Influence of climate and atmospheric circulation changes on water balance of Mount Kenya and surroundings

Climate over Kenya is rather heterogeneous and exceptionally dry for a region located in the tropics. This is related to various large-scale drivers, such as Lake Victoria, the complex topography, and the vicinity to the ocean. In consequence water resources are scarce and several stakeholders depend on these. Hence, it is important to understand how precipitation amounts and patterns change under global warming. A special focus is on Mount Kenya, one of the most important freshwater towers in Kenya. To investigate these changes, we employ the Weather Research and Forecasting (WRF) model V3.8.1 to downscale a 30-year period for the present and the future climate, based on global climate simulations. The present period covers the years 1981–2010, and the future is run once for the mitigation scenario RCP2.6 and for the high-emission scenario RCP8.5 for the years 2071–2100.

Changes in precipitation and temperature are well noticeable in the region of Mount Kenya. The projection indicates an increase in precipitation for the two rainy seasons (March to May, and October to November), while precipitation is reduced in the dry season. Extreme precipitation around Mount Kenya shows increases in the future during the rainy season, whereby the two different scenarios show a similar increase in extreme precipitation. This result is a bit surprising and needs further investigation. As expected, temperatures are projected to increase over all of Kenya, and particularly along the slopes of Mount Kenya in all months. For temperature there is a clear difference in the warming between the two scenarios, as RCP8.5 shows a much stronger change in temperature than RCP2.6. The summit of Mount Kenya reaches temperatures in the future that today are found at an elevation of around 3,200 m above sea level (a.s.l.). This warming can substantially affect the endemic vegetation along the slopes of Mount Kenya. Assuming that the tree line is limited by temperature and not precipitation, as the latter is abundant, it could move from around 3,000 m a.s.l. up to 3,700 m a.s.l. The strong increase in temperature further affects the remaining glacier, which is currently an important water storage during dry months. The projected increase in precipitation over entire Kenya will therefore increase water availability and reduce fire danger. Nevertheless, the combined increase in temperature and precipitation could affect human and animal wellbeing, as heat stress may be increased.

All these results are based on a single regional and global climate model. Preliminary results indicate that the rainy season is clearly underestimated in the present simulation, compared to simulations obtained by a downscaling of the reanalysis ERA5. This indicates that important components of the atmosphere are not correctly captured by the model. These could include land-atmosphere interactions, misrepresentation of land cover, biases in sea surface temperatures and related changes in the atmospheric circulations. Thus, the atmospheric circulation and interactions with the land surface have to be assessed in further studies.