29 March 2014

Regional Model Simulation of the African Monsoon Precipitation Response to Land Use Change


Land use and land cover (LULC) throughout Africa have changed substantially over the last 60 years. Scientists propose this change affects monsoon circulation and precipitation, but climate models do not agree to what degree LULC plays a role compared to other factors that also influence monsoon precipitation. Understanding how land use and land cover may affect precipitation is important to predicting future changes in the water cycle in climate sensitive regimes such as the African monsoon. Using a set of regional model simulations with different combinations of land surface and cumulus parameterization schemes, a team of scientists, led by U.S. Department of Energy researchers at Pacific Northwest National Laboratory, examined the uncertainties of the model simulated response in the African Monsoon system and Sahel precipitation due to LULC change. Although the magnitude of the response covers a broad range of values, most of the simulations show a decline in Sahel precipitation due to the expansion of pasture and croplands at the expense of trees and shrubs, and an increase in surface air temperature. Simulations that are climatologically too dry or too wet compared to observations and re-analyses have a weak response to land use change because they are in moisture- or energy-limited regimes, respectively. The simulations that lie in-between, and are closer to observations, have stronger response to the LULC changes, showing a more significant role in land-atmosphere interactions. Much of the change in precipitation is related to changes in circulation, particularly to the response of the strength and latitudinal position of the African Easterly Jet, which varies with the changes in meridional surface temperature gradients. These findings highlight the need for measurements of the surface fluxes across the meridional cross-section of the Sahel to evaluate models and thereby allowing human impacts such as land use change on the monsoon to be projected more realistically.

Samson Hagos