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Even the most advanced climate models struggle to reproduce the observed wintertime circulation of the atmosphere over the North Atlantic and western Europe. During winter, the large-scale motions of this particularly challenging region are dominated by eddy-driven and highly non-linear flows, whose low-frequency variability is often studied from the perspective of regimes – a small number of qualitatively distinct atmospheric states. Poor representation of regimes associated with persistent atmospheric blocking events, or variations in jet latitude, degrades the ability of models to correctly simulate extreme events. In this paper we leverage a recently developed hybrid approach – which combines both jet and geopotential height data – to assess the representation of regimes in 8400 years of historical climate simulations drawn from the Coupled Model Intercomparison Project (CMIP) experiments, CMIP5, CMIP6, and HighResMIP. We show that these geopotential-jet regimes are particularly suited to the analysis of climate data, with considerable reductions in sampling variability compared to classical regime approaches. We find that CMIP6 has a considerably improved spatial regime structure, and a more trimodal eddy-driven jet, relative to CMIP5, but it still struggles with under-persistent regimes and too little European blocking when compared to reanalysis. Reduced regime persistence can be understood, at least in part, as a result of jets that are too fast and eddy feedbacks on the jet stream that are too weak – structural errors that do not noticeably improve in higher-resolution models. [ABSTRACT FROM AUTHOR]

China’s FengYun 3 (FY-3) polar orbiting satellites are set to become an important source of observational data for numerical weather prediction (NWP), atmospheric reanalyses, and climate monitoring studies over the next two decades. As part of the Climate Science for Service Partnership China (CSSP China) program, FY-3B Microwave Humidity Sounder 1 (MWHS-1) and FY-3C MWHS-2 observations have been thoroughly assessed and prepared for operational assimilation. This represents the first time observations from China’s polar orbiting satellites have been used in the UK’s global NWP model. Since 2016, continuous data quality monitoring has shown occasional bias changes found to be correlated to changes in the energy supply scheme regulating the platform heating system and other transient anomalies. Nonetheless, MWHS-1 and MWHS-2 significantly contribute to the 24-h forecast error reduction by 0.3% and 0.6%, respectively, and the combination of both instruments is shown to improve the fit to the model background of independent sounders by up to 1%. The observations from the Microwave Radiation Imager (MWRI) also are a potentially significant source of benefits for NWP models, but a solar-dependent bias observed in the instrument half-orbits has prevented their assimilation. This paper presents the bases of a correction scheme developed at the Met Office for the purpose of a future assimilation of MWRI data. [ABSTRACT FROM AUTHOR]

The Arctic is a vital component of the global climate, and its rapid environmental evolution is an important element of climate change around the world. To detect and diagnose the changes occurring to the coupled Arctic climate system, a state-of-the-art synthesis for assessment and monitoring is imperative. This paper presents the Arctic System Reanalysis, version 2 (ASRv2), a multiagency, university-led retrospective analysis (reanalysis) of the greater Arctic region using blends of the polar-optimized version of the Weather Research and Forecasting (Polar WRF) Model and WRF three-dimensional variational data assimilated observations for a comprehensive integration of the regional climate of the Arctic for 2000-12. New features in ASRv2 compared to version 1 (ASRv1) include 1) higher-resolution depiction in space (15-km horizontal resolution), 2) updated model physics including subgrid-scale cloud fraction interaction with radiation, and 3) a dual outer-loop routine for more accurate data assimilation. ASRv2 surface and pressure-level products are available at 3-hourly and monthly mean time scales at the National Center for Atmospheric Research (NCAR). Analysis of ASRv2 reveals superior reproduction of near-surface and tropospheric variables. Broadscale analysis of forecast precipitation and site-specific comparisons of downward radiative fluxes demonstrate significant improvement over ASRv1. The high-resolution topography and land surface, including weekly updated vegetation and realistic sea ice fraction, sea ice thickness, and snow-cover depth on sea ice, resolve finescale processes such as topographically forced winds. Thus, ASRv2 permits a reconstruction of the rapid change in the Arctic since the beginning of the twenty-first century-complementing global reanalyses. ASRv2 products will be useful for environmental models, verification of regional processes, or siting of future observation networks. [ABSTRACT FROM AUTHOR]

Arctic climate change in winter is tightly linked to changes in the strength of surface temperature inversions, which occur frequently in the present climate as Arctic air masses form during polar night. Recent work proposed that, in a warmer climate, increasing low-cloud optical thickness of maritime air advected over high-latitude landmasses during polar night could suppress the formation of Arctic air masses, amplifying winter warming over continents and sea ice. But this mechanism was based on single-column simulations that could not assess the role of fractional cloud cover change. This paper presents two-dimensional cloud-resolving model simulations that support the single-column model results: low-cloud optical thickness and duration increase strongly with initial air temperature, slowing the surface cooling rate as the climate is warmed. The cloud-resolving model cools less at the surface than the single-column model, and the sensitivity of its cooling to warmer initial temperatures is also higher, because it produces cloudier atmospheres with stronger lower-tropospheric mixing and distributes cloud-top cooling over a deeper atmospheric layer with larger heat capacity. Resolving larger-scale cloud turbulence has the greatest impact on the microphysics schemes that best represent general observed features of mixed-phase clouds, increasing their sensitivity to climate warming. These findings support the hypothesis that increasing insulation of the high-latitude land surface by low clouds in a warmer world could act as a strong positive feedback in future climate change and suggest studying Arctic air formation in a three-dimensional climate model. [ABSTRACT FROM AUTHOR]

This paper investigates the mean spatial features of the winds in the Mediterranean and Black Seas using the wind fields observed by the SeaWinds scatterometer. Five years (2000–04) of data have been analyzed on annual and seasonal basis, with particular attention paid to the meso- and local scales. The fields show the existence of structured regional wind systems—in particular, the mistral in the western Mediterranean and the etesians in the Levantine Basin, which are characterized, respectively, by high wind variability and moderate steadiness and by low wind variability and high steadiness. Estimated seasonal mean wind stress τ fields show that the values falling in the top range 0.15 < τ < 0.20 N m-2 affect a large portion of the Mediterranean Basin in winter, in the belt extending from the Gulf of Lion up to the Levantine Basin and the northern Black Sea. In the other seasons, only few regions experience such high values of τ. The analysis of the wind vorticity shows and quantifies the main cyclonic and anticyclonic circulations, and the study of the joint features of wind stress and vorticity has identified the strongest and most persisting local-scale wind circulations produced by the interaction between the wind flow and the orography. They occur at the lee side of Sardinia–Corse and Crete–Rhodos Islands and persist in all seasons, with some seasonal variation in strength and extent. The areas affected by the orographic disturbances are characterized by high values of wind stress and by a structure of vorticity showing alternating areas of cyclonic and anticyclonic circulations, whose strength is comparable to those of the regional-scale cyclones. [ABSTRACT FROM AUTHOR]

