The physical and dynamical processes that control, and at times constrain, climate-hydrology connections over the Greater Horn of Africa (GHA) operate on top of highly variable and constantly changing land use and vegetation cover conditions. The central hypothesis of this project is that key mechanisms and drivers of climate-hydrology-land use connections and associated hydro-climatic variability and changes over the GHA can only be well understood through an integrated process-based approach, employing a dynamically coupled climate-hydrology modeling system. Under this support, multiple observation datasets will be utilized to investigate historical climate events, especially droughts. A comparison between an ensemble of coupled Weather Research and Forecasting Model (WRF) simulations and a suite of Coordinated Regional Downscaling Experiments (CORDEX) model simulations will be undertaken. The role of global SST anomalies, change in Congo Air Mass, and changes in land use/cover characteristics in extreme precipitation (droughts/floods) will be examined.
Based on CMIP5 ensemble model analysis of historical simulations (1981-2010) our research showed that duirng MAM season, especially over southern Horn of Africa, the number of extremely wet days varied between 10-40 days with a bimodal peak density of about 20 and 30 days. The density function distribution of wet days was also slightly right-skewed, thus showing a tendency of less extremely wet days during the season. Using thresholds of 5th and 95th percentiles to define extremely dry and wet conditions,respectively, our research however showed no significant projected changes in the key components of the hydrological cycle ( E and P) by the middle of the 21st century, in both RCP4.5 and 8.5 scenarios. This is mostly consistent over the southern parts of the Horn of Africa. Over the northern parts, a slight decreasing trend in precipitation is dominant, with P-E is mostly negative indicating relatively drier conditions are projected to occur over these regions by the middle of the 21st century. Finally, how precipitation pattern will likely change in future over most parts of the Horn of Africa tend to depend significnatly on the balance between P and ET and the critical processes driving that balance or imbalance. Over the Horn of Africa our research showed there is generally a projected surplus of mositure flux(E>P) during the transition between the predominantly bimodal rainfall regime ( MAM and OND seasons), based on velocity potential anaysis of both RCP4.5 and RCP8.5 scenarios from an ensemble of six CMIP5 models . In particular there is projected moisture surplus during DJF over northern parts and during JJAS over the southern and equatorial regions. On the other hand, there is a projected moisture flux deficit especially over equatorial and southern regions during MAM and OND seasons which seem to be as a result of anomalously wet conditions largely occasioned by other local and/or regional moisture sources. Over the northern regions analysis of velocity potential display anomalous moisture influx from the Congo forest, Gulf of Guinea and the Mediterranean that tend to contribute significantly to the anomalous precipitation projected over these areas. Over the southern parts of the Horn of Africa the projected orientation of the Indian Ocean Walker-type of circulation during DJF implies that Indian Ocean is apparently the primary source of moisture. However, there is a projected decrease in mositure flux into the region during MAM season which seems to be a secondary response to increased precipitation over the Congo Basin, which leads to mositure deficit especially over equatorial and parts of southern Horn of Africa due to predominantly northwest transport of mositure into the Congo Basin. .
