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

摘要

This study presents the first quantitative estimate of the mineral dustemission associated with atmospheric depressions and mobile cyclones in NorthAfrica. Atmospheric depressions are automatically tracked at 925 hPa based onERA-Interim data from the European Centre for Medium-Range Weather Forecastsfor 1989–2008. A set of filter criteria is applied to identify mobilecyclones, i.e. migrating and long-lived cyclones. The shorter term cyclone isused as a synonym for mobile cyclones. Dust emission is calculated with adust emission model driven by 10 m winds and soil moisture from ERA-Interim.Emission peaks during winter and spring with spatial averages of250–380 g m−2 per month. Comparison of the dust source activationfrequency from the model against SEVIRI satellite observation shows a goodagreement in the Bodélé Depression but differences in the north and westof North Africa. Depressions are abundant, particularly in summer when theSaharan heat low is situated over West Africa and during spring in the lee ofthe Atlas Mountains. Up to 90% (55% annually and spatiallyaveraged) 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 inagreement with previous studies and over summertime West Africa consistentwith near-surface signatures of African Easterly Waves. Dust emission within10 degrees of cyclones peaks over Libya with up to 25% in spring.Despite the overall small contribution of 4% annually and spatiallyaveraged, cyclones coincide with particularly intense dust emission eventsexceeding the climatological mean by a factor of four to eight. Soil moistureweakens dust emission during cyclone passage by about 10%.