Barry Saltzman was a giant in the fields of meteorology and climate science. A leading figure in the study of weather and climate for over 40 yr, he has frequently been referred to as the “father of modern climate theory.” Ahead of his time in many ways, Saltzman made significant contributions to our understanding of the general circulation and spectral energetics budget of the atmosphere, as well as climate change across a wide spectrum of time scales. In his endeavor to develop a unified theory of how the climate system works, he played a role in the development of energy balance models, statistical dynamical models, and paleoclimate dynamical models. He was a pioneer in developing meteorologically motivated dynamical systems, including the progenitor of Lorenz’s famous chaos model. In applying his own dynamical-systems approach to long-term climate change, he recognized the potential for using atmospheric general circulation models in a complimentary way. In 1998, he was awarded the Carl-Gustaf Rossby medal, the highest honor of the American Meteorological Society “for his life-long contributions to the study of the global circulation and the evolution of the earth’s climate.” In this paper, the authors summarize and place into perspective some of the most significant contributions that Barry Saltzman made during his long and distinguished career. This short review also serves as an introduction to the papers in this special issue of the Journal of Climate dedicated to Barry’s memory. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate horizontal transport processes in the winter stratosphere using data with a resolution relevant for chemistry and climate modeling. For this reason the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) with its model top at 83 km altitude, increased horizontal resolution T42 and the semi-Lagrangian transport scheme for advecting passive tracers is used. A new approach of this paper is the classification of specific transport phenomena within the stratosphere into tropical-subtropical streamers (e.g. Offermann et al., 1999) and polar vortex extrusions hereafter called polar vortex streamers. To investigate the role played by these largescale structures on the inter-annual and seasonal variability of transport processes in northern mid-latitudes, the global occurrence of such streamers was calculated based on a 10- year model climatology, concentrating on the existence of the Arctic polar vortex. For the identification and counting of streamers, the new method of zonal anomaly was chosen. The analysis of the months October-May yielded a maximum occurrence of tropical-subtropical streamers during Arctic winter and spring in the middle and upper stratosphere. Synoptic maps revealed highest intensities in the subtropics over East Asia with a secondary maximum over the Atlantic in the northern hemisphere. Furthermore, tropicalsubtropical streamers exhibited a higher occurrence than polar vortex streamers, indicating that the subtropical barrier is more permeable than the polar vortex barrier (edge) in the model, which is in good correspondence with observations (e.g. Plumb, 2002; Neu et al., 2003). Interesting for the total ozone decrease in mid-latitudes is the consideration of the lower stratosphere for tropical-subtropical streamers and the stratosphere above ∼20 km altitude for polar vortex streamers, where strongest ozone depletion is observed at polar latitudes (WMO, 2003). In the lower stratosphere the FUBCMAM simulated a climatological maximum of 10% occurrence of tropical-subtropical streamers over East-Asia/West Pacific and the Atlantic during early- and mid-winter. The results of this paper demonstrate that stratospheric streamers e.g. large-scale, tongue-like structures transporting tropical-subtropical and polar vortex air masses into midlatitudes occur frequently during Arctic winter. They can therefore play a significant role on the strength and variability of the observed total ozone decrease at mid-latitudes and should not be neglected in future climate change studies. [ABSTRACT FROM AUTHOR]

Constraining the low-frequency (LF) behavior of general circulation models (GCMs) requires reliable observational estimates of LF variability. This two-part paper presents multiproxy reconstructions of Niño-3.4 sea surface temperature over the last millennium, applying two techniques [composite plus scale (CPS) and hybrid regularized expectation maximization (RegEM) truncated total least squares (TTLS)] to a network of tropical, high-resolution proxy records. This first part presents the data and methodology before evaluating their predictive skill using frozen network analysis (FNA) and pseudoproxy experiments. The FNA results suggest that about half of the Niño-3.4 variance can be reconstructed back to A.D. 1000, but they show little LF skill during certain intervals. More variance can be reconstructed in the interannual band where climate signals are strongest, but this band is affected by dating uncertainties (which are not formally addressed here). The CPS reliably estimates interannual variability, while LF fluctuations are more faithfully reconstructed with RegEM, albeit with inevitable variance loss. The RegEM approach is also tested on representative pseudoproxy networks derived from two millennium-long integrations of a coupled GCM. The pseudoproxy study confirms that reconstruction skill is significant in both the interannual and LF bands, provided that sufficient variance is exhibited in the target Niño-3.4 index. It also suggests that FNA severely underestimates LF skill, even when LF variability is strong, resulting in overly pessimistic performance assessments. The centennial-scale variance of the historical Niño-3.4 index falls somewhere between the two model simulations, suggesting that the network and methodology presented here would be able to capture the leading LF variations in Niño-3.4 for much of the past millennium, with the caveats noted above. [ABSTRACT FROM AUTHOR]

Since several destructive floods have occurred in Germany in the last decades, it is of considerable interest and relevance ( e.g., when undertaking flood defense design) to take a closer look at the climatic factors driving the changes in flood hazard in Germany. Even if there also exist non-climatic factors controlling the flood hazard, the present paper demonstrates that climate change is one main driver responsible for the increasing number of floods. Increasing trends in temperature have been found to be ubiquitous in Germany, with impact on air humidity and changes in (intense) precipitation. Growing trends in flood prone circulation pattern and heavy precipitation are significant in many regions of Germany over a multi-decade interval and this can be translated into the rise of flood hazard and flood risk. [ABSTRACT FROM AUTHOR]

The fluctuation–dissipation theorem (FDT) states that for systems with certain properties it is possible to generate a linear operator that gives the response of the system to weak external forcing simply by using covariances and lag-covariances of fluctuations of the undisturbed system. This paper points out that the theorem can be shown to hold for systems with properties very close to the properties of the earth’s atmosphere. As a test of the theorem’s applicability to the atmosphere, a three-dimensional operator for steady responses to external forcing is constructed for data from an atmospheric general circulation model (AGCM). The response of this operator is then compared to the response of the AGCM for various heating functions. In most cases, the FDT-based operator gives three-dimensional responses that are very similar in structure and amplitude to the corresponding GCM responses. The operator is also able to give accurate estimates for the inverse problem in which one derives the forcing that will produce a given response in the AGCM. In the few cases where the operator is not accurate, it appears that the fact that the operator was constructed in a reduced space is at least partly responsible. As an example of the potential utility of a response operator with the accuracy found here, the FDT-based operator is applied to a problem that is difficult to solve with an AGCM. It is used to generate an influence function that shows how well heating at each point on the globe excites the AGCM’s Northern Hemisphere annular mode (NAM). Most of the regions highlighted by this influence function, including the Arctic and tropical Indian Ocean, are verified by AGCM solutions as being effective locations for stimulating the NAM. [ABSTRACT FROM AUTHOR]

The present study examines variations in summertime (July–September) tropical cyclone (TC) activity over East Asia during the period 1951–2003. To represent TC activity, a total of 853 TC best tracks for the period were converted to TC passage frequencies (TPFs) within 5° × 5° latitude–longitude grids; TPFs are defined as the percentage values obtained by dividing the number of TC appearances in each grid box by the total number of TCs each year. Empirical orthogonal function analysis of the TPF showed three leading modes: two tropical modes that represent the long-term trend and the relationship with ENSO and one midlatitude mode that oscillates between south of Korea and southeast of Japan with an interannual time scale. The latter proved to be the most remarkable climatic fluctuation of summertime TC activity in the midlatitudes and is referred to as the East Asian dipole pattern (EADP) in this paper. Anomalous atmospheric flows directly connected to the EADP are an enhanced anticyclonic (cyclonic) circulation centering around Japan when the TPF is high south of Korea (southeast of Japan), thereby showing an equivalent barotropic structure in the entire troposphere. This regional circulation anomaly varies in conjunction with the zonally oriented quasi-stationary Rossby wave train in the upper troposphere. This wave train is meridionally trapped in the vicinity of the summer-mean jet stream; therefore, the mean jet stream alters its internal meandering structure according to the phase of the wave train. [ABSTRACT FROM AUTHOR]

