African Easterly Waves and Atlantic Hurricanes

Rosana Nieto Ferreira Tom Rickenbach

East Carolina UniversityEarle Williams (MIT)

Nick Guy (Colorado State University)

East Carolina University

Connections: African Sahel and North Carolina

DroughtNiger, West Africa

Floyd aftermathGreenville, NC

When West Africa gets more rain, we get more hurricanes!

Hurricane Floyd BioHurricane Floyd (1999)

• Formed from an African Easterly Wave that left the coast of Africa on September 2

Hurricane Dennis (1999)

• Formed from an African Easterly Wave that left the coast of Africa on Aug 17

Dennis

Floyd AEW

Dennis

Floyd AEW

Dennis

Floyd AEW

FloydGert

Atlantic Tropical Cyclones and African Easterly Waves

• More than half of all Atlantic tropical cyclones, including Dennis and Floyd, form in African Easterly Waves

• African Easterly Waves form in the African Sahel.

What is an African Easterly Wave?

Like our own Jet Stream, cyclonic meandering in winds over the African Sahel favors the formation of storms

AEJRain Rain

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, Rennick 77, Mass 77, Reed et al 77, Norquist 77, Thorncroft and Hoskins 94)

Sahara Desert

ITCZ Rain

10 N 20 N 30 NEQ

Latitude

Sahara Air Layer

African Easterly Jet

African Easterly Jet - AEJ

African Easterly Waves1999

~20 AEW passed through Niamey

8 of 12 Atlantic Tropical CyclonesFormed in African Easterly Waves

including Hurricanes Dennis and Floyd

1999 700 mb Relative Vorticity – Africa (5N-15N)

West African Coastline 20W

AMMA Field Campaign - Summer 2006

Goal: Study the connections between Sahel Rainfall and Atlantic Hurricanes

2006 GPCP 1dd Rainfall - Niamey 2006 GPCP 1dd Rainfall - Abuja

Abuja, Nigeria

Niamey, Niger

Two different regimes of rainfall in Equatorial Africa

At the peak of the rainy season (JAS),

• It rains every 3-4 days in Niamey.

• About 12 mm per rainy day

AEW Regime ITCZ Regime

2006 Total : 410 mm 2006 Total : 1436 mm

At the peak of the rainy season (JAS),

• It rains every day in Abuja.

• About 12 mm per rainy day

2006 700 mb Relative Vorticity, Africa (5N-15N)African Easterly Waves2006

~21 AEW passed through Niamey

7 of 8 Atlantic Tropical CyclonesFormed in African Easterly Waves

including Hurricane Ernesto and Tropical Storm Alberto that affected NC

West African ‘Rainmakers’ Organized as squall lines: the largest, rainiest

systems observed over land

• African Easterly Waves African Squall Lines

• Squall lines produce most of the monsoon rain vital to subsistence agriculture in West Africa

MIT Radar - Niamey, Niger, West Africa Squall line precursor to Hurricane Helene

8 September 2006

MIT Radar - Niamey, Niger, West Africa 8 September 2006

Hurricane Precursors:The Squall Line

• A total of 28 squall lines

• Squall lines produced 82% of the 2006 rainfall in Niger

Squall lines are the largest, rainiest systems observed over the Sahel

(Rickenbach et al. 2009, GRL)

Relation between squall lines and African Easterly WavesTwo Tracks

(Nieto Ferreira et al. 2009, MWR)

• 15 squall lines were associated with AEW troughs that propagated along 10-16°N (Northern Track)

• 13 squall lines were associated with AEW troughs that propagated along 2-6°N (Southern Track) 6°N

700 mb relative vorticity (contours), winds and rainfall (shaded)

Structural differences in northern vs. southern track squall lines

Stratiform Rain Fraction

Convective Rain Fraction

Northern 47% 53%Southern 35% 65%

The rainfall produced by northern track and southern track squall lines is very similar but the mode of delivery of rainfall is very different:

Hypothesis: AEW troughs present near Niamey in northern track squall lines favor stratiform rain production

Conclusion

There is a direct relationship between Sahel rain and Atlantic hurricanes

Squall lines are the main rainmakers in the Sahel

Distinct types of squall line form ahead of AEW troughs along two different AEW tracks

Understanding the interaction between squall lines and AEWs in the African Sahel may lead to improved

Atlantic hurricane prediction

Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclone Activity

WET WETDRY

African Easterly Waves and Atlantic Hurricanes

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