Nonlinear Evolution of Axisymmetric Twisted Flux Tubes in the Solar Tachocline

We numerically study the evolution of magnetic fields and fluid flows in a thin spherical shell. We take the initial field to be a latitudinally confined, predominantly toroidal flux tube. For purely toroidal, untwisted flux tubes, we recover previously known radial-shredding instabilities, and show further that in the nonlinear regime these instabilities can very effectively destroy the original field. For twisted flux tubes, including also a poloidal component, there are several possibilities, including the suppression of the radial-shredding instability, but also a more directly induced evolution, brought about because twisted flux tubes in general are not equilibrium solutions of the governing equations.