LETTERS. Vertical structure of recent Arctic warming

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1 Vol 5 Jnury 8 doi:.8/nture65 Verticl structure of recent Arctic wrming LETTERS Rune G. Grversen, Thorsten Muritsen, Michel Tjernström, Erlnd Källén & Gunill Svensson Ner-surfce wrming in the Arctic hs een lmost twice s lrge s the glol verge over recent decdes 5 phenomenon tht is known s the Arctic mplifiction. The underlying cuses of this temperture mplifiction remin uncertin. The reduction in snow nd ice cover tht hs occurred over recent decdes 6,7 my hve plyed role 5,8. Climte model experiments indicte tht when glol temperture rises, Arctic snow nd ice cover retrets, cusing excessive polr wrming 9. Reduction of the snow nd ice cover cuses ledo chnges, nd incresed refreezing of se ice during the cold seson nd decreses in se-ice thickness oth increse het flux from the ocen to the tmosphere. Chnges in ocenic nd tmospheric circultion, s well s cloud cover, hve lso een proposed to cuse Arctic temperture mplifiction 7. Here we exmine the verticl structure of temperture chnge in the Arctic during the lte twentieth century using renlysis dt. We find evidence for temperture mplifiction well ove the surfce. Snow nd ice feedcks cnnot e the min cuse of the wrming loft during the greter prt of the yer, ecuse these feedcks re expected to primrily ffect tempertures in the lowermost prt of the tmosphere, resulting in pttern of wrming tht we only oserve in spring. A significnt proportion of the oserved temperture mplifiction must therefore e explined y mechnisms tht induce wrming ove the lowermost prt of the tmosphere. We regress the Arctic temperture field on the tmospheric energy trnsport into the Arctic nd find tht, in the summer hlf-yer, significnt proportion of the verticl structure of wrming cn e explined y chnges in this vrile. We conclude tht chnges in tmospheric het trnsport my e n importnt cuse of the recent Arctic temperture mplifiction. The recent wrming of the Erth s surfce is most proly due to n increse of tmospheric greenhouse-gs concentrtions 8. Although most greenhouse gses re firly uniformly distriuted round the gloe, the temperture response to greenhouse-gs forcing is thought to e lrger in polr thn equtoril regions. The response depends on vrious feedcks within the climte system. In ddition to snow nd ice processes, the strength of the tmospheric strtifiction constitutes such feedck. The troposphere is more stly strtified in the polr regions thn closer to the Equtor. An increse in downwelling long-wve rdition t the surfce (for exmple, due to n ltered tmospheric CO level) cuses wrming, which t high ltitudes is confined to the lower troposphere 8. In the tropics, in contrst, the wrming is distriuted verticlly y deep convection. It hs lso een proposed tht the increse of polluting mterils (such s lck cron) on Arctic ice nd snow hve cused ledo chnges nd dded to the Arctic wrming 9. Common to ll these processes is tht they re expected to induce the lrgest wrming in the lowermost prt of the tmosphere. The Arctic mplifiction cn lso e cused y other processes. Idelized experiments with models tht hve no surfce-ledo feedck lso revel polr-temperture-mplifiction response to douling of CO concentrtion. It is found tht the excessive Arctic wrming is due to n increse of the tmospheric northwrd trnsport of het nd moisture. These results re supported y oservtionl studies, which suggest tht chnges of the het trnsport hve dded to the recent Arctic surfce wrming. The linkge etween Arctic wrming nd chnges of tmospheric circultion hs een investigted y studying vrious Northern Hemisphere circultion indices, such s tht ssocited with the Arctic Oscilltion. Generlly, different phses of these indices re ssocited with liner temperture responses chrcterized y est west het redistriution etween the mid-ltitude ocen nd lnd, wheres the high ltitudes re less ffected. However, in the winter seson, high phses of the circultion indices re ssocited with wrmer Arctic. This wrming is prticulrly pronounced over the northern rims of the continents 6. From the 97s through to the mid-99s, the indices were in their high phses, while since then, they hve relxed towrds neutrl vlues. The Arctic wrming, on the other hnd, hs shown persistently positive trend over the pst yers. It is therefore difficult to ssocite chnges in these indices, 6 8, c 6 8, d ºC per yr Figure Averged temperture trends round ltitude circles for 979 plotted versus ltitude nd height for the four sesons. Trends re shown for winter (, Decemer Ferury), spring (, Mrch My), summer (c, June August) nd utumn (d, Septemer Novemer). The liner trends re estimted from monthly men dt using lest-squres fit. Deprtment of Meteorology, Stockholm University, S-6 9 Stockholm, Sweden. 7 Nture Pulishing Group 5

