Thomas R. Knutson's research while affiliated with Geophysical Laboratory and other places

Publications (149)

Preprint
Tropical cyclone rapid intensification events often cause destructive hurricane landfalls because they are associated with the strongest storms and the worst forecasts. Multi-decade observational datasets of tropical cyclone behavior have recently enabled documentation of upward trends in tropical cyclone rapid intensification in several basins. Ho...
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In this paper, U.S. landfalling tropical cyclone (TC) activity is projected for the late twenty-first century using a two-step dynamical downscaling framework. A regional atmospheric model, is run for 27 seasons, to generate tropical storm cases. Each storm case is -resimulated (up to 15 days) using the higher-resolution Geophysical Fluid Dynamics...
Article
Here we present a machine learning based wind reconstruction model. The model reconstructs hurricane surface winds with XGBoost which is a decision tree based ensemble predictive algorithm. The model treats the symmetric and asymmetric wind fields separately. The symmetric wind field is approximated by a parametric wind profile model and two Bessel...
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Increasing evidence suggests that climate change impacts are already observed around the world. Global environmental assessments face challenges to appraise the growing literature. Here we use the language model BERT to identify and classify studies on observed climate impacts, producing a comprehensive machine-learning-assisted evidence map. We es...
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In this study, we investigate the response of tropical cyclones (TCs) to climate change by using the Princeton environment-dependent probabilistic tropical cyclone (PepC) model and a statistical-deterministic method to downscale TCs using environmental conditions obtained from the Geophysical Fluid Dynamics Laboratory (GFDL) High-resolution Forecas...
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U.S. landfalling tropical cyclone (TC) activity was projected for late 21 st century conditions using a two-step dynamical downscaling framework. A regional atmospheric model, run for 27 seasons, generated tropical storm cases. Each storm case was re-simulated (up to 15 days) using the higher resolution GFDL hurricane model. Thirteen CMIP3 or CMIP5...
Preprint
An ever-growing body of evidence suggests that climate change is already impacting human and natural systems around the world. Global environmental assessments assessing this evidence, for example by the Intergovernmental Panel on Climate Change (IPCC) ¹ , face increasing challenges to appraise an exponentially growing literature ² and diverse appr...
Preprint
An ever-growing body of evidence suggests that climate change is already impacting human and natural systems around the world. Global environmental assessments assessing this evidence, for example by the Intergovernmental Panel on Climate Change (IPCC) ¹ , face increasing challenges to appraise an exponentially growing literature ² and diverse appr...
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Atlantic hurricanes are a major hazard to life and property, and a topic of intense scientific interest. Historical changes in observing practices limit the utility of century-scale records of Atlantic major hurricane frequency. To evaluate past changes in frequency, we have here developed a homogenization method for Atlantic hurricane and major hu...
Preprint
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Atlantic hurricanes are a major hazard to life and property 1,2,3 , and a topic of intense scientific interest 4,5,6 . Historical changes in observing practices limit the utility of century-scale records of Atlantic major hurricane frequency ⁷⁻¹³ . To evaluate past changes in frequency, we have here developed a homogenization method for Atlantic hu...
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Earlier studies have proposed many semi-empirical relations between climate and tropical cyclone (TC) activity. To explore these relations, this study conducts idealized aqua-planet experiments using both symmetric and asymmetric sea surface temperature (SST) forcings. With zonally symmetric SST forcings that have a maximum at 10°N, reducing meridi...
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Observed sea level pressure (SLP) trends for 1901-2010, 1951-2010, and 1981-2010 are assessed using two observed data sources (HadSLP2_lowvar and 20CRv3) compared to a CMIP5 multi-model ensemble. The CMIP5 simulations include runs with: i) no external forcing (Control runs), ii) natural external forcing only (Natural-Forcing), or iii) natural plus...
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This paper assesses published findings on projections of future tropical cyclone (TC) activity in the ESCAP/WMO Typhoon Committee Region under climate change scenarios. This assessment also estimates the projected changes of key TC metrics for a 2oC anthropogenic global warming scenario for the western North Pacific (WNP) following the approach of...
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The locally accumulated damage by tropical cyclones (TCs) can intensify substantially when these cyclones move more slowly. While some observational evidence suggests that TC motion might have slowed significantly since the mid-20th century ( 1 ), the robustness of the observed trend and its relation to anthropogenic warming have not been firmly es...
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Published findings on climate change impacts on tropical cyclones (TCs) in the ESCAP/WMO Typhoon Committee Region are assessed. We focus on observed TC changes in the western North Pacific (WNP) basin, including frequency, intensity, precipitation, track pattern, and storm surge. Results from an updated survey of impacts of past TC activity on vari...
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Abstract GFDL's new CM4.0 climate model has high transient and equilibrium climate sensitivities near the middle of the upper half of CMIP5 models. The CMIP5 models have been criticized for excessive sensitivity based on observations of present‐day warming and heat uptake and estimates of radiative forcing. An ensemble of historical simulations wit...
