Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns

OVERVIEW

The Arctic has been undergoing significant changes in recent years. Surface temperatures in the region are rising twice as fast as the global mean. The extent and thickness of sea ice is rapidly declining. Such changes may have an impact on atmospheric conditions outside the region. Several hypotheses for how Arctic warming may be influencing mid-latitude weather patterns have been proposed recently. For example, Arctic amplified warming could lead to a weakened jet stream resulting in more persistent weather patterns in the mid-latitudes. Or Arctic sea ice loss could lead to an increase of snow on high-latitude land; snow expands on land in autumn, which in turn impacts the jet stream resulting in cold Eurasian and North American winters. These and other potential connections between a warming Arctic and mid-latitude weather are the subject of active research.

The National Research Council convened a workshop on September 12-13, 2013, to review our current understanding and to discuss research needed to better understand proposed linkages. The workshop participants were encouraged to take a global perspective and consider the influence of the Arctic in the context of forcing from other components of the climate system, such as changes in the tropics, ocean circulation, and mid-latitude sea surface temperature.

Many workshop participants noted that research on these linkages is still in its infancy, making it difficult to draw conclusions regarding their existence or their mechanisms. Workshop presentations highlighted the complex mechanisms and feedbacks that link the tropics, poles, and mid-latitudes while also noting that they can be bidirectional and not necessarily direct (e.g., the stratosphere and oceans may act as intermediaries). Understanding how Arctic warming could impact these linkages or other large-scale circulation patterns is critical, but the large natural variability of the atmosphere and Arctic sea ice makes attribution difficult.

Furthermore, many of the proposed linkages are composed of multistep hypotheses, and our understanding of the various steps is complicated by inconsistent evidence from observations and modeling studies. Some participants said that the time series for observations is too short to detect statistically significant trends. It was noted by some participants that trends could be clarified through the quantification of uncertainty in data products. Atmospheric reanalyses and efforts to extend Arctic sea ice records back in time might also be useful for detecting trends. Under-utilized observations could also be harnessed from often overlooked sources, such as the National Ice Center (NIC) operational products.

There is also no standard for objective measurement of important mechanisms such as wave amplitude or atmospheric blocks, which lead to a stagnation of weather patterns. A better understanding of the range of blocking systems and how they are impacted by



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Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns OVERVIEW The Arctic has been undergoing significant changes in recent years. Surface temperatures in the region are rising twice as fast as the global mean. The extent and thickness of sea ice is rapidly declining. Such changes may have an impact on atmospheric conditions outside the region. Several hypotheses for how Arctic warming may be influencing mid-latitude weather patterns have been proposed recently. For example, Arctic amplified warming could lead to a weakened jet stream resulting in more persistent weather patterns in the mid-latitudes. Or Arctic sea ice loss could lead to an increase of snow on high-latitude land; snow expands on land in autumn, which in turn impacts the jet stream resulting in cold Eurasian and North American winters. These and other potential connections between a warming Arctic and mid-latitude weather are the subject of active research. The National Research Council convened a workshop on September 12-13, 2013, to review our current understanding and to discuss research needed to better understand proposed linkages. The workshop participants were encouraged to take a global perspective and consider the influence of the Arctic in the context of forcing from other components of the climate system, such as changes in the tropics, ocean circulation, and mid-latitude sea surface temperature. Many workshop participants noted that research on these linkages is still in its infancy, making it difficult to draw conclusions regarding their existence or their mechanisms. Workshop presentations highlighted the complex mechanisms and feedbacks that link the tropics, poles, and mid-latitudes while also noting that they can be bidirectional and not necessarily direct (e.g., the stratosphere and oceans may act as intermediaries). Understanding how Arctic warming could impact these linkages or other large-scale circulation patterns is critical, but the large natural variability of the atmosphere and Arctic sea ice makes attribution difficult. Furthermore, many of the proposed linkages are composed of multistep hypotheses, and our understanding of the various steps is complicated by inconsistent evidence from observations and modeling studies. Some participants said that the time series for observations is too short to detect statistically significant trends. It was noted by some participants that trends could be clarified through the quantification of uncertainty in data products. Atmospheric reanalyses and efforts to extend Arctic sea ice records back in time might also be useful for detecting trends. Under-utilized observations could also be harnessed from often overlooked sources, such as the National Ice Center (NIC) operational products. There is also no standard for objective measurement of important mechanisms such as wave amplitude or atmospheric blocks, which lead to a stagnation of weather patterns. A better understanding of the range of blocking systems and how they are impacted by 1

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2 Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns various components of the climate system is needed. Participants suggested deploying process-based studies on the “weak links” of the specific hypotheses as a way forward. Results from modeling studies offer a lot of divergence and are difficult to compare systematically because different boundary and/or initial conditions are often used. To address this challenge, many participants would like the modeling community to organize careful model intercomparisons and conduct model sensitivity studies with identical initial and/or boundary conditions. Moreover, given that sea ice loss is only one driver of Arctic amplification, many participants said attribution studies are needed to understand the proportion of Arctic amplification that is due to processes outside of the Arctic. Although models are useful for providing clues on the mechanisms of potential Arctic linkages, as well as detection and attribution, some participants acknowledged that there are important biases and limitations that must be considered. Many participants said that combining observations with a hierarchy of models is key to making progress on this issue. Furthermore, attention so far has focused on changes in the meridional temperature gradient, but future studies might also consider other mechanisms of Arctic linkages, such as zonal temperature gradients, static stability, and moisture changes. The climate and weather communities could collaborate to apply the sophisticated methods and diagnostics developed in the meteorological community to better understand linkages. Some participants also thought that the possibility of regional impacts of Arctic warming is an important consideration. Some workshop participants noted that a large-scale research program dedicated to understanding the mechanisms that link a warming Arctic and the heavily-populated mid- latitudes is needed, particularly because it could lead to improved seasonal forecasts. Several participants pointed to the analogous example of an improved understanding of El Niño Southern Oscillation (ENSO) resulting in improved seasonal forecast skill. Participants expressed optimism that significant progress could be obtained because of several recent achievements and events: (a) improvements in atmospheric, oceanic, and sea ice models; (b) longer observational records of sea ice and atmospheric circulation; (c) emergence of a reasonably strong sea ice forcing in recent years; and (d) strong interest in the topic from policymakers, scientists, and the public. INTRODUCTION On September 12-13, 2013 the Board on Atmospheric Sciences and Climate and the Polar Research Board held a public workshop that brought together a diverse array of experts to examine linkages between a warming Arctic and mid-latitude weather patterns. The workshop included presentations from leading researchers representing a range of views on this topic. This workshop was planned and organized by an ad hoc committee of the National Research Council1. The committee was tasked to plan a workshop to address the following questions: 1. What do we currently understand about the mechanisms that link declines in Arctic sea ice cover, loss of high-latitude snow cover, changes in Arctic-region                                                              1 This report has been prepared by the workshop rapporteur as a summary of what occurred at the workshop. The planning committee’s role was limited to planning and convening the workshop. The views contained in the report are an interpretation of those presented by individual workshop participants and do not necessarily represent the views of all workshop participants, the planning committee, or the National Research Council.

