Comparative Diagnosis of Blocking Versus Nonblocking Planetary-Scale Circulation Changes during Synoptic-Scale Cyclogenesis

Abstract

A 17-day period during November 1980 is investigated to obtain insight into differing large-scale 500 mb circulation changes during three consecutive synoptic-scale cyclone ?events? Each event is defined by at least one rapidly intensifying surface cyclone over eastern North America or the western Atlantic Ocean. The first event is followed in time by the retrogression of a downstream split-flow type of block at 500 mb. The second event is characterized by the intensification of a large-scale 500 mb trough into a blocking cyclonic vortex. During the third event, no blocking systems are established or intensified; instead, the blocking cyclonic vortex is ejected downstream. Quasi-geostrophic model diagnosis reveals that during the first two (blocking) events the large-scale waves are reinforced by relatively large and spatially and temporally persistent transports of potential vorticity associated with 500 mb synoptic-scale waves linked with the surface cyclones. In the third (nonblocking)event, during which the planetary waves have lower amplitude than before, the potential vorticity transports at 500 mb near the surface cyclones are relatively large but neither spatially nor temporally persistent. It is suggested that, on the basis of these analyses, whether a 500 mb blocking structure occurs and what type of structure (cyclonic or anticyclonic vortex) follows an intense surface cyclone event may depend critically upon the amplitude of existing planetary waves and the phase of these waves relative to the surface cyclones and attendant 500 mb potential vorticity transports.