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Understanding the roles of land surface conditions and atmospheric circulation on continental daily temperature variance is key to improving predictions of temperature extremes. Evaporative resistance (rs, hereafter), a function of the land cover type, reflects the ease with which water can be evaporated or transpired and is a strong control on land–atmosphere interactions. This study explores the effects of rs perturbations on summer daily temperature variance using the Simple Land Interface Model (SLIM) by mimicking, for rs only, a global land cover conversion from forest to crop/grassland. Decreasing rs causes a global cooling. The cooling is larger in wetter areas and weaker in drier areas, and primarily results from perturbations in shortwave radiation (SW) and latent heat flux (LH). Decreasing rs enhances cloud cover due to greater land surface evaporation and thus reduces incoming SW over most land areas. When rs decreases, wetter areas experience strong evaporative cooling, while drier areas become more moisture-limited and thus experience less cooling. Thermal advection further shapes the temperature response by damping the combined impacts of SW and LH. Temperature variance increases in drier areas and decreases in wetter areas as rs decreases. The temperature variance changes can be largely explained from changes in the combined variance of SW and LH, including an important contribution of changes in the covariance of SW and LH. In contrast, the effects of changes in thermal advection variance mainly affect the Northern Hemisphere midlatitudes. Significance Statement This study aims to better understand processes governing daily near-surface air temperature variance over land. We use an idealized modeling framework to explore the effects of land surface evaporative resistance (a parameter that controls how hard it is to evaporate water from the surface) on summer daily temperature variance. We find that a uniform decrease of evaporative resistance across the global land surface causes changes in the temperature variance that can be predicted from changes in the combined variance of shortwave radiation and latent heat flux. The variance of horizontal advection is important in altering the temperature variance in the Northern Hemisphere midlatitudes. Our findings shed light on predicting the characteristics of temperature variability as a function of surface conditions.

In this study, observations and simulations are used to investigate the mechanisms behind the different surface responses over the North Pacific and North Atlantic basins in response to sudden stratospheric warmings associated with a polar-night jet oscillation event (PJO SSWs). In reanalysis and a free-running preindustrial simulation, on average, a negative North Atlantic Oscillation (NAO) response is seen, corresponding to an equatorward shift of the eddy-driven jet. This is considered as the canonical tropospheric response to PJO SSWs. In contrast, the response over the North Pacific is muted. This basin-asymmetric response is shaped by the North Pacific air–sea interactions spun up by the tropospheric precursor to PJO SSWs, which prevent the Pacific eddy-driven jet from responding to the downward influence from the stratosphere. To isolate the downward influence from the sudden warming itself from any preconditioning of the troposphere that may have occurred prior to the warming, a nudging technique is used by which a reference PJO SSW is artificially imposed in a 195-member ensemble spun off from a control simulation. The nudged ensembles show a more basin-symmetric negative Northern Annular Mode (NAM) response, in which the eddy-driven jet shifts equatorward in both the Pacific and Atlantic sectors. Monitoring the atmospheric and oceanic conditions in the North Pacific before and at the onset of PJO SSWs may be useful for forecasting whether a basin-asymmetric negative NAO or basin-symmetric negative NAM response is more likely to emerge. This can be further used to improve subseasonal-to-seasonal predictions of weather and climate. Significance Statement Stratospheric sudden warming events (SSWs) occur when the eastward winds usually found above the Arctic in the winter spontaneously and rapidly reverse. Following their occurrence, the Northern Hemisphere surface westerlies move southward, sometimes over both the North Atlantic and North Pacific and other times over the North Atlantic only. We therefore wanted to understand this uncertainty in the North Pacific surface westerlies response. We find that the North Pacific surface westerlies response to SSWs can be muted by air–sea interactions over the North Pacific. Our results highlight the importance of monitoring the atmospheric and oceanic conditions in the North Pacific before the occurrence of SSWs to forecast whether the Pacific westerlies are likely to respond to SSWs.

Extreme heat events are a threat to human health, productivity, and food supply, so understanding their drivers is critical to adaptation and resilience. Anticyclonic circulation and certain quasi-stationary Rossby wave patterns are well known to coincide with heatwaves, and soil moisture deficits amplify extreme heat in some regions. However, the relative roles of these two factors in causing heatwaves is still unclear. Here we use constructed circulation analogs to estimate the contribution of atmospheric circulation to heatwaves in the United States in the Community Earth System Model version 1 (CESM1) preindustrial control simulations. After accounting for the component of the heatwaves explained by circulation, we explore the relationship between the residual temperature anomalies and soil moisture. We find that circulation explains over 85% of heatwave temperature anomalies in the eastern and western United States but only 75%–85% in the central United States. In this region, there is a significant negative correlation between soil moisture the week before the heatwave and the strength of the heatwave that explains additional variance. Further, for the hottest central U.S. heatwaves, positive temperature anomalies and negative soil moisture anomalies are evident over a month before heatwave onset. These results provide evidence that positive land–atmosphere feedbacks may be amplifying heatwaves in the central United States and demonstrate the geographic heterogeneity in the relative importance of the land and atmosphere for heatwave development. Analysis of future circulation and soil moisture in the CESM1 Large Ensemble indicates that, over parts of the United States, both may be trending toward greater heatwave likelihood.

