Description: The rapid melt of snow and sea ice during the Arctic summer provides a significant source of low-salinity meltwater to the surface ocean on the local scale. The accumulation of this meltwater on, under, and around sea ice floes can result in relatively thin meltwater layers in the upper ocean. Due to the small-scale nature of these upper-ocean features, typically on the order of 1 m thick or less, they are rarely detected by standard methods, but are nevertheless pervasive and critically important in Arctic summer. Observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in summer 2020 focused on the evolution of such layers and made significant advancements in understanding their role in the coupled Arctic system. Here we provide a review of thin meltwater layers in the Arctic, with emphasis on the new findings from MOSAiC. Both prior and recent observational datasets indicate an intermittent yet longlasting (weeks to months) meltwater layer in the upper ocean on the order of 0.1 m to 1.0 m in thickness, with a large spatial range. The presence of meltwater layers impacts the physical system by reducing bottom ice melt and allowing new ice formation via false bottom growth. Collectively, the meltwater layer and false bottoms reduce atmosphere-ocean exchanges of momentum, energy, and material.The impacts on the coupled Arctic system are far-reaching, including acting as a barrier for nutrient and gas exchange and impacting ecosystem diversity and productivity.

Description: Low-salinity meltwater from Arctic sea ice and its snow cover accumulates and creates under-ice meltwater layers below sea ice.These meltwater layers can result in the formation of new ice layers, or false bottoms, at the interface of this low-salinity meltwater and colder seawater. As part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we used a combination of sea ice coring, temperature profiles from thermistor strings and underwater multibeam sonar surveys with a remotely operated vehicle (ROV) to study the areal coverage and temporal evolution of under-ice meltwater layers and false bottoms during the summer melt season from mid-June until late July. ROV surveys indicated that the areal coverage of false bottoms for a part of the MOSAiC Central Observatory (350 by 200 m2) was 21%. Presence of false bottoms reduced bottom ice melt by 7-8% due to the local decrease in the ocean heat flux, which can be described by a thermodynamic model. Under-ice meltwater layer thickness was larger below first-year ice and thinner below thicker second-year ice.We also found that thick ice and ridge keels confined the areas in which under-ice meltwater accumulated, preventing its mixing with underlying seawater. While a thermodynamic model could reproduce false bottom growth and melt, it could not describe the observed bottom melt rates of the ice above false bottoms. We also show that the evolution of under-ice meltwaterlayer salinity below first-year ice is linked to brine flushing from the above sea ice and accumulating in the meltwater layer above the false bottom.The results of this study aid in estimating the contribution of underice meltwater layers and false bottoms to the mass balance and salt budget for Arctic summer sea ice.

Description: First- and second-year sea-ice thickness, draft, salinity, temperature, and density were measured during two ice stations on 24 October 2022 and 30 October 2022 during leg 1 of the GoNorth 2022 expedition. The ice cores were extracted with 7.25-cm (Mark III) internal diameter ice corers (Kovacs Enterprise, US). During each ice station, ice temperature was measured in situ from a separate temperature core, using Ebro TFX 410 Thermometer thermometers in drill holes with a length of half-core-diameter at 5 cm vertical resolution. Ice bulk practical salinity was measured from melted core sections at 5 cm resolution using a Mettler Toledo SevenGo conductivity meter. Sea ice density was measured using the hydrostatic weighing method (Pustogvar and Kulyakhtin, 2016) from several density cores in the freezer laboratory onboard RV Kronprins Haakon at the temperature from –10°C to –14°C. Relative volumes of brine and gas were estimated from ice salinity, temperature, and density using Cox and Weeks (1983) for cold ice and Leppäranta and Manninen (1988) for ice warmer than –2°C. This sea ice physics data was collected during leg 1 of the GoNorth 2022 scientific expedition on 14 October – 3 November 2022 (cruise number 2022713) on the RV Kronprins Haakon. The ice cores were collected at two ice stations (Station 6 and Super Station 14) located at 82°13.56' N and 26°41.43' E for the first and 82°31.05' N and 17°30.04' E for the second sea ice station in the area north of Svalbard. The data contains the event label (1), station (2), time (3), global coordinates (4,5) of each coring measurement, ice type (11), and sample ID (12). Each core has its manually measured ice thickness (6), ice draft (7), snow height (8), and local coordinates for each ice station (7,8). Each core section has the total length of its top (13) and bottom (14) measured in situ. Each core section has the value of its practical salinity (15), each core section of a temperature core has the value of its in situ temperature (16), and each core section of density cores has the value of its ice density (18). Each core section also has laboratory temperature (17), an estimate of brine volume fraction (19), and gas volume fraction (20).

