There has been a flip in the weather patterns over the North Atlantic which has a number of interesting climatic consequences. Not only does it mean a significant interruption in the upward trend in global warming, but also it has more immediate implications for winter weather in Europe.
The change, which occurred about a year ago, is part of a phenomenon known as the North Atlantic Oscillation (NAO). It shows up most strongly during the winter.
Before last winter, the previous eight years featured predominantly strong westerly circulation that brought almost unbroken mild weather to northern Europe, but extremely severe seasons to Greenland. Last winter, for the first time since the mid-Eighties, the hemispheric patterns became stuck in a form that brings much colder conditions here.
The nature of this switch was first recognised by a Danish missionary, Hans Egede Saabye, in the late 18th century. He noted that in Greenland, although all winters are severe, they are not alike. The Danes noted that when the winter in Denmark was severe, the winter in Greenland was (by its standards) mild, and vice-versa. This see-saw behaviour was quantified by Sir Gilbert Walker in the 1920s in terms of pressure differences between Iceland and southern Europe. He named it the North Atlantic Oscillation.
The NAO shifts between a deep depression near Iceland coupled with high pressure around the Azores - which produces strong westerly winds - and the reverse pattern with much weaker circulation. The first, the strong westerly pattern, pushes mild air across Europe and into Russia, while pulling cold air southwards over western Greenland. It also tends to bring mild winters to much of North America. One significant climatic effect is the reduction of snow cover, not only during the winter, but well into the spring.
The reverse, meandering pattern often features high pressure over Iceland or Scandinavia, which pulls cold Arctic air down into Europe, and funnels mild air up towards Greenland. This produces much more extensive continental snow cover, reinforcing the cold weather in Scandinavia and eastern Europe. That is what happened last winter and spring.
Since 1870, the NAO has fluctuated appreciably on timescales from several years to a few decades. It took the strong westerly form between 1900 and 1915, in the Twenties and, most notably, from 1988 to 1995. But it flipped to the sluggish meandering form in the 1940s and Sixties, bringing frequent severe winters to Europe but exceptionally mild weather in Greenland.
These fluctuations are now seen as a major factor in understanding climatic change. They have had a quasi-periodic flavour, in that they have often stuck in one phase for a number of years before switching to opposite form. But they do not show regular cycles, which would provide a clue as to what is driving this oscillation.
The importance of the NAO for understanding climatic change is the effect it exerts on average temperatures in the northern hemisphere. Because winters show the greatest variance of all the seasons, annual temperatures tend to be heavily influenced by whether the winter was very mild or very cold. When the NAO is in its strong westerly phase, its benign impact over much of northern Eurasia and North America outweighs the cooling around Greenland, and this shows up in the annual figures. So, a significant part of the global warming in the past 10 years has been associated with the very mild winters in the northern hemisphere.
One of the challenges for scientists in predicting future climatic change is anticipating the behaviour of the NAO. So far the computer models of the global climate cannot yet handle switches in weather regimes of the type experienced by the NAO last winter. The latest models, such as the one developed at the Hadley Centre at Bracknell, while doing an increasingly good job on the broad features of global warming - including predicting an initial cooling in the vicinity of Greenland - have underestimated the rapid warming over land in the northern hemisphere in the Eighties. This may be due in part to difficulties in handling the NAO.
The solution probably lies in better models of how the atmosphere interacts with the oceans. Because the NAO in its westerly phase produces very cold conditions in the North Atlantic around Greenland, this may, over a number of years, sustain sufficient changes to alter ocean circulation patterns. These, in turn, could create circumstances that trigger a switch to the sluggish phase of the NAO. This process could flip-flop back and forth in an irregular manner, depending on how it was influenced by the behaviour of the rest of the global climate system.
Even more speculative is the possibility that ocean-atmosphere interactions, such as the NAO, are part of much more dramatic changes that occurred at the end of the last Ice Age and at earlier times in the Earth's climatic history. Some global climate models do suggest that it may be possible for changes in atmospheric circulation to produce long-lasting shifts in ocean currents. One such change could effectively divert the Gulf Stream on to a more southerly course, bringing much colder winters to northern Europe. Computer predictions of global warming do hint at regions of both warming and cooling in different parts of the North Atlantic - suggesting that human activities are capable of producing erratic climatic reactions to small perturbations.
It may be some time before we have models that tell us what controls the NAO and to what extent human activities could lead to one or other phase becoming more prevalent, and so to more dramatic changes in the climate. In the meantime, the evidence of the past natural behaviour of the oscillation is that once it switches it is likely to remain in the new phase for several years. Last winter's cold weather may be a harbinger of things to come, despite the global warming of the past two decades. Perhaps it's time to start hunting out those warm clothes again.
Bill Burroughs is the author of `Weather Cycles: Real or Imaginary?', pounds 14.99, Cambridge University Press.