doi:10.1007/s00376-017-7039-9

Baldwin M. P., T. J. Dunkerton, 2001: Stratospheric harbingers of anomalous weather regimes.Science,294,581-584, https://doi.org/10.1126/science.1063315.

10.1126/science.1063315

eef35230b9a42dc2ec1960dde3dbee70

http%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di0276-4741-32-4-431-Baldwin1%26amp%3Bdbid%3D8%26amp%3Bdoi%3D10.1659%252FMRD-JOURNAL-D-12-00062.1%26amp%3Bkey%3D11641495

http://www.sciencemag.org/cgi/doi/10.1126/science.1063315

Baldwin M. P., D. B. Stephenson, D. W. J. Thompson, T. J. Dunkerton, A. J. Charlton, and A. O'Neill, 2003: Stratospheric memory and skill of extended-range weather forecasts.Science,301,636-640, https://doi.org/10.1126/science.1087143.

10.1126/science.1087143

12893941

393e58b950d97c5fdc013c6d497f4d8f

http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FPMED%3Fid%3D12893941

http://www.sciencemag.org/cgi/doi/10.1126/science.1087143We use an empirical statistical model to demonstrate significant skill in making extended-range forecasts of the monthly-mean Arctic Oscillation (AO). Forecast skill derives from persistent circulation anomalies in the lowermost stratosphere and is greatest during boreal winter. A comparison to the Southern Hemisphere provides evidence that both the time scale and predictability of the AO depend on the presence of persistent circulation anomalies just above the tropopause. These circulation anomalies most likely affect the troposphere through changes to waves in the upper troposphere, which induce surface pressure changes that correspond to the AO.

Boland, E. J. D., T. J. Bracegirdle, E. F. Shuckburgh, 2017: Assessment of sea ice-atmosphere links in CMIP5 models.Climate Dyn.,49,683-702, https://doi.org/10.1007/s00382-016-3367-1.

10.1007/s00382-016-3367-1

ab3a251e32d7be734f9bdd52732c59b5

http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00382-016-3367-1

http://link.springer.com/10.1007/s00382-016-3367-1The Arctic is currently undergoing drastic changes in climate, largely thought to be due to so-called `Arctic amplification', whereby local feedbacks enhance global warming. Recently, a number of observational and modelling studies have questioned what the implications of this change in Arctic sea ice extent might be for weather in Northern Hemisphere midlatitudes, and in particular whether recent extremely cold winters such as 2009/10 might be consistent with an influence from observed Arctic sea ice decline. However, the proposed mechanisms for these links have not been consistently demonstrated. In a uniquely comprehensive cross-season and cross-model study, we show that the CMIP5 models provide no support for a relationship between declining Arctic sea ice and a negative NAM, or between declining Barents-Kara sea ice and cold European temperatures. The lack of evidence for the proposed links is consistent with studies that report a low signal-to-noise ratio in these relationships. These results imply that, whilst links may exist between declining sea ice and extreme cold weather events in the Northern Hemisphere, the CMIP5 model experiments do not show this to be a leading order effect in the long-term. We argue that this is likely due to a combination of the limitations of the CMIP5 models and an indication of other important long-term influences on Northern Hemisphere climate.

Christiansen B., 2001: Downward propagation of zonal mean zonal wind anomalies from the stratosphere to the troposphere: Model and reanalysis.J. Geophys. Res.,106,27 307-27 322, https://doi.org/10.1029/2000JD000214.

10.1029/2000JD000214

c783b03c5a5d38dcc5b8b3f21c12683d

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000JD000214%2Fcitedby

http://doi.wiley.com/10.1029/2000JD000214The connection between the Arctic Oscillation and the stratosphere is investigated on intra-annual timescales. Both the National Centers for Environmental Prediction reanalysis data and a general circulation model simulation are used. In the winter half year November-April the dominant variability in the stratosphere in middle and high latitudes has the form of downward propagation of zonal mean zonal wind anomalies. The strength of the anomalies decays below 10 hPa, but often the anomalies reach the surface. The time for the propagation from 10 hPa to the surface is 藴15 days. When positive anomalies reach the surface, the phase of the Arctic Oscillation tends to be positive. The stratospheric variability and the downward propagation is found to be driven by the vertical component of the Eliassen-Palm flux. This flux propagates from the lower troposphere to the tropopause on a timescale of 5 days. Model and reanalysis compare well in the structure of the stratospheric variability and the connection between the stratosphere and troposphere. However, the strength of the stratospheric variability is 藴25% weaker in the model.

Cohen J., J. Jones, J. C. Furtado, and E. Tziperman, 2013: Warm arctic,cold continents: A common pattern related to arctic sea ice melt,snow advance,and extreme winter weather. Oceanography, 26, 150-160,2013. 70.https://doi.org/10.5670/oceanog.

Cohen, J., Coauthors, 2014: Recent arctic amplification and extreme mid-latitude weather.Nature Geoscience,7,627-637, https://doi.org/10.1038/ngeo2234.

10.1038/ngeo2234

4cd471caba2fba3502432bd1eab5ae32

http%3A%2F%2Fwww.nature.com%2Fabstractpagefinder%2F10.1038%2Fngeo2234

http://www.nature.com/doifinder/10.1038/ngeo2234The Arctic region has warmed more than twice as fast as the global average a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncertainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events.

