Abstract:
Fronts, or regions with large horizontal density gradients, are common and important features of the upper ocean. On sufficiently large scales, the hydrostatic pressure gradient associated with the horizontal density gradient is nearly balanced by a (geostrophic) along-front flow. However, fronts in the ocean and atmosphere co-exist with small-scale turbulence which disrupts this balance. I will present a simple asymptotic model in which turbulent mixing is represented using a turbulent viscosity. Leading order solutions show that mixing acts to drive a cross-front flow and maintain a vertical density stratification with the correlation between these two effects leading to hear dispersion. Using the leading order solutions, a nonlinear diffusion equation for the background density can be derived and used to study the long term evolution of the fronts. I will present the case of an unforced front in which shear dispersion leads to self-similar spreading of the frontal region and the case of a forced front in which surface forcing may act to oppose the spreading. Finally I will consider the effects of turbulent mixing on frontal baroclinic instability and show that mixing can act to suppress instability growth and modify the direction of the fastest growing wave-vectors.