Abstract
The West African Summer Monsoon (WASM) supplies around 362 million people with 80% of their total annual rainfall, which is necessary for agriculture, hydroelectric power generation, human consumption, and industrial purposes. Given the population density of this region, a substantial fraction of Africa’s population is therefore critically sensitive to changes and variability in monsoon rainfall, as well as potential future changes under 21st-century warming. This study focuses on understanding the mechanisms controlling summer monsoon rainfall over West Africa, its recent changes and variability, and possible future changes.In summer, three distinct tropospheric circulation features are notable for controlling WASM rainfall: the upper-level Tropical Easterly Jet (TEJ), mid-level African Easterly Jet (AEJ), and low-level westerly flow. This research has revealed that these dynamical features controlling the WASM exhibit an obvious contrast during typical wet and dry years over West Africa; a weaker (stronger) AEJ (TEJ) is observed during wet years. Also, a well-developed and deep low-level westerly flow at about 850 hPa is evident in wet years, while an obvious reversal is observed in dry years. Considering this, the main regions of the two easterly jet streams and low-level westerly wind were proposed for objectively defining an effective WASM index (WASMI). The WASMI defined herein can reflect variations in June – September rainfall over West Africa. The index exhibits both interannual and decadal variability, with high (low) values occurring in the 1950s–1960s (1970s–1980s), suggesting that the WASMI is skilled in capturing the respective wet and dry episodes over the region. Also, the WASMI is significantly correlated with summer monsoon rainfall, which further affirms that it can indicate not only the variability but also the intensity of WASM rainfall. Hence, the WASMI would be a good tool for studying and monitoring the WASM.
Analysis of long-term trends in WASM rainfall provide insight into the possible increase in rainfall that has occurred in the last two decades. However, the possible mechanism responsible for this recent increase has not previously been described in detail. Based on observations and satellite and reanalysis data, a consistent increase in WASM rainfall over the central-eastern Sahel (CES) subregion of West Africa was observed during the period 1980–2012. A moisture budget analysis was employed to assess the controlling mechanisms and to quantify the contributions of different factors to this increasing trend. Results reveal that the majority of the increased rainfall over the CES is balanced by increased vertical moisture advection. Furthermore, the vertical moisture advection term was decomposed into dynamic, thermodynamic, and nonlinear components. The dynamic component exhibits the largest contribution to the trend, followed by the thermodynamic component. These components are modulated by strong convergence in the middle and lower troposphere and increased near-surface specific humidity from a remote source.
Finally, future changes in the WASM rainfall annual cycle, mean rainfall, extremes, variability, and atmospheric circulation were investigated using multi-model ensemble mean (EnsMean) simulations of the best-performing Coupled Model Intercomparison Project phase 5 (CMIP5) models as well as high-resolution model output from the Coordinated Regional Climate Downscaling Experiment (CORDEX). In a warmer climate, the annual cycle exhibits a decrease (increase) in pre-monsoon (post-monsoon) rainfall over the region, especially over the Sahel subregion. Also, summer rainfall is projected to increase in most parts of West Africa, but a decrease is expected in the northwest, around Senegal. A future increase in extreme rainfall events (R95pTOT), consecutive dry days (CDD), and simple rainfall intensity (sdii) are expected. Furthermore, WASM rainfall variability is projected to increase by about 10 – 24% over the entire region, and it is remarkably robust over a wide range of timescales. Changes in the monsoon circulation are opposite in the lower and upper troposphere, with the tipping point at 400 hPa. Specifically, a projected strengthening and northward shift are observed in the lower and mid-troposphere, while weakening is evident in the upper troposphere. A diagnosis based on a moisture budget equation reveals that the robust positive response of West African summer monsoon rainfall to global warming is largely explained by the enhancement of moisture convergence and surface evaporation. The results presented in this thesis can be used as a scientific basis for regional development and adaptation strategies over West Africa.
| Date of Award | 24 Jul 2019 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Wen ZHOU (Supervisor) |
Cite this
- Standard