Previous studies have shown that models overestimate the strength of ENSO teleconnections to the North Pacific during springtime, but the underlying reasons for this bias remain unknown. In this work, the relative contributions from basic-state and thermodynamic/dynamic forcing factors are disentangled through idealized experiments with the Community Earth System Model and a range of stationary wave modeling experiments. It is revealed that in CESM1 the diabatic heating biases over the tropical Indian Ocean and tropical central-western Pacific jointly favor a cyclonic (anticyclonic) circulation bias to occur in the North Pacific during the springtime of El Niño (La Niña) events. On one hand, the difference in the modeled and observed climatological basic state does not lead to the bias formation directly, as the diabatic heating biases are the primary cause. On the other hand, the springtime basic state is conducive to a more vigorous stationary wave response to the biased diabatic heating than the wintertime state, and this explains why the teleconnection bias occurs during springtime but not in winter. An iterative bias-correction approach is then implemented in the atmospheric model component of CESM1 to verify the linkage between the tropical diabatic heating bias and the teleconnection bias. Moreover, this explanation is shown to be relevant in other models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) as a strong relationship is found between biases in ENSO-related tropical central-western Pacific/Indian Ocean precipitation and North Pacific circulation across models in spring. Significance Statement The purpose of this study is to explain why climate models tend to overestimate the springtime ENSO teleconnection to the North Pacific. Through both simplified and comprehensive model experiments, we found that the diabatic heating biases over the tropical Indian Ocean and central-western Pacific basins are the main cause behind the circulation bias. Although similar heating biases also occur in winter, the spring mean climate state is more sensitive to the biased heating than the winter mean state. These findings are useful for developing future climate models that would better simulate the springtime climate response during the ENSO events, as the same problem can be found in many other models.

Anthropogenically forced-warming and La Niña forced-precipitation deficits caused at least a sixfold risk increase for compound extreme low precipitation and high temperature in California–Nevada from October 2020 to September 2021.

Specified dynamics schemes are ubiquitous modeling tools for isolating the roles of dynamics and transport on chemical weather and climate. They typically constrain the circulation of a chemistry–climate model to the circulation in a reanalysis product through linear relaxation. However, recent studies suggest that these schemes create a divergence in chemical climate and the meridional circulation between models and do not accurately reproduce trends in the circulation. In this study we perform a systematic assessment of the specified dynamics scheme in the Community Earth System Model version 2, Whole Atmosphere Community Climate Model version 6 (CESM2 (WACCM6)), which proactively nudges the circulation toward the reference meteorology. Specified dynamics experiments are performed over a wide range of nudging timescales and reference meteorology frequencies, with the model's circulation nudged to its own free-running output – a clean test of the specified dynamics scheme. Errors in the circulation scale robustly and inversely with meteorology frequency and have little dependence on the nudging timescale. However, the circulation strength and errors in tracers, tracer transport, and convective mass flux scale robustly and inversely with the nudging timescale. A 12 to 24 h nudging timescale at the highest possible reference meteorology frequency minimizes errors in tracers, clouds, and the circulation, even up to the practical limit of one reference meteorology update every time step. The residual circulation and eddy mixing integrate tracer errors and accumulate them at the end of their characteristic transport pathways, leading to elevated error in the upper troposphere and lower stratosphere and in the polar stratosphere. Even in the most ideal case, there are non-negligible errors in tracers introduced by the nudging scheme. Future development of more sophisticated nudging schemes may be necessary for further progress.

It has been demonstrated that decadal variations in the North Atlantic Oscillation (NAO) can be predicted by current forecast models. While Atlantic Multidecadal Variability (AMV) in sea surface temperatures (SSTs) has been hypothesised as the source of this skill, the validity of this hypothesis and the pathways involved remain unclear. We show, using reanalysis and data from two forecast models, that the decadal predictability of the NAO can be entirely accounted for by the predictability of decadal variations in the speed of the North Atlantic eddy-driven jet, with no predictability of decadal variations in the jet latitude. The sub-polar North Atlantic (SPNA) is identified as the only potential source of an SST-based signal across the models and reanalysis, and the predictability of the jet speed is shown to be consistent with a forcing from the SPNA visible already within a single season. The pathway is argued to be tropospheric in nature, with the SPNA-induced heating extending up to the mid-troposphere, which alters the meridional temperature gradient around the climatological jet position. The link between SSTs and heatfluxes, which mediates the predictability, is shown to be underestimated in the forecast models by approximately a factor of two, with potential implications for the `signal-to-noise paradox'. The relative roles of anthropogenic aerosol emissions and the AMOC at generating predictable SPNA variability are also discussed. The analysis is extensively supported by the novel use of a set of seasonal hindcasts spanning the 20th century and forced with prescribed SSTs.

