1. East Carolina University Nick Guy (San José State University)
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

3. Typical Hurricane Paths

4. Atlantic Tropical Cyclones
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 Floyd Bio 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)
Statistics
Summertime feature of the African Circulation
10-15 m/s
centered at 15°N between mb
strongest over West Africa and the east Atlantic
AEJ
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. West African Monsoon 10 N 20 N 30 N EQ 600 200 Latitude
Pressure (mb)
Latitude
ITCZ (Monsoon Rain)
Sahara Warm Dry
Air
African Easterly Jet
Cool Gulf of Guinea SSTs

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

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

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

36. African Easterly Waves (AEW)
Statistics
15° N
Wavelength ~ 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
AEW
Rain
AEJ
(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. 1) AEW 2) ITCZ Two different regimes of rainfall in Equatorial Africa
2006 GPCP 1dd Rainfall - Niamey
2006 GPCP 1dd Rainfall - Abuja
Niamey, Niger
Abuja, Nigeria
mean = mm
= mm
It rains almost every day in Abuja
On average, 9mm per rainy day
mean = mm
= mm
It rains every 3-5 days
On average, 9 mm per rainy day

39. Mean rainfall per rainy day 17 mm
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
Wet 99
Dry 97

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

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

42. In general, we should expect more and stronger African Easterly Waves during wet years in the African Sahel but 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 Analysis Field Campaign - Summer 2006

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

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

46. H Helene H Gordon H Florence TS Debby H Ernesto TS Chris
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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

76. West African ‘Rainmakers’
Organized as squall line these are the largest, rainiest systems observed over land
A toatl of 23 squall line-MCS like this one produced over 70 percent of the 2006 rainfall in Niamey

77. 2006 African Monsoon was about average E-mail: ferreirar@ecu.edu
Conclusion
2006 African Monsoon was about average
23 squall line systems associated with African Easterly waves were the major rainmaker in the African Sahel in 2006
Rainfall in the Sahel is a good indicator of the number and strength of African Easterly Waves, but not a good indicator of the number of tropical cyclones in the Atlantic.
The relationship between the strength of the African monsoon and the number of altantic hurricanes is not very strong in interannual timescales.

78. Ongoing Work
Use a combination of the AMMA-Niamey radar, NCEP Reanalysis, GPCP rainfall datasets and a hierarchy of numerical models of the atmosphere to further improve our understanding of the two-way interaction between convection and synoptic-scale flow in the West African Monsoon

79. GPCP Rainfall and NCEP Reanalysis 700 mb Vorticity and Winds
8 Sep
Niamey Squall Line