ALBERT

Description: Changes in the oceanic current system and in the oceanic mass distribution alter the state of the Earth’s free rotation which is characterised by length of day and polar motion. The study of this connection was the challenge and the subject of this thesis. To this end, the oceanic state was estimated by assimilating Earth rotation observations with a global ocean model. The considered time span was 1993 to 2002. Although assimilation is a well established tool in climate science the assimilation of Earth rotation observations with a global ocean model is done for the first time. Before the assimilation, the observations had to be projected onto the angular momentum of the ocean. Non-oceanic contributions were removed. The result of the assimilation procedure is a time varying ocean model state, i.e. a trajectory, that reproduces the Earth rotation observations. This trajectory was studied to understand the generation of Earth rotation deviations by the oceans. The governing physical mechanisms could be identified: First, changes in length of day are attributed to changes in total ocean mass. These changes are determined by the surface freshwater flux from the atmosphere to the ocean. Changes in the ocean current system have a minor contribution to length of day changes. Second, the excitation of polar motion is connected to the inhomogeneous distribution of ocean mass. These inhomogeneities are a consequence of the wind and buoyancy-driven currents. The results of this non-uniform mass distributions are currents too, i.e. geostrophic currents. This way the oceanic excitation of polar motion splits up into contributions from currents and mass distribution. Both contributions entail each other and are highly correlated