North Atlantic Oscillation

North Atlantic Oscillation (NAO), an index of climatic variation in the North Atlantic based on the ratio between the high atmospheric pressure centred over the Azores and the low pressure centred over Iceland. It undergoes long-term cycles of variation with periodicities of 2.1, 8, 24, and 70 years. For example, the index was particularly low during the 1880s and 1960s, and particularly high during the 1920s and 1990s.

When the index is high, winter storms in the North Atlantic are more frequent and more violent, warmer and wetter winters are experienced in Europe and the north-eastern USA, but in Canada and Greenland the winters are colder and drier. When the index is low, winter storms in the North Atlantic are less frequent, Europe and North America experience colder and snowier winters, whereas the Mediterranean regions suffer from wetter conditions. Historically, these events are recorded in variations in the thickness of tree rings. The flow of the Gulf Stream appears to be reduced and no longer extends so far north, which provides a feedback mechanism between the ocean and climate.

The NAO also affects the growth and survival of marine plankton, like the keystone copepod species Calanus finmarchicus, hence the success of many commercial fish species, and may be one of the factors contributing for the spectacular decline and lack of recovery of cod stocks in the North Sea and Grand Banks fisheries. There is a comparable climate index in the North Pacific, and both these indices appear to be linked to El Niño events.

See also climate change; marine meteorology.

www.ldeo.columbia.edu/NAO/www.met.rdg.ac.uk/cag/NAO/

Bibliography

A plot of the index can be found on www.met. rdg.ac.uk/cag/NAO/

M. V. Angel