Subseasonal transition of Barents–Kara sea-ice anomalies in winter related to the reversed warm Arctic–cold Eurasia pattern

Yiji Zhng ,Zhicong Yin,b,c,＊ ,Huijun Wng,b,c

a Key Laboratory of Meteorological Disaster, Ministry of Education / Joint International Research Laboratory of Climate and Environment Change (ILCEC) /Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters(CIC-FEMD),Nanjing University of Information Science&Technology,Nanjing,China

b Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

c Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Keywords: Warm Arctic-Cold Eurasia Barents–Kara sea ice Subseasonal variation Turbulent heat flux Sea-ice drift

ABSTRACT Subseasonal reversal of the warm Arctic–cold Eurasia pattern (WACE) could trigger an extreme cold/warm transition in winter and sandstorms in spring over eastern China.An associated subseasonal transition of the seaice anomaly also occurs in the Barents–Kara seas (BKS) driven by such remarkable high-latitude atmospheric pattern reversals.Under a warm Arctic and enhanced Ural high,abnormal downward turbulent heat flux and increased downward infrared radiation in the BKS are conducive to sea ice melting.The surface southerly wind drives the sea ice to drift from the thin to perennial ice area and further enlarges the open ocean surface.The opposite mechanism occurs in the opposite phase of WACE,causing positive BKS sea-ice anomalies.When WACE reverses on the subseasonal scale,the above mechanisms occur in early and late winter,respectively,resulting in a significant subseasonal transition of BKS sea-ice anomalies.More importantly,in the last decade,with a more frequent reversal of WACE,the subseasonal transition between early winter and late winter in BKS sea ice has enhanced.The findings of this study establish a comprehensive schematic of the subseasonal reversal of WACE and contribute to better understanding and predicting extreme climate in eastern China.

1.Introduction

The constant melting of Arctic sea ice and the accelerated warming in the Arctic are significantly amplifying the effects of global warming(Huang et al.,2017).As an important external forcing in the high-latitude climate system,the change in Arctic sea ice impacts the absorption of solar radiation energy by changing the albedo,thereby causing an adjustment in the atmospheric baroclinicity and planetary waves(Gao et al.,2015),and ultimately contributing to the occurrence of extreme weather and climate events in the mid–low latitudes(Cohen et al.,2014).In September 2020,the Barents–Laptev sea ice reached its lowest level since 1981 (Yang and Fan,2022),which enhanced the Siberian high and caused a record-breaking cold wave in East Asia the following winter (Peng et al.,2022).Also,during severe snowfall in eastern China in 2018,the loss of Barents–Kara sea ice and the associated strengthening of the Siberian high led to the invasion of northerly cold advection(Sun et al.,2019).

While the Arctic is warming rapidly,cold events are occurring frequently in Eurasia,presenting a cooling trend,which is related to the decline in Arctic sea ice(Mori et al.,2019;Kug et al.,2015).The melting of sea ice in autumn in the Barents–Kara seas (BKS) results in more frequent Ural blocking(UB)and a cold anomaly in Eurasia,thus forming a warm Arctic–cold Eurasia pattern (WACE;Mori et al.,2014)—a pattern that also appears on the subseasonal scale(Kug et al.,2015;Nie et al.,2022).As a key driver,UB can affect the subseasonal variability of BKS sea ice and WACE through humidity and temperature advection,heat flux,and downward infrared radiation (IR) (Luo et al.,2016;Kim et al.,2021;Tyrlis et al.,2019).A sustained UB combined with a positive phase of the North Atlantic Oscillation (NAO) provides an optimal circulation pattern for the transport of warm and moist air (Luo et al.,2017),which enhances the response of WACE and the melting of BKS sea ice.WACE and BKS sea-ice reduction lag UB by about one to two days and three to four days,respectively (Yin et al.,2023;Gong and Luo,2017;Chen et al.,2018).However,on the subseasonal scale,UB,WACE,and BKS sea ice are coupled and interact with each other.WACE can cause the gradient of meridional temperature and potential vorticity to weaken,thus leading to a persistent UB and further melting of sea ice(Luo et al.,2019).Meanwhile,the BKS sea ice provides a favorable condition for long-lived UB events (Gong and Luo,2017),thus further maintaining the WACE pattern.The above three processes form a positive feedback relationship within the air–ice system.

