January 2016 extensive summer melt in West Antarctica favoured by strong El Niño

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1 Received 5 Jul Accepted My 7 Pulished 5 Jun 7 DOI:./ncomms5799 OPEN Jnury extensive summer melt in West Antrctic fvoured y strong El Niño Julien P. Nicols, Andrew M. Vogelmnn, Ryn C. Scott, Aron B. Wilson, Mri P. Cdeddu, Dvid H. Bromwich,5, Johnnes Verlinde, Dn Luin, Lynn M. Russell, Colin Jenkinson 7, Heth H. Powers, Mciej Ryczek, Gregory Stone 7 & Jonthn D. Wille Over the pst two decdes the primry driver of mss loss from the West Antrctic Ice Sheet (WAIS) hs een wrm ocen wter underneth costl ice shelves, not wrmer tmosphere. Yet, surfce melt occurs spordiclly over low-lying res of the WAIS nd is not fully understood. Here we report on n episode of extensive nd prolonged surfce melting oserved in the Ross Se sector of the WAIS in Jnury. A comprehensive cloud nd rdition experiment t the WAIS ice divide, downwind of the melt region, provided detiled insight into the physicl processes t ply during the event. The unusul extent nd durtion of the melting re linked to strong nd sustined dvection of wrm mrine ir towrd the re, likely fvoured y the concurrent strong El Niño event. The increse in the numer of extreme El Niño events projected for the twenty-first century could expose the WAIS to more frequent mjor melt events. Byrd Polr nd Climte Reserch Center, The Ohio Stte University, 9 Crmck Rod, Columus, Ohio, USA. Brookhven Ntionl Lortory, Building 9D, PO Box 5, Upton, New York 97, USA. Scripps Institution of Ocenogrphy, University of Cliforni, Sn Diego, 95 Gilmn Drive, L Joll, Cliforni 99, USA. Argonne Ntionl Lortory, 97 South Css Avenue, EVS/Building, Argonne, Illinois 9, USA. 5 Deprtment of Geogrphy, The Ohio Stte University, Dery Hll, 5 North Ovl Mll, Columus, Ohio, USA. Deprtment of Meteorology nd Atmospheric Science, The Pennsylvni Stte University, 5A Wlker Building, University Prk, Pennsylvni, USA. 7 Austrlin Bureu of Meteorology, GPO Box 9, Melourne, Victori, Austrli. Los Almos Ntionl Lortory, PO Box, Los Almos, New Mexico 755, USA. Correspondence nd requests for mterils should e ddressed to J.P.N. (emil: nicols.7@osu.edu). NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

2 NATURE COMMUNICATIONS DOI:./ncomms5799 Episodes of widespred summer melt hve een spordic in West Antrctic since the phenomenon strted eing monitored from spce in the lte 97s (refs,). Their infrequent occurrence nd lck of roust field mesurements to supplement stellite oservtions leve these melt events insufficiently understood. However, oth the geogrphy nd climte of West Antrctic conspire to mke such events more likely to occur under reltively modest tmospheric wrming. Indeed, y virtue of reltively low elevtions nd frequent intrusions of wrm (nd moist) mrine ir, West Antrctic experiences milder climte thn neighoring Est Antrctic. At the pek of ustrl summer (Decemer Jnury), it is reltively common for surfce melt to occur over the fringe of ice shelves ordering the Amundsen Se,, nd for surfce tempertures over low-lying inlnd res to pproch C (ref. 5). In ddition, the West Antrctic climte is suject to the influence of lrge-scle modes of climte vriility such s the Southern Annulr Mode (SAM) nd the El Niño Southern Oscilltion (ENSO). These modes nd their mutul interctions re responsile for importnt disruptions of the regionl tmospheric circultion tht cn sustin wrm ir dvection towrds the continent for extended periods,. Here we document prominent surfce melt event tht occurred in Jnury nd ffected lrge portion of the Ross Ice Shelf. This event hppened while n importnt field cmpign, the Atmospheric Rdition Mesurement West Antrctic Rdition Experiment (AWARE), ws ongoing in centrl West Antrctic. The oservtions collected during this cmpign provided unique insight into some of the physicl mechnisms governing surfce melting in this otherwise dt-sprse region. In prticulr, these oservtions highlighted the presence of low-level liquid-wter clouds, which my hve ided the rditive heting of the snow surfce. Furthermore, we explore the lrge-scle tmospheric fctors ehind the melt event, nmely the role plyed y the strong 5 El Niño event nd the positive SAM. Building on existing literture nd new idelized climte model simultions, we show tht the El Niño event is likely responsile for setting up the tmospheric circultion pttern tht steered wrm ir towrds the Ross Ice Shelf. We lso show tht the positive SAM countercted to some extent the El Niño influence nd thus likely mitigted the overll mgnitude of the melt event. Results Melt event cptured y stellite nd surfce oservtions. Pssive microwve stellite oservtions (Fig. ) indicte tht surfce melt occurred during one or more dys over rod sector of West Antrctic (termed Ross sector herefter) in Jnury, with up to 5 melt dys over prts of the estern Ross Ice Shelf nd Siple Cost. We ssess the significnce of this event in the context of the entire stellite record (97 ) using two common melt indictors : the melt extent (re of ll grid cells with t lest one dy of melting) nd the melt index (MI) (melt re weighted y durtion of the melting), oth clculted for the Ross sector (red outline in Fig. inset). Bering in mind tht the results re sensitive to the choice of indictor nd melt lgorithm, we estimte tht Jnury ws one of the three lrgest melt events in the Ross sector since 97 (second ehind 99 9 for MI, nd virtul tie for first with Jnury 5 for melt extent). The stellite oservtions were corroorted on the ground y numer of utomtic wether sttions (AWSs) tht recorded ner-surfce tempertures ner or ove C for severl consecutive dys during Jnury (Fig. c). The onset of the melt event on Jnury ws ccompnied y n rupt temperture increse t WAIS Divide nd Byrd, in centrl West Antrctic. The temperture time series from these two sites highlight roughly two phses: Phse ( Jnury), during which