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2010-11-03 – SEMINAR by Jan Hagedoorn: The core-mantle coupling contribution to the excitation of the Earth’s rotation variation on decadal time scale

3 November, 2010 (16:00), 5 Merrion Square, Dublin 2.

Speaker: Jan Hagedoorn, Helmholtz Center Potsdam, GFZ German Research Center for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany.
Title: The core-mantle coupling contribution to the excitation of the Earth’s rotation variation on decadal time scale.

Abstract:

Decadal variation of the Earth’s rotation could not be fully explained by surface processes, i.e. by angular momentum exchange of the fluid subsystems of atmosphere and ocean with the mantle. In an extended model, we consider additionally internal coupling mechanism at the core-mantle boundary, the electromagnetic and topographic core-mantle coupling. Based on certain combination of input data like geomagnetic field at the Earth’s surface, the electric conductivity of the Earth’s mantle and the topography of the core-mantle boundary, the electromagnetic and topographic coupling torques are determined for the time interval 1962–2000. Our investigation is restricted to this time interval, because the considered atmospheric and oceanic angular momentum functions are based on the ERA-40 reanalysis provided by the ECMWF, which are given for this time interval. We combine the coremantle coupling torques with the angular momentum exchange at the surface by equivalent excitation functions for the coupling torques, which can be combined with the both angular momentum time series. Our forward computation of the variation of polar motion and length-of-day on decadal time scale leads to model results, which are in the same order of magnitude as the observation in all three components on the decadal time scale.

The differences between the observations and our model results are used as a constraint for the gravitational coupling of the inner core with the Earth’s mantle. For a simplified model, where the inner core is considered as two-axial ellipsoid with constant density, we estimate the angle of misalignment of the equatorial plane of the inner core and the mantle causing a gravitational coupling torque, which would be necessary to explain the differences to the observed decadal variation of polar motion. The values of this angle are below 0.05◦ and in a range, which was also proposed by other authors before. The first results for this torque approach are motivating to incorporate the gravitational coupling torque in a joint coupling model.