Enhancement of fronts by vertical mixing

Microstructure observations near upwelled fronts indicate considerable variation in the structure of vertical mixing across the frontal region. Observations of cool filaments off northern California indicate that within the cool (dense) core of filaments the raised pycnocline inhibits the penetration to middepths of surface‐generated mixing. The microstructure profiles are used to estimate the available wind energy for mixing as a function of pycnocline, or mixed layer depth. A greater portion of energy input at the surface is available for entrainment of dense fluid through the pycnocline and into the surface mixed layer where the pycnocline is shallow. Hence surface‐forced mixing may cause a more rapid increase in mixed layer density within the cool filament than outside the filament, resulting in an enhanced horizontal density gradient in the mixed layer. Assuming the flow adjusts towards geostrophy, the enhanced horizontal density gradient at the front could result in an accelerated mixed layer in the direction of the preexisting geostrophic flow. Proportions relating the gain in potential energy to the wind energy vary with pycnocline depth and differ by as much as an order of magnitude from the findings of Denman and Miyake (1973) and Davis et al. (1981). Horizontal variability of pycnocline erosion may not be properly taken into account in some models and should more realistically be parameterized by including dependence on pycnocline depth.