The Sahel is a vast geographic and ecological transition zone in Africa, stretching as a narrow band from the Atlantic Ocean eastward to the Red Sea. This semi-arid strip, whose name means “the shore” in Arabic, borders the hyper-arid Sahara Desert to the north and the more humid Sudanian Savanna to the south. Spanning over 5,400 kilometers, the region includes parts of Senegal, Mauritania, Mali, Burkina Faso, Niger, Chad, Sudan, and Eritrea. The Sahelian climate is defined by extreme heat and high variability in precipitation, making it one of the most climatically sensitive regions on Earth.
Fundamental Characteristics of the Sahelian Climate
The Sahelian climate is classified as a hot semi-arid or steppe climate (BSh in the Köppen system). This designation reflects the region’s aridity, where precipitation is low but not scarce enough to qualify as a true desert. Annual rainfall generally ranges from 200 millimeters in the north to around 800 millimeters in the south, showing a sharp north-to-south gradient.
Thermal conditions are consistently high, with monthly mean maximum temperatures often reaching between 33°C and 36°C. These high temperatures lead to high rates of potential evapotranspiration, meaning the amount of water that could evaporate is far greater than the water actually received. This imbalance between moisture supply and atmospheric demand is a primary driver of the region’s ecological stress.
A defining feature of the Sahelian climate is the profound variability in precipitation, both year-to-year and across decades. This extreme fluctuation makes the Sahel a global hotspot for climate variability, exceeding that of many other regions worldwide. The low and unpredictable nature of the rainfall dictates the feasibility of rain-fed agriculture and pastoralism, which are the primary livelihoods for millions of people.
The Mechanism: The Intertropical Convergence Zone and Atmospheric Drivers
The primary force controlling the Sahel’s climate is the annual movement of the Intertropical Convergence Zone (ITCZ). The ITCZ is a belt of low pressure near the equator characterized by the convergence of air masses. Specifically, it draws in moist southwesterly flow from the Atlantic Ocean and dry northeasterly flow from the Sahara, bringing the vast majority of the region’s annual rainfall. The latitudinal position and strength of this convergence zone determine the timing, intensity, and duration of the short wet season in the Sahel.
The Sahel’s rainfall is also strongly influenced by sea surface temperatures (SSTs) in distant oceans. The thermal gradient between the northern and southern Atlantic Ocean, for instance, modulates the ITCZ’s position. Warmer than average SSTs in the tropical Atlantic can shift the ITCZ southward, causing reduced rainfall and drought. Conversely, a warmer northern Atlantic relative to the south has historically been linked to increased rainfall.
The Annual Cycle: Seasons and Associated Weather Phenomena
The climate year in the Sahel is sharply divided into three distinct phases, defined by the position of the ITCZ and the dominant wind system.
Cool Dry Season (December to February)
This season occurs when the ITCZ is at its southernmost position near the Gulf of Guinea. The region is dominated by the Harmattan, a dry, dusty northeasterly trade wind originating over the Sahara Desert. Harmattan winds bring extremely low humidity and carry vast plumes of fine sand and dust, creating a persistent atmospheric haze. Temperatures fluctuate dramatically, with cool nights and daytime highs sometimes rising to 35°C. This season is characterized by desiccating conditions that dry out the landscape and increase the risk of fire.
Hot Dry Season (March to May)
The hot dry season follows as the sun moves northward and the ITCZ begins migrating toward the Sahel. This is often the hottest period of the year, with surface temperatures peaking before the arrival of the rains. The intense solar heating of the land creates a thermal low-pressure area over the Sahara, which is a key component in drawing the moist air northward.
Wet Season (June to September)
The wet season, or monsoon season, runs when the ITCZ has moved sufficiently north to bring moisture-laden air via the West African Monsoon (WAM) flow. Most annual precipitation falls during this short window. Rainfall is often delivered by intense Mesoscale Convective Systems (MCSs), which are large complexes of thunderstorms known as squall lines. The rapid, high-intensity nature of this rainfall makes the Sahelian ecosystem vulnerable to runoff and soil erosion.
Climate Variability, Drought, and Desertification
The Sahel’s history is marked by severe, multi-year drought cycles, highlighting the region’s inherent climate variability. The most documented event was the prolonged drought throughout the 1970s and 1980s. This event was one of the most geographically extensive and persistent droughts recorded in the 20th century, resulting in annual rainfall reductions of more than 30% compared to the 1950s, causing widespread famine and ecological damage.
Desertification describes the environmental degradation driven by climate stress and human activity. Prolonged periods of below-average rainfall, combined with land-use pressures, lead to the loss of vegetation and soil fertility, expanding desert-like conditions. While local land-use practices contribute, scientific consensus suggests that large-scale oceanic forcing, particularly changes in Atlantic sea surface temperatures, was the dominant cause of the major historical droughts.
Since the severe dry period, the Sahel has experienced a partial recovery in rainfall beginning in the 1990s, but precipitation remains highly variable. Climate models consistently project a significant increase in surface air temperatures across the Sahel. Warming is expected to be 150% higher than the global average, potentially reaching 3°C to 4°C by the end of the century. Although future rainfall projections are less certain, an increase in the frequency of extreme rainfall events is expected, posing challenges for water management and flood prevention.