A Statistical Linkage between Extreme Cold Wave Events in Southern China and Sea Ice Extent in the Barents-Kara Seas from 1289 to 2017

Arctic sea ice loss and the associated enhanced warming has been related to midlatitude weather and climate changes through modulate meridional temperature gradients linked to circulation. However, contrasting lines of evidence result in low confidence in the influence of Arctic warming on midlatitude climate. This study examines the additional perspectives that palaeoclimate evidence provides on the decadal relationship between autumn sea ice extent (SIE) in the Barents-Kara (B-K) Seas and extreme cold wave events (ECWEs) in southern China. Reconstruction of the winter Cold Index and SIE in the B-K Seas from 1289 to 2017 shows that a significant anti-phase relationship occurred during most periods of decreasing SIE, indicating that cold winters are more likely in low SIE years due to the “bridge” role of the North Atlantic Oscillation and Siberian High. It is confirmed that the recent increase in ECWEs in southern China is closely related to the sea ice decline in the B-K Seas. However, our results show that the linkage is unstable, especially in high SIE periods, and it is probably modulated by atmospheric internal variability.

Changes in the Proportion of Precipitation Occurring as Rain in Northern Canada during Spring–Summer from 1979–2015

Changes in the form of precipitation have a considerable impact on the Arctic cryosphere and ecological system by influencing the energy balance and surface runoff. In this study, station observations and ERA-Interim data were used to analyze changes in the rainfall to precipitation ratio（RPR） in northern Canada during the spring–summer season（March–July）from 1979–2015. Our results indicate that ERA-Interim describes the spring–summer variations and trends in temperature and the RPR well. Both the spring–summer mean temperature [0.4℃–1℃（10 yr）^（-1）] and the RPR [2%–6%（10 yr）^（-1）] increased significantly in the Canadian Arctic Archipelago from 1979–2015. Moreover, we suggest that, aside from the contribution of climate warming, the North Atlantic Oscillation is probably another key factor influencing temporal and spatial differences in the RPR over northern Canada.

Measurements of physical characteristics of summer snow cover on sea ice during the Third Chinese Arctic Expedition

The spatial distribution of snow cover on the central Arctic sea ice is investigated here based on the observations made during the Third Chinese Arctic Expedition. Six types of snow were observed during the expedition： new/recent snow, melt-fi＇eeze crust, icy layer, depth hoar, coarse-grained, and chains of depth hoar. Across most measurement areas, the snow surface was covered by a melt-freeze crust 2-3 cm thick, which was produced by alternate strong solar radiation and the sharp temperature decrease over the summer Arctic Ocean. There was an intermittent layer of snow and ice at the base of the snow pack. The mean bulk density of the snow was 304.01~29.00 kg/m3 along the expedition line, and the surface values were generally smaller than those of the sub- surface, confirming the principle of snow densification. In addition, the thicknesses and water equivalents of the new/recent and total-layer snow showed a decreasing trend with latitude, suggesting that the amount of snow cover and its spatial variations were mainly determined by precipitation. Snow temperature also presented significant variations in the vertical profile, and ablation and evaporation were not the primary factors in the snow assessment in late summer. The mean temperature of the surface snow was -2.01±0.96℃, which was much higher than that observed in the interface of snow and sea ice.

On the Shallowing of Antarctic Low-Level Temperature Inversions Projected by CESM-LE under RCP8.5

Temperature inversions are frequently observed in the boundary layer and lower troposphere of polar regions.Future variations of the low-level temperature inversions in these regions,especially the Antarctic,are still poorly understood.Due to the scarcity of observations in the Antarctic,reanalysis data and numerical simulations are often used in the study of Antarctic climate change.Based on ERA-Interim,ERA5,JRA-55,and NCEP-NCAR reanalysis products,this study examines temporal and spatial variations of Antarctic inversion depth in austral autumn and winter during 1979-2020.Deeper inversions are found to occur over the high plateau areas of the Antarctic continent.Based on the Mann-Kendall test,ERA-Interim and ERA5 data reveal that the Antarctic inversion depth in austral autumn and winter increased during 1992-2007,roughly maintained afterwards,and then significantly decreased since around 2016.The decrease trend is more obvious in the last two months of winter.Overall,JRA-55 better represents the spatial distribution of inversion depth,and ERA-Interim has better interannual variability.The Community Earth System Model Large Ensemble(CESM-LE)30-member simulations in 1979-2005 were first verified against JRA-55,showing reasonable consistency,and were then used to project the future changes of Antarctic low-level inversion depth over 2031-2050 under RCP8.5.The CESM-LE projection results reveal that the temperature inversion will shallow in the Antarctic at the end of the 21st century,and the decrease in depth in autumn will be more pronounced than that in winter.In particular,the temperature inversion will weaken over the ice-free ocean,while it will remain stable over the ice sheet,showing certain spatial heterogeneity and seasonal dependence on the underlying cryospheric surface conditions.In addition,the decrease of inversion depth is found closely linked with the reduction in sea ice,suggesting the strong effect of global warming on the thermal structure change of the Antarctic.

