Improved sea ice parcel trajectories in the Arctic via data assimilation

An assimilated sea ice motion product is used to track ice parcels in several regions of the Arctic over time periods of one day to several weeks during 1992-1993. Motions simulated using a two-dimensional, dynamic-thermodynamic sea ice model are combined with motions derived from daily 85 GHz special sensor microwave/imager (SSM/I) imagery using an optimal interpolation method that minimizes error covariance. Assimilation attenuates the tracking error over the stand-alone model in comparison to buoy trajectories with the same starting location and time. The average 14-day assimilated trajectory's displacement error is as much as 34% lower than the model trajectory, while the RMS direction error is decreased by up to 10 degrees (24%). Assimilation can also yield an estimate of dispersion, which is not retrievable by point buoy observations. An assimilation approach improves estimates of ice drift and has the potential to further the understanding of ice mass flux, freshwater flux, and pollutant transport in the polar regions.     

Pixel‐scale evaluation of SSM/I sea‐ice algorithms in the marginal ice zone during early fall freeze‐up

Observed reduction in recent sea ice areal extent and thickness has focused attention on the fact that the Arctic marine system appears to be responding to global‐scale climate variability and change. Passive microwave remote‐sensing data are the primary source underpinning these reports, yet problems remain in geophysical inversion of information on ice type and concentration. Uncertainty in sea‐ice concentration (SIC) retrievals is highest in the summer and fall, when water occurs in liquid phase within the snow–sea‐ice system. Of particular scientific interest is the timing and rate of new ice formation due to the control that this form of sea ice has on mass, energy and gas fluxes across the ocean–sea‐ice–atmosphere interface. In this paper we examine the critical fall freeze‐up period using in situ data from a ship‐based and aerial survey programme known as the Canadian Arctic Shelf Exchange study combined with microwave and optical Earth observations data. Results show that: (1) the overall physical conditions observed from aerial survey photography were well matched with coincident moderate‐resolution imaging spectroradiometer data and Radarsat ScanSAR imagery; (2) the shortwave albedo was linearly related to old ice concentration derived from survey photography; (3) the three SSM/I SIC algorithms (NASA Team (NT), NASA Team 2 (NT2), and Bootstrap (BT)) showed considerable discrepancies in pixel‐scale comparison with the Radarsat ScanSAR SICs well calibrated by the aerial survey data. The major causes of the discrepancies are attributed to (1) the inherent inability to detect the new thin ice in the NT and BT algorithms, (2) mismatches of the thin‐ice tie point of the NT2 algorithm, and (3) sub‐pixel ambiguity between the thin ice and the mixture of open water and sea ice. These results suggest the need for finer resolution of passive microwave sensors, such as AMSR‐E, to improve the precision of the SSM/I SIC algorithms in the marginal ice zone during early fall freeze‐up. Copyright © 2006 John Wiley & Sons, Ltd.     

Dipole anomaly in the Arctic atmosphere and winter Arctic sea ice motion

Arctic sea ice, an important component of the cli- mate system, has received significant attention re- cently. Arctic sea ice variation is an important indica- tor of changes in the climate system, such as global change and polar amplification, and observation and climate modeling suggests that sea ice can itself be an agent of climate change[1―4]. Previous studies[3,5] have shown that sea ice influences the surface and atmos- pheric boundary layer temperature. Potentially more important is t…     

大气变率对北极地区近期海冰变化趋势影响数值模拟研究

为研究近期21年(1989—2009年)北极地区海冰变化原因,本文利用欧洲中期天气预报中心ERA-Interim数据集资料和美国麻省理工学院MITgcm全球海冰-海洋耦合模式开展了不同大气强迫条件下海冰变化的数值模拟研究.研究工作中共设计了6个数值试验,除1个试验全部采用1989—2009年每日4个时次的大气强迫场外,其余5个试验各有一种大气强迫(地表气温、地表大气比湿、向下短波辐射通量、向下长波辐射通量和地表风)采用1989年月平均结果.分析了各模拟试验结果中3月和9月北极地区海冰面积的年际变化特征及最小二乘拟合意义下的线性变化趋势,并以ERA-Interim结果为参照标准对各模拟试验结果进行了对比和检验,以说明不同大气强迫量变率对海冰变化的作用.结果表明:地表气温变率和向下长波辐射通量变率是造成海冰面积减少的主要原因;向下短波辐射通量变率对海冰面积变化影响几乎可以忽略;地表大气比湿变率对海冰面积线性变化趋势影响较小,但对海冰面积年际变化特征有调制作用;地表风变率对海冰季节变化、海冰面积线性变化趋势及年际变化特征均有明显影响,说明提高大气风应力精度是改善海冰数值模拟结果的重要手段.     

