南极洲冰封的边缘正在迅速消失

<正>一项新研究表明:南极洲冰封的边缘正在溶解于日益变暖的海水中,而且在过去的10年里南极冰的消失速度有了显著的增大。南极冰盖是覆盖南极大陆绝大部分地区的厚厚冰层,其漂浮的边缘由延伸到周围海洋中的冰架构成,这些冰架对南极冰盖起到了固定作用,对于"接地部分"的冰起到了支撑作用,有助于减缓冰盖中的冰川流入海洋的速度。然而,不断变暖的海水一直在侵蚀这些冰架的底部,     

ENVISAT ASAR数据用于大区域稻田识别研究

应用高级合成孔径雷达(ASAR)获取的中等分辨率宽幅模式(Wide-Swath Mode)数据提取了江苏省中北部地区的稻田分布信息.通过分析和比较稻田与其他地物的VV极化后向散射时域变化特征,选择合适的阈值和条件,利用阈值分类算法从多时相宽幅模式影像中提取稻田,并结合地面水稻样方,对稻田识别结果进行验证.结果表明,利用ASAR宽幅模式数据进行大区域范围的稻田识别,其精度达到73.68%,为利用中等分辨率雷达数据进行大区域水稻长势监测提供了重要的保障.     

极地大气科学与全球变化研究进展

南极和北极是地球上的气候敏感地区, 也是多个国际科学计划研究全球气候变化的关键地区。极地大气科学考察与研究是极地科学研究的重要组成部分。中国气象科学研究院的极地大气科学考察与研究始于20世纪80年代, 25年来有较大进展。中国气象科学研究院参加了我国组织的23次南极考察、2次北冰洋考察和3次北极考察; 承担了南极长城站和中山站、北极黄河站气象业务建设和维持, 以及中-澳合作南极冰盖3个无人自动气象站工作; 进行了常规地面气象、Brewer大气臭氧、近地面物理、冰雪和大气化学等观测, 获得了较为系统的极地大气环境资料。开展了有关极地大气科学与全球变化的研究, 在极地天气气候特征及气候变化时空多样性、极地海冰变化和南极海冰涛动、极地近地面物理特征和海-冰-气相互作用、中山站臭氧变化特征及南极臭氧洞和大气化学、气候代用资料获取和古气候环境以及极地大气环境变化对东亚环流和中国天气气候影响等方面的研究取得了新进展。中国极地大气科学正积极通过多学科交叉、走国际合作道路, 努力提高对极地在全球变化中作用的认识水平, 并积极探索极地变化对我国气候、环境的影响。     

研究发现海洋暖流是南极冰雪融化的主要原因

欧盟研发框架计划资助,由荷兰、英国和美国科学家组成的研究团队,利用美国宇航局（NASA）ICESat南极观测卫星提供的数据,通过计算机模拟系统的研究分析,得出3点主要结论：（1）海洋暖流的接触是造成南极冰盖和南极冰架（Ice Shelves）下部融化的主要原因;（2）暖热大气是造成南极表面冰雪融化的主要原因;（3）海洋暖流融化大大高于暖热空气融化,因此是南极冰雪融化的最主要原因。     

大气河对南极冰盖及海冰的影响

南极冰盖与海冰对全球气候具有重要影响。大气河作为高纬度地区经向水汽输送的重要途径,其对南极冰盖与海冰的影响在近年来愈发受到重视。南极大气河通常形成于高压脊(阻塞高压)与温带气旋之间的强向极经向输送带内。低频的大气河活动为南极带来强降雪,有利于冰盖质量增加。然而,强暖湿水汽侵入同时会导致表面融化、冰架崩解和极端高温,对冰盖质量存在潜在负贡献。大气河携带极端暖湿水汽与强风通过热力与动力过程导致海冰密集度下降。目前,大气河的识别算法仍不完善,其对液态降水的直接影响、与南大洋的相互作用等仍不清楚,需要进一步明晰大气河对南极冰盖与海冰的影响机制,以准确预估未来大气河对南极冰盖物质平衡与海冰变化的作用。     

