GRACE observations of rapid mass variations at the core-mantle boundary during deep mantle phase transitions in interaction with core flow

Sudden changes in the secular variation of the geomagnetic field, the geomagnetic jerks, provide information about the dynamics of the core at short timescales. How this dynamics may be coupled to changes in the core-mantle boundary (CMB) topography is not fully understood, due to the difficulty of obtaining direct observations on this region. Yet, it could be a key factor in explaining rapid changes in the geomagnetic field. Here, we use satellite measurements on the Earth’s gravity field variations in order to constrain potential mass redistributions at the CMB. We conduct an analysis of GRACE satellite and Satellite Laser Ranging (SLR) measurements of the Earth's gravity field from 2003 to 2015. The combination of second-order spatial derivatives of the gravity potential with a multi-scale temporal analysis allows for an enhanced separation of superimposed signals in the gravity field, based on their different spatial patterns and timescales. This way, we identify a significant transient north-south gravity anomaly at the boundary between the Atlantic Ocean and the African continent with maximum intensity in January 2007, with a timescale of 2-3 years. This signal cannot be fully explained by variations in surface water mass sources, suggesting an origin within the solid Earth. We show that the observed anomaly may be associated with mass redistributions during perovskite-to-post-perovskite phase transition triggered by moving thermal anomalies in the African Large Low Shear Velocity Province (LLSVP). This dynamic process results in decimetric variations in the CMB topography over months to years. We study how core flows in a stratified layer at the top of the core are impacted by this topography change, and discuss the corresponding signals in the geomagnetic field.