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Understanding precipitation variability over Africa is important because this region experiences rainfall events that influence agriculture and economic infrastructure, and threaten lives regularly. This study aims to advance our understanding of African precipitation across different timescales, from diurnal cycle to interannual variations, using observations, reanalyses, and regional climate model simulations. First, we evaluate the role of MCSs in the total rainfall distribution as a function of season from a climatological perspective (1998-2014) over sub-Saharan northern Africa and examine how the diurnal cycle of rainfall changes with season. The percentages of the full TRMM precipitation delivered by MCSs have meridional structures in spring, fall and winter, while the percentages are homogenous in summer (>80%). The diurnal peaks are classified into three categories: single afternoon peak, continuous afternoon peak, and nocturnal peak. The continuous afternoon peak combines rainfall from two system types –one locally-generated and one propagating. The seasonality of the diurnal cycle is related to the seasonality of MCS lifetimes, and propagation speeds and directions. Second, a low-level jet, the West African westerly jet, is investigated over the West African coast. In the western Sahel (0°-10°W, 8°-18°N), the moisture flux associated with the jet is stronger than that associated with the southerly West African monsoon flow from July 5 to August 20 (45 days). The moisture budget analysis reveals that the seasonal evolution of the rainfall in this analysis region is associated with zonal moisture convergence related to changes of the jet. Finally, three sub-regions of the Indian Ocean in which SSTs significantly influence the equatorial East African short rains on interannual timescales are identified, and the physical processes of this influence are studied using regional climate model simulations. SSTAs in the western Indian Ocean exert a stronger influence on the short rains than central and eastern Indian Ocean SSTAs both in terms of the coverage of significantly-changed precipitation and the magnitude of the precipitation response. The mechanisms of this influence are diagnosed using atmospheric moisture budget and moist static energy analyses, with reference to Kelvin and Rossby wave generation as in the Gill model, but in the presence of complicated topography and nonzero background flows.

Understanding precipitation variability over Africa is important because this region experiences rainfall events that influence agriculture and economic infrastructure, and threaten lives regularly. This study aims to advance our understanding of African precipitation across different timescales, from diurnal cycle to interannual variations, using observations, reanalyses, and regional climate model simulations. First, we evaluate the role of MCSs in the total rainfall distribution as a function of season from a climatological perspective (1998-2014) over sub-Saharan northern Africa and examine how the diurnal cycle of rainfall changes with season. The percentages of the full TRMM precipitation delivered by MCSs have meridional structures in spring, fall and winter, while the percentages are homogenous in summer (>80%). The diurnal peaks are classified into three categories: single afternoon peak, continuous afternoon peak, and nocturnal peak. The continuous afternoon peak combines rainfall from two system types –one locally-generated and one propagating. The seasonality of the diurnal cycle is related to the seasonality of MCS lifetimes, and propagation speeds and directions. Second, a low-level jet, the West African westerly jet, is investigated over the West African coast. In the western Sahel (0°-10°W, 8°-18°N), the moisture flux associated with the jet is stronger than that associated with the southerly West African monsoon flow from July 5 to August 20 (45 days). The moisture budget analysis reveals that the seasonal evolution of the rainfall in this analysis region is associated with zonal moisture convergence related to changes of the jet. Finally, three sub-regions of the Indian Ocean in which SSTs significantly influence the equatorial East African short rains on interannual timescales are identified, and the physical processes of this influence are studied using regional climate model simulations. SSTAs in the western Indian Ocean exert a stronger influence on the short rains than central and eastern Indian Ocean SSTAs both in terms of the coverage of significantly-changed precipitation and the magnitude of the precipitation response. The mechanisms of this influence are diagnosed using atmospheric moisture budget and moist static energy analyses, with reference to Kelvin and Rossby wave generation as in the Gill model, but in the presence of complicated topography and nonzero background flows.