Edmon H. J., B. J. Hoskins, and M. E. McIntyre, 1980: Eliassen-palm cross sections for the troposphere. J. Atmos. Sci., 37, 2600-2616, https://doi.org/10.1175/1520-0469(1980)037 <2600:EPCSFT>2.0,CO;2.

10.1175/1520-0469(1980)0372.0.CO;2

63345326183008ef37da4949ba7e3a17

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1980jats...37.2600e

http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281980%29037%3C2600%3AEPCSFT%3E2.0.CO%3B2`Eliassen-Palm (EP) cross sections' are meridional cross sections showing the Eliassen-Palm flux F by arrows and its divergence by contours. For large-scale, quasi-geostrophic motion F is defined to have and components rcos[, /] where is latitude, pressure, potential temperature, rthe radius of the earth, bars and primes denote zonal means and deviations and (u,v) is horizontal velocity. The theoretical reasons for using EP cross sections diagnostically are reviewed. The divergence of F reflects the magnitude of transient and irreversible eddy processes at each height and latitude, and is proportional to the northward flux of quasi-geostrophic (not Ertel's) potential vorticity. It is a direct measure of the total forcing of the zonal-mean state by the eddies. The direction of F indicates the relative importance of the principal eddy fluxes of heat and momentum. If the eddy dynamics is Rossby wavelike, then F is also a measure of net wave propagation from one height and latitude to another. Observational and theoretical EP cross sections are presented for the layer 1000-50 mb, and discussed in terms of the abovementioned properties. The observational cross sections for transient eddies are more reliably determined than for stationary eddies, and resemble to a significant degree the cross sections given by nonlinear baroclinic instability simulations. They do not resemble those given by linear instability theory for a realistic mean state (verifying the inappropriateness of linear theory as a basis for eddy parameterizations). They provide a direct view of the latitudinal planetary-wave propagation mechanism whereby midlatitudinal instabilities influence the high-tropospheric subtropics. A similar dynamical linkage appears to be depicted by the EP cross sections for stationary eddies in winter. The cross sections for stationary eddies in summer are strikingly different, but not very well determined by the data. Nevertheless, there are reasons why some of the differences might be real, with possible implications for theories of stationary planetary waves. The `residual meridional circulations' associated with the observed EP cross sections are presented and discussed.

Francis J. A., W. H. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron, 2009: Winter northern hemisphere weather patterns remember summer arctic sea-ice extent,Geophys. Res. Lett.,36,L07503, https://doi.org/10.1029/2009GL037274.

10.1029/2009GL037274

d3b87d349c019c3e05604f4fbcfb548c

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009GL037274%2Ffull

http://onlinelibrary.wiley.com/doi/10.1029/2009GL037274/fullThe dramatic decline in Arctic summer sea-ice cover is a compelling indicator of change in the global climate system and has been attributed to a combination of natural and anthropogenic effects. Through its role in regulating the exchange of energy between the ocean and atmosphere, ice loss is anticipated to influence atmospheric circulation and weather patterns. By combining satellite measurements of sea-ice extent and conventional atmospheric observations, we find that varying summer ice conditions are associated with large-scale atmospheric features during the following autumn and winter well beyond the Arctic's boundary. Mechanisms by which the atmosphere “remembers” a reduction in summer ice cover include warming and destabilization of the lower troposphere, increased cloudiness, and slackening of the poleward thickness gradient that weakens the polar jet stream. This ice-atmosphere relationship suggests a potential long-range outlook for weather patterns in the northern hemisphere.

Garfinkel C. I., D. L. Hartmann, and F. Sassi, 2010: Tropospheric precursors of anomalous northern hemisphere stratospheric polar vortices.J. Climate,23,3282-3299, https://doi.org/10.1175/2010JCLI3010.1.

10.1175/2010JCLI3010.1

ef082c9382fca2eb375c19e810fe3fbe

http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103246477.html

http://journals.ametsoc.org/doi/abs/10.1175/2010JCLI3010.1Regional extratropical tropospheric variability in the North Pacific and eastern Europe is well correlated with variability in the Northern Hemisphere wintertime stratospheric polar vortex in both the ECMWF reanalysis record and in the Whole Atmosphere Community Climate Model. To explain this correlation, the link between stratospheric vertical Eliassen-Palm flux variability and tropospheric va...

Honda M., J. Inoue, and S. Yamane, 2009: Influence of low arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett. 36, https://doi.org/10.1029/2008GL037079.

10.1029/2008GL037079

11ff7459b32da24cee92554351efd9cb

http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103055291.html

http://www.cabdirect.org/abstracts/20103055291.htmlInfluence of low Arctic sea-ice minima in early autumn on the wintertime climate over Eurasia is investigated. Observational evidence shows that significant cold anomalies over the Far East in early winter and zonally elongated cold anomalies from Europe to Far East in late winter are associated with the decrease of the Arctic sea-ice cover in the preceding summer-to-autumn seasons. Results fro...

Hopsch S., J. Cohen, and K. Dethloff, 2012: Analysis of a link between fall arctic sea ice concentration and atmospheric patterns in the following winter,Tellus A,64,18624, v64i0. 18624.https://doi.org/10.3402/tellusa.

