North Atlantic Oscillation
The North Atlantic Oscillation (NAO) is a term that refers to large scale changes in pressure that occur naturally in the North Atlantic region. These changes have a massive impact on both weather and climate patterns in the surrounding continents. Prior knowledge of forthcoming changes in the NAO can thus provide information about such impacts on seasonal timescales and beyond.
Changes in local weather patterns such as temperature, rainfall and wind strength/direction are strongly influenced by changing local pressure patterns. Low pressure over the UK is accompanied by unsettled conditions with a tendency to form cloud, while high pressure is associated with more settled conditions and clearer skies.
A giant see-saw across the North Atlantic
The common pressure features seen in the North Atlantic Ocean are for large regions of relatively high pressure centred over the Azores islands (west of Portugal, known as the sub-tropical or Azores high) and low pressure centred over Iceland (the sub-polar or Icelandic low). The NAO describes the relative changes in pressure between these two regions (Azores minus Iceland), and was discovered in the 1920s by Sir Gilbert Walker. This has a strong influence on winter weather and climate patterns in Europe and North America, and can extend further into northern Asia if phases are prolonged. Acting like a giant seesaw, the NAO leads to changes in the intensity and location of the North Atlantic jet stream - ribbons of very fast winds high in the atmosphere that influence the movement of regions of low pressure (depressions) and their associated storms.
Positive NAO phase
The positive NAO phase represents a stronger than usual difference in pressure between the two regions.
Winds from the west dominate, bringing with them warm air, while the position of the jet stream enables stronger and more frequent storms to travel across the Atlantic
These support mild, stormy and wet winter conditions in northern Europe and eastern US. Conversely, northern Canada, Greenland and southern Europe are prone to cold and dry winter conditions.
Negative NAO phase
The negative NAO phase represents the reverse with a weaker than usual difference in pressure.
Winds from the east and north-east are more frequent, bringing with them cold air, while the adjusted position of the jet stream leads to weaker and less frequent storms.
As a result, Europe and eastern US are more likely to experience cold, calm and dry winters. In contrast, northern Canada and Greenland will tend to be mild and wet.
Predicting the NAO and its impacts
The NAO and associated winter climate in Europe and North America have previously been found to be largely unpredictable at seasonal time-scales. However, the Met Office Hadley Centre have now reassessed the seasonal predictability, using a new long range forecast system based on their global climate model. They have identified major sources of predictability, and demonstrated that the winter NAO and key aspects of European and North American winter climate are in fact predictable months ahead. This represents a key breakthrough in climate prediction capability at the Met Office as such forecasts can allow more time to prepare for increased or decreased likelihood of associated weather events, such as damaging winter storms, high near surface wind-speeds and extreme temperatures.