SHORT-WAVELENGTH INSTABILITIES ON COASTAL JETS AND FRONTS

The stability of a coastal jet and front is investigated using the pri mitive equations applied to a continuously stratified flow in geostrop hic balance. A linear stability analysis successfully explains the gro wth of two modes of instability with distinctly different horizontal s cales. A long-wavelength mode (fastest-growing wavelength of 0(100 km) ) is found which is a modified version of a traditional baroclinic ins tability. A second, rapidly growing frontal instability also exists. F or a realistic basic state density and flow structure, this mode has i ts fastest growth at short wavelengths (0(20 km)), e-folds in less tha n 1.5 days and propagates rapidly in the direction of the mean flow. T he frontal instability grows primarily by extracting the available pot ential energy of the mean flow via a baroclinic instability mechanism. A small contribution from vertical Reynolds stress is also found, but the transfer via horizontal Reynolds stress is from the eddy to the m ean kinetic energy. Further evidence shows that the frontal instabilit y is not a result of horizontal shear instability nor is it an inertia l instability. The frontal mode is trapped to the surface front and it s influence is confined to the upper water column (z less than or simi lar 70 m). A significant subsurface vertical velocity maximum (20 m d- 1 at 30 m) is associated with a frontal instability with a reasonable, as judged by satellite sea surface temperature observations, surface temperature perturbation of 0.35-degrees-C. The linear stability predi ctions are verified by and compared with results from a time-dependent , three-dimensional, nonlinear ocean circulation model. Finally, the f rontal instability is discussed in the context of other recent stabili ty analyses that yeld high-wavenumber modes.