Schepanski, K and Knippertz, P (2011) Soudano-Saharan depressions and their importance for precipitation and dust: a new perspective on a classical synoptic concept. Quarterly Journal of the Royal Meteorological Society, 137 (659). 1431 - 1445. ISSN 0035-9009
Abstract
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.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | (c) 2011, Royal Meteorological Society. Reproduced in accordance with the publisher's self-archiving policy. |
Keywords: | Africa; West-African Monsoon; North-Atlantic Oscillation; Heat low; Atmospheric Circulation; Easterly waves; Hoggar Massif; Variability; Rainfall; Onset; Cyclogenesis; Low-pressure system; Lee trough; Cyclone tracking; Climatology |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 17 Oct 2013 10:18 |
Last Modified: | 15 Sep 2014 02:55 |
Published Version: | http://dx.doi.org/10.1002/qj.850 |
Status: | Published |
Publisher: | Royal Meteorological Society |
Identification Number: | 10.1002/qj.850 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:76606 |