  Helioseismology has revealed many details of solar differential rotation and
also its time variation, known as torsional oscillations. So far there is no
generally accepted theoretical explanation for torsional oscillations, even
though a close relation to the solar activity cycle is evident. On the
theoretical side non-kinematic dynamo models (including the Lorentz force
feedback on differential rotation) have been used to explain torsional
oscillations. In this paper we use a slightly different approach by forcing
torsional oscillations in a mean field differential rotation model. Our aim is
not a fully self-consistent model but rather to point out a few general
properties of torsional oscillations and their possible origin that are
independent from a particular dynamo model. We find that the poleward
propagating high latitude branch of the torsional oscillations can be explained
as a response of the coupled differential rotation / meridional flow system to
periodic forcing in mid-latitudes, of either mechanical (Lorentz force) or
thermal nature. The speed of the poleward propagation sets constraints on the
value of the turbulent viscosity in the solar convection zone to be less than
3x10^8 m^2/s. We also show that the equatorward propagating low latitude branch
is very unlikely a consequence of mechanical forcing (Lorentz force) alone, but
rather of thermal origin due to the Taylor-Proudman theorem.
