MONSOONAL WINDS
Apart from the upper level jets, the surface African monsoonal winds are ultimately caused by the differential heating of land and ocean, which inevitably leads to a pressure gradient. Figure 7 shows the global pressure distributions for the months of January and July. The January pressure gradient field on the African continent, in particular the Saharan desert, shows a higher mean pressure (close to 1020mb) then that in July (1010mb). This is the continental portion of the differential heating for the West African monsoon. The southern Atlantic Ocean over the St. Helena island group (see Figure 6) shows a change from the January mean pressure of 1020mb to an average of 1025mb. This high-pressure system sets up an anti-cyclonic system, which flows counterclockwise, in the southern hemisphere. As the summer months progress, the higher pressure over St. Helena related to the lowering pressure over the Saharan desert. This larger pressure gradient causes a stronger flow originating from the southern hemisphere. In the tropics, the Coriolis acceleration is at a minimum so there is close to a straight-line path from the St. Helena high to the low over the Saharan desert. This flow is not a straight-line path, of course, and as it passes the equator a slight shift to the east causes the flow to enter the lower continent from the west.
Relating these three circulations (AEJ, TEJ and monsoonal flow) is Figure 8, which shows a zonal wind profile that clearly shows the height dependence of these three flows. This profile was part of the West African Monsoon Experiment (WAMEX) and was taken at Kano, Nigeria on August 1979. At the surface, the strong westerly flow from the monsoon winds is overlain by the AEJ at approximately 650mb and the TEJ at 200mb. The TEJ and AEJ are prominent features of the African synoptic scale circulations, but these are not what ultimately drive the monsoonal rains. From the northeast and at higher latitudes, the African northeasterly trade winds named the Harmattan overlays the southwesterly monsoon flow. The Harmattan is a very dry gusty flow that comes from the Sahel and the Saharan desert. It is the meeting of these two flows that determine the monsoonal characteristics of the African continent. This meeting is actually the ITCZ but some literature refers to it as the Inter-Tropical Discontinuity (ITD) due to the fact that it behaves much differently than the ITCZ over water. Figure 9 shows a three-dimensional view of each of these circulation systems for a common summer month. The AEJ is not shown in this figure however, but would be situated at approximately 700mb in the same direction as the TEJ. Included in this three-dimensional view is another zonal flow that remains controversial. The Walker Cell is typically a Pacific Ocean and Indian Ocean phenomenon. Schematically these are longitudinal cells where, on one side of the ocean, convection and the associated release of latent heat in the air above lifts isobaric surfaces upward in the upper troposphere and creates a high pressure region there. The lack or lesser degree of the same process on the other side of the ocean results in lower pressure there, and a longitudinal pressure gradient is established which, being on the equator, cannot be balanced by the Coriolis acceleration. Although this primarily exists in the Pacific Ocean, there is some speculation that a Walker Cell may exist in Africa during large El Nino years and thus may affect the precipitation there.
The West African Monsoon is primarily the fluctuations of the southwesterly wind and the Harmattan at the surface. This fluctuation does have a seasonal frequency, but it also varies considerably with the synoptic pressure patterns. The Harmattan comes in spells that mostly last from a few days to more than a week. During the northern hemisphere's winter months, the landmass of Africa cools significantly and the relatively higher pressure from the colder air drives back the southwesterly monsoon flow. It can be driven as far south as the equator in some locations.
It is the location of the ITD that determines the precipitation patterns of the West African monsoon. The advancing fringe of the southwest monsoon is very shallow (below 1000 meters) and therefore cannot produce the large thunderstorms and disturbances that will produce large-scale precipitation. It is the system behind (200-300 kilometers) the ITD where the moist air is much deeper (1000-2000 meters) that produces the rains. Figure 10 shows the position of the ITD at the surface, 850mb, 700mb and 500mb for the month of July, 1973. The front of the ITD is actually within the Saharan desert, but much further back, in the African Sahel the top of the ITD reaches 850mb, or approximately 1km.
It was observed that the onset of the ITD could be broken down into several zones depending on the weather systems observed in each one (Dhonneur, 1970). In the June of 1978, a global attempt was made to further understand the weather patterns that were observed by Dhonneur and others (WMO/ICSU, 1978). Figure 11 shows a meridional view of the typical monsoon flow along with the 4 weather patterns.
Figure 12 further shows the limits of the ITD throughout the year. The northern limit of the ITD undergoes three distinct movements, differing both in amplitude and duration. There exists the diurnal motion consisting of shifts southward during the morning and northwards during the afternoon with average amplitude of 200 km. The second movement is the annual migration, which approximately follows the displacement of the sun but with a delay of approximately six to eight weeks. The third movement is an intermediate oscillation with amplitude approximately an average seasonal position of several hundred kilometers and duration of several days. This movement is often observed during northern winter months.
Figure 13 shows a monthly progression of the ITD. The ITD is shown as a dark line and the dashed lines show other front locations of the other different trade winds. Beginning in the winter months, the ITD on the western regions of Africa remains across the equator, but due to the fact that the land extends below the equator along the easterly side, this allows the low-pressure system to build up over the southern continent (see Figure 7) and maintain the ITD in the southern hemisphere. Due to the fact the ocean maintains its temperature, the St. Helena high maintains the flow in the northern hemisphere on the West African coast. As the summer season approaches, the entire ITD begins to progress northward until the entire ITD is above the equator in April. Eventually, the ITD progresses to its maximum position in August. Although the sun is at its maximum intensity on July 21st, the ocean has a significant time lag that provides the greatest differential heating two months after the summer solstice. Again, as the winter season approaches, the ITD recedes. It is not apparent in these figures, but the ebb of the ITD is much quicker than the flow.
Figure 14 shows the variances of the thunderstorm activity for both the southwest coastal regions of Africa (coast to 7 degrees north) and the inland continent (10 degrees to 13 degrees north). The first obvious difference between the two regions is what is referred to as the "little dry season." As the ITD progresses northward, zone C (the highest precipitation area), greets the coastal area in the early spring. As the ITD progresses further north, Zone C is eventually replaced by Zone D (an area of erratic rainfall) and the thunderstorm activity is reduced. During August, the furthest extent of the ITD, the thunderstorm activity along the coast reaches its minimum. After this time, the ITD ebbs much quicker then its flow and as Zone C revisits the coastal region, the thunderstorm activity is reduced. This cycle repeats itself. The inland areas, being further north, only see Zone C once throughout the year and this maximum occurs at the ITD's extent in August. Figure 15 shows the similar rainfall pattern for both locations with a time lag for the rainfall peak being in June rather than April for the thunderstorm activity. This is primarily due to the fact that convective development is replaced by thick-layered clouds which also provide rain but diminishes thunderstorm activity due to the lowered insolation from the sun.