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Extraordinary heat during the 1930s US Dust Bowl and associated large-scale conditions

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Unusually hot summer conditions occurred during the 1930s over the central United States and undoubtedly contributed to the severity of the Dust Bowl drought. We investigate local and large-scale conditions in association with the extraordinary heat and drought events, making use of novel datasets of observed climate extremes and climate reanalysis covering the past century. We show that the unprecedented summer heat during the Dust Bowl years was likely exacerbated by land-surface feedbacks associated with springtime precipitation deficits. The reanalysis results indicate that these deficits were associated with the coincidence of anomalously warm North Atlantic and Northeast Pacific surface waters and a shift in atmospheric pressure patterns leading to reduced flow of moist air into the central US. Thus, the combination of springtime ocean temperatures and atmospheric flow anomalies, leading to reduced precipitation, also holds potential for enhanced predictability of summer heat events. The results suggest that hot drought, more severe than experienced during the most recent 2011 and 2012 heat waves, is to be expected when ocean temperature anomalies like those observed in the 1930s occur in a world that has seen significant mean warming.

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... Because the recovery of loose, unconsolidated sediment at the top of freeze cores is uneven, varve years were assigned in relation to an interval of particularly prominent calcite laminae, which can be easily identified with the naked eye within any core from Crawford Lake ( Figures 5-7). These distinct laminae were deposited during the warm and dry Dust Bowl years of the 1930s across North America [67][68][69][70]. The 1935 varve, an especially warm and dry year, thus characterized by a particularly thick calcite lamina, was then used as the point from which varve counting ...
... Because the recovery of loose, unconsolidated sediment at the top of freeze cores is uneven, varve years were assigned in relation to an interval of particularly prominent calcite laminae, which can be easily identified with the naked eye within any core from Crawford Lake ( Figures 5-7). These distinct laminae were deposited during the warm and dry Dust Bowl years of the 1930s across North America [67][68][69][70]. The 1935 varve, an especially warm and dry year, thus characterized by a particularly thick calcite lamina, was then used as the point from which varve counting was carried out along the core, both up and down section. ...
... Microscopic examination of strew slides with the light and dark laminae from the thick 1935 couplet has confirmed that crystals of calcite are the dominant component [36,37]. The drought conditions that dominated during the 1930's combined with the frequency of hot days was anomalously high in North America, including Southern Ontario [67][68][69][70]. Dust storms were widespread during this period, with 1935 being the standout year where one estimate puts the loss of topsoil from wind erosion at~771 million metric tons [67]. ...
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Varves accumulating below the chemocline of meromictic Crawford Lake, Milton, Ontario, Canada, consist of dark-colored organic matter laminations that are primarily deposited during the fall plankton die-off, alternating with light-colored laminations comprising calcite crystals that are precipitated during a relatively narrow water temperature and pH-controlled depositional window in the summer. A novel high-resolution imaging protocol was used to photograph the varve record in the 87 cm-long freeze core CRA19-2FT-B2, collected from the deepest part (~23 m) of the lake in February 2019. High-resolution images were used to: (1) characterize varve couplets deposited between AD 1870 and 2000 (chronology verified through 137Cs/210Pb analysis of freeze core CRA22-1FRA-3, and consistent with the historic record of nuclear fallout and other proxies of the Great Acceleration); (2) document distinctive varves that permit a correlation between cores throughout the deep basin of Crawford Lake; (3) measure the thickness of individual dark and light- colored laminations, which were found to vary between 0.04 mm and 3.76 mm; and (4) carry out wavelet and spectral time series analyses based on varve thickness data that can be correlated to climatic trends and cycles. Time series analyses identified cycles with statistically significant periodicities that were attributed to the Quasi-biennial Oscillation (2.3 years), El Niño Southern Oscillation (2–7 years), the 11-year Schwabe Sunspot cycle and a possible Pacific Decadal Oscillation (50–70 years). This research not only provides baseline chronostratigraphic data that allow the correlation between freeze cores subsampled for various proxies, but also documents the dynamics of the climate drivers that influence the deposition of both organic matter and inorganically precipitated calcite. Crawford Lake is currently a candidate site under consideration for the Global boundary Stratotype Section and Point (GSSP) to define the Anthropocene series/epoch.
... These records, like maximum daily Tmax over the central US (Fig. 1a), are unlikely to have resulted from instrumental biases 2,3 . Instead a strong upper-level atmospheric ridge and land-atmosphere interactions may have allowed for extreme heat to build during the Dust Bowl drought [3][4][5][6] . The drought, defined through precipitation and evapotranspiration-based indices 3 , emerged during a period of cooler-than-average North Pacific sea surface temperatures (SSTs) and a warmer North Atlantic 4,6-10 . ...
... Improved representations of precipitation and temperatures during the Dust Bowl period are simulated when AGCMs 4,11 and regional models 13 implement realistic historical land-cover changes and dust aerosol forcing. Following new insights on the observed extreme heat during the Dust Bowl 3,6 , and with future increases in global heatwave activity likely 14 , a comprehensive understanding of what contributed to the Dust Bowl heatwaves is crucial. ...
... Dry spring preconditioning summer heatwaves. A key factor in observed summer heat extremes over the Great Plains is springtime preconditioning 6 . Observational studies suggest that dry springs pre-conditioned the Dust Bowl summer heat extremes 3,6,10 , driven, in part, by mid-tropospheric ridging 10 and reduced moisture advection from the Gulf of Mexico 9 . ...
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The severe drought of the 1930s Dust Bowl decade coincided with record-breaking summer heatwaves that contributed to the socio-economic and ecological disaster over North America’s Great Plains. It remains unresolved to what extent these exceptional heatwaves, hotter than in historically forced coupled climate model simulations, were forced by sea surface temperatures (SSTs) and exacerbated through human-induced deterioration of land cover. Here we show, using an atmospheric-only model, that anomalously warm North Atlantic SSTs enhance heatwave activity through an association with drier spring conditions resulting from weaker moisture transport. Model devegetation simulations, that represent the wide-spread exposure of bare soil in the 1930s, suggest human activity fueled stronger and more frequent heatwaves through greater evaporative drying in the warmer months. This study highlights the potential for the amplification of naturally occurring extreme events like droughts by vegetation feedbacks to create more extreme heatwaves in a warmer world. In the 1930s, the US was hit by a severe drought and record-breaking heatwaves in a period known as the Dust Bowl. Here, the authors present model experiments that suggest that warm North Atlantic temperatures and human devegetation played key roles in making these heatwaves particularly strong.
... The United States (US) Great Plains (GP) are prone to devastating droughts such as the infamous Dust Bowl of the 1930s (e.g., Brönnimann et al., 2009;Donat et al., 2016), the extended drought in the 1950s (e.g., Cook et al., 2011), the Texas drought of 2011 (e.g., Fernando et al., 2016), and the record-breaking drought of 2012 (e.g., Hoerling et al., 2013). Projections of the global climate models (GCMs) that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5) show a robust intensification of dry conditions over the GP under different global warming scenarios in the coming decades (Cook et al., 2015;Teng et al., 2016), which would damage agricultural and food industries throughout the region. ...
... The GP drought and its underlying mechanisms have been studied extensively. Numerous studies have shown that, in the early stages of the GP droughts, the upper-level atmosphere features an anomalous high and anticyclonic vorticity over central North America (Chang and Wallace, 1987;Namias, 1991;Lyon and Dole, 1995;Cook et al., 2011;Donat et al., 2016;Fernando et al., 2016). A dynamical teleconnection between the height anomalies over the US and the North Pacific sea surface temperature (SST) anomalies has been considered as the main driver responsible for the onset of GP summer droughts in 1980 and 1988 (Trenberth et al., 1988;Lyon and Dole, 1995;Chen and Newman, 1998). ...
... Leathers et al. (1991) showed a significant positive correlation between the precipitation anomalies over the US SW and the Pacific/North American teleconnection index (PNA) during April to May. Previous studies have also identified an anomalous high and anticyclonic vorticity in the upper troposphere as an atmospheric driver of summer droughts over central North America (Chang and Wallace, 1987;Namias, 1991;Lyon and Dole, 1995;Cook et al., 2011;Donat et al., 2016;Fernando et al., 2016). For the two droughts of SGP 2011 and NGP 2012, the anomalies of 700 mbar (and also 350 mbar) height feature a dipole pattern with an anomalous low over northwestern North America and an anomalous high over the southeastern US (Fig. S5). ...
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This study addresses the role of the atmospheric moisture budget in determining the onset and development of summer droughts over the North American Great Plains (GP) using two state-of-the-art reanalysis datasets. We identified zonal moisture advection as the main cause of severe tropospheric drying during the extreme droughts in the southern GP in 2011 and northern GP in 2012. For both events, the eastward advection of anomalously dry and warm air in the free troposphere in spring set the stage for summer drought. This led to a sharp drop in relative humidity above the boundary layer, enhancing dry entrainment and suppressing deep convection. Further breakdown of the zonal advection into dynamic (caused by circulation anomalies) and thermodynamic (caused by moisture anomalies) contributions reveals dominance of thermodynamic advection in the tropospheric drying observed during the onset of both 2011 and 2012 droughts. The dependence of thermodynamic advection on the moisture gradient links springtime precipitation in the Rockies and southwestern US, the source region of the anomalous dry advection, to the GP summer precipitation (with correlations > 0.4 using gauge-based data). Identifying this previously overlooked precursor of the GP summer droughts improves our predictive understanding of drought onset mechanisms over the region.