Description: Temperature and heating-induced temperature differences were measured along a chain of thermistors. 2020M26 (a.k.a. Bruncin IMB042) is an autonomous modular instrument that was installed on drifting sea ice in the Arctic Ocean during the 4th leg of MOSAiC in June 2020. The thermistor chain was 5 m long and included 256 sensors. The resulting time series describes the evolution of temperature and temperature differences after three heating cycles of 4, 20 and 24 s as a function of place, depth and time between 26 June 2020 and 19 August 2020 in sample intervals of 1 hour for temperature and 6 hours for temperature differences. In addition, this modular buoy consisted of sensors measuring position (GPS) and barometric pressure at hourly intervals. The buoy was installed on a ridge, called Jaridge that was studied during leg 4, in the MOSAiC Central Observatory. This instrument was deployed as part of the project Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean (HAVOC), funded by the Research Council of Norway (project number 280292).

Description: Snow and first-year sea ice ridge thickness, draft, and morphology were measured using a 2-inch ice drilling auger (Kovacs Enterprises) during walking surveys on the first-year ice ridge during the MOSAiC expedition. Drilling was performed during June and July 2020 across seven drilling transects. The total covered area was approximately 35 m by 25 m. The investigated “Jaridge” was formed on February 4–12, 2020 between first- and second-year ice and consisted mainly of 0.2–0.4 m thick ice blocks. The ridge was located on drifting sea ice in the Arctic Ocean within the Central Observatory of MOSAiC. The table contains the event label (1), event ID (2), time (3), and global coordinates (4,5) of each drilling measurement. Each separate drilling hole has its number (6), and local coordinates X (7) and Y (8) in [m]. Global coordinates are given for the local coordinates of (0,0). For each drill hole, the depth relative to the waterline of the top (9) and bottom (10) interface of each separate layer is given together with its ice type (11). Ice types include snow, ice, and water. The drill hole with local coordinates of (0,0) coincides with the ice mass balance buoy 2020M26 installation described in doi:10.1594/PANGAEA.926580. The drill holes with local coordinates of (7.5,20) and (19,35) coincide with the isotope and salinity data from ice coring described in doi:10.1594/PANGAEA.943746 for events PS122/4_46-178 and PS122/4_47-199.

Description: Oxygen and hydrogen isotopic ratio samples were collected from sea ice and water samples. Coring and sampling were completed at six locations on July 25 for measurement of salinity and stable oxygen isotopic composition to better understand the meltwater sources for under-ice meltwater and false bottoms. Cores were collected using a corer (9 cm inner diameter; Mark II coring system, Kovacs Enterprises, US), and ice samples were collected from the bottom 10 centimeters of the ice core in 5 cm increments (0-5 and 5-10 cm above the bottom) and from the false bottom ice. Under-ice meltwater layer samples and water from directly below the false bottom were collected using a peristaltic pump. In order to minimize contamination from coring activities, under-ice meltwater samples from the meltwater layer in the void space between ice and false bottom were collected prior to coring through the false bottom. Surface melt pond water samples were collected by dipping plastic sample cups (rinsed with milliQ water) in the melt ponds where cores were collected, prior to ice coring. In total, ice samples were collected from six cores –– five of which included false bottoms –– and water samples were collected from four surface melt ponds, three under-ice melt water layers, and seawater beneath false bottoms at four locations. Locations are shown on map (Figure 2) as pink dashes. Ice samples were melted onboard the R/V Polarstern, and salinities of all water and ice samples were measured onboard using a calibrated YSI model 30 probe (salinity is given in PSS-78 scale, unitless). Vials for oxygen isotope composition were shipped to the AlfredWegener Institute (AWI) ISOLAB Facility in Potsdam, Germany, where they were analyzed for stable water isotopes with Finnigan MAT Delta-S mass spectrometers using equilibration techniques. The oxygen isotope composition is given as per mil difference relative to VSMOW (‰, Vienna Standard Mean Ocean Water), with an internal 1 sigma error better than 0.1‰ for delta 18O (Meyer et al., 2000). See attached diagram for a visualization of where the samples were collected.

Description: Snow and first-year sea ice ridge thickness, draft and morphology were measured using a 2-inch ice drilling auger (Kovacs Enterprises) during walking surveys on first-year ice ridge during the MOSAiC expedition. Drilling was performed during January and February 2020 across three drilling transects located 20 m from each other. The investigated "Fort Ridge" was formed from late September to early October 2019 between second- and first-year ice. It was approximately 90–100 m long and 20–30 m wide. The ridge was located on drifting sea ice in the Arctic Ocean within the Central Observatory of MOSAiC. The table contains the event label (1), event ID (2), time (3), and global coordinates (4,5) of each drilling measurement. Each separate drilling hole has its number indicating cross-section and number within cross-section (6), and local coordinate X (7) in [m] and transect name (8). Global coordinates are given for the local coordinates of X = 22 m at the transect 1. For each drill hole, the depth relative to the waterline of the top (9) and bottom (10) interface of each separate layer is given together with its ice type (11). Ice types include snow, ice (with various resistance), and voids. Information about missing freeboard measurements is given in comments (12). In the case of no freeboard measurement, it was assumed as 10% of the maximum keel draft. The drill hole with local coordinates of X = 22 m at the transect 1 coincides with the ice mass balance buoy 2020T60 installation described in doi:10.1594/PANGAEA.924269. These measurements were performed as part of the project Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean (HAVOC), funded by the Research Council of Norway, project number: 280292).