Time-average and time-dependent parts of core flow

We infer fluid motions below the core–mantle 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.