Environment: Atmosphere

Abstract

By 8000 years ago the Pleistocene continental glacial ice still covered a large area in north central Canada as well as northern Labrador and Ungava, but closed-crown spruce forest had become established over far northwestern Canada and the Mackenzie Delta (Ritchie, 1984) and was colonizing southern Quebec and the Maritime Provinces (Macpherson, 1985; Mott, 1985; Filion, 1984; Andrews, 1985). The contrast between the warmer air masses of the Alaska-Mackenzie region and colder air masses of the Hudson Bay region persists to the present day, the consequence of the western (ridge) and eastern (trough) pattern of wide amplitude in the upper atmospheric belt of westerly winds. The forest-tundra is found in the zone where frontal conditions prevail between northern masses of arctic air and more southern air masses of Pacific origin, a zone that fluctuates as a result of weather conditions but which is generally found not far north of a modal position that extends in summer from the Mackenzie Delta to the southern edge of Hudson Bay and thence in a somewhat more diffuse band across northern Quebec and Labrador. The closed-crown circumpolar boreal forest is found generally south of this climatic transition belt, in regions dominated by warm air masses in summer in North America, and warm continental air over Siberia. Tundra is found in regions northward dominated by arctic air masses.