An Arctic influence

21.01.10 18:52

The anomalously cold winter thus far in the UK, parts of Europe, much of the USA and regions of China has been variously explained.

At its simplest, natural year-to-year variability can be cited. However, there is surely a reason why this winter specifically has so far turned out colder than average.

The jet stream and its unusually sluggish and southward path are part of the reason, allied with higher pressure than normal near Iceland and in the Arctic and lower than usual pressure in the Atlantic subtropics. This has caused what is known as negative phase of two climatic variables: the Arctic Oscillation and the North Atlantic Oscillation.

These tend to lead to more frequent and greater incursions of cold from the north at various points around the northern hemisphere. One of those points happens to have been near the UK this winter but other areas have benefited from the reverse side of the coin and experienced unusual winter warmth.

Atmospheric scientists have realised that another mechanism may also be working, something they call the “Arctic Dipole”. This means a build of high pressure in the Arctic near North America and low pressure developing on the Eurasian side of the Arctic.

Such a pattern drags warm air towards the Pole and aids the melting of ice sheets, perhaps accounting for the below-average ice coverage during the past three years. Warming in the Arctic regions even by just 1 degree C means that the usual west to east circulation of the atmosphere is weakened and parts of the northern hemisphere made more susceptible to cold outbreaks from the north.

If these processes feed off each other than who is to say that we will not have to get used to colder winters more frequently? Only time will tell.

By: Stephen Davenport