2 LETTERS NATURE Vol 5 Jnury 8 Temperture nomlies (ºC) Northern Hemisphere North of 65º N Yer Figure Drk-month (Novemer Ferury) nomlies of men temperture reltive to the 85 9 verge s function of yer. Dt were otined from lnd-sttion oservtions. In the min figure, the symols represent mens from individul yers, wheres the lines show the temporl evolution when vriility over timescles smller thn yers hs een removed using wvelet filter. Solid line nd open circles re sed on oservtions north of 65u N, while the dshed line nd dots re for the entire Northern Hemisphere. Inset shows the smoothed temperture time series for the full instrumentl period. The dt were provided y the Climte Reserch Unit (CRU) s 5u 5u gridded dt set. such s the Arctic Oscilltion index, with the recent Arctic wrming trend. The verticl structure of the Arctic wrming during the 98s nd 99s, sed on the ERA- renlysis (see Methods), exhiits trends throughout lrge prts of the troposphere tht re comprle in mgnitude to those t the surfce (Fig. ). In fct, the Arctic wrming in the renlysis dt shows cler mxim well ove the surfce in winter nd in summer, nd the trends re lmost equl t heights elow the hp level in the tmosphere during utumn. This verticl structure is not consistent with the hypothesis tht retreting snow nd ice cover is the min cuse of the mplifiction. Retreting snow nd ice re ssocited with energy input t the surfce, which long with the stle strtifiction conditions often previling in the Arctic mens tht this process would e expected to induce the lrgest temperture response in the lowermost prt of the troposphere. But we only oserve this verticl structure of wrming in spring. It is worth noting tht this is when the trends ove the oundry lyer re of comprle mgnitude to those t the surfce. We note tht the lck of mplifiction ner the surfce in summer is consistent with expecttions ecuse surfce ir tempertures over the Arctic Ocen re constrined to e close to the freezing point owing to the melting of se ice, ut tht the mplifiction loft cnnot e explined y surfce feedcks. It is lso notle tht during the sme period oservtions solely from Arctic lnd sttions revel n mplifiction of the temperture trend during the drk months, Novemer Ferury (Fig. ). This mplifiction cnnot e explined y snow-cover chnges, s the ledo effect is prcticlly sent during this drk period. Moreover, the het flux from the ground is very smll. This is contrry to conditions in the ice-free prts of the Arctic ocens during winter, where convection ensures tht cold wter t the surfce is replced y wrmer wter from elow; this process mintins lrge verticl temperture grdient etween the ocen surfce nd the cold tmosphere. In ddition, reduction of se-ice cover during summer results in incresed se-ice formtion during utumn nd winter, such tht ltent het is stored during the wrm seson nd relesed into the tmosphere during the susequent months. So wht re the mechnisms giving rise to the verticl structure of the Arctic wrming? Chnges in the dvection of tmospheric energy into the Arctic region might imply Arctic wrming with mximum not necessrily locted t the surfce. Mid-tropospheric tempertures in the Arctic re sensitive to dvection of energy cross the Arctic oundry: this is evident from liner regressions of the Arctic 5 hp temperture field on the tmospheric northwrd energy trnsport (ANET) cross 6u N (Fig. ). Positive (negtive) nomlies of the ANET t 6u N re followed y positive (negtive) temperture nomlies over the Arctic re, where the nomlies of Time lg (d) Figure Regressions of the 5 hp temperture field on the tmospheric northwrd energy trnsport (ANET) cross 6u N., Regressions verged round ltitude circles s function of ltitude nd time lg;, regressions for 5-dy lg s function of longitude nd ltitude. Solid nd dotted contours indicte positive nd negtive regressions, respectively. In ech point the regression hs een scled y the sptil 5 7 Nture Pulishing Group stndrd devition of ll regressions. Light- nd drk-grey shding shows res where regressions differ significntly from zero t the 99% nd 99.9% level, respectively. The regressions indicte temperture nomlies ssocited with n ANET nomly t lg zero. For instnce, positive ANET nomly is followed 5 dys lter y wrming nd cooling north nd south of 6u N, respectively.

