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To analyze links among key processes that contribute to Arctic‐midlatitude teleconnections we apply causal discovery based on graphical models known as causal graphs. First, we calculate the causal dependencies from observations during 1980–2021. Observations show several robust connections from early to late winter, such as atmospheric blocking within central Asia via the Ural blocking and Siberian High, the North Atlantic Oscillation phase and the polar vortex (PV). The PV is affected by poleward eddy heat flux at 100 hPa, which is also directly connected with the Aleutian Low. We then evaluate climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) by comparing their causal graphs with those derived from observations. Compared to observations, CMIP6 historical and future simulations do not robustly capture Arctic‐midlatitude teleconnections arising from Arctic sea ice variability. This highlights the role of atmospheric internal variability in modulating the Arctic‐midlatitude teleconnections. However, we find several distinct patterns that are simulated by most of the analyzed climate models. For example, both historical and future model simulations robustly capture observed atmospheric blocking in central Asia. But contrary to observations, model simulations show a robust link between the Arctic temperature and sea ice cover over Barents and Kara seas. The analysis of future changes also reveals that the connection between the Aleutian Low and the poleward eddy heat flux at 100 hPa is expected to become more robust toward the end of the 21st century than in the analyzed past. The role of different mechanisms that link amplified Arctic warming and changes in midlatitude weather remains an open question. Observations and model simulations lead to different conclusions, making interpreting Arctic‐midlatitude connections difficult. To improve the understanding of these processes, this study uses a novel method that goes beyond simple correlation analysis, known as causal discovery. The application of causal discovery in combination with physical reasoning provides a powerful tool to evaluate the performance of climate models and better understand the mechanisms of analyzed teleconnections. Causal discovery detects cause–effect relationships from analyzed data using graphical models known as causal graphs. As a first step, we calculate causal graphs for observations. Then we compare the causal graphs from observations with historical simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6). This comparison shows if the CMIP6 models reproduce the observed connections in the current climate. We also estimate future changes in Arctic‐midlatitude teleconnections toward the end of the century by comparing causal graphs from the CMIP6 historical and Scenario Model Intercomparison Project (ScenarioMIP) simulations. This study demonstrates that the observations and CMIP6 model simulations do not robustly show Arctic‐midlatitude links that arise from Arctic sea ice variability. The robustness of Arctic‐midlatitude teleconnections is evaluated by applying causal discovery to observations and CMIP6 simulationsObservations, CMIP6 historical and SSP5‐8.5 simulations do not robustly show Arctic‐midlatitude links arising from sea ice variabilityCMIP6 SSP5‐8.5 simulations predict a more robust link between poleward eddy heat flux and Aleutian Low toward the end of the 21st century The robustness of Arctic‐midlatitude teleconnections is evaluated by applying causal discovery to observations and CMIP6 simulations Observations, CMIP6 historical and SSP5‐8.5 simulations do not robustly show Arctic‐midlatitude links arising from sea ice variability CMIP6 SSP5‐8.5 simulations predict a more robust link between poleward eddy heat flux and Aleutian Low toward the end of the 21st century

Details of the characteristics of upward planetary wave propagation associated with Arctic sea ice loss under present climate conditions are examined using reanalysis data and simulation results. Recent Arctic sea ice loss results in increased stratospheric poleward eddy heat fluxes in the eastern and central Eurasia regions and enhanced upward propagation of planetary‐scale waves in the stratosphere. A linear decomposition scheme reveals that this modulation of the planetary waves arises from coupling of the climatological planetary wavefield with temperature anomalies for the eastern Eurasia region and with meridional wind anomalies for the central Eurasia region. Propagation of stationary Rossby wave packets results in a dynamic link between these temperature and meridional wind anomalies with sea ice loss over the Barents‐Kara Sea. The results provide strong evidence that recent Arctic sea ice loss significantly modulates atmospheric circulation in winter to modify poleward eddy heat fluxes so as to drive stratosphere‐troposphere coupling processes. Recent Arctic sea ice loss results in increased poleward eddy heat fluxes at 100 hPa over the central and eastern Eurasian regionsAnomalous meridional wind and temperature fields coupled with climatological wave structure lead to the increased eddy heat fluxesThese anomalies are dynamically linked through the propagation of stationary Rossby waves from the Barents‐Kara sea ice reduction

Extreme winters in Northern Hemisphere midlatitudes in recent years have been connected to declining Arctic sea ice and continental snow cover changes in autumn following modified planetary waves in the coupled troposphere‐stratosphere system. Through analyses of reanalysis data and model simulations with a state‐of‐the‐art atmospheric general circulation model, we investigate the mechanisms between Arctic Ocean sea ice and Northern Hemisphere land snow cover changes in autumn and atmospheric teleconnections in the following winter. The observed negative Arctic Oscillation in response to sea ice cover changes is too weakly reproduced by the model. The planetary wave train structures over the Pacific and North America regions are well simulated. The strengthening and westward shift of the Siberian high‐pressure system in response to sea ice and snow cover changes is underestimated compared to ERA‐Interim data due to deficits in the simulated changes in planetary wave propagation characteristics. Changes in sea ice and snow cover induce a negative Arctic Oscillation in winterAn atmospheric model has deficits in reproducing the negative Arctic OscillationDeficits in the simulated planetary wave propagation are identified

The altitude dependent variability of ozone in the polar stratosphere is regularly observed by balloon-borne ozonesonde observations at Neumayer Station (70°S) in the Antarctic and at Koldewey Station (79°N)in the Arctic. The reasons for observed seasonal and interannual variability and long-term changes are discussed. Differences between the hemispheres are identified and discussed in light of differing dynamical and chemical conditions. Since the mid- 1980s, rapid chemical ozone loss has been recorded in the lower Antarctic stratosphere during the spring season. Using coordinated ozone soundings in some Arctic winters, similar chemical ozone loss rates have been detected related to periods of low temperatures. The currently observed cooling trend of the stratosphere, potentially caused by the increase of anthropogenic greenhouse gases, may further strengthen chemical ozone removal in the Arctic. However, the role of internal climate oscillations in observed temperature trends is still uncertain. First results of a 10000 year integration of a low order climate model indicate significant internal climate variability. on decadal time scales, that may alter the effect of increasing levels of greenhouse gases in the polar stratosphere.

We demonstrated atmospheric responses to a reduction in Arctic sea ice via simulations in which Arctic sea ice decreased stepwise from the present‐day range to an ice‐free range. In all cases, the tropospheric response exhibited a negative Arctic Oscillation (AO)‐like pattern. An intensification of the climatological planetary‐scale wave due to the present‐day sea ice reduction on the Atlantic side of the Arctic Ocean induced stratospheric polar vortex weakening and the subsequent negative AO. Conversely, strong Arctic warming due to ice‐free conditions across the entire Arctic Ocean induced a weakening of the tropospheric westerlies corresponding to a negative AO without troposphere‐stratosphere coupling, for which the planetary‐scale wave response to a surface heat source extending to the Pacific side of the Arctic Ocean was responsible. Because the resultant negative AO‐like response was accompanied by secondary circulation in the meridional plane, atmospheric heat transport into the Arctic increased, accelerating the Arctic amplification. A negative AO response appears in winter with more heat transport into the Arctic in either case of realistic or extreme Arctic sea ice lossArctic sea ice impacts midlatitude climate via two pathways: the stratospheric and tropospheric pathways