|
8.2. Regional Climate Information
8.2.1. Current Climate
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.
|