Special Lecture: Response of Regional Climate to Global Forcing: The Role of Synoptic-scale Weather Systems in Shaping Climate Change Over West Africa
Synoptic-scale weather systems known as African easterly waves (AEWs) modulate rainfall variability in West Africa, mobilize and transport Saharan dust globally, and serve as the precursors to over 80% of the most intense (category 3 and above) Atlantic hurricanes. Given relationships between AEWs and the distribution of temperature and moisture over West Africa observed in the current climate, changes in climate conditions over West Africa in response to global warming are expected to modify the characteristics of AEWs in the future. In this talk, I will discuss the role of AEWs within the climate system of West Africa, and explore ways in which we can use our physical understanding of AEWs to elucidate the response of regional climate change in West Africa to enhanced greenhouse gas forcing. With the use of a suite of numerical modeling techniques, including global and nested high-resolution climate models, I first identify the critical processes that are necessary to accurately simulate AEWs and their relationship with rainfall. Results show that model differences in the simulation of AEWs are largely driven by the closure and trigger assumptions employed within each model’s convective parameterization scheme. This finding highlights the physical mechanisms responsible for model biases in variables such as wind speed and precipitation, and can be used to constrain future projections of climate in West Africa by identifying the sources of divergence among climate model projections. I then use an ensemble of climate model simulations to quantify the response of AEWs to projected enhanced greenhouse gas forcing. AEWs exhibit a robust increase in strength in response to enhanced baroclinicity and stronger monsoon flow in West Africa. I will discuss the implications of these changes in AEWs for future precipitation in the drought-vulnerable Sahel region of West Africa, for global Saharan dust transport, and for tropical cyclogenesis in the Atlantic basin. I will conclude my talk with a discussion of the potential role of AEWs in shaping rapid humid-to-arid climate transitions in Northern Africa during Earth’s history.