Why is it snowing so much in the Northern US States this Spring? Two words: Global Warming. Let me ‘splain.

Weather is all about air and moisture. The distribution of air is uneven, with some places having more air in big piles, other places having less air, into which the extra air from the big piles of air tends to spill. The big piles form because the surface beneath is relatively warm, causing the air to expand more than in adjoining areas. As air falls off these giant mounds of seeming nothingness, it forms low pressure systems that consist of swirling moving masses, made up of air of different temperature and humidities, and this is where many storms come from. Where the high mounds of air form depends on the seasons; since it is the relative temperature that matters, we might expect high pressure systems (mounds of air) to form over water during the winter and land over the summer with the low pressure systems being located over the opposite landform, but it is of course way more complex than that.

Using this simplified model of piles of air pouring into low spots, one can understand the climatic concept of “oscillations.” There are large regions of the earth where high pressure tends to form, and spill its air in a certain direction. That air, somewhat cooled off, may then return to the area of the high pressure where it is re-warmed (by surface conditions) to contribute to the high pressure mound. If this happens over the ocean, the movement of air may also drive the movement of surface currents, which can actually increase the level of heat at the base of the high pressure system. If the earth was simple, i.e., had no continents and a sea of even depth over the entire surface, high and low pressure systems might form and last for very long periods of time, merely changing slightly during seasonal changes. Also, since the overall driving force of the climate system involves the movement of heat (in water and air) and the warm water and air itself from equatorial regions (where the sun has a stronger effect) to the poles, and the earth is spinning, this set of high and low pressure systems should be organized in bands encircling the planet, bands that interact with each other at their edges.

As long as we are on the subject we should note that the Jet Streams can be thought of as places where the dynamics of air hearting, rising, thus becoming less dense and releasing heat (and thus becoming more dense per altitude) and so on and so forth runs into a nasty math problem. If you model (again, I oversimplify) the movement of air molecules that are passing through different conditions of altitude, pressure, temperature, etc. there are places where the math seems to get hinkey, and there are air molecules that are definitely not supposed to be where they are, and there are places where there should be more air molecules as well. This causes air molecules to move very quickly from point A to point B, and the result is a set of high altitude, sinuous very rapidly moving bands of air … the Jet Streams. These tend to occur at the boundaries of the hypothetical (but rarely actualized in any clean and neat way) bands of air that would exist around a simplified version of the earth.

And, of course, the earth is not simple; there are continents, mountain ranges, huge glaciers or ice fields, areas where water in the sea is trapped by continental configurations so it gets extra warm and other areas where currents can circumnavigate a land mass pretty easily and redistribute heat efficiently. You have probably already heard that the dynamics of land ice and sea ice formation and melting in the Arctic and the Antarctic are different. Knowing what you know now (see above) a quick glance at the distribution of land and sea in those two regions should lead you to conclude that the Arctic and Antarctic simply can not have the same climatic details, even though both are polar regions.

Getting back to the oscillations… So, we have these areas which may for years at a time have high pressure in one place linked to an adjoining lower pressure in another area, and air and water currents are both affected by and cause this relationship. But, it is also possible that a different configuration could emerge, perhaps with the same basic layout but with the high pressure zone moving to a somewhat different location nearby, or being more widely or narrowly positioned. Then, this alternative configuration could last a few years.

It is the shifting back and forth between two (or more) such configuration that we call an oscillation (usually). ENSO, the El Niño–Southern Oscillation, is one such system in the equatorial pacific. There is a North Atlantic Oscillation and an Arctic Oscillation, and others.

Very simply put, the fact that we are having coldish and snowy weather in the Dakotas, Minnesota, and nearby areas at the same time that the Arctic Sea’s ice is breaking up and melting early is because the Arctic Oscillation … a big giant climatic feature at the northern end of the Earth, is undergoing its “negative” phase, which is kinda rare, instead of its “positive” phase, which at least in the recent past has been more common.

So, what the heck does this have to do with climate change, or in particular, global warming?

Global warming has caused the Arctic Ocean to lose much more of its ice than it has in the recorded past, which leaves more sea water exposed to sun during the Arctic summer. Sea water absorbs (and later releases) sunlight and stores it as heat, while ice and snow reflect sunlight back into the atmosphere and onward into space. For this reason, the Arctic Ocean is warming, and this causes ice to form more slowly and melt more quickly, which in turn allows the summer ice free waters to absorb more heat, and so on and so forth. The Arctic Ocean’s ice cover, which expands every winter and shrinks every summer, is undergoing a sort of climatic death spiral that looks like this:

Remember the part above about how surface conditions determine the location, intensity, and extent of high and low pressure systems? During the “positive” phase of the Arctic Oscillation, highish pressure systems around the Arctic maintain a large low pressure region known as the Arctic Vortex, over the pole. But the Arctic Ocean, being warmer, says “hold on there, a second, I’m also a high(ish) pressure system, stop vortexing me!”

The high pressure region that encircles the Arctic during the positive phase backs off (goes south) and is less effective in maintaining a nice vortex over the Arctic. The strong gradient between a sub-arctic high pressure encircling the polar region and the strong polar vortex, during the positive phase of the oscillation, keeps colder Arctic air near the poles, and regions like the Great Lakes, Upper Plains, and Norther Eurasia enjoy warmer weather. With the weaker gradient during the negative phase, the Arctic air spreads out and encompasses more of the subarctic and temperate land masses, but the cold, as it were, is now distributed over a much larger region, so is it less cold in places that would otherwise be very cold, and less warm in places where it would be more warm.

One analogy that has been knocked around a bit and works pretty well is the traditional refrigerator, with a freezer on top, and the fridge below. Imagine that your refrigerator occasionally develops a modest hole between the two compartments. The freezer would still be colder because there are more cooling coils up there, but it would not be as cold and your ice would be wet and your frozen beans soft, while skims of ice would form on your milk and cranberry juice. In real live, this means that North Dakota bets to be slushy cranberry juice, wile the Arctic Ocean gets to be a bunch of soft, barely frozen Freeze-Pops.

The region of colder air is not a uniform, even circle around the poles in any case, but during the negative phase, it is very uneven because the oceans have a strong effect. Since the oceans are busy moving large amounts of heat from the equator to the north, the cold tends to get bunched up over the continents. In the North Atlantic, changes in the Arctic Oscillation interact with the North Atlantic oscillation and that affects weather in that region.

You’ve already heard that changes in the Arctic likely contributed to the path (and strength) of Hurricane Frankenstorm Sandy last year. Well changes in the Arctic have resulted in some very snowy winters in recent times, some killer cold snaps, and most recently, a series of large winter storms that don’t seem to have gotten the memo about it being Spring. The Weather Channel just recently started naming regular storms like hurricanes are named, in order to keep track of them. Winter Storm Xerxes just passed through my back yard, and Winter Storm Yogi is now forming up on the West Coast.

We’re gonna need a longer alphabet.

Information about Winter Storm Yogi from Weather Underground.

More on the Arctic Oscillation from Paul Douglas

Arctic Warming’s Effect on the Jet Stream by Peter Sinclair

Top image from Wikipedia.

Sea ice death spiral graphic from here.