West Africa experiences a diverse climate, largely influenced by its expansive geography and atmospheric patterns. The region’s varied latitudes lead to significant variations in temperature and rainfall, from northern desert fringes to the southern coast. This climatic diversity supports a wide array of ecosystems, from arid lands to dense tropical forests.
Influences on West Africa’s Climate
West Africa’s climate is shaped by several geographical and atmospheric factors. Latitude plays a significant role, with areas closer to the equator generally experiencing higher temperatures and more consistent rainfall. Mean annual temperatures across the lowlands are typically above 18°C, with areas within 10° of the equator averaging around 26°C.
The Atlantic Ocean to the south and west provides moisture-laden air masses, which are a primary source of seasonal monsoons. Conversely, the vast Sahara Desert to the north and east introduces hot, dry air. This interaction between the moist oceanic air and the dry continental air creates distinct wet and dry seasons across the region. Topography also contributes to local climate variations.
Distinct Climate Regions
West Africa features several distinct climate zones, transitioning from arid conditions in the north to more humid environments closer to the coast.
Sahelian Zone
Located between 11°N and 16°N, the Sahelian zone is semi-arid. It receives 200 mm to 800 mm of rainfall annually, concentrated in a short rainy season from June through September. Vegetation consists of steppe and short grass savanna, with a dry season lasting up to 10 months.
Sudanian Zone
South of the Sahel, between 8°N and 11°N, is the Sudanian zone, also known as the Savanna. This zone is more humid than the Sahel, experiencing annual rainfall between 750 mm and 1250 mm. It is characterized by open woodland and savannas, with a dry season lasting five to seven months.
Guinean Zone
Further south, along the Atlantic Ocean from 4°N to 8°N, lies the Guinean (or coastal) zone. This sub-humid region receives 1,600 mm to 2,000 mm of rainfall in its tropical areas, and up to 5,000 mm in subequatorial parts. The Guinean zone has wet and dry seasons and supports deciduous or semi-deciduous forests.
Annual Weather Cycles
West Africa’s climate is defined by an annual oscillation between wet and dry periods, largely driven by the movement of the Intertropical Convergence Zone (ITCZ). As the ITCZ shifts north and south with the apparent movement of the sun, it dictates the seasonal progression of rainfall across the region.
During the boreal summer (roughly June to September), the ITCZ moves northward, bringing the rainy season to areas like Ghana. This period sees increased rainfall and lower temperatures, particularly in northern West Africa as moist southwesterly monsoon winds penetrate further inland. As the ITCZ retreats southward during the boreal winter (around January to February), dry conditions prevail, and temperatures can rise, especially in northern areas. This southward shift results in the dry season, with minimal cloud cover and reduced rainfall across much of the region.
Defining Climatic Features
The climate of West Africa is shaped by several distinguishing atmospheric features.
The Harmattan
The Harmattan is a hot, dry, and dusty wind originating from the Sahara Desert that blows across West Africa from November to February. This wind results from the tropical continental air mass, extending its influence as far south as 5° to 7°N in January. The Harmattan brings cooler nights and hot days, often carrying dust that reduces visibility.
West African Monsoon (WAM)
The West African Monsoon (WAM) is a major wind system affecting the region between latitudes 9° and 20°N. It is characterized by southwesterly winds during warmer, wetter months and northeasterly winds during cooler, drier months. The WAM is a component of the ITCZ system, bringing seasonal rainfall as moist air flows inland from the Atlantic Ocean.
Intertropical Convergence Zone (ITCZ)
The Intertropical Convergence Zone (ITCZ) is a low-pressure belt near the equator where trade winds from the northern and southern hemispheres meet. This convergence causes air to rise, leading to high cloudiness, frequent thunderstorms, and heavy rainfall. The ITCZ’s seasonal north-south migration, following the sun’s position, directly influences the timing and intensity of West Africa’s wet and dry seasons.