West African Monsoon During Glacial Conditions: A Dynamical Downscaling Approach

Abstract

A regional climate model (RCM) is used to dynamically downscale general circulation model (GCM) data for conditions during the Last Glacial Maximum (LGM) in Africa north of 10°S. Main differences between LGM and modern boundary conditions are (1) the reduction of greenhouse gas concentrations, (2) small changes in Earth's orbit and rotation axis, (3) altered topography and land/sea mask due to large amounts of water being stored in ice sheets and (4) changes in vegetation. The output is compared to data of a control run under modern conditions and to precipitation reconstructions from pollen and plant fossils.
The RCM outperforms the host GCM in terms of spatial detail and representation of vegetation
effects.
The African climate is found to be colder by 0-5 K. Precipitation is reduced almost all over the continent. The West African summer monsoon circulation is less intense by about 15% in terms of mass and energy transport and smaller in geometrical extent. Main climate system components, such as the African Easterly Jet, the Subtropical Westerly Jet, the Intertropical Convergence Zone and the Intertropical Discontinuity are shifted south by about 2-4°. This becomes manifest in a strong late summer precipitation decrease in the Sahel zone of 20%-60%, accounting for up to 50% in the annual mean.
The northern coast of the continent receives a precipitation gain. Both the northern and the southern edge of the Sahara desert are thus shifted south according to the RCM results.
The rainfall in the Sahel (and the absence of such) is found to be closely connected to the amount of mesoscale convective systems (MCSs). MCS activity is strongly decreased in the Sahel and almost completely ceased on its northern edge.
Model output qualitatively agrees with the precipitation proxies on large scales. Regionally, however, they can differ substantially.

Resources

• 20160726_anhaeuser_master-thesis.pdf

Bibliography

Anhäuser, A. (2016): West African Monsoon During Glacial Conditions: A Dynamical Downscaling Approach.