Instability of a tidal mixing front in the presence of realistic tides and mixing
Additional Document Info
Instability and lateral eddy fluxes associated with a tidal mixing front are studied using idealized primitive equation numerical model runs. The front itself develops as a result of turbulence associated with imposed tidal currents over a sloping bottom. Thus, the model includes realistic levels of turbulence and time-dependence in the base-state conditions. In all of the 21 configurations considered, the front is unstable to fluctuations that usually draw energy primarily from the potential energy pool. Scalings are developed to parameterize a) the location of the tidal mixing front; b) the eddy kinetic energy; and c) the lateral eddy coefficients. In all cases, baroclinic instability enhances the lateral mixing relative to the two-dimensional case, but the extent of enhancement varies with the input parameters. Preliminary model runs that include a simple nutrient-phytoplankton-zooplankton-detritus biological model do not suggest any substantial ecological effect of the baroclinic instabilities. The lack of a strong biological effect, despite the enhanced eddy transports, occurs because of the near cancelation of oppositely directed cross-isobath eddy fluxes in the upper and lower parts of the water column. Similarly, shallow and deep cross-frontal eddy heat fluxes also nearly balance: this cancelation appears to help explain how the classical one-dimensional potential energy criterion for frontal location can work so well in a complex ocean.