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Plain Language Summary Recent observational studies suggest that Atlantic hurricane activity is strongly affected by extratropical weather processes, but modeling studies disagree about the importance of such an impact. Using a regional atmospheric model, we conduct idealized experiments to explore the extratropical impact on Atlantic hurricane act...
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Since the Eighth International Workshop on Tropical Cyclones (IWTC-8), held in December 2014, progress has been made in our understanding of the relationship between tropical cyclone (TC) characteristics, climate and climate change. New analysis of observations has revealed trends in the latitude of maximum TC intensity and in TC translation speed....
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Recent climate modeling studies point to an increase in tropical cyclone rainfall rates in response to climate warming. These studies indicate that the percentage increase in tropical cyclone rainfall rates often outpaces the increase in saturation specific humidity expected from the Clausius-Clapeyron relation (~7% °C⁻¹). We explore the change in...
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Responses of tropical cyclones (TCs) to CO2 doubling are explored using coupled global climate models (GCMs) with increasingly refined atmospheric/land horizontal grids (~200 km, ~ 50 km and ~25 km). The three models exhibit similar changes in background climate fields thought to regulate TC activity, such as relative sea surface temperature (SST),...
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An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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Model projections of tropical cyclone (TC) activity response to anthropogenic warming in climate models are assessed. Observations, theory, and models, with increasing robustness, indicate rising global TC risk for some metrics that are projected to impact multiple regions. A 2°C anthropogenic global warming is projected to impact TC activity as fo...
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An assessment was made of whether detectable changes in tropical cyclone (TC) activity are identifiable in observations and whether any changes can be attributed to anthropogenic climate change. Overall, historical data suggest detectable TC activity changes in some regions associated with TC track changes, while data quality and quantity issues cr...
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Atlantic Multidecadal Variability (AMV) is a multivariate phenomenon. Here for the first time we obtain a multivariate AMV index and associated patterns using Multivariate Empirical Orthogonal Function (MEOF) analysis to explore the multivariate nature of AMV. Coherent multidecadal variability that is unique to the Atlantic is found in the observed...
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The original version of this Article contained an error in the second sentence of the first paragraph of the ‘Quantile mapping’ section of the Methods, which incorrectly read ‘We primarily focus on results produced using an additive version of QDM26 by making use of R programming language code contained in the CRAN MBC package version 0.10–438.’ Th...
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Tropical cyclones that rapidly intensify are typically associated with the highest forecast errors and cause a disproportionate amount of human and financial losses. Therefore, it is crucial to understand if, and why, there are observed upward trends in tropical cyclone intensification rates. Here, we utilize two observational datasets to calculate...
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The Atlantic Meridional Overturning Circulation (AMOC) has profound impacts on various climate phenomena. Using both observations and simulations from the Coupled Model Intercomparison Project Phase 3 and 5, here we show that most models underestimate the amplitude of low-frequency AMOC variability. We further show that stronger low-frequency AMOC...
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Over regions where snowmelt runoff substantially contributes to winter-spring streamflows, warming can accelerate snowmelt and reduce dry-season streamflows. However, conclusive detection of changes and attribution to anthropogenic forcing is hindered by the brevity of observational records, model uncertainty, and uncertainty concerning internal va...
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Precipitation trends for 1901-2010, 1951-2010, and 1981-2010 over relatively well-observed global land regions are assessed for detectable anthropogenic influences and for consistency with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CMIP5 historical all-forcing runs are broadly consistent with the o...
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According to CMIP5 simulations, the 2016 record global warmth was only possible due to substantial centennial-scale anthropogenic warming. Natural variability made a smaller contribution to the January–December 2016 annual-mean global temperature anomaly.
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According to CMIP5 simulations, the highly anomalous Arctic warmth during November–December 2016, as estimated in five observed datasets, most likely would not have been possible without anthropogenic forcing.
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Observed Atlantic major hurricane frequency has exhibited pronounced multidecadal variability since the 1940s. However, the cause of this variability is debated. Using observations and a coupled earth system model (GFDL-ESM2G), here we show that the decline of the Atlantic major hurricane frequency during 2005–2015 is associated with a weakening of...
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In this study the mechanisms for low-frequency variability of summer Arctic sea ice are analyzed using long control simulations from three coupled models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three...
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This study aims to understand the relative roles of external forcing versus internal climate variability in causing the observed Barents Sea winter sea ice extent (SIE) decline since 1979. We identify major discrepancies in the spatial patterns of winter Northern Hemisphere sea ice concentration trends over the satellite period between observations...
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1. Human activities have contributed substantially to observed ocean–atmosphere variability in the Atlantic Ocean (medium confidence), and these changes have contributed to the observed upward trend in North Atlantic hurricane activity since the 1970s (medium confidence). 2. Both theory and numerical modeling simulations (in general) indicate an in...
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In this appendix, we present a brief overview of the methodologies and methodological issues for detection and attribution of climate change. Attributing an observed change or an event partly to a causal factor (such as anthropogenic climate forcing) normally requires that the change first be detectable (Hegerl et al. 2010). A detectable observed c...
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1. The tropics have expanded poleward by about 70 to 200 miles in each hemisphere over the period 1979-2009, with an accompanying shift of the subtropical dry zones, midlatitude jets, and storm tracks (medium to high confidence). Human activities have played a role in this change (medium confidence), although confidence is presently low regarding t...