In the recent winters of 2020/21 and 2021/22,WACE presented a noteworthy feature on the subseasonal scale.In both winters,a significant WACE or cold Arctic–warm Eurasia pattern (CAWE)appeared in the early and late winter,but the pattern was opposite,presenting a subseasonal reversal between WACE and CAWE from the early-winter mean to late-winter mean.Further studies have confirmed the existence of subseasonal reversal between WACE and CAWE in winter (Yin et al.,2023),demonstrating it to not be an incidental phenomenon.Accompanying the strongest large-scale subseasonal transition from WACE to CAWE in winter 2020/21(Fig.1(a,b)),the sea ice in the whole of the BKS also showed a consistent and significant subseasonal inversion,from a negative anomaly in early winter to a positive anomaly in late winter (Fig.1(a,b)).However,studies so far have mainly focused on the interaction between the BKS sea ice and WACE on the subseasonal–interannual time scales.Differently,the present paper instead focuses on the subseasonal transition of WACE and BKS sea ice anomalies between the early-winter mean and late-winter mean,and investigates the subseasonal variation and physical mechanism of the BKS sea ice associated with the subseasonal reversal between WACE and CAWE.

Fig.1.(a,b)The SAT(shading)and sea-ice fraction(markers)anomalies of early winter and late winter in 2020/21.The green frames represent the locations of the Arctic and Eurasia that define the SATAE index.(c) Temporal evolution of SICBKS in early and late winter and the standardized difference between early and late winter (late minus early winter;bars) after detrending from 1979/80 to 2021/22.The green and orange bars represent the subseasonal difference in SICBKS in the SATAE-R1 and SATAE-R2 years,respectively,and the dark orange and green bars represent the difference in the SATAE reversal years exceeding plus or minus one standard deviation.(d)Daily variation in average SATAE(blue)and SICBKS(black)in the winter of SATAE-R1 and SATAE-R2 years.(e)Composite SICBKS in early and late winter in SATAE-R1 years,SATAE-R2 years,persistent WACE years,and persistent CAWE years.The linear trend in each panel has been removed.

2.Data and methods

The daily meteorological data from 1979 to 2022 (horizontal resolution: 1?× 1?) were derived from the ERA5 dataset (Hersbach et al.,2023) and included the surface air temperature (SAT),zonal and meridional winds at 10 m,latent and sensible heat flux,and downward IR.The sum of the latent and sensible heat flux is the turbulent heat flux(THF).Daily sea ice data from 1979 to 2022 were also obtained from EAR5.Daily sea-ice motion vectors from 1979 to 2021 (horizontal resolution: 25 km × 25 km) were provided by NSIDC-0116 Polar Pathfinder,version 4.1 (Tschudi et al.,2019).The linear trend and climate mean during 1979–2022 was removed from the daily data before calculation and analysis.The data processing method for the daily data was to remove the trend and climate mean from 1979 to 2022 separately on each day,which removed the seasonal cycle and obtained the daily anomaly.

The difference in area-averaged SAT between the Arctic(70?–85?N,30?–100?E) and Eurasia (40?–60?N,60?–110?E) after detrending was defined as SATAE(the former minus the latter),representing the intensity of WACE (positive SATAE) and CAWE (negative SATAE).The regional selection for the Arctic and Eurasia was based on the second mode of the empirical orthogonal function (EOF2) of the early-winter and late-winter SAT variability (Fig.S1),which reflected the spatial distribution of the WACE/CAWE pattern.The correlation coefficients between the EOF2 time series of early winter and late winter and their SATAEindexes were 0.95 and 0.83,respectively,indicating that the SATAEindex reasonably reflected the variation in WACE/CAWE.The definition of the subseasonal reversal between WACE and CAWE adopted the method in Yin et al.(2023).Among the 43 winters from 1979/80 to 2021/22,14 were identified as years with significant subseasonal reversal (Table S1),including two reversal types: WACE to CAWE(SATAE-R1,six years) and CAWE to WACE (SATAE-R2,eight years).Using similar criteria,persistent WACE and CAWE patterns throughout the whole winter were also selected for seven and eight years,respectively (Table S1).

3.Results

3.1.Subseasonal variation in BKS sea-ice anomalies

Under the subseasonal reversal of SAT in the Arctic–Eurasia,the BKS(70?–82?N,26?–80?E) sea ice also shows a significant subseasonal variation(Fig.1(c,d)).The subseasonal variation in this study refers to the subseasonal difference between the early-winter mean and latewinter mean.The time point of the BKS sea ice subseasonal transition is different under the two inversion types,with 22 January in SATAE-R1 and 15 January in SATAE-R2(Fig.1(d)),and the week between is defined as the transition period of sea ice.As the time points of SATAEtransition are different from that of SICBKS,it is unreasonable to use a single division for early and late winter.Therefore,1 December to 14 (13)January and 23(14)January to 28 February is defined as the early and late winter for BKS sea ice(WACE/CAWE;Yin et al.,2023),respectively.

