The importance of Harmattan surges for the emission of North African dust aerosol

Abstract

Dust aerosol is important in the Earth system, but the relative impact of meteorological mechanisms on North African dust emission remains unclear. This study presents the first climatology of dust emission amounts associated with Harmattan surges (HSs), characterized by postfrontal strengthening of near‐surface winds. A new automated identification uses their strong isallobaric winds as an indicator for HSs in 32 years of ERA‐Interim reanalysis. Their impact on dust aerosol emission is estimated by combining the identified events with derived dust emissions. The estimate highlights that about one third of the total emission mass is associated with HSs. Spring shows the largest associated emissions of 30–50% of the monthly totals consistent with the largest number and duration of HSs. Regional emission contributions of up to 80% in the north coincide with the overall largest emission maxima in spring. The importance of HSs for dust emission implies that aerosol‐climate models need to accurately represent synoptic‐scale storms.

The Role of Jet Adjustment Processes in Subtropical Dust Storms

“…During the northern hemisphere cold season (November to April), the North African climate is influenced by the northeasterly trade winds called the Harmattan, which is the result of meridional pressure gradients at the synoptic scale across the Saharan desert and can be viewed as part of the meridional circulation (Burton et al, ). Fiedler et al () showed that synoptic scale cold air advection over North Africa causes increased intensity and fluctuations in this system, that is, the Harmattan surges. Regionally, the importance of the study of the Harmattan wind system has been increasing since this system advects a significant amount of mineral dust equatorward, causing negative impacts on visibility, agriculture, and human health (e.g., causes meningitis) (Burton et al, ; Kalu, ; Pérez García‐Pando et al, ).…”

Retrieving the global distribution of the threshold of wind erosion from satellite data and implementing it into the Geophysical Fluid Dynamics Laboratory land–atmosphere model (GFDL AM4.0/LM4.0)

“…Dust can serve as ice nuclei and affect the formation, lifetime, and characteristic of clouds (e.g., Levin et al, 1996;Rosenfield et al, 1997;Wurzler et al, 2000;Nakajima et al, 2001;Bangert et al, 2012), perturbing the hydrological cycle. Iron-and phosphorus-enriched dust is also an important nutrient for the marine and terrestrial ecosystems and thus interacts with the ocean and land biogeochemical cycles (e.g., Fung et al, 2000;Jickells et al, 2005;Shao et al, 2011;Bristow et al, 2010;Yu et al, 2015).…”

Large scale upper-level precursors for dust storm formation over North Africa and poleward transport to the Iberian Peninsula. Part I: An observational analysis

“…Northeasterly/easterly low-level trade winds over northern Africa, commonly referred to as Harmattan winds, prevail in the area and therefore normal conditions do not favor the poleward advection to the Mediterranean and Europe. The intensity of these winds can be modulated by cold air outbreaks in Western Europe resulting in stronger dust transport over the Tropical North Atlantic (e.g., Fiedler et al, 2015;Schepanski et al, 2017). As a consequence, intense dust export has a highly episodic nature.…”

“…Moreover, a considerable effort has been made in recent years to identify the dust source areas and the main processes leading to dust entrainment in North Africa (e.g., Knippertz & Todd, 2012, and references therein;Schepanski et al, 2007Schepanski et al, , 2009Fiedler et al, 2014Fiedler et al, , 2015Pokharel et al, 2017). Over the drylands of North Africa, dust is mainly mobilized in deflatable areas by low-level jets, synoptic scale circulations, convective features and downslope winds.…”