  We consider a coupling of the Stommel box model and the Lorenz model, with
the goal of investigating the so-called "crises" that are known to occur given
sufficient forcing. In this context, a crisis is characterized as the
destruction of a chaotic attractor under a critical forcing strength. We
document the variety of chaotic attractors and crises possible in our model,
focusing on the parameter region where the Lorenz model is always chaotic and
where bistability exists in the Stommel box model. The chaotic saddle
collisions that occur in a boundary crisis are visualized, with the chaotic
saddle computed using the Saddle-Straddle Algorithm. We identify a novel
sub-type of boundary crisis, namely a vanishing basin crisis. For forcing
strength beyond the crisis, we demonstrate the possibility of a merging between
the persisting chaotic attractor and either a chaotic transient or a ghost
attractor depending on the type of boundary crisis. An investigation of the
finite-time Lyapunov exponents around crisis levels of forcing reveals a
convergence between two near-neutral exponents, particularly at points of a
trajectory most sensitive to divergence. This points to loss of hyperbolicity
associated with crisis occurrence. Finally, we generalize our findings by
coupling the Stommel box model to other strange attractors and thereby show
that the behaviours are quite generic and robust.
