Systems associated with wet spells over Kenya during the usually dry December-February season

Abstract

Most socio-economic activities in Kenya are rain-dependent with the occurrences of floods and droughts inflicting miseries at many locations year after year. Most of the rains in Kenya are experienced within two major rainy seasons every year and are centred on March-May and October-December. A third rainfall peak is observed at some locations in western Kenya in June-August. The months of late December to early February are however dry in most locations across the country. The main objective of this study was to investigate the climate systems that may be associated with extremely wet conditions that are sometimes observed during the usually dry December-February (DJF) season in Kenya. The datasets used in this study included monthly and daily rainfall data for 32 seasons from DJF 1981/82 to 2012/13. Other datasets are wind downloaded from European Centre for Medium range Weather Forecasting (ECMWF), historical tropical cyclone (name, track, and dates they occurred), Sea Surface Temperature downloaded from National Oceanic and Atmospheric Administration (NOAA), and Madden Julian Oscillation (MJO) proxy values from Bureau of Meteorology (BoM) website. The data were subjected to several analyses including percent anomaly and correlation analysis. The results from the study indicated that during DJF, wet conditions are observed during warm El Niño Southern Oscillation (ENSO) seasons regardless of the Indian Ocean Dipole (IOD) phase though the intensity increases during a positive IOD phase. The intraseasonal rainfall characteristics showed that although wet conditions spread throughout the DJF season, the highest amounts of rainfall is received during specific dekads. For example, during 1989/90 and 2012/13 DJF seasons, the rainfall received during the third dekad of December accounted for 61% and 70% of the long term mean respectively. These peak rainfall occurrences were associated with tropical cyclones occurring in the South West Indian Ocean. This study further observed that MJO’s contribution to rainfall during DJF of the ENSO neutral seasons is insignificant. Results from this study provided information that can be used in water use planning through water harvesting for irrigation, disaster risk reduction by incorporating tropical cyclones in seasonal weather forecasting, among others in support of sustainable livelihoods and national development.