3 NATURE Vol 5 Jnury 8 LETTERS the tempertures lg those of the ANET y out 5 dys (Fig. ). Positive regressions re found for positive time lg north of 6u N nd for negtive time lg south of 6u N, wheres the opposite distriution is found for negtive regressions. In sttisticl sense, this indictes tht lrge energy trnsport follows conditions where lrger-thn-usul north south temperture grdient t 6u N hs previled. This trnsport, in turn, is succeeded y wrming of res north of 6u N nd cooling south of this ltitude signture of energy convergence nd divergence north nd south of 6u N, respectively. The wrming evluted t five-dy lg (Fig. ) is distriuted over the mjor prt of the Arctic re, wheres the cooling in the mid-ltitudes is found over lrge prts of the continents. This linkge etween the ANET cross 6u N nd the temperture field is found through the entire verticl extent of the troposphere (not shown), nd is similr to tht found from regressions of the surfce tempertures on the ANET. The ANET cross 6u N hs incresed during recent decdes, except in Jnury nd Ferury. For the summer hlf-yer, April through to Octoer, the ANET cn explin sustntil prt of the Arctic temperture trends (Fig. ). The prt of the temperture trends tht cn e linked to the ANET (Fig. ) shows roughly the sme verticl distriution s the totl temperture trends (Fig. ), with mximum t round 7 hp. At 6u N, the ANET is minly ccomplished y tmospheric wves, such s Rossy wves nd cyclone systems. Hence, the ANET t mid-ltitudes cn e viewed s n index of tmospheric circultion ptterns. This index ppers to e n efficient indictor of linkge etween circultion chnges nd Arctic temperture trends ºC per yr Other processes tht might e importnt contriutors to the wrming ove the surfce include chnges in cloud cover nd the tmospheric wter vpour content. In the Arctic, except possily for short summer period, persistent low clouds re elieved to induce surfce wrming. Often the greenhouse effect of clouds domintes over the ledo effect, s the clouds cover n lredy highly reflecting surfce. By soring rdition, clouds my furthermore wrm the tmosphere t the height where they re present. As result, it is possile tht n increse in cloud cover t given tmospheric height my cuse wrming there. Oservtions from stellites indeed suggest n increse of Arctic cloud frction in summer during the 98s nd 99s 7. Wrming of the Arctic middle troposphere might lso prtly e n effect of chnges in the tmospheric rditive properties. These chnges could e ssocited with the ove-mentioned increse in dvection of energy, which is siclly trnsport of wrm nd/or moist ir into the Arctic. The dvection in itself ccounts for considerle prt of the mximum wrming t 7 hp, ut dditionl wrming t this height would occur if the dvected ir is more humid thn the mient ir nd hence sors long-wve rdition more efficiently; wter vpour is n efficient greenhouse gs 8. Our results do not imply tht studies sed on models forced y nticipted future CO levels re misleding when they point to the importnce of the snow nd ice feedcks. It is likely tht further sustntil reduction of the summer ice-cover would strengthen these feedcks nd they could ecome the dominnt mechnism underlying future Arctic temperture mplifiction. Much of the present wrming, however, ppers to e linked to other processes, such s tmospheric energy trnsports. METHODS SUMMARY The ERA- renlysis dt re used for Figs, nd. A discussion of the dt qulity nd comprison with two other renlysis dt sets re given in the Supplementry Discussion. The ANET t given ltitude is defined s the totl energy flux cross this prticulr ltitude. Hence, this quntity constitutes one time series. Using dily dt, the