A 16-year climatology of the link between dry air intrusions and large-scale dust storms in Northwest Africa

Abstract:

Large-scale dust storms over Northwest Africa transport dust over thousands of kilometers equatorward and into the Mediterranean, thereby affecting human health and the natural environment. Dry Intrusions (DIs) are synoptic-scale descending airstreams from the midlatitude upper troposphere towards the surface at lower latitudes. DIs occur behind midlatitude troughs and cyclones, and were shown to induce potential instability and enhanced wind in the planetary boundary layer. Thus, DIs can potentially play a major role in the emission and transport of dust over North Africa. Here, we aim to identify the climatological association between DIs and large dust storms, to (i) enhance process understanding of the driving mechanisms and predictability of the dust events, and (ii) provide a climatological quantification of their seasonal frequency, and spatio-temporal variability. Using the Copernicus Atmosphere Monitoring Service (CAMS) reanalyzed dust optical depth (DOD), atmospheric fields from ERA-Interim reanalysis and a Lagrangian-based detection of DIs, we identify dust events with DIs by applying a systematic matching algorithm. We present climatological results for 2003-2018 during the winter to spring transition, the season when the co-occurrence peaks. We find that during DI-related dust storms, the jet stream is shifted poleward, together with isentropic PV anomalies indicative of anticyclonic Rossby wave breaking. The extended upper-tropospheric negative PV anomaly over West Africa occurs together with surface drying, cooling and enhanced southwestward winds, thus favoring a long-range dust transport equatorward. Furthermore, the Intertropical discontinuity (ITD) is positioned further equatorward during the dust events. Overall, our climatological results emphasize the central role of DIs in producing large-scale dust storms. Moreover, specific regions and the seasonal cycle of DI-related dust storms are highlighted, providing valuable information for the events accurate prediction, since those midlatitude systems typically hold a certain degree of predictability (days or more).