african monsoon wassila m. thiaw noaa/ climate prediction center 21 february 2012 cpc international...
TRANSCRIPT
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