Sea ice records over more than a century at an observatory facing the Okhotsk coast of Hokkaido, Japan.

Socio-economical activities around the Okhotsk coast of Hokkaido, Japan are strongly influenced by local sea ice variations, which are determined by larger-scale atmospheric drivers in the North Pacific. Here, we constructed the long-term sea ice observational data from the 1890s by combining data from the 1940s, already released by the Japan Meteorological Agency prior to this study, with further data shared for the first time in this study. For this purpose, old hand-written reports and sketches stored in the Abashiri Local Meteorological Office were investigated. For each year, first and last sea ice dates and daily sea ice variations recorded visually by sighting the ice were digitized to provide continuous time series to the present. In addition, we introduced a new reconstruction approach from satellite data for the date of last annual drift ice occurrence, for which observations are lacking after 2021. The data presented here are of value in understanding the long-term climate variability in the Sea of Okhotsk and the North Pacific.