Out in the middle of the frozen Arctic and Antarctic waters are pockets of open water called polynyas (Russian for ‘ice hole’). I first ran across polynyas when I read ‘Ice Station Zebra’ by Alistair MacLean as a teenager - I understand the book was made into a movie in 1968, but I haven’t seen it. A cold war thriller, the novel centers on a nuclear submarine traveling under the Arctic ice pack on a supposed rescue mission that results in sabotage. Getting through the ice becomes critically important to the submarine’s crew - and normal pack ice is much too thick to break through. A polynya provides the perfect way through the ice, but why are they there?
It seems paradoxical that open water can co-exist with below freezing air temperatures. Shouldn’t the water just freeze? Polynyas form only under very special conditions. First, a physical barrier is needed to stop ice from moving in. A point of land or ice bridge would do the trick. Next, mechanisms to stop ice formation must occur which can be broken into two broad categories.
If the forming ice is removed by some mechanical process, it’s called a mechanically forced polynya. Appropriate mechanical processes include wind, currents and tides. Because ice is being formed, then moved away, the surface waters would become extra-salty - as sea ice forms it rejects the brine. This salty, cold water would then sink.
The second type of polynya is formed by convection. Convection is a common heat-transfer process that can be found in any kitchen. It explains how a pot of water is brought to the boiling point from a heat source below. The element heats the bottom layer of water (conduction) and this water rises heating water further up (convection).
In Arctic waters (and Antarctic waters I think - I haven’t been looking into what happens in the Antarctic), the lowest layer of water is quite warm, about three degrees Celsius. It stays on the bottom because it’s dense (i.e. heavy). If a process, like tides or upwelling, brought this warmer water up to the surface, it would keep the surface waters from freezing. An added bonus when deep waters are brought to the surface is that they tend to be nutrient rich, supporting diverse life.
As with everything in nature, polynya formation is complex. Typically, they form due to a combination of factors and can even create their own feedback loops.