10.3402/tellusa.v64i0.18624

46fe623b20c72ca3fe205effb7547100

http%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.3402%2Ftellusa.v64i0.18624

https://www.tandfonline.com/doi/full/10.3402/tellusa.v64i0.18624The impact of anomalous fall Arctic sea ice concentrations (SICs) on atmospheric patterns in the following winter is revisited by analysing results for two time periods: the most recent, satellite-era period (19792010) and a longer time-period (19502010). On the basis of September SICs for each time-period, an index was constructed which was used to identify anomalous high/low SIC years for both the original, as well as for the linearly detrended sea ice index. Identified years were then used to derive composites for the following winter's monthly atmospheric variables. Mid-troposphere geopotential height composites for winter months are in general reminiscent of the North Atlantic Oscillation pattern with high latitude maximum shifted towards the Barents Sea. Also, lower troposphere temperatures indicate the presence of cooler conditions over the continents during low SIC years. However, differences in the composite patterns are significant only for areas with limited spatial extent. While suggested pathways in previously published studies seem reasonable, our results show that these findings are not yet robust enough from a statistical significance perspective. More data (e.g. provided by longer, climate-quality reanalysis datasets) are needed before conclusions of impacts and feedbacks can be drawn with certainty.

Jaiser R., K. Dethloff, D. Hand orf, A. Rinke, and J. Cohen, 2012: Impact of sea ice cover changes on the northern hemisphere atmospheric winter circulation,Tellus A,64,11595, v64i0. 11595.https://doi.org/10.3402/tellusa.

10.3402/tellusa.v64i0.11595

73cf21d64de757276aa85a42a608612c

http%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.3402%2Ftellusa.v64i0.11595

https://www.tandfonline.com/doi/full/10.3402/tellusa.v64i0.11595The response of the Arctic atmosphere to low and high sea ice concentration phases based on European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) atmospheric data and Hadley Centre's sea ice dataset (HadISST1) from 1989 until 2010 has been studied. Time slices of winter atmospheric circulation with high (1990-2000) and low (2001-2010) sea ice concentration in the preceding August/September have been analysed with respect to tropospheric interactions between planetary and baroclinic waves. It is shown that a changed sea ice concentration over the Arctic Ocean impacts differently the development of synoptic and planetary atmospheric circulation systems. During the low ice phase, stronger heat release to the atmosphere over the Arctic Ocean reduces the atmospheric vertical static stability. This leads to an earlier onset of baroclinic instability that further modulates the non-linear interactions between baroclinic wave energy fluxes on time scales of 2.56 d and planetary scales of 10-90 d. Our analysis suggests that Arctic sea ice concentration changes exert a remote impact on the large-scale atmospheric circulation during winter, exhibiting a barotropic structure with similar patterns of pressure anomalies at the surface and in the mid-troposphere. These are connected to pronounced planetary wave train changes notably over the North Pacific.

Kim B.-M., S.-W. Son, S.-K. Min, J.-H. Jeong, S.-J. Kim, X. D. Zhang, T. Shim, and J.-H. Yoon, 2014: Weakening of the stratospheric polar vortex by arctic sea-ice loss,Nature Communications,5,4646, https://doi.org/10.1038/ncomms5646.

10.1038/ncomms5646

25181390

0c34446785d2f1fe02375ca8641143f2

http%3A%2F%2Fwww.nature.com%2Fncomms%2F2014%2F140902%2Fncomms5646%2Fabs%2Fncomms5646.html

http://www.nature.com/doifinder/10.1038/ncomms5646Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (November-December), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.

Knutti R., D. Masson, and A. Gettelman, 2013: Climate model genealogy: generation CMIP5 and how we got there.Geophys. Res. Lett.,40,1194-1199, https://doi.org/10.1002/grl.50256.

10.1002/grl.50256

6041d4ddba374d30561fd92024b47421

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fgrl.50256%2Ffull

http://doi.wiley.com/10.1002/grl.50256A new ensemble of climate models is becoming available and provides the basis for climate change projections. Here, we show a first analysis indicating that the models in the new ensemble agree better with observations than those in older ones and that the poorest models have been eliminated. Most models are strongly tied to their predecessors, and some also exchange ideas and code with other models, thus supporting an earlier hypothesis that the models in the new ensemble are neither independent of each other nor independent of the earlier generation. On the basis of one atmosphere model, we show how statistical methods can identify similarities between model versions and complement process understanding in characterizing how and why a model has changed. We argue that the interdependence of models complicates the interpretation of multimodel ensembles but largely goes unnoticed.

Koenigk T., M. Caian, G. Nikulin, and S. Schimanke, 2016: Regional arctic sea ice variations as predictor for winter climate conditions.Climate Dyn.,46,317-337, https://doi.org/10.1007/s00382-015-2586-1.

10.1007/s00382-015-2586-1

8ca179b60d5c665d89e9c04c6bb98392

http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-015-2586-1

http://link.springer.com/10.1007/s00382-015-2586-1Seasonal prediction skill of winter mid and high northern latitudes climate from sea ice variations in eight different Arctic regions is analyzed using detrended ERA-interim data and satellite sea ice data for the period 1980–2013. We find significant correlations between ice areas in both September and November and winter sea level pressure, air temperature and precipitation. The prediction skill is improved when using November sea ice conditions as predictor compared to September. This is particularly true for predicting winter NAO-like patterns and blocking situations in the Euro-Atlantic area. We find that sea ice variations in Barents Sea seem to be most important for the sign of the following winter NAO—negative after low ice—but amplitude and extension of the patterns are modulated by Greenland and Labrador Seas ice areas. November ice variability in the Greenland Sea provides the best prediction skill for central and western European temperature and ice variations in the Laptev/East Siberian Seas have the largest impact on the blocking number in the Euro-Atlantic region. Over North America, prediction skill is largest using September ice areas from the Pacific Arctic sector as predictor. Composite analyses of high and low regional autumn ice conditions reveal that the atmospheric response is not entirely linear suggesting changing predictive skill dependent on sign and amplitude of the anomaly. The results confirm the importance of realistic sea ice initial conditions for seasonal forecasts. However, correlations do seldom exceed 0.6 indicating that Arctic sea ice variations can only explain a part of winter climate variations in northern mid and high latitudes.

