Estimation of snow water equivalent using microwave radiometry over Arctic first‐year sea ice

The magnitude and spatial distribution of snow on sea ice are both integral components of the ocean–sea‐ice–atmosphere system. Although there exists a number of algorithms to estimate the snow water equivalent (SWE) on terrestrial surfaces, to date there is no precise method to estimate SWE on sea ice. Physical snow properties and in situ microwave radiometry at 19, 37 and 85 GHz, V and H polarization were collected for a 10‐day period over 20 first‐year sea ice sites. We present and compare the in situ physical, electrical and microwave emission properties of snow over smooth Arctic first‐year sea ice for 19 of the 20 sites sampled. Physical processes creating the observed vertical patterns in the physical and electrical properties are discussed. An algorithm is then developed from the relationship between the SWE and the brightness temperature measured at 37 GHz (55°) H polarization and the air temperature. The multiple regression between these variables is able to account for over 90% of the variability in the measured SWE. This algorithm is validated with a small in situ data set collected during the 1999 field experiment. We then compare our data against the NASA snow thickness algorithm, designed as part of the NASA Earth Enterprise Program. The results indicated a lack of agreement between the NASA algorithm and the algorithm developed here. This lack of agreement is attributed to differences in scale between the Special Sensor Microwave/Imager and surface radiometers and to differences in the Antarctic versus Arctic snow physical and electrical properties. Copyright © 2003 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…     

Simulating pan‐Arctic runoff with a macro‐scale terrestrial water balance model

A terrestrial hydrological model, developed to simulate the high‐latitude water cycle, is described, along with comparisons with observed data across the pan‐Arctic drainage basin. Gridded fields of plant rooting depth, soil characteristics (texture, organic content), vegetation, and daily time series of precipitation and air temperature provide the primary inputs used to derive simulated runoff at a grid resolution of 25 km across the pan‐Arctic. The pan‐Arctic water balance model (P/WBM) includes a simple scheme for simulating daily changes in soil frozen and liquid water amounts, with the thaw–freeze model (TFM) driven by air temperature, modelled soil moisture content, and physiographic data. Climate time series (precipitation and air temperature) are from the National Centers for Environmental Prediction (NCEP) reanalysis project for the period 1980–2001. P/WBM‐generated maximum summer active‐layer thickness estimates differ from a set of observed data by an average of 12 cm at 27 sites in Alaska, with many of the differences within the variability (1σ) seen in field samples. Simulated long‐term annual runoffs are in the range 100 to 400 mm year^?1. The highest runoffs are found across northeastern Canada, southern Alaska, and Norway, and lower estimates are noted along the highest latitudes of the terrestrial Arctic in North America and Asia. Good agreement exists between simulated and observed long‐term seasonal (winter, spring, summer–fall) runoff to the ten Arctic sea basins (r = 0·84). Model water budgets are most sensitive to changes in precipitation and air temperature, whereas less affect is noted when other model parameters are altered. Increasing daily precipitation by 25% amplifies annual runoff by 50 to 80% for the largest Arctic drainage basins. Ignoring soil ice by eliminating the TFM sub‐model leads to runoffs that are 7 to 27% lower than the control run. The results of these model sensitivity experiments, along with other uncertainties in both observed validation data and model inputs, emphasize the need to develop improved spatial data sets of key geophysical quantities (particularly climate time series) to estimate terrestrial Arctic hydrological budgets better. Copyright © 2003 John Wiley & Sons, Ltd.     

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.     

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.     

Ice-induced seasonal variability of tidal constants in the Arctic Ocean

The results for three-dimensional (3D) winter and summer tidal flows in the homogeneous Arctic Ocean, obtained with the use of a modified version of the 3D finite-element hydrothermodynamic model QUODDY-4, are presented. It is shown that seasonal variability of the M₂ tidal constants (amplitudes and phases of tidal sea surface level elevations) in the Central and Canadian parts of the Arctic Ocean is less than the error in the predicted tidal sea surface level elevations. This means that the seasonal variability can be neglected at least as a first approximation. A different situation is encountered in the Siberian continental shelf, where seasonal changes of tidal amplitude are ±5 cm, while those of tidal phase vary from 15° to several tens of degrees.     

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…     

New satellite derived sea ice motion tracks Arctic contamination

Sea ice has been reported to contain contaminants from atmospheric and nearshore sediment resuspension processes. In this study successive passive microwave images from the 85.5 GHz channels on the Special Sensor Microwave Imager (SSM/I) were merged with drifting buoy trajectories from the International Arctic Buoy Program to compute Arctic sea ice motion in the Russian Arctic between 1988 and 1994. Smooth daily motion fields were averaged to prepare monthly maps making it possible to compute the 7-year mean and mean seasonal ice motions as well as principal components of directional variability of sea ice motion for the entire Arctic and surrounding basins. These mean motion vectors are used to simulate the advection of contaminants deposited on or contained within the sea ice and subsequently transported into the Arctic Ocean in order to predict both their mean trajectories and dispersal over time. The 3-year displacement of contaminants from a number of Russian sites and one American site display various behaviours from substantial displacement and dispersal to almost no movement. This computational procedure could be applied to realtime SSM/I and ice buoy data to provide detailed, all-weather, vector motion maps of ice circulation to predict the path and dispersal of any new substance introduced to the sea ice and transported into the Arctic or Antarctic ocean surface.     

