Interleaving motions on a wide, baroclinic front are modeled using a second-moment closure to represent unresolved fluxes by turbulence and salt fingering. A linear perturbation analysis reveals two broad classes of unstable modes. First are scale-selective modes comparable with interleaving as observed in oceanic fronts. These correspond well with observations in some respects but grow by a very different mechanism, which ought to be easily distinguished in hydrographic profiles. The second mode type is the so-called ultraviolet catastrophe, which is expected to lead to steppy profiles even in the absence of interleaving. Both modes are driven by positive feedbacks between interleaving and the underlying small-scale mixing processes. Contrary to expectations, use of the second-moment closure in place of earlier empirical mixing models does not lead to improved agreement with observations.Keywords: Mixing, Diapycnal mixing, Instabilit