Kost J. T., M. P. McDermott, 2002: Combining dependent P-values.Statistics & Probability Letters,60,183-190, .http://doi.org/10.1016/S0167-7152(02)00310-3

Kug J.-S., J.-H. Jeong, Y.-S. Jang, B.-M. Kim, C. K. Folland , S.-K. Min, and S.-W. Son, 2015: Two distinct influences of arctic warming on cold winters over North America and East Asia.Nature Geosci,8,759-762, https://doi.org/10.1038/ngeo2517.

10.1038/NGEO2517

76adf9a50b5913a10d5735fde3c115c3

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv8%2Fn10%2Fngeo2517%2Fmetrics

http://www.nature.com/doifinder/10.1038/ngeo2517Arctic warming has sparked a growing interest because of its possible impacts on mid-latitude climate. A number of unusually harsh cold winters have occurred in many parts of East Asia and North America in the past few years, and observational and modelling studies have suggested that atmospheric variability linked to Arctic warming might have played a central role. Here we identify two distinct influences of Arctic warming which may lead to cold winters over East Asia or North America, based on observational analyses and extensive climate model results. We find that severe winters across East Asia are associated with anomalous warmth in the Barents-Kara Sea region, whereas severe winters over North America are related to anomalous warmth in the East Siberian-Chukchi Sea region. Each regional warming over the Arctic Ocean is accompanied by the local development of an anomalous anticyclone and the downstream development of a mid-latitude trough. The resulting northerly flow of cold air provides favourable conditions for severe winters in East Asia or North America. These links between Arctic and mid-latitude weather are also robustly found in idealized climate model experiments and CMIP5 multi-model simulations. We suggest that our results may help improve seasonal prediction of winter weather and extreme events in these regions.

Martius O., L. M. Polvani, and H. C. Davies, 2009: Blocking precursors to stratospheric sudden warming events. Geophys. Res. Lett. 36, https://doi.org/10.1029/2009GL038776.

10.1029/2009GL038776

8725b8aefe0512d59882521b541f153d

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009GL038776%2Ffull

http://onlinelibrary.wiley.com/doi/10.1029/2009GL038776/fullThe primary causes for the onset of major, midwinter, stratospheric sudden warming events remain unclear. In this paper, we report that 25 of the 27 events objectively identified in the ERA-40 dataset for the period 1957-2001 are preceded by blocking patterns in the troposphere. The spatial characteristics of tropospheric blocks prior to sudden warming events are strongly correlated with the type of sudden warming event that follows. Vortex displacement events are nearly always preceded by blocking over the Atlantic basin only, whereas vortex splitting events are preceded by blocking events occurring in the Pacific basin or in both basins contemporaneously. The differences in the geographical blocking distribution prior to sudden warming events are mirrored in the patterns of planetary waves that are responsible for producing events of either type. The evidence presented here, suggests that tropospheric blocking plays an important role in determining the onset and the type of warmings.

McCusker K. E., J. C. Fyfe, and M. Sigmond, 2016: Twenty-five winters of unexpected Eurasian cooling unlikely due to arctic sea-ice loss.Nature Geoscience,9,838-842, https://doi.org/10.1038/ngeo2820.

10.1038/ngeo2820

86ebb2cb4509993b8b6992484cd04e67

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv9%2Fn11%2Fngeo2820%2Fmetrics

http://www.nature.com/doifinder/10.1038/ngeo2820Winter cooling over Eurasia has been suggested to be linked to Arctic sea-ice loss. Climate model simulations reveal no evidence for such a link and instead suggest that a persistent atmospheric circulation pattern is responsible.

Mori M., M. Watanabe, H. Shiogama, J. Inoue, and M. Kimoto, 2014: Robust arctic sea-ice influence on the frequent Eurasian cold winters in past decades.Nature Geoscience,7,869-873, https://doi.org/10.1038/ngeo2277.

10.1038/ngeo2277

cdef8a86f56c39f6052fde6e5d1dd7bb

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv7%2Fn12%2Fabs%2Fngeo2277.html

http://www.nature.com/doifinder/10.1038/ngeo2277Over the past decade, severe winters occurred frequently in mid-latitude Eurasia, despite increasing global- and annual-mean surface air temperatures. Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline, through excitation of circulation anomalies similar to the Arctic Oscillation. In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation. Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents-Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insufficient ensemble size.

Nakamura T., K. Yamazaki, K. Iwamoto, M. Honda, Y. Miyoshi, Y. Ogawa, Y. Tomikawa, and J. Ukita, 2016: The stratospheric pathway for arctic impacts on midlatitude climate.Geophys. Res. Lett.,43,3494-3501, https://doi.org/10.1002/2016GL068330.

