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

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 6-32 in Ramage

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 CoastIvory CoastGold Coast Slave Coast

Vegetation

NDVI in WAFR

Sahara

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)

V 925mb along 2°E

U 925mb along 2°E

OLR along 2.5°E

Thermal Fields in the WAFR Monsoon

Surface Temperature NCEP Climatology

Temperature on 2-sep-04

Atlantic GuineaSoudanianSahelian Sahara

Vertical Cross-Section Atlantic GuineaSoudanianSahelian 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 GuineaSoudanianSahelian 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 GuineaSoudanianSahelian Sahara In the SAL, dry convection keeps the layer well-mixed: Saharan Air Layer (SAL) Monsoon Layer

Well-Mixed Air Atlantic GuineaSoudanianSahelian 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

Monsoon Layer Atlantic GuineaSoudanianSahelian 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 GuineaSoudanianSahelian 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