Abstract
Enhanced summer insolation over North Africa induced a monsoon precipitation increase during the mid-Holocene, about 6000 years ago, and led to a widespread expansion of lakes and wetlands in the present-day Sahara. This expansion of lakes and wetlands is documented in paleoenvironmental sediment records, but the spatially sparse and often discontinuous sediment records provide only a fragmentary picture. Former simulation studies prescribed either a small lake and wetland extent from reconstructions or focused on documented mega-lakes only to investigate their effect on the mid-Holocene climate. In contrast to these studies, we investigate the possible range of mid-Holocene precipitation changes in response to a small lake extent and a potential maximum lake and wetland extent. Results show that the maximum lake and wetland extent shift the North African rain belt about 3 ° farther northward than the small lake extent. Vegetated wetlands cause a larger precipitation increase than the equally-large lakes due to their high surface roughness. A moisture budget analysis reveals that both, lakes and wetlands, cause an enhanced inland moisture transport and local moisture recycling to their southern side. In contrast, increased moisture advection by the Harmattan winds causes a drying response to the north of the lakes and wetlands. These results indicate that the latitudinal position of the lakes and wetlands influences the northward extension of the African summer monsoon. In the sensitivity experiments, the northern position of West Saharan lakes and wetlands substantially contributes to the strong monsoon northward shift seen in the maximum lake and wetland simulations.
Highlights
Paleoenvironmental sediment records reveal that North African lakes and wetlands spatially expanded during the mid-Holocene, 20 as a result of increased summer monsoon precipitation (Holmes and Hoelzmann, 2017; Lézine et al, 2011)
The West Saharan lakes in the maximum lake experiment are located relatively far northward compared to the lakes in the small lake experiment (Fig. 1)
These results indicate that the latitudinal position of the Saharan lakes likely plays a key role in shifting the simulated rain belt northward, over the western Sahara where the African Easterly Jet is strongest and where the Saharan heat low is located
Summary
Paleoenvironmental sediment records reveal that North African lakes and wetlands spatially expanded during the mid-Holocene, 20 as a result of increased summer monsoon precipitation (Holmes and Hoelzmann, 2017; Lézine et al, 2011). This precipitation increase was initiated by changes in the orbital forcing Kutzbach (1981), but reinforced by surface changes such as the expansion of vegetation (Kutzbach et al, 1996; Claussen and Gayler, 1997), the formation of soil Reconstructions widely differ regarding the existence of mega-lakes (Quade et al, 2018) Given this spatially and temporally limited information from reconstructions, investigating the effect of lakes and wetlands on the mid-Holocene climate becomes a scientific challenge
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