Ice in the North and Baltic Seas:
Ice formation in both seas is an annually recurring process. Ice occurrence and ice extent depend largely on meteorological, oceanographic, and hydrographic conditions, thus coining the typical ice conditions of the different areas of the North and Baltic Seas.
The essential importance of ice occurrence to navigation in the North and Baltic Seas is reflected in the long series of recorded and reconstructed ice data. In order to better understand the ice regime in the Seas and its changes in the past, the ice winters with similar characteristics are merged in 5 ice winter classes: very weak, weak, moderate, strong, and very strong to extremely strong.
There are different methods of ice winter classification, but each of them shows the decrease of strong to extremely strong ice winters since the end of the 80s by increase of the weak ice winters at the same time.
Here the question arises immediately: Is it a unique or periodic process? The study of long-term variations of ice winter severities in the western Baltic Sea, reconstructed back to 1301, may help to understand the changes in the ice regime viz. in the climate.