A Synoptic Investigation of Anticyclogenesis Accompanying North American Polar Air Outbreaks

Abstract

Results are presented for a synoptic-dynamic investigation of certain polar anticyclones originating in northwestern Canada.. A composite, consisting of three similar high pressure systems, is constructed. This cold anticyclone weakens while it moves southeastward towards the Gulf of Mexico. Along the Gulf Coast it curves northeastward and reintensifies as it becomes a warm, dynamic anticyclone. During a second composite case the high moves rapidly southeastward before dissipating quickly over the warm waters of the Gulf of Mexico. No further intensification or recurvature occurs. In both instances, cold air penetrates deeply into the southern latitudes. An upper tropospheric ridge over northwestern Canada and Alaska appears important in strengthening the polar anticyclone. The center of the high moves southward if uninterrupted northwesterly flow is established aloft. The cold air preceding the high pushes towards the Gulf of Mexico as the trough downstream of the Canadian ridge deepens. The eventual history of the anticyclone seems to depend on the interaction between the low-levellee side trough which forms over Idaho and Montana after the anticyclone's passage and the deep trough in the middle and upper troposphere over the East Coast. A weakening East Coast trough and an eastward propagating lee side trough are associated with rising heights in the middle troposphere over the central and eastern United States. Consequently, an anticyclone is supported dynamically as it builds northeastward from the Gulf of Mexico. If, however, the East Coast trough is pronounced, strong northwesterly flow dominates the central United States, the effects of the lee side trough are minimal, and the cold anticyclone moves over the Gulf of Mexico where rapid weakening occurs. In the middle and upper troposphere, vorticity advection is most important in causing anticyclonic tendencies over and downwind of the surface anticyclone. In contrast, divergence usually dominates in producing similar tendencies in the lower troposphere over the surface high. The tipping term of the vorticity equation may also be critical in lower levels near frontal regions or orographic barriers. In the lower troposphere subsidence is generally maximum on the east side of the surface anticyclone in a region or pronounced cold advection. In that area descent of 3 to 6 ?b?s?1 may occur. Subsidence over the high center is weak, seldom exceeding 1 or 2 ?b?s?1.