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Potential Influence of a Developing La Niña on theSea-Ice Reduction in the Barents–Kara Seas

Rui Luo, Zhiwei Wu, Peng Zhang & Juan Dou

To cite this article: Rui Luo, Zhiwei Wu, Peng Zhang & Juan Dou (2019): Potential Influence of aDeveloping La Niña on the Sea-Ice Reduction in the Barents–Kara Seas, Atmosphere-Ocean, DOI:10.1080/07055900.2019.1587375

To link to this article: https://doi.org/10.1080/07055900.2019.1587375

Published online: 18 Mar 2019.

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Potential Influence of a Developing La Niña on the Sea-IceReduction in the Barents–Kara Seas

Rui Luo1, Zhiwei Wu1,*, Peng Zhang1, and Juan Dou2

1Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, FudanUniversity, Shanghai 200438, People’s Republic of China

2College of Atmospheric Science and Key Laboratory of Meteorological Disaster of the Ministry ofEducation, Nanjing University of Information Science and Technology (NUIST), Nanjing 210044,

Jiangsu, People’s Republic of China

[Original manuscript received 1 August 2018; accepted 3 January 2019]

ABSTRACT The potential influence of a developing La Niña on Arctic sea-ice annual variability is investigatedusing both observational data and an atmospheric general circulation model. It is found that during the developingphase of an eastern Pacific (EP) La Niña event in June, July, and August (JJA) and September, October, andNovember (SON), the sea-ice concentration (SIC) over the Barents–Kara Seas declines more than 15%. Thelocal atmospheric circulation pattern associated with the EP La Niña is characterized as a weak decrease ingeopotential height over the Barents–Kara Seas, combined with an anticyclone in the North Atlantic. Thecorresponding southerly winds push warm waters northward into the key sea-ice reduction region and directlyaccelerate sea-ice melt. Meanwhile, the abundant moisture contained in the lower troposphere is transportedinto the Arctic region by winds resulting from the local barotropic structure. The humid atmosphere contributesto both net shortwave and longwave radiation and thus indirectly accelerates the decline in sea ice. Simulationsby the European Centre Hamburg Model, version 5.4, are forced by observed sea surface temperature anomaliesassociated with EP La Niña events. The results of the simulations capture the North Atlantic anticyclone and repro-duce the moisture transport, which supports the premise that an EP La Niña plays a crucial role in sea-icereduction over the Barents–Kara sector from the perspective of atmospheric circulation and net surface heat flux.

RÉSUMÉ [Traduit par la rédaction] Nous étudions l’influence potentielle du développement d’un épisode La Niñasur la variabilité annuelle de la glace de mer arctique, et ce, à partir de données d’observation et d’un modèle decirculation générale de l’atmosphère. Nous notons que pendant la phase de formation d’un épisode La Niña dans lePacifique Est, en juin, juillet et août (JJA), et en septembre, octobre et novembre (SON), la concentration de glacedans les mers de Barents et de Kara s’amoindrit de plus de 15%. La circulation atmosphérique locale associée auphénomène La Niña du Pacifique Est se caractérise par une faible diminution de la hauteur géopotentielleau-dessus des mers de Barents et de Kara, et par un anticyclone dans l’Atlantique Nord. Les vents du sud corre-spondants poussent de l’eau chaude vers le nord dans la région où survient principalement la diminution de laglace de mer et ils accélèrent directement la fonte de cette glace. Parallèlement, l’humidité abondante contenuedans la basse troposphère se déplace vers la région arctique en raison des vents résultant de la structure barotropelocale. L’atmosphère humide influe à la fois sur le rayonnement net des ondes courtes et des ondes longues, etaccélère donc indirectement le déclin de la glace de mer. Les simulations issues du modèle de Hambourgversion 5.4 du Centre européen sont pilotées par les anomalies des températures de surface de la mer observéeslors d’épisodes La Niña du Pacifique Est. Les résultats des simulations reproduisent l’anticyclone de l’AtlantiqueNord et le transport de l’humidité, ce qui étaye l’hypothèse selon laquelle un épisode La Niña exerce une influencecruciale sur la réduction de la glace dans les mers de Barents et de Kara, sur les plans de la circulation atmosphér-ique et du flux net de chaleur superficielle.

KEYWORDS Barents–Kara sea ice; EP La Niña; atmospheric circulation; moisture; net surface heat flux

1 Introduction

Climate change in the Arctic has received unprecedentedattention in the past few decades because of the dramaticreduction in sea ice (Duarte, Lenton, Wadhams, &Wassmann,

2012). Along with sea-ice melt, near-surface air temperature inthe Arctic has risen to twice the global average because of alocal positive feedback, known as “Arctic Amplification”(Screen & Simmonds, 2010). Growing evidence has suggested

*Corresponding author’s email: zhiweiwu@fudan.edu.cn

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that the pronounced changes are caused by a combination ofexternal anthropogenic forcing (Gillett et al., 2008; Polyakov,Walsh, & Kwok, 2012; Serreze, Barrett, Stroeve, Kindig, &Holland, 2009) and internal natural variability. The internalcomponents could be related to the Atlantic MultidecadalOscillation (Chylek, Folland, Lesins, Dubey, & Wang,2009), the Madden–Julian Oscillation (Yoo, Feldstein, &Lee, 2011, 2012), the Pacific–North American (PNA) pattern(L’Heureux, Kumar, Bell, Halpert, & Higgins, 2008), and thechanges in sea surface temperature (SST) and convection inthe Pacific associated with the El Niño–Southern Oscillation(ENSO; Ding et al., 2014; Hu et al., 2016; Lee, 2012; Lee,Gong, Johnson, Feldstein, & Pollard, 2011).Being one of the most prominent large-scale ocean–atmos-

