Monsoon. Arabic word mausim means season. Loose definition: a wind/precipitation pattern that shifts seasonally

Transcription

1 Monsoon 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

2 January July

3 Asian monsoon circulation seasonal reversal of wind direction

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5 Monsoon Definitions Only Asia, Australia, and Africa satisfy Ramage s requirements of monsoons. A more general definition is probably better. One commonly used: Wind must reverse in direction between summer and winter. Summer season must be very wet, and winter season very dry. Using this definition, North America also experiences a monsoon (in the Southwest U. S. and Northern Mexico.) Using this general definition, over ½ of the tropics (hence ¼ of entire globe) experience a monsoon climate.

6 N. H. Summer Other Examples North American South American West African South African East Asian Australian N. H. Winter

7 Driving mechanisms Differential heating of land and ocean Moist processes

8 DIFFERENTIAL HEATING OF LAND AND OCEAN The specific heat of water is much larger than that of dry soil. Effective heat capacity difference is even larger, because mass of ocean is much larger. Only the upper most few centimeters of land are heated, due to slow molecular transfer of heat vertically. In oceans, heat is effectively mixed downward tens of meters via turbulent mixing. The difference in heat capacities, rather than specific heats themselves, is most important. Moist soil has higher specific heat than dry soil. Saturated soil behaves more like ocean than land. Differential heating sets up a horizontal pressure gradient (similar to land/sea breeze only on much larger scale.)

9 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

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11 schematic monsoon

12 average annual rainfall

13 onset and withdrawal

14 onset and withdrawal

15 Indian rainy season Lat/Long = N, E Average Annual Temperature ( C) = Annual Temperature Range ( C) = 3.6 Total Annual Precipitation (mm) = Summer Precipitation (mm) = Winter Precipitation (mm) = 293.3

16 Indian precipitation

17 Indian precipitation

18 African monsoon

19 African Monsoon

20 Moist convection in the south, dry convection in the north (the Sahara) African Easterly Jet (AEJ) 600 mb Monsoon winds Water vapour 17/6/97 African Monsoon

21 African precipitation

22 Australian monsoon circulation winter seasonal reversal of wind direction summer

23 Australia precipitation

24 North American monsoon

25 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"

26 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

27 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.

28 North American Monsoon - rainfall

29 North American monsoon rain