10.1002/2016GL068330

b149e8559dbb4aaa964cec0873755ae5

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2016GL068330%2Ffull

http://doi.wiley.com/10.1002/2016GL068330Recent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic-midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary-scale waves. Those results and consistency with observation-based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase.

Newman P. A., E. R. Nash, 2000: Quantifying the wave driving of the stratosphere.J. Geophys. Res.,105,12 485-12 497, https://doi.org/10.1029/1999JD901191.

10.1029/1999JD901191

2bdca7d63b8ba6404e46f86ce2cd5c04

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F1999JD901191%2Ffull

http://doi.wiley.com/10.1029/1999JD901191Abstract Top of page Abstract References The zonal-mean eddy heat flux is directly proportional to the wave activity that propagates from the troposphere into the stratosphere. This quantity is a simple eddy diagnostic which is calculated from conventional meteorological analyses. Because this “wave driving” of the stratosphere has a strong impact on the stratospheric temperature, it is necessary to compare the impact of the flux with respect to stratospheric radiative changes caused by greenhouse gas changes. Hence we must understand the precision and accuracy of the heat flux derived from our global meteorological analyses. Herein we quantify the stratospheric heat flux using five different meteorological analyses and show that there are 15% differences, on average, between these analyses during the disturbed conditions of the Northern Hemisphere winter. Such large differences result from the planetary differences in the stationary temperature and meridional wind fields. In contrast, planetary transient waves show excellent agreement among these five analyses, and this transient heat flux appears to have a long-term downward trend.

Overland, J. E., Coauthors, 2016: Nonlinear response of mid-latitude weather to the changing arctic.Nat. Clim. Change,6,992-999, https://doi.org/10.1038/nclimate3121.

10.1038/nclimate3121

042c9149fc49589ba72cc4cbab4b151d

http%3A%2F%2Fwww.nature.com%2Fnclimate%2Fjournal%2Fv6%2Fn11%2Ffig_tab%2Fnclimate3121_F5.html

http://www.nature.com/doifinder/10.1038/nclimate3121Are continuing changes in the Arctic influencing wind patterns and the occurrence of extreme weather events in northern mid-latitudes? The chaotic nature of atmospheric circulation precludes easy answers. The topic is a major science challenge, as continued Arctic temperature increases are an inevitable aspect of anthropogenic climate change. We propose a perspective that rejects simple cause-and-effect pathways and notes diagnostic challenges in interpreting atmospheric dynamics. We present a way forward based on understanding multiple processes that lead to uncertainties in Arctic and mid-latitude weather and climate linkages. We emphasize community coordination for both scientific progress and communication to a broader public.

Pedersen R. A., I. Cvijanovic, P. L. Langen, and B. M. Vinther, 2016: The impact of regional arctic sea ice loss on atmospheric circulation and the NAO.J. Climate,29,889-902, https://doi.org/10.1175/JCLI-D-15-0315.1.

10.1175/JCLI-D-15-0315.1

d3da800db08f6a0c8116f9714124a0cc

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2016EGUGA..18.7437A

http://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0315.1Rasmus A. Pedersen, Ivana Cvijanovic, Peter L. Langen, and Bo M. Vinther, 2016: The Impact of Regional Arctic Sea Ice Loss on Atmospheric Circulation and the NAO. J. Climate, 29, 889–902. doi: http://dx.doi.org/10.1175/JCLI-D-15-0315.1

Peings Y., G. Magnusdottir, 2014: Response of the wintertime northern hemisphere atmospheric circulation to current and projected arctic sea ice decline: A numerical study with CAM5.J. Climate,27,244-264, https://doi.org/10.1175/JCLI-D-13-00272.1.

10.1175/JCLI-D-13-00272.1

e9732dd6358dbd0d3b6b5b82193b2c9c

http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1175%2FJCLI-D-13-00272.1

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00272.1The wintertime Northern Hemisphere (NH) atmospheric circulation response to current (2007-12) and projected (2080-99) Arctic sea ice decline is examined with the latest version of the Community Atmospheric Model (CAM5). The numerical experiments suggest that the current sea ice conditions force a remote atmospheric response in late winter that favors cold land surface temperatures over midlatitudes, as has been observed in recent years. Anomalous Rossby waves forced by the sea ice anomalies penetrate into the stratosphere in February and weaken the stratospheric polar vortex, resulting in negative anomalies of the northern annular mode (NAM) that propagate downward during the following weeks, especially over the North Pacific. The seasonality of the response is attributed to timing of the phasing between the forced and climatological waves. When sea ice concentration taken from projections of conditions at the end of the twenty-first century is prescribed to the model, negative anomalies of theNAMare visible in the troposphere, both in early and late winter. This response is mainly driven by the large warming of the lower troposphere over the Arctic, as little impact is found in the stratosphere in this experiment. As a result of the thermal expansion of the polar troposphere, the westerly flow is decelerated and a weak but statistically significant increase of the midlatitude meanders is identified. However, the thermodynamical response extends beyond the Arctic and offsets the dynamical effect, such that the stronger sea ice forcing has limited impact on the intensity of cold extremes over midlatitudes. 2014 American Meteorological Society.

Perlwitz J., M. Hoerling, and R. Dole, 2015: Arctic tropospheric warming: Causes and linkages to lower latitudes.J. Climate,28,2154-2167, https://doi.org/10.1175/JCLI-D-14-00095.1.

