MESOSCALE MODELING OF THE AFRICAN HUMID PERIOD

During the last glacial period conditions over Saharan and Sahelian Africa were drier than those of today. Beginning around 14.5 ky BP, the region became increasingly moist. Until about 5.5 ky BP, numerous large lakes dotted the landscape of what is now the Sahel and the Sahara/Sahel boundary was about 5° farther north. This time, known as the African Humid Period (AHP), occurred at a time of higher summertime insolation in the Northern Hemisphere, which is thought to have been associated with an intensification of the African monsoon. Global Climate Models (GCMs), however, suggest that a consideration of additional factors and/or feedbacks within the climate system is required to fully understand the AHP.

A mesoscale climate model (MCM) is developed for studying the AHP. By modeling climate on smaller space scales (10's of kilometers) than is possible with a GCM, the resolution of the modeling is brought closer to that of the geological record. This provides a better opportunity for model validation, and for the modeling to aid in the interpretation of the geologic evidence. The MCM also produces an improved simulation of the present day climate of northern Africa, as its physical parameterizations can be optimized for the region, and the strong surface moisture and temperature gradients that characterize it can be resolved.

A series of MCM integrations is used to both compare the climate of the AHP with the climate of the present, and to explore the sensitivity to various assumptions about the surface boundary conditions. In contrast to GCM simulations of the AHP, which underestimate the precipitation implied by geologic records, the MCM produces excessive precipitation over the present-day Sahel and Sahara regions when the solar insolation, surface vegetation, and atmospheric CO2 levels are set at their 6 ky BP values. The addition of colder sea surface temperatures off the west coast, as suggested by ocean cores, improves the model's agreement with geological reconstructions, lowering the precipitation. Thus, the MCM results support observations of enhanced upwelling off the west coast of Africa during the African Humid Period.