Estimate of dust emissions in the intertropical discontinuity region of the West African Monsoon

Dust emission over West Africa is known to be highest during summer in coincidence with the annual northward displacement of the Inter Tropical Discontinuity (ITD) over the Sahelian dust sources. This study aims to identify the meteorological mechanisms at the origin of the high dust activity during the monsoon season. More attention is given to dust emissions in the ITD region when it is located over Niger and Mali around 18°N. The study focuses on the period from 2 to 12 July 2006, in the framework of the African Monsoon Multidisciplinary Analysis (AMMA). Two new mechanisms for dust emissions and transport over the Sahel have been identified. The first one is related to the dynamics of the monsoon nocturnal flow where high turbulent winds at the leading edge of the monsoon flow density-current like lead to dust mobilization. The second one is associated with the formation of vortices in the ITD region in response to the strong shear across the front.

Additionally, a three-dimensional mesoscale numerical simulation has been performed to quantify the dust emissions associated with the strong near-surface winds blowing over the Sahelian dust sources during the period of interest. The daily mean values of dust load related to the strong winds on both side of the ITD, as estimated from the simulation within the model domain (2°W–16°E, 12–28°N), are in excess of 2 Tg on some of the days of the 2–12 July 2006 period. In the present case, the dust load associated with the strong winds south of the ITD is simulated to range between 0.5 and 0.8 Tg on average. This study suggests that emissions driven by strong surface winds occurring on both sides of the ITD while lying across the Sahel may contribute significantly to the total dust load over West and North Africa observed annually.