Relationships between Rwandan seasonal rainfall anomalies and ENSO events

Abstract

This study aims primarily at investigating the relationships between Rwandan seasonal rainfall anomalies and El Nino-South Oscillation phenomenon (ENSO) events. The study is useful for early warning of negative effects associated with extreme rainfall anomalies across the country. It covers the period 1935–1992, using long and short rains data from 28 weather stations in Rwanda and ENSO events resourced from Glantz (2001). The mean standardized anomaly indices were calculated to investigate their associations with ENSO events. One-way analysis of variance was applied on the mean standardized anomaly index values per ENSO event to explore the spatial correlation of rainfall anomalies per ENSO event. A geographical information system was used to present spatially the variations in mean standardized anomaly indices per ENSO event. The results showed approximately three climatic periods, namely, dry period (1935–1960), semi-humid period (1961–1976) and wet period (1977–1992). Though positive and negative correlations were detected between extreme short rains anomalies and El Nino events, La Nina events were mostly linked to negative rainfall anomalies while El Nino events were associated with positive rainfall anomalies. The occurrence of El Nino and La Nina in the same year does not show any clear association with rainfall anomalies. However, the phenomenon was more linked with positive long rains anomalies and negative short rains anomalies. The normal years were largely linked with negative long rains anomalies and positive short rains anomalies, which is a pointer to the influence of other factors other than ENSO events. This makes projection of seasonal rainfall anomalies in the country by merely predicting ENSO events difficult.