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1. Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Plains, Midwest, and Northeast. A national average increase of 4% in annual precipitation since 1901 is mostly a result of large increases in the fall season. (Medium confidence) 2. Heavy precipitation events in mos...
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1. The likely range of the human contribution to the global mean temperature increase over the period 1951–2010 is 1.1° to 1.4°F (0.6° to 0.8°C), and the central estimate of the observed warming of 1.2°F (0.65°C) lies within this range (high confidence). This translates to a likely human contribution of 93%–123% of the observed 1951–2010 change. It...
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1. The global climate continues to change rapidly compared to the pace of the natural variations in climate that have occurred throughout Earth’s history. Trends in globally averaged temperature, sea level rise, upper-ocean heat content, land-based ice melt, Arctic sea ice, depth of seasonal permafrost thaw, and other climate variables provide cons...
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1. Recent droughts and associated heat waves have reached record intensity in some regions of the United States; however, by geographical scale and duration, the Dust Bowl era of the 1930s remains the benchmark drought and extreme heat event in the historical record (very high confidence). While by some measures, drought has decreased over much of...
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In 2015, record warm surface temperatures were observed for the global mean, India, and the equatorial central Pacific. CMIP5 simulations suggest that for the globe and India, anthropogenic warming was largely to blame.
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Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade−1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade−1). Here we investigate the prospects for this slower rate to persist for a decade or more. The slower rate could persist if the...
Data
Supplementary Figures 1-5, Supplementary Notes 1-3 and Supplementary References 1-2
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This study investigates spatio-temporal features of multidecadal climate variability, using observations and climate model simulation. Aside from a long-term warming trend, observational SST and atmospheric circulation records are dominated by a ~65yr variability component. Though its center of action is over the North Atlantic, but it manifests al...
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One of the most consequential impacts of anthropogenic warming on humans may be increased heat stress, combining temperature and humidity effects. Here we examine whether there are now detectable changes in summertime heat stress over land regions. As a heat stress metric we use a simplified wet bulb globe temperature (WBGT) index. Observed trends...
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The GFDL hurricane modeling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution will be briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range...
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According to CMIP5 models, the risk of record annual mean warmth in European, northeast Pacific, and northwest Atlantic regions— as occurred in 2014 — has been greatly increased by anthropogenic climate change.
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According to CMIP5 models, the risk of record annual mean warmth in European, northeast Pacific, and northwest Atlantic regions—as occurred in 2014—has been greatly increased by anthropogenic climate change.
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Recent research has strengthened the understanding of the links between cli- mate and tropical cyclones (TCs) on various timescales. Geological records of past climates have shown century-long variations in TC numbers. While no sig- nificant trends have been identified in the Atlantic since the late 19th century, significant observed trends in TC n...
Chapter
We review past, present and future North Atlantic hurricane activity based on analysis of observational records and models projections. When adjusted for likely missed tropical cyclones, the observational record does not show any significant increase or decrease of North Atlantic hurricane frequency. Downscaling results for most available CMIP5 mod...
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Global projections of intense tropical cyclone activity are derived from the Geophysical Fluid Dynamics Laboratory (GFDL) High Resolution Atmospheric Model (HiRAM; 50-km grid) and the GFDL hurricane model using a two-stage downscaling procedure. First, tropical cyclone genesis is simulated globally using HiRAM. Each storm is then downscaled into th...
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We welcome the comments of Landsea (2015, hereafter L15) and we1 applaud his efforts toward reanalyzing past tropical cyclone data in the Atlantic (Landsea et al. 2008, 2012, 2014; Hagen et al. 2012). However, L15 does not substantially change the conclusions stated in Kunkel et al. (2013, hereafter K13). L15 voices two main concerns: 1. The U.S. l...
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The eastern United States is vulnerable to flooding from tropical cyclone rainfall. Understanding how both the frequency and intensity of this rainfall will change in the future climate is a major challenge. One promising approach is the dynamical downscaling of relatively coarse general circulation model results using higher-resolution regional cl...
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Global tropical cyclone (TC) activity is simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) CM2.5, which is a fully coupled global climate model with horizontal resolution of about 50km for atmosphere and 25 km for ocean. The present climate simulation shows fairly realistic global TC frequency, seasonal cycle, and geographical distribut...
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International scientists from seven countries with expertise in decadal climate variability met to review and critically assess recent progress in, and new directions for, understanding decadal climate variability and cross-scale interactions.
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We analyze several U.S. regions with seasonal and annual mean precipitation anomalies in 2013 that we classify as “extreme” (i.e., ranked first, second, or third highest or lowest). Our analysis uses the Global Historical Climatology Network (GHCN) monthly precipitation dataset (Vose et al. 1992), covering 1900–2013, on a 5°× 5° grid effectively li...
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A global survey of surface temperature anomalies occurring during 2013 (Fig. 8.1a; Supplementary Fig. S8.1) in the HadCRUT4 observations (Morice et al. 2012) reveals pronounced warm ann