Chia-Ying Lee

Most climate models simulate greater warming in the equatorial eastern Pacific than in the west under historical forcing, resulting in a reduced zonal sea surface temperature (SST) gradient and a weakened Walker circulation. This contrasts with observed trends and highlights a persistent model–observation discrepancy with far‐reaching impacts on regional climate extremes. The authors' previous work showed that reducing mean‐state biases in a coupled model yields SST trends more consistent with observations. Here, we downscale tropical cyclones (TCs) from these simulations and find that a strengthened zonal SST gradient is associated with increased TC frequency and hazards in the North Atlantic and suppresses activity in the South Pacific. In future projections, both biased and bias‐corrected models simulate relative eastern Pacific warming, yet their TC responses differ regionally, with the bias‐corrected model projecting greater TC activity and hazards over the North Atlantic.

Tropical cyclones (TCs) often undergo extratropical transition (ET) at the midlatitudes, becoming post‐tropical cyclones (PTCs). We combine a logistic regression PTC identification model with synthetic TC tracks and intensities from the Columbia HAZard model (CHAZ) to project future ET completion over the North Atlantic (NA). Projections are based on downscaled outputs from 12 Coupled Model Intercomparison Project Phase 6 (CMIP6) models under three shared socioeconomic pathways (SSPs): SSP2‐4.5, SSP3‐7.0, and SSP5‐8.5. Future changes in PTC numbers largely follow projected TC frequency, while the ET fraction remains nearly unchanged. Despite this stability, the preferred ET completion region shifts northeastward. This shift likely results from weaker vertical wind shear, larger potential intensity, slower overall translation speed at the midlatitudes, and more northward TC genesis. Although storms move more slowly overall, an enhanced eastward storm‐motion component favors the northeastward shift, whereas a weaker northward component partially offsets the displacement.

Scientific assessments play a critical role in synthesizing evidence to support evidence‐based decision‐making in many sectors and topics. For climate change, large‐scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the United States (U.S.) National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While co‐creation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near‐term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or sub‐national climate assessment reports, this paper highlights select U.S.‐relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, sub‐national, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities roadmap aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross‐cutting climate action plans and decisions in the U.S.

We analyze North Atlantic Tropical Cyclone (TC) activity using the Columbia HAZard model to downscale 12 models from CMIP6 under the SSP1‐2.6 and SSP5‐8.5 scenarios—representing low and high radiative forcing, respectively. Despite greater warming under SSP5‐8.5, basin‐wide North Atlantic TC frequency is not consistently higher than under SSP1‐2.6, contrasting the global‐mean response. Landfalling TC frequency increases along the Southeastern U.S. and declines along the Gulf under both SSPs, a contrast that persists throughout the 21st century. Both TC genesis and steering flow changes contribute to this contrast, and appear consistent with projected El Niño‐like shifts in the tropical Pacific mean state, influencing both Atlantic potential intensity and vertical wind shear variability. These results emphasize the sensitivity of regional TC activity to the spatial pattern of surface warming over its global‐mean change, and the significance of the contention around projected trends in the tropical Pacific, in particular.

Scientific assessments play a critical role in synthesizing evidence to support evidence-based decision-making in many sectors and topics. For climate change, large-scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While cocreation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near-term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or subnational climate assessment reports, this paper highlights select U.S.-relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, subnational, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities road map aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross-cutting climate action plans and decisions in the United States. PLAIN LANGUAGE SUMMARY: Scientific assessments are critical in informing households, communities, businesses, institutions, and governments as they respond to intensifying climate hazards. As climate change impacts accelerate, the pace of assessment and integration of science with decision-making and action must accelerate as well. A network of U.S. climate assessment efforts could rapidly synthesize actionable science and case studies to inform climate response strategies. To support future processes, this paper presents a selection of foundational topics needing assessment and near-term research across climate impacts, risks, and actions from the perspective of the United States and some of its key regions, environments, and economic sectors. For example, assessment is needed for the increasing threats posed by compound extremes and cascading climate risks, which can overwhelm systems developed for independent events. The effectiveness of climate responses likewise requires continuous evaluation and improvement, with consideration of the broader impacts (e.g., economic, environmental, health) associated with those strategies. System-level understanding of integrated climate response strategies can be advanced by combining observations and modeling with mitigation, adaptation, climate interventions, disaster management, and resilience decision scenarios. Overall, these research priorities aim to encourage new research and future national, regional, local, and sectoral climate assessments that support climate-informed decision-making in the United States.