the tempertures were t their wrmest; nd Phse (5 Jnury), during which the tempertures grdully decresed towrds their pre-event levels. The trnsition from Phse to Phse is chrcterized y shift of the melt pttern towrds the Trnsntrctic Mountins pprent in the AWS temperture time series nd in the sequence of dily melt mps (Supplementry Fig. ). The Jnury melt event lso coincided with the AWARE field cmpign, during which comprehensive upper-ir, cloud nd surfce rdition oservtions were crried out t the WAIS Divide Field Cmp (str symol in Fig. ). This site ws just downwind nd upslope (, m ove se level) of the min melting region nd ws thus exposed to some of the sme wether conditions, s evidenced y the lrge-scle tmospheric circultion pttern during the melt event (see Fig. nd results section Regionl tmospheric circultion ). The AWARE cmpign ws lso notle in nd of itself for providing the first routine upper-ir oservtions from West Antrctic since 97, when the rdiosonde progrm ended t Byrd Sttion. Cloud nd rditive processes. During the short Antrctic summer, strong onshore winds my y themselves rise the ice sheet s surfce temperture (T s ) up to the melting point (through exchnge of sensile het), especilly t low elevtions. However, T s is ultimtely controlled y the full surfce energy udget (SEB), eing the net of rditive (short- nd longwve) nd turulent (sensile nd ltent) het fluxes. Clouds exert n importnt influence on the SEB y modulting the rditive fluxes, primrily y enhncing downwelling longwve rdition nd ttenuting incoming solr rdition. In prticulr, low-level liquid-ering clouds cn hve determinnt role in either cusing or prolonging melting conditions over ice sheets 5,. Model estimtes from the ERA-Interim Renlysis nd stellite-sed cloud phse retrievls from the Cloud-Aerosol Lidr nd Infrred Pthfinder Stellite Oservtion (CALIPSO) mission for Jnury (Fig. nd Supplementry Fig. nd Supplementry Fig. ) indicte tht liquid-ering clouds were widespred over West Antrctic during the erly stge of the melt event. Note tht prt of the differences etween ERA-Interim (Fig. ) nd CALIPSO (Fig.,c), such s over portions of the Ross Ice Shelf, cn e scried to complete ttenution of the CALIPSO lidr signl through thick upper-level ice cloud lyers. The presence of wrm (tht is, liquid-ering) low-level clouds over the Ross Ice Shelf is lso pprent in Moderte Resolution Imging Spectrordiometer (MODIS) oservtions from Jnury (Supplementry Fig. ). The close mtch etween the pttern of ERA-Interim cloud liquid wter pth (CLWP) nd the contours of the melt re on the Ross Ice Shelf highlights the potentilly importnt role of this type of cloud in mintining melt-prone conditions (compre Supplementry Fig., Supplementry Fig., nd Supplementry Fig. for Jnury ). In ddition, the tongue of CLWP stretching from the estern Ross Ice Shelf to the region of WAIS Divide in Fig. further indictes tht AWARE oservtions my provide insight into the cloud microphysicl properties t lower elevtion. The rdiosonde profiles (Fig.,) from AWARE t WAIS Divide cptured the lrge nd verticlly deep temperture nd moisture perturtions ssocited with the mrine ir intrusion on Jnury. Micropulse lidr mesurements (Fig. d,e) yielded periods of high ttenuted cksctter ( db) nd low depolriztion rtios (o%) elow km, indicting high cloud NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

3 NATURE COMMUNICATIONS DOI:./ncomms5799 S c Phse Phse 9 W W Melt index (dys Mkm ) + WD + BY Amundsen Se 5 W Ross Se Melt dys Trnsntrctic Mountins Ross sector NDJF melt extent Jnury melt index Decemer melt index Yer + EZ Siple cost + SA Ross Ice Shelf + MA + EL Melt extent (Mkm ) Temperture ( C) WAIS Divide/Kominko Slde (WD, m) Byrd (BY, 5 m) Elizeth (EZ, 59 m) Srin (SA, m) Eline (EL, m) Mrgret (MA, 7 m) Dte (Jnury ) Figure The Jnury melt event cptured y stellite nd surfce oservtions. () Mp of West Antrctic showing the numer of melt dys in Jnury estimted from pssive microwve stellite oservtions overlid on MODIS mosic imge. The white crosses denote the loctions of the utomtic wether sttions (AWSs) shown in c. The inset mp outlines the oundries of the ckground MODIS imge (lck line) nd Ross sector (red line). () Time series of Decemer nd Jnury melt index (rs) nd Novemer Ferury melt extent (lue crosses) clculted for the Ross Sector (see inset mp in ) nd estimted from stellite-sed dily melt dt. The yer refers to Jnury (for exmple, the 99 melt indices re Decemer 99 nd Jnury 99). No dt re shown for 9 owing to insufficient oservtions. (c) Time series of -minute ner-surfce tempertures from six West Antrctic AWSs whose loctions re shown in. AWS nme revition nd elevtion ove se level re given in prentheses. Ornge shding highlights tempertures ove C (surfce melting cn occur despite elow-freezing ner-surfce temperture ecuse of rditive heting). liquid wter content nd low ice wter content 7 (mixed-phse clouds). The oserved CLWP (Fig. c) ws frequently within g m, tht is, the rnge where the cloud rditive enhncement effect previously oserved over Greenlnd 5 occurs. In this rnge, the clouds re thick enough to enhnce the downwelling longwve rdition (Fig. ) ut thin enough to lso llow shortwve rdition to rech the surfce (Fig. ). The CLWP ws within this rnge % of the time during Jnury, suggesting tht this enhncement mechnism contriuted to the melt event. This is further supported y the frequent nd widespred occurrence of clouds with CLWP within g m simulted y ERA-Interim during the sme period (Supplementry Fig. nd Supplementry Fig. 5). However, we lso notice significnt frequency of CLWP g m (Fig. c), under which shortwve flux is ttenuted nd longwve flux is similr to lckody rdition t the cloud effective temperture. These opticlly thicker clouds represent contrst to the Greenlnd cloud rditive enhncement effect in tht they signify more prominent role of therml lnketing s consequence of the wrm ir dvection (Fig.,). The totl SEB (Fig. e) shows mrked increse in the net energy input into the snowpck (up to W m ), minly ttriutle to enhnced downwelling longwve rdition (Fig.,c). This dditionl energy input is lso pprent in the stellite rightness tempertures (Fig. e). Regionl tmospheric circultion. We trce the immedite cuses of the melt event to the presence of n mplified high-pressure ridge (locking high) over the 9 W sector of the Southern Ocen (Fig. 5 c). By creting prominent dent in the circumpolr westerly flow, this ridge generted strong north-south dvection of wrm mrine ir towrds West Antrctic. The ridge ws strongest during Jnury (Phse ) ut persisted through Jnury (Fig. 5c), mintining wrm conditions fvourle to surfce melt in the Ross sector (Phse ). Positive se surfce temperture (SST) nomlies of C ner5 S, W (Fig. 5) my hve lso provided dditionl het to the ir trvelling south (note tht the positive geopotentil height nomlies ner S, 9 W fvour nticlockwise motion). Dt from ERA-Interim suggest tht rin fell over prts of the Ross Ice Shelf t the eginning of the event (Supplementry Fig. ), which my hve preconditioned the snow surfce for prolonged melting. Although the renlysis dt should e treted with cution, drizzle ws oserved t WAIS Divide on Jnury (Supplementry Fig. 7) nd rin ws witnessed y one field prty on the Km Ice Strem (lck tringle in Fig. ) on Jnury (Dr Huw Horgn, Victori University of Wellington, personl communiction). Lrge-scle tmospheric context. On roder scle, the melt event occurred during one of the strongest El Niño events on NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

4 NATURE COMMUNICATIONS DOI:./ncomms5799 Altitude (km) S A 9 W WD C D c B 5.S 5W 79.S.9W.S 5.E 7.S 7.E 5.5S 57.E 7.S.E 5.5S 7.79E C.S.W 5 5 g m B 79.S 9.9W 5 W 7.S 5.W Altitude (km) W A.5S 5.W 7.9S.W N/A D Unknown Ice Wter Oriented ice Figure Cloud liquid wter simulted y ERA-Interim nd detected y CALIPSO lidr on Jnury. () Mp of cloud liquid wter pth (CLWP) simulted y ERA-Interim for : UTC on Jnury. The lck str denotes the loction of WAIS Divide. The two pink lines represent the CALIPSO stellite trjectories corresponding to the verticl profiles shown in,c. The points lelled A, B, C nd D long these trjectories re identicl to those shown t the ottom of the verticl profiles. The lck tringle denotes where rin ws witnessed y field prty on Jnury (Dr Huw Horgn, Victori University of Wellington, personl communiction). (,c) Verticl profiles of ice/wter phse retrievls from CALIPSO Lidr Level dt products over West Antrctic for two time windows on Jnury (:5 :9 UTC in nd 7: 7: UTC in c). The thin lck line represents the surfce topogrphy. c Phse e Height (km). Mixing rtio (g/kg) Hours (UTC) on Jn Depolriztion rtio (%) Height (km). 7 9 Dys of Jnury 5 Bcksctter (db). Height (km) d. 5 Dys of Jnury CLWP. Dys of Jnury 5 PWV CLWP (g m ) Height (km) Temperture perturtion (K) Phse. PWV (cm) Figure Cloud, moisture nd temperture oservtions t WAIS Divide during Jnury. (,) Verticl profiles of temperture perturtion nd wter vpour mixing rtio sed on -hourly rdiosoundings. The temperture perturtion is clculted s the difference etween the mesured temperture nd the men temperture during the period spnning Decemer 5 to Jnury (durtion of the field cmpign). (c) Precipitle wter vpour (PWV) nd cloud liquid wter pth (CLWP) estimted from microwve rdiometer mesurements. (d,e) Verticl profiles of ttenuted cksctter nd depolriztion rtio sed on micropulse lidr mesurements from Jnury. For ll verticl profiles, the height is with respect to the ground level (, m ove se level). NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

5 NATURE COMMUNICATIONS DOI:./ncomms5799 LW rdition (W m ) SW rdition (W m ) c Net rdition (W m ) d Net turulent (W m ) e Totl net ground (W m ) 5 5 5, Downwrd Upwrd Downwrd Upwrd SSMIS T Surfce energy gin Surfce energy loss Dys of Jnury Phse Figure Surfce energy udget t WAIS Divide in Jnury. () Downwrd (LWk) nd upwrd (LWm) longwve rdition fluxes. () Downwrd (LWk) nd upwrd (LWm) shortwve rdition fluxes. (c) Net rdition flux (LWk LWm þ SWk SWm). (d) Net turulent flux, clculted s the sum of sensile (SHF) nd ltent (LHF) fluxes, where positive is energy trnsfer wy from the surfce to the tmosphere. (e) Totl net energy flux into the ground (snowpck) clculted s net rdition minus net turulent fluxes. The green crosses represent the SSMIS 9 GHz horizontlly polrized rightness tempertures mesured from spce t the loction of WAIS Divide. record (Figs 5 nd ). Positive (nticyclonic) geopotentil height nomlies in the South Pcific, such s those oserved in Jnury (Fig. 5), re typicl signture of El Niño teleconnections, s seen oth in oservtions,9 nd climte model simultions,. This type of tmospheric pttern promotes wrm ir dvection to the Ross sector, which explins why surfce melt in this re tends to e ssocited with El Niño-like conditions. For instnce, the prominent melt events of Decemer 9 nd Decemer 99 Jnury 99 (see Fig. ) oth occurred in conjunction with El Niño conditions, chrcterized y negtive Equtoril Southern Oscilltion Index (SOI) in Fig.. Conceptully, the fct tht circultion pttern resemling the El Niño teleconnection ws present during the SSMIS 9GHz T (K) melt event nd tht this pttern is fvourle to wrm conditions over the Ross Ice Shelf points towrds cusl link etween the 5 El Niño nd the Jnury melt event. However, in prctice (sed on oservtions ville since 979), the reltionship etween the two phenomen remins complex (see Fig. 7 nd relted discussion elow). The SAM, which chrcterizes the strength of the westerly winds round Antrctic, is n importnt modultor of the tropicl influence in the South Pcific sector of the Southern Ocen 7. During Novemer 5 Jnury, the SAM Index remined predominntly positive (Fig. ), indicting stronger-thn-norml westerly winds. This is reflected in the negtive geopotentil height nomlies over Antrctic in Fig. 5. The conjunction of strong El Niño nd