The Azimuth Project
North Atlantic Oscillation (Rev #10)



According to the introduction by James W. Hurrell et al., cited below:

The North Atlantic Oscillation (NAO) is one of the most prominent and recurrent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment. Understanding the processes that govern this variability is, therefore, of high priority, especially in the context of global climate change. There is no unique way to define the spatial structure of the NAO, or thus its temporal evolution, but several common approaches are often illustrated. The relationship between the NAO and variations in surface temperature, storms and precipitation, and thus the economy, as well as the ocean and ecosystem responses to NAO variability.

Although the NAO is a mode of variability internal to the atmosphere, indices of it exhibit decadal variability and trends. That not all of its variability can be attributed to intraseasonal stochastic atmospheric processes points to a role for external forcings and, perhaps, a small but useful amount of predictability. The surface, stratospheric and anthropogenic processes may influence the phase and amplitude of the NAO.

Here is a plot on the NAO Winter index, showing the difference of normalized sea level pressure with a 5-year moving average (black)



Hurell et al. also state that although the overwhelming indication is that the NAO is a mode of variability internal to the atmosphere, there is some evidence that external factors such as volcanic aerosols, human-caused changes in the atmosphere, and variations in solar activity can influence its phase and amplitude. Moreover, it has been argued that interactions between the atmosphere and the underlying surface, or between the troposphere and stratosphere, can lend a “low-frequency” component to the NAO variability, such that limited prediction is plausible. At present there is no consensus on the relative roles such processes play in NAO variability, especially where long (interdecadal) times scales are concerned. Considering the significant impact the NAO exerts on the climate of the Northern Hemisphere, understanding the mechanisms that control and affect the NAO is therefore crucial to the current debate on climate variability and change.


The ocean integrates the storms in the form of surface waves, so that it exhibits a marked response to long lasting shifts in the storm climate. The recent upward trend toward more positive NAO index winters has been associated with increased wave heights over the northeast Atlantic and decreased wave heights south of 40°N. Hurell et al. also note some ecological impacts Over the last couple of years interest in the ecological impacts of NAO variability has increased markedly and show the NAO affects a broad range of marine, terrestrial and freshwater ecosystems across large areas of the NH, diverse habitats and different trophic levels. Although such effects are far-reaching, the nature of the impacts varies considerably. Similarly, the impact of ENSO on the North Atlantic climate, and the NAO in particular, remains open to debate,although most evidence suggests the effects are small but nontrivial.

Economic effects from the NAO has been studied, e.g. the connection between the NAO and stream flow of the Euphrates and Tigris rivers. A major issue in this region involves water supply shortages and surpluses for irrigation farming in the Middle East. Decreases in rainfall associated with the long term trend in the NAO index have had catastrophic effects on crop yields and have contributed to high level political disputes on water withdrawals from the rivers between Turkey, where most of the rain falls, and Syria, a downstream riparian neighbor.


The paper by Osborn et al., also cited below, states that the realism of the Hadley Centre?’s coupled climate model (HadCM2?) is evaluated in terms of its simulation of the winter North Atlantic Oscillation (NAO). During 1400 years of a control integration with present-day radiative forcing levels, HadCM2 exhibits a realistic NAO associated with spatial patterns of sea level pressure, synoptic activity, temperature and precipitation anomalies that are very similar to those observed. Spatially, the main problem is that the simulated NAO has a teleconnection with the North Pacific that is stronger than observed. Temporally, the simulation is compatible with the observations if the recent observed trend (from low values in the 1960s to high values in the early 1990s) in the winter NAO index (the pressure difference between Gibraltar and Iceland) is ignored. This recent trend is, however, outside the range of variability simulated by the control integration of HadCM2, implying that either there’s a problem with the model, or forcing is responsible for the variation. They write:

It is shown, by analyzing two ensembles, each of four HadCM2 integrations that were forced with historic and possible future changes in greenhouse gas and sulphate aerosol concentrations, that a small part of the recent observed variation may be a result of anthropogenic forcing. If so, then the HadCM2 experiments indicate that the anthropogenic effect should reverse early next century, weakening the winter pressure gradient between Gibraltar and Iceland. Even combining this anthropogenic forcing and internal variability cannot explain all of the recent observed variations, indicating either some model deficiency or that some other external forcing is partly responsible.


Abstract. This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterized by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. This NAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability may be influenced, however, by ocean and sea-ice processes.

  • Rodwell, Oceanic forcing of the wintertime NAO.

  • T. J. Osborn, K.R. Briffa, S.F.B. Tett and P.D. Jones, Evaluation of the North Atlantic Oscillation as simulated by a coupled climate model, Climate Dynamics 15 (1999), 685-702.

  • Wanner and Bronniman, [North Atlantic Oscillation - Concepts and Studies), Springer 2001.]

category: climate