10.1175/JCLI-D-14-00095.1

e1f15b352e80b6968f6ca4e5cee932b6

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.2154P

http://journals.ametsoc.org/doi/10.1175/JCLI-D-14-00095.1Arctic temperatures have risen dramatically relative to those of lower latitudes in recent decades, with a common supposition being that sea ice declines are primarily responsible for amplified Arctic tropospheric warming. This conjecture is central to a hypothesis in which Arctic sea ice loss forms the beginning link of a causal chain that includes weaker westerlies in midlatitudes, more persistent and amplified midlatitude waves, and more extreme weather. Through model experimentation, the first step in this chain is examined by quantifying contributions of various physical factors to October揇ecember (OND) mean Arctic tropospheric warming since 1979. The results indicate that the main factors responsible for Arctic tropospheric warming are recent decadal fluctuations and long-term changes in sea surface temperatures (SSTs), both located outside the Arctic. Arctic sea ice decline is the largest contributor to near-surface Arctic temperature increases, but it accounts for only about 20% of the magnitude of 1000500-hPa warming. These findings thus disconfirm the hypothesis that deep tropospheric warming in the Arctic during OND has resulted substantially from sea ice loss. Contributions of the same factors to recent midlatitude climate trends are then examined. It is found that pronounced circulation changes over the North Atlantic and North Pacific result mainly from recent decadal ocean fluctuations and internal atmospheric variability, while the effects of sea ice declines are very small. Therefore, a hypothesized causal chain of hemisphere-wide connections originating from Arctic sea ice loss is not supported.

Petoukhov V., V. A. Semenov, 2010: A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res. 115, https://doi.org/10.1029/2009JD013568.

10.1029/2009JD013568

dc1ac9e62c94b87f316ae99122829c96

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD013568%2Fpdf

http://onlinelibrary.wiley.com/doi/10.1029/2009JD013568/pdfThe recent overall Northern Hemisphere warming was accompanied by several severe northern continental winters, as for example, extremely cold winter 2005-2006 in Europe and northern Asia. Here we show that anomalous decrease of wintertime sea ice concentration in the Barents-Kara (B-K) seas could bring about extreme cold events like winter 2005-2006. Our simulations with the ECHAM5 general circulation model demonstrate that lower-troposphere heating over the B-K seas in the Eastern Arctic caused by the sea ice reduction may result in strong anticyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents. This causes a continental-scale winter cooling reaching -1.5ºC, with more than 3 times increased probability of cold winter extremes over large areas including Europe. Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment. Furthermore, our results suggest that high-latitude atmospheric circulation response to the B-K sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then back again to cyclonic type of circulation as the B-K sea ice concentration gradually reduces from 100% to ice free conditions. We present a conceptual model that may explain the nonlinear local atmospheric response in the B-K seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area.

Sato K., J. Inoue, and M. Watanabe, 2014: Influence of the gulf stream on the Barents Sea ice retreat and Eurasian coldness during early winter,Environmental Research Letters,9,084009, https://doi.org/10.1088/1748-9326/9/8/084009.

10.1088/1748-9326/9/8/084009

9c5d56ad9ccb94480ed2f47502d57e80

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2014AGUFM.A33D3218S

http://stacks.iop.org/1748-9326/9/i=8/a=084009?key=crossref.6f28bcabaff7bcbdba0494c16983ca82Abnormal sea-ice retreat over the Barents Sea during early winter has been considered a leading driver of recent midlatitude severe winters over Eurasia. However, causal relationships between such retreat and the atmospheric circulation anomalies remains uncertain. Using a reanalysis dataset, we found that poleward shift of a sea surface temperature front over the Gulf Stream likely induces warm southerly advection and consequent sea-ice decline over the Barents Sea sector, and a cold anomaly over Eurasia via planetary waves triggered over the Gulf Stream region. The above mechanism is supported by the steady atmospheric response to the diabatic heating anomalies over the Gulf Stream region obtained with a linear baroclinic model. The remote atmospheric response from the Gulf Stream would be amplified over the Barents Sea region via interacting with sea-ice anomaly, promoting the warm Arctic and cold Eurasian pattern. (letter)

Screen J. A., 2017a: Climate science: Far-flung effects of arctic warming.Nature Geoscience,10,253-254, https://doi.org/10.1038/ngeo2924.

10.1038/ngeo2924

aea36c4b3f00dd6d18be43477a8eb5d6

http%3A%2F%2Fwww.nature.com%2Fngeo%2Fjournal%2Fv10%2Fn4%2Ffull%2Fngeo2924.html

http://www.nature.com/doifinder/10.1038/ngeo2924Abstract Arctic warming affects weather and climate thousands of miles to the south. Scientists are split on how large this effect is.

Screen J. A., 2017b: Simulated atmospheric response to regional and pan-arctic sea ice loss.J. Climate,30,3945-3962, https://doi.org/10.1175/JCLI-D-16-0197.1.

