Verbesserung der Simulation  des westafrikanischen Klimas  durch die Implementierung eines einfachen  dynamischen Vegetationsmodells (SVege)  in das Klimamodell ECHAM5

Between 1970 and 2000 the climate of West Africa was a¿ected by a severe drought. 

The reasons for the sudden change to the dry period are unclear. Therefore, investigations 

based on climate model simulations are undertaken to capture the observed decadal climate 

variability by including as many sub systems of the Earth Climate System as possible. 

Therefore, this work focuses the in¿uence on simulating present day climate by including 

the biosphere into the general circulation model (GCM) ECHAM5 to capture the recent 

climate in a better way. It is well-known, that there is an ampli¿cation by vegetation on 

climate variability, in particular the decadal one. The Simple Vegetation (SVege) model 

is used for these coupled experiments. Furthermore, this optimised version is used to set 

up climate change simulations in an ensemble mode following the SRES (Special Report 

on Emission Scenarios ) greenhouse gas forcings A1B and B1 until the year 2100. It is 

assumed, that there will be a change within vegetation cover by a climate change, therefore 

the coupled version will be responsive to the climate impact. At least very promising results 

by doing this coupling with the former version No. 4 of ECHAM give the motivation for 

this work. 

To estimate the e¿ect of using the vegetation model, simulations for present day climate 

(1960 to 1999) are investigated by using the biosphere-atmosphere model and the stand- 

alone version of the GCM both driven by an observed sea surface temperature (SST) 

data set (AMIP2). Both versions capture the observed climate of West Africa well and 

a small, insigni¿cant shift to a better reproduction of the decadal variability in rainfall is 

noticed by including the vegetation model. There are no signi¿cant di¿erences in simulating 

present day climate compared to the standard version. So, the coupled version using a 

dynamical vegetation model calculating time-dependent values for some surface parameters 

(e.g. albedo) gets the same results as using climatological mean numbers. 

It will be shown, that the coupled biosphere-atmosphere version captures the decadal 

variability better, but even the models are able to simulate the climatological mean state 

quite well, it has to be summarised, that both model versions fail to simulate the strength 

of the observed decadal variability of precipitation amounts in West Africa. Compared 

to similar studies using the previous version of the GCM (ECHAM4), the e¿ect of the 

imbedded biosphere is low. Due to some changes in the surface parametrizations and

formulating the surface scheme in a more sophisticated way, it can be assumed, that the 

used vegetation formulation is to simple for the new complex version of the GCM. 

The analysis of the 240 years (1860 to 2099) climate change simulations concentrates 

on possible shifts in precipitation intensities within a warmer world (SRES A1B and B1). 

Investigations on the yearly, quarterly, and daily precipitation sums and the analysis of 

the simulated tropical rain belt as well as the 2 m temperature are done. For the control 

climate (1960 to 1999) it is shown, that signi¿cant di¿erences between the simulations 

based on observed and modelled SSTs exist. These di¿erences are due to the two unequal 

SST data sets for the period 1960 to 1999 for computing (i) present day climate using 

observed values (AMIP2) and (ii) doing climate change experiments based on simulated 

(OM1, Ocean Model ) SST patterns. The trends indicate, that there is a possible shift of 

daily intensities to higher values. This shift is superimposed by increasing rainfall sums 

in summertime (June to August) at the Guinea Coast and less precipitation in the Sahel 

leading to a more prominent dipole between the wet Coast and dryer interior. With an 

increasing number of extreme precipitation events with respect to monthly rainfall sums 

(dry months in the Sahel and wet in the coastal region), the internal variability will increase 

and the scarce fresh water resources will become more precarious in a warmer climate. The 

changes due to the SRES scenarios are more pronounced in the stronger A1B scenario than 

in B1. In summary, the climate change results are similar to the ECHAM5-investigations 

pointed out by the fourth assessment report of the IPCC (Intergovernmental Panel on 

Climate Change ) without using a vegetation model.