Effects on the transition zone of retreating seasonal sea ice in the Baltic Sea

According to Copernicus data, the last three years (2023–2025) have been the hottest on record globally. In Northern Europe, however, the pace of warming has even exceeded the global average. This case study focuses mainly on the Estonian coastal area of the Baltic Sea, where air temperatures at coastal stations have risen by about 2.5°C between 1950 and 2025. As a result, the annual maximum Baltic Sea ice extent has declined (according to the endpoints of a linear trend) from 212 to 140 thousand km² (34%) over the centennial period between 1924/25 and 2024/25, and ice-cover duration in the Estonian coastal sea has decreased, depending on the station, by roughly 30–60% since 1950. This decline has likely contributed to the intensification of coastal erosion observed along several Estonian coastal stretches and more broadly along the southeastern Baltic Sea.

The study reviews shifts in ice-related processes with coastal geomorphic consequences, including changes in wave conditions due to longer ice-free seasons, reduced effective fetch, and shorter periods of frozen coastal sediments. Two less-studied effects are examined in detail: (1) using the ERA5-forced WRF–FVCOM modelling suite to quantify how the absence of ice affects sea-level patterns in the Gulf of Riga and the suppression of sea-level maxima during winter storms; and (2) analysing the occurrence and impact of winter (“warm”) upwelling on ice dynamics using ADCP measurements, meteorological–oceanographic observations, ice charts, and SST imagery.

The results show that reduced ice cover can help explain the higher storm surges observed in the Baltic Sea. In recent, more ice-free decades, winter storms have more freely produced surges, whereas before the 1980s they were often suppressed by sea ice. The combination of declining ice cover and the increasing probability of undamped storm surges likely contributes to steeper sea-level maxima and higher upper-quantile sea-level trends in long-term records. Secondly, although coastal upwelling in the Baltic Sea is usually considered a summertime process, similar forcing in winter can bring slightly warmer subsurface water (2–4°C) to the surface, contrasting with pre-freezingly cooled (0–1°C) surface water. The process is quite frequent along the straight North Estonian coast of the Gulf of Finland, when during sustained easterly winds this upwelled water creates a stark contrast with cold (–10…–20°C) weather conditions in the area. The phenomenon often delays coastal ice formation relative to the Finnish side and may help explain an anomaly in regional ice-pattern statistics in the mouth section of the Gulf of Finland.

Finally, as the transition zone between ice-free and seasonally frozen seas shifts northward under climate warming, the regions affected by these processes will also migrate. Winter upwelling has little effect in seas that are either fully ice-free or that freeze over rapidly. It is likely that about a century ago these processes were more pronounced south of Estonia (e.g., along the Latvian and Lithuanian coasts), and in the future they may shift farther north, such as into the Bothnian Sea.