On symmetry and anisotropy of Earth-core flows

Abstract

Quasi-geostrophic (QG) flows are a recently developed and very promising paradigm for modeling decadal secular variation (SV). Here we examine the effects of allowing anisotropy and departures of the flow from quasigeostrophy. We perform dedicated numerical experiments of the flow dynamics and magnetic induction inside the Earth's liquid core at time scales characteristic of secular variation of the geomagnetic field. Obtained results motivate new flow inversion regularization featuring an equatorially anti-symmetric component superimposed to quasi-geostrophic columns, and stronger latitudinal than longitudinal flow gradients. Applying these constraints allows to explain the observed SV for the whole period 1840-2010, and most significantly, provides a clearly improvement in prediction for decadal length-of-day variations for the period 1980-2000. Furthermore, the trace of the inner-core appears clearly without any assumption for the 1997-2010 period covered by satellite geomagnetic data. Our results support QG being the appropriate description of the force balance within the core on decadal time scales and large spatial scales. Copyright 2011 by the American Geophysical Union.