... The DBD was a time of elevated summer temperatures and pronounced deficits in evapotranspiration (Donat et al., 2016;Burnette and Stahle, 2013;Lee and Gill, 2015;Donat et al., 2016). Climate modeling studies and reanalysis of climate data places the DBD in a global context and underscores the complex land-oceanatmosphere interactions propagating extreme droughts (Schubert et al., 2004;Fye et al., 2006;Cook et al., 2007;Seager et al., 2008;Cook et al., 2011;Nigam et al., 2011;Seager and Hoerling, 2014;Donat et al., 2016;Hu et al., 2018). ...
... The DBD was a time of elevated summer temperatures and pronounced deficits in evapotranspiration (Donat et al., 2016;Burnette and Stahle, 2013;Lee and Gill, 2015;Donat et al., 2016). Climate modeling studies and reanalysis of climate data places the DBD in a global context and underscores the complex land-oceanatmosphere interactions propagating extreme droughts (Schubert et al., 2004;Fye et al., 2006;Cook et al., 2007;Seager et al., 2008;Cook et al., 2011;Nigam et al., 2011;Seager and Hoerling, 2014;Donat et al., 2016;Hu et al., 2018). ...
... The DBD was a time of elevated summer temperatures and pronounced deficits in evapotranspiration (Donat et al., 2016;Burnette and Stahle, 2013;Lee and Gill, 2015;Donat et al., 2016). Climate modeling studies and reanalysis of climate data places the DBD in a global context and underscores the complex land-oceanatmosphere interactions propagating extreme droughts (Schubert et al., 2004;Fye et al., 2006;Cook et al., 2007;Seager et al., 2008;Cook et al., 2011;Nigam et al., 2011;Seager and Hoerling, 2014;Donat et al., 2016;Hu et al., 2018). However, there is insufficient knowledge of land surface processes, ecosystem changes, atmospheric feedbacks, and concomitant physical controls on dust emissivity across the historically-defined Dust Bowl region (Fig. 1b;Cordova and Porter, 2015;Lee and Gill, 2015;Bolles et al., 2017;Bolles and Forman, 2018), which can limit climate model performance. ...
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Mineral dust aerosols are a key component of the Earth system and a growing public health concern under climate change, as levels of dustiness increase. The Great Plains in the USA is particularly vulnerable to dust episodes, but land-atmosphere interactions contributing to large-scale dust transport are poorly constrained. This study compiled one of the longest quantitative, spatially-comprehensive records of dust events in the core Dust Bowl region never before available. Combined with experiment station reports from the Soil Conservation Service, reanalysis data products, and contemporary field surveys using a Portable In-Situ Wind Erosion Laboratory (PI-SWERL), the study examined meteorological catalysts for dust events and surficial dynamics of particle emission on the Southern High Plains (SHP). Multivariate statistical analyses of dust event variance yield 6 principal components capturing ˜60% of the variance of all dust event days. Results identified four dominant modes of dust events related to the season of occurrence and principal meteorological controls. A broader assessment of the potential emissivity of SHP soils reveals that disturbed surfaces begin to emit dust at a magnitude-higher rate than undisturbed surfaces as soon as the wind velocity reaches the threshold, increasing linearly with windspeed. Conversely, crusted undisturbed soil surfaces do not begin to reach the same flux rate until much higher windspeeds, at which point crusts are broken and emissivity rates increase exponentially. Significantly, the particle emissivity of undisturbed, loose sandy soils mirrors that of disturbed surfaces in relation to windspeed and potential magnitude of dust emission. This finding suggests that the prevalent sandier, rangeland soils of the SHP could be equal or greater dust sources than cultivated fields during periods of sustained, severe aridity.
... Sea surface temperature has been a well-documented forcing of seasonal precipitation and temperature changes in the United States (USGCRP 2018). In general, drought conditions and warmer summertime temperatures in central North America are connected to a cold Pacific Ocean and a warm Atlantic Ocean (Ting and Wang 1997;Schubert et al. 2004;Seager et al. 2008;Schubert et al. 2009;Wang et al. 2009;Findell and Delworth 2010;Ruiz-Barradas and Nigam 2010;Wang et al. 2010;Cook et al. 2011;Seager and Hoerling 2014;Wang and Schubert 2014;Donat et al. 2016;Jia et al. 2016), with many studies emphasizing the role of either the Pacific (Schubert et al. 2004;Alfaro et al. 2006;Koster et al. 2009;Schubert et al. 2009;Findell and Delworth 2010;Wang et al. 2010;Mei and Wang 2011;Burgman and Jang 2015;Jia et al. 2016) or the Atlantic (Enfield et al. 2001;Knight et al. 2006;Kunkel et al. 2006;Weaver and Nigam 2008;Weaver et al. 2009;Nigam et al. 2011). In particular, the correlation to the North Atlantic is strong for the analysis regions in this study (Table 1). ...
... Great Plains precipitation North Atlantic (Donat et al. 2016) 0.31** 0.34** 20.43** 20.20* Tropical Pacific (Mei and Wang 2011) 20.20* 20.18 0.29** 0.24** Central North Pacific (Mei and Wang 2011) 0 impacts on the results. Although anomalous boundary condition features will appear in results that present a single simulation, results that show the changes dues to vegetation and GHG are free of boundary condition error because all simulations are conducted with the same set of boundary conditions. ...
... SST observations are taken from the Extended Reconstructed Sea Surface Temperatures Version 5 (ERSSTv5) (Huang et al. 2017). In this study, the SST regions used are the North Atlantic [378-538N, 3038-3178E, closely matching Donat et al. (2016)], tropical Pacific (18S- 98N, 1978-2098E), and central North Pacific (298-358N, 1798-1958E) where the latter two are roughly derived from regions used in Mei and Wang (2011). ...
Article
Agricultural development is among the most significant forms of land-use change globally. In central North America it has consisted of cropland expansion in the early 1900s, yield intensification starting in the 1930s, and the development of large irrigated areas beginning in the 1950s. The area of this study encompasses the Midwest and Great Plains of the United States not only because significant agricultural change has occurred here but also because of the significant cooling (warming hole) there in the midcentury. This study investigates the relative contribution of agricultural development and greenhouse gas (GHG) emissions on the observed patterns of regional changes in summer temperature, precipitation, and evapotranspiration using a long-term twentieth-century reanalysis dataset (CERA-20C) as boundary conditions for simulations with the MIT Regional Climate Model (MRCM). Temperatures in the Great Plains (33°–43°N, 95°–109°W) and the Midwest (38°–48°N, 82°–109°W) would have been significantly higher in the second half of the twentieth century without the influence of agricultural development, largely due to an increase in evaporative cooling. The simulations of precipitation changes reflect a significant influence of global SST teleconnections at decadal time scales. Numerical simulations also demonstrate the competing effects of cropland expansion and yield intensification on shaping the observed pattern of increases in precipitation. Ultimately, a combination of agricultural development and decadal variability of global sea surface temperatures (SST) explains most of the observed variability of summer temperature and precipitation during the twentieth century over central North America.
... Internal variability also drives the observed negative temperature trends in the north central and northeast U.S. beginning in the 1930s. These regions were strongly influenced by the anomalously warm Dust Bowl of that decade which was most likely caused by variability in Pacific SSTs (Schubert et al 2004, Seager et al 2008, Donat et al 2016. ...
... The 1930s were an exceptionally hot and dry decade in the U.S., commonly known as the 'Dust Bowl' . For example, the summers of 1934 and 1936 remain two of the hottest on record, particularly in the north central U.S.(Peterson et al 2013, Donat et al 2016. The Dust Bowl was most likely the result of internal variability in Pacific SSTs(Schubert et al 2004, Seager et al 2008, Donat et al 2016, potentially amplified by dust aerosol and land use changes(Cook et al 2009), and/or by internal atmospheric variability. ...
... For example, the summers of 1934 and 1936 remain two of the hottest on record, particularly in the north central U.S.(Peterson et al 2013, Donat et al 2016. The Dust Bowl was most likely the result of internal variability in Pacific SSTs(Schubert et al 2004, Seager et al 2008, Donat et al 2016, potentially amplified by dust aerosol and land use changes(Cook et al 2009), and/or by internal atmospheric variability. Negative summertime temperature trends in the north central U.S. starting in the 1930s reflect their start date occurring during the Dust Bowl, and can be interpreted as a recovery from this anomalously warm decade. ...