Accompanied by WACE reversing to CAWE,the sea ice in the BKS increases significantly from early to later winter (Fig.1(d,e)),characterized by significantly and strongly negative anomalies in early winter turning to positive anomalies in later winter (Fig.S2(a,b)).The SAT change from CAWE to WACE corresponds to an anomalous positive to significantly negative sea ice (Fig.S2(c,d)),showing a rapid decline from early to late winter(Fig.1(d,e)).However,when WACE or CAWE persists throughout the whole winter,the sea ice in early and late winter shows significant negative or positive anomalies (Fig.1(e)).The subseasonal variation in BKS sea ice is small (Fig.S2(e–h)),and the large abnormal changes manifest mainly on the winter-mean scale.More importantly,the time points of the area-averaged BKS sea ice (SICBKS)subseasonal transition lag the SATAEboth in SATAE-R1 and SATAE-R2(Fig.1(d)).In the lead–lag correlation coefficients during the 90 days of winter,the SICBKSshows the strongest relationship with SATAEat a lag of 1–3 days whether WACE is reversed or not,which is consistent with previous work by Gong and Luo(2017).The 45-day running correlations between the SATAEand SICBKS,which reflect a relationship change in the period of the running half winter,also present a stable and strongest linkage when the SICBKSlags by 1–2 days in the SATAEreversal years(Fig.S3).This lag relationship suggests that the subseasonal change in BKS sea ice may be mainly driven by atmospheric patterns.

The interannual variations in SATAEand SICBKSin the early-winter mean,late-winter mean,and the difference between the early and late winter are highly related,with correlation coefficients of-0.72,-0.74,and-0.75(all above the 99%confidence level),respectively,during the period from 1979/80 to 2021/22.Although not all SICBKSreverse between positive and negative anomalies on the subseasonal scale in the 14 years of SATAEsubseasonal reversal,all show corresponding increases or decreases,except for two years (1987 and 2016;Fig.1(c)).The SICBKSdifference between early and late winter exceeds plus or minus one standard deviation in 8 of the 14 SATAEreversal years,accounting for 67%of all years that exceed this standard.Especially in the last decade with the increased frequency of SATAEsubseasonal reversal,the amplitude of the SICBKSsubseasonal variation enhances significantly,which weakens the relationship between the SICBKSin early and late winter(Fig.S4(a)).The largest increased and decreased SICBKSfrom early to late winter occurs in the recent winters of 2020/21 and 2021/22,corresponding to the strongest subseasonal reversals of SATAE-R1 and SATAE-R2 (Fig.1(a,b)).This enhanced BKS sea ice subseasonal variability in the last decade is probably due to global warming,which has increased the extremes of variability in the climate system.

3.2.Thermal and dynamic processes associated with the atmosphere driving sea ice

Many studies have revealed that the BKS sea ice variability and WACE are highly correlated with atmospheric processes,especially UB and the NAO,causing warm air and moisture transport to the pole(Luo et al.,2016;Tyrlis et al.,2019).The associated mechanism on the synoptic scale applies well to the subseasonal transition.For the reversal between WACE and CAWE,the inversion between UB and a Ural trough from early to late winter plays an important role in SAT and BKS sea-ice changes (Fig.S5;Yin et al.,2023).The NAO also shows a transition between positive and negative phases.The signal of the NAO is weak in early winter;while in late winter,the combination of a positive NAO phase and an enhanced Ural high or negative NAO phase and a weakened Ural high is significant (Fig.S5),causing an enhanced SATAEresponse of about 0.8?C stronger than in early winter.

Warm and moist air in the Arctic is conducive to sea ice melting through increased downward IR and abnormal downward THF (Gong and Luo,2017),while the loss of sea ice leads to an increase in upward THF that drives a warmer air.Therefore,the direction of THF becomes the key physical process determining whether the atmosphere drives the sea ice or the sea ice drives the atmosphere (Blackport et al.,2019).Because the thermal and dynamic processes associated with atmospheric-driven sea-ice change can be more consistent with SAT variation than the sea ice,the time definition for early and late winter based on SATAEis therefore used to analyze the subseasonal variation in downward IR,THF,and sea-ice drift.