Climate change projections consistently demonstrate that warming temperatures and dwindling seasonal snowpack will elicit cascading effects on ecosystem function and water resource availability. Despite this consensus, little is known about potential changes in the variability of ecohydrological conditions, which is also required to inform climate change adaptation and mitigation strategies. Considering potential changes in ecohydrological variability is critical to evaluating the emergence of trends, assessing the likelihood of extreme events such as floods and droughts, and identifying when tipping points may be reached that fundamentally alter ecohydrological function. Using a single-model Large Ensemble with sophisticated terrestrial ecosystem representation, we characterize projected changes in the mean state and variability of ecohydrological processes in historically snow-dominated regions of the Northern Hemisphere. Widespread snowpack reductions, earlier snowmelt timing, longer growing seasons, drier soils, and increased fire risk are projected for this century under a high-emissions scenario. In addition to these changes in the mean state, increased variability in winter snowmelt will increase growing-season water deficits and increase the stochasticity of runoff. Thus, with warming, declining snowpack loses its dependable buffering capacity so that runoff quantity and timing more closely reflect the episodic characteristics of precipitation. This results in a declining predictability of annual runoff from maximum snow water equivalent, which has critical implications for ecosystem stress and water resource management. Our results suggest that there is a strong likelihood of pervasive alterations to ecohydrological function that may be expected with climate change.

The impacts of summer heat extremes are mediated by humidity. Increases in temperatures due to human-caused climate change are generally expected to increase specific humidity; however, it remains unclear how humidity extremes may change, especially in climatologically dry (low-humidity) regions. Here we show that specific humidity on dry days during summer (defined here as July–September) has decreased over the past seven decades in the United States Southwest, and that the greatest decreases co-occur with the hottest temperatures. Hot, dry summers have anomalously low evapotranspiration that is linked to low summer soil moisture. The recent decrease in summer soil moisture is explained by declines in June soil moisture, whereas the interannual variability is controlled by summer precipitation. Future projections of hot, dry days in the Southwest are uncertain due to the large spread in the Coupled Model Intercomparison Project phase 6 (CMIP6) trends in soil moisture and precipitation through 2100. Global humidity increases with warming, but the United States Southwest has shown summer decreases since 1950, with the largest declines on hot days attributed to decreased soil moisture, not atmospheric moisture transport. Projections are uncertain due to model spread in precipitation trends.

The wintertime ENSO teleconnection over the North Pacific region consists of an intensified (weakened) low pressure center during El Niño (La Niña) events both in observations and in climate models. Here, it is demonstrated that this teleconnection persists too strongly into late winter and spring in the Community Earth System Model (CESM). This discrepancy arises in both fully coupled and atmosphere-only configurations, when observed SSTs are specified, and is shown to be robust when accounting for the sampling uncertainty due to internal variability. Furthermore, a similar problem is found in many other models from piControl simulations of the Coupled Model Intercomparison Project (23 out of 43 in phase 5 and 11 out of 20 in phase 6). The implications of this bias for the simulation of surface climate anomalies over North America are assessed. The overall effect on the ENSO composite field (El Niño minus La Niña) resembles an overly prolonged influence of ENSO into the spring with anomalously high temperatures over Alaska and western Canada, and wet (dry) biases over California (southwest Canada). Further studies are still needed to disentangle the relative roles played by diabatic heating, background flow, and other possible contributions in determining the overly strong springtime ENSO teleconnection intensity over the North Pacific.

The evolving roles of anthropogenic aerosols (AER) and greenhouse gases (GHG) in driving large-scale patterns of precipitation and SST trends during 1920–2080 are studied using a new set of “all-but-one-forcing” initial-condition large ensembles (LEs) with the Community Earth System Model version 1 (CESM1), which complement the original “all-forcing” CESM1 LE (ALL). The large number of ensemble members (15–20) in each of the new LEs enables regional impacts of AER and GHG to be isolated from the noise of the model’s internal variability. Our analysis approach, based on running 50-yr trends, accommodates geographical and temporal changes in patterns of forcing and response. AER are shown to be the primary driver of large-scale patterns of externally forced trends in ALL before the late 1970s, and GHG to dominate thereafter. The AER and GHG forced trends are spatially distinct except during the 1970s transition phase when aerosol changes are mainly confined to lower latitudes. The transition phase is also characterized by a relative minimum in the amplitude of forced trend patterns in ALL, due to a combination of reduced AER and partially offsetting effects of AER and GHG. Internal variability greatly limits the detectability of AER- and GHG-forced trend patterns in individual realizations based on pattern correlation metrics, especially during the historical period, highlighting the need for LEs. We estimate that

El Niño–Southern Oscillation (ENSO) is a main driver of climate variability worldwide, but the presence of atmospheric internal variability makes accurate assessments of its atmospheric teleconnections a challenge. Here, we use a multimodel large ensemble of simulations to investigate the ENSO teleconnection response to a low global warming scenario that represents Paris Agreement targets. The ensemble comprises five atmospheric general circulation models with two experiments (present-day and +2°C) in which the same set of ENSO events is prescribed, which allows for quantification of the uncertainty in the ENSO response due to internal variability. In winter, the teleconnection during the positive ENSO phase features a strong negative anomaly in sea level pressure over the northeast Pacific (and vice versa for the negative phase); this anomaly shifts northeastward and strengthens in the warming experiment ensemble. At least 50–75 ENSO events are required to detect a significant shift or strengthening, emphasizing the need to adequately sample the internal variability to isolate the forced response of the ENSO teleconnection under a low warming scenario. Even more events may be needed if one includes other sources of uncertainty not considered in our experimental setup, such as changes in ENSO itself. Over North America, precipitation changes are generally more robust than temperature changes for the regions considered, despite large internal variability, and are shaped primarily by changes in atmospheric circulation. These results suggest that the observational period is likely too short for assessing changes in the ENSO teleconnection under Paris Agreement warming targets.

