Baltic Sea Ice Extent from Observations Reprocessing

'''DEFINITION'''

Sea ice extent ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 2 (Samuelsen et al, 2018) and is derived from satellite observations product SEAICE_BAL_PHY_L4_MY_011_019. Sea ice extent is defined as the area covered by sea ice, that is the area of the ocean having more than 15% sea ice concentration. Sea ice concentration is the fractional coverage of an ocean area covered with sea ice. Daily sea ice extent values are computed from the daily sea ice concentration maps. All sea ice covering the Baltic Sea is included, except for lake ice. The annual course of the sea ice extent has been calculated as daily mean ice extent for each day-of-year over the period October 1992 – September 2014. Weekly smoothed time series of the sea ice extent have been calculated from daily values using a 7-day moving average filter.

CONTEXT

Sea ice coverage has a vital role in the annual course of physical and ecological conditions in the Baltic Sea. Moreover, it is an important parameter for safe winter navigation. The presence of sea ice cover sets special requirements for navigation, both for the construction of the ships and their behavior in ice, as in many cases, merchant ships need icebreaker assistance. Temporal trends of the sea ice extent could be a valuable indicator of the climate change signal in the Baltic Sea region. It has been estimated that a 1 °C increase in the average air temperature results in the retreat of ice-covered area in the Baltic Sea about 45,000 km2 (Granskog et al., 2006). Decrease in maximum ice extent may influence vertical stratification of the Baltic Sea (Hordoir and Meier, 2012) and affect the onset of the spring bloom (Eilola et al., 2013). In addition, statistical sea ice coverage information is crucial for planning of coastal and offshore construction. Therefore, the knowledge about ice conditions and their variability is required and monitored in Copernicus Marine Service.

CMEMS KEY FINDINGS

Sea ice in the Baltic Sea exhibits a strong seasonal cycle, typically forming in October and persisting until early June. The 2023/24 ice season was notable for an intermediate maximum extent, reaching approximately 105 000 km² on 12 February 2024. Initial ice formation was observed as early as 22 October 2023 in the northern Bay of Bothnia. Expansion accelerated through November and December, with rapid growth continuing into January. The extent remained above the climatological median for much of the freezing period.

Ice coverage peaked in mid-February and then gradually declined, with the Baltic Sea becoming ice-free by 4 June 2024, marking a longer-than-average season. The peak day for sea ice extent varies annually but typically falls between late February and early March (Singh et al., 2024). Over the past 30 years, the highest recorded extent was 260,000 km² during the 2010/11 winter. While there has been a declining trend in sea ice extent from 1993 to 2024, the linear trend remains statistically insignificant.

'''DOI (product):''' https://doi.org/10.48670/moi-00200