African Easterly Waves
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Figure from Chris Landsea
A Hurricane Life
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Tropical Storm Genesis
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AEW early research
Dunn, G. E., 1940: ~ observed in Carribean westward
traveling lower tropospheric “seeds” for tropical cyclones
Riehl, H., 1945: ~ origins of waves were over North Africa
Burpee, R. W., (1972): ~ origin in barotropic/baroclinic
disturbances of African Easterly Jet
Burpee, R. W., (1974): ~ wavelength, period
Satellite Image
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10-15 N Pacific,
June-Aug 1967
AEW basic facts (1)
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Statistics:
Occur April/May through September/October
~ 60 per year in N. Atlantic
60% of tropical storms + Cat 1,2 hurricanes
85% of Cat 3,4, 5 hurricanes
most East Pacific hurricanes traced back to AEW’s
wavelength 2000-2500 km (18o-23o lon)
period 3-5 days
Origin:
disturbance of African Easterly Jet
Riehl model
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AEW features noted early
“Inverted V” horizontal orientation
AEW features (cont)
Mesoscale feature of lower atmosphere (~ 0 - 5 km)
Low level vorticity signal, decreasing with height
Fair weather to west of trough;
Upper level convergence, low level divergence
Subsidence pattern
Active weather to east of trough
Lower level convergence, upper level divergence
Deep layer of moisture
Convective precipitation
Intense waves can draw ITCZ north
Agee 1969 (Tropical Storm Anna)
Example
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African Easterly Jet
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Africa
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Skin Temperature
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850mb -700 mb Temperature
Latitude vs Month
Avg’d 0-10 E
JJA
Lat vs Lon
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June 1
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Sept 1
Zonal wind
JJA 600mb
Height vs Time
600
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June
Sept
Winds averaged:
10 N - 20 N,
0 - 10 E
5E
Meridional (60 S - 60 N)
Aug Temperature
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30 W
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35 E
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Note: low level
T gradient ~ 15 N
Meridional (0-35 N) Aug
T & U at 5 E
Zonal Wind
Temperature
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Circulation Aug, 5 E
Zonal Wind
Merid’l
Wind
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Omega
Hurricane and Positive vorticity center tracks
(Thorncroft &
Hodges 2001),
J. Climate
20 years (1979-98)
ECMWF reanalyses
Tracked systems with
closed vorticity contours
of at least + 5 x 10-6 s-1
Omitted systems that
- lasted less than 2 days
- traveled < 1000 km
Positive vorticity tracks
May-Oct 1995
600 mb
850 mb
(Thorncroft & Hodges 2001)
Positive vorticity tracks
May-Oct 1994
600 mb
850 mb
(Thorncroft & Hodges 2001)
Climatological 850mb
Tracks (1979-98)
Track density
(shading > 6)
Genesis dens.
(shading > 5)
“Growth rate”
(Dark = growth
Light = decay)
density = number/yr/(106 km2 )
(Thorncroft & Hodges 2001)
Climatological 850mb
Tracks (1979-98)
Track density
(shading > 6)
Genesis dens.
(shading > 5)
“Growth rate”
(Dark = growth
Light = decay)
density = number/yr/(106 km2 )
(Thorncroft & Hodges 2001)
Seasonal Cycle of Track
Density (1979-98)
(Thorncroft & Hodges 2001)
May
June
Sept
Oct
May
June
Sept
Oct
850 mb
600 mb
Interannual Variability
AEW activity (May-Oct)
(Thorncroft & Hodges 2001)
June
600 mb
Number
30
850 mb
20
10
75
80
85
Year
90
95
Interannual Variability AEW & Hurricane (May-Oct)
(Thorncroft &
Hodges 2001)