Under the subseasonal reversal from CAWE to WACE,the Arctic SAT changes from cold to warm,accompanied by increased downward IR(Fig.2(b)).Most of the BKS region shows subseasonal variation of anomalous downward THF(Fig.2(b)),which occurs in the ice-free openocean surface of the BKS and at the margins of sea-ice changes.The heat absorbed by the ocean surface warms the water,which can affect the melting of the sea-ice edges and bottom(Liu et al.,2021).This increased THF and downward IR cause the sea ice to melt,indicating the effect of the atmosphere driving the sea ice.From the daily THF variation in the open-ocean surface,the heat exchange of the atmosphere is relatively continuous and stable,which may correspond to the frequent invasion of cold and warm air in early and later winter,respectively (Fig.S6(b)).During the reversal from WACE to CAWE,the downward IR is anomalously negative (Fig.2(a)),and the THF subseasonal variation changes from the downward to upward direction (Fig.S6(a)),which indicates that the atmosphere actively obtains heat to cause the increase in BKS sea ice from early to late winter.Around Nova Zemlya,where the sea ice changes significantly,there is a small downward THF,showing that the significant increase in sea ice has a positive feedback effect on the air to further cool.Although the sea-ice changes in the two reversal types are not exactly opposite in spatial distribution terms,they both manifest mainly as the atmosphere driving the sea ice,which corresponds to the transition point of the SICBKSlagging the SATAEsubseasonal reversal by several days(Fig.1(d)).

Fig.2.(a,b) Differences in sea ice (shading),downward IR (green contours),and THF (red and blue contours represent downwards and upwards,respectively)between early and late winter (latter minus former)composited in SATAE-R1 years and SATAE-R2 years during 1979/80 to 2021/22.(c,d) Correlation coefficients of SATAE and SICBKS with THF in early(shading) and late (contours) winter.(e,f) Differences in sea ice(shading),surface winds(black vectors;units:m s-1) and sea-ice drift (blue vectors;units: cm s-1)between early and late winter (latter minus former)composited in SATAE-R1 years and SATAE-R2 years during 1979/80 to 2021/22.(g,h)The orange and blue bars represent the transition of the zonal (10mU) and meridional (10mV) wind at 10 m and zonal (IceU) and meridional (IceV) sea-ice motion in early and late winter,respectively.The linear trend in each panel has been removed.The shading indicates that the composite results and correlation coefficients are significant above the 95% confidence level.

The correlation between the SATAEand THF on the subseasonal scale indicates that the atmospheric forcing is mainly concentrated in the open ice-free ocean surface(Fig.2(c)),which is at the edge of the sea-ice change.The sea ice obtains or loses heat from/to the adjacent ocean surface,thus causing it to melt or freeze.The process of sea ice forcing the atmosphere appears in the large-value region of sea-ice change,both in early and late winter (Fig.2(d)).Whether the sea ice can drive the atmosphere may be attributable to the intensity of the sea ice anomaly,such as the downward THF anomaly occurring in the area of strongly increasing sea ice when WACE turns to CAWE (Fig.2(a)).Meanwhile,during CAWE reversing to WACE,the reduction in sea ice is relatively weak without the signal of the sea ice driving the atmosphere(Fig.2(b)).It can be clearly seen that the interactions among the atmosphere,ice and ocean are tightly coupled.The subseasonal transition of WACE/CAWE and BKS sea ice presents a holistic subseasonal variation of the Arctic air–ice system.

In addition to the thermal processes,atmospheric patterns can drive sea-ice changes through dynamic processes.The surface wind affects the direction and speed of sea-ice drift and thus causes changes in the sea ice(Yu et al.,2022).The formation of CAWE is accompanied by a weakened UB and Siberian high,which causes a strong northeasterly wind in the BKS (Mori et al.,2019).When the SAT anomaly turns to WACE,the relevant atmospheric circulations also reverse significantly.The direction of surface wind shows an obvious reversal,which becomes southwesterly in late winter (Fig.2(f,h)).Under this effect,the direction of sea-ice drift also changes to the complete opposite.From early to late winter,the sea ice drifts from southwest to northeast in the BKS,from the thin ice near the Arctic Circle to the perennial ice areas at higher latitudes,thus enlarging the open-ocean surface.The drift situation is completely opposite when WACE reverses to CAWE.As the UB changes from being strong in early winter to weak in late winter,the surface wind drives the subseasonal variation in sea ice to drift from perennial ice to thin ice,further increasing the BKS sea ice (Fig.2(e,g)).The regions where the sea-ice drift is strongest correspond to the areas where the sea ice changes most dramatically.Through the surface wind reversals,the atmosphere further forces the subseasonal variation in BKS sea ice.