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The United States 'warming hole' is a region in the southeast/central U.S. where observed long-term surface temperature trends are insignificant or negative. We investigate the roles of anthropogenic forcing and internal variability on these trends by systematically examining observed seasonal temperature trends over all time periods of at least 10 years during 1901-2015. Long-term summer cooling in the north central U.S. beginning in the 1930s reflects the recovery from the anomalously warm 'Dust Bowl' of that decade. In the northeast and southern U.S., significant summertime cooling occurs from the early 1950s to the mid 1970s, which we partially attribute to increasing anthropogenic aerosol emissions (median fraction of the observed temperature trends explained is 0.69 and 0.17, respectively). In winter, the northeast and southern U.S. cool significantly from the early 1950s to the early 1990s, but we do not find evidence for a significant aerosol influence. Instead, long-term phase changes in the North Atlantic Oscillation contribute significantly to this cooling in both regions, while the Pacific Decadal Oscillation also contributes significantly to southern U.S. cooling. Rather than stemming from a single cause, the U.S. warming hole reflects both anthropogenic aerosol forcing and internal climate variability, but the dominant drivers vary by season, region, and time period.
... A complementary approach is to take advantage of certain mid-latitude conditions that may precede hot weather. Monthly-resolution precipitation deficits have been correlated with high temperatures in the following month 9 and anomalous mid-latitude sea surface temperature (SST) patterns have been highlighted as preceding heatwave episodes [10][11][12][13] . In the following we further explore and quantify the degree to which precipitation and sea surface temperatures provide for skillful prediction of high summer temperatures. ...
... Owing to concerns about data availability, we do not extend our analysis to before 1950, and therefore do not assess if our methods could have predicted the extreme heat experienced in the US during the 1930s. Prior work, however, indicates that SST anomalies in the springs of 1934 and 1936 were of opposite sign to those identified here 13 . The prolonged drought and high temperature associated with the Dust Bowl years probably involve mechanisms different from those associated with the synoptic-scale temperature events focused on here 22 . ...
... North Atlantic SSTs have previously been correlated with hot days in the US on seasonal timescales 13 , and we find significant North Atlantic SST anomalies in our composites across a range of lead times (Fig. 3). We perform the same analysis applied to the PEP domain, but using the region identified in ref. 16 (37.5-52.5 • N, 302.5-317.5 • E) to test if Atlantic anomalies are also predictive of hot days, rather than simply correlated with them. ...
Article
Seasonal forecast models exhibit only modest skill in predicting extreme summer temperatures across the eastern US. Anomalies in sea surface temperature and monthly-resolution rainfall have, however, been correlated with hot days in the US, and seasonal persistence of these anomalies suggests potential for long-lead predictability. Here we present a clustering analysis of daily maximum summer temperatures from US weather stations between 1982-2015 and identify a region spanning most of the eastern US where hot weather events tend to occur synchronously. We then show that an evolving pattern of sea surface temperature anomalies, termed the Pacific Extreme Pattern, provides for skillful prediction of hot weather within this region as much as 50 days in advance. Skill is demonstrated using out-of-sample predictions between 1950 and 2015. Rainfall deficits over the eastern US are also associated with the occurrence of the Pacific Extreme Pattern and are demonstrated to offer complementary skill in predicting high temperatures. The Pacific Extreme Pattern appears to provide a cohesive framework for improving seasonal prediction of summer precipitation deficits and high temperature anomalies in the eastern US.
... The 1950s drought lasted for 76 months in the Dust Bowl portion of Texas, 67 months in New Mexico, 58 months in Colorado, 57 months in Oklahoma, and 56 months in Kansas (Table 1). The 1930s drought, however, covered a larger area of North America at its peak , and was accompanied by warmer temperatures (Donat et al., 2015). ...
... noaa.gov/paleo/pdsidata.html. Fig. S4), and synthesizing information from Donat et al. (2015), Schubert et al. (2004), Seager et al. (2008) and other references cited in Section 7. ...
... Globally, warmer than average sea surface temperatures in the subtropical and tropical North Atlantic, a particularly warm North Atlantic, colder-than-average western/central Pacific, and warming in parts of the northeast Pacific all tend to contribute to drought in central and southwestern North America (Woodhouse and Overpeck, 1998;Fye et al., 2004;McCabe et al., 2004;Schubert et al., 2004;Zhang and Mann, 2005;Booth et al., 2005Booth et al., , 2006Cook et al.,2007Cook et al., , 2010Cook et al., , 2011aCook et al., , 2011bSeager et al., 2008;Brönnimann et al., 2009;Kushnir et al., 2010;Shin et al., 2010;Feng et al., 2011;Nigam et al., 2011;Wang and Schubert, 2014;Donat et al., 2015). Longer-term climatic trends such as the Pacific Decadal Oscillation (PDO), Atlantic Multi-decadal Oscillation (AMO), and Arctic Oscillation (AO), all involving changing sea surface temperature and air pressure patterns, also play a role in modulating and amplifying North American drought cycles, including those in the Great Plains (e.g. ...
... To understand the potential driving mechanism behind the present-day amplification of the Dust Bowl heatwave conditions, we consider the influence of spring drought in amplifying heat extremes over the central United States 14,17 . We first re-order the simulations based on their spring-time (March-May) precipitation over the central United States, driest to wettest, and then see how this affects the subsequent summer precipitation and heatwave behaviour (frequency, amplitude and timing) over the central United States (Fig. 3). ...
... model only has fixed historical bare soil fractions across the central United States, making it difficult to assess land-surface feedbacks in the response to rapid land clearing. Modelling studies have shown that the Dust Bowl conditions are amplified by rapidly increasing levels of bare soil and imposed dust 16,30 , via surface energy fluxes accelerating the drought 17 ; the human-induced contribution to the heatwaves is therefore likely to be underestimated here. That is why the focus of the present study is the direct impact of GHGs on the historical heatwaves under comparable conditions. ...
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Substantial warming occurred across North America, Europe and the Arctic over the early twentieth century1, including an increase in global drought2, that was partially forced by rising greenhouse gases (GHGs)3. The period included the 1930s Dust Bowl drought4–7 across North America’s Great Plains that caused widespread crop failures4,8, large dust storms9 and considerable out-migration10. This coincided with the central United States experiencing its hottest summers of the twentieth century11,12 in 1934 and 1936, with over 40 heatwave days and maximum temperatures surpassing 44 °C at some locations13,14. Here we use a large-ensemble regional modelling framework to show that GHG increases caused slightly enhanced heatwave activity over the eastern United States during 1934 and 1936. Instead of asking how a present-day heatwave would behave in a world without climate warming, we ask how these 1930s heatwaves would behave with present-day GHGs. Heatwave activity in similarly rare events would be much larger under today’s atmospheric GHG forcing and the return period of a 1-in-100-year heatwave summer (as observed in 1936) would be reduced to about 1-in-40 years. A key driver of the increasing heatwave activity and intensity is reduced evaporative cooling and increased sensible heating during dry springs and summers. The United States experienced two of its hottest recorded summers in 1934 and 1936, amplified by drier soils associated with the Dust Bowl drought. A large regional climate model ensemble estimates present-day GHGs would cause similarly extreme, 1-in-100-year heatwaves to occur about every 40 years.
... The ETCW featured a pronounced Arctic warming in the 1920s and 1930s, and embedded in this period were several important climatic anomalies such as Indian monsoon failures in the 1900s (Wang, 2006;Zhou et al., 2010), the North American "Dust Bowl" droughts and record-breaking heat waves in the 1930s (Cook, Miller, & Seager, 2009;Cowan et al., 2017;Donat et al., 2016;Schubert, Suarez, Pegion, Koster, & Bacmeister, 2004b), the cold European winters of 1940-1942(Brönnimann et al., 2004, and the World War II period drought in Australia between 1937and 1945(e.g., Verdon-Kidd & Kiem, 2009). The European summer droughts and heat waves of the mid and late 1940s (e.g., Sutton & Hodson, 2005), such as 1947 (Schär et al., 2004), followed the anomalously cold winters during WWII. ...
... Dry springs were hence an important factor during the hottest Dust Bowl summers of 1934 and 1936 (Cowan et al., 2017). However, the role of SSTs in spring drought specifically is less than clear (Donat et al., 2016), as are the driving mechanisms of high decadal heat wave activity overall. Could such heat waves occur again, possibly stronger due to global warming? ...