positive SAM phse ws unusul since, in ustrl summer, the ltter is most often ssocited with L Niñ-like conditions, (conversely, negtive SAM phse most often occurs in conjunction with El Niño-like conditions). Furthermore, positive SAM phse hmpers meridionl het exchnge etween middle nd high ltitudes 7, nd is thus generlly not conducive to surfce melt in West Antrctic. Thus, if nything, the positive SAM phse tht previled efore nd during the melt event should hve fvoured colder-thn-norml, not wrmer-thn-norml, conditions in West Antrctic. Contriution of El Niño nd the SAM. Properly understnding the mechnisms responsile for the Jnury melt event requires investigting the potentil roles of El Niño nd the SAM. This in turn cn provide insight into the recurrence of such event in the future (see Discussion). Figure 7 provides mens to visulize the three-wy reltionships etween West Antrctic summer melt, the SAM, nd the ENSO phenomenon (represented y the Equtoril SOI) since 979. Note tht, in this figure, the two melt indices re clculted for Decemer Jnury, nd the two climte indices re Novemer Jnury verges. Figure 7 shows tht, in generl, less melt tends to occur during L Niñ-like conditions (SOI ) nd positive SAM phse, wheres more melt tends to occur during El Niño-like conditions (SOI o ) nd negtive SAM phse. This qulittive ssessment is confirmed y tllying positive nd negtive melt nomlies depending on the sign of the two climte indices (Supplementry Tle ). It is worth noting tht the directions of the reltionships re similr to those previously found etween Antrctic-wide melt nd the SAM Index nd SOI. These reltionships re y no mens simple. For exmple, not ll mjor El Niño events re ccompnied y widespred surfce melt in West Antrctic (for exmple, 997 9); not ll prominent West Antrctic melt events coincide with strong El Niño events (for exmple, 5); nd the mgnitude of West Antrctic melt does not scle with the intensity of El Niño events. Accordingly, it is not possile to estlish with certinty whether the 5 El Niño cused (in deterministic sense) the Jnury melt event, prolem inherent to wether nd climte phenomen. It is not uncommon for the polr jet round Antrctic to exhiit lrge menders, giving rise to wrm mrine ir intrusions, even in the sence of n El Niño event. Following proilistic pproch, we seek rther to ssess the likelihood of the Jnury melt event occurring given the concurrent strong El Niño nd positive SAM conditions. The sttisticlly smll numer of El Niño events (especilly of strong events such s 9, nd 5 ) oserved since 979 does not permit roust sttisticl nlysis. However, climte model simultions cn llevite this issue y generting lrger smples of events. NATURE COMMUNICATIONS :5799 DOI:./ncomms

6 NATURE COMMUNICATIONS DOI:./ncomms5799 E S hp temperture ( C) 9S E E W W c Dys of Jnury 5E 5W E W 9E 9W S W SST nomlies ( C) E Dily Z5 nomlies (m) N W Phse Phse E E W W Figure 5 Atmospheric circultion ssocited with the Jnury melt event. () Mp showing the ir temperture (shded) nd wind vectors t 5 hp t UTC on Jnury sed on ERA-Interim. The thick lck line outlines the 5, meter geopotentil height contour seprting cold polr ir msses from wrmer mid-ltitudes ir msses. () Se surfce temperture nomlies (shded) overlid with ERA-Interim 5 hp geopotentil height (Z5) nomlies (contour lines) in Jnury. Solid red, dshed lue nd solid lck contour lines represent (respectively) positive, negtive nd zero Z5 nomlies. Anomlies re clculted with respect to the 97 period. (c) Hovmo ller digrm showing dily meridionlly verged Z5 nomlies within ltitudes 5 S during Jnury. The nomlies re with respect to the 979 Jnury monthly men. SAM (NDJ) Yer Equtoril SOI (NDJ) Yer Figure Time series of Novemer-Jnury verged climte indices. () Southern Annulr Mode Index. () Equtoril Southern Oscilltion Index. The series spn 957. The yer corresponds to the month of Jnury. The lst dt point is for Novemer 5-Jnury. Modelling experiment set-up. We used the Community Atmosphere Model (CAM) version to conduct set of four 5-yer idelized simultions (see Methods for detils). Three simultions were forced with nnully repeting SSTs chrcteristic of pst strong El Nin o events, representing totl of 5 El Nin o events. The fourth simultion ws forced with nnully repeting climtologicl SSTs to serve s control run. A model SAM index ws clculted for ech simultion sed on principl component nlysis of Southern Hemisphere monthly 5 hp geopotentil height nomlies. Here gin, we considered the verge SAM Index for Novemer Jnury. Estimting surfce melt occurrence in the CAM model cn e prolemtic s the process is ffected y model grid resolution, model temperture ises nd model deficiencies in the plcement of key tmospheric fetures5. To circumvent these issues, we used nomlies in the model ner-surfce ir temperture s n indictor of melt-prone conditions. We clculted these nomlies for ech simultion y sutrcting the long-term monthly mens of the control simultion, nd considered the men nomlies for Decemer Jnury sptilly verged over rod Ross sector of West Antrctic (75 9 S; 9 W). We lelled these nomlies wrm events or cold events depending on their sign. Results from model simultions. Bsed on the model SAM Index nd series of wrm nd cold events, we generted contingency tle (Tle ) tllying the numer of events per type (wrm or cold) nd phse of SAM (positive, negtive or neutrl) cross ll simulted mjor El Nin o events. Out of 5 El Nin os, wrm events occur times (7.%) versus times (.9%) for cold events. This result is consistent with the known positive impct of the El Nin o teleconnection pttern in the South Pcific on West Antrctic tempertures lredy mentioned. Out of the wrm events, 5 (.9%) occur during negtive SAM phse. Out of the cold events, (.5%) occur during positive SAM phse. The chi-squre sttistic is significnt t Po., mening tht the type of event is significntly dependent on the comined sttes of El Nin o nd SAM. Such dependence confirms findings from previous literture7,9,. Expnding on the previous nlysis, we lso find tht 9 out of the top wrmest events occur during negtive or neutrl SAM phse, while 9 out of the top coldest events occur during positive or neutrl SAM phse. This demonstrtes tht the Ross sector is much more likely to experience conditions fvourle to NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

