  We discuss here a purely hydrodynamical mechanism to invert the sign of the
kinetic helicity, which plays a key role in determining the direction of
propagation of cyclical magnetism in most models of dynamo action by rotating
convection. Such propagation provides a prominent, and puzzling constraint on
dynamo models. In the Sun, active regions emerge first at mid-latitudes, then
appear nearer the equator over the course of a cycle, but most previous
global-scale dynamo simulations have exhibited poleward propagation (if they
were cyclical at all). Here, we highlight some simulations in which the
direction of propagation of dynamo waves is altered primarily by an inversion
of the kinetic helicity throughout much of the interior, rather than by changes
in the differential rotation. This tends to occur in cases with a low Prandtl
number and internal heating, in regions where the local density gradient is
relatively small. We analyse how this inversion arises, and contrast it to the
case of convection that is either highly columnar (i.e., rapidly rotating) or
locally very stratified; in both of those situations, the typical profile of
kinetic helicity (negative throughout most of the northern hemisphere) instead
prevails.
