Ok, this page is called the Earth Story and we wouldn’t be true to our name if we didn’t post something on the major Earth Story impacting most of our readership…the fact that it’s cold as **** (pick whichever 4 letter word you feel is appropriate) in much of the United States.
The Earth’s climate is driven by one basic fact – the tropics receive more energy from the sun than the poles because the planet is a sphere and the poles angle away from the sun. On a simple planet that did not rotate but had an atmosphere, this setup would cause 2 convection cells to form. Air would rise at the equator, move north and south, cool, and sink at the poles. Countering this flow, cold air from the poles would south towards the equator and be gradually heated until it rises.
This simple circulation pattern is known as a Hadley Cell model for the weather. It is what the Earth tries to do – a conveyor belt bringing excess heat from the equator to the poles where it can be radiated to space.
But Earth rotates. Consequently, air can’t simply head north and south – it gets deflected by the rotation of the Earth and the Coriolis Effect. The single cell pattern is broken up and instead generally forms 3 cells in each hemisphere – one near the equator, a central one, and a cell isolated near the pole, with air rising and sinking at the boundaries between the cells.
At the boundaries between these cells, as a consequence of both the differences in pressure and the Coriolis Effect, the winds are sped up into what we know as Jet Streams.
At the North Pole, the jet stream typically keeps the coldest air somewhat in place. Oh, it migrates north and south sometimes but generally, arctic air swirls to the north, somewhat isolated by that jet stream and the fact that it’s in a different circulating cell.
Except…on occasion, something happening in one cell can push into another cell. If, for example, a high pressure system forms in the center cell, it could push north so far as to knock the other cell out of the way. That’s what you’re seeing in this image and that’s the “polar vortex” you may hear about on the news.
Normally the polar vortex sits pretty happily up at the North Pole. Today, it’s been pushed into the United States. The location of the jet stream was bent to the north, in part driven by high pressure over the Pacific and western U.S. As a consequence, the jet stream was forced deep into the southeastern U.S., bringing the air from the arctic all the way down to the Gulf of Mexico.
Of course, there are obviously people laughing at the idea of climate change every time it gets this cold. The first obvious answer is that still ignores what is happening in the southern hemisphere, such as Australia, which is currently mired in another record-breaking heat wave.
Meanwhile, even in a climate changed world, the Arctic will stay extremely cold. The mean average winter temperature in the Arctic is around -40°C (-40°F). So far, the average global temperature has only increased by 1°C; even if the Arctic winter temperature jumped by 10°C it would still be incredibly cold up there. But the science is actually more interesting than just that.
The jet streams are held roughly in place by the temperature differences between the cells – that’s why even in summer it stays pretty cold at the poles, warmer air just doesn't get up there. But, as we've seen over the past few years, the Arctic is one of the fastest-warming places on Earth, with the icecaps shrinking rapidly and the open ocean starting to heat up. That means the temperature difference between the cells is, to some extent, decreasing as the arctic heats up.
There was a recent paper published by Dr. Jennifer Francis from Rutgers University arguing that as a consequence of this change, it is becoming easier to push the jet stream around. If the jet streams are more easily buckled by high pressure systems, the end result could be that extreme events like this one, caused by extreme bends in the jet stream, could become more common.
That science is still work-in-progress, but it is definitely interesting and it provides a mechanism for the increase in “extreme weather events” that has been documented in many countries over the past decade.
So that’s the U.S. right now. Extremely cold because the cold air usually trapped in the Arctic was punched to the south by a high pressure system. The world hasn't suddenly cooled down, this extremely cold air usually just sits somewhere else. And from a climate change perspective, we don’t know if events like this will become more or less common with time, but even in a climate changed world, events like this are certainly possible.
Australian heat wave post:
http://www.wunderground.com/news/polar-vortex-plunge-science-behind-arctic-cold-outbreaks-20140106