Deglacial change of Antarctic Bottom Water in transient simulations

本文利用气候瞬变模拟试验TraCE21k分析了末次冰消期南极底层水(Antarctic Bottom Water,AABW)的演变特征及物理机制.TraCE21k的全强迫试验复现了观测记录里现代大西洋AABW相对于冰期时变薄的特征,其强迫敏感性试验进一步指出冰期-现代AABW的差异主要由大气温室气体的升高及大陆冰川的退缩二者共同驱动.冰消期气候强迫下海洋和大气温度升高,南大洋海冰逐渐消融,后者直接调控了AABW强度和形态的演变.本文还从绝热翻转环流的观点出发,认为冰消期南大洋的气候变化潜在影响了北大西洋深层水的演变.     

南极极冰的时空变化特征及其对大气环流的影响

本文利用1973—1982年南极极冰覆盖面积资料,分析了南极极冰的时空变化特征,并对极冰增长和融化过程的变化进行了谱分析。结果表明,整个南极极冰增长和融化过程的变化存在着准8.2周的主要振荡周期,极冰出现最大增长和融化速率的时间分别在5—6月和11—12月(北半球时间)。还发现南极极冰的变化对北半球西太平洋副高脊线位置的变动及登陆我国台风的数量有重要的影响。     

针对冰盖的定向地球工程研究

针对冰盖的定向地球工程研究旨在增强冰盖稳定性和减缓冰盖物质流失,从源头上减少冰盖对海平面上升的贡献,有望为应对气候变化和保护海岸线争取几百年的时间。冰盖地球工程主要作用在冰底和冰架-海洋界面上,主要途径如下：(1)排干或冻结冰盖底部水来干燥冰床,增强冰盖底部摩擦力;(2)在海洋中建造人造岛来支撑漂浮的冰架;(3)在冰架前端建造水下隔离墙,阻止温暖的海水到达冰川底部以减缓其融化。冰盖地球工程包括数值模拟、方案设计、工程试验和政治法律等诸多方面的研究。国际上的研究团队正在开展数值模拟和方案设计方面的研究,工程试验和政治法律等方面的研究尚未起步。预计工程试验的难度阶梯很可能是从实验室试验开始,到小尺度的野外试验,接着到格陵兰冰盖的入海冰川,最后到南极冰盖的入海冰川。针对冰盖的定向地球工程研究很有可能成为21世纪全球变化领域新兴的研究方向。     

近20年来中国极地大气科学研究进展

南极、北极和青藏高原是地球上的 3大气候敏感地区 ,是多个国际计划研究全球变化的关键地区。中国的南极和北极实地考察研究 ,分别始于 2 0世纪 80和 90年代 ,起步较晚 ,但近 2 0余年来有较大的进展。极地大气科学考察与研究是极地科学研究的重要组成部分。讫今为止 ,中国已组织了 2 0次南极考察和 3次北极考察 ,建立了中国南极长城站、中山站和北极黄河站等 3个常年科学考察站 ;进行了常规地面气象、Brewer大气臭氧、近地面物理、高层大气物理、冰雪和大气化学等观测 ,获得了较为系统的极地大气科学第一手资料 ;开展了有关极地与全球变化的研究 ,取得了新的进展。南极地区大气温度、臭氧和海冰的气候变化在时间和空间上都是多样的。南极地区的增暖主要发生在南极半岛地区 ,在南极大陆主体并不明显 ,近 10余年来还有降温趋势。中国南极长城站和中山站的观测资料也证实了这一点。此外 ,还揭示了南极半岛西侧和罗斯海外围的海冰变化具有“翘翘板”特征 ,由此定义的南极涛动指数可用来讨论南极海冰状况和海冰关键区的活动 ;用实地考察资料研究了极地不同下垫面的近地面物理和海 -冰 -气相互作用特征 ,给出了边界层特征参数 ;讨论了极地天气气候和大气环境特征及其对东亚大气环流和中国天气气候的影响 ;利用     

