NAO and AO

 

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The North Atlantic Oscillation (NAO)

NAO values since 1824, using the WMO normal period 1961-1990 as base period. This produces an artificial dominance of years with positive NAO values. The high NAO values characterizing the period 1990-2000 is however seen not to be unique when the whole data series is considered. NAO data using 1961-1990 as reference period can be downloaded by clicking here. Last update May 23, 2008.

 

The North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation variability in the North Atlantic region ranging from central North America to Europe. The NAO is a seesaw in atmospheric mass between the subtropical high and the polar low. As an atmospheric phenomenon, NAO is known since almost 200 years. Van Loon and Rogers (1978) quote the missionary Hans Egede Saabye who wrote the following observation in his diary (1770–1778): “In Greenland, all winters are severe, yet they are not alike. The Danes have noticed that when the winter in Denmark was severe, as we perceive it, the winter in Greenland in its manner was mild, and conversely”. Today we know that this temperature seesaw is a manifestation of the NAO. The name North Atlantic Oscillation was coined by the British meteorologist and statistician Sir Gilbert Walker during his search for finding links between related anomalies across the globe (Walker 1924; Walker and Bliss 1932) and he was also the pioneer in considering the phenomenon as a pattern of atmospheric circulation variability. The climate anomalies associated with NAO are most pronounced during winter when the NAO is strongest. For the region corresponding to the Icelandic Low, cyclone events are more common under the positive NAO extremes as compared to negative extremes. Cyclones found in this region during the positive phase tend also to be deeper than their low NAO counterparts.

In general terms, NAO is calculated as the difference of normalised surface pressure between a station close to the subtropical high area, usually Ponta Delgada in Azores, Lisbon or Gibraltar, and a station close to the Icelandic low area, typically Reykjavik, Iceland. The NAO is apparent in meteorological data throughout the depth of the troposphere. It has been suggested that the NAO is a regional manifestation of the above mentioned more general Arctic Oscillation (Thompson and Wallace 1998).

When the NAO is in its positive phase, the subtropical high-pressure centre is stronger than usual and the Icelandic low-pressure centre is deeper. The positive phase is associated with stronger-than-average westerlies across mid-latitudes, warm and wet winters in Northern Europe, dry winters in Southern Europe, cold and dry winters in Northern Canada and Western Greenland, and mild and wet winter conditions in Eastern USA. The negative phase is associated with the opposite anomalies.

In certain years both Europe and Greenland experience extraordinary warm or cold winters. Warm winters on both sides of the North Atlantic are typically associated with higher than normal pressures over the Atlantic south of 55oN and a broad region of lower than normal pressures throughout the Arctic, resulting in strong westerly flow onto Europe and an absence of strong meridional flow anywhere from the Polar Regions (van Loon and Rogers 1978). Low surface pressures in the Arctic are usually accompanied by an increased number of cyclonic disturbances that lead to increased cloudiness and mean wind speed, and thus break up the shallow winter surface temperature inversions established during cold spells (see above). In contrast, cold winters on both sides of the North Atlantic are remarkable for the associated low standard deviation of the monthly mean sea level pressure north of 55oN within the region, implying very small cyclonic activity (van Loon and Madden 1983), and a higher frequency of shallow winter surface temperature inversions.

Much interest has been attached to variations in NAO since 1990, and global climate models have forecasted high, positive NAO values to characterise the 21st century. The NAO index has, however, since 2000 shown an overall falling trend, but with variations. NAO data may be downloaded by clicking here.

The NAO index is well suited to illustrate the visual effect of the base period chosen. Typically, the WMO normal period 1961-1990 is used as base period, and zero is defined at the average of NAO index values for this period (see figure above).

When considering the whole series of calculated NAO values since 1824, however, is it obvious that the WMO normal period represents a period with low NAO values. Using 1961-1990 as reference period has the mathematical effect of generating a artificial high number of years with positive NAO values, and only few years with negative values. If instead the whole data series is chosen as reference period a more realistic balance between positive and negative NAO values is obtained, as is shown by the diagram below.  

