  Rotational shear layers at the boundary between radiative and convective
zones, tachoclines, play a key role in the process of magnetic field generation
in solar-like stars. We present two sets of global simulations of rotating
turbulent convection and dynamo. The first set considers a stellar convective
envelope only; the second one, aiming at the formation of a tachocline,
considers also the upper part of the radiative zone. Our results indicate that
the resulting mean-flows and dynamo properties like the growth rate, saturation
energy and mode depend on the Rossby (Ro) number. For the first set of models
either oscillatory (with ~2 yr period) or steady dynamo solutions are obtained.
The models in the second set naturally develop a tachocline which, in turn,
leads to the generation of strong mean magnetic field. Since the field is also
deposited into the stable deeper layer, its evolutionary time-scale is much
longer than in the models without a tachocline. Surprisingly, the magnetic
field in the upper turbulent convection zone evolves in the same time scale as
the deep field. These models result in either an oscillatory dynamo with ~30 yr
period or in a steady dynamo depending on Ro. In terms of the mean-field dynamo
coefficients computed using FOSA, the field evolution in the oscillatory models
without a tachocline seems to be consistent with dynamo waves propagating
according to the Parker-Yoshimura sign rule. In the models with tachoclines the
dynamics is more complex involving other transport mechanisms as well as
tachocline instabilities.