Tropical warm pools appear as the primary mode in the distribution of tropical sea surface temperature (SST). Most previous studies have focused on the role of atmospheric processes in homogenizing temperatures in the warm pool and establishing the observed statistical SST distribution. In this paper, a hierarchy of models is used to illustrate both oceanic and atmospheric mechanisms that contribute to the establishment of tropical warm pools. It is found that individual atmospheric processes have competing effects on the SST distribution: atmospheric heat transport tends to homogenize SST, while the spatial structure of atmospheric humidity and surface wind speeds tends to remove homogeneity. The latter effects dominate, and under atmosphere-only processes there is no warm pool. Ocean dynamics counter this effect by homogenizing SST, and it is argued that ocean dynamics is fundamental to the existence of the warm pool. Under easterly wind stress, the thermocline is deep in the west and shallow in the east. Because of this, poleward Ekman transport of water at the surface, compensated by equatorward geostrophic flow below and linked by equatorial upwelling, creates a cold tongue in the east but homogenizes SST in the west, creating a warm pool. High clouds may also homogenize the SST by reducing the surface solar radiation over the warmest water, but the strength of this feedback is quite uncertain. Implications for the role of these processes in climate change are discussed. [ABSTRACT FROM AUTHOR]

The relative importance of the processes responsible for the annual cycle in the upper-ocean heat content in the Peru Current, in the southeastern tropical Pacific, was diagnosed from an oceanic analysis dataset. It was found that the annual cycle of heat content is forced mainly by insolation. However, the ocean dynamical processes play an important role in producing different regional budget characteristics. In a band 500 km from the coast of Peru, the annual heat content changes in this region are relatively large and can be approximated as sea surface temperature (SST) changes in a fixed-depth mixed layer. The annual cycle of the albedo associated with low-level clouds enhances the annual cycle in insolation, which explains the relatively strong annual cycle of heat content. These clouds, to a large extent, act as a feedback to SST, but a small additional forcing, which is proposed to be cold air advection in this paper, is needed to explain the fact that the maximum cloudiness leads the lowest SST by around a month. Ocean dynamics is important closer to the coast, where upwelling acts partly as damping of the heat content changes and forces it to peak earlier than farther offshore. In a band farther to the southwest, locally wind-forced thermocline motions, which become shallower (deeper) in the warm (cool) season, partially cancel the effect of net surface heat fluxes, whose annual cycle is comparable to that in the region previously mentioned, producing a relatively small annual cycle of heat content. The local forcing appears to be associated with the annual meridional displacements of the South Pacific anticyclone. The annual cycle in SST is also relatively small, which is probably due to the changes in the temperature of the water entrained into the mixed layer associated with the thermocline motions, but also to a mixed layer deeper than that closer to the coast. [ABSTRACT FROM AUTHOR]

This paper reviews recent research that assesses evidence for the detection of anthropogenic and natural external influences on the climate. Externally driven climate change has been detected by a number of investigators in independent data covering many parts of the climate system, including surface temperature on global and large regional scales, ocean heat content, atmospheric circulation, and variables of the free atmosphere, such as atmospheric temperature and tropopause height. The influence of external forcing is also clearly discernible in reconstructions of hemispheric-scale temperature of the last millennium. These observed climate changes are very unlikely to be due only to natural internal climate variability, and they are consistent with the responses to anthropogenic and natural external forcing of the climate system that are simulated with climate models. The evidence indicates that natural drivers such as solar variability and volcanic activity are at most partially responsible for the large-scale temperature changes observed over the past century, and that a large fraction of the warming over the last 50 yr can be attributed to greenhouse gas increases. Thus, the recent research supports and strengthens the IPCC Third Assessment Report conclusion that “most of the global warming over the past 50 years is likely due to the increase in greenhouse gases.” [ABSTRACT FROM AUTHOR]

The regional climate of the western United States shows clear footprints of interaction between atmospheric circulation and orography. The unique features of this diverse climate regime challenges climate modeling. This paper provides detailed analyses of observations and regional climate simulations to improve our understanding and modeling of the climate of this region. The primary data used in this study are the 1/88 gridded temperature and precipitation based on station observations and the NCEP-NCAR global reanalyses. These data were used to evaluate a 20-yr regional climate simulation performed using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (Penn State-NCAR) Mesoscale Model (MM5) driven by large-scale conditions of the NCEP-NCAR reanalyses. Regional climate features examined include seasonal mean and extreme precipitation; distribution of precipitation rates; and precipitation intensity, frequency, and seasonality. The relationships between precipitation and surface temperature are also analyzed as a means to evaluate how well regional climate simulations can be used to simulate surface hydrology, and relationships between precipitation and elevation are analyzed as diagnostics of the impacts of surface topography and spatial resolution. The latter was performed at five east-west transects that cut across various topographic features in the western United States. These analyses suggest that the regional simulation realistically captures many regional climate features. The simulated seasonal mean and extreme precipitation are comparable to observations. The regional simulation produces precipitation over a wide range of precipitation rates comparable to observations. Obvious biases in the simulation include the oversimulation of precipitation in the basins and intermountain West during the cold season, and the undersimulation in the Southwest in the warm season. There is a tendency of reduced precipitation... [ABSTRACT FROM AUTHOR]

The persistent inter‐model spread in the response of global‐mean surface temperature to increased CO2 (known as the "Equilibrium Climate Sensitivity," or "ECS") is a crucial problem across model generations. This work examines the influence of the models' present‐day atmospheric circulation climatologies, and the accompanying climatological cloud radiative effects, in explaining that spread. We analyze the Coupled Model Intercomparison Project Phase 6 (CMIP6) models and find that they simulate a more poleward, and thus more realistic, edge of the Hadley cell (HC) in the Southern Hemisphere than the CMIP5 models, although the climatological shortwave cloud radiative effects are similar in the two generations of models. A few CMIP5 models with extreme equatorward biases in the HC edge exhibited high ECS due to strong Southern midlatitude shortwave cloud radiative warming in response to climate change, suggesting an ECS dependence on HC position. We find that such constraint no longer holds for the CMIP6 models, due to the absence of models with extreme HC climatologies. In spite of this, however, the CMIP6 models show an increased spread in ECS, with more models in the high ECS range. In addition, an improved representation of the climatological jet dynamics does not lead to a new emergent constraint in the CMIP6 models either. [ABSTRACT FROM AUTHOR]

