|Title||The past, present and future of African dust|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Evan AT, Flamant C., Gaetani M., Guichard F.|
|Type of Article||Article|
|Keywords||atmosphere; climate; emission; impact; mineral dust; north-atlantic oscillation; reanalysis project; soil-moisture; tropical atlantic; variability|
African dust emission and transport exhibits variability on diurnal(1) to decadal(2) timescales and is known to influence processes such as Amazon productivity(3), Atlantic climate modes(4), regional atmospheric composition and radiative balance(5) and precipitation in the Sahel(6). To elucidate the role of African dust in the climate system, it is necessary to understand the factors governing its emission and transport. However, African dust is correlated with seemingly disparate atmospheric phenomena, including the El Nino/Southern Oscillation(7,8), the North Atlantic Oscillation(9), the meridional position of the intertropical convergence zone(10,11), Sahelian rainfall(8) and surface temperatures over the Sahara Desert(12), all of which obfuscate the connection between dust and climate. Here we show that the surface wind field responsible for most of the variability in North African dust emission reflects the topography of the Sahara, owing to orographic acceleration of the surface flow. As such, the correlations between dust and various climate phenomena probably arise from the projection of the winds associated with these phenomena onto an orographically controlled pattern of wind variability. A 161-year time series of dust from 1851 to 2011, created by projecting this wind field pattern onto surface winds from a historical reanalysis(13), suggests that the highest concentrations of dust occurred from the 1910s to the 1940s and the 1970s to the 1980s, and that there have been three periods of persistent anomalously low dust concentrations-in the 1860s, 1950s and 2000s. Projections of the wind pattern onto climate models give a statistically significant downward trend in African dust emission and transport as greenhouse gas concentrations increase over the twenty-first century, potentially associated with a slow-down of the tropical circulation. Such a dust feedback, which is not represented in climate models, may be of benefit to human and ecosystem health in West Africa via improved air quality(14) and increased rainfall(6). This feedback may also enhance warming of the tropical North Atlantic(15), which would make the basin more suitable for hurricane formation and growth(16).
This decline in dust over North Africa may result in a slight improvement in air quality in the region, although the effect of regional population growth and urbanization will undoubtedly overshadow the benefits of a reduction in airborne dust. While the radiative forcing of dust may be near zero over North Africa, as shortwave cooling is approximately balanced by the longwave warming, dust transported over the tropical North Atlantic cools the surface via direct and indirect radiative effects. Therefore, a reduction in dust would act as a positive feedback to warming by greenhouse gases in the tropical North Atlantic. Furthermore, since this feedback is not pan-tropical, this additional dust-forced warming could increase hurricane activity by increasing tropical North Atlantic sea surface temperature, relative sea surface temperature (which is the difference between sea surface temperature in the tropical Atlantic and the sea surface temperature averaged over all of the tropics), and the northward meridional sea surface temperature gradient. The radiative and temperature effects from such a reduction in dust are not captured in most CMIP5 simulations; many models do not have interactive dust, and of those models that do, the majority show an increase in simulated African dust concentrations during the twenty-first century. Thus, it is plausible that current temperature projections for the tropical Atlantic through the Caribbean are too conservative.