We report a calculation of time-dependent quasi-geostrophic core flows for1940-2010. Inverting recursively for an ensemble of solutions, we evaluate themain source of uncertainties, namely the model errors arising from interactionsbetween unresolved core surface motions and magnetic fields. Temporalcorrelations of these uncertainties are accounted for. The covariance matrixfor the flow coefficients is also obtained recursively from the dispersion ofan ensemble of solutions. Maps of the flow at the core surface show, upon aplanetary-scale gyre, time-dependent large-scale eddies at mid-latitudes andvigorous azimuthal jets in the equatorial belt. The stationary part of the flowpredominates on all the spatial scales that we can resolve. We retrievetorsional waves that explain the length-of-day changes at 4 to 9.5 yearsperiods. These waves may be triggered by the nonlinear interaction between themagnetic field and sub-decadal non-zonal motions within the fluid outer core.Both the zonal and the more energetic non-zonal interannual motions wereparticularly intense close to the equator (below 10 degrees latitude) between1995 and 2010. We revise down the amplitude of the decade fluctuations of theplanetary scale circulation and find that electromagnetic core-mantle couplingis not the main mechanism for angular momentum exchanges on decadal time scalesif mantle conductance is 3 10 8 S or lower.
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