The large-scale climate system is driven by imbalances of the reservoirs of heat contained in the world oceans. The transport and redistribution of this heat is determined in part by nonlinear mesoscale eddies (radii ~50-200 km), as well as by planetary waves whose widths approach the size of mesoscale eddies away from the equator. In this dissertation, new analysis techniques are developed and implemented to assess oceanic phenomena in two regions: mesoscale eddy-mean flow interaction in the Kuroshio Extension (KE) region, and the effects of coastal Kelvin waves and mesoscale eddies in the Indian Ocean south of Java. In the KE region, a jet-following coordinate reference frame is used to quantify the contributions of eddies to the vorticity budget along the KE jet in a strongly eddying ocean general circulation model simulation, the Parallel Ocean Program (POP). The jet reference frame preserves synoptic gradients of the jet that are not accurately represented in multi-year Eulerian means. This analysis found that eddies tend to accelerate the jet just downstream of crests in the topographically-induced meanders, implying an intensification of frontal gradients in these areas. In the Indian Ocean, a method involving projections of harmonic basis functions onto altimetry-derived sea level anomaly (SLA) is used to estimate Kelvin wave activity along the equatorial-coastal waveguide. The resulting Kelvin wave coefficient presents a more accurate representation of Kelvin wave activity than that from raw SLA. Moreover, values of the Kelvin wave coefficient in April-June are a robust predictor of positive Indian Ocean Dipole (pIOD) event development later in the calendar year. Finally, a temperature budget using a strongly eddying POP simulation isolates the specific contributions of mesoscale processes south of Java. It shows that Kelvin waves and local wind forcing both contribute substantially to anomalous cooling during pIOD years, while mesoscale eddies have a modest warming effect. These results suggest that mesoscale processes in the study regions have an important influence on the ocean's structure and can trigger a climate response; use of the new analysis techniques may help quantify the effects of mesoscale eddies and planetary waves elsewhere in the oceans.

Abstract: The Nile River, the primary water resource and the life artery for the downstream countries, Egypt and Sudan, exhibits strong seasonal fluctuations. The Upper Blue Nile basin (UBNB), the most significant tributary of the Nile, contributes more than half of the Nile’s streamflow. Prompted by the lack of knowledge on the nonstationarity of hydro-climatic processes in the Ethiopian Highlands (EH), and the Oceanic Sea Surface Temperature (SST), this thesis employed the nonstationary techniques of Wavelet principal component analysis (WPCA) and coherence analysis to identify the spatial, temporal and frequency variability regimes of these hydro-climatic processes. A fully distributed, physically-based model, which is a modified version of the Interactions Soil-Biosphere-Atmosphere (MISBA), and a lumped- conceptual SAC-SMA model, was used to model the UBNB streamflow. To study the potential effect of climate anomalies on the UBNB, years of rainfall/temperature data, when climate anomalies are active, were re-sampled and used to drive MISBA and SAC-SMA. An artificial neural network calibrated by a genetic algorithm (ANN-GA) model, is developed to forecast the seasonal rainfall of UBNB through teleconnection with selected sectors of SST. Results show that seasonal rainfall predicted by ANN-GA agrees well with the observed rainfall data of UBNB. The Valencia and Schaake model was used to disaggregate the forecasted seasonal rainfall to weekly rainfall, which was found to reasonably capture the UBNB observed weekly rainfall characteristics. ANN-GA was developed to directly forecast the UBNB seasonal streamflow from seasonal oceanic SST and then disaggregated to weekly streamflow. To improve the streamflow forecast, we combined the forecasted seasonal rainfall, in addition to the SST predictors. Results indicate that forecasts based on climate indices alone possess considerable skill (correlation of 0.66) with up to four months lead time, while combining the rainfall and SST as predictors provides better results (correlation of 0.83). The analysis of nonstationary energy helped to determine the effects of global oceanic anomalies on the rainfall/streamflow of the UBNB. Knowledge on these effects on UBNB will be useful to the planning and water resources management of the Nile River, especially under the impact of both climate variability and impending droughts.