 

NAO values since 1824, using the longer period 1824-2007 as base period. This corrected diagram shows a realistic balance between years with positive and negative NAO values. Since 2000 there has actually been several years with NAO value below the long-term average (1824-2007). Last update May 23, 2008.

 

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The Arctic Oscillation (AO)

Arctic Oscillation (AO) index values since 1950. Pressure variability for the winter is larger than the annual variability. Following a short peak around 1990, the AO index have again decreased to values near zero. AO data can be downloaded from the Goddard Space Flight Center by clicking here. Last update May 23, 2008.

 

The Arctic Oscillation (AO) has been recognised by various names for several years, but has become a topic of keen research interest only since 1995. The AO index refers to opposing atmospheric pressure patterns in northern middle and high latitudes (Serreze et al. 1993; Serreze et al. 1995; Serreze and Barry 1998). A band of upper-level winds circulate around the North Pole, forming a vortex. When the vortex increase and the AO index becomes positive, the winds tighten like a noose around the North Pole, locking cold air masses in place near the pole. A weak vortex and a negative AO allow intrusions of cold air masses to plunge southward into North America, Europe and Asia.

 

Left diagram shows winter (DJF) sea-level pressure (SLP) averaged over the period 1900-2001. Isobars are spaced every 3 hPa with red colours used for SLP values greater or equal than 1013 hPa and blue colors used for lower values. Numbers at circumference indicate SLP values in hPa. Right diagram shows the modern distribution of permafrost in the Northern Hemisphere. Continuous permafrost is shown by dark blue colour. Discontinuous and sporadic permafrost is shown by light blue color. Red and black arrows show main surface air flow (warm and cold, respectively) as generated by the 20th century pattern of SLP. The overall windsystems set up by the average winter sea-level pressure appears to represent one of several controls on the present distribution of permafrost in the northern hemisphere.

 

The Arctic Oscillation thus exhibits a "negative phase" with relatively high pressure over the polar region and low pressure at midlatitudes (about 45oN), especially the oceanic mid-latitudes, and a "positive phase" in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives cyclones farther north toward the Arctic. Changes in the circulation pattern then bring wetter weather to Alaska, Iceland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean, and vice versa while in the negative phase (see diagrams below). In the positive phase, cold winter air masses do not extend as far into North America and Europe as it would during the negative phase of the oscillation. This keeps much of Europe and the United States east of the Rocky Mountains warmer than normal during periods of high AO, but leaves Greenland, Labrador and Newfoundland colder than usual. Over the latter half of the past century, the Arctic Oscillation alternated between its positive and negative phases. From the late 1970s the oscillation tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of North America and northern Eurasia, but colder than normal in the Greenland-Labrador region. Around the turn of the century, a change towards a future negative phase may be beginning to emerge (Polyakov et al. 1999).

Series of surface atmospheric pressure from the Arctic Ocean is only available from 1950, wherefore the AO index values only is available for a hort period, compared to the NAO index. Updated AO values are shown in the diagram at the top of this section. The red graph in this diagram shows annual values, while the blue graph shows index values calculated for the winter (DJF). The pressure pattern in the northern middle and high latitudes is clearly seen to be much more variable during winter than on an annual basis.

AO data may be downloaded from the Goddard Space Flight Center by clicking here.

 

5yr running mean AO and NAO index values calculated for January-March (reference period 1961-1990), compared to 5yr running January-March surface air temperature in Svalbard, since 1950. A certain degree of covariance is visible between the two index values and air temperatures. In general, high AO and NAO values (JFM) tend to correspond to high JFM air temperature in Svalbard, and vice versa. However, the lack of perfect match between the graphs clearly demonstrates that also other factors are important for winter temperatures in Svalbard, especially in the beginning and at the end of the period shown. Last update May 23, 2008.

 

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