Abstract:
Upward feedbacks from the land surface and vegetation to the atmospheric boundary layer over southern Africa are investigated. Satellite derived rainfall and vegetation data and model evaporative fluxes for the period 1981-2000 are used to reveal spatial and temporal inter-relationships via Principal Components Analysis.
Seasonal rainfall and NDVI exhibit distinct unimodal seasonal cycles maximizing during the austral summer over the Zambezi valley and interior plateau. Spectral analysis indicates major cycles of intraseasonal rainfall events at approximately 40 and 20 days. The 40-day oscillation reflects the Madden-Julian Oscillation and is partially phase-locked to the seasonal cycle. The spatial loadings are focused on a region along the eastern edge of the Kalahari (± 23°E), extending from the western Zambezi toward central South Africa, spanning 20° of latitude. The loading pattern is consistent with tropical-temperate troughs and associated northwest (NW) cloud bands. This 40-day mode connects two 20-day modes over the Agulhas current and Angola, hence NW cloud bands are a slower terrestrial harmonic of the faster modes at either end.
NDVI also exhibits intraseasonal cycles operating at approximately 40 days with spatial loadings co-located with the rainfall mode except in the south. Vegetation has a 1-2 dekad lagged correspondence with rainfall. It is hypothesised that an earlier rainfall event and subsequent 'greening' results in an evapo-transpiration flux that affects the next rainfall event. Boundary layer structure is studied along west-east transects and compared with vertical and horizontal fluxes of moisture. The sensitivity of vegetation to rainfall is most pronounced over the eastern Kalahari due to high evaporative losses during the intervening dry spells.
Since vegetation-rainfall interactions can be a result of moisture convergence, surface evapotranspiration, or convection, this study focuses on the vertical moisture flux from the land surface and attempts to separate this from the dominant large-scale horizontal moisture convergence through a budget analysis. The boundary layer deepens but does not diminish even in the absence of external forcing. A most significant finding is the agreement between vegetation and low level velocity potential. A sharp increase in vegetation appears to draw airflow towards itself, in a self sustaining way.
Results of this study contribute to the understanding of land-atmosphere interactions and their role in the climate system of southern Africa.
Description:
Thesis for the Master of Science degree , Department of Geography and Environmental Studies, University of Zululand, 2005.
