Blocking as a local instability to zonally varying flows

An idealized system consisting of a steady-state, axisymmetric vortex patch with topography forcing variations in the basic-state Row along the vortex edge is shown to possess local instabilities for a broad range of parameters. One branch of these instabilities corresponds in scale and structure to observed atmospheric blocks. The nonlinear equilibration of these blocking-type instabilities reproduces many aspects of observed blocking onsets; the potential for rapid non-modal growth and ready excitation of these local instabilities by transients suggests that this instability may lie at the root of observed blocking onsets.