  Although the Sun's torsional oscillation is believed to be driven by the
Lorentz force associated with the sunspot cycle, this oscillation begins 2--3
yr before the sunspot cycle. We provide a theoretical explanation of this with
the help of a solar dynamo model having a meridional circulation penetrating
slightly below the bottom of the convection zone, because only in such dynamo
models the strong toroidal field forms a few years before the sunspot cycle and
at a higher latitude.
