North Atlantic oscillation

• Main Authors: Hanna, Edward, Cropper, Thomas E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: F331 Atmospheric Physics; Arctic; Greenland; Global warming; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://eprints.lincoln.ac.uk/id/eprint/26480/; https://eprints.lincoln.ac.uk/id/eprint/26480/1/acrefore-9780190228620-e-22.pdf; https://doi.org/10.1093/acrefore/9780190228620.013.22

Many variations in the weather in the European and North Atlantic regions are linked with changes in the North Atlantic Oscillation (NAO). The NAO is measured using a south-minus-north index of atmospheric surface pressure variation across the North Atlantic and is closely connected with changes in the North Atlantic atmospheric polar jet stream, as well as wider changes in atmospheric circulation. The physical, human, and biological impacts of NAO changes propagate well beyond weather and climate, with major economic, social, and environmental effects. The NAO index, based on barometric pressure records, now extends as far back as 1851, according to recent work. Although there are few significant overall trends in monthly or seasonal NAO (i.e., for the whole record), there are many shorter term multidecadal variations. A prominent increase in the NAO between the 1960s and 1990s was widely noted in previous work and was thought to be related to human-induced greenhouse gas forcing. However, since then this trend has reversed, with a significant decrease in the summer NAO. In addition, because of a striking increase in variability of the winter (especially December which has resulted, during the period of 2004–2014, in four of the six highest and two of the five lowest NAO December measurements occurring in the 116-year record), NAO has been accompanied by increased variability in year-to-year winter weather conditions over the United Kingdom. These NAO changes are related to an increasing trend in the Greenland Blocking Index (GBI, equals high pressure over Greenland) in summer and a significantly more variable GBI in December. Such NAO and related jet-stream and blocking changes are not generally present in the current generation of global climate models, although recent process studies offer insights into their possible causes. Several plausible climate forcings and feedbacks, including changes in the energy output of the sun and the Arctic amplification of global warming, with accompanying reductions in sea ice, may help explain the recent NAO changes. Recent research suggests significant skill is required to make seasonal NAO predictions, and therefore long-range weather forecasts, for up to several months ahead for northwest Europe, but global climate models overall remain unclear on longer term NAO predictions for the remainder of the 21st century.