North Atlantic Oscillation

Definition of North Atlantic Oscillation: The North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean characterized by fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High . This is the common definition for North Atlantic Oscillation, other definitions can be discussed in the article

Fig. 1. The monthly NAO index over the period 1955-2015. This NAO index corresponds to the first EOF (Empirical Orthogonal Function) of atmospheric pressure variability over the north Atlantic, based on the analysis of monthly data of the mean 500 millibar height anomaly.

Climate fluctuations in north-western Europe

Climate fluctuations in north-western Europe are correlated with fluctuations in the atmospheric sea level pressure difference over the north Atlantic between the Azores and Iceland. A larger than average difference in the sea level pressure at the two regions leads to increased westerlies and, consequently, cool summers and mild and wet winters in central Europe and its Atlantic facade. In contrast, if the pressure difference is smaller than average, westerlies are suppressed, northern European areas suffer cold dry winters and storms track are directed southwards toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.

The North Atlantic Oscillation (NAO) is related to the sea level pressure difference over the north Atlantic expressed in the NAO index. The NAO index is defined in several ways. The most common definition is the difference in atmospheric pressure measured at the weather stations of Lisbon and Reykjavik (more precisely: the deviation from the average difference). Another more mathematical definition is given in the caption of Fig. 1.

The NAO index exhibits considerable interseasonal and interannual variability, and prolonged periods (several months) of both positive and negative phases of the pattern are common (Fig. 1). The wintertime NAO exhibits significant decadal-multidecadal variability (Fig.2). This figure also shows that the winter temperature of the North Sea is correlated with the wintertime NAO. The atmospheric pressure anomaly typical for positive and negative NAO phases is shown in Fig. 3.

Fig. 2. Winter (December through March) index of the NAO based on the difference of normalized sea level pressure (SLP) between Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland since 1864. The SLP values at each station were normalized by removing the long-term mean and by dividing by the long-term standard deviation. Both the long-term means and standard deviations are based on the period 1864-1983. Normalization is used to avoid the series being dominated by the greater variability of the northern station . Green inserted graph: North Sea winter temperature (after Alheit et al., 2005 ). The North Sea winter temperature correlates with the NAO index.

Fig. 3. Location of atmospheric high and low pressure zones during winter, typical for positive (left panel) and negative (right panel) NAO phases. The corresponding distribution of warm (yellow) and cold (blue) water masses over the North Atlantic is also indicated in the figure, together with the dominant storm tracks (black arrow). Adapted from (Wanner et al., 2001).

Low-frequency NAO oscillations

Fig. 4. Global NAO spectrum based on wavelet analysis of the NAO index annual time series for the period 1860-2005. The global spectrum is determined by integrating the time dependent DOG₁₄ wavelet spectra (see Wavelet analysis of coastal processes). Redrawn after Massei et al. (2007 ).

The North Atlantic Oscillation has no a well-defined oscillation periods. According to current understanding, NAO is the result of quasi-random non-linear processes (sometimes described as red noise) that mainly take place in the atmospheric system with minor feedback from the north Atlantic ocean. NAO has a broad frequency spectrum with a major peak around a period of about 8 years, see Fig. 4. The 8-year modulation was particularly pronounced in the second half of the 20th century, but almost absent in other periods. The processes responsible for the low-frequency (decadal - multidecadal timescale) NAO oscillations are not yet fully elucidated and subject of research. There is observational and modeling evidence that low-frequency variability in the North Atlantic has significant implications for the global climate, particularly for the climate of the Northern Hemisphere.

Under a positive NAO index (NAO+), regionally reduced atmospheric pressure causes a regional rise in sea level due to the 'inverse barometer effect'. This effect is important both for the interpretation of historical sea level records and for predictions of future sea level trends, as mean pressure fluctuations in the order of millibars can lead to sea level fluctuations in the order of centimeters.

WEPA

An analysis of winter wave activity along the Atlantic coast of Europe shows a mediocre correlation with NAO based on the surface pressure difference between Lisbon and Reykjavik. An alternative so-called Western Europe Pressure Anomaly (WEPA) index is based on the normalized SLP difference measured between the stations Valentia (Ireland) and Santa Cruz de Tenerife (Canary Islands, Spain). The positive phase of WEPA reflects intensified latitudinal SLP gradient in the NE Atlantic that drives increased W-SW winds around 45^∘ associated with severe storms, many eventually passing over UK, which funnel high-energy waves toward western Europe. WEPA is the most relevant index to capture extreme wave height both spatially and temporally, like for the extreme 2013/2014 that caused severe erosion along the Atlantic coast of Europe.