“Traditional” terminology Arabic word “mausim” means“season” Loose definition: a wind/precipitation pattern that shifts seasonally Classical criteria (Ramage 1971) Prevailing wind shifts 120 o between Jan & July Average frequency of prevailing wind > 40% Speed of mean wind exceeds 3 m/s Pressure patterns satisfy a steadiness criterion Classical Example: Indian Monsoon
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Traditional terminology Arabic word mausim meansseason Loose definition: a wind/precipitation pattern that shifts seasonally Classical criteria (Ramage.
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“Traditional” terminology
Arabic word “mausim” means“season”
Loose definition: a wind/precipitation pattern that shifts seasonally
Classical criteria (Ramage 1971)Prevailing wind shifts 120o between Jan & JulyAverage frequency of prevailing wind > 40%Speed of mean wind exceeds 3 m/sPressure patterns satisfy a steadiness criterion
Classical Example: Indian Monsoon
Other Examples
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North AmericanWest AfricanAfrican Winter(Indian Summer)East AsianN.W. Australian
N. H. Summer
N. H. Winter
Surface temperature range that results in the monsoon
Kump et al. text (2004)
Sea surface temperature variations are much smaller than land surface temperature variations. (Mainly the seasonal cycle)
(*) Eastward progression of regions of both enhanced and suppressed tropical rainfall, mainly over the Indian Ocean and Pacific Ocean.
(*) Anomalous rainfall is usually first evident over the western Indian Ocean; propagates over
warm western and central tropical Pacific.
(*) Pattern of tropical rainfall weakens/disappears over the cooler waters of the eastern Pacific;
reappears over the Atlantic and Indian Ocean.
(*) Each cycle lasts approximately 30-60 days
(*) Apparently anti-correlated with El Nino
MJO-mature
(material from second Webster chapter not included here)
North American Monsoon -1
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North American Monsoon - description
* Summer daytime heating over the Sonoran Desert of Mexico and Arizona forms large surface low-pressure cell over the region,
* The low draws moist tropical air from the Gulfs of California and Mexico, triggering thunderstorms as it is lifted by the mountains and solar heating.
* The monsoonal circulation does not produce thunderstorms every day but rather occurs in a pattern that has "bursts" and "breaks:"
Burst: A movement of a weak trough in the upper level westerly wind into the southwest U.S. which spreads upper level cold air into the region. In the lower levels of the atmosphere, strong surface heating and southerly winds transport moisture into the region, creating unstable conditions and leading to widespread thunderstorm outbreaks.
"Break: An enhanced ridging of the Pacific subtropical High Pressure moves inland, effectively cutting off the moisture flow and stabilizing the atmosphere"
(Late June - Early September)
Burst and Break
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Burst Break
weak disturbances in the upper atmosphere act to focus thunderstorm activity over the 4 corners for a period of a few days to more than a week
Late June - Early September
More on the North American Monsoon
On a typical Arizona Monsoon day, thunderstorms initially develop In the early afternoon over the higher mountains and the Mogollon Rim. Rain-cooled air from these thunderstorms descends from the high country and into the desert. Acting like a cold front, this mesoscale outflow induces the hot desert air to rise, again producing thunderstorms. Over the higher deserts, they usually occur during the mid to late afternoon, while over the lower deserts storm activity is most common during the late afternoon and evening. Generally, thunderstorm activity ceases around midnight.
North American Monsoon - more description
During the monsoon season, the region receives most of its annual precipitation, approximately 35 to 45 percent for Arizona and New Mexico and 60 percent for northern Mexico.
For example, Acapulco rainfall totals 51.8 inches June-October (more than 9 inches each month except October), while only 3.3 inches falls during the rest of the year.
North American Monsoon - rainfall
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July 500 mb height
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High
Anticyclonic circulation around High involves flow from Gulfsof California and Mexico into Northwest Mexico and Southwest US
500 mb heights: May, August, October
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May
August
October
High
North American Monsoon - setup and decay
Q u ic k T im e ™ a n d aT IF F (U n c o m p re s s e d ) d e c o m p re s s o ra re n e e d e d to s e e th is p ic tu re .
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