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The dry and warm Sahara was potentially wet and vegetated in the past within a relatively warm climate during the African Humid Period (AHP). This climatic shift is strongly debated and is a possible scenario in a future warmer climate. Such a dramatic shift in the land surface properties of Northern Africa can induce large feedbacks on the global climate. One of the strongest evidence to the greening of the Sahara is the presence of paleo-lakes in the desert. Even today, Saharan lakes get filled from time to time, but very little is known about the meteorology associated with these events. Gaining knowledge about Saharan lake-filling episodes (LFEs) in the present helps to understand the potential mechanisms involved in the greening of the Sahara in the past and thus, presumably, also in a warmer future. This Master’s thesis investigates the meteorology of LFEs of Sabkhat El-Mellah - a lake in western Central Algeria. Heavy precipitation events (HPEs) and LFEs are identified using remote sensing observations. The fifth generation of the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis dataset (ERA5) is used together with trajectory calculations with the Lagrangian Analysis Tool (LAGRANTO) to analyse weather systems that lead to the HPEs and to assess the conditions necessary for the filling of Sabkhat El-Mellah. Six LFEs are observed between June 2000 and May 2021. In most of these LFEs, an upper-level forcing in combination with an extratropical low-level cyclone at the western coast of Morocco induces wind fields which transport moisture from the tropics and the Atlantic Ocean through a precipitation-evaporation recycling-domino-mechanism over the Sahara into the Sabkhat El-Mellah catchment. The Atlas Mountains play an important role by additionally forcing the air parcels to lift and precipitate. The stationarity of the systems makes it possible to advect enough moisture into the catchment and maintain the precipitation forcing, distinguishing between ’normal HPEs’ and LFE-triggering HPEs. This mechanism of precipitation is most likely similar to the ones that were important during the AHP. This connection to dynamical drivers of Sahara greening motivates to further investigate open questions like the influence of teleconnections on LFEs, and the sequential generation of upper-level forcings reaching far south, also in view of anthropogenic climate change. |