The chaotic nature of the atmospheric dynamics has stimulated the applications of methods and ideas derived from statistical dynamics. For instance, ensemble systems are used to make weather predictions recently extensive, which are designed to sample the phase space around the initial condition. Such an approach has been shown to improve substantially the usefulness of the forecasts since it allows forecasters to issue probabilistic forecasts. These works have modified the dominant paradigm of the interpretation of the evolution of atmospheric flows (and oceanic motions to some extent) attributing more importance to the probability distribution of the variables of interest rather than to a single representation. The ensemble experiments can be considered as crude attempts to estimate the evolution of the probability distribution of the climate variables, which turn out to be the only physical quantity relevant to practice. However, little work has been done on a direct modeling of the probability evolution itself. In this paper it is shown that it is possible to write the evolution of the probability distribution as a functional integral of the same kind introduced by Feynman in quantum mechanics, using some of the methods and results developed in statistical physics. The approach allows obtaining a formal solution to the Fokker-Planck equation corresponding to the Langevin-like equation of motion with noise. The method is very general and provides a framework generalizable to red noise, as well as to delaying differential equations, and even field equations, i.e., partial differential equations with noise, for example, general circulation models with noise. These concepts will be applied to an example taken from a simple ENSO model. [ABSTRACT FROM AUTHOR]

A simple technique is proposed for calculating global mean climate forcing from transient integrations of coupled atmosphere–ocean general circulation models (AOGCMs). This “climate forcing” differs from the conventionally defined radiative forcing as it includes semidirect effects that account for certain short time scale responses in the troposphere. First, a climate feedback term is calculated from reported values of 2 × CO2 radiative forcing and surface temperature time series from 70-yr simulations by 20 AOGCMs. In these simulations carbon dioxide is increased by 1% yr-1. The derived climate feedback agrees well with values that are diagnosed from equilibrium climate change experiments of slab-ocean versions of the same models. These climate feedback terms are associated with the fast, quasi-linear response of lapse rate, clouds, water vapor, and albedo to global surface temperature changes. The importance of the feedbacks is gauged by their impact on the radiative fluxes at the top of the atmosphere. Partial compensation is found between longwave and shortwave feedback terms that lessens the intermodel differences in the equilibrium climate sensitivity. There is also some indication that the AOGCMs overestimate the strength of the positive longwave feedback. These feedback terms are then used to infer the shortwave and longwave time series of climate forcing in twentieth- and twenty-first-century simulations in the AOGCMs. The technique is validated using conventionally calculated forcing time series from four AOGCMs. In these AOGCMs the shortwave and longwave climate forcings that are diagnosed agree with the conventional forcing time series within ∼10%. The shortwave forcing time series exhibit order of magnitude variations between the AOGCMs, differences likely related to how both natural forcings and/or anthropogenic aerosol effects are included. There are also factor of 2 differences in the longwave climate forcing time series, which may indicate problems with the modeling of well-mixed greenhouse gas changes. The simple diagnoses presented provides an important and useful first step for understanding differences in AOGCM integrations, indicating that some of the differences in model projections can be attributed to different prescribed climate forcing, even for so-called standard climate change scenarios. [ABSTRACT FROM AUTHOR]

NW flow snowfall (NWFS) events are common occurrences at higher elevations (1500 to >2000 m a.s.l.) and on windward slopes in the Southern Appalachian Mountain region of the south-eastern USA. The spatial patterns of NWFS are strongly controlled by topography, resulting in pronounced spatial variability. The strong topographic and low-level convective forcing, coupled with low temperatures and strong winds, increases societal impacts. This paper analyzes the relationships between NWFS and various topographic and geographic (TOPO/GEOG) variables in the Southern Appalachians. We identified NWFS events on the basis of low-level wind direction, extracted values for the TOPO/GEOG variables from Digital Elevation Models (DEMs), and developed statistical relationships between NWFS and the TOPO/GEOG variables. Using a multivariate regression model and GIS techniques, we also mapped mean annual NWFS across the region. Results indicate that elevation and NW exposure are most strongly correlated with NWFS; however, the strength of these relationships is mediated by temperature. In the colder NWFS events, the relative importance of elevation is diminished, while NW exposure and distance to a NW slope are relatively more important. Additionally, we demonstrate that multivariate regression modeling in conjunction with GIS techniques can be an effective way to map snowfall patterns associated with specific wind directions, particularly when a strong topographic control is evident. [ABSTRACT FROM AUTHOR]

In this paper we describe the dynamic behaviour of Salajekna, a valley glacier, over the last 200 years using terrestrial observations, in situ measurements, remote sensing observations and glacier reconstructions. The response time of the glacier was calculated using analytical models and field measurements. We were subsequently able to attribute specific dynamic responses to climate trends in the available climate record. The glacier's historical maximum extension was reached around 1880–1910 and was the result of a more continental climate with multi-modal airflows in the late 18th and early 19th centuries. A transition to more maritime conditions in the mid-19th century resulted in a near-continuous 20th century retreat before the glacier adjusted to a near-steady state. [ABSTRACT FROM AUTHOR]

Given uncertainty in the processes involved in polar amplification, elucidating the role of poleward heat and moisture transport is crucial. The Polar Amplification Model Intercomparison Project (PAMIP) permits robust separation of the effects of sea ice loss from sea surface warming under climate change. We utilize a moist isentropic circulation framework that accounts for moisture transport, condensation, and eddy transport, in order to analyze the circulation connecting the mid‐latitudes and the Arctic. In PAMIP's atmospheric general circulation model experiments, prescribed sea ice loss reduces poleward heat transport (PHT) by warming the returning moist isentropic circulation at high latitudes, while prescribed warming of the ocean surface increases PHT by strengthening the moist isentropic circulation. Inter‐model spread of PHT into the Arctic reflects the tug‐of‐war between sea‐ice and surface‐warming effects. Plain Language Summary: A major conundrum in current climate science is to understand what Arctic changes imply for the climate and environment in mid‐latitude regions. The Polar Amplification Model Intercomparison Project (PAMIP) designed a set of climate model experiments to specifically answer this question in a carefully designed, idealized framework. PAMIP's approach is to separate historic and projected climate change into parts associated with Arctic sea ice loss and ocean surface warming and investigate how these two contributions can influence the atmosphere. To isolate these effects, only models can be used, because, in reality, sea ice loss and ocean surface warming are strongly linked together. This letter focuses on what the PAMIP experiments imply for the transport and redistribution of heat and moisture in the atmosphere. In PAMIP, we learn that Arctic sea ice loss causes the atmosphere to reduce the transport of dry and cold air away from the Arctic while ocean warming causes more transport of moist warm air toward the Arctic. These two effects are in a tug‐of‐war, suggesting that climate change can cause a mix of competing impacts on the global energy transport from the tropics to the Arctic. Key Points: Sea ice loss exports static energy from the Arctic by warming the equatorward branch of the global mass circulationSea surface warming imports static energy into the Arctic by increasing the mid‐latitude mass transport, overwhelming the sea ice effectThere is more spread amongst models for the effect of sea‐surface warming than for sea ice loss [ABSTRACT FROM AUTHOR]