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Arctic Warming and Extreme Events in the Mid-Latitudes: a Possible Link?    3  energy fluxes, atmospheric circulation patterns, and the occurrence of extreme weather events? 2. What may be the possible implications of more severe loss (and eventually, total loss) of summer Arctic sea ice upon weather patterns at lower latitudes? 3. What are the major gaps in our understanding, and what sort of observational and/or modeling efforts are needed to fill those gaps? 4. What are the current opportunities and limitations for using Arctic sea ice predictions to assess the risk of temperature/precipitation anomalies and extreme weather events over northern continents? How might these capabilities improve over time? The workshop discussions focused on questions 1 and 3. As the research matures, it may become possible to fully address questions 2 and 4 in the future. Committee members planned the workshop structure, identified speakers and attendees, and developed background materials for attendees. They also led the workshop and served as session facilitators. Furthermore, during the workshop planning, they decided that the workshop should expand to include links to Arctic amplification as well, because Arctic amplification is not exclusively driven by sea ice loss. All workshop presentations are available online at http://dels.nas.edu/global/basc/al- presentations. This summary was written by a rapporteur to present the various ideas and suggestions that arose in the workshop discussion and to synthesize the main discussion items. It does not include any conclusions or recommendations, nor does it cover the full spectrum of issues around this topic. For further information, relevant references that were suggested by workshop participants and planning committee members can be found at http://dels.nas.edu/resources/static-assets/basc/miscellaneous/basc-arctic-linkages- workshopreferences.pdf. Report Roadmap The structure of this report largely follows the structure of the workshop sessions and is divided into three general topics: the big picture context, observations, and modeling and theoretical work. Some speakers touched on all three topics in their remarks so this summary does not step through each presentation in detail, but rather gathers related points thematically. Brief abstracts of each presentation are in Appendix A. The final section lists research needs and key messages that were identified by individuals in breakout groups. ARCTIC WARMING AND EXTREME EVENTS IN THE MID-LATITUDES: A POSSIBLE LINK? Rising global average temperatures, and especially intense warming in the northern polar regions, are leading to a rapid loss of the sea ice cap that covers the Arctic Ocean. The seven summers with the lowest sea ice minimums all occurred during the past seven years. Workshop presenter James Screen, University of Exeter, noted that Arctic sea ice loss in summer 2012 broke the previous record low set in 2007, with ice cover half as extensive as it was only 30 years ago. Although summer sea ice extent in 2013 did not set a new record low, it still was consistent with a long-term trend of rapid decline. The observed 2012 record low in summer Arctic sea ice extent is extremely unlikely to have occurred due to internal climate variability according to Rong Zhang, National Oceanic and Atmospheric Administration (NOAA). In addition to sea ice, spring snow cover has also decreased in the Arctic. As discussed by workshop presenter James Overland, NOAA, many areas of the Arctic experienced almost a 40 percent decrease in snow cover in June 2012 compared to the average from 1971-2000 (Figure 1). David Robinson, Rutgers University, noted that annual snow cover extent over

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4 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 1 Top: Arctic sea ice extent for the years 2007- T t -2013 compare to the 1981-2 ed 2010 average. Bottom: June sn now cover anom malies in June 2012 in the Nor 2 rthern Hemisphhere compared to the long-term 1971-2000 a m average based o the on number of days in the yea when a location was snow covered. Shade of brown ind icate places tha experienced up to ar c es at 40 percent fewer snow-coovered days tha average in Ju 2012. Blue indicates areas that experienc up to 40 pe an une s ced ercent more snoww-covered days than average. SOURCE: Clima S ate.gov, http://w www.climate.goov/news-features/featured- images/reco ord-low-spring--snow-cover-no orthern-hemisphere-2012.

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Arctic Warming and Extreme Events in the Mid-Latitudes: a Possible Link? 5 Northern Hemisphere lands has averaged lower since the late 1980s than earlier in the satellite era which began in the late 1960s. The loss of Arctic sea ice has occurred at rates much faster than climate models have recently projected, even for the worst-case forcing scenarios. The full disappearance of late- summer Arctic sea ice is possible in the coming decades (Massonnet et al., 2012; Stroeve et al., 2012). Many people may consider this stark indicator of climate change to be a distant concern that does not affect our “real lives.” But such trends do indeed matter greatly to society, because the Arctic may play an integral role in the planetary system of climate and weather well beyond the Arctic region. Arctic Amplification Although the workshop topic originally focused on the impact of Arctic sea ice loss, because many workshop participants stressed that sea ice loss is only part of the issue, the discussion was broadened to include the impact of Arctic amplification. This term is commonly defined as amplified warming near the Arctic compared to the rest of the hemisphere or globe in response to a change in global climate forcing (e.g., the concentration of greenhouse gases [GHGs] or solar output). For example, as the sea ice melts because of warmer temperatures in the Arctic, the resulting darker ocean waters are exposed to incoming solar radiation. The darker ocean absorbs significantly more energy than the white, reflective sea ice. The result is an increase in water and air temperatures, which melts even more sea ice. Overland noted that this heat does not stay in the ocean; it eventually is re-emitted to the atmosphere. The result is a smaller difference in temperature between the frozen north and the more temperate mid-latitudes (an important concept for several of the proposed Arctic linkages). The temperature change associated with Arctic amplification occurs not only near the surface, but also through a considerable depth of the troposphere. Some participants noted that there are several other drivers of Arctic amplification and that the impact of albedo on Arctic amplification may not be as significant as previously thought (Pithan and Mauritsen, 2014; Winton, 2006). Martin Hoerling, NOAA, suggested that the estimated observed 1000-500hPa (hectopascal) Arctic warming could be largely due to natural decadal sea surface temperature (SST) forcing. See also Feldstein’s presentation on the role of the tropics in Arctic Amplification (Appendix A). Extreme Weather Events This decade has seen a series of record-breaking extreme weather events taking place around the world (Figure 2). Screen highlighted some severe weather events (e.g., heat waves, drought, flooding) in the mid-latitudes, some of which have been high profile and high impact (e.g., Hurricane Sandy, Russian heat wave). Such events can cause human suffering and have a significant economic impact. In 2011, 14 events in the United States caused losses in excess of US$1 billion each (WMO, 2011). Proposed Arctic Linkages The recent extreme weather events and the recent significant decline in Arctic sea ice have spurred an increasing interest in the relationship between a warming Arctic and large-scale climate dynamics, which may have tremendous implications for society. Of particular concern is emerging research that may indicate that Arctic warming can have dramatic impacts upon weather patterns across the heavily populated northern mid-latitudes, and