phere interactions over the tropical Pacific, ENSO has pro-found effects on both local and remote climate variability(Brönnimann, 2007; Ropelewski & Halpert, 1987; Trenberth& Caron, 2000; Van Loon & Madden, 1981). CanonicalENSO or eastern Pacific (EP) ENSO is characterized by seasurface temperature anomalies (SSTAs) developing in theeastern Pacific and propagating westward (Rasmusson & Car-penter, 1982). Since the 1970s, a new type of ENSO hasoccurred more frequently and become more intense (Lee &McPhaden, 2010; Yu & Kim, 2011, 2013). Because theSSTA begins in the central equatorial Pacific and developsin situ, it is termed the central Pacific (CP) ENSO. Giventhat the atmospheric responses to the EP and CP El Niñosare quite different (Kao & Yu, 2009; Yu & Kim, 2011;Zhang, Wu, & Zhou, 2016), the relationships between thetwo types of El Niño and changes in the Arctic sea ice havebeen discussed separately. Hu et al. (2016) suggested thatthe CP El Niño has a strong impact on Arctic sea-ice variabil-ity over the Canadian basin through an “atmospheric bridge.”The warming associated with the CP El Niño deepens theArctic polar vortex in the troposphere and strengthens the cir-cumpolar westerly wind, thus suppressing summer Arcticwarming and sea-ice retreat.Some studies have also discussed the necessity of La Niña

classification. Using surface currents derived from satelliteobservations, CP La Niña events can be clearly distinguishedfrom EP La Niña events (Shinoda, Hurlburt, & Metzger,2011).Meanwhile, there are large discrepancies in the responseof the extratropical atmosphere over East Asia to EP and CP LaNiña events (Wang,Wang, Zhou, & Li, 2012), the rainfall tele-connections in Australia (Cai & Cowan, 2009), and especiallyhigh-latitude atmospheric circulation (Zhang, Wang, Xiang,Qi, & He, 2015). Therefore, it is necessary to consider theresponse of Arctic sea ice to both EP and CP La Niña events.Based on satellite observations from the National Aeronauticsand Space Administration (NASA), September Arctic sea-iceextent attained record minima four times: 16 September 1984(6.80 million km2); 30 September 1995 (6.10 million km2);22 September 2005 (5.50 million km2); and 14 September2007 (4.20 million km2) (NASA, n.d.). They all coincide intime with four EP La Niña events. Motivated by the obser-vations, we focus mainly on the relationship between the

developing phase of an EP La Niña and the Arctic sea ice inboreal summer and autumn and the underlying mechanism.In the remainder of this paper, datasets, methodology, andmodel experiment settings are described in Section 2. Section3 shows the observational Arctic sea-ice changes associatedwith an EP La Niña. In Section 4, a potential mechanism is pre-sented to illuminate how EP La Niña events impact Arctic seaice from the perspective of both atmospheric circulation and netsurface heat flux. Section 5 verifies the underlying causalityusing the fifth generation of the European Centre–Max PlankInstitute model, version 5.4 (ECHAM5.4), which is an atmos-pheric general circulation model. Major conclusions and dis-cussions are summarized in Section 6.

2 Data, methodology, and model

The major datasets employed in this paper include (i) monthlysea-ice concentration (SIC) data obtained from the HadleyCentre Sea Ice and SST (HadISST) dataset for the 1953–2017 period at 1.0° × 1.0° resolution (Rayner et al., 2003).Although HadISST data starts in 1950, we found thatmonthly sea-ice data from 1950 to 1952 are exactly thesame, so we started our research period in 1953. The otherdatasets used were (ii) monthly SSTs from the NationalOceanic and Atmospheric Administration (NOAA) ExtendedReconstructed SST, version 5 (ERSST.v5), gridded at2.0° × 2.0° resolution (Huang et al., 2017) and (iii) monthlymean atmospheric fields and surface heat flux data from theNational Centers for Environmental Prediction/NationalCenter for Atmospheric Research (NCEP/NCAR) reanalysisat 2.5° × 2.5° resolution (Kalnay et al., 1996). Our researchfocuses on June, July and August (JJA) and September,October, and November (SON) of the EP La Niña developingphase. During this time, sea ice, on one hand, reaches itslargest annual variability (Serreze, Holland, & Stroeve,2007), making it vulnerable to melting and more easilyaffected by other forcings (e.g., Ding et al., 2017; Wettstein& Deser, 2014; Yang & Magnusdottir, 2018); on the otherhand, sea ice has a profound influence on subsequentchanges in atmospheric circulation and climate (Deser, Mag-nusdottir, Saravanan, & Phillips, 2004; Holland, Bitz, &Tremblay, 2006; Li & Wu, 2012; Singarayer, Bamber, &Valdes, 2006; Wang & Overland, 2009; Wu, Li, Li, & Li,2016). Although observational records for sea ice are morereliable after 1979, our research period starts in 1953 so thatwe can maximize the number of EP La Niña events, and wehave verified that the conclusions are consistent with eachother in the two periods.

In this paper, a La Niña is defined to occur when Niño 3.4SSTAs (averaged over 120°W–170°W, 5°N–5°S) are below−0.5°C during December, January, and February (DJF).Selection of EP La Niña events are made if DJF Niño 3SSTAs (averaged over 150°W–90°W, 5°N–5°S) are lessthan the DJF Niño 4 SSTAs (averaged over 160°E–150°W,5°N–5°S) as in Yeh et al. (2009). Therefore, we identify tenEP La Niña events as follows: 1954/55, 1955/56, 1964/65,

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1971/72, 1984/85, 1985/86, 1995/96, 2005/06, 2007/08, and2017/18. The developing phase of an EP La Niña occurs inJJA and SON of the previous year based on the above selection.To better understand the underlying mechanism, ECHAM5.4

is used to evaluate the response of the Arctic atmospheric cir-culation to an EP La Niña. The resolution of ECHAM5.4 usedis triangular 63 with 19 vertical levels (T63L19) (Roeckneret al., 2003). Based on the model, two experiments are set.In the control experiment, ECHAM5.4 is driven byHadISST climatological SSTs, and the results are derived asa reference state. In the sensitivity experiment, the compositeof observational monthly SSTAs (120°E–80°W, 30°N–30°S,June to November) associated with EP La Niña events isadded to ECHAM5.4 as the only forcing superposed on theclimatological SSTs. The integration is conducted for 40years, and the monthly mean of the last 30 years is analyzed.Atmospheric circulation discrepancies between control andsensitivity experiments in the Arctic region are considered tobe the remote responses to EP La Niña forcing. To eliminatethe effect of global warming, all datasets were detrended.Both composite and simulated results in this paper are testedusing a Student’s t-test.

