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Why does pack ice form in the Gulf of Bothnia during many winters?

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Map showing the physical features, major cities, and national boundaries around the Baltic Sea, North Sea, and English Channel regions of northern and western Europe. The map highlights coastal countries including the United Kingdom, Norway, Denmark, Germany, the Netherlands, Belgium, France, Sweden, Finland, Estonia, Latvia, Lithuania, Poland, and Russia, with sea depths indicated and key islands, peninsulas, and underwater trenches labeled. — Baltic Sea The Baltic Sea, the North Sea, and the English Channel.

Pack ice forms in the Gulf of Bothnia during most winters primarily due to the region's extremely low salinity and shallow coastal waters.12

Because the gulf receives water from numerous rivers, its salinity levels are significantly lower than those found in other parts of the Baltic Sea. This reduced salt content, combined with the shallow nature of the coastal areas, allows pack ice to accumulate and persist for up to five months during the long, cold winters characteristic of the region's continental climate. In some instances, this ice can bank up into pressure ridges reaching nearly 50 feet in height.123