The hydrometeorological ingredients needed to fill dry Saharan lakes

The Sahara desert was potentially much wetter and vegetated in the past during the warm African Humid Period. Although debated, this climatic shift is a possible scenario in a future warmer climate. The most prominent reported evidence 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 investigated in a dedicated climatological approach. This study proposes to fill this knowledge gap by investigating 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 remote sensing satellite imagery. 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 runoff coefficient, i.e. the ratio between the increase in lake water volume during LFEs and precipitation volume during the HPEs that triggered the lake-filling, ranges five orders of magnitude and is much smaller than the figures often cited in the literature regarding this arid area. 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 premoistening of the region upstream of the catchment over the Sahara through a recycling-domino-process, (iv) coupled or sequential lifting processes (e.g., orographic lifting and large-scale forcing), and (v) the stationarity of synoptic systems. Based on the insights gained into Saharan LFEs in the present-day climate, future studies will be able to better assess the mechanisms involved in the greening of the Sahara in the past and also in a warmer future.