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The most pronounced warming in the historical global climate record prior to the recent warming occurred over the first half of the 20th century and is known as the Early Twentieth Century Warming (ETCW). Understanding this period and the subsequent slowdown of warming is key to disentangling the relationship between decadal variability and the response to human influences in the present and future climate. This review discusses the observed changes during the ETCW and hypotheses for the underlying causes and mechanisms. Attribution studies estimate that about a half (40–54%; p > .8) of the global warming from 1901 to 1950 was forced by a combination of increasing greenhouse gases and natural forcing, offset to some extent by aerosols. Natural variability also made a large contribution, particularly to regional anomalies like the Arctic warming in the 1920s and 1930s. The ETCW period also encompassed exceptional events, several of which are touched upon: Indian monsoon failures during the turn of the century, the “Dust Bowl” droughts and extreme heat waves in North America in the 1930s, the World War II period drought in Australia between 1937 and 1945; and the European droughts and heat waves of the late 1940s and early 1950s. Understanding the mechanisms involved in these events, and their links to large scale forcing is an important test for our understanding of modern climate change and for predicting impacts of future change. This article is categorized under: • Paleoclimates and Current Trends > Modern Climate Change
... In particular, local boundary conditions may play a vital role. For example, specific anomalous sea surface temperature patterns have been identified to be associated with similar atmospheric flow anomalies, which ultimately significantly contributed to summer extremes (Donat et al., 2016;Lyon & Dole, 1995;McKinnon et al., 2016). In contrast, other studies have also demonstrated the contribution of soil moisture conditions to heat waves by releasing surface diabatic heat (Miralles et al., 2019;Seneviratne et al., 2010) and even of the circumglobal circulation response of Rossby waves Figure 9. Evolution of heat waves 20 days ahead of the events for ROB: Red and dashed blue contours represent composites of 200 hPa streamfunction anomalies at ±0.025, and ±0.07 × 10 7 m 2 s 1 (red is positive). ...
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Plain Language Summary Heat waves are the leading weather‐related killer in the US and primarily affect the most vulnerable communities. These extreme events are statistically related to a mid‐latitudinal planetary‐scale coherent pattern with a wavenumber‐five structure along the longitudes. To determine what controls this wave pattern, we use a simple general circulation model containing only a dry atmosphere in a rotating sphere without complex interactions with moisture or clouds. We modify the model's climatological state of temperature and velocity fields based on the observed thermal structure from the Northern Hemisphere summer. After this modification, we observe a similar wavenumber‐five pattern developing days before the heatwave events, resembling the observations. This result suggests that the climatological state of the Northern Hemisphere summer provides a critical and conducive environment for heat waves in the US because it serves as a “brake” to slow down the propagation speed of the Rossby wave packets. A deeper understanding of its dynamics is crucial and needed because, as this pattern develops up to 20 days ahead of the heat extremes, the underlying physical process governing this pattern may serve as a source of predictability on the a subseasonal‐to‐seasonal (S2S) timescale, a current gap in forecasts between weather and climate.
... In contrast, the Western United States has observed larger warming trends during the last few decades, which has contributed to the formation of prolonged drought risk and favorable environments for wildfire ignition (Abatzoglou & Williams, 2016;Diffenbaugh et al., 2015;Parks & Abatzoglou, 2020;Williams et al., 2020). Although the ratio of new record high temperatures compared to record lows continues to widen (Meehl et al., 2022), the overall detectability of CONUS temperature signals continues to remain challenging, partially due to the anomalous warmth observed in the Dust Bowl era (Donat et al., 2016;Hansen et al., 2001;Peterson et al., 2013). Given the broad range of consequences associated with future projected warming over the CONUS (Program, 2018;Wuebbles et al., 2014), it remains urgent to better characterize the ToE of summertime temperatures in order to aid in future decision-making on regional health hazards and other impacts that could fall outside of historical climate variability (Bevacqua et al., 2023;Deser, 2020;Mankin et al., 2020;Schwarzwald & Lenssen, 2022). ...
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To better understand the regional changes in summertime temperatures across the conterminous United States (CONUS), we adopt a recently developed machine learning framework that can be used to reveal the timing of emergence of forced climate signals from the noise of internal climate variability. Specifically, we train an artificial neural network (ANN) on seasonally averaged temperatures across the CONUS and then task the ANN to output the year associated with an individual map. In order to correctly identify the year, the ANN must therefore learn time‐evolving patterns of climate change amidst the noise of internal climate variability. The ANNs are first trained and tested on data from large ensembles and then evaluated using observations from a station‐based data set. To understand how the ANN is making its predictions, we leverage a collection of ad hoc feature attribution methods from explainable artificial intelligence (XAI). We find that anthropogenic signals in seasonal mean minimum temperature have emerged by the early 2000s for the CONUS, which occurred earliest in the Eastern United States. While our observational timing of emergence estimates are not as sensitive to the spatial resolution of the training data, we find a notable improvement in ANN skill using a higher resolution climate model, especially for its early twentieth century predictions. Composites of XAI maps reveal that this improvement is linked to temperatures around higher topography. We find that increases in spatial resolution of the ANN training data may yield benefits for machine learning applications in climate science.
... In addition to drought, aerosol particles and cloud cover may also account for forest decline. During the 1930s, most of the world experienced periods of severe 'dust bowls', during which atmospheric aerosol levels substantially rose (Donat et al. 2016;Schubert et al. 2004). When the aerosol load increases, it raises the concentration of cloud droplets, and the overall cloud cover becomes larger, leading to an increase in cloud reflectivity and more sunlight being reflected back into space, thus reducing the amount of radiation reaching the surface (Lohmann 2017;Yli-Juuti et al. 2021). ...
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Key message The decline in Qinghai spruce forests was not aggravated by increasing temperatures; rather, excess cloud cover was the main trigger of forest decline. Abstract Forest declines accompanied by global warming have been increasingly reported worldwide. However, such declines are less frequently observed on the Tibetan Plateau. To assess the decline risk in Tibetan Plateau forests, we studied tree rings from two spruce (Picea crassifolia) forests in the Qilian Mountains of the northeastern Tibetan Plateau. We collected increment cores from 35 spruce trees at each sampling site in Qilian and Menyuan counties. Tree-ring widths were measured, crossdated and detrended. Growth decline in trees was identified based on low tree-ring index values lasting at least eight years. The percentage of the number of declining trees in all the samples was used to evaluate the occurrence and degree of forest decline events. The results showed that forest declines occurred at a low level during 1915–1933 and 1994–2005 in Qilian County and during 1996–2003 in Menyuan County. The radial growth of spruce had a significant negative correlation with the amount of cloud cover in the growing season. The results suggest that the growth declines of the spruce forests were not directly induced by the recent climate warming but were more likely related to stress conditions under increased cloud cover. Our findings provide insights into the assessment of forest decline risks and planning of forest protection.
... Previous studies of heat waves have used definitions with a variety of time and spatial scales (National Academies of Sciences, Engineering, and Medicine (US) and National Academies of Sciences, Engineering, and Medicine (US), 2016; Xu et al., 2016). Definitions are usually based on the exceedance of fixed absolute values or a deviation from the normal such as from a daily mean or maximum value (Cowan et al., 2014;Donat et al., 2016;Robinson, 2001). ...
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As global surface temperatures continue to rise, both the duration and the intensity of heat waves across most land areas are expected to increase. The 2022 European summer broke a number of temperature records where a new record daily maximum temperature of 40.3°C was reached on 19th July making it the hottest July heat wave event in the UK. This paper aims to detect and analyse historical heat wave events, particularly prior to 1927 and compare these with recent events, particularly, 2022, which featured four summer heat wave events in the UK. This allows us to understand how noteworthy historical extremes are in comparison to those in recent decades, to place modern events into historical context, and to extend the sample of extreme events. Summer heat wave events have been detected between 1878 and 2022 from long station data in the UK. Heat wave extent, duration, and intensity have been analysed to compare past heat waves to the recent 2022 heat waves. For each of the summer months at least one of the top 10 most intense events between 1878 and 2022 occurred in the earliest third of the dataset (before 1927) emphasising the value of analysing early heat events. In all detected events, the anomalous UK heat was part of large‐scale European extreme heat when examining 20th‐century reanalysis data, associated with a high‐pressure system. The 2022 July event resembles in pattern of warming and circulation some earlier events, for example, in 1925. While there is a clear trend in the monthly data and the overall frequency of anomalously hot days, heat wave activity on daily scales even in the period 1878 and 1926 is considerable and in some cases comparable to modern heat wave events in the UK. The most intense events detected led to societal impacts based on UK newspaper articles from the period including impacts on the agricultural sector, health impacts, and travel disruptions, broadly comparable to impacts from recent events.
... These are not yet fully understood or adequately represented in climate models . The negative trends in eastern North America and parts of California are well understood to be the result of land use changes, irrigation and changes in agricultural practice (Donat et al., 2016(Donat et al., , 2017Thiery et al., 2017;Cowan et al., 2020). The Pacific Northwest shows positive trends twice as large as the global mean temperature trend up to 2019. ...
