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African Easterly Waves

African Easterly Waves. Figure from Chris Landsea. A Hurricane Life. Tropical Storm Genesis. 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

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African Easterly Waves

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  1. African Easterly Waves Figure from Chris Landsea

  2. A Hurricane Life

  3. Tropical Storm Genesis

  4. 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 AEW early research

  5. Satellite Image

  6. 10-15 N Pacific,June-Aug 1967

  7. 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

  8. Riehl model

  9. “Inverted V” horizontal orientation AEW features noted early

  10. 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 AEW features (cont)

  11. Agee 1969 (Tropical Storm Anna)

  12. Example

  13. African Easterly Jet

  14. Africa

  15. Skin Temperature

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

  17. Zonal wind JJA 600mb Height vs Time 600 June Sept Winds averaged: 10 N - 20 N, 0 - 10 E

  18. 5 E Meridional (60 S - 60 N) Aug Temperature 30 W 35 E Note: low level T gradient ~ 15 N

  19. Meridional (0-35 N) Aug T & U at 5 E Zonal Wind Temperature

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

  21. (Thorncroft & Hodges 2001), J. Climate Hurricane and Positive vorticity center tracks 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

  22. (Thorncroft & Hodges 2001) 600 mb Positive vorticity tracksMay-Oct 1995 850 mb

  23. (Thorncroft & Hodges 2001) 600 mb Positive vorticity tracksMay-Oct 1994 850 mb

  24. (Thorncroft & Hodges 2001) Climatological 850mbTracks (1979-98) Track density (shading > 6) Genesis dens. (shading > 5) “Growth rate” (Dark = growth Light = decay) density = number/yr/(106 km2 )

  25. (Thorncroft & Hodges 2001) Climatological 850mbTracks (1979-98) Track density (shading > 6) Genesis dens. (shading > 5) “Growth rate” (Dark = growth Light = decay) density = number/yr/(106 km2 )

  26. (Thorncroft & Hodges 2001) Seasonal Cycle of Track Density (1979-98) May June 850 mb Sept Oct May June 600 mb Oct Sept

  27. (Thorncroft & Hodges 2001) Interannual VariabilityAEW activity (May-Oct) June 600 mb Number 30 850 mb 20 10 75 80 85 90 95 Year

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

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