Simulating Oceanic Contributions to Earth Rotation

Ocean waters are constantly on the move, flowing in complex patterns affected by winds, atmospheric pressure, temperature differences, bottom topography, and the gravitational attraction from celestial bodies. The mass redistributions associated with these circulations have long been recognized as major sources for irregularities in the Earths rotation, changing both the orientation of our planet in space (nutation) as well as the trajectory of a terrestrial reference axis at the surface of the Earth (polar motion). Knowledge of ocean dynamics and the corresponding rotational variations is thus integral to any scientific problem involving spaceborne observations, but particular aspects of the problem are yet unexplored. Project SCORE (Simulating Oceanic Contributions to Earth Rotation) specifically addresses the oceanic circulation on sub-monthly time scales, using two elaborate numerical ocean models that solve the hydrodynamic flow equations for both single-layer and multi-layer configurations. These models describe the nature of the ocean flow in response to atmospheric pressure, wind, and thermodynamic forcing. In SCORE, they are employed to gain insight into two hitherto unconsidered problems of ocean-related Earth rotation research. First, it is suggested that changes in the tropical weather due to the El Niño-Southern Oscillation (ENSO) enhance the daily cycle in the atmosphere and that the resultant pressure forces at the sea surface alter the oceanic tide of diurnal (24-hour) periodicity. The associated redistribution of water masses may be sufficiently strong to affect nutation and tilt our planet in space. Dedicated ocean model simulations are thus performed under ENSO conditions, using multi-year pressure forcing data from numerical weather models as well as barometric benchmark values from in situ sensors at tropical islands and moored buoys. The simulated ocean mass variations and velocities allow for an estimation of recent ENSO signals in Earths nutation and lend themselves to an independent validation against observed Earth orientation changes. The project is complemented by a systematic study of the ocean response to atmospheric wind and pressure input with periods from 2 to 20 days. The particular objective is to clarify the relevance of some key components of the hydrodynamic flow equations in describing ocean- induced polar motion on short time scales. In particular, numerical experiments are performed to assess the dependence of Earth rotation results on the models horizontal resolution, the formulation of internal frictional processes, and the inclusion of feedback effects when water masses attract themselves. The outcome of these simulations will be an optimally-configured ocean model capable of reducing remaining discrepancies between observed and geophysically modeled Earth rotation signals.

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. The key result from our investigation of ocean bottom pressure changes is that daily gravity field solutions, derived mostly from GRACE observations alone, provide a realistic description of water mass motion with periods as short as ~4 days. These novel data products can therefore help identify deficiencies in oceanographic models, pertaining to, e.g., how the energy input from atmospheric winds is lost to friction in the ocean's interior or at the seafloor. Indeed, comparisons of GRACE with ocean model output show that numerical simulations that have fine grid spacing (that is, high horizontal resolution) and focus energy losses over undulating seafloor best capture fast circulation changes. A complementary study of ocean-induced Earth rotation variations also lends credence to numerical models with finer grid spacing than present standards in this research area. Specifically, a global model simulation that accounts for energy pathways in swirls of ~50 km horizontal extent suggests that the ocean's contribution to rapid oscillations of the pole of rotation is 36% in relative terms, significantly higher than previously thought. From a methods' perspective, we presented the first successful inclusion of the attraction effect of changing ocean masses onto themselves in a high-resolution numerical model. This development particularly benefits the accuracy by which we can simulate tides, i.e., the regular rise and fall of the sea surface along the coast. In supplemental work beyond the original project idea, we therefore quantified, by numerical means, long-term changes of ocean tides in response to present-day sea-level rise and different continental configurations over geological eras. The latter analysis shows that both the geometry of the ocean basins and the faster Earth rotation rate billions of years ago had a significant influence on ocean tides and changes in the orbit of the Moon with respect to the Earth.