The knowledge of the geomagnetic field and its secular variation allows us
to compute the fluid flow at the core surface. The poloidal and toroidal co
mponents of the fluid flow at the core-mantle boundary (CMB) have been calc
ulated every year from the Bloxham and Jackson model (1992) and plotted at
50 year intervals over the last three centuries. The flow patterns conserve
some broad features over this whole time-span. The time constant of the de
gree 1 component of the motion is larger than the time constant of the rest
of the flow. The average motion over 300 years appears to be in large part
symmetrical with respect to the equator. This average flow can be represen
ted by the sum of a few geostrophic vectors. The acceleration fields corres
ponding to the well documented jerks of 1969, 1979, 1992 have also been com
puted. The geometry of these acceleration fields is the same, within a chan
ge of sign, for the three events. Moreover, this geometry has close connect
ions with the geometry of the flow itself. The spatial and temporal variati
ons of the flow field can be simply described, in a first approximation; it
is possible to give an analytical schematic representation of the flow fie
ld during the last three decades. Some characteristics of the decadal lengt
h-of-day variations follow if the coupling torque between core and mantle i
s topographic.