Dry intrusions in the rear of Mediterranean cyclones govern large-scale dust storms in North Africa

Large dust storms in the Saharan desert and the subsequent transport of airborne dust over large distances are a major meteorological hazard. Several mechanisms associated to dust emission, occurring on a range of scales, have been previously documented, notably involving Rossby wave breaking and a low-level jet. However, the mechanistic link between the different features and actual dust concentrations has not been coherently established. Here, using a Lagrangian approach, and the conceptual view of extratropical cyclone airstreams, the role of the dry intrusion (DI) airstream for translating the influence of the upper-tropospheric Rossby wave perturbation to near-surface flow conductive for the highest dust concentrations is examined. To this end, illustrative cases and a climatological set of 325 large-scale dust storms accompanied by dry intrusions in west Africa during 2003-2018 are studied. Data from the Copernicus Atmospheric Monitoring Service (CAMS) are combined with atmospheric data from reanalysis and objectively-identified Lagrangian trajectories of dry intrusions. We find that dry intrusions link Rossby wave breaking in the east Atlantic with the lower-tropospheric dry and cold jets. These conditions favor dust uplift and transport along an arc-shaped cold front trailing from a Mediterranean cyclone. Consequently, the southwest side of the front is characterized by the highest near-surface dust concentrations ahead of the dry intrusion outflow. The northeast part of the front is, however, accompanied by southerly, warm conveyor belt-like flow transporting the dust northward to the Mediterranean, Middle East and/or Europe at mid and upper-tropospheric levels. Climatologically, such dust-DI events occur mostly in late winter and spring, when they are also larger in size and last longer, compared to summer events. When occurring with DIs, dust optical depth is generally higher, compared to events that are not accompanied by DIs. Focusing further on March events, we find coherent large-scale precursors of dust-DI events: a northward jet shift over the North Atlantic with anticyclonic Rossby wave breaking occurring on average 4-5 days prior to the events.