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The present study investigates aerosols distributions and a strong Sahara dust-storm event that occurred by early August 2018, in the South of Morocco. We used columnar aerosol optical depth (AOD), Angstrom Exponent (AE) and volume size distributions (VSD) as derived from ground-based observations by 2 AERONET (AErosol RObotic NETwork) sun-photometers at Saada (31.63°N, 8.16°W) and Ouarzazate (30.93°N, 6.91°W) sites, over the periods 2004–2019 and 2012–2015, respectively. The monthly seasonal distributions of AOD, AE, and VSD showed a seasonal trend dominated by the annual cycle, with a maximum aerosol load during summer (July–August) and a minimum in winter (December–January), characterized by a coarse mode near the radius of 2.59 μm and a fine mode at the radius of 0.16 μm, respectively. Indeed, this study showed that aerosol populations in southern Morocco are dominated by Saharan desert dust, especially during the summer season. The latter can sometimes be subject of dust-storm events. The case study presented in this paper reports on one of these events, which happened in early August 2018. The HYSPLIT (HYbrid Single Particle Lagrangian Integrated Trajectory) model was used to simulate air-mass back-trajectories during the event. In agreement with ground-based (AERONET sun-photometers) and satellite (CALIOP, MODIS and AIRS) observations, HYSPLIT back-trajectories showed that the dust air-mass at the 4-km layer, the average height of the dust plume, has crossed southern Morocco over the Saada site, with a westward direction towards the Atlantic Ocean, before it changed northward up to the Portuguese coasts.

K-means cluster analysis of wintertime 500-hPa geopotential height anomalies allowed identifying seven weather regimes (WRs) describing the atmospheric variability over the Euro-Mediterranean domain. The study of transitions between those WRs provided consistent results with the westward displacement of the blocking nearby northern Europe before the onset of the negative phase of the North Atlantic Oscillation (NAO−). The onset of the latter is, indeed, preceded by the North Atlantic blocking regime (NABl). In addition, we detected a preferred transition from the Scandinavian Blocking (ScBl) to NAO+ through the European Ridge regime (EuRG), which is modulated by active phases of the Madden-Julian Oscillation (MJO). The examination of the relationship between WRs and precipitation over Morocco showed that the NAO− (NAO+) regime is accompanied by more (less) rainy episodes. The investigation of the lagged relationships between the MJO and the WRs depicted the role of an active MJO in Phase 2 as a precursor of the ScBl and of an active MJO in Phase 6 as a precursor of the NABl. The exploration of the 10-15 days lagged impact of the MJO on Moroccan rainfall showed an increase (decrease) of wet (dry) conditions 10 to 15 days after the occurrence of an active MJO in Phases 6 and 8 (Phases 2-3-4). The MJO modulation of the WRs and rainfall patterns over Morocco constitutes an important source of predictability at the medium-and the extended-range (subseasonal) time scales, with potential use by decision makers in key socioeconomic sectors in the region.

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