Sea ice physical properties were determined at 21 first‐year ice (FYI) and 20 multiyear ice (MYI) stations in the Pacific sector of the Arctic during summer in 2008–2018. The bulk ice temperature was between −2.7 and −0.3 °C for FYI and between −1.7 and −0.2 °C for MYI. The bulk salinity was 0.4–3.2 practical salinity unit (psu) for FYI and 0.4–2.4 psu for MYI. A low‐salinity layer and an almost fresh layer were always present at the top of FYI and MYI, respectively. The bulk density was 600.3–900.1 kg/m³ for FYI and 686.1–903.3 kg/m³ for MYI. The upper layer density was less than the lower layer density due to higher gas content. Salinity and density were less than previously reported values of summer sea ice, and their parameterization formulae were updated. The brine and gas volume fractions were determined based on the measured ice temperature, salinity, and density. The average bulk brine and gas volume fractions were 11.3 ± 6.2% and 14.9 ± 7.9% for FYI and 9.5 ± 11.0% and 13.5 ± 12.7% for MYI. Typical brine and gas volume fraction profiles were identified and parameterized using regression analysis. The large brine and gas content of sea ice affects the sea ice melting. The energy used to melt a unit volume of summer Arctic sea ice was estimated as (2.26 ± 0.29) × 10⁵ kJ for FYI and (2.36 ± 0.24) × 10⁵ kJ for MYI. This study shows that the current physical state of the Arctic summer sea ice is different from earlier sea ice climatology, and the results will guide further research and modeling of Arctic sea ice.

Interaction of under‐ice physical, chemical, and biological processes with lake ice/snow cover is examined to better understand how changing winter climate may affect lake ecosystems. We derived under‐ice dissolved oxygen (DO) dynamics from high‐frequency observations and modified a widely used lake metabolism model by including the effect of freezing and thawing on DO concentration. Estimates were produced for the production and respiration in a shallow lake on the Mongolian Plateau in three winters. Diel, synoptic, and seasonal variations in DO concentration were detected as responses to solar radiation, episodic snowfall events, and occasional convective mixing. Based on the observations and a radiative transfer model, incident solar radiation was partitioned into reflectance, absorbance, and transmittance by the snow and ice cover. For bare ice, the contributions of these three parts were 35%, 39%, and 26%, respectively, while under a new 4.5 cm thick snow cover, the corresponding values were 79%, 17%, and 3%. This points out the critical role of snow and ice on under‐ice light conditions, which is the primary forcing for the temperature and the rate of photosynthesis under ice. The results showed three principal factors, which influenced under‐ice DO and metabolism: (1) thickness and optical properties of ice and snow, which affected the light transfer and depth of the euphotic zone, (2) mediated radiation and ice‐water heat transfer which controlled water temperature, and (3) DO exclusion during freezing and dilution by melt water. This study highlights the ecosystem characteristics in shallow ice‐covered lakes in arid temperate regions and promotes our understanding of the response of the cold aquatic environment to climate change.

A total of 25 flexural and 55 uniaxial compressive strength tests were conducted in laboratory using landfast sea ice samples collected in the Prydz Bay. Three-point bending tests were performed at ice temperatures of −12 to −3 ^∘C with force applied vertically to original ice surface, and compressive strength tests were performed at −3 ^∘C with a strain-rate level of 10⁻⁶–10⁻² s⁻¹ in the directions vertical and horizontal to ice surface. Judging from crystal structure, the ice samples were divided into congelation ice, snow ice, and a mixture of the two. The results of congelation ice showed that the flexural strength had a decreasing trend depending on porosity rather than brine volume, based on which a mathematical equation was established to estimate flexural strength. Both flexural strength and effective modulus of elasticity increased with increasing platelet spacing. The uniaxial compressive strength increased and decreased with strain rate below and above the critical regime, respectively, which is 8.0 × 10⁻⁴–1.5 × 10⁻³ s⁻¹ for vertically loaded samples and 2.0 × 10⁻³–3.0 × 10⁻³ s⁻¹ for horizontally loaded samples. A drop-off in compressive strength was shown with increasing sea ice porosity. Consequently, a model was developed to depict the combined effects of porosity and strain rate on compressive strength in both ductile and brittle regimes. The mechanical strength of mixed ice was lower than congelation ice, and that of snow ice was much weaker. To provide a safe guide for the transportation of goods on landfast sea ice in the Prydz Bay, the bearing capacity of the ice cover is estimated with the lower and upper envelopes of flexural strength and effective elastic modulus, respectively, which turned out to be a function of sea ice porosity.

To improve the understanding of the seasonal evolution of the mass and heat budget of ice-covered lakes in the cold and arid climate zone, in-situ observations were collected during two winters (2016–2017 and 2017–2018) in Lake Wuliangsuhai, Inner Mongolia, China. The mean snow thickness was 5.2 and 1.6 cm in these winters, due to low winter precipitation. The mean ice thickness was 50.9 and 36.1 cm, and the ice growth rate was 3.6 and 2.1 mm day⁻¹ at the lower boundary of ice. Analyses of mass and heat balance data from two winters revealed that the surface heat budget was governed by solar radiation and terrestrial radiation. The net heat flux loss of the ice was 9–22 W m⁻², affected by the snow and ice thickness. Compared to boreal lakes, Lake Wuliangsuhai received more solar radiation and heat flux from the water. The ice temperature had a strong diurnal variation, which was produced by the diurnal cycles of solar radiation, and air and water temperatures. These results expand our knowledge of the evolution of mass and heat balance in temperate lakes of mid-latitude arid areas.