The Sahel and the ITCZ
Air masses moving over Africa have different characteristics depending on their areas of origin. Air originating over the tropical oceans, known as maritime tropical air, is warm, moist, and unstable. When this moisture-laden air is forced to rise and become cooler, condensation and precipitation often occur. Conversely, the hot, dry air that originates over the African continent (continental tropical air) is incapable of producing rain, even if it is forced to rise.
Over most of Africa, air moves toward the equator. Northern Hemisphere winds from the northeast and Southern Hemisphere winds from the southeast converge at the intertropical convergence zone (ITCZ), a low-pressure zone centred on the equator. Air rises at the ITCZ, spreads out, and descends in the north and south, in subtropical high-pressure zones centred on the Tropic of Cancer and Tropic of Capricorn. After descending, the air flows outward, either pole ward into temperate regions or back toward the equator.
The ITCZ constantly shifts its position, moving north of the equator during the Northern Hemisphere’s summer, and south of the equator during the Northern Hemisphere’s winter (see below diagram). This shift brings about seasonal changes, notably in the tropical summer rainfall zone. During the summer season (July to September in the Northern Hemisphere and December to February in the Southern Hemisphere), moist maritime air is drawn in toward the ITCZ and produces rainfall when it is forced to rise. During the winter season (December to February in the Northern Hemisphere and July to September in the Southern Hemisphere), when the ITCZ is centred in the opposite hemisphere, dry conditions prevail due to the dominance of hot, dry winds coming from the subtropical high-pressure zone. These global, seasonal shifts in wind and pressure zones also bring about the seasonal changes that characterize the Mediterranean climates of Africa’s northern and southern margins.