厄尔尼诺事件和拉尼娜事件的成因与预测

通过对南极气温资料、环南极海冰资料、臭氧变化资料、太平洋海温资料、地球自转速度变化资料、厄尔尼诺和拉尼娜资料的综合验证,发现了构造运动与厄尔尼诺因果关系。大气、海洋与岩石圈的角动量交换在南半球和北半球有不同的形式,这是由陆海分布的差异决定的。南极上空臭氧变化和环南极海冰变化是赤道海温和全球气候准两年振荡的原因。其中,德雷克海峡的海冰变化起主要作用。这个结论给出了作者提出的“海洋锅炉效应”、“臭氧洞漏能效应”、“德雷克海冰气候开关效应”和“大洋地壳跷跷板运动”的相互关系,证明构造运动对厄尔尼诺的重要影响。强潮汐准4a周期的发现,表明南极海冰变化、东太平洋海温变化、地球自转变化和厄尔尼诺都具有4a准周期变化的原因。海温和海冰开关的准2a周期和日食-厄尔尼诺系数理论有较好的预测效果。     

Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations

The Antarctic ice sheet (AIS) has the greatest potential for global sea level rise. This study simulates AIS ice creeping, sliding, tabular calving, and estimates the total mass balances, using a recently developed, advanced ice dynamics model, known as SEGMENT-Ice. SEGMENT-Ice is written in a spherical Earth coordinate system. Because the AIS contains the South Pole, a projection transfer is performed to displace the pole outside of the simulation domain. The AIS also has complex ice-water-granular material-bedrock configurations, requiring sophisticated lateral and basal boundary conditions. Because of the prevalence of ice shelves, a ‘girder yield’ type calving scheme is activated. The simulations of present surface ice flow velocities compare favorably with InSAR measurements, for various ice-water-bedrock configurations. The estimated ice mass loss rate during 2003–2009 agrees with GRACE measurements and provides more spatial details not represented by the latter. The model estimated calving frequencies of the peripheral ice shelves from 1996 (roughly when the 5-km digital elevation and thickness data for the shelves were collected) to 2009 compare well with archived scatterometer images. SEGMENT-Ice’s unique, non-local systematic calving scheme is found to be relevant for tabular calving. However, the exact timing of calving and of iceberg sizes cannot be simulated accurately at present. A projection of the future mass change of the AIS is made, with SEGMENT-Ice forced by atmospheric conditions from three different coupled general circulation models. The entire AIS is estimated to be losing mass steadily at a rate of ~120 km³/a at present and this rate possibly may double by year 2100.     

On the deterioration of icebergs in the marginal ice zone

Abstract

During the Labrador Ice Margin Experiment (LIMEX) of March‐April 1989, the International Ice Patrol (IIP) of the United States Coast Guard deployed two satellite‐tracked TIROS Arctic Drifter (TAD) platforms on two medium‐sized tabular icebergs. The icebergs were drifting in sea ice of about 9/10 concentration east of Newfoundland. These deployments were part of an experiment to examine differential sea‐ice/iceberg motion during spring conditions near the ice margin. Sea‐ice concentration and movement data were collected concurrently by other LIMEX investigators.

The TADs, deployed on 11 March 1989, were tracked using the ARGOS data collection and location system carried on two NOAA polar‐orbiting satellites of the TIROS family. For two months following the deployment, IIP periodically attempted to relocate the icebergs during routine aerial iceberg patrols. One of the TADs stopped transmitting on 23 April 1989 probably because of a major calving event that resulted in the TAD being crushed. As of 24 April the drift rate of the other TAD nearly doubled compared with its drift rate prior to that date, indicating that it had fallen off the iceberg and was floating on water. By 24 April there was no sea ice near either of the two icebergs.

