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Melt pond is one of the most significant and important features of Arctic sea ice in the summer and can dramatically reduce the albedo of ice, promoting more energy into the upper ocean. The observation of the seasonal evolution of melt pond can improve our fundamental understanding of the role and sensitivity of sea ice in the context of global climate change. In this study, an ice-tethered observation system is developed for melt pond evolution with vision and temperature profile measurements. The system composition, structure of the ice-tethered buoy, freeze-resistant camera, and thermistor chain are analyzed. A sealed shell and electric heating wires are used to increase the temperature to around the camera in low-temperature environments. The ice thickness and depth of melt pond can be inverted using a specific interface recognition algorithm. A low-light image enhancement strategy is proposed to improve the quality of images under the low lighting conditions in polar regions. The proposed system was tested in the second reservoir of Fen River, Yellow River, from 15 January to 27 January 2021. An artificial freshwater pond was used as the location for thermistor chain deployment and observation. The differences in mean square error (MSE), peak signal-to-noise ratio (PSNR), and feature similarity index (FSIM) between the original and enhanced images indicate that the proposed algorithm is suitable for low-light image enhancement. The research on the ice-tethered observation system will provide a new framework and technical support for the seasonal observation for melt pond. [ABSTRACT FROM AUTHOR]

ABSTRACTIn this study, we investigated specific components and intra-seasonal dynamic processes that play pivotal roles in the East Antarctica’s central Dronning Maud Land (cDML) region. The present study focuses on harnessing the potential of a multispectral sensor-based Unmanned Aerial Vehicle (UAV) for cryosphere studies over the polar ice sheet in cDML (Schirmacher Oasis) region of East Antarctica, conducted as part of the 42nd and 43rd Indian Scientific Expedition to Antarctica (ISEA), under the aegis of National Centre for Polar and Ocean Research (NCPOR), Ministry of Earth Sciences, Government of India, during austral summer period from 2022 to 2023. The surveyed area (~100 acres) encompasses a segment of the ice sheet frontal edge outer margin, situated near the Maitri Indian research base at Schirmacheroasen, and includes a melt pond/supraglacial lake. The results indicate that the ice sheet frontal edge segment experienced a decrease in elevation of 0.25 meters with a total mass reduction of 13.6 kilotons during one-week period. The supraglacial lake accumulated meltwater with an average depth ranging from 0.25 to 1.6 meters, covering an area spanning from 9.1 × 103 square meters to 24.7 × 103 square meters. The ultra-high resolution multispectral UAV data also revealed dynamic changes in various cryo-facies classes, including water bodies/meltwater, frozen meltwater, dry and wet snow, debris/bare ice, and bedrock during the study period over the study area. This study represents a pioneering effort in assessing the region near the Maitri research base in both spatial and temporal scales. It also marks the first application of UAV surveying techniques with the integration of in-situ data from the Pressure Sensor Assembly (PSA) to examine and understand its complex intra-seasonal dynamics and characteristics. The research team is the first Indian group to employ UAVs for scientific research in the Antarctic region, which is a notable advancement in Indian scientific exploration and contributes to the expanding knowledge of the Antarctic region.

This study analyzes the dynamics of surface melting in Antarctica, which are crucial for understanding glacier and ice sheet behavior and monitoring polar climate change. Specifically, we focus on the Nivlisen ice shelf in East Antarctica, examining melt ponds, supra glacial lakes (SGLs), seasonal surface melt extent, and surface ice flow velocity. Spatial and temporal analysis is based on Landsat and Sentinel-1 data from the austral summers of 2000 to 2023. Between 2000 and 2014, melt ponds and SGLs on the ice shelf covered roughly 1 km2. However, from 2015 to 2023, surface melting increased consistently, leading to more extensive melt ponds and SGLs. Significant SGL depths were observed in 2016, 2017, 2019, and 2020, with 2008, 2016, and 2020 showing the highest volumes and progressive SGL area growth. We also examined the relationship between seasonal surface melt extent and ice flow velocity. Validation efforts involved ground truth data from a melt pond in central Dronning Maud Land (cDML) during the 2022–2023 austral summer, along with model-based results. The observed increase in melt pond depth and volume may significantly impact ice shelf stability, potentially accelerating ice flow and ice shelf destabilization. Continuous monitoring is essential for accurately assessing climate change's ongoing impact on Antarctic ice shelves. [ABSTRACT FROM AUTHOR]

