The African Easterly Jet (AEJ) is a key component of the West African monsoon system, which is intimately connected to the thermodynamic structure and meridional circulations over the continent. Analyses of the AEJ are presented here which are based on meridional transects of high resolution dropsonde observations made during JET2000; an aircraft campaign conducted in the last week of August 2000. A number of dynamical and thermodynamic diagnostics are presented, and used to explore and develop theoretical ideas about the AEJ maintenance and evolution.

The observations have confirmed that the AEJ is closely defined by geostrophic balance. In terms of bulk determinants of the jet location, the baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core. Moreover, the location and morphology of the jet core correspond closely to a locally-defined geostrophic wind measure, to within a degree of latitude and 50 hPa in pressure. The potential vorticity has also been computed, and its structure has been found to accord with theoretical expectations, with distinctive positive and negative PV anomalies equatorward and poleward of the jet core respectively

The thermodynamic structure of the AEJ environment can be categorised into coherent layers. The monsoon layer is a humid zone connected to the land surface, extending northwards into the Sahel and increasing in depth towards the south. This layer is affected by the land surface on diurnal timescales, through the growing convective mixed layer and through shallow cumulus clouds. Above the convective mixed layer, the monsoon layer profile is generally close to pseudoadiabatic. Above the monsoon layer is the Saharan Air Layer (SAL), which can be identified as a layer of low static stability and low potential vorticity. The SAL is deep where it merges with the Saharan boundary layer in the north, and becomes thinner toward the south, terminating in this observational case around 8-12N. It has been shown that the boundaries of the SAL can be approximated to good accuracy as adiabatic surfaces, meaning that the SAL comprises air which is adiabatically connected to the land surface via the Saharan boundary layer. Finally, the troposphere above the SAL is again almost pseudoadiabatic, with small baroclinicity which determines the closure of the AEJ core aloft. To the south of the AEJ, where the SAL is absent (or indistinguishable), the full profile is close to pseudoadiabatic, consistent with an environment characterised by frequent deep convection.

Evidence of transport and exchange between the layers will be presented. Through inspection of thermodynamic tracers, the SAL is identified as a layer in which lateral air motion can exchange dry air from the Sahara with humid air from the ITCZ, and therefore can be an effective route for tropospheric transport. Deep convection is shown to inject air of low equivalent potential temperature into the monsoon layer, and may detrain high humidity air at the top of the SAL. Shallow convection entrains dry air from the SAL into the monsoon layer, and some plumes may penetrate the SAL to detrain at its cap. The upper region of the SAL is identified as a layer of high relative humidity where altocumulus and stratocumulus layers are observed.