Attributing changes in extreme precipitation across the Northeast U.S. under different climate scenarios

The Northeast United States has experienced the most rapidly increasing occurrences of extreme precipitation within the U.S. over recent decades, particularly during the warm season. This trend is primarily linked to events associated with tropical cyclones. Understanding the drivers leading to long-term trends in regional extreme precipitation under different future climate scenarios is critical to adaptation and mitigation planning.

New simulations with the fully-coupled 25-km GFDL (Geophysical Fluid Dynamics Laboratory) SPEAR (Seamless System for Prediction and EArth System Research) model and its 10 ensemble members, present a unique opportunity to study changes in regional extreme precipitation and relevant physical processes. Under the SSP5-8.5 scenario, SPEAR projects top 1% extreme precipitation events over the Northeast U.S. to increase by up to 2.4% by the end of the 21st century. The projected increase is driven by higher anthropogenic radiative forcing and is distinguishable from natural variability by the mid-century. From the meteorological perspective, the occurrences of warm season extreme precipitation related to both atmospheric rivers and tropical cyclones are projected to increase, even though the frequency of tropical cyclones in the North Atlantic is projected to decrease in the model.

The SSP5-8.5 scenario, however, represents a highly unlikely trajectory, prompting the scientific community to explore scenarios with rapid reductions in greenhouse gas (GHG) concentrations through various climate mitigation efforts. Using the SSP5-3.4OS overshoot scenario from the SPEAR model—where GHG emissions decline sharply after 2040 and reach net-negative levels by 2070—we assess the impact of mitigation on extreme precipitation over the Northeast U.S. Our results show that extreme precipitation frequency over the Northeast U.S. is projected to decrease as GHG concentrations decline. However, the timing of this reversal is seasonally dependent: warm-season trends reverse shortly after global mean surface temperature starts to decline, while cold-season trends lag by approximately 15 years. These results suggest that the response of extreme precipitation to GHG reductions may depend on the underlying mechanisms driving these events. For example, cold-season extremes are more often associated with large-scale extratropical cyclones, where dynamical processes play a significant role. Our study underscores the urgent need for a deeper understanding of the physical processes governing regional climate extremes in response to GHG mitigation. Such insights are essential for informing adaptation strategies and policymaking for effective climate risk management.