Melt pond is one of the most significant and important features of Arctic sea ice in the summer and can dramatically reduce the albedo of ice, promoting more energy into the upper ocean. The observation of the seasonal evolution of melt pond can improve our fundamental understanding of the role and sensitivity of sea ice in the context of global climate change. In this study, an ice-tethered observation system is developed for melt pond evolution with vision and temperature profile measurements. The system composition, structure of the ice-tethered buoy, freeze-resistant camera, and thermistor chain are analyzed. A sealed shell and electric heating wires are used to increase the temperature to around the camera in low-temperature environments. The ice thickness and depth of melt pond can be inverted using a specific interface recognition algorithm. A low-light image enhancement strategy is proposed to improve the quality of images under the low lighting conditions in polar regions. The proposed system was tested in the second reservoir of Fen River, Yellow River, from 15 January to 27 January 2021. An artificial freshwater pond was used as the location for thermistor chain deployment and observation. The differences in mean square error (MSE), peak signal-to-noise ratio (PSNR), and feature similarity index (FSIM) between the original and enhanced images indicate that the proposed algorithm is suitable for low-light image enhancement. The research on the ice-tethered observation system will provide a new framework and technical support for the seasonal observation for melt pond.

The Arctic has experienced extensive sea ice retreat in recent decades, as observed from satellite remote sensing and simulated by general circulation models. The significant low albedo of melt pond compared with snow and ice results in more absorption of radiation compared with sea ice without melt pond, which will then enhance sea ice melt to form more melt pond. This so-called positive feedback of melt pond is crucial for sea ice thermal and physical properties. The melt pond coverage varies significantly spatially and temporally and satellite remote sensing provides a possible solution in determining the spatial coverage of melt pond. This work aims to develop a new physically based algorithm for retrieving the cover fraction of melt ponds on sea ice using remote sensing data. In the newly developed algorithm, dynamic sea ice and melt pond reflectances are used in the pixel spectral unmixing for melt pond fraction (MPF) estimation. Radiative transfer models for sea ice and melt pond reflectance modeling are developed for MPF retrieval algorithm. The validation of retrieved MPF and its comparison with static pixel spectral unmixing results show that the dynamic algorithm proposed in this study has the potential of application in wide range of situations by considering dynamic spectral reflectances of melt pond and sea ice. The Arctic MPF and sea ice cover fraction (SIF) in 2000–2021 are generated by applying the proposed algorithm. The spatial and temporal characteristics of MPF and SIF are analyzed. Time series trend analysis of MPF in 2000–2021 reveals the significant increasing trend of relative MPF in the Arctic. The melt pond expansion in 2000–2021 is more obvious in the marginal areas, especially the marginal areas close to Russia coast. Trend analysis for typical regions such as Beaufort Sea, Greenland Sea, the Baffin Bay, Chukchi, East Siberian, Laptev, and Kara seas and center Arctic also reveals that the relative MPF and SIF show contrasting trend. [ABSTRACT FROM AUTHOR]

Comparing helicopter‐borne surface temperature maps in winter and optical orthomosaics in summer from the year‐long Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition, we find a strong geometric correlation between warm anomalies in winter and melt pond location the following summer. Warm anomalies are associated with thinner snow and ice, that is, surface depression and refrozen leads, that allow for water accumulation during melt. Warm surface temperature anomalies in January were 0.3–2.5 K warmer on sea ice that later formed melt ponds. A one‐dimensional steady‐state thermodynamic model shows that the observed surface temperature differences are in line with the observed ice thickness and snow depth. We demonstrate the potential of seasonal prediction of summer melt pond location and coverage from winter surface temperature observations. A threshold‐based classification achieves a correct classification for 41% of the melt ponds. Plain Language Summary: We compare winter surface temperatures from an infrared camera with summer photographs of sea ice with melt ponds. The datasets were recorded from a helicopter during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition. Melt ponds form on sea ice in summer when the snow melts and water accumulates in the lower locations on the ice floes. Melt ponds are very important for the Arctic energy budget because they strongly change the sea ice brightness and thus the amount of solar energy absorbed by the ice. We find surface characteristics with similar size and location between warmer areas in winter and the location of melt ponds in summer. For a better process understanding, we calculate the surface temperature with a simple model and find that the warm temperature anomalies are due to thinner ice and snow. Stronger warm temperature anomalies appear in new cracks in the ice which are covered with newly formed, thin ice. With a temperature‐based classification, we are able to estimate the summer melt pond fraction. Key Points: Winter warm surface temperature anomalies are co‐located with melt pond locations in the following summerWarm anomalies appear in refrozen leads, in refrozen melt ponds, and in troughs between ridges, due to thinner snow and iceWe show the potential for prediction of summer melt pond fraction from winter surface temperatures [ABSTRACT FROM AUTHOR]