Many climate extremes, including heatwaves and heavy precipitation events, are projected to worsen under climate change, with important impacts for society. Future projections required for adaptation are often based on climate model simulations. Given finite resources, trade-offs must be made concerning model resolution, ensemble size, and level of model complexity. Here we focus on the resolution component. A given resolution can be achieved over a region using either global climate models (GCMs) or at lower cost using regional climate models (RCMs) that dynamically downscale coarser GCMs. Both approaches to increasing resolution may better capture small-scale processes and features (downscaling effect), but increased GCM resolution may also improve the representation of the large-scale atmospheric circulation (upscaling effect). The size of this upscaling effect is therefore important for deciding modelling strategies. Here we evaluate the benefits of increased model resolution for both global and regional climate models for simulating temperature, precipitation, and wind extremes over Europe at resolutions that could currently be realistically used for coordinated sets of climate projections at the pan-European scale. First we examine the benefits of regional downscaling by comparing EURO-CORDEX simulations at 12.5 and 50 km resolution to their coarser CMIP5 driving simulations. Secondly, we compare global-scale HadGEM3-A simulations at three resolutions (130, 60, and 25 km). Finally, we separate out resolution-dependent differences for HadGEM3-A into downscaling and upscaling components using a circulation analogue technique. Results suggest limited benefits of increased resolution for heatwaves, except in reducing hot biases over mountainous regions. Precipitation extremes are sensitive to resolution, particularly over complex orography, with larger totals and heavier tails of the distribution at higher resolution, particularly in the CORDEX vs. CMIP5 analysis. CMIP5 models underestimate precipitation extremes, whilst CORDEX simulations overestimate compared to E-OBS, particularly at 12.5 km, but results are sensitive to the observational dataset used, with the MESAN reanalysis giving higher totals and heavier tails than E-OBS. Wind extremes are somewhat stronger and heavier tailed at higher resolution, except in coastal regions where large coastal grid boxes spread strong ocean winds further over land. The circulation analogue analysis suggests that differences with resolution for the HadGEM3-A GCM are primarily due to downscaling effects. [ABSTRACT FROM AUTHOR]

Anthropogenic emissions can modify the frequency and intensity of extreme weather events such as cold spells, heatwaves, and heavy precipitations. A major challenge is to detect changes in the atmospheric circulation patterns associated with those extreme events. The emergence of patterns depends on the chaotic behavior of the atmospheric flow and can also be modified by anthropogenic emissions. By embedding the circulation patterns observed during selected extremes into historical climate simulations and projections based on emission scenarios, we find major changes in probability, predictability, and persistence of atmospheric patterns observed during extreme events using an analog‐based method. The results highlight the need to take into account the role of atmospheric circulation in attribution studies as future extremes will be associated with modified circulation patterns. Plain Language Summary: Weather extreme events greatly impact agricultural, social, and economic activities. In a changing climate, it seems primordial to ask how anthropogenic emissions impact the frequency and intensity of extreme events. Attribution studies focus on this issue, often assuming that the atmospheric circulation associated with extreme events is not itself affected by climate change. We show how temporal properties of the synoptic patterns associated to extremes are affected by climate change. We explain how such changes must be taken into account in future research to perform meaningful attribution studies. Key Points: We find major changes in probability, predictability, and persistence of circulation patterns associated to extreme eventsOur methodology is based on embedding the circulation patterns observed during extremes into historical and future climate simulationsThese results warn that atmospheric circulation changes must be taken into account in future [ABSTRACT FROM AUTHOR]

El Niño induces an anomalous easterly wind along the equator and a pair of anomalous anticyclones straddling the equator over the tropical Indian Ocean (TIO) during the autumn of its developing phase. Based on 30 coupled models participating in CMIP5, these atmospheric circulation anomalies over TIO are substantially weakened by about 12%–13% K−1 under global warming scenarios, associated with a weakened zonal gradient of the sea surface temperature (SST) anomaly. The mechanism for the response is investigated based on a hierarchy of model experiments. Based on stand-alone atmospheric model experiments under uniform and patterned mean-state SST warming, the atmospheric circulation anomaly over TIO during the autumn of the developing El Niño is also substantially weakened by about 8% K−1 even if the interannual variability of SST remains exactly unchanged, suggesting that the primary cause resides in the atmosphere rather than the SST anomaly. The tropospheric static stability is robustly enhanced under global warming, and experiments performed by a linear baroclinic model show that a much weaker atmospheric circulation anomaly over TIO is stimulated by an unchanged diabatic heating anomaly under a more stable atmosphere. The weakened atmospheric circulation anomaly due to enhanced static stability weakens the zonal gradient of the SST anomaly within TIO through local air–sea interaction, and it acts to further weaken the atmospheric circulation anomaly. The enhanced static stability of the troposphere is probably the primary cause and the air–sea interaction within TIO is a secondary cause for the weakened impact of the developing El Niño on atmospheric circulation variability over TIO. [ABSTRACT FROM AUTHOR]

Future changes in geostrophic winds over Europe and the North Atlantic region were studied utilizing output data from 21 CMIP5 global climate models (GCMs). Changes in temporal means, extremes, and the joint distribution of speed and direction were considered. In concordance with previous research, the time mean and extreme scalar wind speeds do not change pronouncedly in response to the projected climate change; some degree of weakening occurs in the majority of the domain. Nevertheless, substantial changes in high wind speeds are identified when studying the geostrophic winds from different directions separately. In particular, in northern Europe in autumn and in parts of northwestern Europe in winter, the frequency of strong westerly winds is projected to increase by up to 50%. Concurrently, easterly winds become less common. In addition, we evaluated the potential of the GCMs to simulate changes in the near-surface true wind speeds. In ocean areas, changes in the true and geostrophic winds are mainly consistent and the emerging differences can be explained (e.g., by the retreat of Arctic sea ice). Conversely, in several GCMs the continental wind speed response proved to be predominantly determined by fairly arbitrary changes in the surface properties rather than by changes in the atmospheric circulation. Accordingly, true wind projections derived directly from the model output should be treated with caution since they do not necessarily reflect the actual atmospheric response to global warming. [ABSTRACT FROM AUTHOR]

The Max Planck Institute Grand Ensemble (MPI‐GE) is the largest ensemble of a single comprehensive climate model currently available, with 100 members for the historical simulations (1850–2005) and four forcing scenarios. It is currently the only large ensemble available that includes scenario representative concentration pathway (RCP) 2.6 and a 1% CO2 scenario. These advantages make MPI‐GE a powerful tool. We present an overview of MPI‐GE, its components, and detail the experiments completed. We demonstrate how to separate the forced response from internal variability in a large ensemble. This separation allows the quantification of both the forced signal under climate change and the internal variability to unprecedented precision. We then demonstrate multiple ways to evaluate MPI‐GE and put observations in the context of a large ensemble, including a novel approach for comparing model internal variability with estimated observed variability. Finally, we present four novel analyses, which can only be completed using a large ensemble. First, we address whether temperature and precipitation have a pathway dependence using the forcing scenarios. Second, the forced signal of the highly noisy atmospheric circulation is computed, and different drivers are identified to be important for the North Pacific and North Atlantic regions. Third, we use the ensemble dimension to investigate the time dependency of Atlantic Meridional Overturning Circulation variability changes under global warming. Last, sea level pressure is used as an example to demonstrate how MPI‐GE can be utilized to estimate the ensemble size needed for a given scientific problem and provide insights for future ensemble projects. Key Points: The 100‐member MPI‐GE is currently the largest publicly available ensemble of a comprehensive climate modelMPI‐GE currently has the most forcing scenarios of all large ensemble projects: RCP2.6, RCP4.5, RCP8.5, and 1% CO2The power of MPI‐GE is to estimate the forced response and internal variability, including changing variability, to unprecedented precision [ABSTRACT FROM AUTHOR]

