At the Last Glacial Maximum (LGM) about 21000 years ago (21 ka BP), theoverall mass balance of the Laurentide and Eurasian ice sheets should havebeen close to zero, since their rate of change of total ice volume wasapproximately zero at that time. The surface mass balance should have beenzero or positive to balance any iceberg/iceshelf discharge and basalmelting, but could not have been strongly negative. In principle this can betested by global climate model (GCM) simulations with prescribed ice-sheetextents and topography.

We describe results from a suite of 21 ka BP simulations using a new GCM(GENESIS version 2.0.a), with sea-surface temperatures (SSTs) prescribedfrom GLIMAP (1981) and predicted by a mixed-layer ocean model, and with icesheets prescribed from both the ICE-4G (Peltier, 1994) and CLIMAP (1981) reconstructions. This GCM iswell suited for ice-sheet mass-balance studies because (i) the surface canbe represented at a finer resolution than the atmospheric GCM, (ii) anelevation correction accounts for spectral distortions of the atmosphericGCM topography, (iii) a simple post-processing correction for the refreezingof meltwater is applied, and (iv) the model's precipitation and massbalances for present-day Greenland and Antarctica are realistic. However,for all reasonable combinations of SSTs and ice-sheet configurations, thepredicted annual surface mass balances of the LGM Laurentide and Eurasianice sheets are implausibly negative. Possible reasons for this discrepancyare discussed, including increased ice-age aerosols, higher CLIMAP-likeice-sheet profiles in the few thousand years preceding the LGM, and asurface of the southern Laurentide just before the LGM to produce fleetinglythe ICE-4G profile at 21 ka BP.

The response of the atmospheric boundary layer (ABL) to subgrid-scale variations of sea ice properties and fracturing is poorly understood and not taken into account in mesoscale Numerical Weather Prediction (NWP) model parametrizations. In this paper we analyze three-dimensional air circulation within the ABL over fragmented sea ice. A series of idealized high-resolution simulations with the Weather Research and Forecasting (WRF) model is performed for several spatial distributions of ice floes and leads for two values of sea ice concentration (0.5 and 0.9) and several ambient wind speed profiles. The results show that the convective circulation within the ABL is sensitive to the subgrid-scale spatial distribution of sea ice. Considerable variability of several domain-averaged quantities – cloud liquid water content, surface turbulent heat flux (THF) – is found for different arrangements of floes. Moreover, the organized structure of air circulation leads to spatial covariance of variables characterizing the ABL. Based on the example of THF, it is demonstrated that this covariance may lead to substantial errors when THF values are estimated from area-averaged quantities, as it is done in mesoscale NWP models. This suggests the need for developing suitable parametrizations of ABL effects related to subgrid-scale sea ice features for these models.

Ice-flow properties within a polar ice sheet are examined using the comprehensive data gathered from ice-core drilling by Australian National Antarctic Research Expeditions (ANARE) at Dome Summit South (DSS), on Law Dome, East Antarctica. Using the shear strain rates derived from borehole inclination measurements we demonstrate the need to modify the ice-flow relations to treat enhanced shear deformation deep within the ice sheet. We show that the relation between enhanced flow and the measured crystallographic properties is generally in accord with expectations, at least in the upper parts of the ice sheet, but it becomes clear that nearer to the bedrock the situation is more complicated. We also compare the observed shear strain-rate profile with results from a model that describes flow enhancement as a function of the applied stresses.

Isochronal layers in firn detected with ground-penetrating radar (GPR) and dated using results from ice-core analyses are used to calculate accumulation rates along a 100 km across-flow profile in West Antarctica. Accumulation rates are shown to be highly variable over short distances. Elevation measurements from global positioning system surveys show that accumulation rates derived from shallow horizons correlate well with surface undulations, which implies that wind redistribution of snow is the leading cause of this variability. Temporal changes in accumulation rate over 25–185 year intervals are smoothed to along-track length scales comparable to surface undulations in order to identify trends in accumulation that are likely related to changes in climate. Results show that accumulation rates along this profile have decreased in recent decades, which is consistent with core-derived time series of annual accumulation rates measured at the two ends of the radar profile. These results suggest that temporal variability observed in accumulation-rate records from ice cores and GPR profiles can be obscured by spatial influences, although it is possible to resolve temporal signals if the effects of local topography and ice flow are quantified and removed.

