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Carl Schreck, III & Fredrick Semazzi
International Journal of Climatology, June 2003
The primary objective in this study is to investigate the recent variability of the Eastern Africa climate. The region of interest is also known as the Greater Horn of Africa (GHA), and comprises the countries of Burundi, Djibouti, Eritrea, Ethiopia, Kenya, Rwanda, Somalia, Sudan, Uganda, and Tanzania.
The analysis was primarily based on the construction of empirical orthogonal functions (EOFs) of gauge rainfall data, and CPC Merged Analysis of Precipitation (CMAP) data derived from a combination of rain gauge observations and satellite estimates.The investigation is based on the period of 1961-2001 for the short rains season of Eastern Africa of October through December. The EOF analysis was supplemented by projection of NCEP wind data on to the rainfall eigenmodes to understand the rainfall-circulation relationships. Furthermore, correlation and composite analyses have been performed with the Climatic Research Unit (CRU) globally averaged surface temperature time series to explore the potential relationship between the climate of Eastern Africa and global warming (GW).
The most dominant mode of variability (EOF1) based on CMAP data over Eastern Africa corresponds to El-Nino Southern Oscillation (ENSO) climate variability. It is associated with above normal rainfall amounts throughout the entire region except Sudan. The corresponding anomalous low-level circulation is dominated by easterly inflow from the Indian Ocean and to a lesser extent the Congo tropical rain forest into the positive rainfall anomaly region that extends across most of Eastern Africa. The easterly inflow into Eastern Africa is part of diffluent outflow from the maritime continent during the warm ENSO events. The second Eastern Africa EOF is associated with GW. In distinct contrast from the ENSO mode pattern, the GW mode is characterized by positive rainfall anomalies over the north-eastern sector of Eastern Africa and opposite conditions over the south-western sector. This GW rainfall mode eluded detection in previous studies that did not include recent decades of data, because the signal was still relatively weaker. The wind projection onto this mode indicates that the primary flow that feeds the positive anomaly region over the northern part of Eastern Africa primarily emanates from the rainfall deficient southern region of Eastern Africa and Sudan.
Comparison between the Eastern Africa and global EOF ENSO modes over Eastern Africa reveals important dependency of the results on the size of the domain over which the eigemode analysis is performed. The EOF analysis confined to Eastern Africa is not capable of separating the variability associated with ENSO and the Indian Ocean Zonal Mode (IOZM) since the corresponding rainfall spatial anomaly patterns are similar. The global EOF analysis can easily distinguish between these two modes of climate variability because of the large differences between them outside the Eastern Africa region.