Rapid reductions in greenhouse gas (GHG) concentrations are increasingly included in scenarios used to project the full range of possible future climate changes, yet the response of regional climate extremes to such reductions remains highly uncertain. Here, we assess projected changes in extreme precipitation over the Northeast US under an aggressive overshoot mitigation pathway (SSP5-3.4OS), simulated by the fully-coupled 25 km Geophysical Fluid Dynamics Laboratory (GFDL) Seamless system for Prediction and EArth system Research (SPEAR) climate model. In this scenario, hypothetical mitigation efforts are introduced starting in 2041, with net-negative GHG emissions achieved by the late 21st century. The frequency of extreme precipitation over the Northeast US increases through mid-century under higher radiative forcing but begins to decline following the sharp reductions in GHG concentrations. However, the rate of decrease exhibits pronounced seasonality. In the warm season, extreme precipitation frequency begins to decline shortly after GHG drawdown begins, returning by 2100 to levels comparable to those of the early 21st century. In the cold season, on the other hand, the response is delayed; the frequency of extreme precipitation continues rising for roughly a decade after the peak global mean warming and exhibits hysteresis behavior. By 2100, cold-season extremes only then return to mid-century levels. This delayed response in the cold season is spatially heterogeneous, suggesting that major metropolitan areas in the Northeast—with dense populations and vulnerable infrastructure—may experience different seasonal changes in response to the same climate migration efforts. These results highlight the benefit of climate mitigation in reducing extreme precipitation events, but also the complexity of regional climate responses, which can be modulated by seasonality, local-scale effects, and other factors.
The Northeast United States (NEUS) has faced the most rapidly increasing occurrences of extreme precipitation within the US in the past few decades. Understanding the physics leading to long-term trends in regional extreme precipitation is essential but the progress is limited partially by the horizontal resolution of climate models. The latest fully coupled 25-km GFDL (Geophysical Fluid Dynamics Laboratory) SPEAR (Seamless system for Prediction and EArth system Research) simulations provide a good opportunity to study changes in regional extreme precipitation and the relevant physical processes. Here, we focus on the contributions of changes in synoptic-scale events, including atmospheric rivers (AR) and tropical cyclone (TC)-related events, to the trend of extreme precipitation in the fall season over the Northeast US in both the recent past and future projections using the 25-km GFDL-SPEAR. In observations, increasing extreme precipitation over the NEUS since the 1990s is mainly linked to TC-related events, especially those undergoing extratropical transitions. In the future, both AR-related and TC-related extreme precipitation over the NEUS are projected to increase, even though the numbers of TCs in the North Atlantic are projected to decrease in the SPEAR simulations using the SSP5-8.5 projection of future radiative forcing. Factors such as enhancing TC intensity, strengthening TC-related precipitation, and/or westward shift in Atlantic TC tracks may offset the influence of declining Atlantic TC numbers in the model projections, leading to more frequent TC-related extreme precipitation over the NEUS.
Extreme precipitation is among the most destructive natural disasters. Simulating changes in regional extreme precipitation remains challenging, partially limited by climate models’ horizontal resolution. Here, we use an ensemble of high-resolution global climate model simulations to study September–November extreme precipitation over the Northeastern United States, where extremes have increased rapidly since the mid-1990s. We show that a model with 25 km horizontal resolution simulates much more realistic extreme precipitation than comparable models with 50 or 100 km resolution, including frequency, amplitude, and temporal variability. The 25 km model simulated trends are quantitatively consistent with observed trends over recent decades. We use the same model for future projections. By the mid-21st century, the model projects unprecedented rainfall events over the region, driven by increasing anthropogenic radiative forcing and distinguishable from natural variability. Very extreme events (>150 mm/day) may be six times more likely by 2100 than in the early 21st century.
