|
|
|
|
|
| The North American
Monsoon. PDF version |
| |
| Until
the late 1970s, there was serious debate about whether a monsoon truly existed
in North America. However, considerable research, which culminated in the
Southwest Arizona Monsoon Project (SWAMP) in 1990 and 1993, established
the fact that a bonafide monsoon, characterized by large-scale wind and
rainfall shifts in the summer, develops over much of Mexico and the intermountain
region of the U.S. Published papers at the time called this pattern by different
names, including the "Summer Thunderstorm Season," The Mexican
Monsoon," "The Southwest Monsoon," and the "Arizona
Monsoon." |
| |
| In 2004,
a major multinational research project was conducted in northwest Mexico
and the southwest U.S. The North American Monsoon Experiment (NAME) sought
to better describe the monsoon in North America, and increase our ability
to predict it on a daily, weekly and seasonal basis. NAME showed that despite
its many names, the weather pattern we see during the summer is not only
a true monsoon, but it also affects the weather over a large portion of
North America. Thus the generally accepted name is now "North American
Monsoon." |
| |
| The North
American Monsoon is not as strong or persistent as its Indian counterpart,
mainly because the Mexican Pleateau is not as high or as large as the Tibetan
Plateau in Asia. However, the North American Monsoon shares most of the
basic characteristics of its Indian counterpart. There is a shift in wind
patterns in summer which occurs as Mexico and the southwest U.S. warm under
intense solar heating. As this happens, the flow reverses from dry land
areas to moist ocean areas. In the Northn American Monsoon, the low level
moisture is transported primarily from the Gulf of California and eastern
Pacific. The Gulf of California, a narrow body of water surrounded by mountains,
is particularly important for low-level moisture transport into Arizona
and Sonora. Upper level moisture is also transported into the region, mainly
from the Gulf of Mexico by easterly winds aloft. Once the forests of the
Sierra Madre Occidental green up from the initial monsoon rains, evaporation
and plant transpiration can add additional moisture to the atmosphere which
will then flow into Arizona. Finally, if the southern Plains of the U.S.
are unusually wet and green during the early summer months, that area can
also serve as a moisture source. This combination causes a distinct rainy
season over large portions of western North America, which develops rather
quickly and sometimes dramatically. Graphic 1 shows the general moisture
sources for the North American Monsoon. |
| |
 |
| Graphic
1: Moisture sources for the North American Monsoon. |
| |
| Rainfall
during the monsoon is not continuous. It varies considerably, depending
on a variety of factors. There are usually distinct "burst" periods
of heavy rain during the monsoon, and "break" periods with little
or no rain. Monsoon precipitation, however, accounts for a substantial portion
of annual precipitation in northwest Mexico and the Southwest U.S. Most
of these areas receive over half their annual precipitation from the monsoon. |
| |
| The North
American Monsoon circulation pattern typically develops in late May or early
June over southwest Mexico. By mid to late summer, thunderstorms increase
over the "core" region of the southwest U.S. and Northwest Mexico,
including the U.S. and Mexican states of Arizona, New Mexico, Sonora, Chihuahua,
Sinaloa and Durango. The monsoon typically arrives in mid to late June over
northwest Mexico, and early July over the southwest U.S. Once the monsoon
is underway, mountain ranges, including the Sierra Madre Occidental and
the Mogollon Rim provide a focusing mechanism for the daily development
of thunderstorms. Thus much of the monsoon rainfall occurs in mountainous
terrain. For example, monsoon rainfall in the Sierra Madre Occidental typically
ranges from 10 to 15 inches. Since the southwest U.S. is at the northern
fringe of the monsoon, precipitation is less and tends to be more variable.