This study analyzes the dynamics of surface melting in Antarctica, which are crucial for understanding glacier and ice sheet behavior and monitoring polar climate change. Specifically, we focus on the Nivlisen ice shelf in East Antarctica, examining melt ponds, supra glacial lakes (SGLs), seasonal surface melt extent, and surface ice flow velocity. Spatial and temporal analysis is based on Landsat and Sentinel-1 data from the austral summers of 2000 to 2023. Between 2000 and 2014, melt ponds and SGLs on the ice shelf covered roughly 1 km2. However, from 2015 to 2023, surface melting increased consistently, leading to more extensive melt ponds and SGLs. Significant SGL depths were observed in 2016, 2017, 2019, and 2020, with 2008, 2016, and 2020 showing the highest volumes and progressive SGL area growth. We also examined the relationship between seasonal surface melt extent and ice flow velocity. Validation efforts involved ground truth data from a melt pond in central Dronning Maud Land (cDML) during the 2022–2023 austral summer, along with model-based results. The observed increase in melt pond depth and volume may significantly impact ice shelf stability, potentially accelerating ice flow and ice shelf destabilization. Continuous monitoring is essential for accurately assessing climate change’s ongoing impact on Antarctic ice shelves.

Abstract Comparing helicopter‐borne surface temperature maps in winter and optical orthomosaics in summer from the year‐long Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition, we find a strong geometric correlation between warm anomalies in winter and melt pond location the following summer. Warm anomalies are associated with thinner snow and ice, that is, surface depression and refrozen leads, that allow for water accumulation during melt. Warm surface temperature anomalies in January were 0.3–2.5 K warmer on sea ice that later formed melt ponds. A one‐dimensional steady‐state thermodynamic model shows that the observed surface temperature differences are in line with the observed ice thickness and snow depth. We demonstrate the potential of seasonal prediction of summer melt pond location and coverage from winter surface temperature observations. A threshold‐based classification achieves a correct classification for 41% of the melt ponds.

In order to satisfy the demand of key sea ice parameters, including melt pond depth Hp and underlying ice thickness Hi, in studies of Arctic sea ice change in summer, four algorithms of retrieving Hp and Hi were compared and validated by using optical data of melt ponds from field observations. The Malinka18 algorithm stood out as the most accurate algorithm for the retrieval of Hp. For the retrieval of Hi, Malinka18 and Zhang21 algorithms could also provide reasonable results and both can be applied under clear and overcast sky conditions, while retrievals under clear sky conditions are more accurate. The retrieval results of Hi for Lu18 agreed better with field measurements for thin ice (Hi < 1 m) than that for thick ice, but those results of Hp were not satisfactory. The König20 algorithm was only suitable for clear sky conditions, and underestimated Hp, while showing a good agreement with Hp < 0.15 m. For Arctic applications, Malinka18 and Zhang21 algorithms provided a basis and reference for the satellite optical data such as WoeldView2 to retrieve Hp and Hi. Malimka18 also showed the ability to retrieve Hi, except for the Lu18 algorithm if pond color captured by helicopters and unmanned aerial vehicles were available. This study identifies the optimal algorithm for retrieval of Hp and Hi under different conditions, which have the potential to provide necessary data for numerical simulations of Arctic sea ice changes in summer. [ABSTRACT FROM AUTHOR]

Melt ponds significantly affect Arctic sea ice thermodynamic processes. The melt pond parameterization scheme in the Los Alamos sea ice model (CICE6.0) can predict the volume, area fraction (the ratio between melt pond area to sea ice area in a model grid), and depth of melt ponds. However, this scheme has some uncertain parameters that affect melt pond simulations. These parameters could be determined through a conventional parameter estimation method, which requires a large number of sensitivity simulations. The adjoint model can calculate the parameter sensitivity efficiently. In the present research, an adjoint model was developed for the CESM (Community Earth System Model) melt pond scheme. A melt pond parameter estimation algorithm was then developed based on the CICE6.0 sea ice model, melt pond adjoint model, and L-BFGS (Limited-memory Broyden-Fletcher-Goldfard-Shanno) minimization algorithm. The parameter estimation algorithm was verified under idealized conditions. By using MODIS (Moderate Resolution Imaging Spectroradiometer) melt pond fraction observation as a constraint and the developed parameter estimation algorithm, the melt pond aspect ratio parameter in CESM scheme, which is defined as the ratio between pond depth and pond area fraction, was estimated every eight days during summertime for two different regions in the Arctic. One region was covered by multi-year ice (MYI) and the other by first-year ice (FYI). The estimated parameter was then used in simulations and the results show that: (1) the estimated parameter varies over time and is quite different for MYI and FYI; (2) the estimated parameter improved the simulation of the melt pond fraction. [ABSTRACT FROM AUTHOR]

