The Interannual Variability of the Ice Conditions in the Southern Sea of Okhotsk and Its Likely Factors

The sea ice in the southern Sea of Okhotsk (south of 46N), which appears at the lowest latitude in the world, has significant contributions to freshwater input amounting to 10-20% of the annual discharge of the Amur River and primary production by releasing micro-nutrient when melting. This region also contains Shiretoko, designated as a UNESCO World Heritage site for the rich ecosystem related to sea ice. Thus, monitoring sea ice conditions is an important issue. However, due to the logistical difficulties, the interannual variability of the ice conditions there is poorly understood. Since 1996, sea ice observation has been conducted in this region every winter for consecutive 25 years in collaboration with Japan Coast Guard. Based on the data records obtained during this period, the interannual variability of the ice conditions in this region and the likely factors responsible for it were investigated. In this study, visual observation records according to the international ASPeCt protocol and ice thickness data derived from a video system were mainly used with the satellite SSM/I ice concentration data (1988-2020). To examine the effects of dynamical and thermodynamical processes, AMSR-derived ice drift data sets (2003-2020) and ERA5 meteorological reanalysis data sets (1988-2020) were also analyzed, respectively. Our analysis revealed that 1) the sea ice area in this region varies differently from that in the central and northern Sea of Okhotsk where decreasing trends are remarkable (~10% per decade), 2) sea ice volume has a remarkable interannual variation and the peaks appeared much affected by deformed ice, 3) such prominent deformed ice can be explained only by taking shear components into account as well as convergence components on the basis of sea ice rheology, and seems to be produced through sporadic events, 4) the freezing conditions are somewhat correlated with sea ice area in this region but much less with sea ice volume. These results suggest the importance of including the proper sea ice rheology in numerical sea ice models to reproduce the real deformation processes, sea ice volume, and the difficulty in predicting the ice condition in this region.