Clarity and Clouds: Progress in Understanding Arctic Influences on Mid-Latitude Weather. (2024)

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As we watch the ongoing rapid loss of Arctic sea ice, freshwaterice, permafrost, and spring snow cover, the corresponding amplifiedwarming of the Arctic region (AAW) continues to increase. Thesedisturbing changes to a key component of the earth's climate systemhas spawned a blizzard of new studies that reveal influences of AAW onweather patterns within and beyond the Arctic. Media and public interestin the topic has also been keen, as the loss of Arctic sea ice is one ofthe most conspicuous symptoms of human-caused climate change (Notz &Stroeve, 2016) and unusual weather events often dominate headlines. Acase in point: damage in the United States caused by extreme weatherevents in 2017 was the costliest in history at over $300 billion (Smith,2018). People are being affected directly by these events, andincreasingly they're asking, "What's up with this? Isclimate change playing a role?" Scientists can now answer aconfident "yes" to that question, though the exact degree ofinfluence is difficult to pin down.

It's clear global warming is increasing the intensity of heatwaves and droughts, as well as the frequency of heavy precipitationevents, but the lines of influence are fuzzier when it comes to theeffects of AAW. Progress in understanding the connections, however, hasbeen steady. In recent years, researchers have learned that it'snot just one connection, but rather several that vary with season,region, and fluctuating natural states of the climate system (e.g.,ocean temperature patterns like El Nino). The general hypothesis is thatwhen the Arctic warms faster than lower latitude regions, thenorth-south temperature difference is reduced. Because that temperaturedifference provides the main fuel for the polar jet stream (a river ofstrong wind at levels where jet aircraft fly), the predominantly westwinds of the jet stream weaken. A slower jet stream tends to favor amore meandering north-south path and to shift the mean jet latitudesouthward. A wavier pattern allows warm air to penetrate farther northand cold air to plunge farther south, compared to when the jet is strongand relatively straight (see Figure 1). Larger waves also tend to lingerin one location, as do the surface weather systems they create, whichresults in persistent weather conditions that can turn into extremeevents.

In addition to AAW, many changes in the climate system arehappening simultaneously and are also affecting mid-latitude weatherregimes. For example, most climate model simulations for the futureindicate that another area of amplified warming will occur in the upperatmosphere over the tropics (though there's no sign of it yet inobservations). This amplification will increase the north-southtemperature difference between the tropics and mid-latitudes and opposethe influence of AAW (McCusker et al., 2017; Peings, Cattiaux, Vavrus,& Magnusdottir, 2018). The spectrum of change in the climate system,along with the inherently chaotic behavior of the atmosphericcirculation, hinders efforts to assess the influence of any one factoron weather patterns (Vavrus 2018).

Despite these obstacles, some aspects of the AAW's influenceon the large-scale circulation have come into sharper focus in the pastfew years. Some recent investigations based on global climate modelshave convincingly demonstrated that including atmosphere-oceaninteractions (Deser, Sun, Tomas, & Screen, 2016) and a well-resolvedstratosphere (Zhang et al., 2018) are necessary to capture realisticatmospheric responses to sea-ice loss and AAW. However, many details oflinkage mechanisms remain elusive (Sun, Alexander, & Deser, 2018).Progress in understanding three specific examples of Arctic/mid-latitudelinkages is summarized here: west-east contrasts in North Americanweather, sea-ice loss and winter extremes over Eurasia, and stagnantconditions over continents during summer. For a more complete recentreview, see Vavrus (2018).

North American Warm/Dry West--Cold/Wet East

Since late 2013, the predominant weather regime over North Americahas featured a strong and persistent jet-stream ridge alignednorth/south in the eastern Pacific, which diverts storms away fromCalifornia and sends abnormally warm winds into Alaska (see Figure 1).This so-called "ridiculously resilient ridge" (Swain et al.,2017) is perpetuating drought, heat waves, and extensive wildfiresacross much of western North America. A strong jet-stream ridge is oftenaccompanied by a downstream (eastward) trough, which allows cold Arcticair to plunge southward, bringing persistent icy conditions to thesoutheastern United States (Cohen, Pfeiffer, & Francis, 2018) andcan spawn a parade of destructive nor'easters along the easternseaboard, as in winters of 2013-14 and 2017-18.

How might Arctic warming help promote this ridge/trough pattern?Recent studies reveal that a western ridge is more likely when oceantemperatures along the west coast of North America are warmer thannormal (Swain et al., 2017). The strength of this ridge may then bebolstered by low sea-ice extent north of Alaska, where oceantemperatures are abnormally warm owing to extra solar heat absorbedduring summer. This heat is then released back to the atmosphere whencold autumn air masses arrive, contributing to AAW in the region andaugmenting the ridge's bulge in upper levels of the atmosphere.Strong ridges usually trigger a southward jet-stream dip to their east.Consequently, this persistent pattern is favored when a naturalfluctuation in Pacific Ocean temperature anomalies combines withregional AAW in the Pacific sector of the Arctic (Francis, Vavrus, &Cohen, 2017), perhaps boosted by a tropical connection (Cvijanovic etal., 2017).

