The instability of density fronts is investigated as a possible generation mechanism for the small-scale, wavelike patterns that are commonly observed along upwelling fronts and filaments. Unstable-wave solutions are obtained in two linearized models: a 1 1/2 layer model, and a continuously stratified model confined to the surface region of the ocean. In both systems the thickness of the upper region is held constant for the background state, the front being specified by allowing the temperature field T within the region to vary zonally. The background state in the layer model consists of vertically oriented isotherms associated with a depth-independent current, whereas in the continuously stratified model it consists of steeply tilted isotherms and a vertically sheared current. Solutions are found both when the background velocity field V is zonally uniform and when it is zonally sheared. When V is weak and zonally uniform, approximate solutions are derived analytically for both models that are valid for low-frequency, low-wavenumber waves. These solutions demonstrate that the unstable waves in the two systems are dynamically related, both being representations of ageostrophic baroclinic instability. Numerical solutions corroborate the analytic results and extend their range of validity.

Energetics analyses confirm that the energy source for the waves is the background potential energy associated with the zonally varying T field. When V is a zonally sheared jet, the models still exhibit a band of instability, which is identifiable with ageostrophic baroclinic instability. The most unstable wave in this band has a short wavelength, a frequency near f/2, and a rapid growth rate consistent with observed features. The layer model also has a band of larger-scale waves that is a mixed, baroclinic-barotropic instability; however, for a typical frontal structure this band is weaker than the baroclinic band. ¿ American Geophysical Union 1995