The dependence of oxygen isotope fractionation on ice growth rate during the freezing of sea water is investigated based on laboratory experiments and field observations in McMurdo Sound, Antarctica. The laboratory experiments were performed in a tank filled with sea water, with sea ice grown under calm conditions at various room temperatures ranging from −5°C to −20°C. In McMurdo Sound, the ice growth rate was monitored using thermistor probes for first-year landfast ice that grew to ∼2 m in thickness. Combining these datasets allows, for the first time, examination of fractionation at a wide range of growth rates from 0.8 × 10⁻⁷ to 9.3 × 10⁻⁷ m s⁻¹. In the analysis a stagnant boundary-layer model is parameterized using these two independent datasets. As a result, the optimum values of equilibrium pure-ice fractionation factor and boundary-layer thickness are estimated. It is suggested that a regime shift may occur at a growth rate of ∼2.0 × 10⁻⁷ m s⁻¹. A case study on sea ice in the Sea of Okhotsk, where the growth rate is modeled by coupling the thermodynamic properties of the sea ice with meteorological data, demonstrates the utility of the fitted models.

Sea ice thickness data and sea ice samples were analyzed to examine the characteristics of the ice thickness distribution and ice texture, and to understand ice growth processes in the southern Sea of Okhotsk. Ice thickness data and samples were obtained aboard the icebreaker Soya in early February, the ice growth season. Ice thickness data, which were obtained with a video monitoring system installed on the side deck of the ship each winter from 1991 to 2000 except 1995, show that the average thickness ranges from 19 ± 7 to 55 ± 23 cm and that it matches the characteristics of a Poisson distribution. Ice structure analysis reveals that granular texture occupies about three quarters of the total ice thickness and that the ice exhibits a layered structure with unit thickness averaging 5 to 10 cm. Stratigraphy and stable isotopic composition of the ice indicate that snow ice accounts for 10% and frazil ice accounts for 64% of the total ice thickness. This suggests that dynamic ice thickening processes such as frazil ice growth and piling up are more significant than congelation growth. On the basis of these characteristics, which resemble more those of Antarctic than Arctic sea ice, we propose a conceptual model for the ice thickening process in this region. It is shown that this model can explain the shape of the ice thickness distribution well, and is analogous with the concept of the “pancake cycle” and multiple rafting of Antarctic sea ice growth and thickening.

The lowest latitude sea ice in the world (excluding coastal freezing) is in the southern Sea of Okhotsk (south of 46°N), where it has significant impacts on freshwater input and primary production. This region is subject to climate change, and accordingly the monitoring of sea ice conditions is important. However, the interannual variability of the region's sea ice is poorly understood due to its logistical challenges. Sea ice observations have been conducted in this region every winter for the period 1996–2020. The interannual variability of the ice conditions and the likely factors responsible for it were investigated using visual observations following the international ASPeCt protocol, combined with satellite SSM/I‐SSMIS ice concentration data (1988–2020). AMSR‐derived ice drift data sets and ERA5 meteorological reanalysis data sets were also analyzed to examine the effects of dynamic and thermodynamic processes. Our analysis revealed that (a) sea ice area in this region varies differently from that in the central and northern Sea of Okhotsk, where decreasing trends are reported, (b) sea ice volume has remarkable interannual variation and the peaks appeared much to more affected by dynamically deformed ice than freezing conditions, and (c) prominently deformed ice can be explained by taking shear components into account based on sea ice rheology. These results suggest the importance of including the proper sea ice rheology in numerical sea ice models to reproduce the realistic sea ice volume and deformation processes, for all seasonal ice zones.

Although satellite data are useful for obtaining ice-thickness distribution for perennial sea ice or in stable thin-sea-ice areas, they are still largely an unresolved issue for the seasonal ice zone (SIZ). We address this problem using L-band synthetic aperture radar (SAR). In the SIZ, ice-thickness growth is closely related to deformation, so surface roughness is expected to correlate with ice thickness. L-band SAR, suitable for detecting such surface roughness, is a promising tool for obtaining thickness distribution. This idea was supported by an airborne polarimetric and interferometric SAR (Pi-SAR) validation. To extend this result to spaceborne L-band SAR with coarser resolution, we conducted in situ measurements of ice thickness and surface roughness in February 2008 in the southern Sea of Okhotsk with an icebreaker in coordination with the Advanced Land Observing Satellite (ALOS)/Phased Array-type L-band SAR (PALSAR) orbit. A helicopter-borne laser profiler was used to improve the estimation of surface roughness. It was found that backscatter coefficients (HH) correlated well with ice thickness (R = 0.86) and surface roughness (R = 0.70), which confirms the possibility of determining ice-thickness distribution in the SIZ. the interannual variation of PALSAR-derived ice-thickness distribution in the southern Sea of Okhotsk is also discussed.