A Theoretical and Diagnostic Study of Solitary Waves and Atmospheric Blocking

Abstract

Theories which associate atmospheric blocking with isolated ?free mode? solutions of the equations of motion are reviewed and the central role played by the potential function ? ≡ dq/d? (where q is the is the quasi-geostrophic potential vorticity and ? is the streamfunction) is emphasized. This function provides the common dynamical link that draws together the weakly nonlinear (soliton) and fully nonlinear (modon) theories of isolated coherent structures. A diagnostic study of the European blocking episode during October 1987 is presented and the relationship between q and ? investigated by plotting scatter diagrams of quasi-geostrophic potential vorticity against the streamfunction on an isobaric surface. An approximate functional relationship is found allowing ? to be defined. Over the blocking region, points on the scatter plot cluster around a straight line which is more steeply sloping than the straight line defined by points from nonblocking regions, demonstrating that the block exhibits a local minimum in ?. Such a signature is characteristic of local fully nonlinear free mode structures, the prototype of which has been termed the ?equivalent barottopic modon.? The data strongly suggest that blocking episodes can exhibit local free-mode dynamics and that their persistence may in part be attributed to the robustness and stationary nature of these local coherent structures.