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North Atlantic Oscillation (NAO)

The North Atlantic Oscillation is one of the better understood climate indices. As the name indicates, the North Atlantic Oscillation occurs over the North Atlantic Ocean. It is an atmospheric pressure phenomenon (variations of the strength of high and lower pressure systems) that controls the strength and direction of east-west winds, which means that it has a large impact on storm tracks. The NAO has no particular pattern of variability and thus is not predictable, though it can be monitored.

Phase Effects of the North Atlantic Oscillation

Two phases of the North Atlantic Oscillation are recognized: positive (NAO+) and negative (NAO-). Each phase has very different effects on the westerly winds that bring air from the Atlantic Ocean into Europe and also on seal levels across the North Atlantic. A negative NAO is also referred to as 'blocked'.

There are two permanent pressure systems that affect the direction of the westerly winds. The first is a low pressure system, which sits over Iceland. The second is a high pressure system that sits over the Azores (west of the opening to the Mediterranean Sea). Though the systems are permanent in location, they fluctuate in size and strength over time. This fluctuation is the NAO.

When the pressure difference between the Azores and Iceland is large, it is known as NAO+. A positive NAO is associated with higher than average pressures over Iceland. When the pressure difference is small, it is known as NAO-. A negative NAO is associated with higher than average pressures over the Azores.

NAO+ leads to increased westerly winds, which bring mild air from south of Iceland and lead to cool summers and mild, wet winters. Indirectly, a positive NAO leads to warmer winters in the northeastern United States and eastern Canada. This indirect effect results from an increase in south-westerly circulation, which blocks the Jet Stream and thus prevents cold arctic air from dipping low into the eastern United States. The NAO- produces the opposite effect, leading to very cold winters and increased storm activity in southern Europe and North Africa. A negative NAO leads to strong snowstorms in the eastern United States with 13 of the 15 heaviest snowstorms since 1891 occurring when the NAO index was negative.

NAO_Schematic

From: http://www.climatewatch.noaa.gov/article/2010/forensic-meteorology-solves-the-mystery-of-record-snows/3

Impact on Ocean Height

Of note is the fact that the NAO+ causes a reduction in atmospheric pressure over the North Atlantic. This reduction in pressure can be thought of as creating a mild suction effect, which leads to increased sea levels in some areas. The fluctuation is measured in terms of centimeters, and is sometimes referred to as the inverse barometer effect.

Hurricanes

Because the North Atlantic is a common tract for hurricanes, as opposed to the southern part of the Atlantic, it is important to consider the effects of NAO phases on hurricane generation. The Azores high (high pressure region over the Azores) plays a large role in directing storms that blow off of the Sahara, which eventually become hurricanes when the conditions are correct and impact the Caribbean and North America.

When the Azores High extends further south than normal, it directs Hurricanes into the Gulf of Mexico. When the Azores High extends further north, it causes Hurricanes to travel up the east coast of the United States. The impact is significant. In periods where the Azores High was positioned further south, catastrophic hurricanes make landfall in the Gulf of Mexico 3-5 times more often than during a more northerly positioned Azores High.

It is important to note that the position of the Azores High is not directly a result of NAO phase changes because the trends happen over very long periods of time (roughly every 1000 years). Nevertheless, more minor steering changes to occur with NAO phases. A positive phase will push hurricanes into the Gulf of Mexico and a negative phase will cause them to travel more to the north. These directional changes will be less dramatic than those that occur every millennium or so.

ENSO and NAO Interactions

The NAO does not operate in a vacuum and is influenced by a number of other climate phenomena. Most notably, the El Nino-Southern Oscillation can have a profound effect on the NAO. For example, a negative phase of the NAO in 2009-2010 corresponded quite well with the unusually cold winter in Europe during that time period. The UK was the coldest it had been in 30 years. This period was also associated with an El Nino event.

Unfortunately, the relationship between the two indices is not fully understood, in part because it is complicated by other factors. For instance, increase snowmelt during Arctic summers also leads to increased snowfall in Europe and cooler temperatures during the following winter. This can happen even during La Nina years, which would traditionally be associated with a blunting of NAO- effects. Clearly there is a lot that is not understood about Earth’s climate.

Nutrients

The ocean is greatly affected by a number of climate indices. For instance, ENSO phases profoundly affect nutrient upwelling off the western coast of South America and thus overall populations of everything from fish to ocean mammals and birds. A similar effect is seen with the NAO on the east coast of the United States.

During NAO+, the east coast of the U.S. is warmer than average with increased rainfall and a decrease in the salinity of surface waters of the Atlantic. Because fresh water is less dense than saltwater, it tends to create a barrier that prevents nutrients from reaching the sea surface. This, in turn, suppresses the growth of algae and diatoms, which will in turn lead to fish kills and decreased abundance of food for the entire food chain of the Atlantic. As with ENSO, climate change that directly affects NAO phases can have profound effects on nutrient exchange in the ocean and thus worldwide food chains.

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