Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 2: The imprint of the atmospheric circulation at different scales

Villiger, Leonie; Aemisegger, Franziska

doi:10.5194/egusphere-2023-450

Cited by 1 publication

(1 citation statement)

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“…This diagnostic has been applied in many atmospheric water cycle studies. More specifically, these studies investigated the tropical and subtropical water cycle and the dynamics of the Saharan heat low in 265 Dahinden et al (2023), and the North Atlantic trade wind water cycle in Aemisegger et al (2021) and Villiger and Aemisegger (2023). This trajectory-based diagnostic identifies the evaporative source footprint of precipitating waters by establishing a mass balance of vapour phase humidity in the air parcels contributing to the precipitation event.…”

Section: Moisture Source Diagnosticsmentioning

confidence: 99%

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

Rieder,

Aemisegger,

Dente

et al. 2024

Preprint

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Abstract. 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 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 that result in long-duration (typically 3 d) HPEs. Based on the insights gained into Saharan LFEs in the present-day climate, we suggest that the initial filling and persistence of Saharan lakes may be related to changes in the intensity and frequency of HPEs, rather than a change to mean precipitation alone. Future studies can leverage these insights to better assess the mechanisms involved in the greening of the Sahara in the past and also in a warmer future.

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Section: Moisture Source Diagnosticsmentioning

confidence: 99%