Around the middle of December I wrote an article discussing the phenomenon becoming known as the upside-down winter or the “hot Arctic, cold continent” winter. The continuation of the unusually snowy weather over much of the central and eastern portions of the U.S., and all the talk about whether it’s being caused by global warming or is proof that global warming is not occurring, made me think that we should revisit this topic.
In that December article I briefly tried to explain what seems to be causing this unusual cold, snowy weather. Officially the pattern is being referred to as an extreme negative episode of the North Atlantic oscillation (NAO). The NAO is a climate pattern over the North Atlantic Ocean and is measured by the difference of air pressure between the Icelandic low (over the north Atlantic) and the Azores high (further south in the Atlantic). The NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. If there is a large difference in the pressure between Iceland and the Azores, we call it a positive NAO and this leads to increased westerly winds and mild, wet winters in Europe. Positive NAO conditions also cause the Icelandic low to draw a stronger southwesterly flow of air over eastern North America, preventing arctic air from plunging southward. If the difference in air pressure between Iceland and the Azores is small, it is referred to as a negative NAO. Under this setup, westerly winds are suppressed and this allows Arctic air to spill southward into eastern North America more readily. Negative NAO winters tend to bring cold winters to Europe and eastern North America, but lead to very warm conditions in the Arctic since all the cold air spilling out of the Arctic gets replaced by warm air flowing poleward.
During the first half of winter we saw an extreme negative phase of the NAO form and this brought cold weather to Europe, and it seems to be what’s behind all of the snow over the U.S. Several states and cities across the central and eastern parts of the U.S. have seen record-breaking snowstorms this winter and last winter. The last time this part of the U.S. saw this much snow was back in the winter of 1960-61. Interestingly, that winter, along with this winter and last winter, all saw a very negative phase of the NAO.
So now why are we seeing this particular weather pattern? As I stated back in December, it could be merely chance. We saw this develop back in 1960-61 and we just happen to have it develop again last winter and this winter. While this happening two years in a row could be just chance, I personally believe it has to do with loss of Arctic sea ice.
If we look at the current state of ice in the Arctic, we would see that at the end of January there were 13.5 million square kilometres of ice – the lowest amount ever recorded in January. This is a full two million sq. km lower than what was being recorded in the late 1970s. On a truly bizarre note, Hudson Bay did not totally freeze over until Jan. 21, a full month later than average, and by far the latest date ever recorded.
As I have stated on more than one occasion over the last few years, less ice means more energy available to the atmosphere over the Arctic. In the summer, lower ice amounts allow more of the sun’s energy to be absorbed. When winter rolls around, this extra energy (heat) is then transferred back into the atmosphere. This increase in energy is going to begin changing weather patterns around the globe. Remember, the Earth’s weather at its very basic level is simply the planet trying to equalize the temperature extremes between the equatorial regions and the poles. Start changing one of these regions and how the planet goes about this process will also begin to change – and not always the way we might expect it. Will this negative NAO become the “norm” for the winter? It’s hard to say, because how the atmosphere behaves is controlled by more than just one thing. I do believe if we continue to see low ice levels in the Arctic we are going to continue to see more bizarre weather occur, and as much as I love interesting or bizarre weather, I have to admit it does make me a little worried.
Onatrulybizarrenote,HudsonBaydidnot totallyfreezeoveruntilJan.21, afullmonthlaterthanaverage,andby farthelatestdateeverrecorded.