Changes to the preferred states, or regime behavior, of the North Atlantic eddy-driven jet (EDJ) following a major sudden stratospheric warming (SSW) is examined using a large ensemble experiment from the Canadian Middle Atmosphere Model in which the stratosphere is nudged toward an SSW. In the 3 months following the SSW (January–March), the North Atlantic EDJ shifts equatorward by ~3°, on average; this arises from an increased occurrence of the EDJ’s south regime and reductions in its north and central regimes. Qualitatively similar behavior is shown in a reanalysis dataset. We show that under SSW conditions the south regime becomes more persistent and that this can explain the overall increase in the EDJ latitude decorrelation time scale. A cluster analysis reveals that, following the SSW, the south EDJ regime is characterized by weaker low-level baroclinicity and eddy heat fluxes in the North Atlantic Ocean. We hypothesize, therefore, that the increased persistence of the south regime is related to the weaker baroclinicity leading to slower growth rates of the unstable modes and hence a slower buildup of eddy heat flux, which has been shown to precede EDJ transitions. In the North Atlantic sector, the surface response to the SSW projects onto a negative North Atlantic Oscillation (NAO) pattern, with almost no change in the east Atlantic (EA) pattern. This behavior appears to be distinct from the modeled intrinsic variability in the EDJ, where the jet latitude index captures variations in both the NAO and EA patterns. The results offer new insight into the mechanisms for stratosphere–troposphere coupling following SSWs.

Understanding the roles of land surface conditions and atmospheric circulation on continental daily temperature variance is key to improving predictions of temperature extremes. Evaporative resistance (rs, hereafter), a function of the land cover type, reflects the ease with which water can be evaporated or transpired and is a strong control on land-atmosphere interactions. This study explores the effects of rs perturbations on summer daily temperature variance using the Simple Land Interface Model (SLIM) by mimicking a global land cover conversion from forest (high rs) to crop/grassland (low rs). Decreasing rs causes a global cooling. The cooling is larger in wetter areas and weaker in drier areas, and primarily result from perturbations in shortwave radiation (SW) and latent heat flux (LH). Decreasing rs enhances cloud cover due to greater land surface evaporation, thus reduces SW over most land areas. Wetter areas experience strong evaporative cooling, while drier areas become more moisture-limited when rs decreases, thus experience relatively less cooling. Thermal advection further shapes the temperature response by dampening the combined impacts of SW and LH. Temperature variance increases in drier areas and decreases in wetter areas as rs decreases. The temperature variance changes can be largely explained from changes in the combined variance of SW and LH, including an important contribution of changes in the covariance of SW and LH. In contrast to changes in the mean, changes in thermal advection variance amplify the changes in temperature variance predicted from the surface energy balance alone, particularly in the Northern Hemisphere mid-latitudes.

Mediterranean-type climate regions are heavily dependent on cool season precipitation for water resources and agriculture. Declines in cool season precipitation have been noted in the Mediterranean, Chile, southwest South Africa and southern Australia while California has also been experiencing recent droughts. These changes have been attributed with some confidence to rising greenhouse gases, a poleward shift of storm tracks and Hadley Cell expansion. However, from the perspectives of climate hazards such as fire and heat and ecosystem impacts, spring and summer climate change are also important. For example, recent work shows that summer burned area in California’s Mediterranean-type climate depends on winter precipitation but also on precipitation, temperature and vapor pressure deficit in spring and early summer. Here we consider trends over past decades in the impacts-relevant quantities of precipitation, surface temperature, humidity and vapor pressure deficit throughout the seasons for all the world’s five Mediterranean-type climate regions. Trends from reanalyses are compared to those from CMIP6 models to attribute changes to radiative forcing and natural variability and the connections between change in thermodynamic quantities and the atmospheric circulation are explored. We show that across the Mediterranean-type climate regions human-driven climate change throughout the year is generating changes in impacts-relevant climate quantities that will create substantial challenges to societies and ecosystems.

Major disruptions of the winter season, high-latitude, stratospheric polar vortices can result in stratospheric anomalies that persist for months. These sudden stratospheric warming events are recognized as an important potential source of forecast skill for surface climate on subseasonal to seasonal timescales. Realizing this skill in operational subseasonal forecast models remains a challenge, as models must capture both the evolution of the stratospheric polar vortices in addition to their coupling to the troposphere. The processes involved in this coupling remain a topic of open research. We present here the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) project. SNAPSI is a new model intercomparison protocol designed to study the role of the Arctic and Antarctic stratospheric polar vortices in sub-seasonal to seasonal forecast models. Based on a set of controlled, subseasonal, ensemble forecasts of three recent events, the protocol aims to address four main scientific goals. First, to quantify the impact of improved stratospheric forecasts on near-surface forecast skill. Second, to attribute specific extreme events to stratospheric variability. Third, to assess the mechanisms by which the stratosphere influences the troposphere in the forecast models, and fourth, to investigate the wave processes that lead to the stratospheric anomalies themselves. Although not a primary focus, the experiments are furthermore expected to shed light on coupling between the tropical stratosphere and troposphere. The output requested will allow for a more detailed, process-based community analysis than has been possible with existing databases of subseasonal forecasts.

