Large Scale Influences on Interannual Variability Physical Mechanisms Potential Predictability and Prediction of Short Rains over East Africa

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Addis Ababa University


The climate of the East Africa exhibits marked interannual uctuations that provoke droughts or ooding which lead to enormous impact on socio-economic activities over the region. Therefore, understanding of the mechanisms that produce this variability and developing both dynamical and statistical approaches for extended range forecast and projected information on future climate is of great importance. In the rst part of this study observational datasets and a series of Sea Surface Temperature (SST) forced Atmospheric General Circulation Model (AGCM) ensemble simulations for the whole 20th century are analyzed to investigate the physical mechanism and potential predictability of East African short rains variability. It is found that there is substantial skill in reproducing the East African short rains variability given the SSTs are known. Consistent with recent previous studies it is found that the Indian Ocean (IO) and in particular the western pole of the Indian Ocean dipole (IOD) play a dominant role for the prediction skill, whereas SST outside the IO play a minor role. The physical mechanism for the western IO in uence on East African rainfall in the model is consistent with previous ndings and consists of a gill-type response to a warm (cold) anomaly that induces a westerly (easterly) low-level ow anomaly over equatorial Africa and leads to moisture ux convergence (divergence) over East Africa. On the other hand a positive El Ni~no-Southern Oscillation (ENSO) anomaly leads to a spatially non coherent reducing e ect over parts of East Africa, but the relationship is not strong enough to provide any predictive skill in our model. The East African short rains prediction skill is also analyzed within a model derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the models potential prediction skill. Low frequency variations of the prediction skill are mostly related to SSTs outside the IO region and likely due to an increased interference of ENSO with the IO in uence on East African short rains after the mid-70s climate shift. Based on results from a series of AGCM experiments, the performance of dynamical seasonal forecast systems are evaluated for the prediction of SSTAs over tropical IO and short rains anomalies over equatorial East Africa. The evaluation is based on observational datasets and the Asia-Paci c Climate Center (APCC) Ocean-Atmosphere- Land coupled Multi-Model Ensemble (MME) retrospective forecasts (hindcasts) using common years for all models from 1982 to 2005. The coupled climate models ensemble reproduces seasonal characteristics of low level wind, the spatial distribution of SON mean rainfall and seasonal climate variations over equatorial East Africa with further improvement in MME mean. Ensemble mean of individual coupled models and MME mean also show statistically signi cant skill in forecasting sea surface temperatures anomalies (SSTAs) over the western and eastern parts of the tropical IO, giving signi cant correlation at 99% con dence level for IOD. Moreover, ve out of ten coupled models and MME mean show statistically signi cant skill in predicting equatorial East Africa short rains. The delity of hindcasts is further measured by Anomaly Correlation Coe cient (ACC) and four models as well as MME mean show signi cant skill over East Africa. It is shown that the reproduction of the observed variability in the East African region is mainly due to a realistic relationship of East African rainfall with the IOD. Overall, the skill of the dynamical models is attributed to the fact that slowly evolving SSTs are the primary source of predictability, and to the fact that coupled climate models produce skillful predictions of SON SST anomalies over tropical IO. This study therefore provides insight into interannual rainfall variability and predictability over East Africa, in view of tropical Indian Ocean-Atmosphere climate patterns and underlying mechanisms. In addition, the information on coupled forecast systems will open the possibility of using readily available seasonal forecasts as skillful predictions of equatorial East Africa short rains. On the whole, the results found in this study will feed into real-time monitoring and forecasting at seasonal to interannual timescales to enhance early warning and disaster preparedness activities and minimize the impacts of climate-related catastrophes that are prevalent in the region



Physical Mechanisms