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6 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 2 World map sho W owing the recor rd-breaking extr reme weather e events in the pa decade. The numbers refer to the ast e r year in the twenty-first cen ntury. Blue sym mbols represent rainfall; red sym mbols represen heat-waves and droughts; ye nt ellow symbols rep present hurricanes and cyclon and the gre symbol repr nes; een resents a tornad outbreak. SO do OURCE: Coumo ou and Rahms storf, 2012. that such impacts co ould increase as ice cover c continues to r retreat and the Arctic contin e nues to warm in the coming decades (s Box 1). see THE ROLE OF ARCTIC WA T ARMING IN TH CONTEXT HE T OF OTHER FORCCING FACTO ORS The climate system reflects a com mplex combin ation of many interconnected physical a y and (often chaotic) dyna amical process operating at a variety of timescales in the atmosph ses f n here, ocean, and land. A significant po , ortion of the w workshop was dedicated to understanding the g role of Arctic warmi compared to other forc ing components of the clim f ing d mate system (ee.g., variatio in the trop and strato ons pics osphere) in inf ather in the m fluencing wea mid-latitudes. Speake discussed the various co ers t onnections an linkages wi nd ithin the clima system and ate d considdered whether a warming Arctic now has a stronger in r A s nfluence on at tmospheric circula ation patterns (e.g., polar vo ortex, jet strea modes of variability) an the progres am, nd ssion of syste (e.g., bloc ems cking). Large-scal Atmospheri Circulation Patterns: le ic n What Drives Mid-L t Latitude Weatther? Workshop presenter Elizabeth Ba r arnes, Colorad State Unive do ersity, provide an overview of ed w atmosppheric dynamics of jet streaams, mid-latitu waves, an blocking. S stressed th ude nd She hat mid-latitude atmosp pheric variability is compos of dynamical interactions between sed circula ations of a ran of spatial and temporal scales. nge a

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The Role of Arctic Warming in the Context of Other Forcing Factors 7 BOX 1 Examples of Proposed Arctic Linkages Below is a list of the proposed linkages that were discussed during the workshop. Further details about some of these linkages can be found later in the report. Increased Arctic warming  weakened temperature gradient  weakened, more meandering jet streama  more persistent weather patterns in the mid-latitude (Francis and Vavrus, 2012) Arctic sea ice loss  increase in autumn high latitude snow cover  more expansive and strengthened Siberian high pressure in autumn and winter  increase upward propagation of planetary waves  more sudden stratospheric warmings  weakened polar vortexb and weakened, more meandering jet stream (Cohen et al., 2012; Ghatak et al., 2012) Arctic sea ice loss  changes in regional heat and other energy fluxes  unstable polar vortex  cold polar air moves to the mid-latitudes (Overland and Wang, 2010) Arctic sea ice loss  amore meandering jet stream and winter atmospheric circulation patterns similar to a negative phase of the winter Arctic Oscillation  frequent episodes of atmospheric “blocking” patterns (Liu et al., 2012). Arctic sea ice loss  southward shift of the jet stream position over Europe in summer  increased frequency of cloudy, cool, and wet summers over northwest Europe (Screen, 2013) Arctic sea ice loss  winter atmospheric circulation response resembling the negative phase of the Arctic Oscillation  rainfall extremes in the Mediterranean in winter (Grassi et al., 2013) Arctic sea ice loss  negative phase of the tripole wind pattern  enhanced winter precipitation and declining winter temperature in East Asia (Wu et al., 2013) a The “typical” jet stream meanders north and south, however it can weaken and slow, allowing the meanders to become larger. b The polar vortex is a large-scale region of air that is contained by a strong west-to-east jet stream that circles the polar region. The polar vortex extends from the tropopause (the dividing line between the stratosphere and troposphere) through the stratosphere and into the mesosphere (above 50 km). Cold temperatures are associated with the air inside the vortex. Definition from http://ozonewatch.gsfc.nasa.gov/facts/vortex_NH.html Barnes focused her presentation on the polar jet stream2 in the Northern Hemisphere, which flows over the middle to northern latitudes (50-60°) of North America, Europe, and Asia and their intervening oceans. The daily jet location pattern of the north Atlantic jet (Figure 3) has three distinct peaks, but it varies significantly over decadal time scales. Jet streams and other north Atlantic and north Pacific circulations and variability behave differently depending on the season. Barnes noted that jet streams and storm tracks are strongest over the oceans during winter and shift further north in the summer. Jet and storm track strength and location are driven by changes in: • wave propagation and momentum fluxes (e.g. Chen and Held, 2007; Chen et al., 2008; Kidston and Gerber, 2010); • meridional temperature gradients (e.g. Haarsma et al., 2013a; Harvey et al., 2013); • static stability (e.g. Frierson et al., 2007; Lu et al., 2010); • ocean influences on the meridional overturning circulation (e.g. Chen and Held, 2007; Chen et al., 2008; Woollings et al., 2012); • eddy phase speed increases (from increasing upper-troposphere/lower stratosphere winds [e.g. Chen and Held, 2007; Chen et al., 2008]);                                                              2Jet streams are fast flowing, narrow air currents in the near tropopause. The major jet streams flow west to east and their paths typically have a wavy shape.