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Towards the end of June 2021, temperature records were broken by several degrees Celsius in several cities in the Pacific Northwest areas of the US and Canada, leading to spikes in sudden deaths and sharp increases in emergency calls and hospital visits for heat-related illnesses. Here we present a multi-model, multi-method attribution analysis to investigate the extent to which human-induced climate change has influenced the probability and intensity of extreme heat waves in this region. Based on observations, modelling and a classical statistical approach, the occurrence of a heat wave defined as the maximum daily temperature (TXx) observed in the area 45–52∘ N, 119–123∘ W, was found to be virtually impossible without human-caused climate change. The observed temperatures were so extreme that they lay far outside the range of historical temperature observations. This makes it hard to state with confidence how rare the event was. Using a statistical analysis that assumes that the heat wave is part of the same distribution as previous heat waves in this region led to a first-order estimation of the event frequency of the order of once in 1000 years under current climate conditions. Using this assumption and combining the results from the analysis of climate models and weather observations, we found that such a heat wave event would be at least 150 times less common without human-induced climate change. Also, this heat wave was about 2 ∘C hotter than a 1-in-1000-year heat wave would have been in 1850–1900, when global mean temperatures were 1.2 ∘C cooler than today. Looking into the future, in a world with 2 ∘C of global warming (0.8 ∘C warmer than today), a 1000-year event would be another degree hotter. Our results provide a strong warning: our rapidly warming climate is bringing us into uncharted territory with significant consequences for health, well-being and livelihoods. Adaptation and mitigation are urgently needed to prepare societies for a very different future.
... It is thought that the drought was initiated and maintained by a combination of decadal timescale internally-generated negative sea surface temperature (SST) anomalies in the tropical Pacific associated with the negative phase of the Interdecadal Pacific Oscillation (IPO), and positive SST anomalies north of the equator in the Atlantic with the positive phase of the Atlantic Multidecadal Oscillation (AMO) 2,3 . The resulting convective heating anomalies in those two basins then forced atmospheric circulation anomalies that produced a naturally-occurring drought over the Great Plains 2-4 along with anomalous heat 5 and associated heat extremes there 6,7 . ...
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Extraordinary heat extremes occurred in the 1930s in areas of the Northern Hemisphere far from the record setting heat over the US associated with the Great Plains Dust Bowl drought. A climate model sensitivity experiment is used to identify a new mechanism involving a warm season circumglobal atmospheric teleconnection pattern that spread heat extremes over far-flung areas of the Northern Hemisphere arising from the intense heating over the desiccated Great Plains themselves. It has only been in the twenty-first century that human populations in these regions of the Northern Hemisphere have experienced heat extremes comparable to the 1930s. This demonstrates that humans influenced Northern Hemisphere temperature and heat extremes through disastrous and unprecedented regional land use practices over the Great Plains, and points to the possibility that future intense regional droughts could affect heat extremes on hemispheric scales.
... This is very common, especially for lower APs (e.g., formation of heatwave flash drought) and humid and semi-humid regions. For example, the increased Arctic warming in the 1920s to 1940s which was mostly concentrated in higher latitudes 71 (near-polar regions), the cold European winters 72 in 1940-1942, the "Dust Bowl droughts and extreme heatwaves in North America [73][74][75][76] in 1930s, the World War II period droughts in Australia 77 between 1937 and 1945 and European summer droughts and heatwaves 78 in 1940s. The changes in the wet and dry conditions in the latter decades could be attributed to global warming and the associated climate change. ...
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In recent decades, human-induced climate change has caused a worldwide increase in the frequency/intensity/duration of extreme events, resulting in enormous disruptions to life and property. Hence, a comprehensive understanding of global-scale spatiotemporal trends and variability of extreme events at different intensity levels (e.g., moderate/severe/extreme) and durations (e.g., short-term/long-term) of normal, dry and wet conditions is essential in predicting/forecasting/mitigating future extreme events. This article analyses these aspects using estimates of a non-stationary standardized precipitation evapotranspiration index corresponding to different accumulation periods for 0.5° resolution CRU grids at globe-scale. Results are analyzed with respect to changes in land-use/landcover and geographic/location indicators (latitude, longitude, elevation) at different time scales (decadal/annual/seasonal/monthly) for each continent. The analysis showed an (i) increasing trend in the frequency/count of both dry and wet conditions and variability of dry conditions, and (ii) contrasting (decreasing) trend in the variability of wet conditions, possibly due to climate change-induced variations in atmospheric circulations. Globally, the highest variability in the wet and dry conditions is found during the Northern hemisphere's winter season. The decadal-scale analysis showed that change in variability in dry and wet conditions has been predominant since the 1930s and 1950s, respectively and is found to be increasing in recent decades.
... (near-polar regions), the cold European winters72 in 1940-1942, the "Dust Bowl droughts and extreme heatwaves in North America[73][74][75][76] in 1930s, the World War II period droughts in Australia 77 between 1937 and 1945 and European summer droughts and heatwaves 78 in 1940s. The changes in the wet and dry conditions in the latter decades could be attributed to global warming and the associated climate change.Among all the decades from 1901-to 2019, those during 1951-1980 showed the lowest variability (SVI DE ) of extreme, severe and moderate wet conditions. ...
Article
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In recent decades, human-induced climate change has caused a worldwide increase in the frequency/intensity/duration of extreme events, resulting in enormous disruptions to life and property. Hence, a comprehensive understanding of global-scale spatiotemporal trends and variability of extreme events at different intensity levels (e.g., moderate/severe/extreme) and durations (e.g., short-term/long-term) of normal, dry and wet conditions is essential in predicting/forecasting/mitigating future extreme events. This article analyses these aspects using estimates of a non-stationary standardized precipitation evapotranspiration index corresponding to different accumulation periods for 0.5° resolution CRU grids at globe-scale. Results are analyzed with respect to changes in land-use/landcover and geographic/location indicators (latitude, longitude, elevation) at different time scales (decadal/annual/seasonal/monthly) for each continent. The analysis showed an (i) increasing trend in the frequency/count of both dry and wet conditions and variability of dry conditions, and (ii) contrasting (decreasing) trend in the variability of wet conditions, possibly due to climate change-induced variations in atmospheric circulations. Globally, the highest variability in the wet and dry conditions is found during the Northern hemisphere's winter season. The decadal-scale analysis showed that change in variability in dry and wet conditions has been predominant since the 1930s and 1950s, respectively and is found to be increasing in recent decades.
... Apart from individual station records showing breaks or spurious trends, there are coherent areas with negative or zero trends. In the Central Plains of the United States, the highest temperatures were observed during the Dust Bowl of the 1930s (Cook et al., 2011;Donat et al., 2016), not in recent years, while in the Central-Eastern United States, hot extremes are also not steadily increasing with global mean temperature during recent decades and daytime maxima show different trends from minima (Portmann et al., 2009). In India, extreme high temperatures have no or very small trends since the 1970s (van Oldenborgh et al., 2018) (Figure 1b). ...
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It sounds straightforward. As the Earth warms due to the increased concentration of greenhouse gases in the atmosphere, global temperatures rise and so heatwaves become warmer as well. This means that a fixed temperature threshold is passed more often: the probability of extreme heat increases. However, land use changes, vegetation change, irrigation, air pollution, and other changes also drive local and regional trends in heatwaves. Sometimes they enhance heatwave intensity, but they can also counteract the effects of climate change, and in some regions, the mechanisms that impact on trends in heatwaves have not yet been fully identified. Climate models simulate heatwaves and the increased intensity and probability of extreme heat reasonably well on large scales. However, changes in annual daily maximum temperatures do not follow global warming over some regions, including the Eastern United States and parts of Asia, reflecting the influence of local drivers as well as natural variability. Also, temperature variability is unrealistic in many models, and can fail standard quality checks. Therefore, reliable attribution and projection of change in heatwaves remain a major scientific challenge in many regions, particularly where the moisture budget is not well simulated, and where land surface changes, changes in short‐lived forcers, and soil moisture interactions are important.
... During this time several remarkable weather extremes and climate anomalies were reported such as, a pronounced warming in the Arctic during 1912-1930s (e.g. Hanssen-Bauer, 2002Drinkwater, 2006), a series of the North American droughts and unusual heat waves in the 1930s (Cook et al., 2009;Cowan et al., 2017;Donat et al., 2016;Schubert et al., 2004), a drought in Australia during 1937-1945(Verdon-Kidd and Kiem, 2009 and the European summer droughts and heat waves of the mid-to-late 1940s (Sutton and Hodson, 2005). Also, higher sea surface temperatures (SST) were reported during the 1930s in the North Sea, the English Channel and the Baltic Sea (Beverton and Lee, 1965) and the Gulf Stream waters were 0.4 • C warmer as compared to the 1912-1918 (Scherhag, 1937). ...
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The pteropod Limacina lesueurii (d'Orbigny 1836) is a subtropical species, which in the Atlantic Ocean shows maximum abundance between 30°N and 25°S. In the North Sea, this species has so far only been occasionally reported from the coastal waters off the northern Scotland and in the English Channel. In this short note, we for the first time report presence of L. lesueurii in two sediment cores taken in the Kosterfjord (Skagerrak, North Sea). The pteropod, generally absent or occurring in low numbers throughout both cores, showed distinctly higher abundances between 100 and 130 cm core depth, which based on dating by ¹³⁷Cs, ²¹⁰Pb and lead pollution records corresponds to ~1920–1950s. During this period positive sea surface temperature anomalies were reported in the North Atlantic and number of oceanic water inflows have been documented for the North Sea. Some of these inflows were accompanied by”enormous shoals” of L. lesueurii observed in the English Channel and NE of Scotland. We hypothesize that L. lesueurii was transported into the Skagerrak in connection with these and propose this pteropod species as a new valuable stratigraphic marker for oceanic water inflows in the study area.
