The effect of a dynamic background albedo scheme on Sahel/Sahara precipitation during the mid-Holocene

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: preindustrial 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.

Simulated climate variability in the region of Rapa Nui during the last millennium

“…The evaluation of a 300-yr control simulation with ECHAM5/MPIOM already showed that the tropical sea surface temperature climatology is well simulated and that global-scale heat and freshwater-transport are in agreement with observations (Jungclaus et al, 2006). Furthermore, ECHAM5/MPIOM has been successfully applied to different paleoclimate states such as Mid-Holocene climate and transient Holocene climate simulations (Fischer and Jungclaus, 2010;Otto et al, 2009;Dallmeyer et al, 2010;Vamborg et al, 2011), Last Glacial Maximum simulations (Mikolajewicz, 2011;Arpe et al, 2011), Eemian climate simulations (Fischer and Jungclaus, 2010;Schurgers et al, 2006) and Eocene climate simulations (Heinemann et al, 2009).…”

The effect of a dynamic soil scheme on the climate of the mid-Holocene and the Last Glacial Maximum

“…The integrated vegetation-soil feedback reinforced glacial cooling to some extent (Jiang, 2008). Further progress is shown in Vamborg et al (2011), in which the authors implemented a dynamic background albedo scheme within a comprehensive fully coupled GCM with vegetation dynamics (earth system model, ESM). In this model, the model additionally accounts for land surface albedo changes based on the calculation of carbon fluxes between terrestrial carbon pools (vegetation, litter and phytomass, and soil).…”

“…In addition to vegetation dynamics, however, various studies also highlight the impact of physical soil characteristics on climate, as it represents another component of the land surface (Kutzbach et al, 1996;Doherty et al, 2000;Levis et al, 2004;Knorr and Schnitzler, 2006;Shellito and Sloan, 2006;Wanner et al, 2008;Brovkin et al, 2009;Micheels et al, 2009;Knorr et al, 2011;Krapp and Jungclaus, 2011;Vamborg et al, 2011;Lohmann et al, 2015). So far, model studies designed to examine soil dynamics have mainly focused on the sensitivity of individual physical soil characteristics and have therefore primarily addressed the monocausal impact of land albedo (Bonfils et al, 2001;Levis et al, 2004;Knorr and Schnitzler, 2006;Schurgers et al, 2007;Vamborg et al, 2011), soil texture, and maximum water-holding field capacity (Wang, 1999;Levis et al, 2004) on climate. Furthermore, Jiang (2008) investigated the role of physical soil characteristics (soil colour, soil texture) for the Last Glacial Maximum (LGM) by applying an atmosphere GCM, which was asynchronously coupled to a terrestrial biosphere model.…”

Implications of land use change in tropical northern Africa under global warming

“…In turn, climate in this region is affected by changes in land surface conditions (e.g. Xue and Shukla, 1993;Claussen, 1997;Zeng et al, 1999;Taylor et al, 2002;Koster et al, 2004;Vamborg et al, 2011;Patricola and Cook, 2010). Therefore, it is conceivable that changes in the type and intensity of anthropogenic land use and land cover change, perhaps caused by local or national conflict, will affect regional climate.…”