PROPERTIES OF STEADY FLOWS AT THE CORE MANTLE BOUNDARY IN THE FROZEN-FLUX APPROXIMATION

Steady flows covering the epochs 1960-1980 produced by the method of Whaler and Clarke are examined in terms of the partitioning of energy between toroidal and poloidal, geostrophic and ageostrophic parts, the spatial and temporal fit to the data and the extent to which the flows are symmetric about the equator. It is found that, if they are not constrained to be so, the flows are neither nearly toroidal nor nearly tangentially geostrophic. This is in contrast to previous results in which the toroidal and geostrophic parts of the flow have been found to be considerably stronger than the poloidal and ageostrophic parts, respectively. The temporal fit reflects the choice of secular variation spherical harmonic models used in the inversion, while the spatial distribution of residuals shows no localised areas of high misfit, in contrast to those for toroidal or tangentially geostrophic models, which show large misfits over Africa or near the poles respectively. The equatorially symmetric component of the flow is more energetic and has less small-scale energy, as is the case for purely toroidal and tangentially geostrophic flows, supporting the assertion that it is the more important and better determined part of the flow.