Dynamical Drivers of Changing Rainfall and Flood Risk in the Mississippi River Basin from the Last Millennium to the 21st Century

The Mississippi River basin has been modified through human intervention for hundreds of years, routing the channels to benefit agriculture and industry and reduce hazards. The impact of human activities on Mississippi River basin (MRB) hydroclimate has been extensively studied. Still, there remains a significant gap in determining what factors will dominate increased or decreased river discharge in the lower MRB during the 21st century. This study aims to document the influence of atmospheric dynamics, such as the Bermuda High (BH) and Great Plains low-level jet (GPLLJ), on moisture transport into the MRB. We address the following research questions: (1) Are there significant differences in the strength and positioning of the BH and GPLLJ between the pre-industrial era, 20th, and 21st centuries? (2) How do seasonal circulation patterns influence large-scale moisture transport into the basin, and how do global climate models project that anthropogenic climate change will alter this outcome? We use previously established indices for the BH and GPLLJ to evaluate these changes over the last millennium and 21st century. Given the limitations of a short observational period, we turn to climate model simulations. We employ the Community Earth System Model (CESM) Last Millennium Ensemble and CESM2 SSP3-7.0 Large Ensemble, both of which provide a more robust pool of statistics and a last millennium baseline documenting the range of natural variability. To ensure the effectiveness of our approach, we first determine whether CESM simulations are sufficiently skilled at representing atmospheric circulation features using ECMWF Reanalysis v5. Initial validation of CESM provides confidence in our ability to accurately simulate winds, precipitation, and moisture convergence, which in turn affect soil moisture, runoff, and river discharge over the basin. We then employ principal component analysis to investigate precipitation patterns and their association with both the BH and GPLLJ. Finally, we additionally explore hydroclimate shifts in gridded paleoclimate reconstructions, such as the Paleo Hydrodynamics Data Assimilation product (PHYDA). PHYDA provides high-resolution reconstructions of the Palmer Drought Severity Index, the standardized precipitation evapotranspiration index, and North Atlantic sea surface temperatures. By capitalizing on these diverse data products, we provide a comprehensive understanding of the factors that influence moisture transport into the MRB. We hope that our findings will inform future flood risk management and infrastructure decisions to better handle extreme events in the 21st century.