Long-term (1980-2018) spatial and temporal variability of the atmospheric dust load and deposition fluxes along the North-African coast of the Mediterranean Sea

The Sahara is a major source of mineral particles for the Mediterranean Sea. The air-suspended particles alter the radiative transfer of solar and terrestrial radiation and the input of nutrients resulting from the deposition of mineral dust into the surface waters is thought to be essential for the development of marine ecosystems. In order to document the spatial and temporal variabilities of the dust atmospheric content and deposition flux close to the North-African coast where the impacts are expected to be the largest but where experimental stations are cruelly lacking, we use the MERRA-2 monthly reanalysis data of the 1980-2018 period. We have extracted the dust optical depth (AOD(dust)), surface concentrations (C-dust), precipitations (Prcp), and deposition fluxes (F-dry, F-wet, and F-tot) of 14 areas (7 coastal and 7 offshore) regularly spaced from the Straights of Gibraltar in the west to Egypt and Cyprus in the east. The principal Component Analysis (PCA) shows that 86% of the variability of AOD(dust) can be explained by two factors only: F1 whose influence is dominant in the Central Mediterranean and peaks in spring, and F2 whose importance decreases from Morocco to Egypt and peaks in summer. The variability of the surface concentration is more complex than that of the optical depth. AOD(dust) and C-dust are maximal downwind of the main Algerian and Libyan dust sources, which is to say on and off the coast of the Central Mediterranean (south-Tunisia, Libya, Malta...). In spite of the climate change already underway, no long-term trend of C-dust or AOD(dust) can yet be evidenced for any of the 14 areas between 1980 and 2018. Along the coast of the eastern basin, the low precipitations (from 6.1 to 10.5 mm month(-1) on average) explain that dry deposition accounts for a significant (45-53%) proportion of the total deposition. Everywhere else, wet deposition dominates and particularly in the marine areas where it represents about 90% of the total. From 1980 to 2018, the monthly averaged F-tot did not increase significantly. It varies from 0.24 (eastern Egypt) to 0.63 gm(-2) month(-1) (south-Tunisia) along the North-African coast, and from 0.26 (Baleares) to 0.48 gm(-2) month(-1) (Malta) in the marine areas. Because of the spatial variability of the precipitations, the largest deposition fluxes do not necessarily coincide with the regions of largest atmospheric dust content.