African Monsoon

Wassila M. Thiaw

NOAA/ Climate Prediction Center

21 February 2012

CPC International Desks Training Lecture Series

Outline

• Forcing of the annual cycle

– Sea Surface Temperature (SST)

– Sea Level Pressure (SLP)

– Wind

– Precipitation

• Convergence Zones

• African Monsoon

SST: Major FeaturesAtlantic Warm Pool

Pacific Warm Pool Equatorial Cold Tongues

SST: Extremes in the Annual Cycle

Equatorial SSTs are warmest in April

Equatorial cold tongues are strongest in Jul.-Oct.

SST: Seasonal Cycle

Sea Level Pressure

• Largest annual cycle over the Northern Hemisphere

• Southern Hemisphere displays weaker annual cycle

Annual Range in SLP (hPa)

Large annual range:

• Mongolia/China

• Aleutian Islands

• Iceland

• Southern Africa

• Australia

• South America

SLP: Major NH Features

H H

H HH

L L

H

H H H

L

L

HHigh pressure dominates continental areas (Asia, North America, Europe) during the NH winter, while low pressure is observed over the North Pacific (near the Aleutian Islands) and North Atlantic (near Iceland).

Low pressure dominates continental areas (Asia and Africa) during the NH summer, while high pressure is observed over the eastern North Pacific and central North Atlantic.

H H

H HH

L L

H

H H H

L

L

H

SLP: Major SH Features

High pressure dominates continental areas (South America and Africa) during the SH summer, while low pressure is observed over the subtropical and lower mid-latitude oceanic areas.

High pressure dominates continental and oceanic regions in the subtropics and lower-mid-latitudes during the SH winter.

• Global precipitation analyses based on station data and satellite-derived estimates

• Greatest precipitation over warm surfaces where ample moisture is available, and in areas of mid-latitude storm activity– Tropical land masses– Intertropical Convergence Zones (ITCZs)– South Pacific Convergence Zone (SPCZ)– South Atlantic Convergence Zone (SACZ)– Mid-latitude winter storm tracks

Precipitation: Major Features

Precipitation: Major Features

• Least precipitation in regions lacking moisture or featuring pronounced subsidence, and in colder regions

– Equatorial cold tongues– Deserts– Subtropical high pressure systems– High latitudes

Precipitation: Major Features

IOCZSPCZ

ITCZ

SACZ

Storm Tracks

Precipitation: July

Monsoons

• Seasonally varying winds

• Typical monsoon regions include:– Asia– Africa– South America– Central America/ North America

CDAS Low level, 850 hPa geopotential heights DJF 1979-2008

Atmospheric Circulation

Subtropicalhighs

CDAS Low level, 850 hPa geopotential heights DJF 1979-2008

Thermallows

CDAS Low level, 850 hPa geopotential heights DJF 1979-2008

Low level, 850 hPa geop. heights & winds DJF 1979-2008

Is this dynamics similar to anywhere else?

South America is the closest analog …

Low level, 850 hPa geop. heights & winds DJF 1979-2008

What is the relationship between the large-scale circulation and the precipitation climatology?

Precipitation Climatology DJF 1981-2010

PREC/L

CMAP

SACZ) SICZ

Boreal summer: June-July-August

Latitude-Height Cross Section (V;W) 5E 1979-2010

Northward low-level flow onto Guinean coastextends well into 20N

Southward flow

• Divergence center over continental thermal low (Saharan High)• Meridional divergence associated with southward velocity near 15N• Eastward Coriolis force generates African Easterly Jet (AEJ) near 600 hPa

• Southward outflow over Region of deep convection near 5N extends Tropical Easterly Jet (TEJ) near 200 hPa

TEJ

AEJ

Shading indicates meridional wind speedThick white line indicates zero meridional velocity contour

Deep Convective System

Longitude-Height Cross Section (U;W) Eq. 1979-2010

Shading indicates vertical wind speedThick white line denotes zero vertical velocity contour

Rising motion in association with Congo rain forestSubsidence over the Gulf of Guinea

Indian Monsoon

West African Monsoon

Somali Jet

Reanalysis 1

Low level, 925 hPa geop. heights & winds, JAS 1979-2008

Somali JetLikely the strongestand mostSustainedlow-level windpatterns in theworld

Accounts for 65%of global crossequatorial mass flux

Links Africanand Asian rainfallsystems

Is there an analog for this system?

Maybe South America again?

No …

Low level, 925 hPa geop. heights & winds, JAS 1979-2008

Somali Jet Characteristics

• Also called the East African Jet (EAJ) or Findlater Jet

• Likely the strongest and most sustained low-level wind patterns in the world (mean maximum wind speeds of 25 m/s – observed up to 50 m/s)

• Low-level wind speed maximum at 1.5 km msl

• Accounts for 65% of global cross equatorial mass flux

• Links African and Asian rainfall systems

Mid level, 600 hPa geop. heights & winds, JAS 1979-2008

Saharanhigh

AfricanEasterlyjet

Indian monsoonupper-level high

TropicalEasterly Jet

Upper level heights and winds 200 hPa JAS

Precipitation Climatology JAS 1981-2010

PREC/L

CMAP

Dry Summer in East Africa

The transition (equinox) seasons bring most rainfall to East Africa

Long rains in spring

Short rains in fall

East Africa Annual CyclePrecipitation & 850 hPa Wind1981-2010

East Africa Annual CyclePrecipitation & 850 hPa Wind1981-2010

African Monsoon: Prominent features and Evolution

• West African Monsoon – NH Summer– Extension of the Asian low– Continental thermal low– Low level southwesterly flow– African Easterly Jet– Tropical Easterly Jet

• Southern Africa– Angola low– Interactions between mid-latitude and tropical systems

• East Africa– ITCZ– Transitional equinox spring and autumn seasons– Long rains in MAM– Short rains in OND