The North Atlantic Oscillation

Since 1966. the annual mean position of the Cuff Stream off Cape Hal teres has been determined IVom observations of surface temperature from satellites, aircraft and ships. These observations have been used to provide statistics oil the position (latitude) of the .so-called north wall' of the Cull Stream (the boundary between the Stream and the colder coastal water to the west or north-west) at particular longitudes eastward from Cape I Iatieras. Given w hat you have read about the variability of current systems, it wilt probably not surprise you to know that some years the path of the Gulf Stream is further south, some years further north. What would probably surprise you is that the position of the Gulf Stream may be correlated with various climate-related ecological responses on the eastern side of the Atlantic - including the size of certain zooplankton populations in parts of the North Sea. the size of the spring zooplankton population in Lake Windermere in Cumbria and - believe it or not - rates of grow th of certain terrestrial plant species in controlled plots in Gloucestershire,1 Such long-distance climatic links - known as teleconnections - are becoming easier to study, with an increasingly mult ¬°disciplinary approach to climate-related research, and continual improvements in archiving and electronic access to databases world-wide.

These correlations between the path of the Gulf Stream in the western Atlantic and climatic conditions in north-western Europe almost certainly arise because, rather than one being caused by the other, hoth are manifestations of the same climatic phenomenon - the North Atlantic Oscillation, or NAO. The NAO is a continual oscillation in the difference in atmospheric pressure helween the Iceland Low and the Azores High (cf. Figure 2.3), and is the most important factor affecting climatic conditions over the northern Atlantic Ocean and the Nordic Seas, particularly in winter.

The state of the NAO at any one time is described in terms of the NAO Index. This is usually expressed simply as the wintertime pressure difference between certain meteorological stations in Iceland and the Azores (or. sometimes. Lisbon in Portugal) (Figure 4.40). The Index may also be expressed in terms of the departure of the observed Iceland-Azores pressure difference from the average. In this case, when the pressure difference is higher than usual, the NAO Index is described as positive, and when the pressure difference is lower than usual the Index is described as negative. Generally, when the pressure difference is large the NAO is described as strong, when it is small the NAO is described as weak.

The state of the NAO affects not just wind speed and direction - westerlies are stronger when the NAO is strong - but also the paths followed by winter storms, rainfall patterns and transport of heat and moisture between the North Atlantic and the surrounding land masses, and even the strength of the North-East Trades. The importance of the state of the NAO is illustrated by Figure 4.41 which compares conditions during the winter of 1994-95, when the NAO was strong (positive Index), with conditions during the winter of 1995-96, when it was weak (negative Index).

Returning to the apparently mysterious links between the position of the Gulf Stream in the western Atlantic and climate-related ecological responses in the British Isles and North Sea, it seems likely that the linking factor is heat transfer between the Gulf Stream/North Atlantic Current and the overlying atmosphere far downstream of Cape Hatteras. When the NAO is strong (positive Index) and surface westerlies are strong, the Gulf Stream and North Atlantic Current flow more strongly than usual, and on average their paths are further north; as a result, more heat and moisture is transferred to north-west Europe, which becomes warmer and wetter. The reverse is true when the NAO is weak. However, as Figure 4.41 suggests, the NAO is more complex, and its effects more wide-ranging, than this simple explanation suggests.

Figure 4.40 Record of the wintertime difference in atmospheric pressure (reduced to sea-level) between the Azores High and the Iceland Low over the course of the twentieth century. The black line is a 5-year running mean, i.e. a line joining average values of the Index through successive 5-year periods.

Figure 4.40 Record of the wintertime difference in atmospheric pressure (reduced to sea-level) between the Azores High and the Iceland Low over the course of the twentieth century. The black line is a 5-year running mean, i.e. a line joining average values of the Index through successive 5-year periods.

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