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All other forcings were kept constant at their preindustial values (i.e. without high-frequency variations) of the atmospheric concentrations of CO 2 and CH 4 for the 9–0 kyr BP period. In the performed experiment, the model was forced by annually varying insolation and long-term trends (i.e. This simplification is inevitable given the long time-scales involved in this study.
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It is important to realize that, compared to comprehensive general circulation models, our model represents a simplified description of the coupled system (e.g., low spatial resolution, prescribed cloud cover). Simulated vegetation changes only influence the land-surface albedo in ECBilt-CLIO, and have no effect on other processes, e.g. It has been recently coupled with VECODE, a model that simulates the dynamics of two main terrestrial plant functional types, trees and grasses, and desert as a dummy type. The ECBilt-CLIO model gives a reasonable representation of the modern climate. The ocean-sea-ice component CLIO consists of a free-surface, primitive-equation oceanic general circulation model with 20 vertical levels and 3° × 3° latitude-longitude resolution, coupled to a dynamic-thermodynamic sea-ice model.
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In the model, precipitation is independent of cloud cover. Cloud cover is prescribed according to present-day climatology.
#Vecode solutions full
ECBilt includes a full hydrological cycle that is closed over land by a bucket model for soil moisture and river runoff. Simple parameterizations for the diabatic heating due to radiative fluxes, the release of latent heat, and the exchange of sensible heat with the surface are incorporated. The atmospheric component is version 2 of ECBilt, a spectral quasi-geostrophic model with three vertical levels and a T21 horizontal resolution. This model consists of three components representing dynamics of the atmosphere, ocean-sea ice and vegetation. To investigate the effect of high-frequency (daily-to-decadal) climate variability on the Holocene climate evolution in the Sahara/Sahel, we have conducted a numerical experiment covering the last 9 thousand years with the 3-dimensional global ECBilt-CLIO-VECODE climate model.
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Here, we use transient simulations performed with a three-dimensional coupled climate model to show that, between 7.5 and 5.5 kyr BP, the interannual variability in precipitation caused similar fluctuations at decadal-to-centennial time-scales. For example, sedimentological and paleobotanical data obtained from former lakes in the Western Sahara give clear evidence for alternating high and low lake levels between ∼9 and 5 kyr BP. This may explain why these experiments do not show the centennial-scale paleohydrological fluctuations found in many paleorecords. So far, the effect of this biogeophysical feedback on climate-vegetation dynamics through the Holocene has only been analyzed in model experiments lacking the influence of explicitly simulated high-frequency (daily-to-decadal) climate variability. Model studies elaborated radiative and hydrological mechanisms of atmosphere-vegetation interaction and suggested that at least two states could be stable in the region: 1) a “desert” equilibrium, with low precipitation and absent vegetation, and 2) a “green” equilibrium, with moderate precipitation and permanent vegetation cover. This positive biogeophysical feedback assumes that a decrease in vegetation cover and the associated increase in surface albedo reduce precipitation in the Sahara/Sahel, thus implying self-induction of deserts. Climate model experiments have linked the abruptness of the AHP termination to an atmosphere-vegetation feedback that amplified the desertification. Marine core-based studies indicate that the AHP termination around 5–6 kyr before present (BP) was much more abrupt than might be expected from the slowly decreasing summer insolation. A current paradigm is that this African Humid Period (AHP) was associated with a strengthening of the summer monsoon due to an increase in the summer insolation. The early-to-middle Holocene was a humid period in Northern Africa, with vegetation cover extending greatly into the Sahara.