Soudano-Saharan depressions and their importance for precipitation and dust: a new perspective on a classical synoptic concept



According to a classical synoptic concept, Soudano-Saharan depressions (SSDs) are surface lows that track westward over tropical West Africa, curve anticyclonically across the Sahara and may then transform into eastward-moving Mediterranean cyclones. Occurrence frequency and track location undergo a marked seasonal cycle. Interactions between troughs in the upper-level westerlies and mid-level African easterly waves have been suggested as a mechanism for their formation. SSDs have been reported to be associated with dust-storms and precipitation over northern Africa.

This paper presents the first-ever systematic investigation of SSDs using re-analysis and satellite data. Depressions are identified and tracked objectively based on closed contours in 0000 UTC fields of 925 hPa geopotential height from the European Centre for Medium-Range Weather Forecasts ERA-Interim re-analysis (1989–2008). To classify as potential SSDs, tracks must: (i) start to the south of 20°N; (ii) intersect 15°–30°N, 10°W–30°E; (iii) cover a meridional distance of at least 10° latitude; and (iv) have a minimum lifetime of 24 hours.

Even with a relatively low threshold of 4 gpm, only 50 potential SSDs are found (annual average 2.5, monthly range 0–6). Lagrangian and Eulerian composite analyses reveal that the identified systems: (i) are mostly shallow lee troughs of the central Saharan and Atlas Mountains during the warm season without a well-defined cyclonic wind field; (ii) do not show the seasonal track variation described in the literature; (iii) mostly occur in association with high-pressure anomalies over the Mediterranean Sea; and (iv) are not associated with significant increases in dustiness and precipitation. These results strongly suggest that the disturbances described as SSDs do not manifest themselves as traceable low-level depressions, calling for a fundamental revision of the classical concept in the literature. Copyright © 2011 Royal Meteorological Society