NONLINEAR EQUILIBRATION OF LOCALIZED INSTABILITIES ON A BAROCLINIC JET

Dynamical mechanisms underlying the equilibration of absolute instabil ity are examined in a nonlinear, quasigeostrophic, two-layer model. Th e key to understanding the nonlinear equilibration is in recognizing t hat linear absolute instabilities can be stabilized both by a reductio n of the vertical shear and by enhancement of the mean barotropic velo city. In a localized domain, the equilibration process proceeds with t he creation of locally convectively unstable regions downstream, which encroach onto the locally absolutely unstable region until the local instability is suppressed. That local instabilities exist only if abso lutely unstable regions span a minimum size is verified by eigenvalue calculations of three-dimensional flows. Numerical examples suggest th at this critical size is at least 9000 km for a wide range of paramete r values chosen to investigate the midlatitude storm tracks. Fluctuati ons arising from local absolute instability obtain maximum amplitude i n the downstream convectively unstable regions rather than in the abso lutely unstable regions themselves. Together, these results suggest th at if an equilibrated absolute instability were to occur in midlatitud es, a zonal band of surface easterlies exceeding 9000 km would be requ ired and the associated enhanced variances would not be found coincide nt with the regions of absolute instability.