... Besides this main definition we also 55 analysed the observations for three stations in Portland, Seattle and Vancouver with long homogeneous time series. (Donat et al., 2016(Donat et al., , 2017Thiery et al., 2017;Cowan et al., 2020). ...
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Towards the end of June 2021, temperature records were broken by several degrees Celsius in several cities in the Pacific northwest areas of the U.S. and Canada, leading to spikes in sudden deaths, and sharp increases in hospital visits for heat-related illnesses and emergency calls. Here we present a multi-model, multi-method attribution analysis to investigate to what extent human-induced climate change has influenced the probability and intensity of extreme heatwaves in this region. Based on observations and modeling, the occurrence of a heatwave with maximum daily temperatures (TXx) as observed in the area 45° N–52° N, 119° W–123° W, was found to be virtually impossible without human-caused climate change. The observed temperatures were so extreme that they lie far outside the range of historically observed temperatures. This makes it hard to quantify with confidence how rare the event was. In the most realistic statistical analysis, which uses the assumption that the heatwave was a very low probability event that was not caused by new nonlinearities, the event is estimated to be about a 1 in 1000 year event in today’s climate. With this assumption and combining the results from the analysis of climate models and weather observations, an event, defined as daily maximum temperatures (TXx) in the heatwave region, as rare as 1 in a 1000 years would have been at least 150 times rarer without human-induced climate change. Also, this heatwave was about 2 °C hotter than a 1 in 1000-year heatwave that at the beginning of the industrial revolution would have been (when global mean temperatures were 1.2 °C cooler than today). Looking into the future, in a world with 2 °C of global warming (0.8 °C warmer than today), a 1000-year event would be another degree hotter. It would occur roughly every 5 to 10 years in such global warming conditions. Our results provide a strong warning: our rapidly warming climate is bringing us into uncharted territory with significant consequences for health, well-being, and livelihoods. Adaptation and mitigation are urgently needed to prepare societies for a very different future.
... Another example of possible use of the database can be found in Figure 5 where we depict the monthly average of the maximum temperature recorded at Extremadura in July and August 1936. We selected this year because its summer was very warm around the world (Conwan et al., 2017;Donat et al., 2016). There are ten Extremadura stations with data available in July 1936. ...
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In this work, we provide instrumental meteorological data recovered for the Extremadura region (interior SW Iberia), from 1826 to mid‐20th century. Meteorological variables such as air temperature, atmospheric pressure, precipitation, wind direction and humidity, among others, were retrieved. In total, more than 750 000 instrumental data in 157 meteorological series belonging to 131 different locations throughout Extremadura were rescued. It must be noted that daily resolution data constitutes 80% of the database. This great effort of digitization and data collection has been carried out with the aim of contributing to a significant expansion of the length of the databases with meteorological information in this region. Therefore, this database will provide a better understanding of climate variability, trends and extreme events of the Extremadura region. We provide instrumental meteorological data recovered for Extremadura region (interior SW Iberia), from 1826 to mid‐20th century. Meteorological variables such as air temperature, atmospheric pressure, precipitation, wind direction and humidity, among others, were retrieved. More than 750 000 instrumental data in 157 meteorological series belonging to 131 different locations throughout Extremadura were rescued.
... However, the application of a LOWESS regression to the data suggested there were general patterns in the heating and cooling of air temperatures over time. The greatest measured air temperatures found in the 1930s during the Dust Bowl years [33] are similar to maximum air temperatures found in the period 2008 to 2018. Given that we and others have found a close correspondence between air temperature and lake water temperatures, we expect that average lake water temperatures will follow the same trends as air temperatures over time. ...
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Because warming water temperatures have widespread consequences for freshwater communities, we were interested in estimating the patterns and rates of change of near-surface summer water temperatures in United States lakes. We developed multiple regression models to relate daily surface water temperatures in lakes of the conterminous United States to 8-day average air temperatures, latitude, elevation, and sampling month and year using data from 5723 lake samples in the months of June-September during the period 1981-2018. Our model explained 79% of the variation with a root-mean-square error of 1.69 • C. We predicted monthly average near-surface water temperatures for 1033 lakes for each year from 1981 through 2018. Lakes across the conterminous United States have been warming for the period 1981-2018 at an average heating rate of 0.32 • C per decade for the summer months (June-September). The average summer warming from 1981-2018 would be the equivalent of a lake decreasing 259 m in elevation or moving 233 km south. On the basis of national air temperatures starting in 1895, it was inferred that lake water temperatures are variable from year to year and have been steadily increasing since 1964, but that maximum temperatures in the 1930s were just as warm as those in 2008-2018.
... as the Dust Bowl drought of the 1930s, highlight how extreme spring and summer temperatures can speed the onset, and worsen the impact, of dry spells and droughts (17,18). Climate change farther east, in the Midwest, also means that summer dry spells will tend not only to become hotter but also to lengthen (19). ...
... For example, previous observational studies have shown the impact of soil moisture deficits on hot extreme temperatures through changes in evapotranspiration over southeastern and western Europe and Russia (Hirschi et al., 2011;Miralles et al., 2012;Hauser et al., 2016). Additionally, soil moisture regimes have been found to alter the energy and water exchanges at the surface, influencing inter-annual summer temperature variability in central parts of North 35 America (Donat et al., 2016), and precipitation events in western North America (Diro et al., 2014). Land-Atmosphere interactions, and consequently near-surface conditions, are influenced by vegetation and snow covers (Stieglitz and Smerdon, 2007;Diro et al., 2018). ...
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Abstract. The representation and projection of extreme temperature and precipitation events in regional and global climate models are of major importance for the study of climate change impacts. However, state-of-the-art global and regional climate model simulations yield a broad inter-model range of intensity, duration and frequency of these extremes. Here, we present a modeling experiment using the Weather Research and Forecasting (WRF) model to determine the influence of the land surface model (LSM) component on uncertainties associated with extreme events. First, we evaluate land-atmosphere interactions within four simulations performed by the WRF model using three different LSMs from 1980 to 2012 over North America. Results show LSM-dependent differences at regional scales in the frequency of occurrence of events when surface conditions are altered by atmospheric forcing or land processes. The inter-model range of extreme statistics across the WRF simulations is large, particularly for indices related to the intensity and duration of temperature and precipitation extremes. Areas showing large uncertainty in WRF simulated extreme events are also identified in a model ensemble from three different Regional Climate Model (RCM) simulations participating in the Coordinated Regional Climate Downscaling Experiment (CORDEX) project, revealing the implications of these results for other model ensembles. This study illustrates the importance of the LSM choice in climate simulations, supporting the development of new modeling studies using different LSM components to understand inter-model differences in simulating temperature and precipitation extreme events, which in turn will help to reduce uncertainties in climate model projections.
... For example, the European 'year without a summer' had a clear impact from the Mount Tambora eruption which greatly enhanced the probability of such a cold summer as occurred in 1816 (Schurer et al., in press). The Dust Bowl heat waves in the central United States in the 1930s set records to date (Cowan et al., 2017;Donat et al., 2016), and have likely been strongly influenced by changes in land cover (Cowan et al., in prep.) consistent with sensitivity of land climate to vegetation and land degradation (Arneth et al., 2019). ...
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This review addresses the causes of observed climate variations across the industrial period, from 1750 to present. It focuses on long-term changes, both in response to external forcing and to climate variability in the ocean and atmosphere. A synthesis of results from attribution studies based on palaeoclimatic reconstructions covering the recent few centuries to the 20th century, and instrumental data shows how greenhouse gases began to cause warming since the beginning of industrialization, causing trends that are attributable to greenhouse gases by 1900 in proxy-based temperature reconstructions. Their influence increased over time, dominating recent trends. However, other forcings have caused substantial deviations from this emerging greenhouse warming trend: volcanic eruptions have caused strong cooling following a period of unusually heavy activity, such as in the early 19th century; or warming during periods of low activity, such as in the early-to-mid 20th century. Anthropogenic aerosol forcing most likely masked some global greenhouse warming over the 20th century, especially since the accelerated increase in sulphate aerosol emissions starting around 1950. Based on modelling and attribution studies, aerosol forcing has also influenced regional temperatures, caused long-term changes in monsoons and imprinted on Atlantic variability. Multi-decadal variations in atmospheric modes can also cause long-term climate variability, as apparent for the example of the North Atlantic Oscillation, and have influenced Atlantic ocean variability. Long-term precipitation changes are more difficult to attribute to external forcing due to spatial sparseness of data and noisiness of precipitation changes, but the observed pattern of precipitation response to warming from station data supports climate model simulated changes and with it, predictions. The long-term warming has also led to significant differences in daily variability as, for example, visible in long European station data. Extreme events over the historical record provide valuable samples of possible extreme events and their mechanisms.
