Contribution of Non-Tidal Oceanic Mass Variations to Polar Motion Determined from Space Geodesy and Ocean Data

In order to assess the contribution of non-tidal oceanic mass changes to polar motion, equatorial oceanic excitation functions are determined from combinations of space geodetic techniques and ocean data. Satellite altimetry provides accurate information on sea level anomalies (SLA) which are caused by mass and volume changes of seawater. Since Earth rotation is solely affected by mass variations the volume (steric) effect has to be reduced from the observations in order to infer oceanic contributions to Earth rotation. Oceanic polar motion excitations from reduced SLA are compared with respective results from ocean models. Contributions of ocean currents, atmospheric and hydrological effects are added in order to validate the oceanic excitations from altimetry with independent geodetic observations which reflect the integral effect of a multitude of geophysical processes in the Earth system. This requires an investigation of accuracy and consistency of the combined data sets. The study reveals that model-only combinations of atmospheric, oceanic and hydrological excitations agree better with geodetic observations than combinations which include excitations from altimetry. Possible reasons could be errors in the steric reduction of SLA and/or the compensation of erroneous patterns in atmosphere data by numerical ocean models.