Monsoon Meteorology
ATS 553
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.
Caused by:
• …the fact that water takes so much longer to heat up or cool down than land does.
Three Reasons
1. Cooling by Evaporation
1. Cooling by Evaporation
2. Cooling by Mixing
2. Cooling by Mixing
2. Cooling by Mixing
2. Cooling by Mixing
3. High Specific Heat of Water
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:
Heat Lows
1000mb
850mb
700mb
500mb
Heat Lows
1000mb
850mb
700mb
500mb
Heat Lows
1000mb
850mb
700mb
500mb
Heat Lows
1000mb
850mb
700mb
500mb
Heat Lows
1000mb
850mb
700mb
500mb
Heat Lows
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Polar Highs
1000mb
850mb
700mb
500mb
Fig 6-27 in Ramage
Fig 6-27 in Ramage
Fig 6-32 in Ramage
Monsoon Regions
Monsoon Regions
Major shift or reversal of the circulation features.
Monsoon Regions
Prevailing wind isn’t just a statistical average—it reflects the real flow in the atmosphere.
Monsoon Regions
The prevailing wind needs to be reasonably robust.
Monsoon Regions
Monsoons are NOT just a shift in the storm track, bringing sequences of highs and lows.
Fig 6-28 in Ramage
•India IND•West Africa WAFR•East Africa EAFR•Southeast Asia SEASIA•Australia AUS
Where don’t monsoons happen?
• South America– 1. Cold upwelling
means that the land is ALWAYS warmer than the surrounding oceans.
Where don’t monsoons happen?
• South America– 2. SAMER is too
narrow at subtropical southern latitudes for a planetary scale anticyclone to form.
Where don’t monsoons happen?
• South America– 3. In the NH, SAMER
doesn’t extend to regions of subsidence (which favor heat low formation).
Where don’t monsoons happen?
• Mexico, SW US– 1. No region has a
120° wind shift
Where don’t monsoons happen?
• Mexico, SW US– 2. Central America is
too narrow for formation of polar highs.
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..
Precipitation Regimes in the Wet Season
What You Don’t Know About West Africa Is A Lot
ATS 553
Political Geography
Historically…
Grain Coast Ivory Coast Gold Coast
Slave Coast
Vegetation
NDVI in WAFR
0°W 5°E 10°E5°W10°W5°N
10°N
15°N
20°N
Parakou
Sahara
Sahara
Sahel
Sahel
Savanna (Soudanian Zone)
baobob trees
Soudanian Zone
Cotton Harvest in Burkina Faso
Rainforest
Niger River Basin
Inland Delta
Great Mosque at Jenne
Landforms
Jos Plateau
Atakora Mountains
Aïr Mountains
Oueme River Valley
Climatology of WAFR Monsoon
925mb Winds in June
925mb Winds in September
Time-latitude diagrams along 2.5°E(Parakou, Benin)
V925mb along 2°E
U925mb along 2°E
OLR along 2.5°E
Thermal Fields in the WAFR Monsoon
Surface TemperatureNCEP Climatology
Temperature on 2-sep-04
Atlantic
Gui
nea
Sou
dani
an
Sah
elia
n
Sahara
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.
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
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
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
http://cimss.ssec.wisc.edu/tropic/real-time/wavetrak/sal-atl.html
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
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