7 NATURE COMMUNICATIONS DOI:./ncomms5799 Melt index Melt extent SAM index Equtoril SOI Std. devition Yers Figure 7 9 stndrdized SAM Index, Equtoril SOI, West Antrctic melt index nd melt extent nomlies. Yers re rnk-ordered from lowest to highest melt extent. For ech yer (x xis), the plot displys (in the following order) the melt index (light lue), the melt extent (drk lue), the SAM Index (ornge), nd the equtoril SOI (red). The plot is dpted from ref.. Tle Contingency tle tllying the count of Decemer Jnury wrm nd cold events s function of the phse of the Novemer Jnury Southern Annulr Mode in idelized CAM model simultions. Type of temperture nomly * Wrm Cold Row totls þ SAM w (7.) z (.) z Neutrl SAM w (.5) z 5 (5.9) z 9 SAM w 5 (.7) z (.) z 5 Column totls 5 *The temperture nomlies re Decemer Jnury mens, sptilly verged over the 75 9 S, 9 W sector of West Antrctic, nd clculted with respect to the 5-yer men of the control simultion. The wrm nd cold columns correspond to positive nd negtive temperture nomlies, respectively. wthe SAM index represents the Novemer Jnury verge. The positive ( þ SAM), neutrl, nd negtive ( SAM) phses re defined sed on the ±.5 stndrd devition of the SAM index. zexpected counts (which ssume independence etween the two vriles) re given in prentheses. The chi-squre sttistic is.7. The P vlue is.5. surfce melting during El Niño events when negtive or neutrl SAM is present. Given tht the SAM ws in predominntly positive phse efore nd during the Jnury melt event, our model results suggest tht the stte of the SAM likely mitigted the mgnitude (rel extent, durtion) of surfce melt in West Antrctic during the ustrl summer of 5. In other words, the 5 melt seson would likely hve een more prominent hd the SAM een in negtive or neutrl phse, more commonly ssocited with El Niño events. Discussion Further reserch is needed to etter understnd the vrious mechnisms ehind mjor West Antrctic melt events nd to ccurtely predict their future occurrence. Accurte prediction is contingent on the ility of climte models to resolve the rod rnge of fctors responsile for these events, from the lrge-scle climte drivers to the regionl tmospheric circultion to the microphysicl nd rditive processes. Among them, the simultion of ENSO nd its teleconnections, nd the representtion of high-ltitude mixed-phse clouds (such s those oserved t WAIS Divide in Jnury ) re two key res in need of improvement in climte models 5,,. Future chnges in the intensity of ENSO events re currently estimted with greter confidence thn ENSO-relted long-distnce climte chnges, owing in lrge prt to model difficulties in simulting the present-dy men stte climte,. The frequency of extreme El Niño events is projected to increse over the course of the twenty-first century,. Given the role of El Niño-relted tmospheric circultion in promoting wrm ir dvection to the Ross sector, greter numer of extreme El Niño events could foster more frequent mjor melt events in this re. One source of uncertinty lies in the modulting effect of the SAM on this teleconnection. This effect hs ecome less cler fter Jnury. Indeed, the conjunction of strong El Niño/strong teleconnection on the one hnd, nd strong westerlies on the other hnd ws t odds with known tropicl-high ltitude interctions 7,,,. Yet, this scenrio my grow more likely in the future 5 s nthropogenic forcings re expected to continue fvouring positive SAM conditions in ustrl summer. Finlly, the Jnury melt event demonstrtes tht the present-dy climte of West Antrctic lredy llows for extensive surfce melt to occur occsionlly. In this regrd, two recent modelling studies 7, hve come to rther different conclusions out the future evolution of surfce melt over the Ross Ice Shelf nd its impct on the WAIS mss lnce. One study 7 suggests tht the phenomenon will remin miniml throughout the twenty-first century, nd is, therefore, unlikely to contriute significntly to the destiliztion of the WAIS. The other study projects tht the Ross Ice Shelf will experience extensive surfce melt nd retret sustntilly y, therey ccelerting the disintegrtion of the WAIS. In this context, the extent to which the Jnury event is precursor of the climte of West Antrctic in the coming decdes is uncertin. But our study highlights some of the key mechnisms tht need to e resolved to ddress this question. Methods Stellite-sed melt dt. Surfce melt over ice sheets cn e esily detected from spce s the ppernce of liquid wter in the snowpck cuses shrp increse in microwve rightness temperture. Here we estimted surfce melt occurrence using dily stellite rightness temperture (T) dt otined from the Ntionl Snow nd Ice Dt Center. The dt consisted of twice-dily oservtions (from scending nd descending stellite psses) from the following sensors: The Scnning Microwve Multichnnel Rdiometer (SMMR) onord the Nimus-7 stellite (97 97); the Specil Sensor Microwve/Imger (SSM/I) onord the Defense Meteorologicl Stellite Progrm (DMSP) F-, F-, nd F stellites (97 9); nd the Specil Sensor Microwve Imger Sounder (SSMIS) onord the DMSP F-7 stellite ( present). We used horizontlly polrized T dt in the K-nd ( GHz for SMMR, 9 GHz for SSM/I SSMIS), commonly used for NATURE COMMUNICATIONS :5799 DOI:./ncomms