Cold Eurasian Winters Linked to Sea-Ice Loss in the Barents andKara Seas

Similar to the North American situation, a strong and persistentridge/trough pattern has occurred with increasing frequency in Eurasiain recent decades (see Figure 2). New studies have strengthened theevidence for a role played by sea-ice loss in the Barents and Kara seas(Kretschmer, Coumou, Donges, & Runge, 2016; Yao, Luo, Dai, &Simmonds, 2017; Ye, Jung, & Semmler, 2018; Zhang et al., 2018),though other studies attribute it to random chance (e.g., McCusker,Fyfe, & Sigmond, 2016). The story again begins with a climatologicalridge (a long-term average pattern) that tends to form near the UralMountains, which acts as an obstacle to westerly jet-stream winds inwestern Russia. North of this ridge are the Barents and Kara seas in theArctic Ocean, regions of rapid sea-ice loss and warming in recent years.As with the North American case, the existing ridge is intensified bythe extra heat absorbed in and subsequently released from the ice-lossregion. A stronger ridge creates a stronger surface high-pressure systemover central-eastern Asia and transports cold Arctic air over thecontinent, which depresses the jet stream southward into a deeper troughover Asia. Arctic air is then free to plunge southward, favoringpersistent cold spells in eastern Asia and contributing to observedcooling trends in the region. Under the right conditions, thisridge/trough pattern can become so intense that the resulting waveenergy in the jet stream is transferred upward into the stratosphere,disrupting the normally circular flow of the stratospheric polar vortex,causing it to bulge southward or perhaps even split into two circulationcenters. When this occurs, large jet-stream waves often persist intolate winter and even early spring, resulting in extended periods ofunusual and sluggish weather patterns around the northern hemisphere(Kretschmer et al., 2018). This mechanism clearly occurred in winter2017-18, perpetuating large ridges and troughs around the NorthernHemisphere well into spring that were responsible for severe cold inEast Asia, heat waves at the North Pole, the "Beast from theEast" in Europe, a swarm of nor'easters in New England, and avariety of other disruptive weather abnormalities.

Persistent Summer Weather Patterns Over Continents

The Arctic meltdown may also be contributing to summer heat waves,drought, wildfires, and flooding over Northern Hemisphere continents.New studies suggest that the disappearing spring snow cover focusesArctic warming during late spring and early summer over the far-northernland areas (Francis & Vavrus 2012). This loss of snow during theseason of most intense sunshine causes the soil to dry out and warmearlier, effectively giving summer heating a "jump start"(York, Bhatt, Thoman, & Ziel, 2018). The zonal belt of warming thatrings high-latitude continents also can create a double peak in thenorth-south temperature trend, which favors the formation of a split jetstream. Weather systems can become trapped between the jet branches,causing prolonged hot spells, drought, or rainy periods that can lead toextreme events (Mann et al., 2017). Recent deadly heat waves (e.g.,Europe in 2003 and 2018, Russia in 2010, Japan in 2018, and the UnitedStates in 2011 and 2018) and floods (e.g., Pakistan 2010 and Europe2013) are consistent with this hypothesis. The general relationshipbetween land-based AAW and summer extremes is also supported by modelprojections of the future analyzed by Vavrus and colleagues (2017). Theyfound a belt of abnormally warm air temperatures overlying the snow-lossareas in North America, which created weakened upper-level westerlywinds south of the belt and stronger winds to the north. The band ofweak winds is associated with amplified jet-stream waviness measured as"sinuosity," which favors persistent drought and heat spellsduring summer in mid-latitude continental areas.

Conclusion

The connections between climate change and extreme weather are atopic of intense scientific interest and of profound societal impact.Some of these effects are clear--more severe heat waves, more frequentheavy precipitation events, and more intense droughts--but theunderstanding of other less-direct influences is still partly to mostlycloudy. The role of a rapidly warming and melting Arctic is one of thesefactors that challenges present computer modeling capabilities andunderstanding of atmospheric dynamics. These limitations are now cominginto better focus as changes in the real world either confirm or opposeexpectations based on computer simulations, offering avenues to resolvedisputes in our understanding of Arctic/mid-latitude linkages. Exactlyhow the northern meltdown will "play ball" with other changesand natural fluctuations in the system presents many questions that willkeep scientists busy for years to come, but it's becomingice-crystal-clear that change in the far north will increasingly affectus all.

Jennifer A. Francis, PhD

Senior Scientist

Woods Hole Research Center

Falmouth, MA

Note: This article originally appeared as part of National Oceanicand Atmospheric Administration's 2018 Arctic Report Card and isreprinted with permission. It has been edited for style and formatting.For more info, visithttps://www.arctic.noaa.gov/reportcard/report-card-2018

References

Cohen, J., Pfeiffer, K., & Francis, J.A. (2018). Warm Arcticepisodes linked with increased frequency of extreme winter weather inthe United States. Nature Communications, 9, Article no. 869.doi:10.1038/s41467-018-02992-9

Coumou, D., Di Capua, G., Vavrus, S., Wang, L., & Wang, S.(2018). The influence of Arctic amplification on mid-latitude summercirculation. Nature Communications, 9, Article no. 2959.doi:10.1038/s41467-018-05256-8

Cvijanovic, I., Santer, B.D., Bonfils, C. Lucas, D.D., Chiang,J.C.H., & Zimmerman S. (2017). Future loss of Arctic sea-ice covercould drive a substantial decrease in California's rainfall. NatureCommunications, 8, Article no. 1947. doi:10.1038/s41467-017-01907-4

Deser, C., Sun, L., Tomas, R.A., & Screen, J. (2016). Doesocean coupling matter for the northern extratropical response toprojected Arctic sea ice loss? Geophysical Research Letters, 43(5),2149-2157. doi:10.1002/2016GL 067792