秋季北极海冰与欧亚冬季气温在年代际和年际尺度上的不同联系

北极海冰的急剧消融在近年来欧亚大陆频发的低温事件中扮演着关键角色.秋季北极海冰的偏少对应着冬季欧亚大陆的低温天气,然而二者的联系在年代际和年际两种时间尺度上存在显著区别.本文运用1979—2012年哈德莱中心第一套海冰覆盖率(HadISST1)、欧洲中心(ERA_Interim)的2m温度、风场、海平面气压场、高度场等资料,分别研究了年代际和年际时间尺度上前期秋季北极海冰与欧亚冬季气温的联系.结果表明,欧亚和北极地区(0°—160°E,15°N—90°N)的冬季气温具有显著的年代际和年际变化.在年代际尺度上,温度异常分布在21世纪初由北极冷-大陆暖转为北极暖-大陆冷.这一年代际转折与前期秋季整个北极地区的海冰年代际减少联系密切.秋季北极全区海冰年代际偏少对应冬季欧亚大陆中高纬地区的高压异常,有利于北大西洋的暖湿气流北上和北极的冷空气南侵,造成北极暖-大陆冷的温度分布;在年际时间尺度上,温度异常分布主要由第一模态的年际变化部分和第二模态组成,且第一模态包含的年际变率信号也存在显著的年代际变化.年际尺度上全区北极海冰对欧亚冬季气温的影响远不及位于北冰洋西南边缘的巴伦支海、喀拉海和拉普捷夫海西部(30°E—120°E,75°N—85°N)的关键区海冰影响显著.关键区内海冰的偏少会引发冬季的北大西洋涛动负位相,导致北大西洋吹往欧亚大陆的暖湿气流减弱和欧亚大陆中高纬地区的气温偏低.     

Arctic sea ice thickness variations from CryoSat-2 satellite altimetry data

Arctic sea ice plays an important role in Earth's climate and environmental system. Sea ice thickness is one of the most important sea ice parameters. Accurately obtaining the sea ice thickness and its changes has great significance to Arctic and global change research. Satellite altimeters can be used to derive long-term and large-scale changes in sea ice thickness. The leads detection is vital in sea ice thickness estimation by using satellite altimetry. Different leads detection methods are compared with remote sensing images, and results show that the detection method that uses waveform parameters can obtain improved results. The model for the conversion of freeboard to thickness is optimized by considering the incomplete penetration of snow for radar altimeters. We derive the estimates of the Arctic sea ice thickness for November 2010 to December 2019 by using the CryoSat-2 altimetry data. The sea ice thickness from the IceBridge and draft data from the upward-looking sonar are used to validate our thickness results. Validations show that the accuracy of our thickness estimates is within 0.2 m. Variations in the Arctic sea ice thickness are analyzed using the PIOMAS model and air and sea surface temperatures. A sharp increase in sea ice thickness is found in 2014.     

Re-calibration of Arctic sea ice extent datasets using Arctic surface air temperature records

A new seasonal and annual dataset describing Arctic sea ice extents for 1901–2015 was constructed by individually re-calibrating sea ice data sources from the three Arctic regions (North American, Nordic and Siberian) using the corresponding surface air temperature trends for the pre-satellite era (1901–1978), so that the strong relationship between seasonal sea ice extent and surface air temperature observed for the satellite era (1979-present) also applies to the pre-satellite era. According to this new dataset, the recent period of Arctic sea ice retreat since the 1970s followed a period of sea ice growth after the mid-1940s, which in turn followed a period of sea ice retreat after the 1910s. Arctic sea ice is a key component of the Arctic hydrological cycle, through both its freshwater storage role and its influence on oceanic and atmospheric circulation. Therefore, these new insights have significance for our understanding of Arctic hydrology.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR not assigned     

