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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 researchDunn, 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

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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)