Baltic Sea Ice Extent from Observations Reprocessing

'''DEFINITION'''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 data used to produce the charts are Synthetic Aperture Radar images as well as in situ observations from ice breakers (Uiboupin et al., 2010). 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 coverage in the Baltic Sea is strongly seasonal. In general, sea ice starts to form in October and may last until June. The ice season 2021/22 had relatively low maximum ice extent in the Baltic Sea. Sea ice extent reached a maximum area of about 65 000 km2. Sea ice started to form already in November, then reached the value of 60 000 km2 at the beginning of January, but then stopped to increase and even decreased slightly. Maximum sea ice extent was observed at the beginning of February. Afterwards, the sea ice extent slowly withdrew, while in average winters the sea ice increased until the end of February. In a case of fully ice covered Baltic Sea the maximum ice extent is 422 000 km2, which was last observed during the 1940s (Vihma and Haapala, 2009). Thus, 15% of the Baltic Sea was covered by ice in 2021/22. Although there is a tendency of decreasing sea ice extent in the Baltic Sea over the period 1993-2022, the linear trend is not statistically significant.'''Figure caption'''(a) Time series of day-of-year average sea ice extent derived from remote sensing and in situ observations ((http://www.smhi.se/klimatdata/oceanografi/havsis, Uiboupin et al., 2010). Long-term mean (black line) and one standard deviation (blue shading) are calculated over the period October 1992 – September 2014. Daily sea-ice extent is for 2021/2022 ice season (red line). (b) Time series of the area integrated daily sea-ice extent for the Baltic Sea in 1993–2022. Initial data that consists of remote sensing and in situ observations ((http://www.smhi.se/klimatdata/oceanografi/havsis, Uiboupin et al., 2010) are smoothed using 7-day window moving average filter"'''DOI (product):''' https://doi.org/10.48670/moi-00200