Dynamic and Synoptic Climatology
Roger G. Barry
Tectonic processes and subsequent geologic history have established the continental framework of North America, but it is the interaction of the coupled atmosphere—ocean system with this framework that generates its climate. In turn, climatic conditions greatly influence the hydrologic cycle, vegetation cover, and soils discussed in subsequent chapters. The climate has also changed over time, most dramatically after the disappearance of the last Laurentide Ice Sheet and associated Cordilleran ice caps and glaciers between about 12,000 and 8,000 years ago (see chapters 2– 4). Subsequent centennial- and multidecadal-scale climatic fluctuations, evidenced by neoglacial episodes in the Western Cordillera and recurrent episodes of severe drought in the western United States, have been no less important, however, in human affairs.
This chapter surveys the dynamic and synoptic aspects of North America's present climate, namely, the planetaryscale controls of atmospheric circulation and the embedded synoptic weather systems, recurrent mesoscale phenomena, and the resulting climatic conditions that affect the continent. The discussion is organized according to four topics: the planetary-scale setting, continental-scale influences and their climatic effects, synoptic regimes, and regional climatic features and anomalies. We begin by considering the overall climatic setting of the continent.
The climate of North America is shaped by geographical, oceanographic, and atmospheric factors and their interactions. The continent and adjacent Canadian Arctic Archipelago extend approximately from 15° to 80°N, and from 170°W in Alaska to 50°–60°W in eastern Canada, narrowing to between 120°W in southern California and 80°W in Florida before tapering southward through Mexico. High mountains along the continent's west coast join with intermontane plateaus and the Rocky Mountains farther east to form the Western Cordillera that extends southward from Alaska to Mexico. Mountains also dominate the eastern Canadian Arctic Archipelago and Labrador. The Arctic Ocean and year-round sea ice are key factors in the climate of the Canadian Arctic Archipelago and northern Alaska. The west coast waters are cold as a result of the southwardflowing California Current, whereas the warm Gulf Stream follows the Atlantic coast of the United States. However, the western Baffin Bay—Labrador Current transports subpolar water and, in late winter and spring, sea ice and icebergs southward to 48°W off Newfoundland. Maritime influences are also present in the heart of the continent in summer and autumn around Hudson Bay and the Great Lakes, as well as more generally as a result of the large number of smaller lakes and bogs in the boreal forest of Canada and the mixed forest of the upper Midwest. Until now, the effects of these smaller water bodies have not been represented in general atmospheric circulation models. The Gulf of Mexico is a prominent factor in the climate of the southern and southwestern United States and much of