8 NATURE COMMUNICATIONS DOI:./ncomms5799 melt detection over ice sheets,,9. The dt were provided on Ntionl Snow nd Ice Dt Center s Southern Hemisphere EASE-Grid with 5 5 km grid cells. We filled the gps in the SMMR dt (ville only every other dy) y linerly interpolting the dt from the two djcent dys. We filled the gps in the SSM/I SSMIS dt only if they did not exceed one dy. To ensure consistency etween the different sensors, we djusted ll SMMR nd SSM/I T dt to SSMIS F-7 using the regression coefficients derived y refs. The only exception ws for the djustment etween SSM/I F- nd SSMIS F-7, for which we derived our own coefficients (see Supplementry Fig. ). All coefficients used in our djustment procedure (long with their references) re listed in Supplementry Tles nd. For given grid cell nd given dy, we determined tht melt ws occurring s soon s one of the two dily T oservtions exceeded threshold vlue (T melt ) defined s T melt ¼ T ref þ DT, where DT ¼ K nd T ref is reference temperture. T ref ws clculted s the -month verge from April Mrch fter filtering out ll melt dys s in ref.. When T ref could not e clculted (for exmple, t the eginning/end of stellite record), we used the T ref vlue from the previous or following yer (whichever mtched the sensor/stellite). This overll lgorithm ws shown to e prticulrly well suited for detecting melt in dry-snow res, such s found in the West Antrctic interior. The MI shown in Fig. ws clculted s follows: MI ¼A X N i¼ m i Here, A is the re of pixel, m i is the numer of melt dys during month for pixel i, nd N is the numer of pixels inside the Ross sector (red outline in Fig. ). Oservtions from West Antrctic Ice Sheet Divide. The 5 AWARE field cmpign rn from Decemer 5 through Jnury, nd deployed ARM Moile Fcility instruments t WAIS Divide. Estimtes of upper-ir temperture nd moisture were otined from six-hourly rwinsonde lunches 5 nd continuous retrievls from profiling microwve rdiometer (MWR),7. A micropulse lidr,9 mesured cloud lyer elevtion nd thermodynmic phse using oth direct nd cross-polrized lser returns. Column-integrted precipitle wter vpour nd CLWP were retrieved using the comined dt from the profiling MWR nd two-chnnel MWR,5. Upwelling shortwve nd longwve rditive flux components were mesured y Surfce Energy Blnce system 5. Downwelling flux components were mesured y Sky Rdition System 5, which consists of norml incidence pyrheliometer nd shded pyrnometers nd pyrgeometers. The glol downwelling shortwve flux (S down ) ws computed s follows: S down ¼ S dir cosy z þ S diff Here, S dir is the direct solr em from the norml incidence pyrheliometer, y z is the solr zenith ngle nd S diff is the diffuse flux from shded pyrnometer. Surfce ltent nd sensile het fluxes were derived using the lgorithm of ref. 5 nd surfce mesurements of temperture, moisture nd wind speed from the ARM surfce meteorologicl instrumenttion 5. The velocity roughness length used in the lgorithm ws derived for the WAIS site using momentum fluxes from n Eddy Correltion Flux Mesurement System 55. Model simultions. A sic description of our modelling experiment is lredy provided in the min text for clrity. A few dditionl detils re given here. We conducted four 5-yer simultions using the CAM version (ref. 5) with se-surfce conditions specified s in refs,. Three simultions were forced with cyclic, nnully repeting -month glol SSTs sed, respectively, on the mjor El Niño events of 9, 997 9, nd scled composite of other historicl El Niño events (see detils in ref. ). The fourth simultion (used s control) ws forced with nnully repeting SSTs sed on climtologicl monthly men SSTs for the period 9. The SAM ws defined in ech simultion using the first principl component of monthly men 5 hp geopotentil height nomlies polewrd of S (these nomlies were otined y removing the 5-yer monthly mens of the control simultion). CALIPSO cloud prticle phse. The verticl profiles of cloud prticle phse shown in Fig.,c re sed on mesurements from the Cloud-Aerosol Lidr with Orthogonl Polriztion (CALIOP) flying onord the CALIPSO stellite. Cloud phse retrievls re prt of CALIPSO Lidr Level Verticl Feture Msk (VFM) products (version.) ville from NASA s Atmospheric Dt Center ( Detils out the cloud phse discrimintion lgorithm re given in ref. 57. Technicl documenttion out the VFM products is provided in ref. 5. Regions denoted s unknown in Fig.,c re where cloud phse determintion is miguous. In this regrd, note tht the lgorithm used for VFM version products does not ttempt to identify mixed-phse clouds. Regions denoted s N/A (not pplicle) re clssified s cler ir in the VFM products. These re regions where no fetures (cloud or erosols) re detected either ecuse none re present or ecuse of lidr cksctter signl ttenution through overlying cloud lyers. Other dt. The AWS temperture estimtes used in Fig. c re -minute dt otined from the Antrctic Meteorologicl Reserch Center t the University of Wisconsin-Mdison (ftp://mrc.ssec.wisc.edu/pu/ws/min/rdr/). On these sttions, the temperture sensor is t height of three meters ove the surfce. The CLWP dt used in Fig. nd the temperture, geopotentil height, nd wind dt used in Fig. 5 c re from the ERA-Interim Renlysis 59 ( Note tht CLWP corresponds to ERA-Interim totl cloud liquid wter field. The SST dt used in Fig. 5 re from NOAA s Optimum Interpoltion / Degree Dily Se Surfce Temperture Anlysis, Version (ref. ; The SAM Index used in Fig. is courtesy of G.J. Mrshll ( sm.html). The Equtoril SOI used in Fig. is provided y NOAA s Climte Prediction Center ( Code vilility. The source code of the CAM. glol tmospheric model cn e otined free of chrge through the Community Erth System wepge ( Dt vilility. All oservtions from the AWARE cmpign re ville from the ARM Dt Discovery wesite ( For quick ccess, users cn enter AWARE in the serch ox (top-left corner of the pge) nd click on AWR AWARE (ARM West Antrctic Rdition Experiment in the serch results. References. Liu, H., Wng, L. & Jezek, K. C. Sptiotemporl vritions of snowmelt in Antrctic derived from stellite scnning multichnnel microwve rdiometer nd Specil Sensor Microwve Imger dt (97 ). J. Geophys. Res., F ().. Tedesco, M. Assessment nd development of snowmelt retrievl lgorithms over Antrctic from K-nd spceorne rightness temperture (979 ). Remote Sens. Env., (9).. Nicols, J. P. & Bromwich, D. H. Climte of West Antrctic nd influence of mrine ir intrusions. J. Clim., 9 7 ().. Trusel, L. D., Frey, K. E., Ds, S. B., Kuipers Munneke, P. & vn den Broeke, M. R. Stellite-sed estimtes of Antrctic surfce meltwter fluxes. Geophys. Res. Lett., (). 