During the boreal summer, the African continent heats up rapidly and becomes warmer than the waters of the equatorial eastern Atlantic in the Gulf of Guinea. This thermal contrast between two surfaces with very different inertias induces a response from the atmosphere on a large scale. A regional-scale “sea breeze” forms, allowing the penetration of a cool, humid south-westerly oceanic air flow called the monsoon flow over the African continent. The latter, going up towards the northeast, encounters in the north of the Sahel a flow of air that is on the contrary hot and dry, coming from the Sahara, as schematically shown in the figure opposite. Monsoon rains occur on the continent in the (grey) zone of interaction between these two air masses.

In spring, the strengthening of the south-easterly trade winds coming from the southern hemisphere and associated with the St Helena anticyclone, favors by vertical mixing and horizontal transport the formation of a cold tongue in the Gulf of Guinea (ocean area in blue), near the equator. Under the effect of the ocean-continent thermal contrast, the southeast trade winds can cross the equator (blue arrows), and become a southwest wind under the action of the earth’s rotation. They enter the continent where they then feed the monsoon flow in the lower layers of the atmosphere (1-2 km). The continental warming is maximum over the Sahara, where an immense dome of warm air is installed, that can reach a thickness of 4-5 km, and called Saharan Thermal Depression (HL thereafter for Heat Low). This surface depression allows, at the level of the Inter Tropical Front (ITF around 20°N), the convergence between the cool and humid monsoon flow and the trade winds from the north, hot and dry (Harmattan – red arrows), coming from the Mediterranean Sea and North Atlantic.

The strong thermal contrast between these two air masses in the Sahelian band is responsible for the formation of the African Easterly Jet (AEJ) at an altitude of 4 km. This is an average pattern, but in reality, this jet cannot remain straight and constant. Weakly unstable and under the effect of external forcings such as convection, it oscillates around its mean position while propagating westward at speeds of the order of 800 km/day thus forming the African Easterly Waves (AEWs thereafter). With a wavelength of 2000 to 3000 km, their passage is associated with time oscillations of the order of 4-5 days.

The diagram opposite recalls the average meridian structure of the monsoon characterized by a strong barocliny in the lower layers decreasing with the altitude consistent with the north-south variations of the convection: the humid convection is predominant in the south of the AEJ, and the dry convection in the north. Deepest moist convection occurs just south of the AEJ peak, and dry convection in the Saharan air layer (SAL, in yellow) occurs north of the AEJ. They are separated by an important transition zone, in which the monsoon flow slips under the Saharan air. The relatively warm Saharan air overhanging the monsoon flow creates significant convective inhibition (CIN), and leads to an increase of the convective available potential energy (CAPE) in the boundary layer. Added to this is the strong AEJ wind shear and the fact that the Saharan air layer is very dry, elements that favor the formation of intense convective subsidences. The region located just north of the AEJ is therefore very favorable to the development of tropical squall lines, although rarer.

The Hovmöller diagram in the Figure opposite documents the meridian migration of rainfall over West Africa during the year. The following elements can be highlighted :

• the rather slow penetration of rains on the continent before August, contrasting with a much more rapid withdrawal
• in the Guinean coast region, a first rainy season in May/June wetter than the second one;
• a monsoon shift (onset) around June 24 followed by the short dry season in the Guinean regions.

The figure opposite illustrates the intra-seasonal variability of the monsoon in terms of rainfall for a particular year 2009 corresponding to the occurrence of an extreme event in Ouagadougou on September 1st. In addition to high daily variability characteristic of the Sahel, the wavelet analysis highlights 4 bands of variability at the synoptic (3–5 days) and short (10-15 days), medium (20-30 days), and long (40-90 days) intra-seasonal scales.

The break at the end of July 2009 corresponds to low variability at scales smaller than 25 days, while the extreme event in Ouagadougou coincides with high variability at all scales, a condition favorable to the occurrence of extremes. The comparison with climatology shows that the delay in the onset of the monsoon in 2009 corresponds to low intra-seasonal variability up to the 60-day scale, while at the end of the very rainy season variability is high at all scales.

This variability at different spatio-temporal scales strongly modulates the breaks and active phases of the monsoon and is associated with equatorial and mid-latitude waves with a more predictable character. It is therefore essential for forecasters to monitor and predict these modes of intra-seasonal variability that increase the predictability of the African monsoon system. These variabilities are documented elsewhere on this MISVA site.