Mineral dust is the most important natural aerosol type by mass, with northern Africa the most prominent source region worldwide. Dust particles are lifted into the atmosphere by strong winds over arid or semiarid soils through a range of emission mechanisms, the most important of which is saltation. Dust particles are mixed vertically by turbulent eddies in the desert boundary layer (up to 6km) or even higher by convective and frontal circulations. The meteorological systems that generate winds strong enough for dust mobilization cover scales from dust devils (~100m) to large dust outbreaks related to low- and high-pressure systems over subtropical northern Africa (thousands of kilometers) and include prominent atmospheric features such as the morning breakdown of low-level jets forming in the stable nighttime boundary layer and cold pools emanating from deep convective systems (so-called haboobs). Dust particles are transported in considerable amounts from northern Africa to remote regions such as the Americas and Europe. The removal of dust particles from the atmosphere occurs through gravitational settling, molecular and turbulent diffusion (dry deposition), as well as in-cloud and sub-cloud scavenging (wet deposition). Advances in satellite technology and numerical dust models (including operational weather prediction systems) have led to considerable progress in quantifying the temporal and spatial variability of dust from Africa, but large uncertainties remain for practically all stages of the dust cycle. The annual cycle of dustiness is dominated by the seasonal shift of rains associated with the West African monsoon and the Mediterranean storm track. In summer, maximum dust loadings are observed over Mauritania and Mali, and the main export is directed toward the Caribbean Sea, creating the so-called elevated Saharan Air Layer. In winter the northeasterly harmattan winds transport dust to the tropical Atlantic and across to southern America, usually in a shallower layer. Mineral dust has a multitude of impacts on climate and weather systems but also on humans (air pollution, visibility, erosion). Nutrients contained in dust fertilize marine and terrestrial ecosystems and therefore impact the global carbon cycle. Dust affects the energy budget directly through interactions with short- and long-wave radiation, with details depending crucially on particle size, shape, and chemical composition. Mineral dust particles are the most important ice-nuclei worldwide and can also serve as condensation nuclei in liquid clouds, but details are not well understood. The resulting modifications to cloud characteristics and precipitation can again affect the energy (and water) budget. Complicated responses and feedbacks on atmospheric dynamics are known, including impacts on regional-scale circulations, sea-surface temperatures, surface fluxes and boundary layer mixing, vertical stability, near-surface winds, soil moisture, and vegetation (and therefore again dust emission). A prominent example of such complex interactions is the anti-correlation between African dust and Atlantic hurricane activity from weekly to decadal timescales, the causes of which remain difficult to disentangle. Particularly in the early 21st century, research on African dust intensified substantially and became more interdisciplinary, leading to some significant advances in our understanding of this fascinating and multifaceted element of the Earth system.
Sharon E. Nicholson
Classic paradigms describing meteorological phenomena and climate have changed dramatically over the last half-century. This is particularly true for the continent of Africa. Our understanding of its climate is today very different from that which prevailed as recently as the 1960s or 1970s. This article traces the development of relevant paradigms in five broad areas: climate and climate classification, tropical atmospheric circulation, tropical rain-bearing systems, climatic variability and change, and land surface processes and climate. One example is the definition of climate. Originally viewed as simple statistical averages, it is now recognized as an environmental variable with global linkages, multiple timescales of variability, and strong controls via earth surface processes. As a result of numerous field experiments, our understanding of tropical rainfall has morphed from the belief in the domination by local thunderstorms to recognition of vast systems on regional to global scales. Our understanding of the interrelationships with land surface processes has also changed markedly. The simple Charney hypothesis concerning albedo change and the related concept of desertification have given way to a broader view of land–atmosphere interaction. In summary, there has been a major evolution in the way we understand climate, climatic variability, tropical rainfall regimes and rain-bearing systems, and potential human impacts on African climate. Each of these areas has evolved in complexity and understanding, a result of an explosive growth in research and the availability of such investigative tools as satellites, computers, and numerical models.