In this presentation, an overview was given of model developments going on as a collaboration between the Max Planck Institute for Meteorology, Hamburg and the Max Planck Institute for Biogeochemistry, Jena. The goal of this effort is to build a flexible framework (JSBACH = The Jena Scheme for Biosphere Atmosphere Coupling in Hamburg) for modeling the feedbacks between the atmospheric circulation, or "climate" in its long-term mean, and processes at the land surface including soil hydrology and dynamic vegetation. The coupled system naturally also includes the oceans and the cycling of biogeochemical tracers. The framework will be based on existing models and consists of the following components:

• The Hamburg ECHAM-5 general circulation model.
• The Variable Infiltration Capacity (VIC) macroscale hydrologic model with a complete description of soil heat and moisture fluxes, including frozen soils (see e.g. Nijssen et al., 2001).
• The Biosphere Energy-Transfer and Hydrology (BETHY) scheme for fast vegetation processes including photosynthesis, plant respiration, and a cold and dry phenology scheme.
• The Lund-Potsdam-Jena (LPJ) model for slow vegetation processes including vegetation dynamics (dispersal, establishment, growth, dieback), soil respiration, carbon-nutrient interactions and nutrient cycling, fire and other disturbances, and land-use impacts

The importance of land surface and vegetation feedbacks was illustrated with two applications to the North African monsoon system, using the ECHAM-4 model version. One study (Knorr et al., 2001) investigated the importance of land surface albedo feedbacks for the North African Monsoon system. Using a new data set of albedo derived from the Meteosat satellite using multi-angular techniques it is concluded that desert albedo has so far been largely underestimated leading to a twofold overestimate of current Saharan precipitation compared to observations. With the new albedo data the ECHAM4 climate model reproduces the observed Saharan precipitation very well. Simulations with the desert albedo changed to likely mid-Holocene conditions (covered by Savanna and Steppe) agree well with proxy records and the inferred amplitude of Holocene-to-present climate changes approximately doubles compared to previous simulations, demonstrating a much larger sensitivity of the regional climate system to land-surface feedbacks.

The second application (Schnitzler et al., 2001) explored climate variability on decadal time scales due to feedbacks between physical climate processes and dynamical vegetation in the Sahel. A model experiment with the ECHAM4 atmospheric model coupled to the simple dynamic vegetation model SVEGE (Zeng et al., 1999) highlighted the importance of vegetation changes for the strength of the monsoon oscillation, through variation in the albedo in the Sahel and Saharan regions. It is shown that the interdecadal Sahelian rainfall variablity is enhanced significantly by vegetation interaction in the ECHAM4 GCM, bringing it closer to the observed variability.
 

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