This year’s polar vortex hasn’t gone away yet and the centre of North America now adds windchill to the Arctic cold.

Warnings have been issued of severe cold in central Canada and some northern US states, as windchill figures of -40 degrees Celsius in Alberta and nearly -50C in Montana were quoted.

For much of this northern hemisphere winter, Siberia has been the coldest place on the Earth. Now, as the sun comes north, that title is being reclaimed by Antarctica. Nevertheless, pockets of extreme cold persist in the northern hemisphere, particularly in North America.

The winter ice cover over the Arctic Ocean is becoming less reliable. Areas of open water where once there was ice completely disrupt the formerly settled winter freeze.

The term polar vortex is sometimes bandied about when the weather gets cold in Canada and the US in mid-winter. A vortex in the atmosphere is a rotation around a central low pressure. A tornado is a most obvious example, or a hurricane. The polar vortex is a rotation in the stratosphere, influencing the weather pattern below it, and is the reason for the outbreaks of severe cold.

When the polar vortex is strong, it is well defined and Arctic air is well contained, winter is “normal”. When it is weak, which it generally now is, it will break into two or more vortices, one likely over northern Canada, and another over northeast Siberia, the sites of this winter’s Arctic cold.

The Arctic air travels with the polar vortex, giving a sudden and significant drop in temperature on the ground. This happened in January 2019 and resulted in record cold in both eastern Siberia and the US. The cold air is still there, over the snow-covered ground; add some wind and there is the recipe for severe windchill.

Environment Canada has issued warnings for most of Alberta, southern Manitoba and southern Saskatchewan for the occurrence of windchill temperatures between -40 and -45C. Over the border in the US, figures in Montana and North Dakota as low as -48C are suggested.

Whatever happens, these conditions cannot persist into April. The polar vortex collapses as the sun comes north and the strength of that sun overcomes the cooling of shortening nights.

Extremes of weather are an expectation as the climate changes even though such outbreaks of cold might seem contrary to global warming. The reason for this weakening of the polar vortex is the change in the temperature difference between the equator and the Pole.

The bigger the difference, the stronger the vortex, but the surface water of the Arctic is much warmer than the ice that used to cover it all. Thus the difference in temperature between the equator and the Pole can be as much as 60C in a traditional winter (+30C to -30C), or as little as 30C in what is becoming the new normal winter (+30C to 0C).

The Arctic is warming at a much greater rate than anywhere else on the Earth and as such is biasing future winters towards a weak and split polar vortex. In other words, a tendency towards outbreaks of Arctic air into North America and Eurasia.