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Evolution of the African Monsoon during 2006 (Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclones) Rosana Nieto Ferreira Tom Rickenbach.

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Presentation on theme: "Evolution of the African Monsoon during 2006 (Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclones) Rosana Nieto Ferreira Tom Rickenbach."— Presentation transcript:

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 Jos é 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 Floyd Bio 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 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

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

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30 How do African Easterly Waves form?

31 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 African Easterly Jet (AEJ) (e.g., Carson 69, Burpee 74, Reed et al 77, Norquist 77, Thorncroft and Hoskins 94) Formation Mechanisms reversed meridional temperature gradient between warm Sahara and cool Gulf of Guinea Intertropical convergence zone (ITCZ) convection

32 West African Monsoon 10 N20 N30 NEQ 600 200 Pressure (mb) Latitude ITCZ (Monsoon Rain) Sahara Warm Dry Air Sahara Warm Dry Air African Easterly Jet Cool Gulf of Guinea SSTs

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

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

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

36 AEJ 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 African Easterly Waves (AEW) (e.g., Carson 69, Burpee 74, Norquist 77, Thorncroft and Hoskins 94) Rain

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

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

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

41 + Rainfall > 5 mm Rainfall > 20 mm Two contrasting years in the Sahel - Tropical Cyclones 1997 1 of 8 Tropical Cyclones Formed in African Easterly Waves 6 1997 700 mb Relative Vorticity (5N-15N) 1999 7 of 12 Tropical Cyclones Formed in African Easterly Waves H4 5 8 H3 H7 H6 H10 1999 700 mb Relative Vorticity (5N-15N) 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 + Rainfall > 5 mm Rainfall > 20 mm 27 rainy days with > 5 mm Mean rainfall per rainy day 15.4 mm Total ~ 416 mm H6 H7 4 5 H8 H10 H9 2006 700 mb Relative Vorticity (5N-15N) 2006 7 of 10 Atlantic Tropical Cyclones Formed in African Easterly Waves

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

47 The 2006 African Monsoon in Niamey H Helenes 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 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. E-mail: ferreirar@ecu.edu

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


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