North America possesses a multitude of diverse regional climates as a consequence of its vastness, its topography, and its being surrounded by oceans and seas with widely varying thermal characteristics. The North American region as analyzed in this report (see Figure A-3 in Annex A) extends latitudinally from approximately the Arctic Circle to the Tropic of Cancer and longitudinally from the Aleutian Islands in the west to the Canadian maritime provinces in the east. The regional atmospheric circulation is dominated by disturbances (waves) in the upper-level westerly winds. The development of these waves defines the position of the main upper-level jet stream over the continent and thus the position of the so-called Polar Front at the surface, which generally separates colder, drier air to the north from warmer, moister air to the south. In the colder half of the year, the position of the Polar Front can vary greatly, from southern Canada to the southern reaches of the United States. Such large shifts in the Polar Front are associated with long, high-amplitude waves that often cause one part of the continent to experience warm, moist, southerly airflow while another part experiences a blast of dry and cold Arctic air (meridional flow). These conditions may persist for lengthy periods because of the typically slow movement of these longer waves. At other times, however (mainly in the fall and spring), shorter, weaker waves move more quickly across the continent-producing highly variable weather with rapidly changing, but not extremely high or low, temperatures and short wet and dry periods. In the summer, the Polar Front retreats well into Canada, for the most part, and two oceanic semipermanent high-pressure systems tend to dominate the North American weather; as a result, there typically are fewer and weaker synoptic-scale disturbances in the westerlies. In summer and autumn, tropical storms of Atlantic, Caribbean, or Gulf of Mexico origin occasionally impact the Atlantic and Gulf coasts.
The temperature regime over North America varies greatly. Over all seasons, mean temperatures generally increase from the extreme north along the Arctic Ocean to the southern United States. Mean annual and wintertime temperatures along the west coast of the continent generally are higher than at equivalent latitudes inland or on the east coast because of the warming influence of Pacific air. During the winter in the far north, the long polar nights produce strong radiative cooling over the frozen Arctic Ocean and the typically snow-covered reaches of Alaska and Canada. This results in very cold surface temperatures and a temperature inversion that acts to inhibit cloud development, creating a positive feedback on the radiational cooling process. In this way, vast pools of cold, dense air (Arctic high-pressure systems) are formed and move over central and eastern North America; they sometimes move southward as far as the Gulf of Mexico. These extreme cold air outbreaks usually are confined to areas east of the Rocky Mountains; they often can produce temperatures below -40°C in the heart of the continent, with attendant sea-level pressure readings in excess of 1050 mb. To the west of the Rockies, warmer maritime airflow off the Pacific Ocean produces milder winters along the coast; the western cordillera effectively restricts this mild air from reaching and thus modifying temperatures in the interior. The eastern maritime regions of the continent enjoy much less warming influence from the Atlantic Ocean during these cold air outbreaks because the prevailing air flow is off the land (Schneider, 1996). Nevertheless, in winter the east and west coastal regions of Canada and the United States usually are warmer than inland regions, with the Pacific and Gulf coasts and Florida experiencing the shortest and mildest winters (Schneider, 1996).
In summertime, the large amount of insolation received over the very long days in the northern reaches of North America acts to raise temperatures there so that these areas are more in line with much of the rest of the continent, thus decreasing the north-south temperature gradient. The coldest areas are found in the western Canadian mountains and along the Labrador coast (Schneider, 1996). The highest continental temperatures are found in the U.S. desert southWest and southern plains states, where temperatures routinely exceed 38°C (~100°F). Occasionally, extreme summer heat waves spread over much of the central United States and parts of central and eastern Canada. These conditions can persist for days or weeks when occasional blocking high-pressure ridges form; these ridges may extend from the central United States to the western Atlantic. The hot air can be extremely humid because of low-level southerly airflow off warm Gulf of Mexico waters. The combination of heat and humidity produces dangerous health conditions that have resulted in significant numbers of fatalities (e.g., the July 1995 heat wave over the midwestern United States).
Annual precipitation amounts over North America show large spatial variations. The wettest regions lie along the Pacific coast, extending generally from Oregon to southern Alaska-with mean annual totals exceeding 300 cm at several Canadian locations (Environment Canada, 1995). The other main continental maximum in annual precipitation is located in the southeastern United States. It is centered mainly along the central Gulf coast states during winter, spring, and autumn and over Florida in the summer (Higgins et al., 1997). Mean annual precipitation amounts along the central Gulf coast exceed 150 cm.
Another precipitation maximum typically is observed over the midwestern United States (centered roughly over Missouri and Iowa) (Higgins et al., 1997) in the summer months, where mean rainfall (mainly convective in nature) typically exceeds 25 cm. This feature is associated with convection that often is fueled by a strong low-level southerly jet stream bringing abundant moisture from the Gulf of Mexico. The active convection often begins in the spring and continues through the summer, causing severe local- to regional-scale flooding. The convective activity observed over the Midwest, the Great Plains, and the southeastern United States also is responsible for the fact that this part of the United States experiences more severe weather (in the form of thunderstorms, tornadoes, and hail) than any other part of the world. Although North American annual precipitation is much more climatologically consistent than in many other parts of the world (e.g., northern Africa and eastern Australia), extremely damaging large-scale droughts and floods sometimes occur, often in association with blocking patterns in the large-scale circulation.
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