The NAO is the largest, most powerful climate system on Earth. It affects the climate of the Northern Atlantic and the regions that surround it—mainly Europe and the eastern United States. Unlike El Niño, the NAO does not generate violent weather phenomenon; its effects are more widespread than local.
In order to understand the NAO, it is important to understand the concept of high and low pressure. Air pressure is simply a measure of how much air is pushing down at a particular location. Air mass and temperature differences between the Earth's surface and the upper atmosphere create vertical currents. This is what initially creates high- and low-pressure systems.
In a low-pressure system, the weight of the air is low at the Earth's surface. The vertical winds in this system travel upward, sucking air upward with it, away from the Earth's surface. Because the air is being pulled upward, it lessens the pressure at the ground. Because it leaves a low void at the Earth's surface, atmospheric currents moving along the Earth's surface are pulled inward from the surrounding areas at the base of the low and spin counterclockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere).
In a high-pressure system, air is pushed down toward the ground, adding pressure from the atmosphere. The air moves vertically downward. This causes the atmospheric currents to spin clockwise in the Northern Hemisphere (counterclockwise in the Southern Hemisphere). The air is pushed away from the center of a high-pressure system.
Both the low- and high-pressure systems can evolve into huge circular rotating systems. The higher in pressure a high-pressure system becomes or the lower in pressure a low-pressure system becomes, the stronger it gets, and the stronger it gets, the more intense the circulation pattern becomes.
The driving mechanism that runs the NAO is a difference in pressures: a high-pressure system over the Azores Islands and a low pressure system over Iceland. Although both systems are present all year long, it is during the winter season that the NAO has a significant effect on climate. During the winter months, both the high pressure and low pressure fluctuate in intensity, and it is the way they fluctuate relative to each other that causes distinctive variations in climate. When the difference between both the high pressure and low pressure is large—both pressures are strong—then their effect is to make winters warmer in northern Europe, make it warm in the northeast United States, and cause drought in the Middle East. This is considered a positive NAO.
When the pressure difference between the low and high systems is small—both pressure systems are weak—then they make the Mediterranean countries of Europe rainy and wet, the Scandinavian countries of northern Europe (Finland, Sweden, Norway, Denmark) extremely cold, and the eastern coast of the United States cold. This is a negative NAO.
The NAO oscillates on a cyclic pattern on a decade-to-decade timescale. Scientists have been monitoring it since the 1960s and have determined that the NAO tends to be positive for three to five years and then shifts and becomes negative for three to five years.
The NAO is being studied today by several scientists at National Aeronautics and Space Administration (NASA), as well as the Climate Prediction Center at National Oceanic and Atmospheric Administration (NOAA), trying to understand its behavior: why it behaves as it does, what causes it, what drives it from year to year, whether global warming is involved, and what are the possible short- and long-term consequences. Some scientists believe it may be a response to formation of sea ice or currents in the sea. With supercomputers becoming more capable, these scientists are currently trying to build mathematical models to mimic and predict its behavior.
Even though the NAO has an impact on the entire Atlantic, its largest impact is in Europe where it causes winter storms. When the high over the Azores and the low over Greenland both develop into extremely large, strong circulation patterns, there is a zone that forms between them with a strong-moving current of air that effectively channels the weather systems that pass from the United States directly into Europe. As the pressure systems vary from week to week, they affect the storms, as well as the amount of rain and snowfall that is generated for winter storms by the air passing over the Atlantic and the warm Gulf Stream, determining the amount of precipitation and temperatures experienced in Europe.
When the difference between the high- and low-pressure systems is large (a positive NAO), the strong winds in the channel push winter storms north toward Scandinavia and northern France. Conversely, when the pressure difference is small (a negative NAO), the storms typically travel from the southern United States to southern Europe, the Middle East, and northern Africa.
According to Jim Hurrell at the National Center for Atmospheric Research, the direction these storms take during the winter months can have a dramatic effect on Europe's weather. A positive NAO can make winters warmer by 5°F (3°C). During winters where Europe experiences a consistently positive NAO, it can lengthen the growing season by 20 days in Sweden, provide sunny, warm beaches on the French Riviera, but cause water shortages in the fertile crescent of Egypt. According to NOAA, a long duration of a negative NAO (small pressure difference) does not change southern Europe's rainfall, but significantly cools off northern Europe. The Alps experience superb skiing conditions, and the Mediterranean remains warm, increasing the production of olives and grapes in Greece. Although the most pronounced effects are felt by the European countries, the United States also feels an effect. When the NAO is positive, the eastern United States generally experiences warmer temperatures with less snowfall.
One of the biggest challenges of the NAO for scientists is their ability to predict it. They believe if they could predict a year in advance just how positive or negative the NAO was going to be, they could advise European farmers when to plants crops, advise the ski and tourist industry on predicted snowfall patterns, advise communities of predicted winter temperatures, and advise transportation departments of predicted maintenance workloads. Scientists are currently trying to use computer modeling to find answers to their questions.
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