Royal Dutch Meteorological Institute; Ministery Of Infrastructure And The Environment

 
Research
Climate change in Africa
East Africa
23 August 2007
East Africa is defined as the area astride the Equator from 10°S to ~5°N and eastward of 28°E. Over much of Kenya, Uganda, Rwanda, Burundi and southern Somali there are indications for an upward trend in rainfall under global warming. Wet extremes (defined as high rainfall events occurring once every 10 years) are projected to increase during both the September to December (SOND) rain season and the March to May (MAM) rain season, locally referred to as the short- and long-rains, respectively. For reasons given by Mutai and Ward (2000), we omitted September in most of our analysis for the short-rains season. In general, a positive shift in the whole rainfall distribution is simulated by the models over most of east Africa during both rainy seasons. However, we place less emphasis on the MAM simulations as the signal-to-noise ratios in the model predictions during this season are relatively low.
Background information
Because East Africa lies astride the equator, much of the region experiences two rainy seasons occurring when the Inter-tropical convergence zone (ITCZ) traverses the region in its southward and northward migrations. A longer rainy season starts around March through to June, with the peak centred between March and May. The shorter rainy season runs from September and tapers off in November or December. However some areas along the Lake Victoria coast, experience a prolonged rainy season with significant rains received even during boreal summer as evidenced by a local mean rainfall maxima centred around Lake Victoria. Areas south of about 5°S have a single rain season with most rainfall received during austral summer.

The presence of large water bodies such as the Indian Ocean to the east, and Lake Victoria and Lake Tanganyika, as well as high mountains such as Mount Kilimanjaro and Mount Kenya induces localised climatic patterns in the region. In these analyses, homogeneous rainfall zones identified by Indeje et al (2000) have been adopted. Central Kenya (Zone 3) than most models' horizontal resolution. This area has therefore been omitted in the analyses.

Left: mean precipitation [mm/day] calculated from the CRU TS 2.1 1971-2000 climatology. Right: the selected rainfall regions in East Africa
Mean precipitation [mm/day] calculated from the CRU 1971-2000 climatology and the selected rainfall regions in East Africa
Projected changes in wettest events that occur once every 10 years on average
Short-rains (September-December)
In the warmer climate around 2100, the IPCC models show evidence of an increase in the intensity of extreme rainfall events in much of East Africa, notably in Kenya, Uganda, Rwanda, Burundi and southern Somali. During the short-rains, there are indications of the possibility of increases in excess of 50% in 10-year high rainfall events over the north of East Africa. In southern Tanzania the wettest rainfall events are projected to decrease by 0% to 20%.
Changes in the amount of rainfall around 2100 in 'short-rains' high rainfall events that occur once every 10 years. Shown are the 4 climate models that simulate the current climate most realistically.
GFDL CM2.1 10yr wet differenceMPI ECHAM5 10yr wet difference UKMO HadGEM1 10yr wet differenceGFDL CM2.0 10yr wet difference colourbar from -50% to +50% in steps of 10%
Long-rains (March-May)
Even during the 'long-rains' season, the IPCC models continue to simulate an increase in the 10-year highest rainfall events in large parts of East Africa. Over north-eastern Kenya and southern Somali during this season only HadGEM1 does not simulate large increases in the amount of rain in extremely wet seasons. Over southern Tanzania, most models give an indication of an increase in high rainfall events but this is not a major concern as this part of the region is predominantly dry during March-May.

It should be kept in mind that these climate models all severely underestimate the strength of the long-rains in the current climate, limiting the trust one can put in these projections.

Changes in the amount of rainfall around 2100 in 'long-rains' high rainfall events that occur once every 10 years. Shown are the 4 climate models that simulate the current climate most realistically.
GFDL CM2.1 10yr wet differenceMPI ECHAM5 10yr wet difference UKMO HadGEM1 10yr wet differenceGFDL CM2.0 10yr wet difference colourbar from -50% to +50% in steps of 10%
Projected changes in driest events that occur once every 10 years on average
Short-rains (September-December)
Simulated changes in low-rainfall extremes show that these events are becoming less severe in Burundi, Rwanda, Uganda, northern Kenya and southern Somali during the September to December season in the most realistic models (with the exception of the Rift Valley in HadGEM1). The simulated increase is far more than 50% in certain parts of the region. Noting that increases in both wettest and driest rainfall events have been found over the same areas, this shows an overall shift in the rainfall distribution, with floods becoming more likely than the opposite extreme.

In southern Tanzania, droughts are projected to increase in severity in all these models.

Changes in the amount of rainfall around 2100 in 'short-rains' lowest rainfall events that occur once every 10 years. Shown are the 4 climate models that simulate the current climate most realistically.
GFDL CM2.1 10yr dry differenceMPI ECHAM5 10yr dry difference UKMO HadGEM1 10yr dry differenceGFDL CM2.0 10yr dry difference colourbar from -50% to +50% in steps of 10%
Long-rains (March-May)
During March-May there is no consensus between the models on the likely changes in the severity of dry events. While some models show an increase in the severity of extremely low rainfall events in northern Kenya, others simulate a decrease over the same areas. Because of the inaccuracy of the model simulations of the 20th century climatology during this season, model projections of future climate during this season are judged to be unreliable at the moment.
Changes in the amount of rainfall around 2100 in 'long-rains' lowest rainfall events that occur once every 10 years. Shown are the 4 climate models that simulate the current climate most realistically.
GFDL CM2.1 10yr dry differenceMPI ECHAM5 10yr dry difference UKMO HadGEM1 10yr dry differenceGFDL CM2.0 10yr dry difference colourbar from -50% to +50% in steps of 10%