Summary: Physics of the Earth and Planetary Interiors 155 (2006) 120139
Time-average and time-dependent parts of core flow
Hagay Amit, Peter Olson
Department of Earth and Planetary Sciences, Johns Hopkins University,
Baltimore, MD 21218, USA
Received 15 February 2005; received in revised form 30 October 2005; accepted 30 October 2005
We infer fluid motions below the coremantle boundary by inverting geomagnetic secular variation data over a 150 years, assuming
helical-geostrophic flow. We obtain snapshot images of core flow at 5-year intervals, which we combine to give time-average and
time-dependent parts of the motion over this time interval. The most prominent time-average flow structure is a large anti-cyclonic
vortex in the southern hemisphere beneath the Atlantic and Indian Oceans. The time-average zonal core flow outside the inner core
tangent cylinder is significantly westward in the southern hemisphere but nearly zero in the northern hemisphere. Westward polar
vortices occur inside the tangent cylinder in both hemispheres, particularly in the north. In terms of mantle versus core origins, mantle
driving appears to be responsible for the mid-latitude asymmetry in the zonal core flow, whereas core driving appears to be responsible
for the flow at high latitudes. We also compare changes in the core's angular momentum calculated from our time-dependent core flow
with changes in the mantle's angular momentum derived from decade-scale length-of-day variations and find adequate agreement.
A fit of our time-dependent core flow to a torsional oscillations model yields dominant periods of 110 and 53 years.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Core flow; Geomagnetic secular variation; Thermal wind; Mantle tomography; Geodynamo; Length-of-day variations; Torsional oscil-