African Easterly Waves and Atlantic Hurricanes Rosana Nieto - - PowerPoint PPT Presentation

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african easterly waves and atlantic hurricanes

African Easterly Waves and Atlantic Hurricanes Rosana Nieto - - PowerPoint PPT Presentation

Oct 14, 2022 334 likes •957 views

African Easterly Waves and Atlantic Hurricanes Rosana Nieto Ferreira Tom Rickenbach East Carolina University Earle Williams (MIT) Nick Guy (Colorado State University) East Carolina University Connections: African Sahel and North Carolina

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African Easterly Waves and Atlantic Hurricanes

Rosana Nieto Ferreira Tom Rickenbach

East Carolina University

Earle Williams (MIT) Nick Guy (Colorado State University) East Carolina University

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Connections: African Sahel and North Carolina

Drought Niger, West Africa Floyd aftermath Greenville, NC

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

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Hurricane Floyd Bio

Hurricane 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

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Dennis

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Dennis Floyd AEW

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Dennis Floyd AEW

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Dennis Floyd AEW

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Floyd AEW

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Floyd AEW

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Floyd

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Floyd

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Floyd

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Floyd

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Floyd Gert

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Floyd Gert

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Floyd Gert

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Floyd Gert

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Floyd Gert

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Floyd Gert

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Floyd Gert

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Gert

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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.

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What is an African Easterly Wave?

Like our own Jet Stream, cyclonic meandering in winds

ver the African

Sahel favors the formation of storms

AEJ Rain Rain

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AEJ

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 N EQ 600 200

Pressure (mb)

Latitude ITCZ Sahara Air Layer

African Easterly Jet

African Easterly Jet - AEJ

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African Easterly Waves

1999

~20 AEW passed through Niamey 8 of 12 Atlantic Tropical Cyclones Formed in African Easterly Waves including Hurricanes Dennis and Floyd

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

West African Coastline 20W

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AMMA Field Campaign - Summer 2006

Goal: Study the connections between Sahel Rainfall and Atlantic Hurricanes

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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

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

African Easterly Waves

2006

~21 AEW passed through Niamey 7 of 8 Atlantic Tropical Cyclones Formed in African Easterly Waves including Hurricane Ernesto and Tropical Storm Alberto that affected NC

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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

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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

8 September 2006

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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

bserved over the Sahel

(Rickenbach et al. 2009, GRL)

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Relation between squall lines and African Easterly Waves Two 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 10°N

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

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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

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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

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Sahel Rainfall, African Easterly Waves and Atlantic Tropical Cyclone Activity

WET WET DRY