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African Easterly Waves
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
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AEW basic facts (1)
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
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JJALat vs Lon
Latitude vs Month Avg’d 0-10 E
June 1 Sept 1
Zonal wind
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JJA 600mb
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Winds averaged: 10 N - 20 N, 0 - 10 E
600
Height vs Time
SeptJune
Meridional (60 S - 60 N) Aug Temperature
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30 W
35 E
5 E
Note: low level T gradient ~ 15 N
Meridional (0-35 N) Aug T & U at 5 E
Zonal Wind
TemperatureQuickTime™ and a
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Circulation Aug, 5 E
Zonal WindMerid’lWind
Omega
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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 tracksMay-Oct 1995
600 mb
850 mb
(Thorncroft & Hodges 2001)
Positive vorticity tracksMay-Oct 1994
600 mb
850 mb
(Thorncroft & Hodges 2001)
Climatological 850mbTracks (1979-98)
Track density(shading > 6)
Genesis dens.(shading > 5)
(Thorncroft & Hodges 2001)
density = number/yr/(106 km2 )
“Growth rate”(Dark = growthLight = decay)
Climatological 850mbTracks (1979-98)
Track density(shading > 6)
Genesis dens.(shading > 5)
(Thorncroft & Hodges 2001)
density = number/yr/(106 km2 )
“Growth rate”(Dark = growthLight = decay)
Seasonal Cycle of Track Density (1979-98)
850 mb
600 mb
(Thorncroft & Hodges 2001)
May
Sept
June
Oct
May
Sept
Oct
June
Interannual VariabilityAEW activity (May-Oct)
(Thorncroft & Hodges 2001)
Number
Year
850 mb
600 mb
June
85 9080 9575
20
10
30
Interannual Variability AEW & Hurricane (May-Oct)
(Thorncroft & Hodges 2001)