Observations of preferential summer melt of Arctic sea-ice ridge keels from repeated multibeam sonar surveys

Sea-ice ridges constitute a large fraction of the total Arctic sea-ice area (up to 40 %–50 %); nevertheless, they are the least studied part of the ice pack. Here we investigate sea-ice melt rates using rare, repeated underwater multibeam sonar surveys that cover a period of 1 month during the advanced stage of sea-ice melt. Bottom melt increases with ice draft for first- and second-year level ice and a first-year ice ridge, with an average of 0.46, 0.55, and 0.95 m of total snow and ice melt in the observation period, respectively. On average, the studied ridge had a 4.6 m keel bottom draft, was 42 m wide, and had 4 % macroporosity. While bottom melt rates of ridge keel were 3.8 times higher than first-year level ice, surface melt rates were almost identical but responsible for 40 % of ridge draft decrease. Average cross-sectional keel melt ranged from 0.2 to 2.6 m, with a maximum point ice loss of 6 m, showcasing its large spatial variability. We attribute 57 % of the ridge total (surface and bottom) melt variability to keel draft (36 %), slope (32 %), and width (27 %), with higher melt for ridges with a larger draft, a steeper slope, and a smaller width. The melt rate of the ridge keel flanks was proportional to the draft, with increased keel melt within 10 m of its bottom corners and the melt rates between these corners comparable to the melt rates of level ice.</p

Biophysical characterization of summer Arctic sea-ice habitats using a remotely operated vehicle-mounted underwater hyperspectral imager

publishedVersio

Different mechanisms of Arctic first-year sea-ice ridge consolidation observed during the MOSAiC expedition

Sea-ice ridges constitute a large fraction of the ice volume in the Arctic Ocean, yet we know little about the evolution of these ice masses. Here we examine the thermal and morphological evolution of an Arctic firstyear sea-ice ridge, from its formation to advanced melt. Initially the mean keel depth was 5.6 m and mean sail height was 0.7 m. The initial rubble macroporosity (fraction of seawater filled voids) was estimated at 29% from ice drilling and 43%–46% from buoy temperature. From January until mid-April, the ridge consolidated slowly by heat loss to the atmosphere and the total consolidated layer growth during this phase was 0.7 m. From mid-April to mid-June, there was a sudden increase of ridge consolidation rate despite no increase in conductive heat flux. We surmise this change was related to decreased macroporosity due to transport of snow-slush to the ridge keel rubble via adjacent open leads. In this period, the mean thickness of the consolidated layer increased by 2.1 m. At the peak of melt in June–July we suggest that the consolidation was related to the refreezing of surface snow and ice meltwater and of ridge keel meltwater (the latter only about 15% of total consolidation). We used the morphology parameters of the ridge to calculate its hydrostatic equilibrium and obtained a more accurate estimate of the actual consolidation of the keel, correcting from 2.2 m to 2.8 m for average keel consolidation. This approach also allowed us to estimate that the average keel melt of 0.3 m, in June–July, was accompanied by a decrease in ridge draft of 0.9 m. An ice mass balance buoy in the ridge indicated total consolidation of 2.8 m, of which 2.1 m was related to the rapid mode of consolidation from April to June. By mid-June, consolidation resulted in a drastic decrease of the macroporosity of the interior of keel while the flanks had little or no change in macroporosity. These results are important to understanding the role of ridge keels as meltwater sources and sinks and as sanctuary for ice-associated organisms in Arctic pack ice

Snowmelt contribution to Arctic first-year ice ridge mass balance and rapid consolidation during summer melt

Sea ice ridges are one of the most under-sampled and poorly understood components of the Arctic sea ice system. Yet, ridges play a crucial role in the sea ice mass balance and have been identified as ecological hotspots for ice-associated flora and fauna in the Arctic. To better understand the mass balance of sea ice ridges, we drilled and sampled two different first-year ice (FYI) ridges in June–July 2020 during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). Ice cores were cut into 5 cm sections, melted, then analyzed for salinity and oxygen (d18O) isotope composition. Combined with isotope data of snow samples,we used a mixing model to quantify the contribution of snow to the consolidated sea ice ridge mass. Our results demonstrate that snow meltwater is important for summer consolidation and overall ice mass balance of FYI ridges during the melt season, representing 6%–11% of total ridged ice mass or an ice thickness equivalent of 0.37–0.53 m.These findings demonstrate that snowmelt contributes to consolidation of FYI ridges and is a mechanism resulting in a relative increase of sea ice volume in summer. This mechanism can also affect the mechanical strength and survivability of ridges, but also contribute to reduction of the habitable space and light levels within FYI ridges. We proposed a combination of two pathways for the transport of snow meltwater and incorporation into ridge keels: percolation downward through the ridge and/or lateral transport from the under-ice meltwater layer. Whether only one pathway or a combination of both pathways is most likely remains unclear based on our observations, warranting further research on ridge morphologypublishedVersio

Snowmelt contribution to Arctic first-year ice ridge mass balance and rapid consolidation during summer melt

An assessment of potential groundwater areas in the Ifni basin, located in the western AntiAtlas range of Morocco, was conducted based on a multicriteria analytical approach that integrated a set of geomorphological and hydroclimatic factors influencing the availability of this resource. This approach involved the use of geographic information systems (GIS) and hierarchical analytical process (AHP) models. Different factors were classified and weighted according to their contribution to and impact on groundwater reserves. Their normalized weights were evaluated using a pairwise comparison matrix. Four classes of potentiality emerged: very high, high, moderate, and low, occupying 15.22%, 20.17%, 30.96%, and 33.65%, respectively, of the basin’s area. A groundwater potential map (GWPA) was validated by comparison with data from 134 existing water points using a receiver operating characteristic (ROC) curve. The AUC was calculated at 80%, indicating the good predictive accuracy of the AHP method. These results will enable water operators to select favorable sites with a high groundwater potential

