Ageostrophic Stability of Divergent Jets

Abstract

The barotropic stability of divergent jets is studied under the condition of small but nonzero Rossby number. Small disturbances to the basic state consist of fast waves of inertio-gravity type and slow waves with speeds comparable to those of the basic current. It is shown that the fast waves are stable and do not amplify. It is also shown, for a velocity profile resembling that of the Gulf Stream, that the slow waves are destabilized by ageostrophic effects in the sense that waves of fixed wavenumber which are neutral for zero Rossby number amplify for nonzero Rossby number. For very shallow jets it is found that the complex wave velocity is small and that disturbances will be neutrally stable unless the jet has a countercurrent.