Dry air intrusions link Rossby wave breaking to large-scale dust storms in North Africa

Large-scale dust storms over North Africa transport mineral dust over thousands of kilometers equatorward and into the Mediterranean, thereby affecting human health and infrastructures. Dry Intrusions (DIs) are synoptic-scale descending airstreams from the midlatitude upper troposphere towards the surface. DIs occur behind midlatitude troughs and cyclones, and were shown to induce potential instability and enhance surface 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 understand whether DIs are a common element that can link Rossby wave breaking, a known precursor of large emission events, to the high surface dust concentrations in Lagrangian sense, and to further understand the role of DIs in dust emission. By focusing on selected events and compiling a climatology for the years 2003-2018 we specifically aim to quantify the link between the co-occurrence of DIs and dust events, and identify common precursors. Using the Copernicus Atmosphere Monitoring Service (CAMS) reanalyzed dust optical depth (DOD), vertical dust mixing ratios, atmospheric fields from ERA-Interim reanalysis and a Lagrangian-based detection of DIs, we identify DI-dust events by applying a systematic matching algorithm.

We find that DI-dust events typically peak in winter to spring, and are associated with the maximal dust concentrations in the region. Multiple Rossby wave breakings in the eastern North Atlantic is a common precursor to DI-dust events. The DI airstream is found to connect the upper-tropospheric ridge/trough to the highest surface dust concentrations. Typically, a Mediterranean cyclone further steers the dust over North Africa and northward into the Mediterranean and Europe/Middle East. Vertical profiles of dust mixing ratios show that dust can reach the upper troposphere in the vicinity of the cyclone, attesting to long-range dust transport into the Mediterranean Sea and Europe.

Overall, our detailed case studies and climatological results emphasize the central role of DIs in producing large-scale dust storms. The distinct regional and seasonal frequency of DI-dust occurrence and their coherent precursor signals over the North Atlantic provide valuable information for understanding the predictability of such hazardous events.