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.

No additional funding sources recorded.