3 Observational changes in the Arctic sea-iceassociated with an EP La Niña

Although EP La Niña events are determined based on DJFNiño indices, the spatial distribution displayed in Fig. 1shows that cold SSTAs have already emerged in the easternPacific in JJA (Fig. 1a). They develop off the coast of SouthAmerica and propagate westward into the central Pacific.The cooling centre is confined along the equator with anamplitude less than −1.2°C (Fig. 1d) and is located mainlyin the eastern Pacific near 120°W, determining the subsequentconvective activity pattern and location (Zhang et al., 2016).

The development and spatial distribution of SSTAs in JJAand SON indicate the validity of the selection of an EP LaNiña. Based on the selection, a composite analysis techniqueis used to assess the impacts of EP La Niña events on Arcticsea-ice change.

Figure 2 displays composites of the Arctic sea-ice anomalies.As an EP La Niña develops over the tropical Pacific, Arctic sea-ice concentration (SIC) over the Barents–Kara sector decreasesby 10% in JJA (Fig. 2a). The decrease strengthens andreaches its peak of more than 15%, mainly in the Kara Sea,during late JJA and early SON (Figs 2b and 2c). In SON, thedecline stretches and covers the entire Barents and Kara Seas(Fig. 2d). The sea-ice loss here is presented simultaneously,the reduction signal actually lags SSTAs associated with anEP La Niña by about one to two months, even though thislead-lag relationship is not easily recognized in the three-month running average results displayed in Fig. 2. Becausethe sea-ice decline is evident, concentrated, and persistent, thelocal atmospheric circulation that changes the Arctic thermodyn-amics may thus play a key role in the melt process (e.g., Dinget al., 2017; Hu et al., 2016; Ogi, Rigor, McPhee, & Wallace,2008; Wettstein & Deser, 2014).

4 Possible mechanism of Barents–Kara sea-icereduction associated with an EP La Niña

To illuminate the linkage between sea-ice loss over theBarents and Kara Seas and local thermodynamics, in thissection, the analyses are performed using both local atmos-pheric circulation and the net surface heat flux.

a Atmospheric CirculationIn the lower troposphere (Fig. 3), the sea level pressure (SLP)anomaly distribution is very uneven. A wide range of negative

Fig. 1 Composite of SSTAs (colour bar; °C) during the developing phase of an EP La Niña: (a) June–August (JJA), (b) July–September (JAS), (c) August–October(ASO), and (d) September–November (SON). Stippling indicates statistical significance at the 90% confidence level based on a Student’s t-test.

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pressure anomalies appear over the west coast of Canada, theBeaufort Sea, the Barents–Kara Seas, and eastern Greenland(Fig. 3a). The negative signal over the North Americanmargin weakens promptly and disappears in SON (Fig. 3d).The lower pressure anomaly around the Barents–Kara Seasalso weakens progressively. On the contrary, higher thannormal SLP develops around 50°N in the North Atlantic inJJA (Fig. 3a). The anticyclone strengthens in situ (Figs 3b–3d) and intensifies the local meridional pressure (Fig. 3c). Inthe mid-troposphere at 500 hPa (Z500; Fig. 4), the northernhemisphere features a more regular wavelike pattern withanomalous centres of positive and negative Z500 anomalies.Though pressure anomalies over the key region (outlined inred) of sea-ice reduction are more neutral, an area of weaklow pressure can still be seen over eastern Greenland (Figs 4band 4c). In the southeast of Greenland, a stronger anticyclonein the North Atlantic mirrors the one at the surface and sustainsthe local poleward pressure gradient. A similar pattern can alsobe found at 200 hPa (not shown), which indicates a barotropicstructure that extends from upper levels to the surface.Associated with the poleward pressure gradient in the entire

troposphere, the North Atlantic is dominated by southerlywinds, which can push the North Atlantic warm waters into

the Barents–Kara Seas, increasing the water temperaturethere directly and accelerating sea-ice melt (Fig. 5). Whatremains unclear is whether the crucial southerly winds associ-ated with the North Atlantic anticyclone are a response to thesea-ice reduction or vice versa. In the Arctic region, atmos-pheric circulation in the upper troposphere is relatively insen-sitive to surface temperature related to sea-ice changes(Graversen, Mauritsen, Tjernström, Källén, & Svensson,2008; Screen, 2013; Screen, Deser, & Simmonds, 2012).Given that the circulation in the middle and upper troposphereis stronger and more regular than the surface component, itseems that the barotropic structure here is not likely forcedby surface sea-ice reduction, which, in turn, is a result oflocal atmospheric forcing. Then what is the role of the EPLa Niña in the anomalous circulation around the Barents–Kara Seas?

On further analysis of the mid-troposphere (Fig. 4), weidentify a canonical PNA pattern in that the anomalouscentres arc northeastward over the North Pacific Ocean, thewest coast of Canada, and North America (Figs 4b–4d;Wallace & Gutzler, 1981). Although the PNA patternreaches peak amplitude during DJF, its atmospheric presencecan be found throughout the year, with the wave train varying

Fig. 2 Composite of the sea-ice concentration (SIC) anomaly (%) during the developing phase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO, and (d) SON. Shadingindicates statistical significance at the 90% confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-ice reduction in thisstudy is outlined in red.

