A steady velocity field at the top of the Earth's core in the frozen-flux approximation

• Published in: Geophysical Journal International
• Main Authors: Whaler, K. A., Clarke, S. O.
• Format: Text
• Language: English
• Published: Oxford University Press 1988
• Subjects: Articles; Indian; Pacific; North Atlantic
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/94/1/143; https://doi.org/10.1111/j.1365-246X.1988.tb03434.x

A steady fluid flow at the Earth's core-mantle boundary has been calculated over the 20 year period from 1960 to 1980. The underlying assumption of the geomagnetic field being frozen-in at the core surface is satisfied for the models of Bloxham & Gubbins (1986) used to specify the main field, using a 1980 field model (Gubbins & Bloxham 1985) based on MAGSAT data to define the null-flux patch integrals that are conserved. A new method of determining the coefficients in the spherical harmonic expansion of the toroidal and poloidal parts of the assumed steady velocity from main field and secular variation spherical harmonic coefficients is developed; the coefficients are then solved for by stochastic inversion. Statistical analysis of the residuals suggests that the flow cannot be assumed constant between 1960 and 1980, but there are unaccounted-for errors and correlations in the inversion. Neglecting the covariances, the errors would have to be a factor 2-3 times larger for the flows to fit the data, but this implies an average error per secular variation spherical harmonic coefficient of only 1 per cent. The steady flows predict the radial secular variation at the core-mantle boundary (CMB) better than individual single epoch models. The calculated steady velocity fields show westward flow in a broad equatorial band in the hemisphere centred on 0° longitude. Up- and downwelling occurs at the CMB below the Western edge of the Pacific, upwelling beneath southern Africa [possibly associated with a ‘core spot’ (Bloxham & Gubbins 1985) there], and downwelling beneath central and North America, acting as a sink for the westward-flowing fluid. The fluid moves more slowly, and predominantly latitudinally, in the hemisphere centred on 180° longitude. There is very prominent anticlockwise circulation beneath the Southern Indian Ocean, clockwise flow beneath the North Atlantic, and a series of three eddy-like features (with anticlockwise circulation) running NW from below South America, through central to North ...