Meteorological ingredients of heavy precipitation and subsequent lake filling episodes in the northwestern Sahara

• Main Authors: Rieder, Joëlle C., Aemisegger, Franziska, Dente, Elad, Armon, Moshe
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2024
• Subjects: article; Verlagsveröffentlichung; North Atlantic
• Online Access: https://doi.org/10.5194/egusphere-2024-539; https://noa.gwlb.de/receive/cop_mods_00072637; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00070839/egusphere-2024-539.pdf; https://egusphere.copernicus.org/preprints/2024/egusphere-2024-539/egusphere-2024-539.pdf

The dry Sahara was potentially wetter in the past during the warm African Humid Period. Although debated, this climatic shift is a possible scenario in a future warmer climate. One major line of evidence reported for past green periods in the Sahara is the presence of paleo-lakes. Even today, Saharan desert lakes get filled from time to time. However, very little is known about these events due to the lack of available in-situ observations. In addition, the hydrometeorological conditions associated with these events have never been systematically investigated. This study proposes to fill this knowledge gap by examining the meteorology of lake-filling episodes (LFEs) of Sebkha el Melah – a commonly dry lake in the northwestern Sahara. Heavy precipitation events (HPEs) and LFEs are identified using a combination of precipitation observations and lake volume estimates derived from satellite remote sensing. Weather reanalysis data is used together with three-dimensional trajectory calculations to investigate the moisture sources and characteristics of weather systems that lead to HPEs and to assess the conditions necessary for producing LFEs. Results show that hundreds of HPEs occurred between 2000 and 2021, but only 6 LFEs eventuate. The ratio between the increase in lake water volume during LFEs and precipitation volume during the HPEs that triggered the lake-filling, known as the runoff coefficient, provides a very useful characteristic to assess storm impacts on water availability. For the 6 LFEs investigated in this study, the runoff coefficient ranges across five orders of magnitude and is much smaller than the figures often cited in the literature for the Sahara. We find that LFEs are generated most frequently in autumn by the most intense HPEs, for which the key ingredients are (i) the formation of surface extratropical cyclones to the west of the Atlantic Sahara coastline in interplay with upper-level troughs and lows, (ii) moisture convergence from the tropics and the extratropical North Atlantic, (iii) a ...