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Rosana Nieto Ferreira Tom Rickenbach East Carolina University Earle Williams (MIT)

Evolution of the African Monsoon during 2006 (Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclones). Rosana Nieto Ferreira Tom Rickenbach East Carolina University Earle Williams (MIT) Nick Guy ( San Jos é State University). East Carolina University.

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Rosana Nieto Ferreira Tom Rickenbach East Carolina University Earle Williams (MIT)

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  1. Evolution of the African Monsoon during 2006(Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclones) Rosana Nieto Ferreira Tom Rickenbach East Carolina University Earle Williams (MIT) Nick Guy (San José State University) East Carolina University

  2. http://www.nhc.noaa.gov/gifs/strikes_us.gif

  3. Typical Hurricane Paths

  4. Atlantic Tropical Cyclones 97 98 99 00 01 02 03 04 05 06 07 60% of all TCs during this period formed from AEW

  5. North Carolina Tropical Cyclones Season peak in Aug/Sep Half come from African Easterly Waves

  6. Hurricane Dennis (1999) • Formed from an African Easterly Wave that left the coast of Africa on Aug 17 • Category 2 hurricane • Tropical Storm at landfall in NC on Sep 1 • Hurricane Floyd (1999) • Formed from an African Easterly Wave that left the coast of Africa on September 2 • Category 4 hurricane • category 2 (105 mph winds) at landfall on Sep 16 Hurricane Floyd Bio

  7. What is an African Easterly Wave? Like our own Jet Stream, cyclonic meandering in winds over West Africa favor the formation of storms

  8. African Easterly Wave precursor to Hurricane Helene AEW

  9. African Easterly Wave precursor to Hurricane Helene

  10. African Easterly Wave precursor to Hurricane Helene

  11. African Easterly Wave precursor to Hurricane Helene

  12. African Easterly Wave precursor to Hurricane Helene

  13. African Easterly Wave precursor to Hurricane Helene

  14. African Easterly Wave precursor to Hurricane Helene

  15. African Easterly Wave precursor to Hurricane Helene

  16. African Easterly Wave precursor to Hurricane Helene

  17. African Easterly Wave precursor to Hurricane Helene

  18. African Easterly Wave precursor to Hurricane Helene

  19. African Easterly Wave precursor to Hurricane Helene

  20. African Easterly Wave precursor to Hurricane Helene

  21. African Easterly Wave precursor to Hurricane Helene

  22. African Easterly Wave precursor to Hurricane Helene

  23. African Easterly Wave precursor to Hurricane Helene

  24. African Easterly Wave precursor to Hurricane Helene

  25. African Easterly Wave precursor to Hurricane Helene

  26. African Easterly Wave precursor to Hurricane Helene

  27. African Easterly Wave precursor to Hurricane Helene

  28. African Easterly Wave precursor to Hurricane Helene

  29. African Easterly Wave precursor to Hurricane Helene

  30. How do African Easterly Waves form?

  31. African Easterly Jet (AEJ) AEJ • Statistics • Summertime feature of the African Circulation • 10-15 m/s • centered at 15°N between 600-700 mb • strongest over West Africa and the east Atlantic Formation Mechanisms • reversed meridional temperature gradient between warm Sahara and cool Gulf of Guinea • Intertropical convergence zone (ITCZ) convection (e.g., Carson 69, Burpee 74, Reed et al 77, Norquist 77, Thorncroft and Hoskins 94)

  32. 200 Pressure (mb) African Easterly Jet ITCZ (Monsoon Rain) 600 Sahara Warm Dry Air Cool Gulf of Guinea SSTs EQ 10 N 20 N 30 N Latitude West African Monsoon

  33. West African Monsoon Circulation in the NCEP Reanalysis NCEP Reanalysis (Sep 1979-1997, 20W-10E) TEJ AEJ Monsoon Westerlies Nieto-Ferreira and Suarez, in preparation

  34. Simple Physics Model WITHOUT Easterly Waves ITCZ only Both Sahara only NCEP Reanalysis (Sep 1979-1997, 20W-10E) TEJ AEJ Monsoon Westerlies Nieto-Ferreira and Suarez, in preparation

