North Atlantic Oscillation

Introduction

The North Atlantic Oscillation (NAO) is a natural shift in weather patterns that occurs in the North Atlantic Ocean and influences the weather of surrounding areas from the eastern United States to Siberia. The phenomenon has been observed for decades, but its mechanism is still poorly understood and the shifts of the NAO remain difficult to predict.

Historical Background and Scientific Foundations

NAO is a shift in the pattern of atmospheric pressures over the Atlantic. Over the Azores Islands in the sub-tropical Atlantic (at about the same latitude as Virginia), there is a permanent high-pressure center; in the vicinity of Iceland, there is a low-pressure center. NAO has two phases, called positive and negative. During NAO’s positive phase, the pressure of the high-pressure Icelandic center is higher and that of the low-pressure Azores center is lower. Also, the Icelandic center shifts to the east, and the Azores center to the west. All of these effects are reversed during negative NAO. There is generally about one NAO oscillation per year.

By themselves, NAO pressure shifts would not be significant. However, they are part of a system of changes that alters the exchange of heat and moisture between the Atlantic and the landmasses surrounding it. Storms moving eastward across the Atlantic, in the direction of the prevailing westerly winds, are routed differently depending on the state of NAO. During positive NAO, these westerly winds are strengthened, which has particularly strong effects on the weather, making it cooler and drier over the northwestern Atlantic and Mediterranean countries and warmer and wetter in northern Europe, the eastern United States, and much of Scandinavia. During negative NAO, these effects are reversed.

Weather is the most important direct effect of NAO from the human point of view, since it influences agriculture and water management. However, NAO affects much more than the weather. Because NAO affects surface winds over the Atlantic, it alters the exchange of heat and freshwater at the surface. This affects the strength of the large-scale ocean circulation in the Atlantic that exchanges surface water for deep water, the thermohaline circulation or Atlantic conveyor belt. Slowdown of this conveyor belt may be caused by global climate warming, which accelerates melting of Greenland’s ice cap and so dumps larger amounts of freshwater into the far northern Atlantic. A strengthening trend in NAO might, for a while, mask any such slowdown of the conveyor belt. NAO also affects ecosystems on land and at sea. Changes include movements of populations of fish, shellfish, and zooplankton, and later or earlier blooming of plants and reproductive behaviors of animals on land.

Interest in NAO was low for decades because it was considered highly unpredictable. However, since the late 1990s, increasingly sophisticated computer climate models have made it possible for scientists to probe possible cause-and-effect mechanisms controlling the timing and strength of NAO. Reliable forecasts of NAO would be useful for agriculture in Europe, but NAO remains difficult to forecast.

Impacts and Issues

A strong trend has been observed in NAO over almost the last 30 years. The trend is toward a more strongly positive NAO. This trend accounts for a number of climate shifts across the Northern Hemisphere, including changes in Arctic ocean water composition, shifts in regional precipitation patterns (causing some European glaciers to advance and others to retreat), more severe winters in eastern Canada, changes in zooplankton production in the Atlantic, and shifted fish distributions. Although this shift coincides with global climate change caused by anthropogenic emissions of gases such as carbon dioxide, scientists are uncertain about whether NAO’s trend toward more a positive phase has been caused by global climate change or by the variability of the natural process itself.

Some scientists have pointed out that much of the observed climate warming over Europe and Asia in recent decades can be explained by the NAO trend, and suggested that this has contributed to overestimation of human-caused global warming. However, most climate scientists agree that NAO’s own changes may well be driven by climate change, making it a link in the global warming process. By 2001, some scientists were arguing that the recent trend in NAO is driven by warming of tropical sea-surface temperatures, especially in the tropical Indian Ocean and Pacific, which in turn is attributed to increased levels of greenhouse gases in the atmosphere. In short, global warming is probably causing the positive trend in NAO, rather than the other way around. However, as of 2008, NAO remained one of the more puzzling large-scale features of Earth’s climate.

See Also El Niño and La Niña; Climate Change

BIBLIOGRAPHY

Periodicals

Hurrell, James W. "The North Atlantic Oscillation.” Science 291 (2001): 605-605.

WORDS TO KNOW

ANTHROPOGENIC: Made by humans or resulting from human activities.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters; also termed meridional overturning circulation.

ZOOPLANKTON: Small, herbivorous animal plankton that float or drift near the surface of aquatic systems and that feed on plant plankton (phytoplankton and nanoplankton).

Visbeck, Martin H., et al. “The North Atlantic Oscillation: Past, Present, and Future.” Proceedings of the National Academy of Sciences 98 (2001): 12876–12877.

Web Sites

Climate Research Unit, School of Environmental Sciences, University of East Anglia, UK. “North Atlantic Oscillation.” http://www.cru.uea.ac.uk/cru/info/nao/ (accessed April 12, 2008).