When a lot of arctic air pours south from the North Pole, and invades more southern climes with the wolves of winter, it actually gets milder at the Pole, because the exported air must be replaced. Even when the air is replaced by flows aloft, the air warms as it descends to the surface. Quite often the air is transported north by gales that bring warm sectors northwards, and at times these warm sectors can travel over sea-ice before they occlude and are lifted higher up in the atmosphere, where the air cools and then descends.
In other words, the more harsh, arctic weather comes south to the places where most people live, the more milder air is sucked north. The jet stream becomes loopy, or “meridianal”, rather than being “zonal” and simply circling around and around the Pole. Rather than calm conditions and extreme cold, the Pole sees more wind and relative warmth.
Besides leading to a completely different pattern in the formation of sea-ice, (a subject for another post), the loopy pattern can so drain the Arctic of its reserves of most-cold air that, for a time, the Arctic has less ammunition to throw at the south. To the south the cruel arctic outbreaks may slacken, temperatures may moderate, and there may even be thaws. People are tempted to think Winter is over, when it is merely reloading.
Such a reloading is shown by a plunge in the temperatures up at the Pole, as calm allows the cold to pool and reach its coldest temperatures of the Winter. We can see this in the DMI graph of temperatures surrounding the Pole, and also their map of what those temperatures are.
It is important to remember two things, when the air gets down below -30° (-34° Celsius) at the Pole. The first thing is that it is difficult for air to get any colder, due to the fact the water under the ice is around 28° (-2° Celsius) and is constantly radiating heat up through the ice. The second thing is that the air-mass’s temperatures can get colder, as soon as the air moves south over tundra and taiga that have no such warming-from-beneath, and instead have snow-cover that allows radiational cooling, and the air also experiences very little sunshine, this deep in the winter.
Even though the statistical end to dropping temperatures is reached around now, and statistically temperatures start to rise from now on, as spring approaches, some of the coldest arctic outbreaks can pour south from Poles as chilled as the Pole is now chilled.
Therefore it is wise to be wary of the second half of winter. When these super-cold air-masses charge south, and meet up with the first springtime warm air-masses coming north, they can generate some of our deepest snows and wildest storms.
As an interesting side thought, I’ve always wondered if the so-called “January Thaw” may be a period like the slack waters between a rising tide and a sinking tide. It is the time between temperatures getting colder and temperatures getting warmer, and a reflection of that change may be seen in a calm, but it is a calm that allows the Pole to get very cold.