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8 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 3 Jet latitude inde 3 PDF (probability distributio function) for NCEP/NCAR ( J ex on r (National Cente for Environm ers mental Prediction/ /National Cente for Atmosphe Research) reanalysis, wher the black lin represents the climatologica er eric re ne e al distribution blue line, Gre n; eenland blockin days;4 green line, European blocking days and orange line, Iberian wa ng n n s; ave blocking da The dotted line represents jet latitude ind when no b locking in the t ays. d s dex three sectors is detected. The b black vertical line shows the lati e itude of the cen ntral peak. SOU URCE: (Davini e al., 2013) et • rising tropopause5 (e.g. Lorenz and De L eWeaver, 200 and 07); • changes in wind speed over the region (e.g. Haarsm et al., 2013 Mizuta, 20 o n ma 3b; 012). Modes of Atmosphe Variability s eric y Modes of variability were discuss at the wor s y sed rkshop in the ccontext of link kages (see Box 2 for add ditional details). The dominnant mode of a atmospheric v variability in t Northern the Hemisphere extratro opics is the Noorthern Annu lar Mode/Arct Oscillation (NAM/AO), tic n which can influence surface temp e perature and pprecipitation, especially the frequency o e of extrem events. The may be po me ere otential for sea asonal predict tability of the AO (Riddle et al., t 2013), but it has bee found to be shorter in pr en e ractice (7-10 ddays). It is not clear why th t here seems to be skill in predicting the wintertime A e AO. The manifestation of the AO in th Atlantic sec is commo he ctor only referred t as the North to h Atlantic Oscillation (NAO). The NAO is also so N ometimes refe erred to as a “ “Greenland Block.” The NA is the larg contributing pattern to European inte AO gest erannual varia ability and pla a ays signific cant role in pr redictions of European wint climate. The predictability of the NAO is E ter limited to seasonal timescales (NRC, 2010). d t While not extensive discussed at this worksh ely a hop, participan identified ENSO as an nts examp of a mode of variability in the tropics that can influ ple uence weather in the mid- latitudes (i.e., teleco onnection). Pa articipants not that seasonal forecasts i ted improved as oour unders standing and predictability of ENSO grew a greater un p w; nderstanding of Arctic influen nces on mid-la atitude weather might allow seasonal (3 months) clim w mate forecasts t be to further improved. r                                                              3 “Jetla atitude index” is defined as the daily latitude of maximum lo s e ‐averaged 925– ow‐level (mass‐ –700 hPa) zoonal winds zona averaged over the Atlantic sector (0°–60°W, 15°–75°N; W ally o c Woollings et al l., 2010). 4 A Greeenland Block is a very strong area of high pr ressure located over Greenlan nd. 5 The boundary in the atmosphere be b etween the trop posphere and th stratosphere. he .

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The Role of Arctic Warming in the Context of Other Forcing Factors 9 BOX 2 Characteristics of NAM/AO, NAO, and ENSO NAM/AO is a measure of the surface pressure gradient between the polar and subpolar regions of the Northern Hemisphere (Thompson and Wallace, 2000). When the AO index is positive, surface pressure is low in the Arctic region, which is associated with a strong North Atlantic jet (and a strengthening of the polar vortex), thus keeping cold Arctic air in the polar region. When the AO index is negative, the Arctic tends to have high pressure, weaker zonal winds, resulting in a greater movement of polar air into middle latitudes (Overland et al., 2011). The AO was strongly positive in the early 1990's compared to the previous 40 years; however, the AO has been low and variable for the past 9 years. The NAO is characterized by changes in the North Atlantic jet stream and storm track location and intensity as well as zonal and meridional heat and moisture transport (Hurrell, 1995). The positive phase of the NAO is associated with strengthened westerlies due to an increase in the pressure gradient over the North Atlantic. Such strong westerlies allow cold air to drain off the North American continent allowing the air to flow northeast across the Atlantic into Europe. The weather for both North America and Europe is mild compared to the weather during a negative NAO. The negative phase of the NAO is characterized by a decrease in the pressure gradient across the North Atlantic, which results in a weakening of the westerlies allowing cold air to build over Canada and the eastern parts of the United States. NAO phases vary from year to year, but they can also remain in one phase for intervals lasting several years. ENSO refers to a set of weather patterns associated with variations in sea surface temperatures (SSTs) of the tropical eastern Pacific Ocean and in the sea level pressure in the tropical western Pacific. During an El Niño phase, the tropical easterly winds decrease, warm SSTs cover the central and eastern tropical Pacific, and the heaviest rainfall moves east. El Niño is also characterized by sea-level pressure that is lower in the eastern Pacific and higher in the western Pacific. The opposite pattern occurs during a La Niña phase. In North America, El Niño winters tend to be warmer and drier than average in the northern parts of the United States, whereas northern Mexico and the southern United States typically experience wetter and cooler winters On the other hand, La Niña causes above-average precipitation across the northern United States, with precipitation in the southwestern and southeastern states typically below average. El Niño recurs at irregular intervals ranging from 2 years to a decade. Barnes noted that the Atlantic jet stream variability is affected by the NAO and the east Atlantic pattern,6 which are important to study in the context of Arctic linkages because they are close to the Arctic (and thus more likely to be influenced). Rossby waves, or large meanders within the jet stream, typically travel eastward. Rossby wave breaking leads to jet stream variations, where cyclonic breaking dominates poleward of the jet and tends to drive the jet southward and anticylonic breaking dominates equatorward of the jet and tends to drive the jet northward (Benedict et al., 2004; Strong and Magnusdottir, 2008). High-latitude wave-breaking is correlated with NAO on decadal timescales. Rossby wave breaking also serve as the mechanism for the reversal of the meridional gradient of geopotential height fields that is typical of blocks. Atlantic blocking patterns are a critical component of NAO variability (Woollings et al., 2008). In general, a block is characterized by an atmospheric phenomenon in which a large, quasi-stationary anticyclone develops in the mid-latitudes and persists for several days or longer, blocking the ambient westerly winds and weather systems (Berrisford et al., 2007; Woollings et al., 2008). Currently, there is no consensus on exactly what type of system should be classified as a block. Blocks lead to a stagnation of weather systems and are associated with extreme weather events in the mid-latitudes (e.g. cold snaps, heat waves). They can persist for days to weeks (e.g. Black et al., 2004; Dole et al., 2011).                                                              6 The (EA) pattern is a prominent mode of low-frequency variability over the North Atlantic, appearing in all seasons except for summer. The pattern is structurally similar to the NAO. The anomaly centers of the EA pattern are displaced southeastward to the approximate nodal lines of the NAO pattern. Definition from: http://www.cpc.ncep.noaa.gov/data/teledoc/ea.shtml

