Sea-ice discontinuity in ERA5 during 1978–1979 and its impacts in the Sea of Okhotsk

The European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis version 5 (ERA5) is one of the most widely used atmospheric reanalysis datasets provided by the ECMWF. However, the transition in the sea ice dataset between 1978 and 1979 may introduce inconsistencies that affect various surface meteorological variables. This study evaluated ERA5 sea ice data in the Sea of Okhotsk (SO), focusing on the years 1978 and 1979. Furthermore, the impact of this sea ice transition on meteorological variables was evaluated. In 1979, ERA5 sea ice coverage reached the coastal areas of Hokkaido in the southern SO, with an extent of approximately 1.1 × 10 6 km². In contrast, an unrealistically low sea ice cover of approximately 0.5 × 10 6 km² was observed before 1978. This discontinuity in sea ice stems primarily from issues with assimilated sea ice data used in ERA5. In 1978, the unrealistic negative bias in sea ice cover arguably contributed to positive biases in significant wave height, sea surface temperature, surface air temperature, and surface winds. In the case of wave observations, from 1975 to 1978, ERA5 overestimated significant wave height by more than 60% compared to observations from February to April when sea ice was present. These findings highlight the need for caution when analyzing long-term changes in ice-covered areas when using ERA5 data, particularly for periods before 1979.