  We present nonlinear three-dimensional simulations of the stably-stratified
portion of the solar tachocline in which the rotational shear is maintained by
mechanical forcing. When a broad toroidal field profile is specified as an
initial condition, a clam-shell instability ensues which is similar to the
freely-evolving cases studied previously. After the initial nonlinear
saturation, the residual mean fields are apparently too weak to sustain the
instability indefinitely. However, when a mean poloidal field is imposed in
addition to the rotational shear, a statistically-steady state is achieved in
which the clam-shell instability is operating continually. This state is
characterized by a quasi-periodic exchange of energy between the mean toroidal
field and the instability mode with a longitudinal wavenumber m=1. This
quasi-periodic behavior has a timescale of several years and may have
implications for tachocline dynamics and field emergence patterns throughout
the solar activity cycle.
