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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 flow 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.