To study the partitioning of solar radiation over the melting Arctic sea ice and address the lack of systematic measurements of the optical properties of melt ponds and the underlying ice, four experiments in non-Arctic regions were designed to explore the distribution of solar irradiance on pond surface, pond bottom, and underlying ice. These experiments also investigated the influence of different pond depths (H p), underlying ice thicknesses (H i), and melt pond sidewall conditions (i.e., the black sidewalls, ice sidewalls, and pond horizon size) on the apparent optical properties (AOPs) of melt ponds. The black sidewalls with a small pond radius (barrel experiment) showed the greatest influence on the AOPs, significantly lowering the albedo and transmittance, and increasing the energy absorbed by the pond (Ψ p) and underlying ice (Ψ i). Different sidewall types, such as the black and ice sidewalls, did not affect the AOPs' trend, but affected their values. The AOPs were also influenced by pond size. The albedo linearly decreased and Ψ p linearly increased with the increase of pond radius. The albedo and Ψ p were almost independent of pond radius as pond radius larger than 0.58 m and 1.25, respectively. The black sidewalls experiment, in which the pond radius was 2.5 m, and the pond size experiment showed that solar energy was mainly absorbed by the melt pond. Ψ p increased from 19% to 62%, which corresponded to H p increasing from 5 cm to 35 cm in the pond size experiment. The results of the four experiments were consistent with the values obtained from numerical simulations and in situ measurements in the Arctic. The experiments conducted in this study were shown to be effective tools to complement and verify the optical measurements of Arctic melt ponds. They allow us to obtain the key information in Arctic pond measurements, which is difficult to achieve due to the severe weather conditions and marine environment. • The experiments vary a number of pond parameters and observe the change of optical properties of melt ponds. • The experiments can help confirming pond models. • The results separate the distribution of solar irradiance in the pond. [ABSTRACT FROM AUTHOR]

Given the importance that melt ponds have on the energy balance of summer sea ice, there have been several efforts to develop pan-Arctic datasets using satellite data. Here we intercompare three melt pond data sets that rely on multi-frequency optical satellite data. Early in the melt season, the three data sets have similar spatial patterns in melt pond fraction, but this agreement weakens as the melt season progresses despite relatively high interannual correlations in pond fractions between the data products. Most of the data sets do not exhibit trends towards increased melt pond fractions from 2002 to 2011 despite overall Arctic warming and earlier melt onset. Further comparisons are made against higher resolution optical data to assess relative accuracy. These comparisons reveal the challenges in retrieving melt ponds from coarse resolution satellite data, and the need to better discriminate between leads, small open water areas and melt ponds. Finally, we assess melt pond data sets as a function of ice type and how well they correlate with surface albedo. As expected, melt pond fractions are negatively correlated with surface albedo, though the strength of the correlation varies across products and regions. Overall, first-year ice has larger melt pond fractions than multi-year ice. • Melt pond products did not show a positive trend in May to July from 2002 to 2011. • While melt pond products agree at times, they start to deviate as melt progresses. • All melt pond products are compared against Worldview-2, 3 images. • Melt pond negatively correlates with albedo. [ABSTRACT FROM AUTHOR]

• Formulations of variable sea ice salinity and melt pond processes in an OGCM. • Observed sea ice salinity distributions are broadly reproduced in both hemispheres. • Satellite-based Arctic pond area variations are well reproduced with new scheme. • Ice/snow albedo reductions in summer become consistent with satellite observations. • Impacts on a large-scale sea ice area and thickness fields are discussed quantitatively. Enhanced representation of sea ice processes in ocean modeling studies is required for advancing our understanding and prediction of the climate variability. In the present study, we improved the sea ice thermodynamics in an OGCM by introducing processes for variable sea ice salinity and melt ponds on sea ice. The former affects the latent heat and conductivity of sea ice as well as the salt budget in the ocean–sea ice system. We formulated salinification and desalination processes based on previous observational studies. Melt ponds enhance summertime surface melting by reducing the surface albedo. In our formulation of the melt pond thermodynamics, the shortwave radiation absorbed in the pond was partly released to the air and partly conducted to the underlying sea ice. By adopting these parameterizations, seasonal cycles of the sea ice salinity distributions in both hemispheres were generally reproduced. Simulated seasonal and interannual variations in the Arctic melt pond area and associated surface albedo reductions were basically consistent with observational estimates. The melt pond effect reduced the Arctic sea ice volume by about 1.5 × 103 km3 in the summer; half of this impact remained throughout the year. Our results suggest that the new parameterizations have the potential to improve the seasonal evolution of the sea ice thickness and concentration distributions, which would contribute to prediction studies. [ABSTRACT FROM AUTHOR]