The interior of western Canada, up to and including the Arctic, has experienced rapid change in its climate, hydrology, cryosphere, and ecosystems, and this is expected to continue. Although there is general consensus that warming will occur in the future, many critical issues remain. In this first of two articles, attention is placed on atmospheric-related issues that range from large scales down to individual precipitation events. Each of these is considered in terms of expected change organized by season and utilizing mainly "business-as-usual" climate scenario information. Large-scale atmospheric circulations affecting this region are projected to shift differently in each season, with conditions that are conducive to the development of hydroclimate extremes in the domain becoming substantially more intense and frequent after the mid-century. When coupled with warming temperatures, changes in the large-scale atmospheric drivers lead to enhancements of numerous water-related and temperature-related extremes. These include winter snowstorms, freezing rain, drought, forest fires, as well as atmospheric forcing of spring floods, although not necessarily summer convection. Collective insights of these atmospheric findings are summarized in a consistent, connected physical framework. [ABSTRACT FROM AUTHOR]

Recent studies have focused on the role of cloud radiative effects (CRE) in governing the mean atmospheric circulation and its response to climate change. This study instead examines the role of CRE in climate variability in the extratropics. Cloud locking experiments are performed using the Community Earth System Model. In these experiments, CRE are scrambled, such that they maintain the same climatology but no longer match the model's dynamical fields. The results of these experiments indicate that high‐frequency interactions between CRE and dynamics have a small (≤5–10%) but statistically significant damping effect on the intensity of the extratropical storm tracks, particularly in the Southern Hemisphere. Individual midlatitude cyclones have decreased intensity and shorter lifetime. These effects arise largely from clouds' radiative modification of static stability below 700 hPa. The coupling among clouds, radiation, and dynamics thus has a modest but potentially important influence on the extratropical storm tracks. Plain Language Summary: Clouds are one of the largest sources of uncertainty in properly simulating Earth's climate system. Recent studies have shown that biases in clouds not only affect temperature but also can affect the representation of weather systems. This study focuses on the linkages between clouds and weather systems in the extratropics (poleward of 30° latitude). The results show that if clouds are not properly represented, they can introduce small biases in the number, strength, and lifetime of the low‐pressure systems that drive much of the precipitation at midlatitudes. Consequently, improving the representation of clouds in computer models may help to provide more accurate future projections of midlatitude weather systems. Key Points: High‐frequency coupling of cloud radiative effects and dynamics suppresses the extratropical storm tracks by up to 5–10%Extratropical cyclones have decreased intensity and shorter lifetime if cloud radiative effects are properly coupled to dynamicsIn‐atmosphere longwave radiative heating associated with clouds modifies lower tropospheric stability to suppress eddy kinetic energy [ABSTRACT FROM AUTHOR]

The large-scale circulation anomalies associated with extreme heat (EH) in South Korea and southern–central Japan are examined using data during the time period 1979–2016. Statistical analysis indicates that EH days in these two regions are concentrated in July and August and tend to occur simultaneously. These EH days are therefore combined to explore the physical mechanisms leading to their occurrence. The composite results indicate that the anomalous atmospheric warming during EH days is dominantly caused by a significant subsidence anomaly, which is associated with a deep anomalous anticyclone over East Asia. Further investigation of the evolution of circulation anomalies suggests that the anomalous anticyclone over East Asia related to EH is primarily initiated by wave trains originating from upstream regions, which propagate eastward along the Asian westerly jet in the upper troposphere. These wave trains can be categorized into two types that are characterized by the precursor anticyclonic and cyclonic anomalies, respectively, over central Asia. The distinction between these two types of wave train can be explained by the wavenumbers of the Rossby waves, which are modulated by both the intensity and the shape of the Asian westerly jet as the background basic flow. [ABSTRACT FROM AUTHOR]

Given the recent changes in the Arctic sea ice, understanding the effects of the resultant polar warming on the global climate is of great importance. However, the interaction between the Arctic and midlatitude circulation involves a complex chain of mechanisms, which leaves state-of-the-art general circulation models unable to represent this interaction unambiguously. This study uses an idealized general circulation model to provide a process-based understanding of the sensitivity of the midlatitude circulation to the location of high-latitude warming. A simplified atmosphere is simulated with a single zonally localized midlatitude storm track, which is analogous to the storm tracks in the Northern Hemisphere. It is found that even small changes in the position of the forcing relative to that storm track can lead to very different responses in the midlatitude circulation. More specifically, it is found that heating concentrated in one region may cause a substantially stronger global response compared to when the same amount of heating is distributed across all longitudes at the same latitude. Linear interference between climatological and anomalous flow is an important component of the response, but it does not explain differences between different longitudes of the forcing. Feedbacks from atmospheric transient eddies are found to be associated with this strong response. A dependence between the climatological jet latitude and the jet response to polar surface heating is found. These results can be used to design and interpret experiments with complex state-of-the-art models targeted at Arctic–midlatitude interactions. [ABSTRACT FROM AUTHOR]

How the atmospheric overturning circulation is projected to change is important for understanding changes in mass and energy budget. This study analyzes the overturning circulation by sorting the upward mass transport in terms of the moist static energy (MSE) of air parcels in an ensemble of coupled climate models. It is found that, in response to greenhouse gas increases, the upward transport of MSE increases in order to balance the increase in radiative cooling of the mass transport. At the same time, the overall mass transport decreases. The increase in energy transport and decrease in mass transport can be explained by the fact that the MSE of rising air parcels increases more rapidly than that of subsiding air, thus allowing for a weaker overturning circulation to transport more energy. Plain Language Summary: The atmospheric circulation transports energy upward to balance the energy loss due to radiative cooling. This energy transport is the direct result of the fact that rising air parcels are typically warmer and moister than subsiding air and thus have a higher‐energy content. It is shown here that as the amount of greenhouse gases increases, the upward energy transport increases while the mass transport weakens. This apparent discrepancy can be explained by the fact that, in a warmer world, the energy content of rising air increases more rapidly than that of subsiding air, making the atmospheric circulation more efficient at transporting energy upward. Key Points: Isentropic stream function shows a robust expansion toward smaller pressure and broadening of MSE in a warming climateWhile upward mass transport decreases in global average, upward MSE transport increases, mostly due to the eddiesAn increase of effective MSE range is found globally, especially in the subtropics [ABSTRACT FROM AUTHOR]

