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Paper on the pulsating nature of large-scale Saharan dust transport

by Enric Terradellas last modified Oct 17, 2017 05:40 PM

The study titled “The pulsating nature of large-scale Saharan dust transport as a result of interplays between mid-latitude Rossby waves and the North African Dipole Intensity” has been published in Atmospheric Environment.

The main objective of this study is to explain the atmospheric processes behind the intra-seasonal variations of dust outflows towards the Mediterranean and the subtropical North Atlantic Ocean. Daily and monthly data of MODIS Aerosol Optical Depth MODIS aerosols, NCEP / NCAR reanalysis, ECMWF reanalysis, and MACC reanalysis (today CAMS) have been used.

The North African Dipole Intensity (NAFDI) concept recently introduced by Rodríguez et al. (2015) has been used to achieve this goal. It is defined as the difference of the 700 hPa geopotential height anomaly between the subtropics and the tropics over Northern Africa. Essentially, NAFDI represents the large-scale anomaly of the geostrophic wind at 700 hPa (about 3,000m altitude) over Northern Africa. Under positive NAFDI index, there are greater dust outflows towards the subtropical North Atlantic, whereas under negative NAFDI a positive dust anomaly is observed over the central and western Mediterranean. The first aim was to demonstrate that there is a summertime intra-seasonal variation of NAFDI (June-September), and that it modulates Saharan dust outflows not only to the Atlantic but also to the Mediterranean basin.

The second challenge is to identify the physical mechanisms behind the connection between NAFDI and the Saharan Heat Low (SHL), which is well known to modulate numerous meso-scale meteorological processes that cause Saharan dust mobilization. The location and intensity of dust sources and dust outflows over Northern Africa depend to a large extent on the SHL position. The SHL can be found shifted farther east or west, and therefore conditioning the location and intensity of dust sources but, until now, the processes controlling these longitudinal shifts were unknown. On the other hand, the anomalies of different meteorological fields, such as temperature and wind at 925 hPa, as well as the geopotential thickness 700-925 hPa for positive (negative) NAFDI coincided with those found by other authors associated to the West (East) phase of SHL.

However, the most important question still was unanswered: What atmospheric mechanism explains the variations of NAFDI and SHL  showing a variable period between 10 and 30 days? Based on the very interesting work of Chauvin et al. (2010), and bearing in mind the general principle in physics by which larger spatial scale processes modulate smaller processes, it is shown that mid-latitude Rossby waves (MLRWs) penetrating into the lower troposphere over Northern Africa may be the key.

Cuevas, E., A.J. Gómez-Peláez, S. Rodríguez, E. Terradellas, S. Basart, R.D. García, O.E. García, S. Alonso-Pérez, The pulsating nature of large-scale Saharan dust transport as a result of interplays between mid-latitude Rossby waves and the North African Dipole Intensity, Atmospheric Environment, 167, 586-602, ISSN 1352-2310, https://doi.org/10.1016/j.atmosenv.2017.08.059, 2017.

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