Extremes of the Atmospheric Water Balance: Extreme Floods in the Lower Mississippi River

Abstract:

As the economic artery of the United States, the Mississippi River is bound by one of the most elaborate engineering systems in the world. The 2019 opening of the Bonnet Carre Spillway marked the 6^th opening of the most downstream emergency spillway on the Mississippi River since 2008, providing a sharp contrast to the 8 times the spillway had been operated in the previous 7 decades. Extreme floods in the Lower Mississippi are tied to episodes of long-duration intense rainfall. In this study, we examine the anomalous behavior of precipitable water, column-integrated water vapor transport (IVT) and water vapor convergence across multiple timescales leading to and during the extreme flood periods since 2008. Our analyses of the climatology of atmospheric water balance extremes across various timescales using multi-decadal reanalysis fields provide a basis for assessing water balance anomalies during extreme events. We find evidence of increasing trends in the three variables across Central US. Large regions of “thick-tailed” IVT and water vapor convergence distributions are also found in the region through extreme value analyses, pointing to increasing flood risk in the Lower Mississippi River basin.