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in intensity and location from season to season (L’Heureuxet al., 2008). Over the Atlantic Ocean, atmospheric responsesto the EP La Niña are also found. Lower pressure over easternGreenland and the North Atlantic anticyclone resemble a weakNorth Atlantic Oscillation (NAO) pattern (Fig. 3). It is consist-ent with a previous study showing that JJA NAO has a smallbut significant correlation with a La Niña SST pattern appear-ing in the eastern Pacific (Folland et al., 2009). Because thePNA and NAO are often cited as mid-to-high-latituderesponses to ENSO (Gouirand & Moron, 2003; Livezey &Mo, 1987; Straus & Shukla, 2002), therefore, we supposethat the critical southerly winds related to the Atlantic anticy-clone might be forced by the tropical SST associated with theEP La Niña.

b Net Surface Heat FluxAs well as atmospheric circulation, the energy exchange isalso thought to be a crucial component of sea-ice decreasein the Arctic region because it determines the local radiationbudget that controls the growth and melt of Arctic sea ice.The net surface heat flux is expressed in Eq. (1) in whichtotal net flux consists of downwelling (upwelling) shortwave

radiation Qdsw (Qusw), downwelling (upwelling) longwaveradiation Qdlw (Qulw), latent heat flux Qlh, and sensible heatflux Qsh.

Qnet = Qdsw − Qusw + Qdlw − Qulw − Qlh − Qsh. (1)

Spatial distribution of total net surface heat flux is shown inFig. 6. There is a net heat flux over the entire Arctic region inJJA because of polar day. Over the Barents–Kara Seas, consider-able energy is absorbedby the surface contributing to sea-icemelt(Fig. 6a). The signal then fades and even reverses to energy dis-sipation inSON (Fig. 6d).Tounderstand the underlying causalityof the net surface heat flux changes, we next assess the contri-bution of each component to the net flux change in Eq. (1) by cal-culating domain-averaged indices over the Barents–Kara Seas(65°−85°N, 30°−120°E) as shown in Fig. 7.

Downwelling solar radiation is highly correlated with localmoisture content (Yang & Magnusdottir, 2018). Water vapourabsorbs four times as much infrared radiation as carbondioxide and is thus a critical greenhouse gas. As the vertical dis-tribution in Fig. 8 shows, positive water vapour anomalies arecontained in the lower troposphere (below 700 hPa), resulting

Fig. 3 Composite of the sea level pressure (SLP) anomaly (hPa) during the developing phase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO, and (d) SON. Cross-hatching indicates statistical significance at the 90% confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-icereduction in this study is outlined in red.

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in lower downwelling shortwave radiation (indicated by nega-tive dsw) in the region throughout the research period (Fig. 7).While the lower upwelling solar radiation (indicated by positive-usw), which is likely due to a reduction in surface albedo (lesssea ice), compensates for the lower downwelling solar radiation,resulting in net downward shortwave radiation in JJA(1.84 W m−2; dsw −1.250 W m−2; -usw 3.097 W m−2) andJAS (1.702 W m−2; dsw −1.213 W m−2; −usw 2.915 W m−2).This net flux weakens in ASO (0.464 W m−2; dsw−1.370 W m−2; −usw 1.834 W m−2) and SON(−0.158 W m−2; dsw −0.685 W m−2; −usw 0.527 W m−2).Downwelling longwave radiation, as the primary contribu-

tor to net surface flux, is also highly dependent on watervapour content, especially in the lower atmosphere in theArctic region. It reaches its maximum in ASO (Fig. 7c,4.048 W m−2), coinciding with the vertical distribution ofspecific humidity shown in Fig. 8. Upwelling longwaveradiation, which is associated with surface temperature,closely resembles the SST changes shown in Fig. 5.The Barents–Kara Seas are dominated by sea surfacewarming, and the widespread positive anomalies in ASO(Fig. 5c) result in the maximum upwelling longwave radiationin Fig. 7c (-ulw −3.287 W m−2). Latent heat flux is a result of

a change in the sea-ice phase, namely, melting and freezing;sensible heat flux relies on the temperature differencebetween the air and the water surface, which is also controlledby the sea-ice change. Therefore, latent and sensible heatfluxes here are delayed atmospheric responses to sea-icemelt. They are initially weak because sea-ice retreat starts inJJA (Fig. 7a, -lh −0.401 W m−2, -sh −0.091 W m−2) andstrengthens in SON when the sea-ice reduction reaches itspeak (Fig. 7d, -lh −0.742 W m−2, -sh −0.730 W m−2).

In summary, both net shortwave and longwave radiation con-tribute to total net flux positively. However, latent and sensibleheat flux work the other way around and are responsible for thesignal reversing in Fig. 6d due to sea-ice reduction. The aboveanalysis is consistent with the conclusions of Yang and Magnus-dottir (2018) and indicates that local moisture plays a critical butindirect role in sea-ice melt by contributing to the surface net heatflux. Based on the vertical distribution of water vapour (Fig. 8),we next integrate specific humidity from the surface to 700 hPato trace its source, and the results are presented in Fig. 9.

During JJA (Fig. 9a), moisture originating from the NorthAtlantic is transported poleward into the Arctic region alongthe southwest margin of Greenland. As the anticyclonestrengthens and broadens (Fig. 3), the corresponding southerly

Fig. 4 Composite of the 500 hPa geopotential height (Z500) anomaly (hPa) during the developing phase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO, and (d) SON.Cross-hatching indicates statistical significance at the 90% confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-icereduction in this study is outlined in red.

