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:
Streamlines at 200mb; colors are U winds at 200mb
Determine the heat budget for each of these three regions to determine why there is a temperature gradient in West Africa.
Maybe the Sahara is hotter than the Sahel and the ITCZ simply because:
SW↓ is greatest in the Sahara.
However, the Sahara has a much higher albedo than the Sahel or the ITCZ region.
SW↑ is greatest in the Sahara.
Still, the net shortwave radiation at the surface is greatest in the Sahara.
Most LW↓ comes from:
Therefore, the ITCZ region has the most LW↓, whereas the Sahara gains relatively little.
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.
The ITCZ has very little loss of heat by longwave radiation.
The Sahara loses a great deal of heat by longwave radiation.
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.)
Sensible Heat flux:
Latent Heat flux:
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)
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.
And it does! In fact, notice that the peak winds are directly above the strongest gradient in the precipitation pattern!