Snow cover and snowmelt of an extensive High Arctic wetland: spatial and temporal seasonal patterns

Abstract

This study examined the end-of-winter snow storage, its distribution and the spatial and temporal melt patterns of a large, low gradient wetland at Polar Bear Pass, Bathurst Island, Nunavut, Canada. The project utilized a combination of field observations and a physically-based snowmelt model. Topography and wind were the major controls on snow distribution in the region, and snow was routinely scoured from the hilltop regions and deposited into hillslopes and valleys. Timing and duration of snowmelt at Polar Bear Pass were similar in 2008 and 2009. The snowmelt was initiated by an increase in air temperature and net radiation receipt. Inter-annual variability in spatial snowmelt patterns was evident at Polar Bear Pass and was attributed to a non-uniform snow cover distribution and local microclimate conditions. In situ field studies and modelling remain important in High Arctic regions for assessing wetland water budgets and runoff, in addition to model parameterization and validation of satellite imagery.

Editor Z.W. Kundzewicz

Citation Assini, J. and Young, K.L., 2012. Snow cover and snowmelt of an extensive High Arctic wetland: spatial and temporal seasonal patterns. Hydrological Sciences Journal, 57 (4), 738–755.     

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.     

Seasonal hydrochemistry of a high Arctic wetland complex

The seasonal evolution in the hydrochemistry of four types of wetland (pond, ice wedge trough, wet meadow and mesic site) was studied in a 0·5 km² wetland complex in the Canadian High Arctic on the Fosheim Peninsula, Ellesmere Island. In the spring, a large influx of overland runoff from snowmelt quickly flushed away the solutes of the ice stored on the wetland surface over the winter, and homogenized the hydrochemistry across the entire wetland complex. As the surface flow receded, various wetland patches became hydrologically disconnected and their hydrochemical characteristics evolved differently. Although underlain by marine sediments and saline permafrost, solute concentrations in much of the wetland complex remained dilute compared with many Arctic wetlands. Through continued evaporation, melting of ground ice and localized thermokarst activities, the hydrochemistry of different wetland types acquired their distinctive characteristics as the summer progressed. This study demonstrates that large diversity in wetland hydrochemistry occurs even within a limited area, indicating the need to caution against generalizations based on limited spatial and temporal samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Role of glacial Arctic Ocean ice sheets in Pleistocene oxygen isotope and sea level records

A comparison of the oxygen isotope signal in deep-sea benthic foraminifera with the record of glacio-eustatic sea level for the last 160,000 years reveals that the amplitude of the benthic δ¹⁸O records predicts more continental ice volume than appears to be reflected in lowered sea level stands. These differences between the benthic δ¹⁸O ice volume estimates and radiometrically-dated records of eustatic sea level are consistent with the presence of a large floating Arctic Ocean ice mass during glacial intervals. The presence of an Arctic Ocean ice sheet during glacial intervals may account for the two climatic modes observed in oxygen isotope records which span the entire Pleistocene. The early Pleistocene (1.8 to 0.9 Myr B.P.) interval is characterized by low-amplitude, high-frequency δ¹⁸O fluctuations between glacial and interglacial periods, while the late Pleistocene (0.9 Myr B.P. to present) is characterized by large-amplitude, low-frequency δ¹⁸O changes. These two climatic modes can be explained by the initiation of earth orbital conditions favoring the co-occurrence of glacial period Arctic Ocean ice sheets and large continental ice sheets approximately 900,000 years before present.     

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.     

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.     

Observational estimation of heat budgets on drifting ice and open water over the Arctic Ocean

It is of major scientific interests to determine the parameters of momentum, heat and vapor exchange in the planetary boundary layer in order to study the effects of ocean-ice-atmosphere interactions and their feedback mechanisms on global climate[1]. Lin…     

Distribution of <Superscript>226</Superscript>Ra in the Arctic Ocean and the Bering Sea and its hydrologic implications

Radium-226 (²²⁶Ra) activities were measured in the surface water samples collected from the Arctic Ocean and the Bering Sea during the First Chinese National Arctic Research Expedition. The results showed that ²²⁶Ra concentrations in the surface water ranged from 0.28 to 1.56 Bq/m³ with an average of 0.76 Bq/m³ in the Arctic Ocean, and from 0.25 to 1.26 Bq/m³ with an average of 0.71 Bq/m³ in the Bering Sea. The values were obviously lower than those from open oceans in middle and low latitudes, indicating that the study area may be partly influenced by sea ice meltwater. In the Bering Sea, ²²⁶Ra in the surface water decreased northward, probably as a result of the exchange between the ²²⁶Ra-deficient sea ice meltwater and the ²²⁶Ra-rich Pacific water. In the Arctic Ocean, ²²⁶Ra in the surface water increased northward and eastward. This spatial distribution of ²²⁶Ra reflected the variation of the ²²⁶Ra-enriched river component in the water mass of the Arctic Ocean. The vertical profiles of ²²⁶Ra in the Canadian Basin showed a concentration maximum at 200 m, which could be attributed to the inputs of the Pacific water or/and the bottom shelf water with high ²²⁶Ra concentration. This conclusion was consistent with the results from ²H, ¹⁸O tracers.     

