The bitterly cold Arctic air that spilled into the Tri-State last week, and again this morning, runs counter to the message we've been hearing from meteorologists and climate scientists around the world: 2016 is on track to become the hottest year on record. While most people understand fully that it can occasionally get cold, despite an overall warming pattern, it still is a curiosity. If the Earth is getting warmer, where is the unusually cold air coming from?
The answer lies with the Polar Vortex. This weather phenomenon is certainly nothing new-- it has existed as long as Earth has had an atmosphere-- but has only gained widespread recognition outside meteorology circles in recent years. This term is used for the frigid air that semi-permanently resides over both the North and South Pole. The Polar Jet Stream keeps this cold, dense air contained to the high latitudes through most of the year. The strong winds of the Polar Jet are created in part by the contrast in temperatures between the high latitudes and mid-latitudes. When the temperature gradient from north to south is weakened, the jet-level winds respond by slowing down. Slower winds are a weaker barrier and are easier for a polar airmass to penetrate (see video).
In the Northern Hemisphere, the Polar Vortex sits over the Arctic Ocean, which is a much less stable surface than at the South Pole, where the landmass of Antarctica encompasses nearly the entire Antarctic Circle.
The Arctic Ocean has shown its susceptibility to climate change, especially in recent decades. Sea ice, which typically covered the ocean's surface year-round during the first half of the 20th Century, has been melting more thoroughly than in decades past. The thinning sea ice is not just a sign of milder ocean temperatures beneath; it also exacerbates the warming process because snow and thick ice reflect the sun's rays, whereas thin ice and ocean water absorb much more solar radiation, making the Arctic ocean and surrounding air that much milder. Though the entire Earth has seen a rise in average temperature since the beginning of the 20th century, the change is starkest north of the Arctic Circle, as seen in the picture below.
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| Global average temperature anomaly in 2015. Red and orange areas have the highest positive temperature anomaly. (Courtesy of NASA)
This has profound effects on the Polar Jet's strength. The strength of the jet stream is directly proportional to the temperature gradient, or change in temperature over a given space. So when the Arctic gets disproportionately warmer than the rest of the Northern Hemisphere, the gradient weakens, and so does the Polar Jet. Weaker winds make it easier for Arctic air to head south. So, a weak Polar Vortex can bring a strong Arctic outbreak to the continental United States.
If global temperatures continue to rise, or even if they remain steady, we can expect that the Polar Vortex will become more unstable, since Arctic sea ice extent continues to shrink to new record lows. http://svs.gsfc.nasa.gov/4494 This could result in even more frequent and dramatic temperature swings than the 33-degree drop in temperature we endured between Sunday afternoon and Monday morning.
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