Abstract
Abstract. We have implemented a new albedo scheme that takes the dynamic behaviour of the surface below the canopy into account, into the land-surface scheme of the MPI-ESM. The standard (static) scheme calculates the seasonal canopy albedo as a function of leaf area index, whereas the background albedo is a gridbox constant derived from satellite measurements. The new (dynamic) scheme additionally models the background albedo as a slowly changing function of organic matter in the ground and of litter and standing dead biomass covering the ground. We use the two schemes to investigate the interactions between vegetation, albedo and precipitation in the Sahel/Sahara for two time-slices: pre-industrial and mid-Holocene. The dynamic scheme represents the seasonal cycle of albedo and the correspondence between annual mean albedo and vegetation cover in a more consistent way than the static scheme. It thus gives a better estimate of albedo change between the two time periods. With the introduction of the dynamic scheme, precipitation is increased by 30 mm yr−1 for the pre-industrial simulation and by about 80 mm yr−1 for the mid-Holocene simulation. The present-day dry bias in the Sahel of standard ECHAM5 is thus reduced and the sensitivity of precipitation to mid-Holocene external forcing is increased by around one third. The locations of mid-Holocene lakes, as estimated from reconstructions, lie south of the modelled desert border in both mid-Holocene simulations. The magnitude of simulated rainfall in this area is too low to fully sustain lakes, however it is captured better with the dynamic scheme. The dynamic scheme leads to increased vegetation variability in the remaining desert region, indicating a higher frequency of green spells, thus reaching a better agreement with the vegetation distribution as derived from pollen records.
Highlights
Investigating climates of the past offers an opportunity to improve our understanding of future Earth system dynamics
The dependence of albedo on the canopy is modelled in the way common for most general circulation models (GCMs), using an exponential function of leaf area index (LAI) to define the area covered by green leaves (Beer’s law; Monsi and Saeki, 1953)
The correspondence between annual mean albedo and the vegetation seen in observations is wellcaptured in the pre-industrial for both schemes, but it is only captured with the dynamic scheme for the mid-Holocene
Summary
Investigating climates of the past offers an opportunity to improve our understanding of future Earth system dynamics. The vegetation cover extended at least up to 23◦ N (Jolly et al, 1998a), if not across the whole of North Africa (Hoelzmann et al, 1998), compared to about 12◦ N today (de Noblet-Ducoudre et al, 2000). The largest differences in external forcing between the mid-Holocene and today are radiative forcing anomalies arising from changes in the Earth’s orbital parameters (Berger, 1978). It is largely accepted that the main reason for changes in the palaeomonsoon was an increased seasonal cycle of insolation in the Northern Hemisphere due to a change in these parameters (de Noblet-Ducoudre et al, 2000). As demonstrated by several modelling experiments, a change in just the orbital parameters is not sufficient to induce strong enough changes in the monsoon to agree with reconstructed vegetation and precipitation (Joussaume et al, 1999). The combined effect of orbital change, ocean feedbacks and vegetation feedbacks corresponds best to palaeoreconstructions (Braconnot et al, 2007a)
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