Physical structures and interior melt of the central Arctic sea ice/snow in summer 2012

• Vertical stratifications of ice crystal, temperature, salinity, and density were quantified.
• Volumetric fractions of gas and brine within sea ice were presented.
• Fractions of inclusions (gas and brine) indicated internal melt of Arctic sea ice.
• Interior reduction of sea ice has increased compared to the previous findings.

The physical structures, solar irradiance and reflectance of snow and sea ice in high Arctic were investigated based on snow pit, ice coring, and borehole measurements from August 29 to September 2 in 2012. In all of the six short-term stations, the snow cover was stratified: fresh snow in the top (if any) with dendritic grains (1–4 mm in grain size), fine-grained snow in the middle (4–9 mm), and coarsely spherical grained snow in the bottom (8–16 mm). The incident and reflected irradiance were wavelength-dependent, whereas the albedo and its dependence on wavelength were relatively diverse and scattering. The integrated albedos were 0.47–0.92, largely depending on the snow type, grain size, thickness, and water content. Ice coring and thin sections indicated that all ice covers were first-year, and were dominated by the columnar grained ice. And all the ice cores were characterized by a large number of gas and brine pockets and tubes, the size and content of which were much greater than those of sea ice cover during the winter congelation. The vertical stratifications of ice density, temperature and salinity were quantified using linear regression. Overall, the salinity and density increased with an increasing depth, while the temperature decreased against depth except the top 10–20 cm, where the temperature was dependent on the air and snow temperatures. The estimates of inclusion volume (including gas and brine) gave insight into the sea ice composition (ice-brine/gas matrix). The comparison of the present volumetric fraction of inclusions with previous ones indicated to some degree that the sea ice inclusion volume increased in the ice interior, thus the volumetric fraction of pure ice decreased at the end of summer melt, indicating a rigorous melting within the Pacific Arctic sea ice covers. Our work suggests that the volumetric fraction of sea ice inclusions as a function of temperature, density and salinity can be considered as an alternative indicator for Arctic sea ice reduction, just like the sea ice extent, thickness, age and melting duration.