monsoon meteorology ats 553. monsoon: a reversal of the wind direction at the surface, usually...
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Monsoon Meteorology
ATS 553
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Monsoon:
• A reversal of the wind direction at the surface, usually accompanied by the change in the precipitation regime, that occurs in the tropics on a seasonal time scale.
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Caused by:
• …the fact that water takes so much longer to heat up or cool down than land does.
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Three Reasons
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1. Cooling by Evaporation
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1. Cooling by Evaporation
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2. Cooling by Mixing
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2. Cooling by Mixing
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2. Cooling by Mixing
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2. Cooling by Mixing
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3. High Specific Heat of Water
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Therefore…
• In the SUMMER hemisphere, tropical land masses are much hotter than the surrounding ocean.
• In the WINTER hemisphere, tropical land masses are much colder than the surrounding ocean.
• Formation of HEAT LOWS and POLAR HIGHS:
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Heat Lows
1000mb
850mb
700mb
500mb
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Heat Lows
1000mb
850mb
700mb
500mb
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Heat Lows
1000mb
850mb
700mb
500mb
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Heat Lows
1000mb
850mb
700mb
500mb
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Heat Lows
1000mb
850mb
700mb
500mb
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Heat Lows
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Polar Highs
1000mb
850mb
700mb
500mb
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Fig 6-27 in Ramage
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Fig 6-27 in Ramage
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Fig 6-32 in Ramage
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Monsoon Regions
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Monsoon Regions
Major shift or reversal of the circulation features.
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Monsoon Regions
Prevailing wind isn’t just a statistical average—it reflects the real flow in the atmosphere.
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Monsoon Regions
The prevailing wind needs to be reasonably robust.
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Monsoon Regions
Monsoons are NOT just a shift in the storm track, bringing sequences of highs and lows.
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Fig 6-28 in Ramage
•India IND•West Africa WAFR•East Africa EAFR•Southeast Asia SEASIA•Australia AUS
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Where don’t monsoons happen?
• South America– 1. Cold upwelling
means that the land is ALWAYS warmer than the surrounding oceans.
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Where don’t monsoons happen?
• South America– 2. SAMER is too
narrow at subtropical southern latitudes for a planetary scale anticyclone to form.
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Where don’t monsoons happen?
• South America– 3. In the NH, SAMER
doesn’t extend to regions of subsidence (which favor heat low formation).
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Where don’t monsoons happen?
• Mexico, SW US– 1. No region has a
120° wind shift
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Where don’t monsoons happen?
• Mexico, SW US– 2. Central America is
too narrow for formation of polar highs.
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Where don’t monsoons happen?
• Mexico, SW US– 3. Weather is more
driven by synoptic disturbances.
However, a monsoon trough does form over the eastern Pacific..
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Precipitation Regimes in the Wet Season
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What You Don’t Know About West Africa Is A Lot
ATS 553
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Political Geography
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Historically…
Grain Coast Ivory Coast Gold Coast
Slave Coast
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Vegetation
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NDVI in WAFR
0°W 5°E 10°E5°W10°W5°N
10°N
15°N
20°N
Parakou
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Sahara
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Sahara
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Sahel
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Sahel
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Savanna (Soudanian Zone)
baobob trees
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Soudanian Zone
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Cotton Harvest in Burkina Faso
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Rainforest
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Niger River Basin
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Inland Delta
Great Mosque at Jenne
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Landforms
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Jos Plateau
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Atakora Mountains
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Aïr Mountains
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Oueme River Valley
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Climatology of WAFR Monsoon
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925mb Winds in June
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925mb Winds in September
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Time-latitude diagrams along 2.5°E(Parakou, Benin)
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V925mb along 2°E
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U925mb along 2°E
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OLR along 2.5°E
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Thermal Fields in the WAFR Monsoon
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Surface TemperatureNCEP Climatology
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Temperature on 2-sep-04
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Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
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Vertical Cross-Section
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
Boundary between the warn and cold air mass is sloped with height, just like a cold front or a warm front.
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Vertical Cross-Section
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
Boundary between the warn and cold air mass is sloped with height, just like a cold front or a warm front.
Saharan Air Layer (SAL)
Monsoon Layer
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Vertical Cross-Section
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
In the SAL, dry convection keeps the layer well-mixed:
Saharan Air Layer (SAL)
Monsoon Layer
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Well-Mixed Air
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
•Temperature decreases according to the dry adiabatic lapse rate (DALR).
•Potential temperature is constant with height.
•Stability: Statically Neutral!
Saharan Air Layer (SAL)
Monsoon Layer
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http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal-atl.html
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Monsoon Layer
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
•Tends to be CONDITIONALLY UNSTABLE due to cool conditions at the surface and higher potential temperatures aloft.•Temperature decreases by the moist adiabatic lapse rate.•Potential temperature increases slowly with height.
Saharan Air Layer (SAL)
Monsoon Layer
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Isentropes
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
•SAL: Neutral at SFC, stable in upper trop, very stable above tropopause•Monsoon Layer: Conditionally unstable in lower trop, stable in upper trop, very stable above tropopause
Low θ
High θ
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