雷达卫星SAR与防卫气象卫星SSM/I对渤海海冰的观测研究

用雷达卫星(RADARSAT)合成孔径雷达(SAR)和防卫气象卫星(DMSP)特别微波成像辐射计(SSM/I)在1999年1月23日同一天对中国渤海区域海冰的观测数据，进行了主动SAR与被动SSM/I的组合研究.用一层海冰电磁散射辐射的建模与数值模拟，分析了中纬度渤海沿岸海冰的特征性变化，提出用SSM/I辐射亮度温度的散射指数、极化指数、极化比率来识别渤海海冰；用RADARSAT高分辨率SAR数据(水平极化后向散射系数)对渤海海冰物理特征进行识别与分类.这些特征指数在时间与空间尺度上的相关变化可有效地应用于渤海海冰的监察.     

Shelf-basin exchange in the Laptev Sea in the warming climate: a model study

GCM-based forecast simulations predict continuously increasing seasonality of the sea ice cover and an almost ice-free, summer-time, Arctic Ocean within several decades from the present. In this study we use a primitive equation ocean model: NEMO, coupled with the sea ice model LIM2, to test the hypothesis that under such an increased range in seasonal ice cover the intensity of shelf-basin water exchange will significantly increase. We use the simulated results for the Laptev Sea from a global model run 1958–2007 and compare results for two years with anomalously high and low summer sea ice extents: 1986–1987 and 2006–2007. The shelf–basin fluxes of volume, heat and salt during specific seasons are evaluated and attributed to plausible driving processes, with particular attention to dense water cascading. Analyses of the model temperature distribution at the depth of the intermediate maximum, associated with Atlantic Water, have shown a marked increase of the amount of the local origin cold water in late winter 2007 in the region, where dense water typically appears as a result of its formation on the shelf and subsequent downslope leakage. Calculation of the shelf-basin exchange during March-May in both years confirmed a substantial increase (a factor of two) of fluxes in “ice-free” 2007 compared to the “icy” 1987. According to several past model studies, dense water production on Arctic shelves in winter driven by ice freezing and brine rejection is not likely to cease in a warmer climate, but rather to increase. There is also observational evidence that cascading in the seasonally ice covered seas (e.g. the Barents Sea) is much more efficient than it is in the permanently ice covered Arctic Ocean, which supports these model results.     

Structure of summer atmospheric boundary layer in the center of Arctic Ocean and its relation with sea ice extent change

The atmospheric vertical structure and changed characteristics of boundary layer parameters, as well as their relations with sea ice and temperature changes in the center of Arctic Ocean(80°–88°N) are presented by adopting GPS sounding data obtained by the 4th–6th Arctic expeditions of China and NCEP(National Centre for Environmental Prediction) reanalysis data. Obvious differences are observed regarding the tropopause, boundary layer height, temperature inversion, and vertical structure of wind speed and direction in the center Arctic Ocean in the summer of 2012, 2010, and 2014. These differences can be explained by the relations between temperature and changes in sea ice extent in September from 1979 to 2014. In September 2012, the Arctic sea ice extent decreased by 44% an with obvious warming process. In September 2010 and 2014, it decreased by 22.6% and 17% with an obvious cooling process, respectively. A comparison of the two processes shows that sea ice change has a significant influence on the structure of the atmospheric boundary layer. In the recent 30 years, the temperature changes of 1000 and 850 h Pa in the center of the Arctic Ocean have displayed an obvious warming trend and negative correlation with sea ice extent. These changes indicate that the continuous reduction of Arctic sea ice will continue the warming of the troposphere middle layer.     

Ice and Snow Cover of Continental Water Bodies from Simultaneous Radar Altimetry and Radiometry Observations

We show how the studies of ice and snow cover of continental water bodies can benefit from the synergy of more than 15 years-long simultaneous active (radar altimeter) and passive (radiometer) observations from radar altimetric satellites (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On) and how this approach can be complemented by SSM/I passive microwave data to improve spatial and temporal coverage. Five largest Eurasian continental water bodies—Caspian and Aral seas, Baikal, Ladoga and Onega lakes are selected as examples. First we provide an overview of ice regime and history of ice studies for these seas and lakes. Then a summary of the existing state of the art of ice discrimination methodology from altimetric observations and SSM/I is given. The drawbacks and benefits of each type of sensor and particularities of radiometric properties for each of the chosen water bodies are discussed. Influence of sensor footprint size, ice roughness and snow cover on satellite measurements is also addressed. A step-by-step ice discrimination approach based on a combined use of the data from the four altimetric missions and SSM/I is presented, as well as validation of this approach using in situ and independent satellite data in the visible range. The potential for measurement of snow depth on ice from passive microwave observations using both altimeters and SSM/I is addressed and a qualitative comparison of in situ snow depth observations and satellite-derived estimates is made.     

