SEASONAL VARIATIONS IN REGIONAL CIRCULATION SYSTEMS: THE MONSOONS.
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Transcript of SEASONAL VARIATIONS IN REGIONAL CIRCULATION SYSTEMS: THE MONSOONS.
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SEASONAL VARIATIONS IN REGIONAL CIRCULATION SYSTEMS: THE MONSOONS
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The word “Monsoon” • The word "monsoon" is derived from the
Arabic word "mausim" which means season. Ancient traders sailing in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer. Thus, the term monsoon actually refers solely to a seasonal wind shift, and not to precipitation
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Outline• Definition of Monsoon
The Monsoon MakersAnnual Monsoon CycleVariability of the monsoons
Regional Monsoon SystemsThe Indian MonsoonThe East Asia MonsoonThe Australian Monsoon / East Asia Winter
MonsoonThe African MonsoonThe American Monsoon
Summary
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Let’s examine first the Indian Monsoon
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DEFINITION
• Prevailing wind direction shifts by at least 120º betwen
January and July
• Average frequency of prevailing directions in January and
July exceeds 40%
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The Monsoon Makers
1) Seasonal Heating
2) Moisture Processes
3) The Earth’s Rotation
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The Monsoon Makers: 1) Seasonal HeatingSeasonal contrasts in land surface temperatures produce atmospheric pressure changes.
~ 30C
~ 86F
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H
Seasonal changes in sea level pressure
H H
L
L
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The Monsoon Makers 1) Seasonal Heating
As a result, there are major seasonal wind reversals referred to as “the monsoons”
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The Monsoon Makers 2) Moisture Processes
http://www.geo.arizona.edu/Antevs/ecol438/monsoon.gif
Energy released in the form of latent heat of condensation raises summer land-ocean pressure differences to a point higher than they would be in the absence of moisture in the atmosphere.
Moisture Monsoon Magnitude
Heat
Heat
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The Monsoon Makers 3) The Earth’s Rotation
The air in monsoon currents moves in curved paths
Winds change direction as they cross the equator because of changes in the Coriolis Force
Equator
Fc
Fc
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The Monsoon Makers
1) Seasonal Heating
2) Moisture Processes
3) The Earth’s Rotation
Moiture releases energy (latent heat) that intensifies the monsoon
Seasonal temperature and pressure changes produce seasonal wind reversals
Air moves in curved paths and winds change direction as they cross the equator
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The Annual Monsoon Cycle
Mean annual cycle of precipitation over several major monsoon areas:[North America Monsoon (NAMS, 20°–37°N, 248°–257°E); South America Monsoon (SAMS, 20°–37°N,40°–60°W); India (6°–37°N, 68°–98°E); Sahel (10°–20°N, 15°– 15°W)]. For comparison, one non monsoon region with a large annual cycle is also shown [Pacific Northwest (PNW: 42°–50°N, 112°–124°W)].
FIG. 1. from Vera et al (2006)
Apr – northern hemisphere tropical-subtropical latitudes are beginning to warm upMay to Jun – heating of northern tropical lands intensifies.
Jun to Jul – sensible heat input at the surface is close to a maximumSep – cessation of the northern hemisphere monsoon
Weak Vertical Motion
Maximum Vertical
Motion and Moisture
Monsoon Maximum Intensity
Vertical Motion
Intensifies Predomina
nt wind direction
is onshore Onset of
the monsoon
Weak Vertical Motion
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Variability of the MonsoonsThere is considerable variability in the onset, duration and magnitude of the monsoons
Mechanisms:
forcingBoundary
dynamics Internal
Internal Dynamics = variations in the atmospheric circulation(e.g. travelling disturbances, thermal and orographic forcing, and tropical-extratropical interactions)
Boundary Forcing = changes in surface conditions(e.g. extent of snow cover, soil moisture, and sea surface temperature) energy balance geographical distribution of heat and moisture
Internal Dynamics + Boundary Forcing = Monsoon variations
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Internal Dynamics: Thermal and Orographic forcing
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Boundary Forcing: What is going on?
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Boundary Forcing: What’s going on?