temperture field hs een regressed on the ANET t 6u N for different time lgs of the temperture field reltive to the ANET (Fig. ). When these regressions re multiplied y the ANET time series, projections of the temperture field on the ANET re otined. On the sis of monthly men dt, liner trends of these projections hve een estimted (Fig. ) using lest squres fit. A Monte Crlo pproch with lrge numer of rtificil ANET time series hs een used in order to estimte the significnce of the results in Figs nd.,. Full Methods nd ny ssocited references re ville in the online version of the pper t Received 8 Mrch; ccepted 9 Novemer , Figure Averged temperture trends round ltitude circles for 979 plotted versus ltitude nd height for April Octoer., Totl trends;, trends tht re linked to the ANET cross 6u N. The shdings indicte trends, nd the white contours indicte res where trends differ significntly from zero t the 99% nd 99.9% level, respectively. ºC per yr 7 Nture Pulishing Group. Simon, C., Arris, L. & Hel, B. Arctic Climte Impct Assessment (Cmridge Univ. Press, New York, 5).. Johnnessen, O. M. et l. Arctic climte chnge: oserved nd modeled temperture nd se-ice vriility. Tellus A 56, 8 ().. Polykov, I. V. et l. Oservtionlly sed ssessment of polr mplifiction of glol wrming. Geophys. Res. Lett. 9, doi:9/gl ().. Rigor, I. G., Colony, R. L. & Mrtin, S. Vritions in surfce ir temperture oservtions in the Arctic, J. Clim., (). 5. Serreze, M. C. & Frncis, J. A. The Arctic mplifiction dete. Clim. Chnge 76, 6 (6). 6. Cvlieri, D. J., Gloersen, P., Prkinson, C. L., Comiso, J. C. & Zwlly, H. J. Oserved hemispheric symmetry in glol se ice chnges. Science 78, 6 (997). 7. Stroeve, J. C. et l. Trcking the Arctic s shrinking ice cover: Another extreme Septemer minimum in. Geophys. Res. Lett., doi:.9/gl8 (5). 8. Solomon, S., et l. (eds) Climte Chnge 7: The Physicl Science Bsis (Cmridge Univ. Press, Cmridge, UK, 7). 9. Hollnd, M. M. & Bitz, C. M. Polr mplifiction of climte in coupled models. Clim. Dyn., ().. Hnsen, J. et l. Efficcy of climte forcing. J. Geophys. Res., doi:.9/ 5JD5776 (5).. Chpmn, W. L. & Wlsh, J. E. Simultion of Arctic temperture nd pressure y glol coupled models. J. Clim., 69 6 (7).. Alexeev, V. A., Lngen, P. L. & Btes, J. R. Polr mplifiction of surfce wrming on n quplnet in ghost forcing experiments without se ice feedcks. Clim. Dyn., (5). 55

4 LETTERS NATURE Vol 5 Jnury 8. Thompson, D. W. J. & Wllce, J. M. Regionl climte impcts of the Northern Hemisphere nnulr mode nd ssocited climte trends. Science 9, ().. Moritz, R. E., Bitz, C. M. & Steig, E. J. Dynmics of recent climte chnge in the Arctic. Science 97, 97 5 (). 5. Wu, Q. & Strus, D. M. AO, COWL, nd oserved climte trends. J. Clim. 7, 9 56 (). 6. Qudrelli, R. & Wllce, J. M. A simplified liner frmework for interpreting ptterns of Northern Hemisphere wintertime climte vriility. J. Clim. 7, 78 7 (). 7. Wng, X. & Key, J. R. Arctic surfce, cloud, nd rdition properties on the AVHRR polr pthfinder dtset. Prt II: Recent trends. J. Clim. 8, (5). 8. Mne, S. & Wetherld, R. T. The effect of douling the CO concentrtions on the climte of generl circultion model. J. Atmos. Res., 5 (975). 9. Hnsen, J. et l. Dngerous humn-mde interference with climte: GISS modele study. Atmos. Chem. Phys. 7, 87 (7).. Grversen, R. G. Do chnges in the midltitude circultion hve ny impct on the Arctic surfce ir temperture trend? J. Clim. 9, 5 58 (6).. Thompson, D. W. J. & Wllce, J. M. The Arctic Oscilltion signture in the wintertime geopotentil height nd temperture fields. Geophys. Res. Lett. 9, 97 (998).. Intrieri, J. M. et l. An nnul cycle of Arctic surfce cloud forcing t SHEBA. J. Geophys. Res. 7, doi:.9/jc9 ().. Uppl, S. M. et l. The ERA- re-nlysis. Q. J. R. Meteorol. Soc., 96 (5).. Jones, P. D., New, M., Prker, D. E., Mrtin, S. & Rigor, I. G. Surfce ir temperture nd its vritions over the lst 5 yers. Rev. Geophys. 