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winds become stronger and reach farther into the Arctic overthe edge of the Barents–Kara sector, humidifying the localatmosphere with plentiful water vapour (Fig. 9b,c). Althoughthe amplitude is weaker in SON (Fig. 9d), an evident moistureconveyor belt has been formed along the Barents–Kara Seas,and the positive transport still contributes to the downwellinglongwave radiation flux (Fig. 7d).Based on the discussion of atmospheric circulation and net

surface heat flux above, we provide the following plausiblemechanism: during the EP La Niña developing phase ofJune to November, cold SSTAs over the eastern Pacific canforce a remote anomalous atmospheric pattern of polewardpressure gradient associated with the North Atlantic anticy-clone. This pattern favours the transport of both the NorthAtlantic warm waters and lower troposphere water vapourinto the Barents–Kara Seas. The former accelerates sea-icemelt directly, and the latter, by contributing to net surfaceheat flux, facilitates sea-ice reduction indirectly. The statisticallinks of the EP La Niña with the Barents–Kara sea-ice retreatare robust in both JJA and SON, prompting the followinginvestigation to examine the possible mechanism by conduct-ing numerical experiments with ECHAM5.4.

5 Modelling support from ECHAM5.4

For decades, there have been large uncertainties in predicting theArctic climate in general circulation models (e.g. Randall et al.,1998). These discrepancies are mostly a result of our incompleteunderstanding of thermodynamic processes in the ocean–ice–atmosphere system in the Arctic region both horizontally andvertically (Uttal et al., 2002). Therefore, in this section, weomit discussion of net surface heat flux and focus mainly on

the simulated general circulation pattern over the Barents–KaraSeas. As described in Section 2, we perform two experiments:a control run and a sensitivity experiment. The sensitivity exper-iment is driven by monthly observational EP La Niña SSTAsover the eastern tropical Pacific region (120°E–80°W, 30°N–30°S) from June to November. The differences between thetwo experiments are the response to EP La Niña forcing.

Figure 10 displays the simulated 500 hPa geopotential heightand the 850 hPa winds. They are perfectly matched, indicating abarotropic structure as in the observations. In JJA (Fig. 10a),atmospheric circulation over the mid-high latitudes is mostlyconsistent with observations (Fig. 4a), especially the positiveanomaly centres over North America, the west coast ofEurope, and the East Siberian Sea, as well as the negativeanomaly centres around the key area of sea-ice reduction.Around this area, the low pressure is accompanied by a positiveanomalous centre over Europe. Although the location of thecyclone is shifted southward from the Barents Sea to easternGreenland compared with observations, the pattern still facili-tates a local pressure gradient and evident southerly winds thatpenetrate far over the Barents–Kara Seas. Associated with thedistribution of the winds, the warm waters can be driven intothe Arctic region and heat the water there. A weak PNApattern appears in ASO (Fig. 10c); this pattern strengthens inSON with a negative anomalous centre located over westernCanada, sandwiched between positive anomalies over theBering Sea and the eastern coast of North America (Fig. 10d).

In the North Atlantic Ocean, the positive anomaly is strikinglysimilar to that observed in Fig. 4d. The anticyclone appears in JJA(Fig. 10a), and the model used here reproduces the southerlywinds that contribute directly to sea-ice reduction (Figs 10a and10b). Further insight into the moisture source is achieved in the

m s–1

Fig. 5 Composite of SSTAs (colour bar; °C) and 850 hPa winds (UV850; vector; m s−1) during the developing phase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO,and (d) SON. The thick dark blue line and wind anomalies indicate statistical significance at the 90% confidence level based on a Student’s t-test. The keyregion (65°−85°N, 30°−120°E) of sea-ice reduction in this study is outlined in red.

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same manner of vertical integration of local water vapour fluxfrom the surface up to 700 hPa (Fig. 11). The moisture is trans-ported poleward into the Arctic region in JJA by the southerlywinds (Fig. 11a). The water vapour belt along southern Greenlandbecomes stronger and extends northeastward into the Barents Sea(Figs 11b and 11c), which is highly consistent with the observa-tional results in Fig. 9. The humid atmosphere favours local netheat flux and consequently indirectly accelerates sea-ice melt.The general ECHAM 5.4 simulation here is not perfect

because the location of the JJA anticyclone over the NorthAtlantic is shifted eastward. But the discrepancies do notmean that the simulation has failed. The model captures themajor characteristics of the southerly winds and the moisturetransport throughout the research period, which confirms ourhypothesis for the underlying mechanism. The results improveour understanding of the physical processes involved in theArctic thermodynamics and help us bridge the gap betweenthe model and observations in the study of the Arctic climate.

6 Discussion and conclusions

The noticeable warming associated with sea-ice retreat in theArctic has provoked a surge of research interest worldwide

(e.g. Ding et al., 2017; Duarte et al., 2012; Screen & Sim-monds, 2010; Serreze et al., 2009). It has been shown thatthe rapid change in sea ice is partly triggered by tropicalforcing related to ENSO (Ding et al., 2014; Lee, 2012; Leeet al., 2011). Based on the premise of ENSO classification(Yeh et al., 2009), important progress has been made in therelationship between the CP El Niño and Arctic sea-ice melt(Hu et al., 2016). However, we notice that the response ofsea ice to the canonical EP La Niña is also evident with fourexceptional record minimum sea-ice extents that all occurduring EP La Niña years. Inspired by these records, thispaper demonstrates the linkage between Arctic sea-icechange and the EP La Niña during its developing phase inJJA and SON. To illuminate the potential causality, observa-tional data and simulation results from ECHAM5.4 are ana-lyzed from the perspective of both atmospheric circulationand net surface heat flux with the following conclusions.

Along with the EP La Niña development in JJA, sea-icedecreases significantly in the Barents–Kara Seas. The signal per-sists to SON and reaches its peak amplitude of over 15%reduction in ASO. A local barotropic structure of an anticycloneexists over the North Atlantic Ocean and a low-pressure centre islocated over eastern Greenland. This pattern facilitates a

Fig. 6 Composite of the net surface heat flux anomaly (W m−2) during the developing phase of an EP La Niña; (a) JJA, (b) JAS, (c) ASO, and (d) SON. Cross-hatching indicates statistical significance at the 90% confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-icereduction in this study is outlined in red.

