRACE and best capture fast circulation changes (Schindelegger et al. 2021).

  • Modeled oceanic angular momentum changes explain as much as 80% of the sub-seasonal, non-atmospheric oscillations of the pole of rotation (Figure 1), significantly higher than previously thought. Both finer horizontal resolution and greater dissipation levels, e.g., due to scattering of enhanced bottom velocities at sharper topographic gradients, are playing a role (Harker et al. 2021).

  • First successful inclusion of the ocean self-attraction and loading term in a high-resolution barotropic tide model (Schindelegger et al. 2018) based on two-way spherical harmonic transforms of surface elevations at each time step.


  • Fig3_Harker_etal

    Figure 1: Evaluation of non-tidal oceanic contributions to polar motion (mas) in different spectral bands and from four different ocean models (MPIOM, ECCOv4, LLC540 - eddy-permitting, DEBOT - barotropic model with topographic wave drag scheme). Vertical bars, referred to by the left axis, show the percentage of variance explained by each model in observed and atmosphere-corrected polar motion. Solid lines display the RMS of the residual rotation series, after subtraction of atmospheric and oceanic contributions (adapted form Harker et al. 2021).


    References

    • Schindelegger, M., Green, J.A.M., Wilmes, S.-B., Haigh, I.D. (2018), Can we model the effect of observed sea level rise on tides? Journal of Geophysical Research: Oceans, 123, 4593–4609, doi: https://doi.org/10.1029/2018JC013959.

    • Schindelegger, M., Harker, A.A., Ponte, R.M., Dobslaw, H., Salstein, D.A. (2021), Convergence of daily GRACE solutions and models of submonthly ocean bottom pressure variability. Journal of Geophysical Research: Oceans, 126, e2020JC017031, https://doi.org/10.1029/2020JC017031.

    • Harker, A.A., Schindelegger, M., Ponte, R.M., Salstein, D.A. (2021). Modeling ocean-induced rapid Earth rotation variations: An update. Journal of Geodesy, 95, 100, https://doi.org/10.1007/s00190-021-01555-z.

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