Hydroclimate changes over the Great Plains since the Last Glacial Maximum




Sun, Chijun

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The weather extremes associated with mesoscale convective systems (MCSs) over the Great Plains have substantial socio-economic impacts on the regions’ diverse and growing society. It remains unclear how anthropogenic warming will affect the climate of this region in the future due to large uncertainties in simulated precipitation changes by climate models and a lack of knowledge about the long-term variability of these storms. This dissertation seeks to improve our understanding of the climate dynamics of the Great Plains, focusing on using past changes in MCS activity and hydroclimate to understand their responses to external forcings. To do so, I generate a 20,000-year record of MCS activity and hydroclimate in central Texas using the D/H ratios of sedimentary leaf waxes coupled with trace element analysis and carbon isotope composition of bulk organic matter. To improve our confidence in the interpretation of hydrogen isotope data, I first investigate the meteorological controls on the isotopic composition of precipitation in the Great Plains, which suggests that the isotopic ratios of rainwater mainly reflect the magnitude of the dominating MCS storms over this region, with larger and stronger MCSs producing more negative isotopic values and vice versa. Based on our improved understanding of the regional isotope systematics, our multi-proxy record shows pronounced responses of MCS to external forcings (i.e., retreating ice sheets, solar insolation) on orbital to centennial timescales. Results from transient climate model simulations indicate that variations in the intensity of the Great Plains Low-Level Jet in response to springtime land surface warming can account for these changes in MCS activity. Differential springtime warming over the continent and the North Atlantic drives steeper zonal temperature and pressure gradients over the Great Plains, intensifying the Great Plains Low-Level Jet which consequently enhances the magnitude of Great Plains MCSs via increased moisture availability. These results provide important implications for the potential responses of Great Plains MCS activity to anthropogenic warming, where more pronounced land surface warming relative to the ocean may dynamically intensify future storms with increased risks for extreme weather in the Great Plains


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