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northward pressure gradient and strong southerly winds. Drivenby these winds, the warm waters from the North Atlantic arepushed into the Barents–Kara Seas and accelerate sea-ice meltdirectly. In addition, abundant water vapour associated withthe warm waters are contained in the lower troposphere and isalso transported to the Arctic region because of the barotropicstructure. The moisture contributes to both JJA net shortwaveand JJA longwave radiation, increasing local net heat flux andultimately indirectly contributing to the decline in sea ice. Ana-lyses indicate that local southerly winds, moisture transport,and net surface heat flux are all affected by atmospheric circula-tion and, in turn, affect SIC directly or indirectly.

Further inspection shows that SON atmospheric anomalycentres in the mid-troposphere located over the North Pacificand western and eastern North America qualitatively agreewith observational results, suggesting that anomalous circula-tion in the Arctic is likely a remote response to EP La NiñaSST forcing. In the numerical experiment, we omit the netsurface heat flux simulation because of model uncertaintiesand bias. But the simulation results provide robust evidenceand helpful indicators. The ECHAM5.4 model, driven byobserved EP La Niña SSTAs, can reproduce major character-istics of the North Atlantic anticyclone, the southerly winds,and the moisture transport that are crucial for the sea-icereduction in the observations.

Fig. 7 Each component of the net surface heat flux anomaly (W m−2) averaged over the key region (65°−85°Ν, 30°−120°E) of sea-ice reduction during the devel-oping phase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO, and (d) SON (net: net surface heat flux; dsw: downwelling shortwave radiation; usw: upwellingshortwave radiation; dlw: downwelling longwave radiation; ulw: upwelling longwave radiation; lh: latent heat flux; sh: sensible heat flux). Positive (nega-tive) values correspond to increased (decreased) surface net heat flux and are plotted as upwelling (downwelling) bars.

Fig. 8 Vertical distribution (hPa) of composite specific humidity anomaly(g kg−1) averaged over the key region (65°−85°N, 30°−120°E) ofsea-ice reduction during the developing phase of an EP La Niña inJJA, JAS, ASO, and SON.

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Fig. 9 Composite of vertically integrated water vapour flux (vector; surface to 700 hPa) and its amplitude (colour bar; g cm−1 hPa−1 s−1) during the developingphase of an EP La Niña: (a) JJA, (b) JAS, (c) ASO, and (d) SON. The thick dark blue line and vector anomalies indicate statistical significance at the 90%confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-ice reduction in this study is outlined in red.

m s–1

Fig. 10 Simulated responses of (a) JJA, (b) JAS, (c) ASO, and (d) SON 500 hPa geopotential height (Z500; colour bar; hPa) and 850 hPa winds (UV850; vector;m s−1) to EP La Niña forcing in the ECHAM5.4 model. The thick dark blue line and wind anomalies indicate statistical significance at the 90% confidencelevel based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-ice reduction in this study is outlined in red.

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In addition, we examined the linkage between the CP La Niñaand the Arctic sea-ice change, and the results are quite different.Sea ice increases only in JJA over the Chukchi Sea during thedevelopment of CP La Niña events. However, both the atmos-pheric circulation and the net surface heat flux are muchweaker. It seems that wind-induced flow is responsible for theregional sea-ice increase in JJA by causing pack ice to drift outof the central Arctic Ocean into the East Siberian Sea, andsurface net heat flux is almost neutral throughout the process.Because the sea-ice increase signal appears only in JJA, we arenot sure whether it indicates potential connections between theCP LaNiña and the Arctic sea ice or whether it is simply a coinci-dence. Because of the unclear relationship between sea ice andthe CP La Niña, we will defer this question for future study.We would also like to note that the extent to which the EP La

Niña events account for the evident Barents–Kara sea-ice lossremains unclear. Examining sea-ice changes for each EP LaNiña event, we find the linkage between ice melt and SSTAintensity varies. For example, there were weak EP La Niñaevents in 1984/85 and 2005/06; however, JJA and SON seaicereduction in 1984 is much stronger than in 2005. The Septembersea-ice extent reached record minima during four EP La Niñayears. In 2012, a new historical record minimum of3.40 million km2 was detected, but there was no apparentENSO pattern over the Pacific Ocean. Therefore, this article

may provide a new opportunity for predicting Arctic sea-icechange by showing that remote forcing from the PacificOcean, like EP La Niña events, also plays a crucial role in sea-ice variability. Themechanismwe put forward in this article pro-vides valuable insights into the understanding of the surfaceenergy exchange and air–sea–ice interactions in the Arctic.

Acknowledgements

We thank the model developers at the Max Planck Institute forMeteorology for making ECHAM5.4 available.

Funding

This work is jointly supported by the National Natural ScienceFoundation of China (NSFC) [Grant number 41790475], theNational Key Research & Development Program of China[Grant number 2016YFA0601801], the Ministry of Scienceand Technology of China [Grant numbers 2015CB953904and 2015CB453201], and the NSFC [Grant numbers91637312, 41575075 and 91437216].

Disclosure Statement

No potential conflict of interest was reported by the authors.

ReferencesBrönnimann, S. (2007). Impact of El Niño–Southern Oscillation onEuropean climate. Reviews of Geophysics, 45, RG3003. doi:10.1029/2006RG000199

Cai, W., & Cowan, T. (2009). La Niña Modoki impacts Australia autumn rain-fall variability. Geophysical Research Letters, 36, L12805. doi:10.1029/2009GL037885

Fig. 11 Simulated responses of (a) JJA, (b) JAS, (c) ASO, and (d) SON vertically integrated water vapour flux (vector) and its amplitude (shading;50 g cm−1 hPa−1 s−1) to EP La Niña forcing in the ECHAM5.4 model. The thick dark blue line and wind anomalies indicate statistical significance atthe 90% confidence level based on a Student’s t-test. The key region (65°−85°N, 30°−120°E) of sea-ice reduction in this study is outlined in red.

