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Length of day decade variations, torsional oscillations and inner core superrotation: evidence from recovered core surface zonal flows
Authors: A. Pais and G. Hulot
Ref.: Physics of the Earth and Planetary Interiors 118, 291-316 (2000)
Abstract: We consider the core surface flow derived from geomagnetic models [Bloxham, J., Jackson, A., 1992. Time-dependent mapping of the magnetic field at the coremantle boundary. J. Geophys. Res. 97, 1953719563.] under the frozen flux and tangentially geostrophic assumptions, and focus on the significance of its toroidal zonal component. This component represents a small fraction of the whole flow, but is believed to contain important information concerning core dynamics and the way the liquid core interacts with both the mantle and the inner core. We consider the 150-year period 18401990 for which the best data is available. Our results show that with the current methodology and data, recovered core surface zonal flows can successfully be used to address three issues in core dynamics, provided that one properly takes data and methodology uncertainties into account. The first issue deals with the possibility of testing the theory of Jault et al. [Jault, D., Gire, C., Le Mouel, J.-L., 1988. Westward drift, core motions and exchanges of angular momentum between core and mantle. Nature 333, 353356.] which predicts a relationship between core surface zonal flows and length of day (LOD) variations on decade time scales. We recover the known fact that this theory leads to a successful prediction of LOD variations after 1920 and not early on, and show that this failure can entirely be attributed to the (partly correlated) uncertainties affecting the t_1 and t_3 flow components required to carry on the prediction. The second issue deals with the possibility of detecting torsional oscillations of the kind that Braginsky [Braginsky, S.I., 1970. Torsional magnetohydrodynamics vibrations in the Earths core and variations in day length. Geomagn. Aeron. 10, 312 (Eng. transl. 18).] predicted should occur in the core on decade time scales. We show that the large scale component of the relevant equatorial symmetric zonal flow ( t_1 , t_3 , t_5 and very marginally t_7 ) displays significant time variations that can be attributed to such oscillations. But uncertainties affecting these coefficients are quite large and should therefore be taken into account. The third and final issue deals with the possibility of identifying a surface signature of the inner core superrotation that some seismologists claim to have detected and that is predicted by most dynamo numerical simulations. We show that the average zonal flow recovered over the 18401990 period displays a strong westward rotation (at 0.31º/yr) within the inner core tangent cylinder, which can possibly be interpreted as the surfacic counterpart of an opposite eastward inner core superrotation.
