Arabic word “mausim” means“season” “Traditional” terminology 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%
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
Ocean: range of 3 to 5°CSurface temperature range that results in the monsoon
Sea surface temperature variations are much smaller than land surface temperature variations. (Mainly the seasonal cycle)
Kump et al. text (2004)
Wallace and Hobbs textbook (1977)
Wind up to 100 mb
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
* 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
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.
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.
Anticyclonic circulation around High involves flow from Gulfs
of California and Mexico into Northwest Mexico and Southwest US