Evolution of the African Monsoon during 2006 (Sahel Rainfall, African Easterly Waves and Atlantic...

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

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

Typical Hurricane Paths

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

African Easterly Wave precursor to Hurricane Helene

AEW

African Easterly Wave precursor to Hurricane Helene

How do African Easterly Waves form?

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


Sahara only

AEJ

Monsoon Westerlies

TEJ


Simple Physics Model

WITH Easterly Waves


Both ITCZ only

Sahara only

AEJ

Monsoon Westerlies

TEJ


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

How is African Easterly Wave Activity affected by rainfall in

Africa?

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


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

+

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


20067 of 10 Atlantic Tropical CyclonesFormed in African Easterly Waves


TS DebbyH Ernesto

H Helene

H GordonH Florence

TS Chris

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

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: [email protected]

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 SepNiamey Squall Line

Evolution of the African Monsoon during 2006 (Sahel Rainfall, African Easterly Waves and Atlantic...

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