BRIEF RESEARCH REPORT article
Front. Mar. Sci.
Sec. Coastal Ocean Processes
Sea-ice discontinuity in ERA5 during 1978–1979 and its impacts in the Sea of Okhotsk
Muroran Institute of Technology, Muroran, Japan
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Abstract
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 × 10⁶ km². In contrast, an unrealistically low sea ice cover of approximately 0.5 × 10⁶ 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.
Summary
Keywords
ERA5, Ocean surface wave, sea ice, Sea of Okhotsk, Surface meteorologicalvariables
Received
04 December 2025
Accepted
10 February 2026
Copyright
© 2026 Yusuke and Iwasaki. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Shinsuke Iwasaki
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