Patterns of Thickness Anomaly for Explosive Cyclogenesis over the West-Central North Atlantic Ocean

Abstract

Hemispheric anomaly patterns of 1000?500 mb thickness were obtained for 67 cases of explosive cyclogenesis over the western North Atlantic Ocean in December-February during 1962?77, beginning between latitudes 30°?40°N and between longitudes 70°?80°W. Composite patterns for the 26 strongest cases of cyclogenesis differed markedly from those for the 22 weakest. After a filtering to remove the shortest waves, those for the strongest developments showed a prominent negative anomaly area of large scale, centered over western Canada 5 days before the event, moving southeastward to the western Atlantic days after cyclogenesis. No such pervasive anomaly pattern was seen for the weakest cases. The most intense cyclogenesis occurred when the air over the region of development was slightly colder than the 15-year average, while the least intense occurred in slightly anomalous warmth. In the zonal average from 25° to 125°W, the strongest cases occurred with warmth in polar latitudes, coldness in middle latitudes and anomalously strong westerly thermal wind in the cyclogenetic area. The weakest cases occurred with cold polar latitudes, warmth in upper middle latitudes, and slightly cold anomalies but no excessive thermal wind in the latitudes of cyclogenesis. It is implied that both baroclinic forcing and heat and moisture flux from the sea surface were enhanced in the strongest cases, but neither effect was obviously dominant.