Projections of future changes in tropical cyclone (TC) rainfall are critical for understanding evolving flood risks and infrastructure impacts under climate change. This study couples a physics-based Tropical Cyclone Rainfall (TCR) model with the Columbia HAZard (CHAZ) model, a statistical–dynamical TC downscaling framework, which generates synthetic storm tracks and intensity, to produce large synthetic ensembles of TC generated rainfall downscaled from 12 CMIP6 models. TC rainfall is simulated in the North Atlantic basin based on the CMIP6 historical and future simulations under three Shared Socioeconomic Pathways (SSP2-4.5, SSP3-7.0, SSP5-8.5). Projections from this integrated framework are evaluated against the Geophysical Fluid Dynamics Laboratory (GFDL) Rainfall Climatology and Persistence model (R-CLIPER), a statistical TC rainfall model that provides an alternative to the dynamical simulations. TCR projects widespread increases in TC rainfall, with the strongest changes along the US East and Gulf coasts and for major hurricanes (Categories 3–5). By late century, average rainfall increases reach 40–100% in TCR compared to 30–60% in R-CLIPER, with the largest increases in the Northeast regions. Extreme 24-hour rainfall increases by up to 55% in TCR, roughly twice the magnitude simulated by R-CLIPER. Key drivers include higher TC intensities, increased atmospheric moisture, and projected slower translation speeds, which together contribute to increasing flood risks, especially when combined with more frequent sequential TC events.

Tropical Cyclones (TCs) are intense storms that pose a persistent and considerable risk to coastal communities and infrastructure in the global tropics and subtropics, including the United States (US). With known limitations associated with observations and high-resolution earth system models, synthetic TC models that capture a wide spectrum of storm possibilities have been developed to robustly quantify TC risk. Here we examine the simulation of various TC features in the North Atlantic relevant for US coastal risk in three synthetic TC models forced with ERA5 reanalysis: Massachusetts Institute of Technology (MIT), Columbia HAZard (CHAZ) and Risk Analysis Framework for Tropical Cyclones (RAFT). While there is a broad agreement among these models in terms of their representation of salient TC characteristics, certain differences do exist. To connect these modeling uncertainties with energy infrastructure resilience, we apply fragility curves that link simulated TC intensities to damage probabilities, demonstrating how uncertainty in storm states may translate into that in coastal impacts. Our study indicates that acknowledging and accounting for inter-model uncertainty leads to more reliable risk assessments, strengthening science-to-action pathways for managing risks associated with TCs.

This study analyzes the climatology, variability, and key characteristics of tropical cyclones (TC) in future projections from the Seamless System for Prediction and EArth System Research (SPEAR) Large Ensemble, compared with historical simulations. TC genesis numbers are projected to significantly decrease, a trend that is partly explained by the increase in saturation deficit. Spatially, TC genesis generally declines across most basins under warming, except in the central Pacific where SPEAR projects an increase. Correspondingly, accumulated cyclone energy (ACE), landfall frequency, and TC precipitation (TCP) also show decreasing trends. The projected decrease in climatological TCP is driven mainly by reduced TC occurrence, whereas precipitation per TC increases at a scaling of 4.64%/K, comparable to the TCP “climate scaling.” TCP size is projected to expand by approximately 31.5 km in radius by the end of the century. Normalized TC landfall frequency relative to the total number of basin TCs indicates fewer low‐latitude landfalls due to a poleward shift in genesis regions. However, the intensity of landfalling TCs shows a slight increase. TC translation speed is projected to slow at higher latitudes due to weakened westerlies and to accelerate at lower latitudes. Standard modes of internal climate variability show no fundamental changes under warming, and the TC responses to these modes remain largely unchanged compared to present conditions.