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American Monsoon
Wed Aug 27, 2003
Listen in RealAudio 
We generally associate "Monsoon" with heavy rains in India; however, a monsoon climate is also
found in the American Desert of the southwest United States and Mexico. Hi, I'm Bryan Yeaton
for The Weather Notebook.
In the northern Mexico, as well as southern Arizona and New Mexico, winds aloft flow from the
west to northwest for most of the year. But during the summer-from July through September-they
swing to the south and southeast.
This reversal forms the monsoon, sometimes known as the Southwest or Arizona Monsoon. The new
airflow brings in tropical, maritime air-and its associated high humidity-to combine with
summer's solar heating.
Over the Sonoran Desert of Mexico and Arizona, just as with the Indian monsoon, heat generated
by the summer sun forms a surface low-pressure cell over the region. This draws moist tropical
air northward from the Gulf of California, and thunderstorms develop as the moist air is
lifted by the mountains and daytime convection.
The region receives most of its annual precipitation during the monsoon: about 35 to 45
percent for Arizona and New Mexico and 60 percent for northern Mexico. But the rains do not
begin until the surface water temperature in the Gulf of California warms above 79 degrees
Fahrenheit.
Recent studies have linked strong American Monsoons to summer drought in the Midwest and
relatively wet summers in the Southeast, though the connection remains unclear. Heavy winter
precipitation in the southwestern mountains, or a mature El Niņo event, can presage a
weak monsoon season. During a La Nina, however, a strong monsoon brings soaking conditions to
the Southwest while the Great Plains are bone dry.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is funded
by Subaru of America, and The National Science Foundation.
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