Understanding the drivers of surface melting in West Antarctica is crucial for understanding future ice loss and global sea level rise. This study identifies atmospheric drivers of surface melt on West Antarctic ice shelves and ice sheet margins and relationships with tropical Pacific and high-latitude climate forcing using multidecadal reanalysis and satellite datasets. Physical drivers of ice melt are diagnosed by comparing satellite-observed melt patterns to anomalies of reanalysis near-surface air temperature, winds, and satellite-derived cloud cover, radiative fluxes, and sea ice concentration based on an Antarctic summer synoptic climatology spanning 1979–2017. Summer warming in West Antarctica is favored by Amundsen Sea (AS) blocking activity and a negative phase of the southern annular mode (SAM), which both correlate with El Niño conditions in the tropical Pacific Ocean. Extensive melt events on the Ross–Amundsen sector of the West Antarctic Ice Sheet (WAIS) are linked to persistent, intense AS blocking anticyclones, which force intrusions of marine air over the ice sheet. Surface melting is primarily driven by enhanced downwelling longwave radiation from clouds and a warm, moist atmosphere and by turbulent mixing of sensible heat to the surface by föhn winds. Since the late 1990s, concurrent with ocean-driven WAIS mass loss, summer surface melt occurrence has increased from the Amundsen Sea Embayment to the eastern Ross Ice Shelf. We link this change to increasing anticyclonic advection of marine air into West Antarctica, amplified by increasing air–sea fluxes associated with declining sea ice concentration in the coastal Ross–Amundsen Seas. [ABSTRACT FROM AUTHOR]

Climate change in recent decades has been identified as a significant threat to natural environments and human wellbeing. This is because some of the contemporary changes to climate are abrupt and result in persistent changes in the state of natural systems; so called regime shifts (RS). This study aimed to detect and analyse the timing and strength of RS in Estonian climate at the half-century scale (1966−2013). We demonstrate that the extensive winter warming of the Northern Hemisphere in the late 1980s was represented in atmospheric, terrestrial, freshwater and marine systems to an extent not observed before or after the event within the studied time series. In 1989, abiotic variables displayed statistically significant regime shifts in atmospheric, river and marine systems, but not in lake and bog systems. This was followed by regime shifts in the biotic time series of bogs and marine ecosystems in 1990. However, many biotic time series lacked regime shifts, or the shifts were uncoupled from large-scale atmospheric circulation. We suggest that the latter is possibly due to complex and temporally variable interactions between abiotic and biotic elements with ecosystem properties buffering biotic responses to climate change signals, as well as being affected by concurrent anthropogenic impacts on natural environments. [ABSTRACT FROM AUTHOR]

Subtropical stationary waves may act as an important bridge connecting regional hydrological extremes with global warming. Observations show that the boreal summer stationary-wave amplitude (SWA) had a significantly positive trend during 1979–2013. Here, we investigate the past and future responses of SWA to increasing climate forcing using 31 CMIP5 GCMs. Twenty-four out of 31 models display a consistent increase in climatological-mean SWA in response to warming. To assess the detectability of a trend in SWA, we compared half-century trends between preindustrial control (PiControl), historical, and RCP8.5 simulations. The probability distribution of the normalized SWA trend obtained through bootstrapping shows neither positive nor negative tendencies of SWA trend in PiControl simulations. Twenty-two of 31 historical simulations exhibit a positive SWA trend. The SWA trends in 26 of 31 RCP8.5 simulations are positive. The finding supports the hypothesis that the positive SWA trend is at least partially driven by increasing external forcing. The linear regression of interannual variability in hydrological extreme frequency on SWA suggests that high SWA is related to increased heavy-rainfall-day frequency over South Asia, the Indochinese Peninsula, and southern China (SA-EA), and to increased dry-spell-day frequency over the northwestern and central United States (NUS) and the southern United States and Mexico (SUS-MEX). The projected amplification of SWA, combined with the relationships between SWA and number of hydrological extremes, may partially explain projected increases in the number of dry spells over NUS and SUS-MEX and the number of heavy-rainfall days over SA-EA. [ABSTRACT FROM AUTHOR]

The variation in the interannual relationship between the boreal winter Hadley circulation (HC) and El Niño–Southern Oscillation (ENSO) during 1948–2014 is investigated. The interannual variability of the HC is dominated by two principal modes: the equatorial asymmetric mode (AM) and the equatorial symmetric mode (SM). The AM of the HC during ENSO events mainly results from a combined effect of the ENSO sea surface temperature (SST) anomalies and the climatological background SST over the South Pacific convergence zone. Comparatively, the SM shows a steady and statistically significant relationship with ENSO; however, the interannual relationship between the AM and ENSO is strengthened during the mid-1970s, which leads to a HC regime change—that is, the interannual pulse of the HC intensity and its response to ENSO are stronger after the mid-1970s than before. The long-term warming trend of the tropical western Pacific since the 1950s and the increased ENSO amplitude play vital roles in the HC regime change. Although the tropical eastern Pacific also experienced a long-term warming trend, it has little influence on the HC regime change due to the climatologically cold background SST over the cold tongue region. [ABSTRACT FROM AUTHOR]

The fidelity of the simulated Indian summer monsoon is analysed in the UK Met Office Unified Model Global Ocean Mixed Layer configuration (MetUM-GOML2.0) in terms of its boreal summer mean state and propagation of the boreal summer intraseasonal oscillation (BSISO). The model produces substantial biases in mean June–September precipitation, especially over India, in common with other MetUM configurations. Using a correction technique to constrain the mean seasonal cycle of ocean temperature and salinity, the effects of regional air–sea coupling and atmospheric horizontal resolution are investigated. Introducing coupling in the Indian Ocean degrades the atmospheric basic state compared with prescribing the observed seasonal cycle of sea surface temperature (SST). This degradation of the mean state is attributable to small errors (±0.5 ∘ C) in mean SST. Coupling slightly improves some aspects of the simulation of northward BSISO propagation over the Indian Ocean, Bay of Bengal, and India, but degrades others. Increasing resolution from 200 to 90 km grid spacing (approximate value at the Equator) improves the atmospheric mean state, but increasing resolution again to 40 km offers no substantial improvement. The improvement to intraseasonal propagation at finer resolution is similar to that due to coupling. [ABSTRACT FROM AUTHOR]

Abstract: Glacial‐interglacial cycles are often described as an amplified global response of the climate to perturbations in solar radiation caused by oscillations of Earth's orbit. However, it remains unclear whether internal feedbacks are large enough to account for the radically different glacial and interglacial states. Here we provide support for an alternative view: Glacial‐interglacial states are multiple equilibria of the climate system that exist for the same external forcing. We show that such multiple equilibria resembling glacial and interglacial states can be found in a complex coupled general circulation model of the ocean‐atmosphere‐sea ice system. The multiple states are sustained by ice‐albedo feedback modified by ocean heat transport and are not caused by the bistability of the ocean's overturning circulation. In addition, expansion/contraction of the Southern Hemisphere ice pack over regions of upwelling, regulating outgassing of CO2 to the atmosphere, is the primary mechanism behind a large pCO2 change between states. [ABSTRACT FROM AUTHOR]

General circulation models display a wide range of future predicted changes in the Northern Hemisphere winter stratospheric polar vortex. The downward influence of this stratospheric uncertainty on the troposphere has previously been inferred from regression analyses across models and is thought to contribute to model spread in tropospheric circulation change. Here we complement such regression analyses with idealized experiments using one model where different changes in the zonal-mean stratospheric polar vortex are artificially imposed to mimic the extreme ends of polar vortex change simulated by models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The influence of the stratospheric vortex change on the tropospheric circulation in these experiments is quantitatively in agreement with the inferred downward influence from across-model regressions, indicating that such regressions depict a true downward influence of stratospheric vortex change on the troposphere below. With a relative weakening of the polar vortex comes a relative increase in Arctic sea level pressure (SLP), a decrease in zonal wind over the North Atlantic, drying over northern Europe, and wetting over southern Europe. The contribution of stratospheric vortex change to intermodel spread in these quantities is assessed in the CMIP5 models. The spread, as given by 4 times the across-model standard deviation, is reduced by roughly 10% on regressing out the contribution from stratospheric vortex change, while the difference between models on extreme ends of the distribution in terms of their stratospheric vortex change can reach up to 50% of the overall model spread for Arctic SLP and 20% of the overall spread in European precipitation. [ABSTRACT FROM AUTHOR]

