Evidence for Stable Stratification at the Top of the Core from Core Surface Flows

Abstract:

We investigate the possibility of a stably stratified layer through core surface flow modelling. Motivated by the results of Buffett (2014), we seek a constant background flow, with time-varying flow which is both zonally and equatorially symmetric. Part of the time-varying flow includes an zonal, azimuthal (poloidal) motion. This feature is novel, as the standard flow non-uniqueness assumptions (for example, tangential geostrophy and toroidal flow) exclude this flow component. Indeed, a steady flow in a stably stratified medium does not allow such a flow; however, a poloidal flow is expected through wave motion in the stably stratified layer. The resulting flow is well able to explain features of the magnetic secular variation (for example, geomagnetic jerk signatures that are not explained by purely torsional motion). That a particular flow model can explain the data is not of particular significance - many different flow assumptions can provide such a fit. However, when a simple harmonic wave is fit to each flow component, then good variance reduction is achieved for a 50 year period, and the different modes display strong coherence in phase - broadly, successive toroidal modes are 180 degrees out of phase, and the poloidal components generally lag the toroidal by about 6 years. These coherent properties provide evidence in favour of this interpretation. We use a simple physical model of wave motion to relate the toroidal and poloidal components of flow, and interpret the strong phase coherence in terms of the properties of the layer, including the layer thickness and strength of stratification.

 Buffett, B., 2014. Geomagnetic fluctuations reveal stable stratification at the top of the Earth’s core. Nature, 507, 485-487.