11 December 2010
If you live anywhere from the Mississippi Valley to the East Coast in America, you are about to get cold.
A major Arctic blast is on it’s way south. The leading edge of the cold air (The Arctic front) will reach South Florida by Sunday night. There will be tons of lake effect snow, and flurries as far south as Birmingham and Atlanta. In the meantime, way up north along the Arctic Circle in Greenland, it’s raining.
Yup, it’s raining. Furthermore I’m not surprised at all. The reason for this is what weather geeks call the North Atlantic Oscillation.
No, this is not an issue of bad morals, or something in the restricted section of the local library. It’s a pressure pattern well known to meteorologists, and right now it’s tanking.
Here are the basics.
Normally, there is low pressure in the atmosphere over Greenland and higher pressure near the Azores. This allows storm systems to move rapidly from North America, into and across the Atlantic. It also allows the cold air building up over the polar regions of Northern Canada to stream mainly eastward, and not penetrate that far into the southern portions of North America.
This “normal” pattern is called the positive phase of the North Atlantic Oscillation (NAO from here on out).
Sometimes it reverses itself. When that happens the air pressure over Greenland increases; especially in the upper atmosphere. This pushes the jet stream far to the North.
When the NAO goes negative, and a big upper atmosphere high builds over Greenland the normal west to east flow of weather systems stops. This forces the cold air from the polar regions in Canada deep into the Southeastern U.S. That, of course, leads to frozen oranges in Florida, and causes southern states to dismiss school because of a barely visible dusting of snow on the roads.
With the deeply negative NAO, it’s incredibly mild in Greenland today, and the -30C air over northern Saskatchewan is headed south faster than a Canadian snowbird on I-95.
For those of you in the UK, who are still warming up after last week’s deep freeze and heavy snow, you can blame the NAO as well. A negative NAO can also cause Arctic air in Western Russia to slide into France and England. The event last week buried Gatwick Airport in snow.
One last point.
Anytime it gets cold in the Eastern portion of America some scientifically illiterate politician will say something along the lines of “Tut! Tut! What happened to global warming??”
The answer is simple. It’s raining in Greenland and NASA just announced today that the meteorological year of 2010 was the WARMEST on record. This past November was the warmest November on record as well.
The warmth is even more incredible considering the strong La Nina in the Pacific. A strong La Nina actually cools the planet a few tenths of a degree. A strong El Nino warms it likewise.
We broke the record anyhow. The greenhouse gases are overriding the natural oscillations of the ocean/atmosphere system. Something NASA expert James Hansen predicted would be very noticeable by now.
Except, he did it 29 years ago. (Hansen’s paper from 1981 is here.)
More on the NAO is available here.