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
http://www.nhc.noaa.gov/gifs/strikes_us.gif
Typical Hurricane Paths
Atlantic Tropical Cyclones
30
25
20
# TCs
15
TCs from AEWs
10
5
0
1
2
3
4
5
6
7
8
9
10
11
97 98 99 00 01 02 03 04 05 06 07
60% of all TCs during this period formed from AEW
North Carolina Tropical
Cyclones
Storms within 25 miles of NC (1950-2007)
20
18
16
# of Storms
14
12
Tropical Cy clones
10
Hurricanes
8
6
4
2
0
Jun
Jul
Aug
Sep
Oct
Nov
Season peak in Aug/Sep
Half come from African Easterly Waves
Hurricane Dennis (1999)
Hurricane Floyd (1999)
Hurricane Floyd Bio
• Formed from an African Easterly Wave that
left the coast of Africa on Aug 17
• Formed from an African Easterly Wave that left
the coast of Africa on September 2
• Category 2 hurricane
• Category 4 hurricane
• Tropical Storm at landfall in NC on Sep 1
• category 2 (105 mph winds) at landfall on Sep 16
What is an African Easterly
Wave?
Like our own Jet Stream, cyclonic
meandering in winds over West Africa
favor the formation of storms
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
African Easterly Wave precursor to Hurricane Helene
How do African Easterly Waves
form?
African Easterly Jet (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)
AEJ
West African Monsoon
ITCZ (Monsoon Rain)
Pressure (mb)
200
600
Cool Gulf of Guinea SSTs
EQ
10 N
African
Easterly
Jet
Sahara
Warm
Dry
Air
20 N
Latitude
30 N
West African Monsoon Circulation in the NCEP Reanalysis
NCEP Reanalysis
(Sep 1979-1997, 20W-10E)
TEJ
AEJ
Monsoon
Westerlies
Simple Physics Model
WITHOUT Easterly Waves
ITCZ only
Sahara only
Both
NCEP Reanalysis
(Sep 1979-1997, 20W-10E)
TEJ
AEJ
Monsoon
Westerlies
Simple Physics Model
WITH Easterly Waves
ITCZ only
Sahara only
Both
NCEP Reanalysis
(Sep 1979-1997, 20W-10E)
TEJ
AEJ
Monsoon
Westerlies
Nieto-Ferreira and Suarez, in preparation
African Easterly Waves (AEW)
Statistics
• 15° N
• Wavelength ~ 2000-4000 km
• Propagate westward at 8 m/s or 8°/day
Rain
• 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)
AEJ
Rain
How is African Easterly Wave
Activity affected by rainfall in
Africa?
Two different regimes of rainfall in Equatorial Africa
1) AEW
2) ITCZ
2006 GPCP 1dd Rainfall - Abuja
2006 GPCP 1dd Rainfall - Niamey
Niamey, Niger
Abuja, Nigeria
1979 - 2006 mean =
440
mm
1979-2006 mean = 1432 mm
2006
445
mm
2006
=
= 1627 mm
It rains every 3-5 days
It rains almost every day in Abuja
On average, 9 mm per rainy day
On average, 9mm per rainy day
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 m
Total ~ 528 mm
99
97 Wet
Dry
African Easterly Waves
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
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
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
H6
H7
8
6
H3
5
H4
Rainfall > 5 mm
+
Rainfall > 20 mm
In good agreement with Thorncroft and Hodges 2001
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…
AMMA - African Monsoon
Multidisciplinary Analysis
Field Campaign - Summer 2006
In 2006 Niamey rainfall was
slightly below average for the
1997-2007 period
2006
7 of 10 Atlantic Tropical Cyclones
Formed in African Easterly Waves
2006 700 mb Relative Vorticity (5N-15N)
H10
H9
H8
H7
5
H6
27 rainy days with > 5 mm
Mean rainfall per rainy day 15.4 mm
Total ~ 416 mm
Rainfall > 5 mm
+
Rainfall > 20 mm
4
2006 700 mb Relative Vorticity (5N-15N)
The 2006 African Monsoon in
Niamey
H Helene’s African
Easterly wave was
captured by the Niamey
radar
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
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
MIT Radar - Niamey, Niger, West Africa
Squall line precursor to Hurricane Helene
8 September 2006
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
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
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
GPCP Rainfall and NCEP Reanalysis 700 mb Vorticity and Winds
8 Sep
Niamey
Squall Line