Geoscience Reference
In-Depth Information
h e two pressure cells that most inl uence climate over the American West
are the “Pacii c High,” which extends from just east of Hawaii to the West
Coast, and the “Aleutian Low,” which extends from the Gulf of Alaska to
the Aleutian Islands in the North Pacii c. h e basic physics of our spinning
planet result in the circulation of upper-level winds in a counterclockwise
direction around the Aleutian Low and in a clockwise direction around the
Pacii c High. h e Aleutian Low is strongest during the winter, when the
Pacii c High is weakened and displaced to the south. In the mid-latitudes—
roughly midway between the equator and the North Pole—movement from
west to east of air masses that form over the Pacii c creates the prevailing
westerly wind pattern over the West. In wintertime, most of the rain-bearing
air masses bring moisture from the ocean across the northern coastal states
(Washington, Oregon, and northern California).
Much of the moisture falls as the air masses are forced up the slopes of
i rst the coastal mountains and then the Cascade or Sierra Nevada moun-
tain ranges. In the higher elevations, this precipitation ot en falls as snow.
In the northern Pacii c Ocean, the Aleutian Low occasionally moves farther
west, and thus a high-pressure cell may form over the eastern Pacii c just of
the West Coast, preventing storms from entering the West. h
is situation is
ot en associated with drought.
During the summer, the Pacii c High strengthens and moves farther
north of the West Coast, del ecting storms into the Gulf of Alaska and
Canada. California is positioned in just such a way that it lies under the
eastern side of the Pacii c High during the summer months, which ef ectively
blocks storms from moving into the state. For this reason, California is one
of the few regions in the world that receives virtually no summer rain.
In contrast, the southwestern region of North America—including
Arizona, New Mexico, and Colorado—receives moisture in the summer
during what is called the “North American Monsoon.” h e monsoonal rains
occur when moist air from the Gulf of Mexico, the Gulf of California, and
the eastern Pacii c Ocean moves northward into the southwestern United
States, which has undergone intense solar heating during the months of July,
August, and September. Monsoonal rainfall can account for as much as half
of the annual precipitation in the U.S. Southwest, although it is highly vari-
able since the region lies at the northern border of the monsoon (with the
core located over northern Mexico).
Precipitation in the West is highly variable from year to year and decade to
decade. Research over the past few decades has revealed that these seemingly