5. Reusch, D. B. & Alley, R. B. A 5-yer West Antrctic climtology from six utomtic wether sttion temperture nd pressure records. J. Geophys. Res. 9, D ().. Bromwich, D. H., Monghn, A. J. & Guo, Z. Modeling the ENSO modultion of Antrctic climte in the lte 99s with the Polr MM5. J. Clim. 7, 9 (). 7. Fogt, R. L., Bromwich, D. H. & Hines, K. M. Understnding the SAM influence on the South Pcific ENSO teleconnection. Clim. Dyn., 5557 ().. Ding, Q., Steig, E. J., Bttisti, D. S. & Wllce, J. M. Influence of the tropics on the Southern Annulr Mode. J. Clim. 5, (). 9. Rphel, M. N. et l. The Amundsen se low: vriility, chnge, nd impct on Antrctic climte. Bull. Am. Meteorol. Soc. 97, ().. Wilson, A. B., Bromwich, D. H. & Hines, K. M. Simulting the mutul forcing of nomlous high southern ltitude tmospheric circultion y El Niño flvors nd the Southern Annulr Mode. J. Clim. 9, 9 9 ().. Witze, A. Antrctic clouds studied for first time in five decdes. Nture 59, ().. Arking, A. The rditive effects of clouds nd their impct on climte. Bull. Am. Meteorol. Soc. 7, 795 (99).. Bromwich, D. H. et l. Tropospheric clouds in Antrctic. Rev. Geophys. 5, RG ().. Scott, R. C., Luin, D., Vogelmnn, A. M. & Kto, S. West Antrctic Ice Sheet cloud cover nd surfce rdition udget from NASA A-trin stellites. J. Clim. (in the press) doi:.75/jcli-d Bennrtz, R. et l. July Greenlnd melt extent enhnced y low-level liquid clouds. Nture 9, ().. Vn Tricht, K. et l. Clouds enhnce Greenlnd ice sheet meltwter runoff. Nt. Commun. 7, (). 7. Sssen, K. The polriztion lidr technique for cloud reserch: review nd current ssessment. Bull. Am. Meteorol. Soc. 7, (99).. Doyle, S. H. et l. Amplified melt nd flow of the Greenlnd ice sheet driven y lte-summer cyclonic rinfll. Nt. Geosci., 7 5 (5). 9. Turner, J. The El Niño-southern oscilltion nd Antrctic. Int. J. Climtol., ().. Wilson, A. B., Bromwich, D. H., Hines, K. M. & Wng, S. El Niño flvors nd their simulted impcts on tmospheric circultion in the high southern ltitudes. J. Clim. 7, ().. Hurwitz, M. M. et l. Extr-tropicl tmospheric response to ENSO in the CMIP5 models. Clim. Dyn., 7 7 (). NATURE COMMUNICATIONS :5799 DOI:./ncomms5799

9 NATURE COMMUNICATIONS DOI:./ncomms5799. Tedesco, M. & Monghn, A. J. An updted Antrctic melt record through 9 nd its linkges to high-ltitude nd tropicl climte vriility. Geophys. Res. Lett., L5 (9).. Schneider, D. P., Okumur, Y. & Deser, C. Oserved Antrctic internnul climte vriility nd tropicl linkges. J. Clim. 5, ().. Trenerth, K. E., Fsullo, J. T. & Shepherd, T. G. Attriution of climte extreme events. Nt. Clim. Chnge 5, 75 7 (5). 5. Lndrum, L., Hollnd, M. M., Schneider, D. P. & Hunke, E. Antrctic se ice climtology, vriility, nd lte twentieth-century chnge in CCSM. J. Clim. 5, 7 ().. Wilson, A. B. Using the NCAR CAM to Confirm SAM s Modultion of the ENSO Teleconnection to Antrctic nd Assess Chnges to this Interction during Vrious ENSO Flvor Events. PhD disserttion, Deprtment of Geogrphy, The Ohio Stte University (). 7. Nele, R. B. et l. The men climte of the Community Atmosphere Model (CAM) in forced SST nd fully coupled experiments. J. Clim., 55 ().. Lenerts, J. T. M., Vizcino, M., Fyke, J., vn Kmpenhout, L. & vn den Broeke, M. R. Present-dy nd future Antrctic ice sheet climte nd surfce mss lnce in the Community Erth System Model. Clim. Dyn. 7, 7 (). 9. L Heureux, M. L. & Thompson, D. W. J. Oserved reltionships etween the El Niño--Southern Oscilltion nd the extrtropicl zonl-men circultion. J. Clim. 9, 7 7 ().. Stmmerjohn, S. E., Mrtinson, D. G., Smith, R. C., Yun, X. & Rind, D. Trends in Antrctic nnul se ice retret nd dvnce nd their reltion to El Niño-Southern Oscilltion nd Southern Annulr Mode vriility. J. Geophys. Res., CS9 ().. Lwson, R. P. & Gettelmn, A. Impct of Antrctic mixed-phse clouds on climte. Proc. Ntl Acd. Sci. USA, 5 ().. Ci, W. et l. ENSO nd greenhouse wrming. Nt. Clim. Chnge 5, 9 59 (5).. Lngenrunner, B. & Neelin, J. D. Anlyzing ENSO teleconnections in CMIP models s mesure of model fidelity in simulting precipittion. J. Clim., ().. Ci, W. et l. Incresing frequency of extreme El Niño events due to greenhouse wrming. Nt. Clim. Chnge, (). 5. Wng, G. & Ci, W. Climte-chnge impct on the th-century reltionship etween the Southern Annulr Mode nd glol men temperture. Sci. Rep., 9 ().. Previdi, M. & Polvni, L. M. Climte system response to strtospheric ozone depletion nd recovery. Q. J. R. Meteorol. Soc., 9 (). 7. Trusel, L. D. et l. Divergent trjectories of Antrctic surfce melt under two twenty-first-century climte scenrios. Nt. Geosci., 97 9 (5).. DeConto, R. M. & Pollrd, D. Contriution of Antrctic to pst nd future se-level rise. Nture 5, 5997 (). 9. Picrd, G. & Fily, M. Surfce melting oservtions in Antrctic y microwve rdiometers: correcting -yer time series from chnges in cquisition hours. Remote Sens. Env., 5 ().. Jezek, K. C. et l. Comprison Between SMMR nd SSM/I Pssive Microwve Dt Collected Over the Antrctic Ice Sheet, Byrd Polr Reserch Center Technicl Report 9 (Byrd Polr Reserch Center, The Ohio Stte University, 99).. Adlti, W., Steffen, K., Otto, C. & Jezek, K. C. Comprison of rightness tempertures from SSMI instruments on the DMSP F nd FII stellites for Antrctic nd the Greenlnd ice sheet. Int. J. Remote Sens., 9 (995).. Stroeve, J., Mslnik, J. & Xioming, L. An intercomprison of DMSP F- nd F-derived se ice products. Remote Sens. Environ., (99).. Torinesi, O., Fily, M. & Genthon, C. Vriility nd trends of the summer melt period of Antrctic ice mrgins since 9 from microwve sensors. J. Clim., 7 ().. Mther, J. H. & Voyles, J. W. The ARM climte reserch fcility: A review of structure nd cpilities. Bull. Am. Meteorol. Soc. 9, 77 9 (). 