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In recent years, sea‐ice conditions in the Arctic Ocean changed substantially toward a younger and thinner sea‐ice cover. To capture the scope of these changes and identify the differences between individual regions, in situ observations from expeditions are a valuable data source. We present a continuous time series of in situ measurements from the N‐ICE2015 expedition from January to June 2015 in the Arctic Basin north of Svalbard, comprising snow buoy and ice mass balance buoy data and local and regional data gained from electromagnetic induction (EM) surveys and snow probe measurements from four distinct drifts. The observed mean snow depth of 0.53 m for April to early June is 73% above the average value of 0.30 m from historical and recent observations in this region, covering the years 1955–2017. The modal total ice and snow thicknesses, of 1.6 and 1.7 m measured with ground‐based EM and airborne EM measurements in April, May, and June 2015, respectively, lie below the values ranging from 1.8 to 2.7 m, reported in historical observations from the same region and time of year. The thick snow cover slows thermodynamic growth of the underlying sea ice. In combination with a thin sea‐ice cover this leads to an imbalance between snow and ice thickness, which causes widespread negative freeboard with subsequent flooding and a potential for snow‐ice formation. With certainty, 29% of randomly located drill holes on level ice had negative freeboard. During N‐ICE2015 the snow‐to‐ice thickness ratio was exceptionally high due to thick snow on thin iceModal sea‐ice thickness was below historical observations in the area north of Svalbard confirms the general trend of continued thinningThick snow limited ice growth in winter, resulting in flooding and widespread negative freeboard

We present freeboard measurements from airborne laser scanner (ALS), the Airborne Synthetic Aperture and Interferometric Radar Altimeter System (ASIRAS), and CryoSat‐2 SIRAL radar altimeter; ice thickness measurements from both helicopter‐borne and ground‐based electromagnetic‐sounding; and point measurements of ice properties. This case study was carried out in April 2015 during the N‐ICE2015 expedition in the area of the Arctic Ocean north of Svalbard. The region is represented by deep snow up to 1.12 m and a widespread presence of negative freeboards. The main scattering surfaces from both CryoSat‐2 and ASIRAS are shown to be closer to the snow freeboard obtained by ALS than to the ice freeboard measured in situ. This case study documents the complexity of freeboard retrievals from radar altimetry. We show that even under cold (below −15°C) conditions the radar freeboard can be close to the snow freeboard on a regional scale of tens of kilometers. We derived a modal sea‐ice thickness for the study region from CryoSat‐2 of 3.9 m compared to measured total thickness 1.7 m, resulting in an overestimation of sea‐ice thickness on the order of a factor 2. Our results also highlight the importance of year‐to‐year regional scale information about the depth and density of the snowpack, as this influences the sea‐ice freeboard, the radar penetration, and is a key component of the hydrostatic balance equations used to convert radar freeboard to sea‐ice thickness. In the N‐ICE2015 study region, freeboard retrievals from airborne and satellite radar altimeters are close to the snow freeboardRadar penetration in snow can be low even at low temperatures, with consequences for radar‐based sea‐ice thickness and mass estimatesIn our region treating radar freeboard as sea‐ice freeboard results in a sea‐ice thickness double that measured in the field

