ENSO and interannual rainfall variability in Uganda: implications for agricultural management

Abstract

The El Niñ o-Southern Oscillation (ENSO) is correlated with the short rainy season (September -December) in East Africa. Seasonal climate forecasts made on this basis are being disseminated in the hope that this information will be useful in regional or even local planning and resource management. In order to improve the likelihood of success in using regional forecasts in Uganda, particularly in the agricultural sector, climate analysis was performed at the sub-regional level with distinctions being made between unimodal (short season peak in August) and bimodal (short season peak in November) rainfall zones. Monthly climate data from 1931 to 1960 were available for 33 sites. Averaging across all stations, it is shown that Pacific Ocean NINO3 region sea-surface temperatures (SSTs) from July to September (JAS) are significantly correlated with both the concurrent August -September (AS) rainfall (r= −0.75) and the following November-December (ND) rainfall (r= 0.57), but with opposite signs. When station data are separated into uni-and bimodal sites, it becomes clear that the importance of ENSO events is different in the two zones. In the unimodal zone, El Niñ o events are associated with a depression of the August peak in rainfall, but a lengthening of the season, potentially providing an opportunity for growing later-maturing crops. At bimodal sites, there is very little impact in August but November rainfall is enhanced in El Niñ o years and depressed in La Niña years. Given a forecast of ENSO, the primary strategies that will be useful in farm management will differ by rainfall zone and will revolve around the choice of crop or cultivar and the timing of planting in order to make optimal use of the growing period.

Analysis of mid-twentieth century rainfall trends and variability over southwestern Uganda

“…On average the MAM and SON totals accounted for 76 % of the mean rainfall in the water management area. Evidence of this seasonality was also reported by Ogallo (1984), Basalirwa (1995), and Nicholson (1996) and later used by Phillips and McIntyre (2000) to delineate Uganda into sites of uni-and bimodal rainfall distribution patterns. Beltrando (1990) attributed this distribution pattern to the seasonal movements of the ITCZ.…”

“…These features introduce significant modification in the general wind patterns over the region, coupled with convection processes that generate a climatic pattern, which is complex and changes rapidly over short distances (Nicholson 1996). These climatic processes result into a bimodal pattern of rainfall, with the primary rainy season occurring in March-May and a shorter secondary rainy season occurring in October-December (Phillips and McIntyre 2000).…”

“…For example, on the temporal distribution of rainfall in the Lake Victoria basin, Kizza et al (2009) reported that the Lake Victoria basin experienced a predominantly positive trend over the twentieth century. Phillips and McIntyre (2000) in-vestigated the subregional significance of El Niño-Southern Oscillation (ENSO) events in Uganda, distinguishing between unimodal and bimodal rainfall zones from 1931 to 1960 for 33 sites. They detected a significant correlation between ENSO events and rainfall seasons.…”

The nature of rainfall in the main drainage sub-basins of Uganda

“…This kind of rainfall pattern over Uganda is related to the movement of the ITCZ, which brings rainfall approximately 1 month after the sun's migration north or south resulting in a bi-modal rainfall pattern, with the primary rain season occurring in MAM, and a shorter secondary rainy season occurring in October-November-December (OND) (Nicholson 1996, Kizza et al 2009). Phillips and Mcintyre (2000) attribute moisture during OND to the northeasterly winds originating in the Indian Ocean. However, the northern sub-basins, which have peaks in August, receive moisture from the Congo/ Zaire basin to the southwest.…”

“…Nicholson (1996) also revealed that interannual variability of rainfall is remarkably coherent throughout most of eastern Africa, especially during the October-December season, with a dominant time scale of variability of 5-6 years. Phillips and Mcintyre (2000) investigated Pacific El Niño-Southern Oscillation (ENSO) and inter-annual rainfall variability in relation to agricultural management in Uganda. They discovered that Pacific Ocean NINO3 region sea-surface temperatures (SSTs) from July to September are significantly correlated with both August-September and November-December rainfall, but with opposite signs.…”