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

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850mb

700mb

500mb

Heat Lows

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Heat Lows

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Heat Lows

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Heat Lows

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Heat Lows

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Polar Highs

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Polar Highs

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Polar Highs

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Polar Highs

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Polar Highs

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Polar Highs

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Polar Highs

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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

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Sahara

Vertical Cross-Section

Atlantic

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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

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Sah

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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

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Sah

elia

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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

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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

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Sou

dani

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Sah

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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

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Sou

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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