“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 & 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

Other Examples North American West African African Winter N. H. Summer Other Examples North American West African African Winter (Indian Summer) East Asian N.W. Australian N. H. Winter

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°C Ocean: range of 3 to 5°C Kump et al. text (2004)

Monsoon schematic Wallace and Hobbs textbook (1977)

Combined Monsoon System (Webster et. al. 1988)

Orography (Webster et. al. 1988)

Upper Tropospheric Temperature JJA DJF

Upper Tropospheric Temperature Seasonal Variation Jan July 110 W Dec 90 E 110 W

250 mb Zonal Wind (JJA) Wind Upper Trop Temp

250 mb Zonal Wind (DJF) Wind Upper Trop Temp

Vertical Motions (Webster et. al. 1988)

Meridional Circulation, 90E 1978-79 (Yanai et. al.)

June-August winds Dec-Feb winds (SW monsoon) (NW monsoon) Trade winds SW monsoon winds NE monsoon winds

Meridional Wind 5N Jan 925 mb 5N - 925 - July 5N 925 mb 5N - 925 -

Zonal Wind (JJA) Wind up to 100 mb (0-120 E) (0-40 E)

3-d View (Webster et. al. 1988)

Outgoing Longwave Radiaton SON JJA SON DJF DJF MAM

Indian Monsoon - Rainfall 1963, 71

Indian Monsoon - Interannual Variability -1

Indian Monsoon - Interannual Variability -2

Madden-Julian Oscillation (*) 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

North American Monsoon - description (Late June - Early September) * 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"

Burst and Break Late June - Early September 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

More on the North American Monsoon On a typical Arizona Monsoon day, thunderstorms initially develop I n 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

July 500 mb height High Anticyclonic circulation around High involves flow from Gulfs of California and Mexico into Northwest Mexico and Southwest US

500 mb heights: May, August, October High

North American Monsoon - setup and decay