Sea ice thickness measurement and its underside morphology analysis using radar penetration in the Arctic Ocean

Sea ice, as an important component of the Arctic climate system, has drawn significant sci-entific interest. Sea ice thickness and its morphology have dramatic impacts on ocean-atmos- phere-ice interactions[1—4], which directly affect the exchange proces…     

星载微波ERS-1和多通道SSM/I观测海洋的相关性和随机粗糙面复合模型的数值模拟

欧洲遥感卫星（ERS）和美国防卫气象卫星计划（DMSP）空对地微波遥感是当今研究全球大气地表微波散射辐射和反演地球物理与水文参数的主要数据来源．本文研究了ERS－1散射计和SSM／I多通道辐射计在中国海域观测到的后向散射和热辐射数据,论证了同一地区同一时间段内ERS主动散射计和SSM／I被动辐射计观测数据的相关性．用海域主、被动遥感数据的比较,阐述了主、被动联合多通道分析方法有利于监视和分析复杂地表和海面在时间和空间尺度上的变化．用带泡沫散射层的双尺度随机粗糙面的复合模型计算后向散射和热发射,用以数值模拟ERS和SSM／I数据．并讨论了后向散射与热辐射数值模拟结果的相关性,以及与星载微波遥感器实际观测结果的比较．     

星载微波SSM/I对南中国海海面遥感的辐射特征及其风场反演

研究了星载微波DMSPSSM／I对南中国海海面遥感数据的定量辐射特征．与现有的经验性统计反演不同,本文用37GHz垂直和水平极化两个通道的辐射亮度温度,提出了物理意义明确,又易于计算和参数标定的海面风速反演公式．海面风速及演结果与现有的海面浮标风速观测记录作了很好的比较．这一方法应用于南中国海海面风速的反演,得到了南海海面风速在空间和时间尺度上的分布和变化．     

Toward Improved Estimation of the Dynamic Topography and Ocean Circulation in the High Latitude and Arctic Ocean: The Importance of GOCE

The Arctic plays a fundamental role in the climate system and shows significant sensitivity to anthropogenic climate forcing and the ongoing climate change. Accelerated changes in the Arctic are already observed, including elevated air and ocean temperatures, declines of the summer sea ice extent and sea ice thickness influencing the albedo and CO₂ exchange, melting of the Greenland Ice Sheet and increased thawing of surrounding permafrost regions. In turn, the hydrological cycle in the high latitude and Arctic is expected to undergo changes although to date it is challenging to accurately quantify this. Moreover, changes in the temperature and salinity of surface waters in the Arctic Ocean and Nordic Seas may also influence the flow of dense water through the Denmark Strait, which are found to be a precursor for changes in the Atlantic meridional overturning circulation with a lead time of around 10 years (Hawkins and Sutton in Geophys Res Lett 35:L11603, 2008). Evidently changes in the Arctic and surrounding seas have far reaching influences on regional and global environment and climate variability, thus emphasizing the need for advanced quantitative understanding of the ocean circulation and transport variability in the high latitude and Arctic Ocean. In this respect, this study combines in situ hydrographical data, surface drifter data and direct current meter measurements, with coupled sea ice–ocean models, radar altimeter data and the latest GOCE-based geoid in order to estimate and assess the quality, usefulness and validity of the new GOCE-derived mean dynamic topography for studies of the ocean circulation and transport estimates in the Nordic Seas and Arctic Ocean.     