The TAD data provide a unique datasetfor modelling the deterioration of icebergs while they emerge out of the marginal ice zone and travel in open water. It is shown that a good knowledge of the environmental conditions, pariicularly water temperature and sea state, are critical to model successfully the deterioration and calving of the two icebergs.     

Antarctica and global change

The Antarctic region of the globe is of special importance for a wide range of studies of global change. The IGBP research activities needing special focus for global change should be multidisciplinary, should involve both the geosphere and the biosphere, and should be of global as well as local interest. There are a number of important Antarctic research topics which fit these criteria.A decrease of Antarctic sea ice has a positive feedback on global warming. Reduction in the sea ice also impacts on deep ocean circulation and can give a positive feedback to the increase of atmospheric carbon dioxide by the reduction of a deep ocean sink. Changes in the mass balance of the Antarctic ice sheet impact on global sea level. A unique historic record of past climate and global environmental changes is being obtained from deep core drilling in the Antarctic ice sheet. Decreases of stratospheric ozone are most pronounced over the Antarctic in spring. The impact of increases in ultraviolet radiation on the biosphere can be studied in the Antarctic as a precurser to possible changes developing elsewhere around the globe. Changes in the atmosphere and ocean circulations resulting from the decrease in Antarctic sea ice cover can have important effects on ocean surface temperatures which impact on the climates of the continents.These topics are discussed briefly and a number of Antarctic research areas are highlighted which build on existing or planned international programmes and which can make critical contributions to multidisciplinary studies of global change.     

Icebergs in the Southern Ocean and Factors Defining Their Distribution

Using the data of ~58 000 ship observations of Antarctic icebergs in 1947–2014, the map of average summertime concentration of icebergs (namely, of their number within a circle with the radius of 15 nautical miles) in the Southern Ocean was constructed. The main features of the iceberg distribution are revealed, and their possible reasons are investigated. It is shown that in the open ocean sea currents play a key role in the iceberg distribution. Wind effects are pronounced when ocean currents are weak or absent. According to the authors’ estimates, wind plays a decisive role only in the formation of one wide quasimeridional tongue of high iceberg concentration in the Weddell Sea. It is difficult to assess the impact of Antarctic glaciers’ productivity on the iceberg distribution, because currents, wind, and breaking and jamming of icebergs in shallow water areas cause their rapid redistribution. The clear physical explanation of the main features of iceberg concentration distribution on the constructed map indicates that it provides a rather real pattern.     

Information analysis of ice conditions in the water area for determining the monitoring quality and risk of ice formation collision with marine structures

A potential risk is considered for the marine structures caused by the collision with drifting ice formations (icebergs, stamukhas, and pack ice). An informational assessment is applied to the estimation of the ice threat risk. Informational entropy of the system of random influencing factors is assumed as a quantitative measure of uncertainty. It enables to estimate quantitatively the risks within the complex of various critical circumstances. The characteristics of the complex information system consisting of four elementary information systems with different variants of negative circumstances are considered and the information analysis of the risk category for the marine structures of the Barents Sea water area caused by the collision with icebergs is given. It is demonstrated that the reality of the threat of the collision of icebergs with marine structures is significantly lower because the probability of this event q = 1.57 × 10⁻³ is much smaller than the probability of the fatal threat settled “by lot” p* = 0.0625.     