In the past, ionic analyses of deep ice cores tended to consist of a few widely spaced measurements that indicated general trends in concentration. the ion-chromatographic methods widely used provide well-validated individual data, but are time-consuming. the development of continuous flow analysis (CFA) methods has allowed very rapid, high-resolution data to be collected in the field for a wide range of ions. In the European Project for Ice Coring in Antarctica (EPICA) deep ice-core drilling at Dome C, many ions have been measured at high resolution, and several have been analyzed by more than one method. the full range of ions has been measured in five different laboratories by ion chromatography (IC), at resolutions of 2.5–10 cm. In the field, CFA was used to measure the ions Na+, Ca2+, nitrate and ammonium. Additionally, a new semi-continuous in situ IC method, fast ion chromatography (FIC), was used to analyze sulphate, nitrate and chloride. Some data are now available to 788 m depth. In this paper we compare the data obtained by the three methods, and show that the rapid methods (CFA and FIC) give an excellent indication of trends in ionic data. Differences between the data from the different methods do occur, and in some cases these are genuine, being due to differences in speciation in the methods. We conclude that the best system for most deep ice-core analysis is a rapid system of CFA and FIC, along with in situ meltwater collection for analysis of other ions by IC, but that material should be kept aside for a regular check on analytical quality and for more detailed analysis of some sections.

A local two-dimensional flow model which accounts for the anisotropic behaviour of polar ice and the evolution of its strain-induced anisotropy is briefly reviewed. Due to its complexity, it is not yet possible to use this model to simulate the flow of a whole ice sheet, and its potential applications are presently restricted to limited spatial domains around existing drilling sites. In order to calculate the local flow of ice, boundary conditions must be applied on the lateral edges of the studied domain. Since these limits correspond to fictitious sections of the ice sheet, the type of boundary condition to adopt is not obvious. In the present paper, different kinds of boundary conditions of the Dirichlet type, applied at the lateral boundary of an idealized ice sheet of simplified geometry, are discussed. This will serve as a first step towards the coupling of the local flow model with a global ice-sheet flow model.

Conditions for passive folding near ice-sheet centers are derived treating the ice as an anisotropic viscous medium. Vertical uniaxial compression at a dome, or pure shear stress near a ridge divide, both tend to stretch and flatten folds, while horizontal simple shear deformation tends to overturn folds. Overturned folding is likely in a given vicinity in a steady-state flow field, if the initial slope of a layer disturbance exceeds the ratio of compressive/extensive deformation to shear deformation. Analytical equations for particle tracks in steady state allow us to model the evolution of layers with initial slope disturbances (``wrinkles’’). the effects of anisotropy are explored using an analytical solution for the strain rate as a function of a vertically symmetric c-axis orientation distribution, called a cone fabric. Stronger anisotropy (small cone angle) makes the material softer in horizontal shear, and facilitates folding, i.e. elements with smaller slopes can be overturned for the same stress. the relation between anisotropy and folding is complicated by the fact that, for a range of cone angles, the material is also softer in compression, which opposes folding. Simulating a layer with spatially variable tilt of cone symmetry axes, and accounting for fabric development, demonstrates that variations in the fabric cause localized flow variations that could create the initial perturbations.

A strong volcanic-acid signal is clearly registered, using an acidity-measuring technique, in the A.D. 1259 ice layer in four different Greenland ice cores (Camp Century, Milcent, Crête and Dye 3). This signal is similar in amplitude to the Laki (Iceland) A.D. 1783 volcanic event as recorded in the central and south Greenland ice cores. Measurement of ice layers from corresponding age levels in Antarctic ice cores (Byrd Station, South Pole and J–9 on the Ross Ice Shelf) provides similar strong acid signals. There is no historical record of a significant volcanic eruption for the period around A.D. 1260 in the Northern Hemisphere. Subsequent chemical analyses of all A.D. 1259 ice layers show similar compositions. We suggest that the A.D. 1259 signals registered in both Greenland and Antarctica were caused by the same volcanic disturbance and that its epicenter was located at the Earth’s equatorial zone, which enabled global distribution of the acid gases. These results indicate that inter-hemispheric dating of ice sheets is possible by the chemical identification of major eruptive volcanic events in the equatorial zone.