... We highlight three significantly different patterns of SST anomaly in the Pacific and North Atlantic. Occurrence of warm SST anomaly in the northern Pacific and Atlantic Oceans could shift the atmospheric pressure patterns and consequently northerly wind anomalies reducing the flow of moist air into the continent and leading to reduced precipitation and consequently result in high exceedances in the frequency of hot days over much of United States (Donat et al., 2016). ...
Article
Low flow events can cause significant impacts to river ecosystems and water‐use sectors; as such it is important to understand their variability and drivers. In this study, we characterize the variability and timing of annual total frequency of low streamflow days across a range of headwater streams within the continental United States (US). To quantify this, we use a metric that counts the annual number of low flow days below a given threshold, defined as the Cumulative Dry days Occurrence (CDO). First, we identify three large clusters of streamgauge locations using a Partitioning Around Medoids (PAM) clustering algorithm. In terms of timing, results reveal that for most clusters, the majority of low streamflow days occur from the middle of summer until early fall, though several locations in Central and Western US also experience low flow days in cold seasons. Further, we aim to identify the regional climate and larger‐scale drivers for these low streamflow days. Regionally, we find that precipitation deficits largely associate with low streamflow days in the western US, while within the central and eastern US clusters, high temperature indicators are also linked to low streamflow days. In terms of larger‐scale, we examine sea surface temperature (SST) anomalies, finding that extreme dry years exhibit a high degree of co‐occurrence with different patterns of warmer SST anomalies across the Pacific and northern Atlantic Oceans. The linkages identified with regional climate and SSTs offer promise towards regional prediction of changing conditions of low streamflow events.
... Temperature anomalies were also more extreme in summer, and these conditions acted to exacerbate drought conditions during extreme summer droughts like in 1934 and1936 (Figs. 1c, d, 2c, d;Donat et al. 2016). The intensification of drought from spring to summer during the 1930s and 1950s is also well represented by the instrumental Palmer's Z-index (Figs. 1e, f, 2e, f;Palmer 1965); the atmospheric moisture balance calculated from precipitation and temperature measurements but without the strong monthly persistence prescribed for the soil moisture formulation of the full Palmer Drought Severity Index (PDSI). ...
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The two most severe droughts to impact the Great Plains in the twentieth century, the 1930s Dust Bowl and 1950s Drought, were the result of multiyear moisture deficits during the spring and especially the summer season. Tree-ring reconstructions of the Palmer Drought Severity Index indicate similar droughts in magnitude have occurred in previous centuries, but these reconstructions do not capture the potential distinct seasonal drought characteristics like those of the 1930s and 1950s. Separate tree-ring reconstructions of the spring and summer Z-index based on earlywood, latewood, and adjusted latewood width chronologies have been developed for two regions in the northern and southern Great Plains of the US. The reconstructions extend from 1651 to 1990 and 1698–1990, respectively, with instrumental data added from 1991 to 2017. The four reconstructions explain from 39 to 56% of the variance during the 1945–1990 calibration interval and are significantly correlated with independent moisture balance observations during the 1900–1944 validation period. The reconstructions reproduce similar seasonal sea-surface temperature and 500 mb geopotential height spatial correlation patterns detected with the instrumental data. The 1930s is estimated to have been the most extreme decadal summer drought to impact the two regions concurrently in the last few centuries. On average, spring moisture deficits were more severe during the multidecadal droughts of the mid- to late-nineteenth century, but the timing of drought onset and termination differed between the study regions. In the recent two decades the spring moisture balances for the two study regions have largely been opposite, and this has been one of the most extreme periods of anti-phasing in the last few centuries. Seasonal moisture reversals are not randomly distributed in time based on the reconstructed estimates and are related to sea-surface temperature anomalies in the tropical Pacific and to mid-tropospheric circulation changes over the North Pacific–North American sector during May and June.
... [O .SOC]). The decreases in the eastem half of 4 Nation , particularly in dIe Great Plains, are mainly tied to dIe unprecedented summer heat of the 5 19305 Dust Bowl era, which was exacerbated by land-surface feedbacks driven by springtime 6 precipitation deficits and land mismanagement ( Donat et al. 2016). However, anthropogenic 7 aerosol forcing may also have reduced summer temperatures in the Nordlea st and Soudleast from 8 the early 19505 to the mid-1 970s ( Mascioli et al. 20 17), and agricultural intensification may have 9 suppressed the hottest extremes in the Midwest ( Mueller et al. 20 16). ...
Article
1. Average annual temperature over the contiguous United States has increased by 1.2°F (0.7°C) for the period 1986–2016 relative to 1901–1960 and by 1.8°F (1.0°C) based on a linear regression for the period 1895–2016 (very high confidence). Surface and satellite data are consistent in their depiction of rapid warming since 1979 (high confidence). Paleo-temperature evidence shows that recent decades are the warmest of the past 1,500 years (medium confidence). 2. There have been marked changes in temperature extremes across the contiguous United States. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (the Dust Bowl remains the peak period for extreme heat). The number of high temperature records set in the past two decades far exceeds the number of low temperature records. (Very high confidence) 3. Average annual temperature over the contiguous United States is projected to rise (very high confidence). Increases of about 2.5°F (1.4°C) are projected for the next few decades in all emission scenarios, implying recent record-setting years may be “common” in the near future (high confidence). Much larger rises are projected by late century: 2.8°–7.3°F (1.6°–4.1°C) in a lower emissions scenario (RCP4.5) and 5.8°–11.9°F (3.2°–6.6°C) in a higher emissions scenario (RCP8.5) (high confidence). 4. Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures. The temperatures of extremely cold days and extremely warm days are both expected to increase. Cold waves are projected to become less intense while heat waves will become more intense. The number of days below freezing is projected to decline while the number above 90°F will rise. (Very high confidence)
... We considered sites with available data from 1930 to 2015, 1950 to 2015, and 1970 to 2015 for each of our analyses as these ranges of data span substantially different climatic periods (e.g., Pacific Decadal Oscillation; Graumlich et al. 2003). Analyses beginning in 1930 (hereafter 1930 POR ) provide a means to evaluate contemporary patterns in the context of historical climate anomalies (i.e., the ''Dust Bowl''; Donat et al. 2016). Next, we evaluated trends beginning in the 1950s (hereafter 1950 POR ), given this was a period of relatively cool climate and a range consistent with numerous climate analyses-largely due to the initiation of data collection at many U.S Geological Survey (USGS) gaging stations (Leppi et al. 2012). ...
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Despite the importance of hydrologic regimes to the phenology, demography, and abundance of fishes such as salmonids, there have been surprisingly few syntheses that holistically assess regional, species-specific trends in hydrologic regimes within a framework of climate change. Here, we consider hydrologic regimes within the Greater Yellowstone Area in the Rocky Mountains of western North America to evaluate changes in hydrologic metrics anticipated to affect salmonids, a group of fishes with high regional ecological and socioeconomic value. Our analyses assessed trends across different sites and time periods (1930–, 1950–, and 1970–2015) as means to evaluate spatial and temporal shifts. Consistent patterns emerged from our analyses indicating substantial shifts to (1) earlier peak discharge events; (2) reductions of summer minimum streamflows; (3) declines in the duration of river ice; and (4) decreases in total volume of water. We found accelerated trends in hydrologic change for the 1970–2015 period, with an average peak discharge 7.5 days earlier, 27.5% decline in summer minimum streamflows, and a 15.6% decline in the annual total volume of water (1 October–September 30) across sites. We did observe considerable variability in magnitude of change across sites, suggesting different levels of vulnerability to a changing climate. Our analyses provide an iterative means for assessing climate predictions and an important step in identifying the climate resilience of landscapes.
... Tropical SSTs and convection in the Atlantic Ocean have also been associated with Rossby wave train formation and atmospheric blocking conditions that can lead to summer HW formation across Europe and Russia 5,21,22 . Warm SST anomalies in the north Atlantic and northeast Pacific during boreal spring are also thought to have contributed to record heat extremes during the severe Dust Bowl drought over central United States in the 1930s 23 . A recent study has also found that HWs in eastern United States are associated with characteristic SST patterns in the extratropical Pacific and that these anomalies may provide skill in predicting HWs at lead times up to 50 days 24 . ...
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Scientific Reports 6 : Article number: 29599 10.1038/srep29599 ; published online: 14 July 2016 ; updated: 24 August 2016 This Article contains an error in the legend of Table 1.
... Tropical SSTs and convection in the Atlantic Ocean have also been associated with Rossby wave train formation and atmospheric blocking conditions that can lead to summer HW formation across Europe and Russia 5,21,22 . Warm SST anomalies in the north Atlantic and northeast Pacific during boreal spring are also thought to have contributed to record heat extremes during the severe Dust Bowl drought over central United States in the 1930s 23 . A recent study has also found that HWs in eastern United States are associated with characteristic SST patterns in the extratropical Pacific and that these anomalies may provide skill in predicting HWs at lead times up to 50 days 24 . ...