The salinity and water oxygen isotope composition (δ18O) of 29 first‐year (FYI) and second‐year (SYI) Arctic sea ice cores (total length 32.0 m) from the drifting ice pack north of Svalbard were examined to quantify the contribution of snow to sea ice mass. Five cores (total length 6.4 m) were analyzed for their structural composition, showing variable contribution of 10–30% by granular ice. In these cores, snow had been entrained in 6–28% of the total ice thickness. We found evidence of snow contribution in about three quarters of the sea ice cores, when surface granular layers had very low δ18O values. Snow contributed 7.5–9.7% to sea ice mass balance on average (including also cores with no snow) based on δ18O mass balance calculations. In SYI cores, snow fraction by mass (12.7–16.3%) was much higher than in FYI cores (3.3–4.4%), while the bulk salinity of FYI (4.9) was distinctively higher than for SYI (2.7). We conclude that oxygen isotopes and salinity profiles can give information on the age of the ice and enables distinction between FYI and SYI (or older) ice in the area north of Svalbard. The role of snow in sea ice mass balance is largely two fold. Firstly, it can slow down growth and melt due to its high insulation and high reflectance, but secondly it can actually contribute to sea ice growth if the snow cover is turned into ice. The latter is largely a consequence of high mass of snow on top of sea ice that can push the surface of the sea ice below sea level and seawater can flood the ice. This mixture of seawater and snow can then freeze and add to the growth of sea ice. This is very typical in the Antarctic but not believed to be so important in the Arctic. In this work we show, for the first time, that snow actually contributes significantly to the growth of Arctic sea ice. This is likely a consequence of the thinning of the Arctic sea ice. The conditions in the Arctic, with thinner and more seasonal ice thus resemble the ice pack in the Antarctic. Studies on the role of snow in the Arctic are critical to be able to understand the ongoing changes of the Arctic sea ice pack. Snow contributed 7.5–9.7% (by mass) in a thinner Arctic sea ice pack north of Svalbard in winter and springSecond‐year ice contained significantly more snow than first‐year ice (12.7–16.3% versus 3.3–4.4%)With thinner Arctic sea ice, the snow:ice thickness ratio is similar than in the Antarctic, increasing the potential for snow‐ice formation

The Barents Sea is one of the fastest changing regions of the Arctic, and has experienced the strongest decline in winter‐time sea‐ice area in the Arctic, at −23±4%decade−1. Sea‐ice thickness in the Barents Sea is not well studied. We present two previously unpublished helicopter‐borne electromagnetic (HEM) ice thickness measurements from the northwestern Barents Sea acquired in March 2003 and 2014. The HEM data are compared to ice thickness calculated from ice draft measured by ULS deployed between 1994 and 1996. These data show that ice thickness varies greatly from year to year; influenced by the thermodynamic and dynamic processes that govern local formation vs long‐range advection. In a year with a large inflow of sea‐ice from the Arctic Basin, the Barents Sea ice cover is dominated by thick multiyear ice; as was the case in 2003 and 1995. In a year with an ice cover that was mainly grown in situ, the ice will be thin and mechanically unstable; as was the case in 2014. The HEM data allow us to explore the spatial and temporal variability in ice thickness. In 2003 the dominant ice class was more than 2 years old; and modal sea‐ice thickness varied regionally from 0.6 to 1.4 m, with the thinner ice being either first‐year ice, or multiyear ice which had come into contact with warm Atlantic water. In 2014 the ice cover was predominantly locally grown ice less than 1 month old (regional modes of 0.5–0.8 m). These two situations represent two extremes of a range of possible ice thickness distributions that can present very different conditions for shipping traffic; or have a different impact on heat transport from ocean to atmosphere. Sea‐ice thickness in the northwestern Barents Sea is strongly variable, both interannually and regionallySea‐ice thickness depends on the balance between local ice formation and long‐range advection of old iceRelations between ice age and thickness can be complicated by specific environmental factors; e.g., warm Atlantic water

