Presentation on theme: "“Traditional” terminology"— Presentation transcript:
1 “Traditional” terminology Arabic word “mausim” means“season”Loose definition: a wind/precipitation patternthat shifts seasonallyClassical 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 criterionClassical Example: Indian Monsoon
2 Other Examples North American West African African Winter N. H. SummerOther ExamplesNorth AmericanWest AfricanAfrican Winter(Indian Summer)East AsianN.W. AustralianN. H. Winter
3 Surface temperature range that results in the monsoon Sea surface temperature variations are much smaller than land surface temperature variations. (Mainly the seasonal cycle)Tibetan plateau: range of 60°COcean: range of 3 to 5°CKump et al. text (2004)
4 Monsoon schematicWallace and Hobbs textbook (1977)
21 Madden-Julian Oscillation (*) Eastward progression of regions of both enhancedand suppressed tropical rainfall,mainly over the Indian Ocean and Pacific Ocean.(*) Anomalous rainfall is usually first evident overthe western Indian Ocean; propagates overwarm western and central tropical Pacific.(*) Pattern of tropical rainfall weakens/disappearsover the cooler waters of the eastern Pacific;reappears over the Atlantic and Indian Ocean.(*) Each cycle lasts approximately days(*) Apparently anti-correlated with El Nino
25 North American Monsoon - description (Late June - Early September)* Summer daytime heating over the Sonoran Desert of Mexico andArizona 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 butrather occurs in a pattern that has "bursts" and "breaks:"Burst: A movement of a weak trough in the upper level westerly windinto the southwest U.S. which spreads upper level cold air into the region. Inthe lower levels of the atmosphere, strong surface heating and southerly windstransport moisture into the region, creating unstable conditions and leadingto widespread thunderstorm outbreaks."Break: An enhanced ridging of the Pacific subtropical High Pressuremoves inland, effectively cutting off the moisture flow and stabilizing theatmosphere"
26 Burst and Break Late June - Early September Burst Break weak disturbances in the upper atmosphere act to focus thunderstorm activity over the 4 cornersfor a period of a few days to more than a week
27 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 MogollonRim. Rain-cooled air from these thunderstorms descends from thehigh country and into the desert. Acting like a cold front, thismesoscale outflow induces the hot desert air to rise, againproducing thunderstorms. Over the higher deserts, they usuallyoccur during the mid to late afternoon, while over the lower desertsstorm activity is most common during the late afternoon andevening. Generally, thunderstorm activity ceases around midnight.
28 North American Monsoon - more description During the monsoon season, the region receives most of itsannual precipitation, approximately 35 to 45 percent for Arizonaand 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 only3.3 inches falls during the rest of the year.