Areas further west of the core monsoon region, namely California and Baja
California, typically receive only spotty monsoon-related rainfall. In those
areas, the intense solar heating isn't strong enough to overcome a continual
supply of cold water from the North Pacific Ocean moving down the west coast
of North America. Winds do turn toward the land in these areas, but the
cool moist air actually stabilizes the atmosphere. |
| |
| In addition
to the lower level monsoon circulation, an upper level monsoon (or subtropical)
ridge develops over the southern High Plains and northern Mexico. In June,
this ridge is too far south over Mexico and actually blocks deep moisture
from moving north into Arizona (Graphic 2). |
| |
 |
| Graphic
2: Mean 500mb height pattern, June. Subtropical high is strengthening
over northern Mexico |
| |
| However
by late June or early July, this ridge shifts north into the southern Plains
or southern Rockies (Graphic 3). As this shift takes place, mid and
upper level moisture streams into Arizona, and low level moisture surges
from Mexico meet less resistance. |
| |
 |
| Graphic
3: Mean 500mb height pattern, July. Subtropical high is near maximum
seasonal strength over New Mexico. |
| |
| This monsoon
ridge is almost as strong as the one which develops over Asia during the
summer. However, since the lower level moisture flow is not as persistent
as in the Indian monsoon, the upper level steering pattern and disturbances
around the ridge are critical for influencing where thunderstorms develop
on any given day. The exact strength and position of the subtropical ridge
also governs how far north the tropical easterly winds aloft can spread.
If the ridge is too close to a particular area, the sinking air at its center
suppresses thunderstorms and can result in a significant monsoon "break."
If the ridge is too far away or too weak, the east winds around the high
are inadequate to bring tropical moisture into the mountains of Mexico and
southwest U.S. However, if the ridge sets up in a few key locations, widespread
and potentially severe thunderstorms can develop. |
| |
| It is
very important to note that the monsoon is not an individual thunderstorm.
While the word "monsoon" accurately conjures up images of torrential
rains and flooding, calling a single thunderstorm a "monsoon"
is incorrect. A monsoon is a large scale weather pattern which causes
our summer thunderstorms. |
| |
| 500 mb graphics were
produced using NCEP Reanalysis Derived data provided by the NOAA/OAR/ESRL
PSD, Boulder, Colorado, USA. [Available on line at: Web site at http://www.cdc.noaa.gov
]. |
| |
|
For further reading:
Adams, D.K., and A.C.
Comrie, 1997: The North American Monsoon. Bull. Amer. Meteor. Soc., 78,
2197-2213.
Douglas, M.W., R.A.
Maddox, K Howard, and S. Reyes, 1993: The Mexican monsoon. J. Climate,
6, 1665-1667.
____, 2004: The North
American Monsoon. Reports to the Nation on our Changing Planet. NOAA/National
Weather Service. [Available on line at: http://www.cpc.noaa.gov/products/outreach/Report-to-the-Nation-Monsoon_aug04.pdf
]
Tang, M., and E.R.
Reiter, 1984: Plateau monsoons of the Northern Hemisphere: a comparison
between North America and Tibet. Mon Wea Rev., 112, 617-637.
|
|
| What
is a monsoon? | North American Monsoon
| Gulf Surges | Monsoon
progression | Monsoon Inter-annual
variability | Severe Thunderstorm and
Flash Flooding patterns | Upper
Level Lows and the Monsoon | Monsoon Safety
| For more reading |
| |
|
|
|
Webmaster
US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
Tucson Weather Forecast Office
520 North Park Ave, Suite 304
Tucson, AZ 85719
Tel: (520) 670-6526
|
Disclaimer
Information Quality
Credits
Glossary |
Privacy Policy
Freedom of Information Act
About Us
Career Opportunities
|
National Weather Service Mission: "The
National Weather Service (NWS) provides weather,
hydrologic, and climate forecasts and warnings for
the United States, its territories, adjacent waters
and ocean areas, for the protection of life and
property and the enhancement of the national economy.
NWS data and products form a national information
database and infrastructure which can be used by
other governmental agencies, the private sector,
the public, and the global community." |
|
|
www.weather.gov 
