Climate Prediction Center Merged Analysis of Precipitation (CMAP) data was used to characterize the recent climate variability of the Greater Horn of Africa (GHA) for the October, November, December (OND) season from 1979-2001. Investigation of the spatial and temporal variability of the climate was diagnosed using Empirical Orthogonal Function (EOFs). EOF analysis was performed on monthly and pentad CMAP data. We focus on the recent GHA climate variability because of the enhanced temporal and spatial distribution of rainfall data from satellites and possible impacts associated with global warming.

It is shown that there are two significant modes of variability during the OND season for GHA. The dominant mode of variability characteristically represents the interannual variability of ENSO with zonal SST anomalies over the Indian Ocean, commonly termed the Indian Ocean Zonal Mode (IOZM). The loading pattern and time series is consistent with previous studies in which the majority of the GHA experiences positive rainfall anomalies during warm ENSO events. The intra-seasonal analysis illustrates warm and cold ENSO events have larger intra-seasonal fluctuations compared to normal years. Compositing the intra-seasonal variations during the major El Nino events of 1982/1997 and the positive IOZM of 1994 reveals the entire season is predominantly wet despite the large deviations between pentads. Whereas during La Nina events of 1988, 1998, 1999 and 2000, the season can fluctuate between wet and dry spells. As for the second mode of variability, a decadal trend mode is established with a dipole loading pattern. The loading pattern suggests a wetter northern GHA and a drier southern GHA. The trend has been compared and found to be similar to the increasing global warming signal of the Hadley Climate Research Unit global surface temperature analysis. We also find the spatial pattern and trend to be well correlated with the Tropical South Atlantic Index (TSAI). However, there is the possibility for the interannual variability of the trend to be out of phase with the TSAI. We suggest the decadal trend is associated with a low background frequency in which the moisture capacity increases as the Atlantic Ocean warms. As for the interannual variations, a significantly warm south Atlantic can alter the intensity or direction of the wind plausibly explaining the out of phase relationship between the GHA rainfall and the SST over the tropical south Atlantic. The intra-seasonal variability of the trend varies throughout the season with strongest trend during the month of October. October is the onset month of the rainy season for much of GHA. We suggest that the increasing trend is possibly related to an early onset of the rainfall over northern GHA where the loadings are positive, and a decreasing rainfall trend over southern GHA possibly related to a late start in the seasonal rainfall.