How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

Abstract

Abstract. This study presents the first quantitative estimate of the mineral dust emission associated with atmospheric depressions and mobile cyclones in North Africa. Atmospheric depressions are automatically tracked at 925 hPa based on ERA-Interim data from the European Centre for MediumRange Weather Forecasts for [1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003][2004][2005][2006][2007][2008]. A set of filter criteria is applied to identify mobile cyclones, i.e. migrating and long-lived cyclones. The shorter term cyclone is used as a synonym for mobile cyclones. Dust emission is calculated with a dust emission model driven by 10 m winds and soil moisture from ERA-Interim. Emission peaks during winter and spring with spatial averages of 250-380 g m −2 per month. Comparison of the dust source activation frequency from the model against SEVIRI satellite observation shows a good agreement in the Bodélé Depression but differences in the north and west of North Africa. Depressions are abundant, particularly in summer when the Saharan heat low is situated over West Africa and during spring in the lee of the Atlas Mountains. Up to 90 % (55 % annually and spatially averaged) of dust emission occurs within 10 degrees of these depressions, with embedded mechanisms such as nocturnal low-level jets playing a role. Cyclones are rarer and occur primarily north of 20 • N in spring in agreement with previous studies and over summertime West Africa consistent with near-surface signatures of African Easterly Waves. Dust emission within 10 degrees of cyclones peaks over Libya with up to 25 % in spring. Despite the overall small contribution of 4 % annually and spatially averaged, cyclones coincide with particularly intense dust emission events exceeding the climatological mean by a factor of four to eight. Soil moisture weakens dust emission during cyclone passage by about 10 %.

North African dust transport toward the western Mediterranean basin: atmospheric controls on dust source activation and transport pathways during June–July 2013

“…In particular, from winter to early summer and autumn, low-pressure systems are associated with midlatitudinal troughs migrating from west to east and affecting both the Mediterranean Sea and northern Africa (Dayan et al, 1991;Moulin et al, 1997Moulin et al, , 1998. The so-called Mediterranean cyclones favor dust uplift from northern dust source regions and ultimately due to the southerly wind direction dust export toward the Mediterranean basin (Rodriguez et al, 2001;Schepanski et al, 2009bSchepanski et al, , 2011Fiedler et al, 2014;Flaounas et al, 2015). In particular, during late winter to early summer, when the sea surface temperature of the Mediterranean Sea is still cooler, the baroclinic gradient may enhance the strength of the cyclogenesis (Alpert et al, 1990).…”

“…As a high soil moisture content reduces the susceptibility of a soil for wind erosion, dust mobilization is inhibited for soil moisture content > 99 %. This becomes particularly important in regions where dust emission is associated with synoptic situations accompanied by rainfall, such as cyclogenesis and mesoscale convective systems (Fiedler et al, 2014).…”

A process-based evaluation of dust-emitting winds in the CMIP5 simulation of HadGEM2-ES

“…Other external driving data sets, e.g. potential dust sources, are chosen as in previous studies (summarized above, Fiedler et al 2013Fiedler et al , 2014Heinold et al 2013) and are the same for all offline calculations. This approach allows to assign differences in the offline dust emission calculations to the wind speed.…”

“…Hourly output from HadGEM2-ES in combination with the automated detection tool for NLLJs by Fiedler et al (2013) offers the unique opportunity to statistically analyze this dust-emitting process on sub-daily scales over a climatological time period of 30 years for new insights into the model performance. Since a long-term observational data set of dust emission does not exist, the dust emission climatology from the model by Tegen et al (2002) calculated with wind speeds from ERA-Interim as in Fiedler et al (2013Fiedler et al ( , 2014 is used here as a benchmark. Quantifying the relative importance of NLLJs as a dust-emitting process from the models helps identifying systematic model behavior affecting dust emission, providing the basis for further advances in Earth system model development.…”

New developments in the representation of Saharan dust sources in the aerosol–climate model ECHAM6-HAM2

“…From February to May (FMAM), intense Saharan cyclones along the North African coast (Hannachi et al, 2011;Fiedler et al, 2014) cause major dust emissions in the northern Sahara. In the central and southern Sahara and the Sahel, dust events are often related to enhanced northeasterly harmattan winds resulting from intensifications of the subtropical high (Kalu, 1979;Knippertz et al, 2011).…”