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10 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 4 Atmospheric variables listed in order of as v scending diffic ulty in detectio of change. Low values im on mply a response th is easier to detect than high values. SOUR hat d h RCE: Adapted frrom Screen et a 2013. al., Barnes said that there are three main blocking centers in the Northern He s m e emisphere: the e Atlantic Ocean, Europe, and eastern Asia, and most occur in winter seaso because of n on f greater wave-breaking activity in winter. r Barnes stressed that jet stream shi Rossby w ave propagati and wave breaking, and s ifts, ion e d upled togethe r. Mid-latitude weather var blocking events are all tightly cou e riability is also o dominated by interaactions of the large-scale flo with synop ow ptic-scale edd dies. The latter help balanc the global energy budget by “stirring” the atmosphe and movin cold air tow ce e t ere ng ward the equ uator and war air toward the poles, wh rm hich in turn, reduces the eqquator-to-pole e temperature contrasst. Challenges in Detecting and Attributi ng Changes in Mid-latitude Weather s g e Variabbility of the climate system may be due to non-linear d m o dynamical proocesses intrins to sic the atm mosphere (inte ernal variability; as discusse above), or to variations in natural or ed anthropogenic exter rnal forcing (e external variabbility; e.g. GH emissions, volcanoes). HG Barnes noted that th internal var s he riability of the atmosphere is high, which complicates e h s attribution of Arctic influences on the mid-latit n tudes. Screen et al. (2013) s suggest that presennt-day trends in mid-latitude weather driv by Arctic change are m likely masked e ven most by inte ernal variabilit and that the detection of extreme even is even mo difficult (F ty e f nts ore Figure 4). Som participant said that the patterns asso me ts e ociated with a warming Arctic do not match well with any know natural variability modes (e.g., NAO, A w wn s AO), which suupports that id dea that the recent trend are largely anthropogenic ds a c. Barnes noted that mid-latitude jet position has a large amou of internal variability, on s m t unt n varying timescales (i.e., daily, yea g arly, decadal) . Furthermore blocking fre e, equency is hig ghly variable. She sugges sted that, give the large in ternal variability of the jet stream and en

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The Role of Arctic Warming in the Context of Other Forcing Factors 11 blocking, any potential effect of Arctic warming on mid-latitude weather is unlikely to be detectable with current observations. Furthermore, Overland noted that the time series is too short (i.e., significant loss of Arctic sea ice has only been occurring for about the past 6- 7 years) to robustly differentiate Arctic forcing of mid-latitude extremes from random events. Overland noted that mid-latitude attribution remains difficult and controversial because interactions between Arctic forcing and chaotic mid-latitude flow are complex. One would not expect events to happen the same way every year even with similar Arctic forcing. Ultimately, participants noted that the issue of Arctic warming impacting regions outside the Arctic is much larger than just sea ice retreat and includes processes encapsulated under the larger umbrella of Arctic amplification. In addition, participants stressed that it is critical to assess both natural and anthropogenic mechanisms leading to sea ice loss. It is unknown to what extent rapid sea ice loss, particularly after 2005, is a part of multi-decadal oscillation or a result of Arctic warming. Examples of Connections in the Global Climate System Several workshop speakers presented examples of the connections and feedbacks linking the tropics, mid-latitudes, and the poles, which highlight the interconnectedness and complexity of the global climate system. Polar Stratosphere Influence on the Mid-Latitude Troposphere In his remarks, workshop speaker Paul Kushner, University of Toronto, discussed the coupling between the polar stratosphere and the extratropical troposphere. Stratospheric circulation variability occurs on longer time scales than tropospheric variability and can be characterized by the strength of the polar vortex, which peaks during winter in the Northern hemisphere and late spring in the Southern Hemisphere (Baldwin and Dunkerton, 2001). Kushner said that it is well known that extratropical stratospheric variability is controlled by atmospheric composition (e.g., ozone and GHGs) as well as planetary waves (i.e. Rossby waves) propagating upward from the troposphere (Shindell et al., 1999; Sigmond et al., 2008). However, there is also “stratospheric influence”, that is, variability of the stratosphere impacting tropospheric circulation on intraseasonal timescales. Kushner discussed the pathway for this coupling, which is known as the “NAM/wave mean flow interaction pathway.” Essentially, a strengthened polar vortex leads to positive AO/NAM with a 2- to 4-week lag due to wave mean flow interactions. This interaction has been well studied (i.e., Baldwin and Dunkerton, 2001; Polvani and Waugh, 2004). Kushner also discussed a second pathway for stratosphere-troposhere coupling: the “wave reflection pathway.” The polar vortex can create a reflective surface for upward propagating planetary waves (e.g., Rossby waves). The reflective signal in the troposphere shows up as regional circulation anomalies such as the NAO. This connection has been less well studied, but it continues to be an active area of research (e.g., Perlwitz and Harnik, 2004; Shaw and Perlwitz, 2013). Kushner said that it is important to examine whether the stratosphere serves as a bridge that connects Arctic sea ice loss to changes in the mid-latitude temperature, storm track, and circulation. As workshop presenter Steve Feldstein, Pennsylvania State University, noted, given the large area (i.e., large spatial scales and long timescales) influenced by Arctic sea ice anomalies, it is likely that the stratosphere plays a role.