Response of the Antarctic ice sheet to future greenhouse warming

Possible future changes in land ice volume are mentioned frequently as an important aspect of the greenhouse problem. This paper deals with the response of the Antarctic ice sheet and presents a tentative projection of changes in global sea level for the next few hundred years, due to changes in its surface mass balance. We imposed a temperature scenario, in which surface air temperature rises to 4.2° C in the year 2100 AD and is kept constant afterwards. As GCM studies seem to indicate a higher temperature increase in polar latitudes, the response to a more extreme scenario (warming doubled) has also been investigated. The mass balance model, driven by these temperature perturbations, consists of two parts: the accumulation rate is derived from present observed values and is consequently perturbed in proportion to the saturated vapour pressure at the temperature above the inversion layer. The ablation model is based on the degree-day method. It accounts for the daily temperature cycle, uses a different degree-day factor for snow and ice melting and treats refreezing of melt water in a simple way. According to this mass balance model, the amount of accumulation over the entire ice sheet is presently 24.06 × 10¹¹ m³ of ice, and no runoff takes place. A 1°C uniform warming is then calculated to increase the overall mass balance by an amount of 1.43 × 10¹¹ m³ of ice, corresponding to a lowering of global sea level with 0.36 mm/yr. A temperature increase of 5.3°C is needed for the increase in ablation to become more important than the increase in accumulation and the temperature would have to rise by as much as 11.4°C to produce a zero surface mass balance. Imposing the Bellagio-scenario and accumulating changes in mass balance forward in time (static response) would then lower global sea level by 9 cm by 2100 AD. In a subsequent run with a high-resolution 3-D thermomechanic model of the ice sheet, it turns out that the dynamic response of the ice sheet (as compared to the direct effect of the changes in surface mass balance) becomes significant after 100 years or so. Ice-discharge across the grounding-line increases, and eventually leads to grounding-line retreat. This is particularly evident in the extreme case scenario and is important along the Antarctic Peninsula and the overdeepened outlet glaciers along the East Antarctic coast. Grounding-line retreat in the Ross and Ronne-Filchner ice shelves, on the other hand, is small or absent.     

大气对南极冰异常的低频遥响应

运用ＬＡＰＧＣＭ模式证实了大气对南极冰异常的强迫遥响应是激发全球大气季节内振荡的重要机制,进而着重考察候平均偏差结果的时间序列,并且通过带通以处理,特别分析了响应场中３０－６０天低频振荡的重要特征,发现：大报对南极冰减退的响应是一种具有３０－６０天周期的低频遥响应。强迫场中的重要成分是３０－６０天季节内振荡,并且具有同实际大气中的低频振荡相类的垂直结构以及传播和分布特征。     

Generation and atmospheric heat exchange of coastal polynyas in the Weddell Sea

The forcing mechanisms for Antarctic coastal polynyas and the thermodynamic effects of existing polynyas are studied by means of an air-sea-ice interaction experiment in the Weddell Sea in October and November 1986.Coastal polynyas develop in close relationship to the ice motion and form most rapidly with offshore ice motion. Narrow polynyas occur frequently on the lee side of headlands and with strong curvature of the coastline. From the momentum balance of drifting sea ice, a forcing diagram is constructed, which relates ice motion to the surface-layer wind vector v _z and to the geostrophic ocean current vector c _g . In agreement with the data, wind forcing dominates when the wind speed at a height of 3 m exceeds the geostrophic current velocity by a factor of at least 33. This condition within the ocean regime of the Antarctic coastal current usually is fulfilled for wind speeds above 5 m/s at a height of 3 m.Based on a nonlinear parameter estimation technique, optimum parameters for free ice drift are calculated. Including a drift dependent geostrophic current in the ice/water drag yields a maximum of explained variance (91%) of ice velocity.The turbulent heat exchange between sea ice and polynya surfaces is derived from surface-layer wind and temperature data, from temperature changes of the air mass along its trajectory and from an application of the resistance laws for the atmospheric PBL. The turbulent heat flux averaged over all randomly distributed observations in coastal polynyas is 143 W/m². This value is significantly different over pack ice and shelf ice surfaces, where downward fluxes prevail. The large variances of turbulent fluxes can be explained by variable wind speeds and air temperatures. The heat fluxes are also affected by cloud feedback processes and vary in time due to the formation of new ice at the polynya surface.Maximum turbulent fluxes of more than 400 W/m² result from strong winds and low air temperatures. The heat exchange is similarly intense in a narrow zone close to the ice front, when under weak wind conditions, a local circulation develops and cold air associated with strong surface inversions over the shelf ice is heated above the open water.     