Significant salinity anomalies have been observed in the Arctic Ocean surface layer during the last decade. Our study is based on an extensive gridded dataset of winter salinity in the upper 50 m layer of the Arctic Ocean for the periods 1950–1993 and 2007–2012, obtained from ~20 000 profiles. We investigate the interannual variability of the salinity fields, identify predominant patterns of anomalous behavior and leading modes of variability, and develop a statistical model for the prediction of surface-layer salinity. The statistical model is based on linear regression equations linking the principal components of surface-layer salinity obtained through empirical orthogonal function decomposition with environmental factors, such as atmospheric circulation, river runoff, ice processes and water exchange with neighboring oceans. Using this model, we obtain prognostic fields of the surface-layer salinity for the winter period 2013–2014. The prognostic fields generated by the model show tendencies of surface-layer salinification, which were also observed in previous years. Although the used data are proprietary and have gaps, they provide the most spatiotemporally detailed observational resource for studying multidecadal variations in basin-wide Arctic salinity. Thus, there is community value in the identification, dissemination and modeling of the principal modes of variability in this salinity record.

Supraglacial lakes are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focused on lakes on Greenland's floating termini. Here, we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intraseasonal evolution of supraglacial lakes across Petermann Glacier's (81°N) floating tongue from 2014 to 2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment.

In this paper an algorithm for ice/water classification of C- and L-band dual polarization synthetic aperture radar data is presented. A comparison of the two different frequencies is made in order to investigate the potential to improve classification results with multi-frequency data. The algorithm is based on backscatter intensities in co- and cross-polarization and autocorrelation as a texture feature. The mapping between image features and ice/water classification is made with a neural network. Accurate ice/water maps for both frequencies are produced by the algorithm and the results of two frequencies generally agree very well. Differences are found in the marginal ice zone, where the time difference between acquisitions causes motion of the ice pack. C-band reliably reproduces the outline of the ice edge, while L-band has its strengths for thin ice/calm water areas within the icepack. The classification shows good agreement with ice/water maps derived from met.no ice-charts and radiometer data from AMSR-2. Variations are found in the marginal ice zone where the generalization of the ice charts and lower accuracy of ice concentration from radiometer data introduce deviations. Usage of high-resolution dual frequency data could be beneficial for improving ice cover information for navigation and modelling.

Sea-ice thickness in the Sea of Okhotsk is estimated for 2004–2008 from ICESat derived freeboard under the assumption of hydrostatic balance. Total ice thickness including snow depth (htot) averaged over 2004–2008 is 95 cm. The interannual variability of htot is large; from 77.5 cm (2008) to 110.4 cm (2005). The mode of htot varies from 50–60 cm (2007 and 2008) to 70–80 cm (2005). Ice thickness derived from ICESat data is validated from a comparison with that observed by Electromagnetic Induction Instrument (EM) aboard the icebreaker Soya near Hokkaido, Japan. Annual maps of htot reveal that the spatial distribution of htot is similar every year. Ice volume of 6.3 × 1011 m3 is estimated from the ICESat derived htot and AMSR-E derived ice concentration. A comparison with ice area demonstrates that the ice volume cannot always be represented by the area solely, despite the fact that the area has been used as a proxy of the volume in the Sea of Okhotsk. The ice volume roughly corresponds to that of annual ice production in the major coastal polynyas estimated based on heat budget calculations. This also supports the validity of the estimation of sea-ice thickness and volume using ICESat data.

Using repeat GPS measurements during 2005–16, we calculated and updated two-dimensional high-resolution decadal ice surface velocity estimates along the traverse route from Zhongshan Station to and around Dome Argus, East Antarctica. Along the 71 sites of the transect, the magnitudes of ice velocity increased from near 0 in Dome Argus to 1, 10 and ~100 m a−1 at the sites DT416, DT333 and LT980, respectively. The comparison between GPS and interferometric synthetic aperture radar (InSAR) derived results agree well when the magnitude of the ice surface velocities is faster than 5 m a−1, and disagree for slower flow velocities. A scale value 1.15 and 0.12 can be applied to InSAR derived results over this region with ice surface velocity larger and <5 m a−1, respectively. We attributed the cause of the discrepancy to the insensitivity of InSAR to the magnitude of low ice surface velocities, thus confirming the importance of GPS fieldwork-based ground truth high-resolution ice velocity estimates to constrain ice-sheet dynamics.