5. Coulter, R., Kyrouc, J. & Holdridge, D. ARM: Blloon-Borne Sounding System (BBSS): Visl-Processed Winds, Press., Temp, nd RH (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99).. Cdeddu, M. P., Liljegren, J. C. & Turner, D. D. The Atmospheric rdition mesurement (ARM) progrm network of microwve rdiometers: instrumenttion, dt, nd retrievls. Atmospheric Mes. Tech., 59 7 (). 7. Cdeddu, M. ARM: G-nd ( GHz) Vpor Rdiometer Profiler: 5 Microwve Brightness Tempertures From 7. to. GHz (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), ).. Flynn, C. J., Mendoz, A., Zheng, Y. & Mthur, S. Novel polriztionsensitive micropulse lidr mesurement technique. Opt. Express 5, (7). 9. Mrtin, T., Ermold, B., Koontz, A., Flynn, C. & Coulter, R. ARM: Micropulse Lidr (MPL) Polrized, Fst Smpling: Cloud Bse nd Top Heights (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), ). 5. Cdeddu, M. ARM: Microwve Wter Rdiometer (MWR): Wter Liq. nd Vpor Along Line of Sight (LOS) Pth (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99). 5. Cook, D. ARM: Surfce Energy Blnce System. (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99). 5. Doorghi, M. et l. ARM: Sky Rdition Sensor: -Second Downwelling Irrdinces (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99). 5. Andres, E. L. et l. Prmetrizing turulent exchnge over summer se ice nd the mrginl ice zone. Q. J. R. Meteorol. Soc., 97 9 (). 5. Holdridge, D. & Kyrouc, J. ARM: ARM-Stndrd Meteorologicl Instrumenttion t Surfce (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99). 55. Cook, D. ARM: ECOR: Surfce Verticl Fluxes of Momentum, Sensile Het, nd Ltent Het, -Min Avg (Atmospheric Rdition Mesurement (ARM) Archive, Ok Ridge Ntionl Lortory (ORNL), 99). 5. Gent, P. R. et l. The community climte system model version. J. Clim., (). 57. Hu, Y. et l. CALIPSO/CALIOP cloud phse discrimintion lgorithm. J. Atmos. Ocen. Technol., 9 9 (9). 5. NASA Atmospheric Dt Center. CALIPSO Qulity Sttements Lidr Level Verticl Feture Msk Product Version Releses:.,. (). 59. Dee, D. P. et l. The ERA-Interim renlysis: configurtion nd performnce of the dt ssimiltion system. Q. J. R. Meteorol. Soc. 7, 5597 ().. Reynolds, R. W. & Bnzon, V. F. NOAA Optimum Interpoltion / Degree Dily Se Surfce Temperture (OISST) Anlysis, Version (NOAA Ntionl Climtic Dt Center, ).. Mrshll, G. J. Trends in the Southern Annulr Mode from oservtions nd renlyses. J. Clim., ().. Hrn, T., Bohlnder, J., Scmos, T., Pinter, T. & Fhnestock, M. MODIS Mosic of Antrctic -9 (MOA9) Imge Mp (Ntionl Snow nd Ice Dt Center, ). Acknowledgements J.P.N., A.B.W., nd D.H.B. were supported y Ntionl Science Foundtion (NSF) grnts PLR- nd PLR-95. A.M.V. is supported y the U.S. Deprtment of Energy (DOE) under Contrct DE-SC7. R.C.S. ws supported y NASA grnt NNX5AN5H. M.P.C. is supported y the DOE under Contrct DE-AC-CH57. J.V. ws supported y NSF grnt PLR-95. AWARE is supported y the DOE ARM Climte Reserch Fcility nd NSF Division of Polr Progrms. We thnk WAIS Divide Sttion Mnger E. Bezley nd her crew for their field support. We thnk the ARM dt mngement nd dt qulity personnel (prticulrly N. Keck, C. Sturt, J. King nd A. Theisen) for timely review nd delivery of the WAIS dt, nd instrument mentors (prticulrly D. Cook nd J. Kyrouc) for ssistnce with the dt. CAM model simultions were conducted t the Ohio Supercomputer Center. The University of Wisconsin-Mdison Automtic Wether Sttion Progrm, provider of the AWS dt, is supported y NSF Grnt ANT-5. This pper is contriution 57 of the Byrd Polr nd Climte Reserch Center. Author contriutions J.P.N. processed the SMMR-SSM/I-SSMIS, CALIPSO nd AWS dt nd some of the ERA-Interim dt nd climte indices. A.M.V. processed the AMF rdition nd energy flux dt. R.C.S. ws the AWARE Site Scientist t WAIS Divide nd processed the MODIS dt. A.B.W. conducted the CAM model simultions, processed the CAM output nd some of the ERA-Interim dt nd climte indices. M.P.C. processed the AMF microwve rdiometer dt. J.V. processed the AMF micropulse lidr nd sonde dt. J.P.N., A.B.W., D.H.B., nd J.D.W. provided the meteorologicl nlysis. AWARE ws conceived nd plnned in collortion with ARM nd the U.S. Antrctic Progrm (USAP) y D.L., D.H.B., A.M.V., J.V., nd L.M.R. The ARM WAIS Divide field prty ws C.J., R.C.S., H.H.P., M.R., G.S. nd D.L., with C.J. serving s ARM Site Engineer. J.D.W. served s wether oserver for USAP t WAIS Divide. J.P.N. wrote the pper with help from A.B.W., D.H.B., D.L. nd A.M.V. Additionl informtion Supplementry Informtion ccompnies this pper t nturecommunictions Competing interests: The uthors declre no competing finncil interests. NATURE COMMUNICATIONS :5799 DOI:./ncomms

10 NATURE COMMUNICATIONS DOI:./ncomms5799 Reprints nd permission informtion is ville online t reprintsndpermissions/ How to cite this rticle: Nicols, J. P. et l. Jnury extensive summer melt in West Antrctic fvoured y strong El Niño. Nt. Commun., 5799 doi:./ncomms5799 (7). Pulisher s note: Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions. Open Access This rticle is licensed under Cretive Commons Attriution. Interntionl License, which permits use, shring, dpttion, distriution nd reproduction in ny medium or formt, s long s you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde. The imges or other third prty mteril in this rticle re included in the rticle s Cretive Commons license, unless indicted otherwise in credit line to the mteril. If mteril is not included in the rticle s Cretive Commons license nd your intended use is not permitted y sttutory regultion or exceeds the permitted use, you will need to otin permission directly from the copyright holder. To view copy of this license, visit licenses/y/./ r The Author(s) 7 NATURE COMMUNICATIONS :5799 DOI:./ncomms5799