Post on 31-Jan-2016
description
Evolution of the African Evolution of the African Monsoon during 2006Monsoon during 2006
(Sahel Rainfall, African Easterly Waves and (Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclones)Atlantic Tropical Cyclones)
Rosana Nieto Ferreira Tom Rickenbach
East Carolina University Earle Williams (MIT)
Nick Guy (San José State University)
East Carolina University
Atlantic Tropical Cyclones
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11
# TCs
TCs from AEWs
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)
0
2
4
6
8
10
12
14
16
18
20
Jun Jul Aug Sep Oct Nov
# of Storms
Tropical Cyclones
Hurricanes
Season peak in Aug/Sep Half come from African Easterly Waves
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
What is an African Easterly Wave?
Like our own Jet Stream, cyclonic meandering in winds over West Africa favor the formation of storms
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
West African Monsoon
10 N 20 N 30 NEQ
600
200
Pre
ssur
e (m
b)
Latitude
ITC
Z (
Mo
ns
oo
n R
ain
)IT
CZ
(M
on
so
on
Ra
in)
Sahara Warm DryAir
Sahara Warm DryAir
African Easterly
Jet
African Easterly
Jet
Cool Gulf of Guinea SSTsCool Gulf of Guinea SSTs
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
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
Simple Physics Model
WITH Easterly Waves
NCEP Reanalysis (Sep 1979-1997, 20W-10E)
Both ITCZ only
Sahara only
AEJ
Monsoon Westerlies
TEJ
Nieto-Ferreira and Suarez, in preparation
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 Rain
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) AEW2) ITCZ
Wet
99
Dry
97
Two contrasting years in the Sahel
GPCP Apr-Oct Rainfall
1999 29 rainy days with > 5 mmMean rainfall per rainy day 18.8 mmTotal ~ 528 mm
199715 rainy days with > 5 mm
Mean rainfall per rainy day 17 mmTotal ~ 255 mm
+
Rainfall > 5 mm
Rainfall > 20 mm
West African Coastline 20W
West African Coastline 20W
Niamey2.5ENiamey2.5E
Tim
eT
i me
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
+
Rainfall > 5 mm
Rainfall > 20 mm
Two contrasting years in the Sahel - Tropical Cyclones
19971 of 8 Tropical Cyclones
Formed in African Easterly Waves
6
1997 700 mb Relative Vorticity (5N-15N)
19997 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
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…
+
Rainfall > 5 mm
Rainfall > 20 mm
27 rainy days with > 5 mmMean rainfall per rainy day 15.4 mmTotal ~ 416 mm
H6
H7
4
5
H8
H10
H9
2006 700 mb Relative Vorticity (5N-15N)
20067 of 10 Atlantic Tropical CyclonesFormed in African Easterly Waves
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
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