北极地区春季降水呈现固态向液态转变的态势

降水形态的变化可以影响地表的温度和反照率,对下垫面物质和能量平衡、陆地水文及生态系统均产生极大影响.基于美国阿拉斯加8站和加拿大11站日平均气温和固态、液态降水资料拟合的固-液态降水临界气温,辨析了1961~2010年环北极地区253个站点的降水形态时空变化特征.结果表明:60°N以北地区,降雨量占总降水量的比值(rainfall to total precipitation ratio,RPR)随纬度升高而减小.RPR气候平均态在夏季最高,秋季、春季次之,冬季最小.在不同季节,RPR变化趋势存在明显的区域差异.在春季,RPR变化趋势较为一致,在北极大部分地区(82.46%站点)呈增加趋势,且有22.37%站点通过显著性检验,表明北极大部分地区春季降水在过去50多年间呈现由固态向液态转变的趋势.使用95%置信区间上限和下限临界温度对降水形态进行划分和趋势分析,其结果与使用最优解的计算结果一致.在北极冰雪开始消融的春夏季节转换期(3~7月),阿拉斯加、中西伯利亚和北欧部分地区存在明显的固态降水向液态降水转变的趋势,这一趋势可能正在对北极地-气相互作用施加着影响.

Sources, evolution and impacts of EC and OC in snow on sea ice： a measurement study in Barrow, Alaska

Based on the field measurements in Barrow, Alaska within the period of April-May 2015, we investigate the sources and variations of elemental carbon(EC) and organic carbon(OC) in the surface layer of snowpack on sea ice, and estimate their effects on the sea ice albedo. Results show that the snow OC in Barrow are from natural sources(e.g. terrestrial higher plants and micro-organisms) mainly, as well as biomass burning(e.g. forest fires and straw combustion) as an important part. Both EC and OC can accumulate at the snow surface with snow melt. The variations in EC and OC and liquid water content in the snow layer are well consistent during the snow-melting period. A higher rate of snow melt implied a more efficient enrichment of EC and OC. In the last phase of snow melt, the concentration increased to a maximum of 16.2 ng/g for EC and 128 ng/g for OC, which is ~10 times larger than those before snow melt onset. Except for the dominant influence of melt amplification mechanism, the variation in concentrations of EC and OC could be disturbed by the air temperature fluctuation and snowfall. Our study indicates that the lightabsorbing impurities contributed 1.6%-5.1% to the reduction in sea ice albedo with melt during the measurement period. The significant period oflight-absorbing impurities influencing on sea ice albedo begins with the rapid melting of overlying snow and ends before the melt ponds formed widely, which lasted for about 10 days in Barrow, 2015.

Reconstruction of autumn sea ice extent changes since AD1289 in the Barents-Kara Sea, Arctic

Using high-resolution ice core and tree ring proxies for sea ice extent （SIE）, we reconstructed a robust time series of autumn SIE over the Barents-Kara （B-K） sector of the Arctic from AD 1289-1993. After intercomparing the results and statistical parameters using the ordinary least squares regression （OLSR）, the principle component regression （PCR） and the partial least squares regression （PLSR） methods, SIE time series were synthesized into a more robust series using the weighted average method, which used the explained variances as weights. The results showed that from the end of the 13th century to the end of 18th century, the autumn B-K SIE was large, with large variations and a slightly expanding trend overall. This reflected significant multideeadal oscillations under the Little Ice Age （LIA） background. The B-K SIE began to decrease at the end of the 18th century, and a shrinking trend became significant during the second half of the 19th century, which lasted into the 1930s- 1940s. The 1930s-1940s was a period with a relatively low SIE in the B-K Sea, and the SIE had a short period of expansion from the 1940s-1970s. However, the B-K SIE has continuously and significantly shrank since the 1970s. The reduction in the B-K SIE since the end of the 18th century has been unprecedented in both duration and speed over the last 700 years. The B-K SIE has retreated significantly since the 1970s, with a speed 6.18 times greater than the former mean retreating speed. The industrial revolution may be a dominant factor in this result. The Arctic SIE in recent years may be the lowest it has been over the last millennium.

大气冰冻圈总量与时空结构

大气冰冻圈是大气层内所有冰体的总称,包括冰晶、雪花等,对全球辐射平衡、水循环和灾害天气的形成均具有重要影响,长期以来受到广泛关注.然而,将大气冰冻圈作为一个整体圈层的定量研究较少,相关认识基本停留在概念和理论阶段.本文基于CloudSat和CALIPSO卫星数据联合反演的DARDAR产品,估算了全球大气冰冻圈的总量、三维空间分布和季节变化特征.结果显示:大气冰冻圈总质量约为63[58.8~65.7]Gt,上界为高层冰云的云顶,下界略低于零温层.大尺度环流系统决定了大气冰冻圈的纬向带状分布特征,冰晶形成与增长的微物理机制导致大气冰体在热带上空–8°C等温层出现高值中心.大气冰体的高值区主要分布于低纬度强对流区,从垂直剖面上看,主要分布于对流层中层和低层.痕量冰区(冰水路径<15.0 g/m^(2))主要分布于副热带等高压系统控制区.在高海拔地区,由于水汽含量较低,大气冰含量整体偏低,如青藏高原冬季冰水路径仅为31.3 g/m^(2),约为全球平均值的四分之一.该项研究为进一步认识大气冰冻圈的天气气候效应奠定了基础.