Linkages Between Arctic Warming and Mid-Latitude Weather Patterns Summary of a Workshop (2014) / Chapter Skim
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Linkages Between Arctic Warming and MidLatitude Weather Patterns
Linkages Between Arctic Warming and MidLatitude Weather Patterns
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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.
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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.
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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.
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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.
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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]
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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)
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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)
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; • changes in wind speed over the region (e.g. Haarsm et al., 2013 Mizuta, 20 o n ma 3b; 012)
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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)
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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.
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. 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.
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is a proposed mechanism for tropical convection, Arctic amplification, and a reduction of Arctic sea ice. The trend toward stronger and more localized convection over the tropical Indo-Pacific Warm Pool results in the propagation of Rossby waves toward the poles.
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Arctic Influence on the Hydrological Cycle in the Mid-Latitudes and Tropics l s s Ian Eisenmman, Universit of Californi San Diego, said that cha ty ia, , ange in surfac albedo (due ce e to Arctic sea ice loss) ha been sugge s as ested to affect the hydrolog ical cycle in t mid the latitudes and tropics.
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attributable to ice loss. Oceanic Feedbacks Influence on Arctic Amplification Clara Deser, NCAR, discussed the role of oceanic feedbacks in the atmospheric response to GHG-induced Arctic sea ice loss.
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This section highlights obse T ervational evid dence of some mechanisms that were e raised at the workshop as possibly lin nking Arctic amplification a mid-latitu weather: a and ude Arctic warming faster than th Northern Hemisphere A g he H Decrease in the temperature gradient betw D e e ween the Arct and the mi tic id-latitudes Sl lowing of uppper-level zona winds al Upper-level flo becoming more meridio U ow onal In ncrease in the amplitude of large-scale waves w In ncrease in blocking events La arge-scale waaves progress more slowly eastward m e In ncrease in extr reme events Weakening of the polar vortex W t Arcti Warming Faster Than the Northern He ic e emisphere As discuss in the prev sed vious section, the Arctic is warming faste than the tro er opics and midlatitudes. This has resulted in a greate flux of ener from the n T er rgy newly exposed open ocean d n (relative to ice-covered waters)
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FIGURE 7 A negative trend in poleward geopotential thickness gradien (1000-500 h Pa) from 1979 to 2012.
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. This shift coin cides with wa el T armer temperatu and greate geopotentia thickness ov northeaste Canada, m ures er al ver ern major sea ice loss during October in Baffin Bay, a positive Greenland Blockin Index (grea 500 hPa g ng ater geopotent heights)
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fr rom 2000 to 20 012. Right: Anomalies in zonal winds at 500 hPa fro 2000 to 2012.
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, but there is st a correspon i till ndence between a weakening temperature gradient n and a more meandering upper-level flow e w.
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, which coupled with the decrease in zonal win w h e nds, results in a jet stream that is more meandering. Francis said there is a pos m e .
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. Barnes noted 0 that some studies show that high-latittude blocking over the Nor Atlantic ha decreased i g rth as in the past 40 years (Barne 2013; Croc 0 es, ci-Maspoli et al., 2007; Da avini et al., 20 012; Figure 166)
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Block ays/year; blue line) king events (da plotted with zonal wind sp h peed at 500 hP (dashed line)
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, which s e shows that win sea ice nter anomalies have a strong association with extreme cold events o s g over the Unite States. ed Weakening of the Polar Vortex P Workshop participants briefly discussed the hypot p thesis that Arc sea ice los leads to ctic ss changes in regional hea and other energy fluxes, which result i a weak (or unstable)
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24 Linka kages Between A Arctic Warming and Mid‐Latit g tude Weather P Patterns FIGURE 18 Seasonal trend of (left) Z500 phase speeds and (right)
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Overland noted tha the externa l forcing of Ar f at rctic sea ice loss and snow loss occu on local sc urs cales and on short timescal so, in his o s les, opinion, takinng zonal averages and seasonal average will smooth out some of the effects tha we would b es h at be looking fo in trying to identify trends or s. cknowledged more work is needed to ass Francis ac sess the follow wing:
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The followin variables w ng were discussed in the context of mode that were fo els orced with Arrctic sea ice lo or Arctic w oss warming: temperature gradients, e upper-level zonal winds, , large-scale wave amplitu udes, blocking, annd weather pat tterns and stor tracks. rm 9 The degree to which models agree with one anoth It is often q d h her.
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to understand the effect of recent extensive sea-ice loss years (2007-2012; denoted as 2010C) on the atmospheric circulation in winter and then compared those results to response to projected sea-ice change at the end of the century (2080-99; denoted as 2090C)
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(a) 2010C zonal mean D er C DJF temperature response (K)
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ribution of the 5400 m isoplet th daily wave amplitude chaange in winter (DJF)
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. 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.
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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)
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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.
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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 midlatitude weather. Because the modeling studies are largely divergent, many participants believe research in this area should continue.
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. It is also important to differentiate between sea ice loss and Arctic amplification (which has multiple causes)
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. 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.
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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)
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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.
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