In order to satisfy the demand of key sea ice parameters, including melt pond depth Hp and underlying ice thickness Hi, in studies of Arctic sea ice change in summer, four algorithms of retrieving Hp and Hi were compared and validated by using optical data of melt ponds from field observations. The Malinka18 algorithm stood out as the most accurate algorithm for the retrieval of Hp. For the retrieval of Hi, Malinka18 and Zhang21 algorithms could also provide reasonable results and both can be applied under clear and overcast sky conditions, while retrievals under clear sky conditions are more accurate. The retrieval results of Hi for Lu18 agreed better with field measurements for thin ice (Hi < 1 m) than that for thick ice, but those results of Hp were not satisfactory. The König20 algorithm was only suitable for clear sky conditions, and underestimated Hp, while showing a good agreement with Hp < 0.15 m. For Arctic applications, Malinka18 and Zhang21 algorithms provided a basis and reference for the satellite optical data such as WoeldView2 to retrieve Hp and Hi. Malimka18 also showed the ability to retrieve Hi, except for the Lu18 algorithm if pond color captured by helicopters and unmanned aerial vehicles were available. This study identifies the optimal algorithm for retrieval of Hp and Hi under different conditions, which have the potential to provide necessary data for numerical simulations of Arctic sea ice changes in summer.

The formation of melt ponds is pervasive on sea ice and ice shelves prior to their disintegration. Such process should be critical for the deglaciation of a snowball Earth but has never been considered in previous studies. Here we develop a module to explicitly track the initiation, growth and refreezing of melt ponds on ice. Incorporation of the module into a climate model indicates that it provides a strong positive feedback to the climate. Deglaciation of a snowball Earth will start when the annual mean equatorial surface temperature is only −7.7°C rather than 0°C. At this point, seasonal melt ponds start to appear in the mid‐latitude region. Its positive feedback induces the appearance of perennial melt ponds within the equatorial region and can increase the annual mean temperature there to 6.1°C in less than 10 years. Thus, our results indicate that the threshold CO2 required to deglaciate a snowball Earth will be greatly overestimated (by a factor of ∼4 for the model we use) if the annual mean surface temperature reaching 0°C is used as the criteria. The results also demonstrate unambiguously that the deglaciation of snowball Earth should start from the equator. We then speculate on what will happen to the tropical sea glacier after the surface melting starts. Plain Language Summary: The whole Earth was covered by thick ice during a snowball Earth event. When and where the deglaciation of such an event should start have not been answered. Previous studies judged whether atmospheric CO2 level was high enough to deglaciate a snowball Earth by checking whether the annual mean surface temperature at the equator approached 0 °C. However, seasonal melting of ice must have appeared at a much lower CO2 level in the subtropical region where summer is warm. Melt water reduces surface reflectivity and warms the surface, which will induce more melting and form a positive feedback loop. We find that this feedback is very strong; widespread melting occurs once the annual mean equatorial surface temperature approaches ‐8 °C, much earlier than thought before. Therefore, the threshold CO2 level that is required to deglaciate a snowball Earth was greatly overestimated in previous studies. Moreover, our results show that although melt water appears first in the subtropical region during summer, pervasive perennial melt water appears only around the equator due to stable high solar insolation there. This melt water is conjectured to break the thick sea ice near the equator and start the deglaciation of a snowball Earth there. Key Points: Melt ponds must be considered when studying snowball Earth deglaciationSnowball Earth deglaciation starts when the annual mean equatorial surface temperature reaches approximately −8°CSnowball Earth deglaciation starts from the equator not the subtropical region [ABSTRACT FROM AUTHOR]

