Spring 2013 and Arctic Oscillation

What is with this weather over the last two months? What is the AO?

The so-called Arctic Oscillation describes the mechanism that holds the cold air up in the arctic, near the north pole. It's a single-number index designed to summarize past and present weather conditions on a global scale and, so far, fits into a scale from roughly positive 3 to negative 3. Positive AO numbers tell us it is working particularly well; negative numbers mean it is not. That's what we have been seeing this year, with mid-latitudes frequently experiencing colder weather than the pole itself.

Generally, a positive index corresponds with a jet stream which flows mostly west to east with little variation. Meteorologists call this a zonal flow. A meridional flow, denoted by a negative AO index value, eventually makes its way from the west to the east also. Before it does so, though, it make make a few side trips, swinging from up in the arctic down to near tropical latitudes once or twice on its way across each continent.

Big Loops

For example, while the great plains of Canada and the USA endured endless weeks of anomalous cold temperatures and snow cover right up until the end of April 2013 in spite of a warm fourth weekend, Arctic stations including Yellowknife and especially Eureka enjoyed above average and even record warm temperatures during the same period, which is not good news for those concerned about sea-ice.

The data examined compares each locale's temperature to its climactic average.

How about east-west variations?

The US coastal mountains (south of BC) had virtually no snow in them at the end of April, long before the end of the usual ski season in many regions there.

The same comparison can be made between western Europe and the Atlantic this year. Thule Greenland experienced rarely-seen above-freezing temperatures in March, while the UK struggled with that same range of temperatures.