The following is paraphrased from a talk Chris Thorncroft (of SUNY Albany) gave at Climatea today. Any mistakes are certain to be based on my misunderstandings.
The jet arises from thermal wind balance between the hot saharan air and the cooler sale atmosphere (The gradient points south, so the Jet is Easterly). But why is the jet at such a low altitude (600hPa?) – other jets are nearer the tropopause.
The answer stems from the unusually dry Saharan air. The dryness is such that the temperature profile is essentially a dry adiabatic – there is essential no latent heat released to counteract the cooling by expansion as air rises above the sahara.
Above the Sahel, the air is moist, so there is plenty of latent heating and the temperature drops less rapidly.
Thermal-Wind Balance requires that the wind is increases with height in proportion to (and perpendicular to) the horizontal temperature gradient. So, the Jet peaks when the temperature gradient vanishes – where the temperature above the Sahel and the Sahara are the same.
For other jets, there is moist convection on both sides of the jet, so the temperatures equalize much higher than over West Africa.