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28 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 22 Increased geopotential thickn 2 ness of the lowe troposphere due to sea ice loss. (a) 2010C zonal mean D er C DJF temperature response (K). (b) 2010C-CTL response of th DJF atmosph eric geopotenti thickness be L he ial etween 1000 an 500 nd hPa; (c) 209 zonal mea DJF temperat 90C an ture response. Contour interva 0.5 K; light (d C al dark) shading in ndicates the 90 percent (95 percent) signif 5 ficance level. (d 2090C-CTL response of the DJF atmosphe ric geopotentia thickness betw d) r al ween 1000 and 500 hPa. SOUR 5 RCE: Peings and Magnusdottir, 2014. d ,

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Theoretical and Modeling Studies al g 29 FIGURE 23 Percentage ch 3 hange in 500hP zonal winds in 4xCO2 run o CCSM4. Blac (summer), gr Pa of ck reen (spring), ye ellow (autumn), blue (winter). SO b OURCE: Jennife Francis. er FIGURE 24 A weak but sta 4 atistically signif ficant increase in wave amplit tude for 2090C Top: Range of the 5400 m is C. soline of height on the 500 hPa surface for one day of Februar in the contro Bottom: Distr s ry ol. ribution of the 5400 m isoplet th daily wave amplitude chaange in winter (DJF). White (co ontrol), light gre (2010C), dar grey (2090C) Red diamond ey rk ). ds show the mean of the distribution, and asterisks indicate the significan level of the change of the m m e nce mean in 2010C and C 2090C, com mpared to the control value. SOURCE: Peing and Magnusd c S gs dottir, 2014.

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30 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 25 RCP8.5 minus historical multimodel mean. Left: Seasonal cycle of the No 5 s orthern Hemisp phere (NH) bloc cking frequency as a function of longitude. Units are days per month. Right: Climatology of NH annual-m a f r f mean blocking frequency. Units are numb of blocked days per year. SOURCE: Dun n-Sigouin and Son (2013). ber Block king Barnes noted that so s ome modeling studies find a decrease in high-latitude blocking eve g ents over th oceans, but an increase over Asia. The decreases a largest ove the oceans i he t e are er in autumn and early winter. A coup of modelin studies find that decrease in blocking w ple ng d es g events are linked to changes in th jet stream ( he (i.e. Barnes et al., 2012; de Vries et al., 2 2013). She nooted that similar responses are found usin other block a ng king identifyin schemes (d ng de Vries et al., 2013; Masato et al., 2013). Dunn-S e M 2 Sigouin and S (2013) fou RCP 8.5 Son und ations compar to historical12 integratio integra red ons, showing s significant decreases in blo ocking frequency over both the North Pa h acific and Nor Atlantic re rth egions, with slight increasin ng blocking frequency over western Russia (Figure 25). e ather Patterns a Storm Tra Wea and acks Workshop participa discussed several mode ants eling studies o changing w on weather patter rns and sto tracks. Francis presente output from a CCSM4 R orm ed m RCP8.5 scenar that projec rio cted an increased ridging over contine associated with higher surface air te g ents d emperatures. S She noted that the ridgin over the No Atlantic c t ng orth cools Western Europe. She also presente n ed modeling evidence that projects that light and easterly wind will become more comm at t ds mon the exppense of west winds for the 2090s. This w result in h e will higher surface air temperatures e in the mid-latitudes, which makes extremes mo likely. , ore Tim Woollings, University of Oxf W ford, discussed a set of expe d eriments with one atmosph h heric model (HadGAM2) to investigate sources of sp e pread in CMIP model projections of the P Atlantic storm track response to climate chang This work f c ge. focused on the late 21st century howed that uncertainty in th magnitude of sea ice retreat is a large source of and sh he uncertainty in predicting the response of the sttorm track.                                                              12 Histo orical runs are 20th century cli 2 imate integratio with all obs ons served climate f forcings.

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Theoretical and Modeling Studies al g 31 FIGURE 26 In the Norther Hemisphere (NH), the storm track is weak est during summer (JJA). Proje 6 rn m ected changes f from 1980 to 1999 to 2081 to 2100 in zonal mean pressure-level variance s 2 m statistics by CM MIP5 multimode ensemble based el on RCP8.5 scenario, as a function of latit f tude and pressu (a) DJF, (b) JJA, (c) MAM, (d) SON. Black contours indic ure. k cate model climmatology. Red and blue contou indicate pro urs ojected changes Shadings indi s. icate regions ov which ≥80 ver percent (lig or 100 perc ght) cent (dark) of th models agree on the sign of the change. SO he e f OURCE: Chang et al., 2012. g Barnes preesented a mod study that projects that winter Northe Hemisphe upper-leve del ern ere el storm tracks will weake on equatorw en ward flank by 2100 under RCP8.5; the s y storm track is weakest during summer (JJA) in the Northern Hem N misphere (Figu re 26; Chang et al., 2012). Magnusdo said that current Arctic sea-ice conditions (2010C favor more intense cold ottir c c C) extremes over mid-latitudes (mostly confined to th Asian secto Peings and Magnusdottir o c he or; 2014). Wi stronger se ith ea-ice forcing (2090C), the intensity of co extremes d old decreases everywhere north of 45 because of the extensio of the Arcti c warm anom 5°N o on maly over northern continents. In 2090C (as in 2010C) cold extremes are more intense south of 45°N c N. Despite fa stronger forc ar cing in 2090C compared to 2010C, the intensity of co extremes C o old does not change signific c cantly. Magnu usdottir noted that this imp lies that there is a nonlinea d e ar relationship between seea-ice retreat and mid-latitu temperatu Liu et al. ( a ude ure. (2012) also found that a reduction in Arctic sea ice leads to more frequent c t i m cold weather outbreaks in the northe mid-latitud ern des. dditional modeling studies were briefly presented whic highlight p Several ad w p ch possible regional climate impact when forced with a reduction in Arctic sea ice (Box 3). c ts c x

