Assessment of spatial and temporal characteristics of the December-February seasonal rains over Uganda

Abstract

Uganda’s social economic activities are closely influenced by the seasonal rains since the Country is heavily rainfall dependant. Therefore, the performance of the rains is key to the Country’s success or failure of the economy. Uganda experiences two main rainfall seasons namely March-April-May (MAM) and October-December (OND). The Country also experiences two dry seasons that of June-July (JJ) and December-January-February (DJF). However the dry season of DJF is the longest and driest over most parts of the Country. The main objective of this study was to assess the spatial and temporal characteristics of the unusually extreme DJF rainfall that sometimes occur in Uganda with far reaching socio-economic implications. The study used daily and monthly datasets of rains for the main period 1961-2013 and that of the sea surface temperatures (for Pacific and Indian Ocean), cyclones and winds for the South Western Indian Ocean (SWIO). The data was used to make various analyses which included percent of normal, distribution of wet spell lengths, correlation analysis. The results from the study revealed a positive trend in the frequency of unusually wet DJF seasons, in the recent years which may be associated with a signal of Climate Change over Uganda. The occurrences of the unusually wet DJF seasons were widespread over the Country when the El Niño and positive IOD conditions prevailed at the same time. The lengths of the wet spells were mostly frequently of 1-2 days while for those more than two days reduced following an exponential distribution. The study further revealed that for Uganda, the unusually wet DJF seasons occurred regardless whether the Cyclones in the SWIO were present or not. The anomalous wind patterns at 700-850mb over Uganda, during the unusual wet spells comprised of westerlies blowing from the neighboring country Democratic Republic of Congo. The wind anomalies at 200mb over Uganda were mainly south easterlies. The directions of the anomalies at the various heights over Uganda were the same regardless of whether the Tropical Cyclones over the SWIO are present or not. The results from this study are useful in planning and managing the risks and disasters associated with Climate Change in order to enhance community resilience against the unusual rainfall shortages or floods, for purposes of sustaining National Development of the Country.