African Easterly Waves Figure from Chris Landsea

A Hurricane Life

Tropical Storm Genesis

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

10-15 N Pacific, June-Aug 1967

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 km (18 o -23 o lon) period 3-5 days Origin: disturbance of African Easterly Jet

Riehl model

AEW features noted early “Inverted V” horizontal orientation

AEW features (cont) Mesoscale feature of lower atmosphere (~ 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

African Easterly Jet

Africa

Skin Temperature

850mb -700 mb Temperature JJA Lat vs Lon Latitude vs Month Avg’d 0-10 E June 1Sept 1

Zonal wind JJA 600mb Winds averaged: 10 N - 20 N, E 600 Height vs Time Sept June

Meridional (60 S - 60 N) Aug Temperature 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 Temperature

Circulation Aug, 5 E Zonal Wind Merid’l Wind Omega

Hurricane and Positive vorticity center tracks (Thorncroft & Hodges 2001), J. Climate 20 years ( ) ECMWF reanalyses Tracked systems with closed vorticity contours of at least + 5 x s -1 Omitted systems that - lasted less than 2 days - traveled < 1000 km

Positive vorticity tracks May-Oct mb 850 mb (Thorncroft & Hodges 2001)

Positive vorticity tracks May-Oct mb 850 mb (Thorncroft & Hodges 2001)

Climatological 850mb Tracks ( ) Track density (shading > 6) Genesis dens. (shading > 5) (Thorncroft & Hodges 2001) density = number/yr/(10 6 km 2 ) “Growth rate” (Dark = growth Light = decay)

Climatological 850mb Tracks ( ) Track density (shading > 6) Genesis dens. (shading > 5) (Thorncroft & Hodges 2001) density = number/yr/(10 6 km 2 ) “Growth rate” (Dark = growth Light = decay)

Seasonal Cycle of Track Density ( ) 850 mb 600 mb (Thorncroft & Hodges 2001) May Sept June Oct May Sept Oct June

Interannual Variability AEW activity (May-Oct) (Thorncroft & Hodges 2001) Number Year 850 mb 600 mb June

Interannual Variability AEW & Hurricane (May-Oct) (Thorncroft & Hodges 2001)