The Sea of Okhotsk is the southernmost seasonal sea-ice region in the Northern Hemisphere and is characterized by high biological productivity, including massive spring phytoplankton blooms. It also plays a key role in driving the intermediate thermohaline circulation of the North Pacific through the formation of high-density water associated with sea-ice production. In recent years, however, rapid global warming has become evident, raising concerns about severe impacts on marine ecosystems, such as the contraction of the seasonal sea-ice area during winter and the occurrence of unprecedented marine heatwaves in summer. Despite its importance, reanalysis datasets based on ocean–sea ice models that can be applied to oceanographic analyses and interdisciplinary studies (e.g., fisheries science) in the Sea of Okhotsk remain limited. To address this gap, we have initiated the development of a coupled ocean–sea ice model covering the entire Sea of Okhotsk at 1/60° resolution using open-source tools, with the goal of producing a freely accessible data product for the international research community, covering the period from 1993 to the present. The ocean model is based on the Regional Ocean Modeling System (ROMS), and lateral boundary conditions are provided by GLORYS12 (1/12° global ocean–sea ice reanalysis). The model employs 45 vertical layers, and the vertical coordinate parameters were carefully tuned through trial and error to suppress numerical instabilities. For spin-up, a 1/12° coupled ocean–sea ice model configured over the same domain was integrated from 1 January 1993 to 1 January 2010, and its output was used as the initial condition for the 1/60° model on 1 January 1993. Surface forcing fields were generated as a customized product by combining JRA-55DO (global 0.5625° grid) and DSJRA-55 (approximately 5-km grid over the Japanese region). Freshwater forcing was also derived from JRA-55DO for rivers from the Eurasian continent, while daily mean river discharge for Japanese rivers was estimated using an original method. As a result, the model successfully simulates coupled ocean–sea ice processes. For instance, the model visualizes small-scale dynamical features, including the generation and advection of topographically induced anticyclonic eddies along the eastern coast of Kamchatka, enhanced submesoscale eddies around straits in the northern Kuril Islands, and pronounced submesoscale variability around the northwestern Okhotsk Sea shelf polynya and other ice-edge regions. In this presentation, we will also share future perspectives on data dissemination, planned routine system operation, and further development toward coupling with lower-trophic-level ecosystem models to support a freely accessible international modeling platform.
 

Sea of Okhotsk, Ocean–Sea Ice Coupled Model, Open-Source Modeling, Regional Ocean Modeling System (ROMS)