Explosive Cyclogenesis and Large-Scale Circulation Changes: Implications for Atmospheric Blocking

Large-scale circulation changes attending explosive surface cyclogenesis are quantitatively examined in two cases selected from recent winter seasons. Both cases feature a rapidly deepening surface cyclone over the western Atlantic Ocean, but changes in the 500 mb geopotential height field near the cyclone differ in each case. One event, during January 1977, is characterized by the retrogression of an anticyclonic vortex (blocking high) in the 500 mb height field downstream of the surface cyclone. The second case, in February 1978, is distinguished by the formation of a 500 mb cyclonic vortex (cutoff low) upstream of the surface cyclone, but no downstream anticyclonic vortex is observed. The retrogression of the January 1977 block over the Atlantic Ocean coincides with the migration of a 500 mb synoptic-scale perturbation (associated with the surface cyclone) from a planetary- scale trough over North America toward a planetary-scale ridge over Europe. In the February 1978 case, the blocking cyclonic vortex evolves out of a synoptic-scale perturbation migrating from a long-wave ridge over western North America toward a long-wave trough over eastern North America, initiating the oceanic surface cyclone event. Quasi-geostrophic model diagnosis of atmospheric data during these cases reveals that the middle tropospheric geopotential height tendencies in the blocking systems are forced by the superposition of thermal and vorticity advections. Thermodynamically, the observed temperature increase in the blocking anticyclone case is forced both by warm air advection and subsidence warming, while the temperature decrease observed in the blocking cyclone case is forced by adiabatic cooling attending strong ascent. These and other case studies are consistent with the results of previous theoretical and observational work which have shown that atmospheric blocking patterns can arise due to the interaction of transient, synoptic-scale perturbations with the planetary-scale environment. In this context, blocking may be understood as a response of the planetary waves to synoptic-scale perturbations, which act as sources of energy and vorticity for the incipient blocks. This paper shows, however, that the type of response may depend critically on the location of the synoptic scale perturbation relative to the planetary waves. Specifically, synoptic-scale waves migrating from long-wave ridge (trough) to long-wave trough (ridge) can, in certain instances, favor blocking cyclonic (anticyclonic) vortices.