10.1175/JCLI-D-16-0197.1

5a55386fa74a2b1cde0e04944b938f1b

http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F312642732_Simulated_Atmospheric_Response_to_Regional_and_Pan-Arctic_Sea-Ice_Loss

http://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0197.1Theloss ofArctic sea-iceis already having profound environmental, societal and ecologicalimpactslocally.A highlyuncertainareaofscientificresearch,however,iswhethersuchArcticchange has a tangible effect on weather and climate at lower latitudes. There is emergingevidencethat thegeographicallocation of sea-icelossis criticallyimportantin determiningthe large-scale atmospheric circulation response and associated mid-latitude impacts.However,such regionaldependencieshavenotbeenexploredinathorough andsystematicmanner.Tomake progress on thisissue, this study analyses ensemble simulations with anatmosphericgeneralcirculationmodel prescribedwithsea-icelossseparatelyinnine regionsof theArctic, toelucidate thedistinctresponses toregionalsea-iceloss. Theresultssuggestthatin some regions sea-iceloss triggerslarge-scale dynamical responseswhereasin otherregionssea-icelossinducesonlylocalthermodynamicalchanges.Sea-icelossintheBarents-KaraSeaisuniqueindrivingaweakeningofthestratosphericpolarvortex,followedintimeby a tropospheric circulation response that resembles the North Atlantic Oscillation. ForOctober-to-March, the largest spatial-scale responses are driven by sea-ice loss in theBarents-KaraSeaandSeaofOkhotsk;however,differentregionsassumegreaterimportanceinotherseasons.Theatmosphererespondsverydifferentlytoregionalsea-icelossesthantopan-Arcticsea-iceloss,andthelattercannotbeobtainedbylinearadditionoftheresponsesto regional sea-ice losses. The results imply that diversity in past studies of the simulatedresponse toArcticsea-icelosscanbepartlyexplainedbythedifferentspatialpatternsofsea-iceloss imposed.

Screen J. A., C. Deser, and I. Simmonds, 2012: Local and remote controls on observed Arctic warming. Geophys. Res. Lett. 39, https://doi.org/10.1029/2012GL051598.

10.1029/2012GL051598

0d2bcba75b9feef142830ce668ae7653

http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012GL051598%2Fpdf

http://onlinelibrary.wiley.com/doi/10.1029/2012GL051598/pdfThe Arctic is warming two to four times faster than the global average. Debate continues on the relative roles of local factors, such as sea ice reductions, versus remote factors in driving, or amplifying, Arctic warming. This study examines the vertical profile and seasonality of observed tropospheric warming, and addresses its causes using atmospheric general circulation model simulations. The simulations enable the isolation and quantification of the role of three controlling factors of Arctic warming: 1) observed Arctic sea ice concentration (SIC) and sea surface temperature (SST) changes; 2) observed remote SST changes; and 3) direct radiative forcing (DRF) due to observed changes in greenhouse gases, ozone, aerosols, and solar output. Local SIC and SST changes explain a large portion of the observed Arctic near-surface warming, whereas remote SST changes explain the majority of observed warming aloft. DRF has primarily contributed to Arctic tropospheric warming in summer.

Sjoberg J. P., T. Birner, 2012: Transient tropospheric forcing of sudden stratospheric warmings.J. Atmos. Sci.,69,3420-3432, https://doi.org/10.1175/JAS-D-11-0195.1.

10.1175/JAS-D-11-0195.1

7f1bcf10e6d9a72df676addb849fd891

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JAtS...69.3420S

http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-11-0195.1The amplitude of upward-propagating tropospherically forced planetary waves is known to be of first-order importance in producing sudden stratospheric warmings (SSWs). This forcing amplitude is observed to undergo strong temporal fluctuations. Characteristics of the resulting transient forcing leading to SSWs are studied in reanalysis data and in highly truncated simple models of stratospheric wave搈ean flow interaction. It is found in both the reanalysis data and the simple models that SSWs are preferentially generated by transient forcing of sufficiently long time scales (on the order of 1 week or longer). The time scale of the transient forcing is found to play a stronger role in producing SSWs than the strength of the forcing. In the simple models it is possible to fix the amplitude of the tropospheric forcing but to vary the time scale of the forcing. The resulting frequency of occurrence of SSWs shows dramatic reductions for decreasing forcing time scales.

Smith K. L., P. J. Kushner, and J. Cohen, 2011: The role of linear interference in northern annular mode variability associated with Eurasian snow cover extent.J. Climate,24,6185-6202, https://doi.org/10.1175/JCLI-D-11-00055.1.

10.1175/JCLI-D-11-00055.1

3d829219cb8c882ab29fd95a1067fd4e

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JCli...24.6185S

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-11-00055.1ABSTRACT One of the outstanding questions regarding the observed relationship between October Eurasian snow cover anomalies and the boreal winter northern annular mode (NAM) is what causes the multiple-week lag between positive Eurasian snow cover anomalies in October and the associated peak in Rossby wave activity flux from the troposphere to the stratosphere inDecember. This study explores the following hypothesis about this lag: in order to achieve amplification of the wave activity, the vertically propagating Rossby wave train associated with the snow cover anomaly must reinforce the climatological stationary wave, which corresponds to constructive linear interference between the anomalous wave and the climatological wave. It is shown that the lag in peak wave activity flux arises because the Rossby wave train associated with the snow cover is in quadrature or out of phasewith the climatological stationary wave fromOctober tomid-November. Beginning inmid-November the associated wave anomaly migrates into a position that is in phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes until mid-January. Climate models from the Coupled Model Intercomparison Project 3 (CMIP3) do not capture this behavior. This linear interference effect is not only associated with stratospheric variability related to Eurasian snow cover anomalies but is a general feature of Northern Hemisphere troposphere-stratosphere interactions and, in particular, dominated the negative NAM events of the fall-winter of 2009/10.