Geoscientific Model Development, 15 (13), ISSN:1991-9603, ISSN:1991-959X

Projected changes in the Northern Hemisphere stratospheric polar vortex are analyzed using Climate Model Intercomparison Project Phase 6 experiments. Previous studies showed that projections of the wintertime zonally averaged polar vortex strength diverge widely between climate models with no agreement on the sign of change, and that this uncertainty contributes to the regional climate change uncertainty. Here, we show that there remains large uncertainty in the projected strength of the polar vortex in experiments with global warming levels ranging from moderate (SSP245 runs) to large (Abrupt-4xCO2 runs), and that the uncertainty maximizes in winter. Partitioning of the uncertainty in wintertime polar vortex strength projections reveals that, by the end of the 21st century, model uncertainty contributes half of the total uncertainty, with scenario uncertainty contributing only 10%. Regression analysis shows that up to 20% of the intermodel spread in projected precipitation over the Iberian Peninsula and northwestern US, and 20%–30% in near-surface temperature over western US and northern Eurasian, can be associated with the spread in vortex strength projections after accounting for global warming. While changes in the magnitude and sign of the zonally averaged vortex strength are uncertain, most models (>95%) predict an eastward shift of the vortex by 8°–20° degrees in longitude relative to its historical location with the magnitude of the shift increasing for larger global warming levels. There is less agreement across models on a latitudinal shift, whose direction and magnitude correlate with changes in the zonally averaged vortex strength so that vortex weakening/strengthening corresponds to a southward/poleward shift., Journal of Geophysical Research: Atmospheres, 127 (18), ISSN:0148-0227, ISSN:2169-897X

By summer 2021 moderate to exceptional drought impacted 28% of North America, focused west of the Mississippi, with serious impacts on fire, water resources, and agriculture. Here, using reanalyses and SST-forced climate models, we examine the onset and development of this southwestern drought from its inception in summer 2020 through winter and spring 2020/21. The drought severity in summer 2021 resulted from four consecutive prior seasons in which precipitation in the southwest United States was the lowest on record or, at least, extremely dry. The dry conditions in summer 2020 arose from internal atmospheric variability but are beyond the range of what the studied atmosphere models simulate for that season. From winter 2020 through spring 2021 the worsening drought conditions were guided by the development of a La Niña in the tropical Pacific Ocean. Decadal variability in the Pacific Ocean aided drought in the southern part of the region by driving the cool season to be drier during the last two decades. There is also evidence that the southern part of the region in spring is drying due to human-driven climate change. In sum the drought onset was driven by a combination of internal atmospheric variability and interannual climate variability and aided by natural decadal variability and human-driven climate change.

Many recent studies have confirmed that variability in the stratosphere is a significant source of surface sub-seasonal prediction skill during Northern Hemisphere winter. It may be beneficial, therefore, to think about times in which there might be windows-of-opportunity for skillful sub-seasonal predictions based on the initial or predicted state of the stratosphere. In this study, we propose a simple, minimal model that can be used to understand the impact of the stratosphere on tropospheric predictability. Our model purposefully excludes state dependent predictability in either the stratosphere or troposphere or in the coupling between the two. Model parameters are set up to broadly represent current sub-seasonal prediction systems by comparison with four dynamical models from the Sub-Seasonal to Seasonal Prediction Project database. The model can reproduce the increases in correlation skill in sub-sets of forecasts for weak and strong lower stratospheric polar vortex states over neutral states despite the lack of dependence of coupling or predictability on the stratospheric state. We demonstrate why different forecast skill diagnostics can give a very different impression of the relative skill in the three sub-sets. Forecasts with large stratospheric signals and low amounts of noise are demonstrated to also be windows-of-opportunity for skillful tropospheric forecasts, but we show that these windows can be obscured by the presence of unrelated tropospheric signals.

The Whole Atmosphere Community Climate Model version 6 (WACCM6) is a major update of the whole atmosphere modeling capability in the Community Earth System Model (CESM), featuring enhanced physical, chemical and aerosol parameterizations. This work describes WACCM6 and some of the important features of the model. WACCM6 can reproduce many modes of variability and trends in the middle atmosphere, including the Quasi‐Biennial Oscillation, Stratospheric Sudden Warmings and the evolution of Southern Hemisphere springtime ozone depletion over the 20th century. WACCM6 can also reproduce the climate and temperature trends of the 20th century throughout the atmospheric column. The representation of the climate has improved in WACCM6, relative to WACCM4. In addition, there are improvements in high latitude climate variability at the surface and sea ice extent in WACCM6 over the lower top version of the model (CAM6) that come from the extended vertical domain and expanded aerosol chemistry in WACCM6, highlighting the importance of the stratosphere and tropospheric chemistry for high latitude climate variability.