A set of homogenized monthly mean surface air temperature (SAT) series at 32 stations in China back to the 19th century had previously been developed based on the RHtest method by Cao et al., but some inhomogeneities remained in the dataset. The present study produces a further-adjusted and updated dataset based on the Multiple Analysis of Series for Homogenization (MASH) method. The MASH procedure detects 33 monthly temperature records as erroneous outliers and 152 meaningful break points in the monthly SAT series since 1924 at 28 stations. The inhomogeneous parts are then adjusted relative to the latest homogeneous part of the series. The new data show significant warming trends during 1924-2016 at all the stations, ranging from 0.48 to 3.57°C (100 yr)−1, with a regional mean trend of 1.65°C (100 yr)−1; whereas, the previous results ranged from a slight cooling at two stations to considerable warming, up to 4.5°C (100 yr)−1. It is suggested that the further-adjusted data are a better representation of the large-scale pattern of climate change in the region for the past century. The new data are available online at http://www.dx.doi.org/10.11922/sciencedb.516. [ABSTRACT FROM AUTHOR]

It has been challenging to project the tropical cyclone (TC) intensity, structure and destructive potential changes in a warming climate. Here, we compare the sensitivities of TC intensity, size and destructive potential to sea surface warming with and without a pre-storm atmospheric adjustment to an idealized state of Radiative-Convective Equilibrium (RCE). Without RCE, we find large responses of TC intensity, size and destructive potential to sea surface temperature (SST) changes, which is in line with some previous studies. However, in an environment under RCE, the TC size is almost insensitive to SST changes, and the sensitivity of intensity is also much reduced to 3% °C−1-4% °C−1. Without the pre-storm RCE adjustment, the mean destructive potential measured by the integrated power dissipation increases by about 25% °C−1 during the mature stage. However, in an environment under RCE, the sensitivity of destructive potential to sea surface warming does not change significantly. Further analyses show that the reduced response of TC intensity and size to sea surface warming under RCE can be explained by the reduced thermodynamic disequilibrium between the air boundary layer and the sea surface due to the RCE adjustment. When conducting regional-scale sea surface warming experiments for TC case studies, without any RCE adjustment the TC response is likely to be unrealistically exaggerated. The TC intensity-temperature sensitivity under RCE is very similar to those found in coupled climate model simulations. This suggests global mean intensity projections under climate change can be understood in terms of a thermodynamic response to temperature with only a minor contribution from any changes in large-scale dynamics. [ABSTRACT FROM AUTHOR]

Recent studies have indicated the importance of fall climate forcings and teleconnections in influencing the climate of the northern mid- to high latitudes. Here, we present some exploratory analyses using observational data and seasonal hindcasts, with the aim of highlighting the potential of the El Niño–Southern Oscillation (ENSO) as a driver of climate variability during boreal late fall and early winter (November and December) in the North Atlantic–European sector, and motivating further research on this relatively unexplored topic. The atmospheric ENSO teleconnection in November and December is reminiscent of the east Atlantic pattern and distinct from the well-known arching extratropical Rossby wave train found from January to March. Temperature and precipitation over Europe in November are positively correlated with the Niño-3.4 index, which suggests a potentially important ENSO climate impact during late fall. In particular, the ENSO-related temperature anomaly extends over a much larger area than during the subsequent winter months. We discuss the implications of these results and pose some research questions. [ABSTRACT FROM AUTHOR]

We use two groups of 100-member ensemble AGCM experiment to investigate the robustness and probabilistic nature of the Warm Arctic/Cold Eurasian (WACE) pattern with or without strong warming SST trend and sea-ice reduction. Model ensembles successfully simulate a distribution of trend coefficients close to that of observation. Results show that the recent trend in WACE pattern is driven by the warming of the Arctic SST, but the pattern itself is not amplified between the warming and non-warming experiment and cannot explain the current cooling trend of the mid-latitudes. We argue that the difference in sea-ice condition regulates the more extreme cases of the pattern thereby contributing to the positive trend in WACE pattern similar to that of observation. [ABSTRACT FROM AUTHOR]

Large-scale tracer transport in the upper troposphere and lower stratosphere (UTLS) is investigated using simulations of the Whole Atmosphere Community Climate Model (WACCM) over the period 1955-2099. The analyses are based on e90, an artificial passive tracer with constant emissions and atmospheric loss rates. The separate contributions of advection by the residual circulation, eddy mixing, and subgrid convection to total transport are explicitly evaluated. The results highlight distinct large-scale transport regimes in the tropics, characterized by efficient vertical tracer transport, and the extratropics, dominated by isentropic mixing. One novel result is the important role of vertical eddy mixing in the tropical upper troposphere. It is shown that interannual variability in e90 is largely driven by El Niño-Southern Oscillation and the quasi-biennial oscillation. The long-term trends emphasize a strong impact of a rising tropopause with climate change on UTLS dynamics and tracer transport. The analyses directly attribute the e90 trends to changes in the different transport components. Stronger residual circulation in the future leads to increased tracer concentrations in the tropical lower stratosphere. Enhanced eddy mixing increases e90 in the extratropical lowermost stratosphere, linked to an upward shift of wave dissipation tied to the tropopause rise. In the troposphere, reduced concentrations in the future are due to weaker convective transport out of the boundary layer and weaker extratropical isentropic eddy mixing. [ABSTRACT FROM AUTHOR]

Estimates of the climate response to anthropogenic forcing contain irreducible uncertainty due to the presence of internal variability. Accurate quantification of this uncertainty is critical for both contextualizing historical trends and determining the spread of climate projections. The contribution of internal variability to uncertainty in trends can be estimated in models as the spread across an initial condition ensemble. However, internal variability simulated by a model may be inconsistent with observations due to model biases. Here, statistical resampling methods are applied to observations in order to quantify uncertainty in historical 50-yr (1966-2015) winter near-surface air temperature trends over North America related to incomplete sampling of internal variability. This estimate is compared with the simulated trend uncertainty in the NCAR CESM1 Large Ensemble (LENS). The comparison suggests that uncertainty in trends due to internal variability is largely overestimated in LENS, which has an average amplification of variability of 32% across North America. The amplification of variability is greatest in the western United States and Alaska. The observationally derived estimate of trend uncertainty is combined with the forced signal from LENS to produce an 'Observational Large Ensemble' (OLENS). The members of OLENS indicate the range of observationally constrained, spatially consistent temperature trends that could have been observed over the past 50 years if a different sequence of internal variability had unfolded. The smaller trend uncertainty in OLENS suggests that is easier to detect the historical climate change signal in observations than in any given member of LENS. [ABSTRACT FROM AUTHOR]