Potential Impact of a Developing La Niña on Barents–Kara Sea-Ice Reduction / 11

ATMOSPHERE-OCEAN iFirst article, 2019, 1–13 https://doi.org/10.1080/07055900.2019.1587375Canadian Meteorological and Oceanographic Society

Chylek, P., Folland, C. K., Lesins, G., Dubey, K. M., & Wang, M. (2009).Arctic air temperature change amplification and the Atlantic MultidecadalOscillation. Geophysical Research Letters, 36, L14801. doi:10.1029/2009GL038777

Deser, C., Magnusdottir, G., Saravanan, R., & Phillips, A. (2004). The effectsof North Atlantic SST and sea ice anomalies on the winter circulation inCCM3. Part II: Direct and indirect components of the response. Journalof Climate, 17, 877–889.

Ding, Q., Schweiger, A., L’Heureux, M., Battisti, D. S., Po-Chedley, S.,Johnson, N. C.,… Steig, E. J. (2017). Influence of high-latitude atmos-pheric circulation changes on summertime Arctic sea ice. Nature ClimateChange, 7, 289–295.

Ding, Q., Wallace, J. M., Battisti, D. S., Steig, E. J., Gallant, A. J. E., Kim, H.,& Geng, L. (2014). Tropical forcing of the recent rapid Arctic warming innortheastern Canada and Greenland. Nature, 509, 209–212.

Duarte, C. M., Lenton, T. M., Wadhams, P., & Wassmann, P. (2012). Abruptclimate change in the Arctic. Nature Climate Change, 2, 60–62.

Folland, C. K., Knight, J., Linderholm, H. W., Fereday, D., Ineson, S., &Hurrell, J. W. (2009). The summer North Atlantic Oscillation: Past,present, and future. Journal of Climate, 22, 1082–1103.

Gillett, N. P., Stone, D. A., Stott, P. A., Nozawa, N., Karpechko, A. Y.,Hegerl, G. C.,… Jones, P. D. (2008). Attribution of polar warming tohuman influence. Nature Geoscience, 1, 750–754.

Gouirand, I., & Moron, V. (2003). Variability of the impact of El Niño-Southern Oscillation on sea-level pressure anomalies over the NorthAtlantic in January to March (1874–1996). International Journal ofClimatology, 23, 1549–1566.

Graversen, R. G., Mauritsen, T., Tjernström, M., Källén, E., & Svensson, G.(2008). Vertical structure of recent Arctic warming. Nature, 451, 53–56.

Holland, M. M., Bitz, C. M., & Tremblay, B. (2006). Future abrupt reductionsin the summer Arctic sea ice. Geophysical Research Letters, 33, L23503.doi:10.1029/2006GL028024

Hu, C., Yang, S., Wu, Q., Li, Z., Chen, J., Deng, K.,… Zhang, C. (2016).Shifting El Niño inhibits summer Arctic warming and Arctic sea-icemelting over the Canada Basin. Nature Communications, 7, 11721.doi:10.1038/ncomms11721

Huang, B., Thorne, P. W., Banzon, V. F., Boyer, T., Chepurin, G., Lawrimore,J. H.,… Zhang, H. (2017). Extended reconstructed sea surface temperature,version 5 (ERSSTv5): Upgrades, validations, and intercomparisons.Journal of Climate, 30, 8179–8205.

Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L.,… Zhu, Y. (1996). The NCEP/NCAR 40-year reanalysis project. Bulletin ofthe American Meteorological Society, 77, 437–471.

Kao, H., & Yu, J. (2009). Contrasting eastern-Pacific and central-Pacific typesof ENSO. Journal of Climate, 22, 615–632.

Lee, S. (2012). Testing of the tropically excited Arctic warming mechanism(TEAM) with traditional El Niño and La Niña. Journal of Climate, 25,4015–4022.

Lee, S., Gong, T., Johnson, N., Feldstein, S. B., & Pollard, D. (2011). On thepossible link between tropical convection and the Northern HemisphereArctic surface air temperature change between 1958 and 2001. Journal ofClimate, 24, 4350–4367.

Lee, T., & McPhaden, M. J. (2010). Increasing intensity of El Niño in thecentral-equatorial Pacific. Geophysical Research Letters, 37, L14603.doi:10.1029/2010GL044007

L’Heureux, M. L., Kumar, A., Bell, G. D., Halpert, M. S., & Higgins, R. W.(2008). Role of the Pacific-North American (PNA) pattern in the 2007Arctic sea ice decline. Geophysical Research Letters, 35, L20701. doi:10.1029/2008GL035205

Li, J., &Wu, Z. (2012). Importance of autumn Arctic sea ice to northern wintersnowfall. Proceedings of the National Academy of Sciences, 109, E1898.doi:10.1073/pnas.1205075109

Livezey, R. E., & Mo, K. C. (1987). Tropical-extratropical teleconnectionsduring the Northern Hemisphere winter. Part II: Relationshipsbetween monthly mean Northern Hemisphere circulation patternsand proxies for tropical convection. Monthly Weather Review, 115,3115–3132.

NASA. (n.d.). Arctic sea ice minimum. Retrieved from the Global ClimateChange website: https://climate.nasa.gov/vital-signs/arctic-sea-ice/

Ogi, M., Rigor, I. G., McPhee, M. G., & Wallace, J. M. (2008). Summerretreat of Arctic sea ice: Role of summer winds. Geophysical ResearchLetters, 35, L24701. doi:10.1029/2008GL035672

Polyakov, I. V., Walsh, J. E., & Kwok, R. (2012). Recent changes of Arcticmultiyear sea ice coverage and the likely causes. Bulletin of the AmericanMeteorological Society, 93, 145–151.

Randall, D., Curry, J., Battisti, D. S., Flato, G., Grumbine, R., Hakkinen, S.,…Weatherly, J. (1998). Status of and outlook for large-scale modeling ofatmosphere–ice–ocean interactions in the Arctic. Bulletin of the AmericanMeteorological Society, 79, 197–219.

Rasmusson, E. M., & Carpenter, T. H. (1982). Variations in tropical seasurface temperature and surface wind fields associated with the SouthernOscillation/El Niño. Monthly Weather Review, 110, 354–384.