Hemisphere‐wide observations of melt ponds on sea ice are needed to understand their influence on the surface radiation budget of the Arctic Ocean and to extend the satellite sea ice thickness data record. Here we present a first assessment of NASA's Ice, Cloud, and land Elevation Satellite‐2 (ICESat‐2) over individual Arctic sea ice melt ponds with different reflective properties. We use coincident high‐resolution satellite imagery from WorldView‐2 and Sentinel‐2 over different sea ice topographies to show that smooth ponds are highly reflective and can saturate the ICESat‐2 photon detection system. Rougher ponds have a more varied backscatter signal, and in some cases both the water and underlying ice surfaces are visible in photon height distributions. Characterizing the complex photon backscatter signals of melt ponds on sea ice is a first step toward automating retrievals of pond parameters such as width, melt pond fraction and depth, and improving higher‐level ICESat‐2 sea ice height and freeboard products. Plain Language Summary: In summer, surface melt water forms ponds that can cover up to half of the Arctic sea ice surface. Because melt ponds reduce the reflective and insulative properties of sea ice, they can alter the Arctic energy budget and ecosystem. The presence of melt ponds also complicates satellite altimeter (radar and laser) estimates of sea ice freeboard (the height of the ice above the ocean) and thickness. Here we combine data from NASA's Ice, Cloud, and land Elevation Satellite‐2 (ICESat‐2) laser altimeter and high‐resolution satellite imagery to locate individual melt ponds on Arctic sea ice. We explore how melt ponds affect the return laser signal and discuss how ICESat‐2 data could be used to retrieve parameters of scientific interest such as melt pond width, fraction, and depth. We finish by summarizing the complications and potential solutions associated with sea ice height and freeboard retrievals from ICESat‐2 in summer. Key Points: We present a first assessment of NASA's ICESat‐2 over a range of individual Arctic sea ice melt pondsICESat‐2 backscatter signatures over melt ponds vary significantly, depending on surface roughnessOur ability to estimate melt pond width, fraction, and depth and improve ICESat‐2 sea ice freeboard products depends on surface backscatter [ABSTRACT FROM AUTHOR]

To identify the major source and melt flux of Hg in sea ice, we measured total Hg (THg) and methylmercury (MeHg) concentrations from sea ice cores collected from the Chukchi Plateau, Arctic Ocean. The THg maxima were found in the surface section (0–40 cm) of the cold cores (−2.3 ± 0.19 °C) and middle section (30–75 cm) of the warm cores (−1.9 ± 0.38 °C), whereas MeHg concentrations were nearly constant by core depth in the cold and warm cores. Using the enrichment factor mixing model, we estimated seawater to be the dominant source of THg over atmospheric deposition, both in the cold (89.1–99.6 %) and warm cores (98.4–99.7 %). The preliminary input estimation for MeHg showed that in situ methylation, instead of seawater input, was the potential source of MeHg in sea ice. The sea ice melt flux of MeHg in the Arctic Ocean was estimated to be 231 ± 77 kg yr−1 in the period of 2010–2020, which is insignificant compared with other external sources, such as atmospheric deposition, river discharge, and coastal erosion. Based on the results of the present study, the ongoing decline of sea ice extent related to polar amplification is likely to enhance MeHg production in sea ice in late spring and summer as a result of the improved light availability and consequent promotion of algal and microbial activities. • Sources and melt flux of MeHg in sea ice were studied on the Chukchi Plateau. • THg maxima were found at the surface of cold cores and middle of warm cores. • Vertical MeHg trends were stable in the cold and warm cores. • Major source of THg was seawater and MeHg was in situ methylation in sea ice. [ABSTRACT FROM AUTHOR]

We report continuous observations in the high Arctic (north of 84°N) over the full 2013 summer season at two nearby sites with distinct initial snow depth, ice thickness, and altitude with respect to the local ice topography. The two sites, subject to similar atmospheric conditions that did not favor strong ice melt, showed contrasting evolutions. One site, with initially thin sea ice (1.40 m) at a relatively low location of the floe, witnessed the formation of a spectacular 1.20‐m‐deep melt pond, a pond‐enhanced erosion of the ice surface, and a sudden pond drainage into the ocean. Then, the outpoured fresh water rapidly froze, heated the old ice from below, and also acted as a temporary shield from the ocean heat flux while it was progressively ablated through dissolution. Eventually, the site almost recovered its initial ice thickness. In contrast, the other site, with initially thicker sea ice (1.75 m) at a relatively high location on its floe, did not support any significant meltwater and underwent over 0.5 m of continuous basal ablation. The two sites experienced formation of superimposed and interposed ice. Sea ice survived summer melt at the two sites, which entered the refreezing season with similar snow and ice thicknesses. For the first time, processes associated with the formation of a deep melt pond and subsequent false bottom evolution are continuously documented with ice mass balance instruments. Plain Language Summary: Summer processes in the sea ice in a changing Arctic are documented at two nearby sites in the high Arctic (north of 84°N) in summer 2013. We report the first continuous observations of the formation of a melt pond more than 1.2 m deep, and the evolution of the fresh water after it outpoured to the ocean through a drainage hole at one site located in a topographic low. A nearby site, located on a topographic high, experienced very different evolution with no meltwater retention at the surface and continuous basal melting. Key Points: We present continuous observations in the high Arctic over the full summer season at two nearby sites with distinct initial conditionsOne site experienced a deep melt pond, sudden pond drainage, "false bottom" formation, and migration; the other site experienced continuous basal meltWe report surface, internal, and basal processes in summer leading to drastic transient or more persistent ice structure transformations [ABSTRACT FROM AUTHOR]