Spectral albedo and transmittance in the range 400–900nmwere measured on three separate dates on less than 15cmthick new Arctic sea ice growing on Kongsfjorden, Svalbard at 78.9°N, 11.9°E. Inherent optical properties, including absorption coefficients of particulate and dissolved material, were obtained from ice samples and fed into a radiative transfer model, which was used to analyze spectral albedo and transmittance and to study the influence of clouds and snow on these. Integrated albedo and transmittance for photosynthetically active radiation ( 400–900nm) were in the range 0.17–0.21 and 0.77–0.86, respectively. The average albedo and transmittance of the total solar radiation energy were 0.16 and 0.51, respectively. Values inferred from the model indicate that the ice contained possibly up to 40% brine and only 0.6% bubbles. Angular redistribution of solar radiation by clouds and snow was found to influence both the wavelength‐integrated value and the spectral shape of albedo and transmittance. In particular, local peaks and depressions in the spectral albedo and spectral transmittance were found for wavelengths within atmospheric absorption bands. Simulated and measured transmittance spectra were within 5% for most of the wavelength range, but deviated up to 25% in the vicinity of 800nm, indicating the need for more optical laboratory measurements of pure ice, or improved modeling of brine optical properties in this near‐infrared wavelength region. Observations and modeling of albedo and transmittance of thin ( <15cm) sea iceHigh brine volumes impact the spectral optical properties

Norske Øer Ice Barrier (NØIB) is a vast region of fast ice off northeast Greenland. To understand its thermodynamics, an ice mass balance buoy (IMB) was deployed in late August 2012. The IMB operated for a full year, providing unique year‐round observations of air, snow, ice, and upper ocean. Here, we examine atmospheric conditions and snow and sea ice evolution, derive the ocean heat flux at the deployment site, and investigate the effect of snow on the sea ice mass balance. Although an extremely low surface air temperature of −52°C was recorded at the IMB site, the sum of freezing‐degree days (4,848 K·d) is similar to that in the central Arctic. The maximum snow depth 0.73 m (on 29 May 2013, when ice was 2.19 m thick) is thicker than typically observed on level Arctic landfast ice and drift ice, and thick snow appears common in this region. Ocean heat flux was large in autumn and early winter (>7 W m−2), attributed in part to local solar heating of surface waters in summer and advected ocean heat later in the year. The combination of high ocean heat flux and thick snow resulted in a net loss of ice at the ice bottom over the year. A one‐dimensional sea ice model suggests that the thick snow cover limits bottom ice growth significantly. The heavy snow load could potentially have contributed 0.29 m snow ice; however, we cannot unambiguously prove this happened. Ice mass balance buoy was deployed for the first time in the Norske Øer Ice Barrier fast ice off the northeast Greenland coast at 79°NIn autumn and early winter ocean heat flux was high (>7 W m−2) and thick snow resulted from heavy snow accumulationOver a full year there was a net loss of ice at the ice bottom due to the high ocean heat flux and thick snow

The Fram Strait sea ice volume export 1992–2014 is derived by combining sea ice thickness from upward looking sonars (ULS) with satellite observations of sea ice drift and area. Fram Strait is the main gate for sea ice export from the Arctic. The average yearly sea ice export is 2,400 ± 640 km3. The mean and modal ULS ice thickness in Fram Strait decreased by 15% and 21% per decade, respectively, during 1990–2014. Combined with sea ice drift and area this leads to a decrease of the Arctic sea ice volume export of 27 ± 2% per decade between 1992 and 2014. Thus, for the given time period, changes in sea ice export do not drive the sea ice volume decrease in the Arctic Basin. However, for individual years like 2007 and 2012 the ice export likely has contributed to the loss of summer sea ice. Combined with PIOMAS model simulation we estimate that 14% of the total Arctic sea ice volume is exported every year through Fram Strait. This fraction of the total sea ice volume exported per year does not show a trend because the Arctic Basin ice volume is decreasing at a similar rate as the Fram Strait ice volume export. Compared to ice velocities from Acoustic Doppler Current Profiler (ADCP) the satellite ice drift shows good correspondence in variability but a negative bias. Ice volume transport estimates presented here thus should be considered a conservative estimate. We show, however, that the transport estimates are not sensitive to the exact flux gate location. A new, consistent, observation‐based time series of sea ice volume export through Fram Strait is derived for the years 1992 to 2014During that time period the sea ice volume export decreased by 54±4  km3month−1per decade or 27±2% per decadeThe percentage of total available Arctic sea ice volume exported per year stayed at a constant value of about 14%