7, 7 99 (999). Supplementry Informtion is linked to the online version of the pper t Acknowledgements We thnk P. Lunderg for comments on the mnuscript. The ERA- dt were otined from the Europen Centre for Medium-Rnge Wether Forecsts (ECMWF) dt server, wheres the Climte Reserch Unit (CRU) t the University of Est Angli provided the oservtionl dt used for Fig.. Author Contriutions The nlysis ws performed nd the mnuscript written y R.G.G., nd to some extent T.M. The originl ide to use ERA- dt to study Arctic wrming ws due to R.G.G., M.T. nd E.K. All uthors contriuted with ides, discussions nd text. Author Informtion Reprints nd permissions informtion is ville t Correspondence nd requests for mterils should e ddressed to R.G.G. (rune@misu.su.se) Nture Pulishing Group

5 doi:.8/nture65 METHODS Energy trnsport. The tmospheric energy cn e divided into four components 5 : potentil, gz; internl, c v T; kinetic, u: u; nd ltent, Lq. Here g is grvity, z is height, c v is specific het cpcity for constnt volume, T is solute temperture, u is the three-dimensionl wind vector, L is the specific het of condenstion or sulimtion, nd q is specific humidity. The ANET cross ltitude w o is defined y: þ w~w g ð u: uzc p TzgzzLq v Lp Lg dgdx where w is ltitude, c p is specific het cpcity for constnt pressure, v is the northwrd wind component, p is pressure, g is the verticl hyrid coordinte used within the ERA- frmework, nd x is the est west coordinte. Liner regression. The sensitivity of the Arctic temperture field to the ANET cross 6u N is estimted s follows: first, the ANET cross 6u N sed on 6-hourly dt is determined using eqution (). Then rotropic mss correction,6 is pplied to dily verges of the ANET. At ech grid point nd sed on the dily dt, the temperture time series hve een regressed on the msscorrected ANET time series for different time lgs of the temperture reltive to the ANET. The nnul cycle ws removed nd 7-dy running men pplied to ll dt efore performing the regressions. Normlized regressions t the 5 hp level re shown in Fig.. For estimtions of the Arctic temperture trends, which re linked to the ANET cross 6u N, monthly men rther thn dily dt re used s the wrming response lgs the ANET y round 5 dys (Fig. ). The regressions multiplied y the ANET time series constitute the projection of the temperture field on the ANET. Liner trends of these projections re shown in Fig.. Significnce test. The shding in Fig. nd the white contours in Fig. show the results of sttisticl significnce test sed on Monte Crlo pproch. The temperture field is regressed on rtificil time series with the sme power spectrum s the ANET, ut with ritrry phses of the modes. These regressions re compred to the originl regression t ech sptil grid point. At given grid point, the temperture trend linked to the ANET (Fig. ) is tken to differ significntly from zero on, sy, 99% level, if less thn % of the rtificil regressions show trends tht re numericlly lrger thn the one from the originl regression. A corresponding procedure is used when estimting significnce of the lgged regressions (Fig. ). We hve compred with t lest, rtificil projections. 5. Oort, A. H. & Peixóto, J. P. Glol ngulr momentum nd energy lnce requirements from oservtions. Adv. Geophys. 5, 55 9 (98). 6. Trenerth, K. E. Climte dignostics from glol nlysis: Conservtion of mss in ECMWF nlysis. J. Clim., 77 7 (99). ðþ 7 Nture Pulishing Group

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