“A phenomenon of exceptional magnitude and intensity”

The cross : How to explain such a polar deluge in the north of the United States?

Gerhard Krinner: The causes that lead to this kind of meteorological episode are well known. It happens quite often: a descent of air from the Arctic creates a low pressure system in its encounter with a mass of warm air coming from the South, from the Gulf of Mexico in this case.

It’s relatively classic in North America, where unlike the European geography, there isn’t really any obstacle that prevents this shock of temperatures. The Rocky Mountains, which follow a North-South axis, do not block, and beyond, they are great plains. What is extraordinary here is the scale and intensity of the meteorological phenomenon.

To the point of talking about “low pressure bomb” ?

GK: The meeting of the two air masses generated a cyclone over the Great Lakes region, with a pressure differential of 1,060 hectopascals (hPa) in Montana against 969 hectopascals at the level of the Great Lakes. It is very rare to observe such a fall. This leads to impressive drops in temperature, violent winds, powerful snowfalls, increased risk of blizzard. It could last up to a week.

This is the paradox: even in a climate that is warming globally, low temperature peaks can be observed locally. In the specific case, the temperatures « felt » which take wind speed into account are reaching record levels.

Is global warming to blame?

GK: The research is not yet strong enough to be very positive. This kind of phenomenon is very rare, and therefore, it takes time to integrate them into a statistical model that is reliable. The hypothesis is that the weakening of the temperature difference could weaken the polar vortex, which would undulate more, which could increase the frequency of this type of event. But opinions differ.

What is certain is that each year, the number of global cold records set is much lower (about 10 times lower) than the number of new hot records. So, even if there may still be new cold records sometimes in some places, they are becoming much rarer, a logical consequence of global warming.

The Arctic is warming up much faster (+3°C since the 1970s) than the global average (+1°C over the same period). This results in relatively fine changes in the polar jet (upper current). Not enough to prevent cyclones from forming, as the temperature differential is still high.