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Jim Hurrell, Senior Scientist

North Atlantic Oscillation (NAO) Indices Information

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NAO Station-Based Indices NAO PC-Based Indices NAM/AO Index North Pacific (NP) Indices

Since there is no unique way to define the spatial structure of the NAO, it follows that there is no universally accepted index to describe the temporal evolution of the phenomenon. Most modern NAO indices are derived either from the simple difference in surface pressure anomalies between various northern and southern locations, or from the PC time series of the leading (usually regional) EOF of sea level pressure (SLP). Many examples of the former exist, usually based on instrumental records from individual stations near the NAO centers of action, but sometimes from gridded SLP analyses. A major advantage of most of these indices is their extension back to the mid-19th century or earlier.

A disadvantage of station-based indices is that they are fixed in space. Given the movement of the NAO centers of action through the annual cycle, such indices can only adequately capture NAO variability for parts of the year. Moreover, individual station pressures are significantly affected by small-scale and transient meteorological phenomena not related to the NAO and, thus, contain noise.

An advantage of the PC time series approach is that such indices are more optimal representations of the full NAO spatial pattern; yet, as they are based on gridded SLP data, they can only be computed for parts of the 20th century, depending on the data source. Here we provide station-based NAO indices as well as the PC time series of the leading EOF (PC1) of both Atlantic-sector and Northern Hemisphere (NH) SLP. The latter is sometimes referred to as the Northern Annular Mode (NAM) or Arctic Oscillation (AO). For a more detailed discussion of these issues, see:

Hurrell, J. W., and C. Deser, 2009: North Atlantic climate variability: The role of the North Atlantic Oscillation. J. Mar. Syst., 78, No. 1, 28-41, DOI:10.1016/j.jmarsys.2008.11.026.
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Hurrell et al. (2003) in The North Atlantic Oscillation: Climate Significance and Environmental Impact, Geophysical Monograph 134, American Geophysical Union.

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Jim Hurrell, Senior Scientist and NCAR Director