The water balance of northern Africa during the mid-Holocene: an evaluation of the 6 ka BPPMIP simulations

Runoff fields over northern Africa (10-25degreesN, 20degreesW-30degreesE) derived from 17 atmospheric general circulation models driven by identical 6 ka BP orbital forcing, sea surface temperatures, and CO2 concentration have been analyzed using a hydrological routing scheme (HYDRA) to simulate changes in lake area. The AGCM-simulated runoff produced six-fold differences in simulated lake area between models, although even the largest simulated changes considerably underestimate the observed changes in lake area during the mid-Holocene. The inter-model differences in simulated lake area are largely due to differences in simulated runoff (the squared correlation coefficient, R-2, is 0.84). Most of these differences can be attributed to differences in the simulated precipitation (R-2 = 0.83). The higher correlation between runoff and simulated lake area (R-2 = 0.92) implies that simulated differences in evaporation have a contributory effect. When runoff is calculated using an offline land-surface scheme (BIOME3), the correlation between runoff and simulated lake area is (R-2 = 0.94). Finally, the spatial distribution of simulated precipitation can exert an important control on the overall response.