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... Vautard et al. [2007]; Haarsma et al. [2009] and Zampieri et al. [2009] also identify non-local mechanisms relating dry soils and hot extremes, finding that dry soils in southern Europe enhance the northward spread of heat and dryness. In the U.S., previous studies suggest that drought conditions induce warming in central parts of the continent [Durre et al., 2000;Donat et al., 2015] and that this effect is larger when the Pacific Ocean is in a cold phase [Koster et al., 2009]. Finally, a study for Australia identifies that soil moisturetemperature feedbacks are most relevant for daily maximum temperatures in that region . ...
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... Extreme weather events have severe impacts on societies, economies, and ecosystems and are often the focus of risk management [Benestad et al., 2008]. Over recent decades extreme climate events such as heat waves and droughts have become more frequent, intense, and widespread across the United States and the entire globe [Intergovernmental Panel on Climate Change (IPCC), 2012; Donat et al., 2015]. Because of these events' disproportionate impact on society and ecosystems, understanding climate extremes has become more urgent than tracking changes in mean climate due to climate change [Zhang et al., 2005; IPCC AR5: Climate Change, 2013]. ...
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The impacts of the Atlantic Multidecadal Variability (AMV) on the summer North American climate are investigated using three Coupled Global Climate Models (CGCMs) in which the North Atlantic sea surface temperatures (SSTs) are restored to observed AMV anomalies. Large ensemble simulations are performed in order to estimate how AMV can modulate the occurrence of extreme weather events like heat waves. We show that, in response to an AMV warming, all models simulate a precipitation deficit and a warming over Northern Mexico and Southern US that lead to an increased number of heat wave days by about 30% compared to an AMV cooling. The physical mechanisms associated with these impacts are discussed. The positive tropical Atlantic SST anomalies associated with the warm AMV drive a Matsuno-Gill-like atmospheric response that favors subsidence over Northern Mexico and Southern US. This leads to a warming of the whole tropospheric column, and to a decrease in relative humidity, cloud cover, and precipitation. Soil moisture also plays a role in the modulation of heat wave occurrence by AMV. An AMV warming favors dry soil conditions over Northern Mexico and Southern US by driving year-round precipitation deficit through atmospheric teleconnections coming both directly from the North Atlantic SST forcing and indirectly from the Pacific. The indirect AMV teleconnections highlight the importance of using CGCMs to fully assess the AMV impacts on North America. Given the potential predictability of the AMV, the teleconnections discussed here imply a source of predictability for the North American climate variability and in particular for the occurrence of heat waves at multi-year timescale.
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Record-breaking summer heat waves were experienced across the contiguous United States during the decade-long “Dust Bowl” drought in the 1930s. Using high-quality daily temperature observations, the Dust Bowl heat wave characteristics are assessed with metrics that describe variations in heat wave activity and intensity. Despite the sparser station coverage in the early record, there is robust evidence for the emergence of exceptional heat waves across the central Great Plains, the most extreme of which were pre-conditioned by anomalously dry springs. This is consistent with the entire 20th century record: summer heat waves over the Great Plains develop on average ~15-20 days earlier after anomalously dry springs, compared to summers following wet springs. Heat waves following dry springs are also significantly longer and hotter, indicative of the importance of land surface feedbacks in heat wave intensification. A distinctive anomalous continental-wide circulation pattern accompanied exceptional heat waves in the Great Plains, including those of the Dust Bowl decade. An anomalous broad surface pressure ridge straddling an upper level blocking anticyclone over the western United States forced significant subsidence and adiabatic warming over the Great Plains, and triggered anomalous southward warm advection over southern regions. This prolonged and amplified the heat waves over the central United States, which in turn gradually spread westwards following heat wave emergence. The results imply that exceptional heat waves are pre-conditioned, triggered and strengthened across the Great Plains through a combination of spring drought, upper level continental-wide anticyclonic flow and warm advection from the north.
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Regional droughts are common in North America, but pan-continental droughts extending across multiple regions, including the 2012 event, are rare relative to single-region events. Here, the tree-ring-derived North American Drought Atlas is used to investigate drought variability in four regions over the last millennium, focusing on pan-continental droughts. During the Medieval Climate Anomaly (MCA), the central plains (CP), Southwest (SW), and Southeast (SE) regions experienced drier conditions and increased occurrence of droughts and the Northwest (NW) experienced several extended pluvials. Enhanced MCA aridity in the SW and CP manifested as multidecadal megadroughts. Notably, megadroughts in these regions differed in their timing and persistence, suggesting that they represent regional events influenced by local dynamics rather than a unified, continental-scale phenomena. There is no trend in pan-continental drought occurrence, defined as synchronous droughts in three or more regions. SW, CP, and SE (SW+CP+SE) droughts are the most common, occurring in 12% of all years and peaking in prevalence during the twelfth and thirteenth centuries; patterns involving three other regions occur in about 8% of years. Positive values of the Southern Oscillation index (La Nina conditions) are linked to SW, CP, and SE (SW+CP+SE) droughts and SW, CP, and NW (SW+CP+NW) droughts, whereas CP, NW, and SE (CP+NW+SE) droughts are associated with positive values of the Pacific decadal oscillation and Atlantic multidecadal oscillation. While relatively rare, pan-continental droughts are present in the paleo record and are linked to defined modes of climate variability, implying the potential for seasonal predictability. Assuming stable drought teleconnections, these events will remain an important feature of future North American hydroclimate, possibly increasing in their severity in step with other expected hydroclimate responses to increased greenhouse gas forcing.
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The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.
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Weather and climate extremes have been varying and changing on many different time scales. In recent decades, heat waves have generally become more frequent across the United States, while cold waves have been decreasing. While this is in keeping with expectations in a warming climate, it turns out that decadal variations in the number of U.S. heat and cold waves do not correlate well with the observed U.S. warming during the last century. Annual peak flow data reveal that river flooding trends on the century scale do not show uniform changes across the country. While flood magnitudes in the Southwest have been decreasing, flood magnitudes in the Northeast and north-central United States have been increasing. Confounding the analysis of trends in river flooding is multiyear and even multidecadal variability likely caused by both large-scale atmospheric circulation changes and basin-scale “memory” in the form of soil moisture. Droughts also have long-term trends as well as multiyear and decadal variability. Instrumental data indicate that the Dust Bowl of the 1930s and the drought in the 1950s were the most significant twentieth-century droughts in the United States, while tree ring data indicate that the megadroughts over the twelfth century exceeded anything in the twentieth century in both spatial extent and duration. The state of knowledge of the factors that cause heat waves, cold waves, floods, and drought to change is fairly good with heat waves being the best understood.
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Significance Northeast Pacific coastal warming since 1900 is often ascribed to anthropogenic greenhouse forcing, whereas multidecadal temperature changes are widely interpreted in the framework of the Pacific Decadal Oscillation (PDO), which responds to regional atmospheric dynamics. This study uses several independent data sources to demonstrate that century-long warming around the northeast Pacific margins, like multidecadal variability, can be primarily attributed to changes in atmospheric circulation. It presents a significant reinterpretation of the region’s recent climate change origins, showing that atmospheric conditions have changed substantially over the last century, that these changes are not likely related to historical anthropogenic and natural radiative forcing, and that dynamical mechanisms of interannual and multidecadal temperature variability can also apply to observed century-long trends.
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During the summer of 1934, over 70% of Western North America experienced extreme drought, placing this summer far outside the normal range of drought variability and making 1934 the single worst drought year of the last millennium. Strong atmospheric ridging along the West Coast suppressed cold season precipitation across the Northwest, Southwest, and California, a circulation pattern similar to the winters of 1976–1977 and 2013–2014. In the spring and summer, the drying spread tothe Midwest and Central Plains, driven by severe precipitation deficits downwind from regions of major dust storm activity, consistent with previous work linking drying during the Dust Bowl to anthropogenic dust aerosol forcing. Despite a moderate La Niña, contributions from sea surface temperature forcing were small, suggesting that the anomalous 1934 drought was primarily a consequence of atmospheric variability, possibly amplified by dust forcing that intensified and spread the drought across nearly all of Western North America.
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1] Future climate predictions by global circulation models in the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive indicate that the recent poleward shift of the eddy‐driven jet streams will continue throughout the 21st century. Here it is shown that differences in the projected magnitude of the trend in the Southern Hemisphere are well correlated with biases in the latitude of the jet in the simulation of 20th century climate. Furthermore, the latitude of the jet in the models' 20th century climatology is correlated with biases in the internal variability of the jet stream, as quantified by the time scale of the annular mode. Thus an equatorward bias in the position of the jet is associated with both enhanced persistence of the annular mode, and an increased poleward shift of the jet. These relationships appear to be robust throughout the year except in the austral summer, when differences in forcing, particularly stratospheric ozone, make it impossible to compare the response of one model with another. These results suggest that the fidelity of a model's simulation of the 20th century climate may be related to its fitness for climate prediction. The cause of this relationship is discussed, as well as the implications for climate change projections. Citation: Kidston, J., and E. P. Gerber (2010), Inter-model variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in 20th century climatology, Geophys. Res. Lett., 37, L09708, doi:10.1029/2010GL042873.
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