The presence of leads with open water or thin ice is an important feature of the Arctic sea ice cover. Leads regulate the heat, gas and moisture fluxes between the ocean and atmosphere and are areas of high ice growth rates during periods of freezing conditions. Here, an algorithm providing an automatic lead detection based on synthetic aperture radar images is described that can be applied to a wide range of Sentinel-1 scenes. By using both the HH and the HV channels instead of single co-polarised observations the algorithm is able to classify more leads correctly. The lead classification algorithm is based on polarimetric features and textural features derived from the grey-level co-occurrence matrix. The Random Forest classifier is used to investigate the importance of the individual features for lead detection. The precision–recall curve representing the quality of the classification is used to define threshold for a binary lead/sea ice classification. The algorithm is able to produce a lead classification with more that 90% precision with 60% of all leads classified. The precision can be increased by the cost of the amount of leads detected. Results are evaluated based on comparisons with Sentinel-2 optical satellite data.

Pressure ridges impact the mass, energy and momentum budgets of the sea-ice cover and present an obstacle to transportation through ice-infested waters. Quantifying ridge characteristics is important for understanding total sea-ice mass and for improving the representation of sea-ice dynamics in high-resolution models. Multi-sensor measurements collected during annual Operation IceBridge (OIB) airborne surveys of the Arctic provide new opportunities to assess the sea ice at the end of winter. We present a new methodology to derive ridge sail height from high-resolution OIB Digital Mapping System (DMS) visible imagery. We assess the efficacy of the methodology by mapping the full sail height distribution along 12 pressure ridges in the western and central Arctic. Comparisons against coincident Airborne Topographic Mapper (ATM) elevation anomalies are used to demonstrate the methodology and evaluate DMS-derived sail heights. Sail heights and elevation anomalies were correlated at 0.81 or above. On average mean and maximum sail height agreed with ATM elevation to within 0.11 and 0.49 m, respectively. Of the ridges mapped, mean sail height ranged from 0.99 to 2.16 m, while maximum sail height ranged from 2.1 to 4.8 m. DMS also delivered higher sampling along ridge crests than coincident ATM data.

As the focus of intensive glaciological studies in the 1960–70s, White Glacier on Axel Heiberg Island, Canada, has played an important role in understanding the dynamics of mostly-cold polythermal glaciers in the high Arctic. In this study, we examine the magnitude, duration and timing of peak velocity events in the summers of 2013–15 using continuous dual-frequency GPS observations, and compare them with similar measurements made in 1968. Summer speed-up events in 1968 and 2014, in which ice velocities reached 200% above winter values, were found to occur in conjunction with formation and drainage of an ice-marginal lake. Despite thinning of the glacier by >20 m and a decrease in annual surface velocities of 15–35% since the 1960s, the relative magnitude and duration of these peak events has increased, particularly at lower elevations, in comparison with the observations at the same locations many decades ago. Given the long-term slowdown of the glacier, the relative contribution of summer displacement to the net annual motion has therefore increased significantly, with summer motion over the span of <2 months now accounting for nearly half of the total annual displacement.

Seasonal snow-cover modulates water and energy budgets across large areas of the Northern Hemisphere. Previous research, based on satellite imagery interpreted and curated by the Rutgers University Snow Laboratory, has identified significant negative and positive trends in annual snow-covered duration and area at hemispheric and continental scales between 1971 and 2014. This study uses the same dataset to generate more detailed descriptions of spatial variations in these trends, maps intraannual variations in sign, statistical significance and strength, and quantifies associations with latitude and elevation. It also considers the limitations and uncertainties associated with a binary classification of this type, and the implications for trend magnitudes of adopting alternatives to the conventional assumption of 100% (0%) actual fractional snow-covered area in ‘snow-covered’ (‘snow-free’) spatial units at different stages of the snow-season. This prompts adoption of alternative terminology, referring to ‘snow-dominated’ area and duration. In response to questions about the dataset's veracity raised by some prior studies, it discusses climatological factors of potential relevance in explaining spatio-temporal trend patterns, and considers how biases might possibly have been introduced as a result of extraneous influences.