Each spring, the climatological westerly winds of the Northern Hemisphere (NH) stratospheric polar vortex turn easterly as the stratospheric equator‐to‐pole temperature gradient relaxes. The timing of this event is dictated both by the annual return of sunlight to the pole and by dynamical influences from the troposphere. Here we consider the predictability of NH final stratospheric warmings in multimodel hindcasts from the Subseasonal to Seasonal project database. We evaluate how well the Subseasonal to Seasonal prediction systems perform in capturing the timing of final warmings. We compare the predictability of early warmings (which are more strongly driven by wave forcing) and late warmings and find that late warmings are more skillfully predicted at longer lead times. Finally, we find significantly increased predictive skill of NH near‐surface temperature anomalies at Weeks 3–4 lead times following only early final warmings.

The characteristics of the North Atlantic jet stream play a key role in the weather and climate of western Europe. Although much of the year-to-year variability in the jet stream arises from internal atmospheric processes that are inherently unpredictable on timescales beyond a few days to weeks, any low-frequency variability or long-term trends that can be considered forced by slowly varying boundary conditions offer the potential for extended range predictability of climatological conditions in western Europe. Here we demonstrate that station-based precipitation observations have displayed pronounced multidecadal variability over the past century in western Europe during the late winter. We then use these precipitation observations as an independent verification of the multidecadal Atlantic jet stream variability found in reanalysis products. Both signals are highly correlated with sea surface temperature variability in the North Atlantic that is well predicted in initialized decadal prediction experiments with a coupled general circulation model. Combining the model-based predictions of the sea surface temperature with the observed relationship between precipitation and sea surface temperature, we show that there is great potential for skilful predictions of the forthcoming decadal average of March precipitation in western Europe, with hindcasts for the UK and Portugal yielding anomaly correlation coefficients of 0.82 and 0.69, respectively. Decadal averages of March precipitation in western Europe can be predicted by exploiting links with the jet stream and ocean along with skilful predictions of sea surface temperatures, according to an analysis of observations and reanalysis products.

The Single Column Atmosphere Model (SCAM) is a single column model version of the Community Atmosphere Model (CAM). Here we describe the functionality and features of SCAM6, available as part of CAM6 in the Community Earth System Model, version 2 (CESM2). SCAM6 features a wide selection of standard cases, as well as the ability to easily configure a case specified by the user based on a particular point in a CAM 3‐D simulation. This work illustrates how SCAM6 reproduces CAM6 results for physical parameterizations, mostly of moisture and clouds. We demonstrate how SCAM6 can be used for model development through different physics selections, as well as with parameter sweep experiments to highlight the sensitivity of cloud properties to the specification of the vapor deposition process in the cloud microphysics. Furthermore, we use SCAM6 to illustrate the sensitivity of CAM6 cloud radiative properties and precipitation to variable drop number (cloud microphysics properties). Finally, we illustrate how SCAM6 can be used to explore critical emergent processes such as cloud feedbacks and show that CAM6 cloud responses to surface warming in stratus and stratocumulus regimes are similar to those in CAM5. CAM6 has a larger response in the shallow cumulus regime than CAM5. CAM6 cloud feedbacks in the shallow cumulus regime are sensitive to turbulence parameters. SCAM6 is thus a valuable tool for model development, evaluation, and scientific analy sis and an important part of the model hierarchy in Community Earth System Model, version 2.

Previous studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.

Earth system models are complex and represent a large number of processes, resulting in a persistent spread across climate projections for a given future scenario. Owing to different model performances against observations and the lack of independence among models, there is now evidence that giving equal weight to each available model projection is suboptimal. This Perspective discusses newly developed tools that facilitate a more rapid and comprehensive evaluation of model simulations with observations, process-based emergent constraints that are a promising way to focus evaluation on the observations most relevant to climate projections, and advanced methods for model weighting. These approaches are needed to distil the most credible information on regional climate changes, impacts, and risks for stakeholders and policy-makers. ISSN:1758-6798

An ‘emergent constraint’ (EC) is a statistical relationship, across a model ensemble, between a measurable aspect of the present day climate (the predictor) and an aspect of future projected climate change (the predictand). If such a relationship is robust and understood, it may provide constrained projections for the real world. Here, Coupled Model Intercomparison Project 6 (CMIP6) models are used to revisit several ECs that were proposed in prior model intercomparisons with two aims: (1) to assess whether these ECs survive the partial out-of-sample test of CMIP6 and (2) to more rigorously quantify the constrained projected change than previous studies. To achieve the latter, methods are proposed whereby uncertainties can be appropriately accounted for, including the influence of internal variability, uncertainty on the linear relationship, and the uncertainty associated with model structural differences, aside from those described by the EC. Both least squares regression and a Bayesian Hierarchical Model are used. Three ECs are assessed: (a) the relationship between Southern Hemisphere jet latitude and projected jet shift, which is found to be a robust and quantitatively useful constraint on future projections; (b) the relationship between stationary wave amplitude in the Pacific-North American sector and meridional wind changes over North America (with extensions to hydroclimate), which is found to be robust but improvements in the predictor in CMIP6 result in it no longer substantially constrains projected change in either circulation or hydroclimate; and (c) the relationship between ENSO teleconnections to California and California precipitation change, which does not appear to be robust when using historical ENSO teleconnections as the predictor.