  35. Simple Physics Model WITH Easterly Waves ITCZ only Both Sahara only NCEP Reanalysis (Sep 1979-1997, 20W-10E) TEJ AEJ Monsoon Westerlies Nieto-Ferreira and Suarez, in preparation

  36. AEW AEJ Rain Rain African Easterly Waves (AEW) • Statistics • 15° N • Wavelength ~ 2000-4000 km • Propagate westward at 8 m/s or 8°/day • 29 waves during May-October (Thorncroft and Hodges, 01) • Rainfall occurs ahead of the trough (e.g., Reed et al. 77, Gu et al. 04) Formation Mechanism • Combined barotropic and baroclinic instability of the African Easterly Jet (e.g., Carson 69, Burpee 74, Norquist 77, Thorncroft and Hoskins 94)

  37. How is African Easterly Wave Activity affected by rainfall in Africa?

  38. 2006 GPCP 1dd Rainfall - Niamey 2006 GPCP 1dd Rainfall - Abuja Abuja, Nigeria Two different regimes of rainfall in Equatorial Africa 1) AEW 2) ITCZ Niamey, Niger 1979-2006 mean = 1432 mm 2006 = 1627 mm It rains almost every day in Abuja On average, 9mm per rainy day 1979 - 2006 mean = 440 mm 2006 = 445 mm It rains every 3-5 days On average, 9 mm per rainy day

  39. 99 97 Wet Dry Two contrasting years in the Sahel GPCP Apr-Oct Rainfall 1997 15 rainy days with > 5 mm Mean rainfall per rainy day 17 mm Total ~ 255 mm 1999 29 rainy days with > 5 mm Mean rainfall per rainy day 18.8 mm Total ~ 528 mm

  40. African Easterly Waves Time Rainfall > 5 mm + Rainfall > 20 mm 1999 ~20 AEW passed through Niamey 1997 ~12 AEW passed through Niamey 1997 700 mb Relative Vorticity (5N-15N) 1999 700 mb Relative Vorticity (5N-15N) West African Coastline 20W Niamey 2.5E

  41. Two contrasting years in the Sahel - Tropical Cyclones Rainfall > 5 mm + Rainfall > 20 mm 1997 1 of 8 Tropical Cyclones Formed in African Easterly Waves 1999 7 of 12 Tropical Cyclones Formed in African Easterly Waves 1997 700 mb Relative Vorticity (5N-15N) 1999 700 mb Relative Vorticity (5N-15N) H10 H7 8 H6 6 5 H3 H4 In good agreement with Thorncroft and Hodges 2001

  42. In general, we should expect more and strongerAfrican Easterly Waves during wet years in the African Sahelbut it is not the case that more African Easterly Waves result in more Atlantic tropical cyclones, at least not in interannual timescales…

  43. AMMA - African Monsoon Multidisciplinary AnalysisField Campaign - Summer 2006

  44. In 2006 Niamey rainfall was slightly below average for the 1997-2007 period

  45. H10 H9 H8 H7 5 H6 4 Rainfall > 5 mm + Rainfall > 20 mm 2006 7 of 10 AtlanticTropical Cyclones Formed in African Easterly Waves 2006 700 mb Relative Vorticity (5N-15N) 27 rainy days with > 5 mm Mean rainfall per rainy day 15.4 mm Total ~ 416 mm

  46. 2006 700 mb Relative Vorticity (5N-15N) H Helene H Gordon H Florence TS Debby H Ernesto TS Chris

  47. The 2006 African Monsoon in Niamey H Helene’s African Easterly wave was captured by the Niamey radar

  48. West African ‘Rainmakers’ Organized as squall lines: the largest, rainiest systems observed over land • Squall lines feed back to reinforce African Easterly Waves • Squall lines produce most of the monsoon rain vital to subsistence agriculture in West Africa • African squall lines are “seedlings” for about half of all Atlantic hurricanes

  49. MIT Radar - Niamey, Niger, West Africa Squall line precursor to Hurricane Helene 8 September 2006

  50. MIT Radar - Niamey, Niger, West Africa Squall line precursor to Hurricane Helene 8 September 2006

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