The Interannual Variability of Sea Ice Area, Thickness, and Volume in the Southern Sea of Okhotsk and Its Likely Factors

The lowest latitude sea ice in the world (excluding coastal freezing) is in the southern Sea of Okhotsk (south of 46 degrees 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. Plain Language Summary The southern Sea of Okhotsk (south of 46 degrees N) is the southernmost area of the northern hemisphere, excluding coastal freezing, where there is wintertime sea ice cover. Although this sea ice has significant impacts on the surface heat and freshwater balance and biogeochemical cycles across a wide region, its interannual variability is not well understood yet. Long-term field observations from the patrol vessel in this region for 1996-2020, in combination with satellite data, reveals that the variability of sea ice area south of 46 degrees N is uncorrelated with that in the northern and central regions, and that the sea ice thickness and volume is mainly controlled by dynamical pile-up processes, rather than the thermodynamical freezing conditions. Analysis of satellite-derived ice drift datasets shows that the rheology traditionally used in many sea ice models, in which sea ice behaves like a plastic under the ordinary stress, holds consistently explaining the yearly variations of deformed ice contribution. Considering that sea ice rheology is essential to the dynamical part of sea ice models, our result promotes the possibility of accurately predicting sea ice thickness and volume in the seasonal ice zone.