Modulation of Saharan dust export by the North African dipole
Abstract. Desert dust aerosols influence air quality and climate on a global scale, including radiative forcing, cloud properties and carbon dioxide modulation through ocean fertilisation. North Africa is the largest and most active dust source worldwide; however, the mechanisms modulating year-to-year variability in Saharan dust export in summer remains unclear. In this season, enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. The objective of this study is to identify the relationship between the long term interannual variability in Saharan dust export in summer and large scale meteorology in western North Africa. We address this issue by analysing ~25 yr (1987–2012) dust concentrations at the high altitude Izaña observatory (2373 m a.s.l.) in Tenerife Island, satellite and meteorological reanalysis data. Because in summer Saharan dust export occurs at altitudes 1–5 km, we paid special attention to the summer meteorological scenario in the 700 hPa standard level, characterised by a high over the subtropical Sahara and lower geopotential heights over the tropics; we measured the intensity of this low-high dipole like pattern in terms of the North AFrican Dipole Index (NAFDI): the difference of the 700 hPa geopotential heights anomalies averaged over central Morocco (subtropic) and over Bamako region (tropic). The correlations we found between the 1987–2012 NAFDI with dust at Izaña, satellite dust observations and meteorological re-analysis data, indicates that increase in the NAFDI (i) results in higher wind speeds at the north of the Inter-Tropical Convergence Zone which enhances dust export over the subtropical North Atlantic, (ii) influences on the size distribution of exported dust particles, increasing the load of coarse dust and (iii) are associated with higher rainfall over tropical North Africa and the Sahel. Because of the North African dipole modulation, inter-annual variability in Saharan dust export is correlated with monsoon rainfall in the Sahel. High values of the NAFDI enhance dust export at subtropical latitudes. Our results suggest that long term variability in Saharan dust export may be influenced by global oscillations in the climate of the tropics and subtropics and that this may have influenced dust transport pathways in the last decades.