Introduction
Tropical and subtropical hot deserts (Köppen's BWh climate) are primarily concentrated between 15° and 30° latitudes in both the Northern and Southern Hemispheres, predominantly along the western margins of continents. Major examples include the Sahara, the Atacama, the Namib, the Kalahari, and the Great Australian Desert. Their unique distribution and extreme aridity are governed by an interplay of atmospheric circulation, oceanic currents, and physiographic features.
Primary Factors Responsible for Hot Desert Formation
Hot deserts owe their existence and persistence to several synergistic planetary-scale atmospheric and oceanic mechanisms:
- Subtropical High-Pressure Subsidence: The descending limbs of the atmospheric Hadley cells create persistent subtropical anticyclones around 20°–30° latitudes. As the air subsides, it undergoes adiabatic warming, lowering its relative humidity and stabilizing the atmosphere. This strong anticyclonic stability suppresses vertical convection and cloud formation, as observed in the Sahara and Arabian deserts.
- Offshore Trade Winds: These regions lie within the trade wind belt. The tropical easterlies blow from east to west across continents. As they traverse vast landmasses, they shed moisture along eastern continental margins and become dry, offshore winds by the time they reach the western margins (e.g., Great Australian Desert and the Kalahari).
- Desiccating Effects of Cold Ocean Currents: Cold oceanic currents flowing along western coasts (such as the Peru/Humboldt Current alongside the Atacama, the Benguela Current near the Namib, and the Canaries Current near the Sahara) chill the lower layers of marine air. This creates atmospheric thermal inversions where dense, cold surface air is trapped beneath warm subsiding air, preventing vertical convection and precipitation despite high coastal relative humidity.
- Rain-Shadow Effect: Orographic barriers effectively block moisture-bearing winds from reaching continental leeward basins. For instance, the towering Andes mountains block moist Atlantic air, creating hyper-arid conditions in the Atacama Desert, while the Great Dividing Range shields the central Australian interior from Pacific moisture.
- Continentality: Sheer distance from oceanic moisture sources significantly reduces relative humidity. Moist maritime air masses lose their vapor content long before reaching deep continental interiors, intensifying aridity in regions like the deep Sahara and the Thar Desert.
Conclusion
The distribution of tropical and subtropical hot deserts is thus an outcome of planetary atmospheric subsidence, ocean-atmosphere interactions, and orographic shelter. However, climate change and human activities are accelerating desertification along semi-arid fringes like the Sahel, necessitating coordinated global action under the UNCCD to achieve Land Degradation Neutrality through initiatives such as the Great Green Wall.