Climate Watch
The annual cycle of sea ice development, movement and melt has been an integral part of people’s lives in western Alaska for the past 10,000 or more years. How this happens is a fascinating but frustratingly complex phenomena.
For the Bering and southern Chukchi Seas there are three main factors that affect the timing of sea ice development: ocean surface temperature, ocean water salinity and of course weather.
Let’s start with the ocean conditions. We usually think of water freezing at 32°F, but while this strictly applies to pure water, fresh water in the environment freezes very close to this value. Ocean water is a different story. The temperature that ocean water freezes at depends on how salty the water is, so ocean water near the mouths of rivers is normally less salty than water farther from the coast. For practical purposes, ocean surface water not directly influenced by fresh water from rivers freezes at a temperature about 28.5°F, 3.5°F lower than that of fresh water. These salinity differences are one of the reasons that waters near the mouths of rivers freeze-up before more open water: it does not have to cool as much.
Another ocean issue is the depth of the water, especially when the water is very shallow. The much lower volume of water that needs to cool before ice can form at the surface results in earlier ice development in these shallows. You see this on land, too, in the early autumn, when puddles will have a solid skim of ice on a cold morning while lakes remain ice free. And of course, these factors may combine, so for instance at Safety Lagoon, which becomes ice covered long before the ocean side has any ice.
As for weather, that may seem obvious: south winds and temperatures in the 30°s (or higher) not only prevent ice development but can easily melt (mostly through wave action) thin ice. However, in deep water, even cold winds can prevent the formation of ice as the winds keep mixing up the water, bringing slightly warmer water from depth to the surface. This process delays ice development but actually cools the water to a greater depth, so that when winds abate a “flash freeze” can occur, with tens of thousands of square miles going from open water to ice covered in the course of 12 to 24 hours. This is especially common in the Chukchi Sea but sometimes happens in the northern Bering Sea, too.