Sustained magneto-shear instabilities in the solar tachocline

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.