Thermodynamic scaling of ice ridge consolidation

Laboratory and field experiments together with analytical and numerical simulations were performed to study the scaling of the ice ridge consolidation. Such experiments and corresponding thermodynamic models are an important method for describing and predicting morphological, physical, and mechanical properties of the consolidated layer, corresponding atmospheric heat fluxes, and structural loads. The laboratory-scale experiments covered ice ridges, grown from freshwater, seawater, and water-ethanol solution with different types of morphology including with parallel blocks. Such morphology was used to decrease effects from the ridge inhomogeneity, and to increase the measurement accuracy of the ridge macroporosity and the ice thickness. This allowed for separate investigations of the effects from the other ridge parameters including block thickness, ice initial temperature, and the ridge sail height. The effect of the faster growth rate of the consolidated layer over the level ice for small-scale ridges observed experimentally was found to be related to the difference in convective-conductive coupling for the two types of ice, which can be increased by the extended ridge sail surfaces. The experiments with water-ethanol ice showed no significant difference in consolidation rates with the freshwater ice ridges. The full-scale experiments covered saline ice ridges artificially made from the surrounding level ice. This method allowed us to increase the accuracy of the macroporosity and initial ice temperature values. The results of the field measurements confirmed the thickness overestimation based on the measured temperature profile in the ridge blocks in comparison to the ridge voids. This thickness overestimation was also observed in small-scale experiments. The effect of slower consolidation rates for the full-scale ridges during the initial phase observed experimentally was found to be related to the significant deviation of those ridges from the homogeneous approach. Simulations of the ridge consolidation were performed using a two-dimensional finite element method with the moving boundary and the discrete rubble blocks. It was validated by the performed laboratory and field experiments for different scales and different types of ice. It allowed deeper investigations of the effects from the ridge sail, rubble block initial temperature and thickness, ridge keel, and the thickness estimation methods for the consolidated layer. It has also been able to describe the scale-effects in the previous ridge experiments. The simulations helped to provide insight into the analysis of the ice ridge thermal investigations, the estimation algorithms for the consolidated layer thickness, and on the distribution of the heat transfer through the different ridge parts. The difference between fresh and saline ice growth was equally important for level ice and ice ridges, but its values were becoming significant during the initial and warming phases. The analytical model of ridge consolidation was also formulated and validated using numerical simulations, field, and laboratory experiments. This model also allows to consider sail height, block thickness, initial ice temperature, ice salinity, and snow thickness, but cannot consider the thermal inertia. This analytical ridge model could be used for the prediction of the consolidated layer thickness in the probabilistic analysis of ice actions on structures

Temperature difference after the heating cycle from the sea ice mass balance buoy DTC52 during MOSAiC 2019/2020

Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC52 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 27 August 2020. The thermistor chain was 2.36 m long and included sensors with a regular spacing of 2 cm. The initial ice thickness was 1.32 m, the snow thickness was 0.05 m, and the freeboard was 0.17 m. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 27 August 2020 and 20 September 2020 in sample intervals of 6 hours. It also contains manually estimated positions of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in the undeformed second/multiyear ice next to the Remote Sensing site

Temperature difference after the heating cycle from the sea ice mass balance buoy DTC55 during MOSAiC 2019/2020

Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC55 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 26 August 2020. The thermistor chain was 5.12 m long and included sensors with a regular spacing of 2 cm. The initial ice thickness was 1.14 m, the snow thickness was 0.03 m, and the pond depth was 0.17 m. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 26 August 2020 and 20 September 2020 in sample intervals of 6 hours. It also contains manually estimated positions of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in the undeformed second/multiyear ice next to the Met City and the ice mass balance buoy 2020T84: doi:10.1594/PANGAEA.958397

Temperature after the cooling cycle from the sea ice mass balance buoy DTC52 during MOSAiC 2019/2020

Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC52 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 27 August 2020. The thermistor chain was 2.36 m long and included sensors with a regular spacing of 2 cm. The initial ice thickness was 1.32 m, the snow thickness was 0.05 m, and the freeboard was 0.17 m. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 27 August 2020 and 20 September 2020 in sample intervals of 6 hours. It also contains manually estimated positions of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in the undeformed second/multiyear ice next to the Remote Sensing site

Temperature after the cooling cycle from the sea ice mass balance buoy DTC51 during MOSAiC 2019/2020

Temperature and heating-induced temperature were measured along a chain of thermistors. Digital Thermistor Chain DTC51 is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the MOSAiC expedition on 23 August 2020. The thermistor chain was 4.16 m long and included sensors with a regular spacing of 2 cm. The initial ice thickness was 1.52 m, the snow thickness was 0.02 m, and the freeboard was 0.16 m. The resulting time series describes the evolution of temperature during the heating cycle of 20 s and after the heating cycle during the following 40 s as a function of geographic position (GPS), depth, and time between 23 August 2020 and 17 September 2020 in sample intervals of 6 hours. It also contains manually estimated positions of air-snow, snow-ice, and ice-water interfaces. The DTC was installed in the undeformed second/multiyear ice next to ice mass balance buoys 2020T78: doi:10.1594/PANGAEA.958438