East African Rainfall Anomaly Patterns in Association with El Niño/Southern Oscillation

By applying harmonic analysis to El Niño–Southern Oscillation (ENSO) composites of the 6-month Standardized Precipitation Index (SPI) and rainfall anomalies for the 1900–1996 period, and based on the 90% confidence limits established from bootstrap resampling, it was found that ENSO responses in East African rainfall are region and season dependent, and the influence of El Niño is stronger and opposite that of La Niña. Among five regions of unique ENSO responses identified, northeastern (R4) and southern Tanzania (R5) seem to have the most consistent (in terms of vector coherence, percentage of variance extracted by the first harmonic, and SPI magnitude) ENSO responses. R5 experiences positive (negative) response under La Niña (El Niño) influence during January and June of the post-ENSO year. Southern Uganda and much of the Lake Victoria basin show some significant positive ENSO response for November, December, and January. The temporal and regional patterns of ENSO response periods were also analyzed using the index time series and boxplots on the 6-month SPI. Boxplots confirm a shift in the distribution of 6-month SPI between ENSO and non-ENSO affected seasons.