Sea ice is one of the determining parameters of the climate system. The presence of melt ponds on the surface of Arctic sea ice plays a critical role in the mass balance of sea ice. A total of nine cores was collected from multiyear ice refrozen melt ponds and adjacent hummocks during the 2015 Arctic Sea State research cruise. The depth profiles of water isotopes, salinity, and ice texture for these sea ice cores were examined to provide information about the development of refrozen melt ponds and water balance generation processes, which are otherwise difficult to acquire. The presence of meteoric water with low oxygen isotope values as relatively thin layers indicates melt pond water stability and little mixing during formation and refreezing. The hydrochemical characteristics of refrozen melt pond and seawater depth profiles indicate little snowmelt enters the upper ocean during melt pond refreezing. Due to the seasonal characters of deuterium excess for Arctic precipitation, water balance calculations utilizing two isotopic tracers (oxygen isotope and deuterium excess) suggest that besides the melt of snow cover, the precipitation input in the melt season may also play a role in the evolution of melt ponds. The dual‐isotope mixing model developed here may become more valuable in a future scenario of increasing Arctic precipitation. The layers of meteoric origin were found at different depths in the refrozen melt pond ice cores. Surface topography information collected at several core sites was examined for possible explanations of different structures of refrozen melt ponds. Plain Language Summary: The connection between precipitation and the retreat and expansion of sea ice is poorly understood. This paper brings a new approach to investigate meteoric origins of sea ice in the polar regions. We propose to investigate moisture sources for precipitations and the accumulation of snow based on isotopic analysis of sea ice samples from the Arctic. Better understanding of the moisture sources of precipitation in the polar regions will help us constrain models and predictions of glacial and sea ice coverage of the polar regions under current and future climate conditions. In this paper, depth profiles of stable isotopes, salinity, and ice texture of sea ice samples were described to serve as illustrations of the structures of the refrozen melt ponds in the Arctic. An isotope tracer mixing model was developed to determine the seasonality of meteoric water input for sea ice water balance. Key Points: The presence of meteoric origin isotopic signals as relatively thin layers indicates melt pond water stability during refreezingA dual‐isotope (oxygen isotope and deuterium excess) mixing model was utilized to determine the seasonality of meteoric water inputThe meteoric water signals of refrozen melt ponds were found near the surface or bottom, which may be due to different surface topography [ABSTRACT FROM AUTHOR]

Aerial photography was conducted in the high Arctic Ocean during a Chinese research expedition in summer 2010. By partitioning the images into three distinct surface categories (sea ice/snow, water and melt ponds), the areal fraction of each category, ice concentration and the size and geometry of individual melt ponds, are determined with high-spatial resolution. The ice concentration and melt pond coverage have large spatial deviations between flights and even between images from the marginal ice zone to the pack ice zone in the central Arctic. Ice concentration and pond coverage over high Arctic (from 84°N to north) was ~75% and ~6.8%, respectively, providing ‘ground truth’ for the unusual transpolar reduction strip of ice indicated concurrently by AMSR-E data and for the regions (north of 88°N) where no passive microwave sensors can cover. Melt pond size and shape distributions are examined in terms of pond area (S), perimeter (P), mean caliper dimension (MCD) (L), roundness (R), convex degree (C), the ratio of P/S and fractal dimension (D). Power-law relationships are developed between pond size and number. Some general trends in geometric metrics are identified as a function of pond area including R, C, P/S and D. The scale separation of pond complexity is demonstrated by analyzing area-perimeter data. The results will potentially help the modelling of melt pond evolution and the determination of heterogeneity of under-ice transmitted light fields.

