Recent winters brought extreme cold to North West Europe and parts of the US. It is not likely that consecutive severe winters occurred purely by chance. If not, then what was the cause? And will it happen again this year?
Blocking and the Arctic Oscillation
Extreme conditions can occur when the normal variable circulation of the atmosphere gets stuck into a stationary “blocking” pattern. A block can extend over 1000km’s and persist for days or weeks. In 2010, the block which brought a heat wave to Russia also caused floods in Pakistan.
Blocking anti-cyclones bring severe winter cold to NW Europe. Mild moist air is kept away and replaced by cold dry air. Under clear skies and with long winter nights, the land surface radiates away more heat than it receives. Lower and lower temperatures are reached and relief only comes when the blocking pattern breaks down.
The formation of Northern hemisphere winter blocks is associated with the negative phase of the Arctic Oscillation (AO). The AO index describes the strength of a ring or vortex of air which circulates around the pole, shown in (a) below. The origin of this westerly wind or “jet” is the North-South temperature gradient and the rotation of the earth (coriolis force). In winter, when the temperature gradient is large and the AO is positive, the jet helps confine very cold Arctic night air to polar latitudes.
A negative AO index means that the polar vortex is weaker than normal. In this case, the situation shown in (c) is likely to occur. The difference between high and low AO is analogous to an ice-skater who is very stable when spinning fast, but more likely to wobble and fall over when spinning slowly. The earth-scale meanderings of the jet stream are called Rossby waves. Larger amplitude meanderings such as (c) favour the formation of atmospheric blocks, which is why severe winter weather is associated with negative AO.
The above chart of daily AO index since 1950 shows the recent period of negative index. The chart also shows that Arctic Oscillation is highly variable and irregular. Most of this derives from internal variability of the earth’s climate system. However researchers have known for some time that there is a connection between solar activity and AO e.g. the Little Ice Age (or Maunder Minimum ~1645-1715) was a period low solar activity and severe winters in NW Europe and the US. This has been a puzzle, because the brightness of the sun is nearly constant (variation ~ 0.1%).
Arctic Oscillation and Solar UV
Although only a small part of the sun’s output is in the ultra-violet (UV), variations in this part of the solar spectrum are now known to be fairly large (~8%).The above figure (from the UK Met Office) describes research linking the strength of the polar vortex to solar UV variability. The proposed mechanism works as follows. Solar UV heats the stratosphere at ~ 25km where most of it is absorbed by ozone. Heating is greatest over the tropics where sunlight is most intense. Lower UV radiation means less heating, which means a lower North-South temperature gradient, which means weaker thermal winds in the stratosphere. If this effect is transmitted down to low levels (troposphere), it might weaken the entire polar vortex and push AO negative.
While this is an area of active research, it is probably a good idea to keep an eye on solar UV. Here is a plot of the total UV radation in the range 115-310nm using satellite data from SORCE. (This R script will download the latest data from SORCE and reproduce the plot.)
While total solar radiation has picked up this year, UV radiation is still weak. If the new research is right, the odds are shifted in favour of another severe winter.
 Solar forcing of winter climate variability in the Northern Hemisphere, S. Ineson, A.Scaife, J. Knight, J. Manners, N. Dunstone, L. Gray, J. Haigh Nature Geoscience 4, 753–757 (2011)