The Quasi-biennial Oscillation (QBO) dominates the interannual variability of the tropical stratosphere and influences other regions of the atmosphere. The high predictability of the QBO implies that its teleconnections could lead to increased skill of seasonal and decadal forecasts provided the relevant mechanisms are accurately represented in models. Here modelling and sampling uncertainties of QBO teleconnections are examined using a multi-model ensemble of QBO-resolving atmospheric general circulation models that have carried out a set of coordinated experiments as part of the Stratosphere-troposphere Processes And their Role in Climate (SPARC) QBO initiative (QBOi). During Northern Hemisphere winter, the stratospheric polar vortex in most of these models strengthens when the QBO near 50 hPa is westerly and weakens when it is easterly, consistent with, but weaker than, the observed response. These weak responses are likely due to model errors, such as systematically weak QBO amplitudes near 50 hPa, affecting the teleconnection. The teleconnection to the North Atlantic Oscillation is less well captured overall, but of similar strength to the observed signal in the few models that do show it. The models do not show clear evidence of a QBO teleconnection to the Northern Hemisphere Pacific-sector subtropical jet. We acknowledge the UK's Centre for Environmental Data Analysis (CEDA) for hosting the QBOi data archive. This work was supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement no. 1852977. Portions of this study were supported by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the U.S. Department of Energy's Office of Biological & Environmental Research (BER) via National Science Foundation IA 1844590. NB was supported by the Met Office Hadley Centre Programme funded by BEIS and Defra. SO and LG were supported by the National Centre for Atmospheric Science and the Natural Environment Research Council (NE/P006779/1, NE/N018001/1). FL and SO were supported by the JPI-Climate/Belmont Forum 423 project GOTHAM (ANR-15-JCLI-0004-01, NERC NE/P006779/1). CC and FS were supported by the Copernicus Climate Change Service, funded by the EU and implemented by ECMWF. HN was supported by the Environment Research and Technology Development Fund (S-12) of the Environmental Restoration and Conservation Agency of Japan and by Grant-in-Aid (JP18K03748, JP20H05171) for Science Research (KAKENHI) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. For the EMAC simulations, PB, TK, and SV acknowledge support by the state of Baden-Württemberg through bwHPC. YK was supported by the Japan Society for Promotion of Science (JSPS) KAKENHI (grant nos. JP15KK0178, JP17K18816, JP18H01286 and 19H05702) and by the Environment Research and Technology Development Fund (JPMEERF20192004) of the Environmental Restoration and Conservation Agency of Japan. YK and KH were supported by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of research at the International Pacific Research Center. SW and YK were partly supported by the “Integrated Research Program for Advancing Climate Models (TOUGOU program)” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The Earth Simulator was used for MIROC-ESM and MIROC-AGCM-LL simulations. We acknowledge the scientific guidance of the World Climate Research Programme (WCRP) for helping motivate this work, coordinated under the framework of the Stratosphere-troposphere Processes And their Role in Climate (SPARC) QBO initiative (QBOi) led by JA, NB, KH and SO. Peer Reviewed "Article signat per 29 autors/es: James A. Anstey, Isla R. Simpson, Jadwiga H. Richter, Hiroaki Naoe, Masakazu Taguchi, Federico Serva, Lesley J. Gray, Neal Butchart, Kevin Hamilton, Scott Osprey, Omar Bellprat, Peter Braesicke, Andrew C. Bushell, Chiara Cagnazzo, Chih-Chieh Chen, Hye-Yeong Chun, Rolando R. Garcia, Laura Holt, Yoshio Kawatani, Tobias Kerzenmacher, Young-Ha Kim, Francois Lott, Charles McLandress, John Scinocca, Timothy N. Stockdale, Stefan Versick, Shingo Watanabe, Kohei Yoshida, Seiji Yukimoto"

Research is increasingly showing how drought, heat, and wildfire influence each other. Ongoing collaborations provide templates for how best to study these phenomena and plan for their future impacts.

Specified dynamics schemes are ubiquitous modeling tools for isolating the role of dynamics and transport on chemical weather and climate. They typically constrain the circulation of a chemistry-climate model to the circulation in a reanalysis product through linear relaxation. However, recent studies suggest that these schemes create a divergence in chemical climate and the meridional circulation between models and do not accurately reproduce trends in the circulation. In this study we perform a systematic assessment of the specified dynamics scheme in the Community Earth System Model version 2, Whole Atmosphere Community Climate Model version 6 (CESM2 (WACCM6)), which proactively nudges the circulation toward the reference meteorology. Specified dynamics experiments are performed over a wide range of nudging timescales and reference meteorology frequencies, with the model's circulation nudged to its own free-running output - a clean test of the specified dynamics scheme. Errors in the circulation scale robustly and inversely with meteorology frequency, and have little dependence on nudging timescale. However, the circulation strength and errors in tracers, tracer transport, and convective mass flux scale robustly and inversely with nudging timescale. A 12 to 24 hour nudging timescale at the highest possible reference meteorology frequency minimizes errors in tracers, clouds, and the circulation, even up to the practical limit of one reference meteorology update every timestep. The residual circulation and eddy mixing integrate tracer errors and accumulate them at the end of their characteristic transport pathways, leading to elevated error in the upper troposphere/lower stratosphere and in the polar stratosphere. Even in the most ideal case, there are non-negligible errors in tracers introduced by the nudging scheme. Future development of more sophisticated nudging schemes may be necessary for further progress.

