  Temporal variations in the Suns internal velocity field with a periodicity of
about 11 years have been observed over the last four decades. The period of
these torsional oscillations and their latitudinal propagation roughly
coincides with the period and equatorward propagation of sunspots which
originate from a magnetohydrodynamic dynamo mechanism operating in the Suns
interior. While the solar differential rotation plays an important role in this
dynamo mechanism by inducting the toroidal component of magnetic field, the
impact of torsional oscillations on the dynamo mechanism and hence the solar
cycle is not well understood. Here, we include the observed torsional
oscillations into a flux transport dynamo model of the solar cycle to
investigate their effect. We find that the overall amplitude of the solar cycle
does not change significantly on inclusion of torsional oscillations. However,
all the characteristics of the Waldmeier effect in the sunspot cycle are
qualitatively reproduced by varying only the amplitude of torsional
oscillations. The Waldmeier effect, first noted in 1935, includes the important
characteristic that the amplitude of sunspot cycles is anti-correlated to their
rise time; cycles with high initial rise rate tend to be stronger. This has
implications for solar cycle predictions. Our results suggest that the
Waldmeier effect could be a plausible outcome of cycle to cycle modulation of
torsional oscillations and provides a physical basis for sunspot cycle
forecasts based on torsional oscillation observations. We also provide a
theoretical explanation based on the magnetic induction equation thereby
connecting two apparently disparate phenomena.