Hydrological simulation of the Betwa River basin (India) using the SWAT model

A seasonal water budget analysis was carried out to quantify various components of the hydrological cycle using the Soil and Water Assessment Tool (SWAT) model for the Betwa River basin (43?500 km²) in central India. The model results were satisfactory in calibration and validation. The seasonal water budget analysis showed that about 90% of annual rainfall and 97% of annual runoff occurred in the monsoon season. A seasonal linear trend analysis was carried out to detect trends in the water balance components of the basin for the period 1973–2001. In the monsoon season, an increasing trend in rainfall and a decreasing trend in ET were observed; this resulted in an increasing trend in groundwater storage and surface runoff. The winter season followed almost the same pattern. A decreasing trend was observed in summer season rainfall. The study evokes the need for conservation structures in the study area to reduce monsoon runoff and conserve it for basin requirements in water-scarce seasons.

EDITOR Z.W. Kundzewicz

ASSOCIATE EDITOR F. Hattermann     

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.     

New insights into the spatial variability of the surface water carbon dioxide in varying sea ice conditions in the Arctic Ocean

In the summer of 2005, continuous surface water measurements of fugacity of CO₂ (fCO₂^sw), salinity and temperature were performed onboard the IB Oden along the Northwest Passage from Cape Farwell (South Greenland) to the Chukchi Sea. The aim was to investigate the importance of sea ice and river runoff on the spatial variability of fCO₂ and the sea–air CO₂ fluxes in the Arctic Ocean. Additional data was obtained from measurements of total alkalinity (A_T) by discrete surface water and water column sampling in the Canadian Arctic Archipelago (CAA), on the Mackenzie shelf, and in the Bering Strait. The linear relationship between A_T and salinity was used to evaluate and calculate the relative fractions of sea ice melt water and river runoff along the cruise track. High-frequency fCO₂^sw data showed rapid changes, due to variable sea ice conditions, freshwater addition, physical upwelling and biological processes. The fCO₂^sw varied between 102 and 678 μatm. Under the sea ice in the CAA and the northern Chukchi Sea, fCO₂^sw were largely CO₂ undersaturated of approximately 100 μatm lower than the atmospheric level. This suggested CO₂ uptake by biological production and limited sea–air CO₂ gas exchange due to the ice cover. In open areas, such as the relatively fresh water of the Mackenzie shelf and the Bering Strait, the fCO₂^sw values were close to the atmospheric CO₂ level. Upwelling of saline and relatively warm water at the Cape Bathurst caused a dramatic fCO₂^sw increase of about 100 μatm relative to the values in the CAA. At the southern part of the Chukchi Peninsula we found the highest fCO₂^sw values and the water was CO₂ supersaturated, likely due to upwelling. In the study area, the calculated sea–air CO₂ flux varied between an oceanic CO₂ sink of 140 mmol m⁻² d⁻¹ and an oceanic source of 18 mmol m⁻² d⁻¹. However, in the CAA and the northern Chukchi Sea, the sea ice cover prevented gas exchange, and the CO₂ fluxes were probably negligible at this time of the year. Assuming that the water was exposed to the atmosphere by total melting and gas exchange would be the only process, the CO₂ undersaturated water in the ice-covered areas will not have the time to reach the atmospheric CO₂ value, before the formation of new sea ice. This study highlights the value of using high-frequency measurements to gain increased insight into the variable and complex conditions, encountered on the shelves in the Arctic Ocean.     

Regionalisation of hydrological responses under land-use change and variable data quality

ABSTRACT

Estimating river flows at ungauged sites is generally recognised as an important area of research. In countries or regions with rapid land development and sparse hydrological gauging networks, three particular challenges may arise—data scarcity, data quality, and hydrological non-stationarity. Using data from 44 gauged sub-catchments of the upper Ping catchment in northern Thailand from the period 1995–2006, three relevant flow response indices (runoff coefficient, base flow index and seasonal elasticity of flow) were regionalised by regression against available catchment properties. The runoff coefficient was the most successfully regionalised, followed by base flow index and lastly the seasonal elasticity. The non-stationarity (represented by the differences between two 6-year sub-periods) was significant both in the flow response indices and in land use indices; however relationships between the two sets of indices were weak. The regression equations derived from regionalisation were not helpful in predicting the non-stationarity in the flow indices except somewhat for the runoff coefficient. A partly subjective data quality scoring system was devised, and showed the clear influence of rainfall and flow data quality on regionalisation uncertainty. Recommendations towards improving data support for hydrological regionalisation in Thailand include more relevant soils databases, improved records of abstractions and investment in the gauge network. Widening of the regionalisation beyond the upper Ping and renewed efforts at using remotely sensed rainfall data are other possible ways forward.

EDITOR Z.W. Kundzewicz ASSOCIATE EDITOR T. Wagener