Rayner, N. A., Parker, D. E., Horton, E. B., Folland, C. K., Alexander, L. V.,Rowell, D. P.,…Kaplan, A. (2003). Global analyses of sea surface temperature,sea ice, and night marine air temperature since the late nineteenth century.Journal of Geophysical Research, 108(D14), 4407. doi:10.1029/2002JD002670

Roeckner, E., Bäuml, G., Bonaventura, L., Brokopf, R., Esch, M., Giorgetta,M.,… Tompkins, A. (2003). The atmospheric general circulation modelECHAM5. PART I: Model description (Report 349). Hamburg: MaxPlanck Institute for Meteorology.

Ropelewski, C. F., & Halpert, M. S. (1987). Global and regional scale precipi-tation patterns associated with the El Niño/Southern Oscillation. MonthlyWeather Review, 115, 1606–1626.

Screen, J. A. (2013). Influence of Arctic sea ice on European summer precipi-tation. Environmental Research Letters, 8(4), 044015. doi:10.1088/1748-9326/8/4/044015

Screen, J. A., Deser, C., & Simmonds, I. (2012). Local and remote controls onobserved Arctic warming. Geophysical Research Letters, 39, L10709.doi:10.1029/2012GL051598

Screen, J. A., & Simmonds, I. (2010). The central role of diminishing sea ice inrecent Arctic temperature amplification. Nature, 464, 1334–1337.

Serreze, M. C., Barrett, A. P., Stroeve, J. C., Kindig, D. N., & Holland, M. M.(2009). The emergence of surface-based Arctic amplification. TheCryosphere, 3, 11–19.

Serreze, M. C., Holland, M. M., & Stroeve, J. (2007). Perspectives on theArctic’s shrinking sea-ice cover. Science, 315, 1533–1536.

Shinoda, T., Hurlburt, H. E., & Metzger, E,J. (2011). Anomalous tropicalocean circulation associated with La Niña Modoki. Journal ofGeophysical Research: Oceans, 116, C12. doi:10.1029/2011JC007304

Singarayer, J. S., Bamber, J. L., & Valdes, P. J. (2006). Twenty-first-centuryclimate impacts from a declining Arctic sea ice cover. Journal of Climate,19, 1109–1125.

Straus, D. M., & Shukla, J. (2002). Does ENSO force the PNA? Journal ofClimate, 15, 2340–2358.

Trenberth, K. E., & Caron, J. M. (2000). The Southern Oscillation revisited:Sea level pressures, surface temperatures, and precipitation. Journal ofClimate, 13, 4358–4365.

Uttal, T., Curry, J. A., McPhee, M. G., Perovich, D. K., Moritz, R. E.,Maslanik, J. A.,…Heiberg, A. (2002). Surface heat budget of the ArcticOcean. Bulletin of the American Meteorological Society, 83(2), 255–275.

Van Loon, H., & Madden, R. A. (1981). The Southern Oscillation. Part I:Global associations with pressure and temperature in northern winter.Monthly Weather Review, 109, 1150–1162.

Wallace, J. M., & Gutzler, D. S. (1981). Teleconnections in the geopotentialheight field during the Northern Hemisphere winter. Monthly WeatherReview, 109, 784–812.

Wang,M.,&Overland, J. E. (2009).A sea ice free summerArcticwithin 30years?Geophysical Research Letters, 36, L07502. doi.10.1029/2009GL037820

Wang, X., Wang, D., Zhou, W., & Li, C. (2012). Interdecadal modulation ofthe influence of La Niña events on mei-yu rainfall over the Yangtze RiverValley. Advances in Atmospheric Sciences, 29, 157–168.

Wettstein, J. J., & Deser, C. (2014). Internal variability in projections oftwenty-first-century Arctic sea ice loss: Role of the large-scale atmosphericcirculation. Journal of Climate, 27, 527–550.

12 / Rui Luo et al.

ATMOSPHERE-OCEAN iFirst article, 2019, 1–13 https://doi.org/10.1080/07055900.2019.1587375La Société canadienne de météorologie et d’océanographie

Wu, Z., Li, X., Li, Y., & Li, Y. (2016). Potential influence of Arctic Sea ice tothe interannual variations of East Asian spring precipitation. Journal ofClimate, 29, 2797–2813.

Yang, W., & Magnusdottir, G. (2018). Year-to-year variability in Arcticminimum sea ice extent and its preconditions in observations and theCESM large ensemble simulations. Scientific Reports, 8, 9070. doi:10.1038/s41598-018-27149-y

Yeh, S. W., Kug, J. S., Dewitte, B., Kwon, M., Kirtman, B. P., & Jin, F.(2009). El Niño in a changing climate. Nature, 461, 511–514.

Yoo, C., Feldstein, S., & Lee, S. (2011). The impact of the Madden-JulianOscillation trend on the Arctic amplification of surface air temperatureduring the 1979–2008 boreal winter. Geophysical Research Letters, 38,L24804. doi:10.1029/2011GL049881

Yoo, C., Lee, S., & Feldstein, S. B. (2012). Mechanisms of Arctic surface airtemperature change in response to the Madden–Julian Oscillation. Journalof Climate, 25, 5777–5790.

Yu, J., & Kim, S. T. (2011). Relationships between extratropical sea levelpressure variations and the central Pacific and eastern Pacific types ofENSO. Journal of Climate, 24, 708–720.

Yu, J., & Kim, S. T. (2013). Identifying the types of major El Niño eventssince 1870. International Journal of Climatology, 33, 2105–2112.

Zhang, L., Wu, Z., & Zhou, Y. (2016). Different impacts of typical and atypi-cal ENSO on the Indian summer rainfall: ENSO-developing phase.Atmosphere-Ocean, 54, 440–456.

Zhang, W., Wang, L., Xiang, B., Qi, L., & He, J. (2015). Impacts of two types ofLa Niña on the NAO during boreal winter.Climate Dynamics, 44, 1351–1366.

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