Abstract
Abstract. Dust from Africa strongly perturbs the radiative balance over the Atlantic, with emissions that are highly variable from year to year. We show that the aerosol optical depth (AOD) of dust over the mid-Atlantic observed by the AVHRR satellite has decreased by approximately 10% per decade from 1982 to 2008. This downward trend persists through both winter and summer close to source and is also observed in dust surface concentration measurements downwind in Barbados during summer. The GEOS-Chem model, driven with MERRA re-analysis meteorology and using a new dust source activation scheme, reproduces the observed trend and is used to quantify the factors contributing to this trend and the observed variability from 1982 to 2008. We find that changes in dustiness over the east mid-Atlantic are almost entirely mediated by a reduction in surface winds over dust source regions in Africa and are not directly linked with changes in land use or vegetation cover. The global mean all-sky direct radiative effect (DRE) of African dust is −0.18 Wm−2 at top of atmosphere, accounting for 46% of the global dust total, with a regional DRE of −7.4 ± 1.5 Wm−2 at the surface of the mid-Atlantic, varying by over 6.0 Wm−2 from year to year, with a trend of +1.3 Wm−2 per decade. These large interannual changes and the downward trend highlight the importance of climate feedbacks on natural aerosol abundance. Our analysis of the CMIP5 models suggests that the decreases in the indirect anthropogenic aerosol forcing over the North Atlantic in recent decades may be responsible for the observed climate response in African dust, indicating a potential amplification of anthropogenic aerosol radiative impacts in the Atlantic via natural mineral dust aerosol.
Highlights
Mineral dust aerosol is ubiquitous in the atmosphere and arguably the greatest source of particulate matter
While we do find a positive correlation between the North Atlantic Oscillation (NAO) index and the DAOD in the outflow regions during winter, we find no significant trend in the NAO index between 1982 and 2008, the question remains as to what is driving the stilling of winds over Africa
This research and previous studies have found that satellite observations across the Atlantic show a significant downward trend in DAOD since the 1980s, persisting through both summer and winter seasons
Summary
Mineral dust aerosol is ubiquitous in the atmosphere and arguably the greatest source of particulate matter. Since the 1950s, dust emissions from Africa have increased (Evan and Mukhopadhyay, 2010; Mbourou et al, 1997; Prospero et al, 2002), peaking in the 1980s at the same time as the extreme droughts experienced in the Sahel region. During this period a robust correlation was observed between dust transported to Barbados in the summer and Sudano–Sahel Precipitation Index of the previous year (Prospero and Lamb, 2003). We discuss a potential driver of changes in surface winds using three reanalysis data sets and 15 CMIP5 model simulations
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