Sorokina S. A., C. Li, J. J. Wettstein, and N. G. Kvamst, 2016: Observed atmospheric coupling between Barents Sea ice and the warm-arctic cold-Siberian anomaly pattern.J. Climate,29,495-511, https://doi.org/10.1175/JCLI-D-15-0046.1.

10.1175/JCLI-D-15-0046.1

b29ffa8838c909be8dfad5ed9ad398ff

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2016JCli...29..495S

http://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0046.1The decline in Barents Sea ice has been implicated in forcing the “warm-Arctic cold-Siberian” (WACS) anomaly pattern via enhanced turbulent heat flux (THF). This study investigates interannual variability in winter [December–February (DJF)] Barents Sea THF and its relationship to Barents Sea ice and the large-scale atmospheric flow. ERA-Interim and observational data from 1979/80 to 2011/12 are used. The leading pattern (EOF1: 33%) of winter Barents Sea THF variability is relatively weakly correlated (r = 0.30) with Barents Sea ice and appears to be driven primarily by atmospheric variability. The sea ice–related THF variability manifests itself as EOF2 (20%, r = 0.60). THF EOF2 is robust over the entire winter season, but its link to the WACS pattern is not. However, the WACS pattern emerges consistently as the second EOF (20%) of Eurasian surface air temperature (SAT) variability in all winter months. When Eurasia is cold, there are indeed weak reductions in Barents Sea ice, but the associated THF anomalies are on average negative, which is inconsistent with the proposed direct atmospheric response to sea ice variability. Lead–lag correlation analyses on shorter time scales support this conclusion and indicate that atmospheric variability plays an important role in driving observed variability in Barents Sea THF and ice cover, as well as the WACS pattern.

Sun L. T., C. Deser, and R. A. Tomas, 2015: Mechanisms of stratospheric and tropospheric circulation response to projected arctic sea ice loss.J. Climate,28,7824-7845, org/10.1175/JCLI-D-15-0169.1.https://doi.

10.1175/JCLI-D-15-0169.1

ba274dff919f751130d49ac33e13a325

http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.7824S

http://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0169.1The impact of projected Arctic sea ice loss on the atmospheric circulation is investigated using the Whole Atmosphere Community Climate Model (WACCM), a model with a well-resolved stratosphere. Two 160-year simulations are conducted: one with surface boundary conditions fixed at late 20th century values, and the other with identical conditions except for Arctic sea ice which is prescribed at late 21st century values. Their difference isolates the impact of future Arctic sea ice loss upon the atmosphere. The tropospheric circulation response to the imposed ice loss resembles the negative phase of the Northern Annular Mode, with largest amplitude in winter, while the less well-known stratospheric response transitions from a slight weakening of the polar vortex in winter to a strengthening of the vortex in spring. The lack of a significant winter stratospheric circulation response is shown to be a consequence of largely cancelling effects from sea ice loss in the Atlantic and Pacific sectors, which drive opposite-signed changes in upward wave propagation from the troposphere to the stratosphere. Identical experiments conducted with Community Atmosphere Model Version 4, WACCM's low-top counterpart, show a weaker tropospheric response and a different stratospheric response compared to WACCM. An additional WACCM experiment in which the imposed ice loss is limited to August through November reveals that autumn ice loss weakens the stratospheric polar vortex in January, followed by a small but significant tropospheric response in late winter and early spring that resembles the negative phase of the North Atlantic Oscillation, with attendant surface climate impacts.

Sun L., J. Perlwitz, and M. Hoerling, 2016: What caused the recent "warm arctic,cold continents" trend pattern in winter temperatures? Geophys. Res. Lett.,43,5345-5352,org/10.1002/2016GL069024.https://doi.

Vihma T., 2014: Effects of arctic sea ice decline on weather and climate: A review.Surveys in Geophysics,35,1175-1214, https://doi.org/10.1007/s10712-014-9284-0.

10.1007/s10712-014-9284-0

77da958a10209d7918a429ac5f8df01c

http%3A%2F%2Flink.springer.com%2F10.1007%2Fs10712-014-9284-0

http://link.springer.com/10.1007/s10712-014-9284-0The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air temperature, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.

Walsh J. E., 2014: Intensified warming of the arctic: Causes and impacts on middle latitudes.Global and Planetary Change,117,52-63, 2014. 03. 003.https://doi.org/10.1016/j.gloplacha.

10.1016/j.gloplacha.2014.03.003

0492af2361aef1eb6ba44877938ed087

http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0921818114000575

http://linkinghub.elsevier.com/retrieve/pii/S0921818114000575Over the past half century, the Arctic has warmed at about twice the global rate. The reduction of sea ice and snow cover has contributed to the high-latitude warming, as the maximum of the amplification during autumn is a fingerprint of the ice-albedo feedback. There is evidence that atmospheric water vapor, a greenhouse gas, has increased in the Arctic over the past several decades. Ocean heat fluxes into the Arctic from the North Atlantic and North Pacific have also contributed to the Arctic warming through a reduction of sea ice. Observational and modeling studies suggest that reduced sea ice cover and a warmer Arctic in autumn may affect the middle latitudes by weakening the west-to-east wind speeds in the upper atmosphere, by increasing the frequency of wintertime blocking events that in turn lead to persistence or slower propagation of anomalous temperatures in middle latitudes, and by increasing continental snow cover that can in turn influence the atmospheric circulation. While these effects on middle latitudes have been suggested by some analyses, natural variability has thus far precluded a conclusive demonstration of an impact of the Arctic on mid-latitude weather and climate.