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Numerical investigation of the impact of vegetation indices on the variability of West African summer monsoon


The climate impacts of leaf area index (LAI) and fractional vegetation cover (FVC) on the West African summer monsoon in 1987 and 1988 were investigated through a series of numerical experiments conducted with satellite products and the Simplified Simple Biosphere land surface model (SSiB). The SSiB was run in the offline mode as well as in the coupled mode with the National Centers for Environmental Prediction General Circulation Model (GCM). Prescribed monthly LAI and FVC from a look-up table based on limited ground surveys and those derived from satellite remote sensing were employed in the control and test runs, respectively. Compared with the control runs, both the GCM and offline test runs with satellite measured LAI/FVC produce higher soil moisture and lower surface temperature in tropical West Africa to the south of about 15°N, with the maximum deviations being located around 12°N. This leads to the northward shift of the maximum of positive latitudinal temperature gradient. The associated easterly shear in the lower troposphere results in the African easterly jet and summer rainfall band in West Africa shifting to the north, which partially corrects the dry bias in the GCM control runs. In addition, the GCM test run simulates a relatively stronger West African summer monsoon in 1988, with a northward shifted African easterly jet, a stronger tropical easterly jet, and more rainfall than in 1987, consistent with observations. However, due to small differences in satellite measured LAI/FVC between 1987 and 1988, the model fails to produce interannual variations of precipitation as large as seen in the observation. Water balance analysis has also been carried out to investigate the dominant processes affecting the changes in precipitation. The relative contributions of moisture flux convergence and surface evapotranspiration are identified. The comparisons between GCM and offline results in this study demonstrate a possible future application of CEOP data in African climate study. © 2007, Meteorological Society of Japan.

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