4.Discussion and conclusion

In the context of the significant and frequent subseasonal reversal of WACE occurring in the last decade,we explored the subseasonal variation in BKS sea ice under such remarkable SAT reversal at a finer scale.The BKS sea ice shows a significant increase and decrease from early to late winter,respectively,under WACE to CAWE and CAWE to WACE(Fig.1).Based on an analysis of the thermal and dynamic processes(Fig.2),the subseasonal variation in BKS sea ice is mainly driven by the atmosphere,reflecting in the transition of the BKS sea ice lagging the SATAEby several days (Fig.1(d)).There is also a signal of the sea ice driving the atmosphere in a large-value area of sea-ice change to form positive feedback,which indicates that changes in sea ice and temperature form a more holistic and enriched WACE pattern from different perspectives.However,in each year of WACE/CAWE subseasonal reversal,the BKS sea ice does not always show a transition between a positive and negative anomaly.This may be related to the different atmospheric circulation formed by different configurations of UB intensity and NAO phase in each year,which deserves further study.

Based on these explorations,we depict a more complete subseasonal variation in the overall “warm Arctic–cold Eurasia” structure(Fig.3(b,c));that is,the WACE temperature pattern extending from the surface to the troposphere in early winter reverses to CAWE in late winter,accompanied by changes in the atmospheric circulation of the UB from strong to weak and the westerly jet from weak to strong (Yin et al.,2023),and the BKS sea ice shows significant subseasonal variation owing to downward IR,heat flux,and sea-ice drift.While on the winter-mean scale,these significant subseasonal reversals are largely masked (Fig.3(a)).Such a comprehensive schematic establishes the subseasonal transition phenomenon of the air–ice system in the Arctic–Eurasia region between early winter and late winter from multiple variables such as temperature,sea ice,and atmospheric circulations.

Fig.3.Subseasonal variation in the overall warm Arctic–cold Eurasia structure,which contains the winter-mean and reversal of the temperature extending from the surface to the troposphere and the BKS sea ice,accompanied by opposite changes in the westerly jet and the Ural high in the troposphere.“”markers represent the large value areas of easterly wind anomalies and “⊙” markers represent the large value areas of westerly wind anomalies.The values in the figure have been composited between SATAE-R1 years and SATAE-R2 in the winter mean,early winter,and late winter (SATAE-R1 years minus SATAE-R2 years).

The warm Arctic–cold Eurasia pattern and Arctic sea ice are the source and accumulation area of cold air,whose subseasonal variation has a significant effect on extreme weather and climate events in lower latitudes.In the winter of 2020/21,when the subseasonal reversal of WACE and BKS sea ice was strongest,the SAT in eastern China also showed a strong subseasonal transition owing to their influence(Zhang et al.,2021),with record-breaking cold and warm events in early and late winter,respectively.In addition,the change from cold first to warm made the soil over the sand source looser and barer,transporting abundant and sufficient sand for the super sandstorm that took place in North China in March 2021 (Yin et al.,2022).Attention to the finer scales of temperature in the mid–high latitude and Arctic sea ice has also provided potential predictors of extreme events in the lower latitudes.

The subseasonal variation of WACE and BKS sea ice has enhanced significantly in the last decade.The BKS sea ice shows no significant trend on the winter-mean scale (Fig.S4(b)),corresponding to a weakened “Arctic warming–Eurasia cooling” trend (Yin et al.,2023).While in the period of the significant“Arctic warming–Eurasian cooling”trend,the frequency of SATAEsubseasonal reversal is very low,and at the same time the BKS sea ice changes weakly in terms of subseasonal variation but melts rapidly on the winter-mean scale.In different time periods,WACE and the BKS sea ice show consistent trend changes and correspond to the intensity of subseasonal variation.In the context of global warming,the trend changes of “Arctic warming–Eurasian cooling” and Arctic sea ice are still uncertain,whose prediction faces huge challenges (Wang et al.,2022).Whether subseasonal variation in BKS sea ice will continue to be as strong as that shown in the last decade under different warming scenarios in the future is worthy of further investigation,thus revealing the role of global warming in extreme events.

Funding

This research was supported by the National Natural Science Foundation of China [grant number 42088101] and the Postgraduate Research and Practice Innovation Program of Jiangsu Province [grant number KYCX22_1147].

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.aosl.2023.100392.