  Recent direct numerical simulations (DNS) of large-scale turbulent dynamos in
strongly stratified layers have resulted in surprisingly sharp bipolar
structures at the surface. Here we present new DNS of helically and
non-helically forced turbulence with and without rotation and compare with
corresponding mean-field simulations (MFS) to show that these structures are a
generic outcome of a broader class of dynamos in density-stratified layers. The
MFS agree qualitatively with the DNS, but the period of oscillations tends to
be longer in the DNS. In both DNS and MFS, the sharp structures are produced by
converging flows at the surface and might be driven in nonlinear stage of
evolution by the Lorentz force associated with the large-scale dynamo-driven
magnetic field if the dynamo number is at least 2.5 times supercritical.
