Understanding preciptation changes over West Africa and North America under global warming and identifying a Congo Basin Walker circulation




Neupane, Naresh

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Studies have shown that the Atlantic Ocean affects West African rainfall. However, the response of the West African monsoon to Atlantic warming is not understood clearly. My dissertation explores this by analyzing idealized simulations with a regional climate model. Below 1.5 K warming of the Atlantic, rainfall increase by 30-50% over the Sahel. In contrast, above 2 K, rainfall decreases substantially. Atlantic warming is accompanied by decreases in low-level geopotential heights in the Atlantic decreasing the large-scale meridional geopotential height gradient across West Africa. This leads to easterly wind anomalies in the Sahel. Below 2 K, these easterly anomalies allow moisture transport to the Sahel. Above 2 K, the easterly anomalies reverse the westerly flow and reduce precipitation in the Sahel. Models predict increases of precipitation in the future under global warming. Theoretical understanding of this is founded in the Clausius-Clapeyron equation. I compare precipitation from the theory with the model simulations over the U.S. for the mid-21st century and investigate the physics of the departure of the model from theory. In the spring and fall precipitation increases, up to 30%/ K, and the modeled precipitation matches the theoretical prediction. In contrast, rainfall lowers and prediction fails in the summer. These differences are associated with the soil moisture distributions. Associated with the increased soil moisture, model follows theoretical prediction in the spring and fall, while reduced soil moisture is associated with failure of the theoretical prediction in the summer. The Gulf of Guinea in the eastern Atlantic shows subsidence. This subsidence becomes well established in the low-level from July-September. Using observations, I contribute to understanding its cause and relationship with the regional precipitation and circulation. This subsidence is associated with the subsiding-branch of a Congo basin Walker circulation identified here. The circulation has a rising-branch over the Congo basin, and is driven by temperature gradient. Basin temperature remains almost uniform throughout the year, but in association with the Atlantic cold tongue formation, Guinean temperature cools up to -4 K in the summer. This gradient drives the circulation. A strong Walker circulation is associated with enhanced northward moisture transport.



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