The 1996 Mid-AtlanticWinter Flood: Exploring climate risk through a storyline approach

Abstract This article explores the application of thermodynamic perturbations to a historical mid-latitude, winter-time, rain-on-snow flood event to evaluate how similar events may evolve under different climate forcings. In particular, we generate a hindcast of the 1996 Mid-Atlantic flood using an ensemble of 14km variable-resolution simulations completed with the Department of Energy’s global Energy Exascale Earth System Model (E3SM). We show the event is skillfully reproduced over the Susquehanna River Basin (SRB) by E3SM when benchmarked against in situ observational data and high-resolution reanalyses. In addition, we perform five counterfactual experiments to simulate the flood under pre-industrial conditions and four different levels of warming as projected by the Community Earth System Model Large Ensemble. We find a nonlinear response in simulated surface runoff and streamflow as a function of atmospheric warming. This is attributed to changing contributions of liquid water input from a shallower initial snowpack (decreased snowmelt), increased surface temperatures and rainfall rates, and increased soil water storage. Flooding associated with this event peaks around +1 to +2K of global average surface warming and decreases with additional warming beyond this. There are noticeable timing shifts in peak runoff and streamflow associated with changes in the flashiness of the event. This work highlights the utility of using storyline approaches for communicating climate risk and demonstrates the potential non-linearities associated with hydrologic extremes in areas that experience ephemeral snowpack, such as the SRB.