It has been shown that European heatwaves can be significantly intensified by dry land surface conditions. While local temperature intensification through soil moisture feedbacks are well understood, the role of nonlocal soil moisture conditions has yet to be studied in more detail. Studies suggest that through modification of the atmospheric circulation, soil moisture conditions can enhance local temperatures and even evoke a remote temperature response. In this study, we analyze how nonlocal soil moisture – atmosphere feedbacks contribute to the development of high temperature extremes during a seasonally persistent European heatwave. We use a CESM-based global circulation model framework described in Merrifield et al. (2019), in which near-identical heatwave-inducing atmospheric circulation patterns encounter different land surface conditions. In one ensemble the whole atmosphere is constrained, allowing only local temperature intensification by the land surface. In the other ensemble only the upper atmosphere is constrained, enabling the land surface to modify the atmospheric circulation below 300hPa.To understand what variables and processes contribute to regional heatwave intensification in some members and damping in others on a daily timescale, a refined spatio-temporal analysis of the data set was performed. We find that local daily temperature spread across ensemble members amounts to 5°C in the ensemble with unconstrained lower atmosphere, which is 4°C more than in the ensemble where the whole atmosphere is constrained, highlighting the importance of nonlocal land – atmosphere interactions. We identify atmospheric pathways through which the land surface state in one region affected the intensity of the heatwave in another region during the initiation, peak, and decay phases of the event. These heatwave intensification storylines may help to inform seasonal prediction and improve preparedness for future European heat events.

We present a new version of the Met Office Hadley Centre/Climatic Research Unit global surface temperature dataset, HadCRUT5. HadCRUT5 presents monthly average near-surface temperature anomalies, relative to the 1961-1990 period, on a regular 5° latitude by 5° longitude grid from 1850 to 2018. HadCRUT5 is a combination of sea-surface temperature measurements over the ocean from ships and buoys and near-surface air temperature measurements from weather stations over the land surface. These data have been sourced from updated compilations and the adjustments applied to mitigate the impact of changes in sea-surface temperature measurement methods have been revised. Two variants of HadCRUT5 have been produced for use in different applications. The first represents temperature anomaly data on a grid for locations where measurement data are available. The second, more spatially complete, variant uses a Gaussian process based statistical method to make better use of the available observations, extending temperature anomaly estimates into regions for which the underlying measurements are informative. Each is provided as a 200-member ensemble accompanied by additional uncertainty information. The combination of revised input datasets and statistical analysis results in greater warming of the global average over the course of the whole record. In recent years, increased warming results from an improved representation of Arctic warming and a better understanding of evolving biases sea-surface temperature measurements from ships. These updates result in greater consistency with other independent global surface temperature datasets, despite their different approaches to dataset construction, and further increase confidence in our understanding of changes seen.

The impacts of summer heat extremes are mediated by the moisture content of the atmosphere. Increases in temperatures due to human-caused climate change are generally expected to increase specific humidity; however, it remains unclear how humidity extremes may change, especially in climatologically dry regions. Here, using in situ measurements and reanalyses, we show that specific humidity on low humidity days in the American Southwest has decreased over the past seven decades, and that the greatest decreases co-occur with the hottest temperatures. Hot, dry summers have anomalously low evapotranspiration that is linked to low summer soil moisture. The recent decrease in summer soil moisture is explained by declines in pre-summer soil moisture, whereas the interannual variability is controlled by summer precipitation. Climate models project continued declines in pre-summer soil moisture but increases in summer precipitation, leading to uncertainty as to how summer soil moisture and hot, dry days will change in the future.

It has been suggested that increased stratospheric sulfate aerosol loadings following large, low latitude volcanic eruptions can lead to wintertime warming over Eurasia through dynamical stratosphere-troposphere coupling. We here investigate the proposed connection in the context of hypothetical future stratospheric sulfate geoengineering in the Geoengineering Large Ensemble simulations. In those geoengineering simulations, we find that stratospheric circulation anomalies that resemble the positive phase of the Northern Annular Mode in winter is a distinguishing climate response which is absent when increasing greenhouse gases alone are prescribed. This stratospheric dynamical response projects onto the positive phase of the North Atlantic Oscillation, leading to associated side-effects of this climate intervention strategy, such as continental Eurasian warming and precipitation changes. Seasonality is a key signature of the dynamically-driven surface response. We find an opposite response of the North Atlantic Oscillation in summer, when no dynamical role of the stratosphere is expected. The robustness of the wintertime forced response stands in contrast to previously proposed volcanic responses.