Kinematic structure of the wafr monsoon
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Kinematic Structure of the WAFR Monsoon. ATS 553. 600mb NCEP Climatology Zonal Winds. Cross section of zonal winds along 0°E. Atlantic. Sahara. Guinea. Soudanian. Sahelian. Atlantic. Sahara. Guinea. Soudanian. Sahelian. Cross section of zonal winds along 0°E. Westerlies at 200mb:

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Cross section of zonal winds along 0 e
Cross section of zonal winds along 0°E

Atlantic

Sahara

Guinea

Soudanian

Sahelian


Cross section of zonal winds along 0 e1

Atlantic

Sahara

Guinea

Soudanian

Sahelian

Cross section of zonal winds along 0°E

  • Westerlies at 200mb:

  • A weak subtropical jet?


Cross section of zonal winds along 0 e2

Atlantic

Sahara

Guinea

Soudanian

Sahelian

Cross section of zonal winds along 0°E

  • Easterlies at 200mb:

  • Near the coastline

  • TROPICAL EASTERLY JET (TEJ)


Tej equatorward flank of upper level high pressure systems above heat lows
TEJ=Equatorward Flank of Upper Level High Pressure Systems (Above Heat Lows)

H

H

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


Heat lows
Heat Lows (Above Heat Lows)

500mb

700mb

850mb

1000mb

L


Cross section of zonal winds along 0 e3

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Cross section of zonal winds along 0°E

  • Easterlies at 600mb:

  • Centered near 12°N

  • AFRICAN EASTERLY JET (AEJ)


Cross section of zonal winds along 0 e4

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Cross section of zonal winds along 0°E

  • Low level westerlies:

  • Extend to about 20°N

  • THE MONSOON WESTERLIES


Cross section of zonal winds along 0 e5

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Cross section of zonal winds along 0°E

  • Low level easterlies:

  • Centered near 27°N

  • HARMATTAN


Why is there an african easterly jet
Why is there (Above Heat Lows)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 jet1
Why is there (Above Heat Lows)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

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Thermal Wind Relationship


Thermal wind relationship1

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Thermal Wind Relationship

400mb

500mb

400mb

500mb


Thermal wind relationship2

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Thermal Wind Relationship

400mb

400mb

500mb

500mb

400mb

400mb

500mb

500mb


Thermal wind relationship3

Atlantic (Above Heat Lows)

Sahara

Guinea

Soudanian

Sahelian

Thermal Wind Relationship

400mb

400mb

400mb

500mb

500mb

500mb

Pressure

Gradient

Force

400mb

400mb

400mb

500mb

500mb

500mb



Kinematic structure of the wafr monsoon

Coriolis existence of the AEJ!

AEJ

PGF


Kinematic structure of the wafr monsoon


Define 3 regions
Define 3 Regions Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

  • 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
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget1
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

SW↑:

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

SW↑ is greatest in the Sahara.


Heat budget2
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

Net SW:

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


Heat budget3
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget4
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget5
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget6
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget7
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget8
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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 budget9
Heat Budget Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

  • 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

Atlantic Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

Sahara

Guinea

Soudanian

Sahelian

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.


Latent heat flux in the itcz

Atlantic Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

Sahara

Guinea

Soudanian

Sahelian

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.


An important feedback
An Important Feedback Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

Soil Moisture in the ITCZ Region

Atmospheric Temperature Gradients Near the Surface

Convection in the ITCZ Region (Squalls, MCSs, etc.)

African Easterly Jet (AEJ)

African Easterly Waves (AEW)


Based on this feedback
Based on this feedback… Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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


Rain rate mm d
Rain Rate (mm/d) Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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
Zonal Wind at 700mb along 2.5°E (m/s) Cook (1999) to explain the feedback between the temperature gradient and the AEJ.

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