Simulating Heinrich event 1 with interactive icebergs

During the last glacial, major abrupt climate events known as Heinrich events left distinct fingerprints of ice rafted detritus, and are thus associated with iceberg armadas; the release of many icebergs into the North Atlantic Ocean. We simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere–ocean model. In our model, dynamic-thermodynamic icebergs influence the climate through two direct effects. First, melting of the icebergs causes freshening of the upper ocean, and second, the latent heat used in the phase-transition of ice to water results in cooling of the iceberg surroundings. This cooling effect of icebergs is generally neglected in models. We investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off. At the peak of the simulated Heinrich event, icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects. The cause of this intriguing result must be sought in the involvement of a secondary mechanism: facilitation of sea-ice formation, which can disturb deep water production at key convection sites, with consequences for the thermohaline circulation. We performed additional sensitivity experiments, designed to explore the effect of the more plausible distribution of the dynamic icebergs’ melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid-latitude North Atlantic (NA) Ocean. The early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments, which must be a distribution-effect: the dynamically distributed icebergs quickly affect western NADW formation, which synergizes with direct sea-ice facilitation, causing an earlier sea-ice expansion and climatic response. Furthermore, compared to dynamic-thermodynamic icebergs, a homogeneous hosing overestimates the fresh water flux in the Eastern Ruddiman belt, causing a fresh anomaly in the Eastern North Atlantic, leading to a delayed recovery of the circulation after the event.     

Book review

Summary and Conclusions The Last Great Ice Sheets provides a valuable compilation for current estimates of the extent and timing of the maximum ice extent of the last ice age period around the globe. The areas of uncertainty are indicated and emphasis is given to points of controversy where further research is needed to resolve the most important problems still remaining. The work thereby presents plausible upper and lower limit estimates for the maximum extent of the ice corresponding to about 18 ka BP. These limits can be used for the boundary conditions required for atmospheric climate modelling studies. For this type of application the differences in the upper and lower limits are not serious and consequently the possible errors in the reconstruction estimates are not so important. The greatest uncertainties in the ice cover reconstructions occur for the northerly limits of the grounded ice which tend to be in off-shore regions where reliable data is sparse. This signals the requirement for a much greater research effort to collect off-shore sediment sequences and other data to help clarify the ice cover fluctuation record. The possibility of an extensive arctic ice shelf system and marine ice sheet cover interconnecting with the various grounded ice sheets is put forward as a working hypothesis along with arguments in favour of the upper limit estimates for the maximum ice extent. The extent of the ice age ice shelves is an important question which should be examined by dynamic ice sheet modelling with explicit ice shelf formulation. The extensive growth of the ice sheets to seaward margins leads naturally to ice shelf formation. The major questions are: how extensive were the ice shelves, and to what extent did coalescence occur? The further hypothesis that the ice shelves may have had a major role in the growth and decay of the ice sheets is more difficult to support since the results of the dynamic modelling indicate that the ice shelves form readily as a result of extensive growth of the ice sheets but it is difficult to start extensive ice growth near sea level. The information presented for the timing of the advance towards the maximum ice cover and the subsequent pattern of retreat provides further valuable material to test the dynamic ice sheet and climate models including the reactions to the Earth's orbital radiation changes. These results for the rates of change of the ice cover taken together with the modelling results of Budd and Smith indicate that neither the ice sheets nor the bedrock depression had time to reach equilibrium states. The non-equilibrium nature of the ice sheets with only short periods between relatively rapid advance and retreat phases is an important factor that needs to be taken into account in assessing the reconstructions of the ice sheets. This emphasises the need for further work with dynamic ice sheet models and coupled global atmosphere ocean models to determine more clearly the sequence of changes during the growth and decay of the large ice sheets. ‘The Last Great Ice Sheets’ provides a timely data base and compilation to support these studies.