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32 Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns BOX 3 Highlights of Other Modeling Studies Showing Regional Effects Cold Winters in Northern Europe Yang and Christensen (2012) found that cold early winter weather in northern Europe is associated with 20-40 percent reduction in Barents-Kara sea ice. Petoukhov and Semenov (2010) found that sea ice loss triples the probability of cold winter over large areas of the mid-latitudes including Europe. Wet European Summers There is modeling evidence that sea ice loss may lead to a southward shift of the summer jet stream over Europe, resulting in increased summer precipitation in the United Kingdom (Screen, 2013). Screen noted that the simulated increase is small and closely resembles the spatial pattern of precipitation anomalies in recent summers. East Asian Monsoon Wu et al. (2013) found that composites of high ice minus low ice lead to dry air over western Europe and far-east Asia and increased precipitation over west Asia. Winter Mediterranean Rain Grassi et al. (2013) utilized simulations that project that sea ice reduction will lead to stronger and more frequent extreme cold events over continental Europe and extreme precipitation events over the entire Mediterranean Basin. In particular, simulations suggest an increased risk of winter flooding in southern Italy, Greece, and the Iberian Peninsula. Changes in Energy Transport Due to Arctic Amplification Frierson discussed how poleward energy transport into the Arctic might change in the future (because of global warming). He noted that global circulation models indicate that more Arctic amplification is associated with less heat transport into the Arctic (Hwang et al., 2011). He presented modeling studies that show that energy actually transports toward the mid-latitudes (rather than toward the poles as we might expect) under Arctic amplification. He believes that this anti-correlation is due to the heat transport responding to temperature gradients. Arctic amplification causes weaker temperature gradients which leads to less dry static energy transport into the Arctic. More gradient results in more transport (i.e., transport is diffusive). This “diffusive framework” implies that Arctic warming should spread to lower latitudes (i.e., warmer Arctic leads to warmer mid-latitudes). Models with more Arctic warming have anomalous dry static energy transport southward, back toward the mid-latitudes. This occurs relatively independently of the storm track amplitude and location change within the model. Arctic Sea Ice Predictability Some workshop participants noted that Arctic sea ice predictions could eventually be used to assess the risk of temperature and precipitation anomalies and extreme weather events over northern continents. In her remarks, Holland discussed the potential to predict Arctic sea ice on seasonal timescales. She noted that seasonal sea ice prediction is an initial value problem (i.e., the predictability characteristics are largely dependent on the initial conditions). Potential predictability of sea ice decreases during spring and then is regained

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Theoretical and Modeling Studies 33 during summer and the following winter. Perfect model studies13 consistently show that ocean heat content (temperature) leads to predictability in winter sea ice, in all cases. Summer sea ice predictability is associated with memory that resides in the ice itself—ice thickness. Therefore, there is reduced summer sea ice predictability in thin ice conditions. Model studies suggest limited predictability in sea ice area on 1- to 2-year timescales. Holland said that realizing even a fraction of the potential predictability of sea ice requires an “adequate” observational network and “adequate” (and compatible) models. However, it is currently unclear what “adequate” means. We do not even know the spatial coverage and variables that are required in an “adequate” observational network. What model biases impact predictability, and what model complexity is required? However, Holland noted that some studies have shown some predictive skill (e.g. Sigmond et al., 2013; Wang et al., 2013). Holland argued that the inherent limits of sea ice predictions coupled with the need for system improvements to realize predictability should be acknowledged as an important limitation with respect to the possible influence of sea ice loss on extreme weather. Model Limitations: CMIP Biases As highlighted in this workshop, CMIP5 model projections can be a useful tool for studying linkages between Arctic amplification and mid-latitude circulation. However, as Barnes noted in her remarks, they contain biases that should be considered. For example, models tend to place the Northern Hemisphere jet too far equatorward (no model is poleward of the observed North Atlantic jet in the annual-mean). Barnes also said that models tend to overestimate the seasonal cycle in the north Atlantic and storm track intensity is too weak (Zappa et al., 2014). It is also well-known that models underestimate observed high-latitude blocking frequencies. These underestimates are largest in the cool seasons. In general, increased horizontal and vertical resolution improves model blocking distributions (Anstey et al., 2013; Matsueda et al., 2009). Blocking biases in global circulation models can be reduced by correcting for mean circulation biases, but this does not remove all errors, especially at high latitudes, said Barnes. Conclusion The modeling studies discussed represent a fraction of the work that has been done to test possible mechanisms of the proposed linkages between Arctic amplification and mid- latitude weather. Because the modeling studies are largely divergent, many participants believe research in this area should continue. In particular, many participants said that the modeling community should organize careful model intercomparisons and model sensitivity studies with similar boundary and/or initial conditions to allow for a more systematic review and comparison of the results. Peings and Magnusdottir (2014) provide a detailed comparison of some of the recent modeling work. Although the workshop presentations highlighted several model limitations and biases, many participants asserted that models are the best tools available to complement observational studies, and to test robustness of the proposed mechanisms.                                                              13 A perfect model study is an idealized exercise where an ensemble of forecasts from a single model are compared to one another. The ensemble is typically initialized from some point in time from the same model from an earlier run. The perfect model exercise is not initialized with observations or any attempt to assimilate observations into the model, and hence the initial state is a state of the model, not nature. The evaluation of the forecast therefore avoids any inaccuracy in knowledge of the observed initial state, and it avoids model errors in reproducing nature.

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34 Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns ACID TEST TO ASSESS PROPOSED LINKAGES It was clear from the discussion that research into the linkages is still emerging and will continue in the future. Screen proposed using an “ACID test” as a framework for assessing confidence in proposed linkages: Attribution, Corroboration, Informed by mechanistic understanding, Detection. The first step is to ask whether the proposed link is attributable to sea ice loss (or another forcing). It is important to note that correlation does not necessarily imply causation. Screen stressed that it is difficult (if not impossible) to identify cause and effect in the real world. Carefully designed simulations are key for determining attribution (e.g., decreased mid-tropospheric north-south temperature gradient cannot be attributed to sea ice loss alone). It is also important to differentiate between sea ice loss and Arctic amplification (which has multiple causes). Also, observations do not tell us much about cause and effect, but models can. The next step is to determine if the proposed link is corroborated by multiple lines of evidence. Is there agreement among multiple studies? Is there agreement between models? Different models with the same forcing can actually give opposite responses. All of the different factors that may affect the model outcome must be considered. Is there agreement among metrics? Third, is the proposed linkage informed by mechanistic understanding? Confidence in the existence of a particular linkage is clearly strengthened by an in-depth understanding of the mechanisms. Screen acknowledged that much work is left to be done in this area. Do we understand the mechanisms driving the linkages? The final step in Screen’s ACID test is to ask whether the link is detectable in the real world? Is the signal large enough to be detected over intrinsic/natural variability? Is the signal strong enough to detect over other forcings? Some types of responses are easier to detect than others (see Figure 4). Screen believes that few, if any, of the proposed linkages pass the ACID test and that caution is needed in linking sea ice loss to recent events and trends. More research is required to make more confident statements. Other participants noted that the ACID test approach is sound, but, given the limitations of available information, there are inherent limitations to the analyses that can be conducted. FUTURE NEEDS AND OPPORTUNITIES Workshop participants divided into three breakout groups to discuss future needs and opportunities in (a) observations, (b) models, and (c) a big picture context. A summary of the discussions in each of these three breakout groups follows. Observations One theme that emerged was the need to more effectively utilize existing observational capabilities. For example, there was a sense that research aircraft surveys targeted to key seasons and processes are underutilized. Participants also noted that a vast amount of data is never analyzed. Department of Defense products such as Visible and Infrared Scanner (VIRS) and National Ice Center (NIC) operational products could be useful and are not saved unless there is a perceived need for them. Furthermore, making observations from data available more quickly would allow researchers enough time to analyze them. Individual participants identified other specific observational and data needs, including

