4. Predominance of Westerly Weather
M Hulme

The 'North Atlantic Oscillation Index' for the extended-winter period (November to March) from 1900/01 to 1996/97 (dated by January). The Index is the normalised sea-level pressure difference between Gibraltar and southwest Iceland. Units are dimensionless. Positive values are associated with a strong westerly flow over the UK, bringing warm air from the Atlantic resulting in mild winter temperatures.

Specific Indicator
The specific indicator is the difference in air pressure (measured at sea level) during winter (November to March) between Gibraltar and southwest Iceland - an important measure of the state of the atmospheric circulation over the North Atlantic, which has a powerful effect on winter weather in the UK. This pressure difference is an index of the North Atlantic Oscillation, which is explained further below.
[Source: Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ]

Relevance
The North Atlantic Oscillation is a major disturbance of the atmospheric circulation and climate of the North Atlantic-European region, linked to a waxing and waning of the dominant middle-latitude westerly wind flow. The North Atlantic Oscillation index used here is based on the monthly mean sea-level pressure difference between stations to the south (Azores and/or Iberian peninsula - we use Gibralter) and north (Iceland) of the middle latitude westerly flow. It is therefore a measure of the strength of these zonal winds across the Atlantic.

When the North Atlantic Oscillation index is positive, the westerly flow across the North Atlantic and western Europe is enhanced. During the winter half-year, the strengthened westerly winds bring warmer, maritime air over northwest Europe causing a rise in temperature. When the Index is low or negative, the opposite occurs with temperatures falling over northwest Europe and rising over the northwest Atlantic. The net result is a "see-saw" or oscillation in temperatures across the North Atlantic-European region, as well as changes in other climate variables such as precipitation and sea-ice extent

Sensitivity to climatic and other factors
The North Atlantic Oscillation index is therefore a summary measure of the state of the mid-latitude circulation over the North Atlantic. It is subject to decadal and longer time-scale variability. However, the relationship between the annual North Atlantic Oscillation and global climate change is not well understood and is the subject of current research.
The North Atlantic Oscillation exerts a strong influence on year-to-year climate variability in the British Isles. For example, the correlation between the North Atlantic Oscillation index and winter temperature in the UK is about 0.67.

Change over time
The period until about 1970 saw a generally decreasing trend in the winter (November to March) North Atlantic Oscillation index, towards more negative values. The winter of 1968/69 yielded the second lowest index value this century, being exceeded only by 1995/96. The period from about 1970 recorded rising index values, with the highest value being recorded in 1994/95.

The change in North Atlantic Oscillation condition between the winters of 1994/95 and 1995/96 was quite remarkable - from the highest twentieth century value to the lowest twentieth century value in successive years. The very low index value in winter 1995/96 was associated with a cold winter in the UK. Since the extraordinary fluctuations of the mid 1990s, five of the last six winters have had positive values of the NAO, with a maximum, but by no means exceptional, deviation of 1.54 in 1999/2000. The exception is 2000/01, which had a slightly negative NAO value.

It is difficult to predict how the North Atlantic Oscillation may change in the future as the world warms. It displays variations on a number of different timescales, most of which may be unrelated to global warming. However, given its importance in determining winter weather over the UK, trends in UK climate cannot fully be understood without reference to the North Atlantic Oscillation.