Organochlorine pesticide and trace metal monitoring of Russian rivers flowing to the Arctic Ocean: 1990-1996

Information is presented on the concentrations of organochlorine pesticides (HCHs and DDT residues) and trace metals (Fe, Cu, Zn and Ni) in waters of 15 large Russian rivers flowing to the Arctic Ocean during 1990-1996. Estimates of the corresponding annual fluxes are made. Other contaminants (Hg, Pb, Cr, Mn, beta-HCH and dihydroheptachlor) were examined briefly. Concentration data are presented as averaged annual means for each of the seven years with the ranges, standard deviations and numbers of samples. Also given are data on locations, the methods of analysis and limited quality assurance data. Data on discharges to the Northern Seas for the more frequently monitored contaminants are given for rivers accounting for >70% of the total northerly flows. Scaled-up fluxes to account for unmonitored rivers as well are given for each sea; totals over the period were: Fe, 1452; Cu, 15; Zn, 59 (x 10(3) t yr(-1)); alpha-HCH, 25; gamma-HCH, 44 (t yr(-1)). Ni was monitored at too few rivers to estimate its total Russian flux. The fluxes for the HCHs considerably exceed previous estimates and indicate that the Arctic Ocean is not in balance as much as was previously believed.     

Predictability of storm wave heights in the ice-free Beaufort Sea

A predictability study on wave forecast of the Arctic Ocean is necessary to help identify hazardous areas and ensure sustainable shipping along the trans-Arctic routes. To assist with validation of the Arctic Ocean wave model, two drifting wave buoys were deployed off Point Barrow, Alaska for two months in September 2016. Both buoys measured significant wave heights exceeding 4 m during two different storm events on 19 September and 22 October. The NOAA-WAVEWATCH III^? model with 16-km resolution was forced using wind and sea ice reanalysis data and obtained general agreement with the observation. The September storm was reproduced well; however, model accuracy deteriorated in October with a negative wave height bias of around 1 m during the October storm. Utilising reanalysis data, including the most up-to-date ERA5, this study investigated the cause: grid resolution, wind and ice forcing, and in situ sea level pressure observations assimilated for reanalysis. The analysis has found that there is a 20% reduction of in situ SLP observations in the area of interest, presumably due to fewer ships and deployment options during the sea ice advance period. The 63-member atmospheric ensemble reanalysis, ALERA2, has shown that this led to a larger ensemble spread in the October monthly mean wind field compared to September. Since atmospheric physics is complex during sea ice advance, it is speculated that the elevated uncertainty of synoptic-scale wind caused the negative wave model bias. This has implications for wave hindcasts and forecasts in the Arctic Ocean.     

Scavenging of thorium isotopes in the Arctic regions: implications for the fate of particle-reactive pollutants

The sources of inorganic pollutants to the Arctic areas are reviewed using previously published results. The removal of particle-reactive pollutants is discussed using thorium scavenging as an analog. The scavenging of 234Th from the upper water column (approximately 100 m) and sediment inventory of 230Th from the deep Arctic waters is compared to different ocean basins in the subarctic areas. Such a comparison shows that 234Th is in equilibrium with its parent, 238U, in certain regions of the Canada Basin of the Arctic Ocean, while it is deficient in other regions of the arctic as well as in sub-polar ocean basins. This implies that the particle-reactive pollutants in the deep Arctic of the Canada Basin are less likely to be removed from the deep waters and will eventually be transported out of this area. We have utilized the 230Th inventory in sediments from the Arctic area to determine the removal rates of particle-reactive nuclides. The 230Th inventory in the deep Arctic Ocean of the Canada Basin is much lower than the Norwegian Sea and the Fram Strait of the Arctic as well as all other sub-polar world oceans. These observations suggest that any pollutants into the deep Arctic areas of the Canada Basin are less likely to be removed locally and may be transported out of this area. In those areas, the colloidal material could potentially play a major role in the removal of particle-reactive contaminants.     

Comparison of a simple planetary boundary layer model with measurements of a turbulent boundary layer under pack ice

A simple analytical theory which describes the motion in a turbulent planetary boundary layer near a rough sea bed by using a two-layer eddy viscosity model is presented. The vertical structure of the current in the boundary layer is presented, and comparisons are made with data fromMcPhee and Smith (1976, Journal of Physical Oceanography,6, 696–711) obtained from measurements of the turbulent boundary layer under drifting Arctic ice.     

Effects of Arctic Sea Ice Decline on Weather and Climate: A Review

The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air temperature, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.