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Future Needs and Opportunities 35  quantifying uncertainties in data products to clarify trends (e.g., sea ice concentration),  reprocessing historical snow products,  utilizing a more robust analysis of hourly temperature data to accurately represent the diurnal cycle in the Arctic,  deploying field programs that target extreme storm events (especially moisture measurements to clarify role of latent heating),  obtaining measurements that target: o clouds and surface fluxes (energy/radiation, moisture, momentum), especially over an increasingly open Arctic, from Arctic buoys (especially under high-wind conditions) and autonomous remote surface observations; o atmosphere in the Arctic to better understand vertical temperature structure, stratification (e.g. using dropsondes from unmanned aircraft) and changes in cyclone/storm tracks; and o the Arctic Ocean to improve seasonal predictions, and  deploying process driven studies, for example, to understand the processes that drive sea ice. Modeling Another theme that emerged from the workshop discussion was the importance of model sensitivity studies and model inter-comparison studies to provide clues on the mechanisms of potential Arctic linkages, as well as for detection and attribution. These studies require models that are consistently developed with similar boundary conditions (e.g., sea ice extent and thickness). Individual workshop participants identified other needs related to modeling:  Conducting idealized modeling studies to: o understand how Arctic amplification and the atmospheric circulation response are coupled (e.g. baroclinic vs. barotropic, seasonality); o understand future wave propagation with respect to Arctic change and the possible NAO-type response to sea ice loss; and o capture seasonality and longitudinal variations (by trying perturbed physics runs, and not just fixed boundary conditions).  Conducting attribution studies to understand the proportion of Arctic amplification that is due to sea ice vs. the proportion that is due to processes outside of the Arctic.  Identifying the resolution that is required to correctly model fluxes (including horizontal and vertical atmosphere and boundary conditions).  Applying the sophisticated methods and diagnostics that have been used in the meteorological community (e.g., see John Gyakum’s presentation) to this issue.  Developing higher resolution models to study processes in the planetary boundary layer (below 850 hPa or roughly 1,500 m above sea level).  Developing convection-permitting global climate models that resolve to better simulate vertical wind shear. Big Picture Context The third breakout group discussed the key points that emerged from the workshop discussions. Individuals in the group identified the following key messages:

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36 Linkages Between Arctic Warming and Mid‐Latitude Weather Patterns  Participants still disagree about the influence of recent Arctic warming on mid- latitude weather patterns.  Research on the Arctic influence on mid-latitudes is at an early stage compared to influences such as tropical, mid-latitude SST, or the stratosphere. The recent proposed influences are best viewed as hypotheses.  It is important to retain a global perspective by considering the influence of the Arctic in the context of forcing from other components of the climate system, such as the tropics and the stratosphere. Changes in ocean circulation may be particularly important in Arctic change and its influence on the mid-latitudes. A key limitation is lack of knowledge of the Arctic Ocean energy budget.  Attention has so far focused on changes in the meridional temperature gradient. Some consideration could be given to other mechanisms, such as zonal temperature gradients, static stability and moisture changes. The possibility of distinct regional responses could also be considered (e.g., east and west Atlantic).  Application of more sophisticated methods of analysis to determine whether mid- latitude wave activity is changing would be useful. A long history of dynamical analysis of wave activity can be exploited in this field. Additionally, the link between Arctic warming and weakened westerlies is not clear, because of transient eddy feedbacks, which often act to shift the westerlies instead.  More effort should be directed to understanding the synoptic aspects of possible linkages, considering the variety of synoptic weather systems active in the region, and to bridging the gap between case study analysis and filtered variance approaches. This topic could be further developed through collaboration between weather and climate scientists.  Blocking is important for weather impacts and was a recurring feature of the workshop discussion, yet it is often interpreted differently by different people or analyses. Improving our understanding of the range of blocking systems and how they may be influenced by forcing is critical.  It is possible that recent Arctic changes have pushed the atmosphere into a new state with different variability. The strong Arctic forcing has emerged only in the past few years, and development of new methods and approaches may be required to test or account for it. For example, it might be useful to adopt a probabilistic approach to determine whether there are changes in the probability of extreme events.  Understanding of natural variability should be improved to determine whether changes are taking place. Reanalyses of the atmosphere are generally trustworthy for synoptic and large-scale features, so relatively long records are available. Work to extend sea ice records back in time is also useful. Ocean observations are seriously limited.  Combining observations with a hierarchy of models is key to making progress on this issue. The detection and attribution framework provides one formal approach.  Arctic influences could provide additional sources of skill for seasonal forecasting through model improvements or statistical methods.  A recurring issue is how to respond to the public when asked questions such as “I heard that sea ice loss caused the wet summer, why don’t you include that in your forecasts?”  A large, policy-relevant research program on these issues would bring the subject to maturity in the same way that has occurred for similar subjects (e.g., stratospheric, ocean influences). The program could focus on specific topics, for example, identifying something we are trying to explain, or by being provocative and picking on the weak links of chains of arguments.

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Future Needs and Opportunities 37  Although many challenges exist, several new opportunities will make significant progress in this area possible in the near future. These include (a) improvements in atmospheric models, some of which can now simulate blocking well, (b) similar improvements in ocean and sea ice models, (c) longer records back into the past of some sea ice quantities and atmospheric circulation, (d) emergence of reasonably strong sea ice forcing in recent years, and (e) strong interest in the topic from both public and governments. Overall, many participants thought that, given the far-reaching societal impacts of changes in our climate system and the speed at which the Arctic sea ice cover is disappearing, it is imperative to advance understanding of this issue and to assess what it may mean for the frequency, severity, and persistence of severe weather events in the coming years. The workshop discussions made it clear that progress on this front requires more observational work and modeling experiments to place Arctic linkages within a larger context of the other factors that affect mid-latitude weather.

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