LARGE-AMPLITUDE COHERENT ANOMALIES IN BAROCLINIC ZONAL FLOWS

In a previous paper the authors developed an asymptotic time-dependent theory for coherent structures on a marginally stable baroclinic flow . It was shown that solitary waves, as well as other more general dist urbances of sufficient amplitude, could be explosively unstable and th at the asymptotic theory provided no mechanism for equilibration. The asymptotic theory is reexamined and shows that the instability can be interpreted as a problem in local instability. A discussion of the app ropriate boundary conditions for localized instability problems is als o given. Direct numerical integration of the full quasigeostrophic equ ations is then undertaken to determine the ultimate fate of the explos ive instability. The instability equilibrates as a locally steady, zon ally uniform O(1) alteration of the original zonal flow that expands s lowly in time. This region is connected to the original flow by narrow regions of strong meridional flow. The finite-amplitude region develo ps both closed streamlines and uniform potential vorticity. The ultima te state is a finite-amplitude attractor; it is independent of the ini tial disturbance and depends only on the original zonal dow. A simple model demonstrates that the finite-amplitude state is a conjugate solu tion of the original zonal potential-vorticity-streamfunction relation . The model solution is described and compared with the numerical calc ulations. It is suggested that this transition from a weakly nonlinear to finite-amplitude modonlike state is a possible mechanism for the g eneration of atmospheric blocking events that requires neither forcing nor dissipation.