North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) is a critical atmospheric circulation pattern that influences weather and climate across the North Atlantic region, stretching from central North America to Europe and northern Asia. Typically manifesting in winter, the NAO arises from fluctuations in atmospheric pressure between a low-pressure system near Iceland and a high-pressure system near the Azores-Gibraltar area. The NAO is quantified using the North Atlantic Oscillation index, which tracks the difference in pressure between these two centers.
When the NAO index is in a positive phase, it is characterized by a stronger high-pressure zone and a more pronounced low-pressure zone, leading to warmer and wetter conditions in northern Europe while causing colder and drier winters in northern Canada and Greenland. Conversely, a negative NAO phase results in weakened pressure systems, producing fewer winter storms and delivering colder air to Europe, while northeastern Canada experiences milder winters.
The NAO plays a significant role in climate variability, affecting agricultural yields, water resources, and ecosystems. Its fluctuations are influenced by various factors, including sea surface temperatures and greenhouse gas emissions. As climate change progresses, understanding the mechanisms behind the NAO becomes increasingly important, as its variability has potential implications for global warming trends and winter climate patterns in the Northern Hemisphere.
Subject Terms
North Atlantic Oscillation (NAO)
Definition
The North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation in the North Atlantic region ranging from central North America to Europe and northern Asia. The NAO usually develops in the winter and is caused by fluctuations in atmospheric pressure between a subpolar, low-pressure center near Iceland and a subtropical, high-pressure center near the Azores-Gibraltar region. The NAO is generally described by the North Atlantic Oscillation index, which is a weighted measurement of the difference between the subpolar low-pressure zone and the subtropical high-pressure center during the winter season of the North Atlantic region.
The positive NAO index phase corresponds with time periods when a stronger subtropical high-pressure center and a deeper-than-normal subpolar low-pressure zone exist in the North Atlantic region, increasing the atmospheric pressure in this region. During positive-NAO-index years, the western subtropical North Atlantic Ocean is warm. Strengthened westerly winds blow warmth and moisture into north-central Europe. The warm, moisture-bearing winds arriving from the subtropical Atlantic Ocean make Europe warmer and wetter. In the meantime, northern Canada and Greenland experience cold and dry winters. Cooler temperatures occur off the west coast of Africa. Strong send more dust out across the ocean toward the Caribbean Sea. The eastern United States undergoes a mild and wet winter season.
The negative NAO index phase corresponds with time periods when both the subtropical high-pressure center and the subpolar low-pressure zone are weakened, which reduces the atmospheric pressure gradient in the North Atlantic region. As a result, fewer and weaker winter storms occur in this region. More moist air is brought to the Mediterranean, and cold air is brought to northern Europe. Northeastern Canada and Greenland experience mild and wet winters, and the eastern United States undergoes a cold and dry winter season.
Significance for Climate Change
The NAO index varies from year to year, though loose patterns exist in the data. The NAO index was persistently positive in the early 1900s, negative in the 1960s and 1970s, and considerably more positive during the 1980s and early 1990s. Again from 2010 to 2024, mostly positive trends in the NAO index were noted. Since the heat capacity of the ocean is much greater than that of a continent, the NAO accounts for approximately one-third of the changes in average winter surface temperatures in the Northern Hemisphere. Variations in the NAO have significant impacts on many aspects of North Atlantic societies and the environment, such as agricultural harvests, water resources, fishery yields, industrial energy production, and ecosystems. Significant changes in the NAO may influence climatic changes, including changes in sea surface temperature (SST), ocean circulation patterns, and Arctic sea-ice coverage.
Many mechanisms have been proposed to account for NAO index variability, including atmospheric response to changes in SST, variability of atmospheric convection in the tropics, internal and nonlinear dynamics of the extratropical atmosphere, and anthropogenic forcing caused by greenhouse gas (GHG) emissions and ozone depletion. Tropical heating influences the atmospheric circulation over the North Atlantic region. Since tropical convection is sensitive to the underlying SST distribution, warming of the tropical oceans may lead to persistently positive values for the NAO index.
Some scientists posit that changes in atmospheric circulation associated with the NAO index contributed to the winter warming of the Northern Hemisphere, particularly the winters of 2009–10, 2015–16, and 2020–2021. Statistical evidence has demonstrated that the forcing of increased GHG concentration in the atmosphere may have affected the long-term variability of the NAO. Comparisons of NAO index records between the 1800s and the late twentieth century demonstrate that global warming may cause the increased variability of the NAO. Though studies have linked climate change to the NAO, the mechanism of the NAO is still not fully understood. The NAO needs to be further investigated to advance understanding of the linkages between forcing and NAO variability.
Bibliography
Appenzeller, C. et al. “North Atlantic Oscillation Dynamics Recorded in Greenland Ice Cores.” Science vol. 282, 1998, pp. 446–49. doi: 10.1126/science.282.5388.446. Accessed 17 Dec. 2024
Goodkin, N. F.et al. “Increased Multidecadal Variability of the North Atlantic Oscillation Since 1781.” Nature Geoscience vol. 1, 2008, pp. 844–8. www.nature.com/articles/ngeo352. Accessed 17 Dec. 2024.
Holland, D. M., et al. “Acceleration of Jakobshavn Isbrae Triggered by Warm Subsurface Ocean Waters.” Nature Geoscience 1 2008, pp. 659–64. www.nature.com/articles/ngeo316. Accessed 17 Dec. 2024.
Hurrell, J. W., et al., eds. The North Atlantic Oscillation: Climate Significance and Environmental Impact. Washington, D.C.: American Geophysical Union, 2003.
Kim, Hyo-Jeong, et al. "North Atlantic Oscillation Impact on the Atlantic Meridional Overturning Circulation Shaped by the Mean State." npj Climate and Atmosphere Science, vol. 6, no. 2, 25 Mar. 2023, doi.org/10.1038/s41612-023-00354-x. Accessed 17 Dec. 2024.
Lindsey, Rebecca, and LuAnn Dahlman. “Climate Variability: North Atlantic Oscillation.” Climate, National Oceanic and Atmospheric Administration, 2009, www.climate.gov/news-features/understanding-climate/climate-variability-north-atlantic-oscillation. Accessed 17 Dec. 2024.
“North Atlantic Oscillation (NAO).” National Centers for Environmental Information (NCEI), National Oceanic and Atmospheric Administration, Dec. 2024, www.ncei.noaa.gov/access/monitoring/nao/. Accessed 17 Dec. 2024.
Osborn, Tim. “North Atlantic Oscillation (NAO).” Climatic Research Unit Data, University of East Anglia, Climatic Research Unit, Jan. 2019, crudata.uea.ac.uk/cru/data/nao/. Accessed 17 Dec. 2024.
Paeth, H., et al. “The North Atlantic Oscillation as an Indicator for Greenhouse-Gas Induced Regional Climate Change.” Climate Dynamics 15, no. 12 (1999): 953–960.