In this topic, geophysical causes of decadal variations of the Earth's rotation are investigated. After subtraction of contributions of surface processes due to, e.g., atmospheric and oceanic dynamics, from observed Earth orientation parameter (EOP) significant residuals are obtained. These can only be explained by internal processes, mainly by the exchange of angular momentum between core and mantle due to core-mantle coupling. This hypothesis is confirmed by the well-known finding that the decadal variations of the EOPs are strongly correlated with those of the geomagnetic field.
In this research subject, two types of core-mantle coupling are considered: the electromagnetic and the topographic coupling. Both mechanisms produce significant variations of the components of the vector of the Earth's rotation, the length of day and polar motion, if the assumed values of the parameters of the core-mantle transition zone, i.e., the electrical conductivity of the lowermost mantle and the topographic height of the core-mantle boundary (see figure), are sufficiently high.
To compute the associated torques, the geomagnetic field at the CMB and in the mantle as well as the velocity of the fluid flow at the top of the core must be known (flow chart). To infer these fields from the geomagnetic surface field, we use the non-harmonic downward continuation (NHDC) method, developed at GFZ for the poloidal field, with is observable at the Earth's surface. Moreover, a method was developed to compute the toroidal field from the poloidal one and the surface flow of the liquid core (for further information, see STR08/06). The velocity of the fluid flow at the top of the core is also required to compute the topographic coupling torque: The fluid flow exerts a dynamic pressure on the CMB topography, which causes a coupling torque between mantle and core (for further information, see STR08/11). In addition to the use of these new methods for studies of the coupling and core motions, this gives new insight into the geomagnetic field variations at the CMB and supports studies of transient phenomena like geomagnetic jerks at the CMB.
Systematic investigations of different conductivity models of the Earth's mantle and CMB topography models show significant contributions of the electromagnetic and topographic coupling to the decadal variation of EOPs. Moreover, this comparison highlights the necessity to consider further kinds of coupling. Recently, we developed a theoretical description of the gravitational coupling torques, which considers the interactions between the gravitational potential of the mantle and the relative motion (rotation) of the inner core. This new coupling mechanism should be combined consistently with the other coupling mechanisms in a joint-coupling model, which also considers interactions between the coupling processes.
This investigation was embedded into the project 4 of the DFG research unit FOR 584. The geomagnetic field at the Earth's surface, as input for the downward continuation to the CMB, and the velocity field at the core surface were provided within a cooperation with section 2.3 "Earth's Magnetic Field" at the GFZ.