Meltwater drainage onset (DO) timing and drainage duration (DD) related to snowmelt-water redistribution are both important for understanding not only the Arctic energy and heat budgets but also the salt/heat balance of the mixed layer in the ocean and sea-ice ecosystem. We present DO and DD as determined from the time series of Advanced Microwave Scanning Radiometer-Earth observing system (AMSR-E) melt pond fraction (MPF) estimates in an area with Canadian landfast ice. To address the lack of evaluation on a day-by-day basis for the AMSR-E MPF estimate, we first compared AMSR-E MPF with the daily Medium Resolution Imaging Spectrometer (MERIS) MPF. The AMSR-E MPF estimate correlates significantly with the MERIS MPF (r = 0.73−0.83). The estimate has a product quality similar to the MERIS MPF only when the albedo is around 0.5−0.7 and a positive bias of up to 10% in areas with an albedo of 0.7−0.9, including melting snow. The DO/DD estimates are determined by using a polynomial regression curve fitted on the time series of the AMSR-E MPF. The DOs/DDs from time series of the AMSR-E and MERIS MPFs are compared, revealing consistency in both DD and DO. The DO timing from 2006 to 2011 is correlated with melt onset timing. To the best of our knowledge, our study provides the first large-scale information on both DO timing and DD.

A one-dimensional thermodynamic model of melt pond is established in this paper. The observation data measured in the summer of 2010 by the Chinese National Arctic Research Expedition (CHINARE-2010) are used to partially parameterize equations and to validate results of the model. About 85% of the incident solar radiation passed through the melt pond surface, and some of it was released in the form of sensible and latent heat. However, the released energy was very little (about 15%), compared to the incident solar radiation. More than 58.6% of the incident energy was absorbed by melt pond water, which caused pond-covered ice melting and variation of pond water temperature. The simulated temperature of melt pond had a diurnal variation and its value ranged between 0.0°C and 0.3°C. The melting rate of upper pond-covered ice is estimated to be around two times faster than snow-covered ice. At same time, the change of melting rate was relatively quick for pond depth less than 0.4 m, while the melting rate kept relatively constant (about 1.0 cm/d) for pond depth greater than 0.4 m. [ABSTRACT FROM AUTHOR]

In summer 2014, sea ice morphological characteristics were investigated in the 135°–175°W sector of the Arctic Ocean using in situ, shipborne, and remote sensing measurements. Sea ice in this sector was deformed and compact compared to previous observations. The average ice area in the region (70°–82.5°N, 135°–175°W) was 7.6 × 10 5 km 2 for 29 July through 6 September 2014, the fourth largest record between 2003 and 2014. This can be attributed to the enhanced multiyear sea ice inflow from north of the Canadian Arctic Archipelago from September 2013 to August 2014. Multiyear ice coverage in the study region on 30 April 2014 was 55%, which was larger than the values in 2005–2013. During the melt season of 2014, the Arctic Dipole had a positive anomaly, associated with enhanced southerly wind. In summer 2014 the marginal ice zone exhibited a distinct ice retreat, whereas the pack ice zone (PIZ) showed strong persistence due to the large coverage of multiyear ice. The northward retreat of the PIZ boundary was < 100 km from late July through early September 2014. In the PIZ of 76–80.5°N, average ice thickness weighted by ice concentration, obtained by shipborne measurements in August 2014 was 0.54 m thicker than that obtained in August 2010 due to enhanced ice deformation and less open waters in 2014. At 81°N, sea ice with modal thickness of 1.48 m reached thermodynamic balance by late August 2014, which was much earlier than that in 2010. [ABSTRACT FROM AUTHOR]

An aerial photography has been used to provide validation data on sea ice near the North Pole where most polar orbiting satellites cannot cover. This kind of data can also be used as a supplement for missing data and for reducing the uncertainty of data interpolation. The aerial photos are analyzed near the North Pole collected during the Chinese national arctic research expedition in the summer of 2010 (CHINARE2010). The result shows that the average fraction of open water increases from the ice camp at approximately 87°N to the North Pole, resulting in the decrease in the sea ice. The average sea ice concentration is only 62.0% for the two flights (16 and 19 August 2010). The average albedo (0.42) estimated from the area ratios among snow-covered ice, melt pond and water is slightly lower than the 0.49 of HOTRAX 2005. The data on 19 August 2010 shows that the albedo decreases from the ice camp at approximately 87°N to the North Pole, primarily due to the decrease in the fraction of snow-covered ice and the increase in fractions of melt-pond and open-water. The ice concentration from the aerial photos and AMSR-E (The Advanced Microwave Scanning Radiometer-Earth Observing System) images at 87.0°-87.5°N exhibits similar spatial patterns, although the AMSR-E concentration is approximately 18.0% (on average) higher than aerial photos. This can be attributed to the 6.25 km resolution of AMSR-E, which cannot separate melt ponds/submerged ice from ice and cannot detect the small leads between floes. Thus, the aerial photos would play an important role in providing high-resolution independent estimates of the ice concentration and the fraction of melt pond cover to validate and/or supplement space-borne remote sensing products near the North Pole. [ABSTRACT FROM AUTHOR]