Kinematic Structure of the WAFR Monsoon

ATS 553

600mb NCEP Climatology Zonal Winds

Cross section of zonal winds along 0°E

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Sahara

Cross section of zonal winds along 0°E

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Sahara

Westerlies at 200mb:

• A weak subtropical jet?

Cross section of zonal winds along 0°E

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Sahara

Easterlies at 200mb:

• Near the coastline

• TROPICAL EASTERLY JET (TEJ)

TEJ=Equatorward Flank of Upper Level High Pressure Systems

(Above Heat Lows)

Streamlines at 200mb; colors are U winds at 200mb

HH

Heat Lows

1000mb

850mb

700mb

500mb

Cross section of zonal winds along 0°E

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Easterlies at 600mb:

• Centered near 12°N

• AFRICAN EASTERLY JET (AEJ)

Cross section of zonal winds along 0°E

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Low level westerlies:

• Extend to about 20°N

• THE MONSOON WESTERLIES

Cross section of zonal winds along 0°E

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Low level easterlies:

• Centered near 27°N

• HARMATTAN

Why is there an “African Easterly Jet”?

• Any time there is a change of wind speed with respect to height, you should be thinking “Thermal Wind Relationship”.

• Thermal Wind Relationship: U increases with height when T decreases to the north.

Why is there an “African Easterly Jet”?

• In West Africa, we have the opposite situation:– Temperature increases to the north– Therefore, U decreases with height.

– A negative U is a wind from the east—the AEJ!

Thermal Wind Relationship

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Thermal Wind Relationship

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500mb

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Thermal Wind Relationship

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Sahara

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Thermal Wind Relationship

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500mb

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400mb Pressure

Gradient

Force

Meridional temperature gradient at the surface is key to the existence of the AEJ!

PGF

Coriolis

AEJ

• At this point, the students should have read the paper by Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

Define 3 Regions

• Sahara

• Sahel

• ITCZ

Determine the heat budget for each of these three regions to determine why there is a temperature gradient in West Africa.

Heat Budget

SW↓:

Maybe the Sahara is hotter than the Sahel and the ITCZ simply because:•the sun angles are higher•the days are longer•the sky is more clear

SW↓ is greatest in the Sahara.

Heat Budget

SW↑:

However, the Sahara has a much higher albedo than the Sahel or the ITCZ region.

SW↑ is greatest in the Sahara.

Heat Budget

Net SW:

Still, the net shortwave radiation at the surface is greatest in the Sahara.

Heat Budget

LW↓:

Most LW↓ comes from:•cloud cover•greenhouse gases like water vapor

Therefore, the ITCZ region has the most LW↓, whereas the Sahara gains relatively little.

Heat Budget

LW↑:

Longwave radiation emitted by the surface is determined primarily by the temperature of the surface.

Therefore, the Saharan region loses the most LW↑, whereas the ITCZ region loses relatively little.

Heat Budget

Net LW:

The ITCZ has very little loss of heat by longwave radiation.

The Sahara loses a great deal of heat by longwave radiation.

Heat Budget

Net Radiative Heating:

The ITCZ region actually gains more heat by radiative processes than the Sahara does!

So, why is the Sahara warmer? (It seems like the coastal region should be warmer.)

Heat Budget

Sensible Heat flux:

•In the Sahara, the ground is much hotter than the air during the day, so SHF is very great.•In the ITCZ region, the air temperature and the ground temperature are approximately equal, so SHF is nearly zero.

Heat Budget

Latent Heat flux:

•In the Sahara, the ground is completely dry—there is basically no cooling by evaporation.•In the ITCZ region, soil moisture is high, so there is a great deal of evaporative cooling at the surface.

Heat Budget

• About equal amounts of heat are transferred from the ground to the atmosphere in the Sahara and in the ITCZ regions.– Sahara: Sensible Heat Flux– ITCZ: Latent Heat Flux

• But the difference between these two methods is very important…

Sensible Heat Flux in the Sahara

• Heat transfers from the surface into the atmosphere by conduction, warming only the air very near the surface.

• All of the heat becomes trapped in a very shallow layer.

• This layer becomes very hot.

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Latent Heat Flux in the ITCZ

• Heat transfers from the surface into the atmosphere by evaporation.

• This latent heat is released to the atmosphere at great heights.

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An Important Feedback

Atmospheric Temperature

Gradients Near the Surface

African Easterly Jet (AEJ)

African Easterly Waves (AEW)

Convection in the ITCZ

Region (Squalls,

MCSs, etc.)

Soil Moisture in

the ITCZ Region

Based on this feedback…

• …we should be able to make certain predictions about the AEJ.

Rain Rate (mm/d)

Over the course of the monsoon season, the precipitation moves progressively farther to the north, moistening the soil as it goes.

Therefore, we expect that the AEJ should north between June and August.

Zonal Wind at 700mb along 2.5°E (m/s)

And it does! In fact, notice that the peak winds are directly above the strongest gradient in the precipitation pattern!