TOWARDS A BETTER UNDERSTANDING OF THE PALEOCLIMATIC REGIMES OVER THE AFRICAN SAHARAN DESERT: INFERENCES FROM REMOTE SENSING AND STABLE ISOTOPE DATA

The isotopic composition of modern precipitation (from IAEA stations) over North Africa is generally enriched (e.g., average δD ± 1δ: Algeria: - 17.05‰ ± 8.81; Tunisia: -24.2‰ ± 4.23; Libya: -17.7‰ ± 2.72; Egypt: -15.46‰ ± 3.45) compared to fossil groundwater across North Africa that is progressively depleted from west to east (e.g., Morocco: δD = -20 to -30‰; Algeria δD = -30 to -50‰; Egypt δD = -72 to -81‰) suggesting that precipitation during previous wet climatic periods (450,000 yr B.P to 10,000 yr B.P) resulted from wind regimes different from those prevailing nowadays. A few exceptions to the modern isotopic values were noted from the IAEA stations in Egypt, where the average δD values for five months (March 1982, December 1987, December 1991, March 2002 and April 2002) collected from Sidi Barrani, Cairo, Rafah, and El-Arish IAEA stations all showed isotopic depletions (δD = -43.7‰, to -56‰) similar to those of the fossil waters in the Western and Eastern Deserts of Egypt. The origin and nature of these paleoclimatic regimes were investigated by examining in a GIS environment the direction of propagation of clouds throughout each of the identified five events and the isotopic composition of the rain precipitating from these events. Rectified Infra-red EUMETSAT image data that were acquired every 30 minutes by the Meteosat First Generation Satellites (MSG) and Atmospheric Motion Vectors (AMV) showing wind vectors at various heights were brought to the same projection (WGS84) and scale (1:50,000,000). Individual images were georeferenced to the global MODIS for Africa as a base scene using ESRI’s ArcGIS 9.2 commercial software package and applying GCP-based georeferencing methodologies. For each of the five events, cloud progression directions were extracted in a GIS environment from (1) shape files (8 to 14 scenes were processed for each of the five events) displaying polygons representing digitized cloud boundaries in individual infrared scenes, and (2) wind direction vectors. This study supports models that attribute previous wet climatic periods to the intensification of paleowesterlies and are inconsistent with others that relate these wet climatic periods to intensification of monsoons.