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WIREs Clim Change
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Past and recent changes in the North Atlantic oscillation

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Abstract The North Atlantic oscillation (NAO) is under current climate conditions the leading mode of atmospheric circulation variability over the North Atlantic region. While the pattern is present during the entire year, it is most important during winter, explaining a large part of the variability of the large‐scale pressure field, being thus largely determinant for the weather conditions over the North Atlantic basin and over Western Europe. In this study, a review of recent literature on the basic understanding of the NAO, its variability on different time scales and driving physical mechanisms is presented. In particular, the observed NAO variations and long‐term trends are put into a long term perspective by considering paleo‐proxy evidence. A representative number of recently released NAO reconstructions are discussed. While the reconstructions agree reasonably well with observations during the instrumental overlapping period, there is a rather high uncertainty between the different reconstructions for the pre‐instrumental period, which leads to partially incoherent results, that is, periods where the NAO reconstructions do not agree even in sign. Finally, we highlight the future need of a broader definition of the NAO, the assessment of the stability of the teleconnection centers over time, the analysis of the relations to other relevant variables like temperature and precipitation, as well as on the relevant processes involved. WIREs Clim Change 2012, 3:79–90. doi: 10.1002/wcc.150 This article is categorized under: Paleoclimates and Current Trends > Earth System Behavior

A schematic overview of the two states of the NAO presented by Wanner et al.1 Shading indicates SSTs (bordered by dashed contours) and sea‐ice extension; arrows show the flow systems in ocean, atmosphere and rivers; solid blue and red contours indicate sea level pressures; white rectangles describe either characteristic climate conditions or important processes: (a) The positive phase and (b) the negative phase of the NAO. Source: Wanner et al.1

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Teleconnection axes deduced for a 30‐year running window from teleconnectivity maps in winter (DJF).85 The teleconnectivity is based on 500 hPa geopotential height88 (for the region 30°W–40°E and 30°N–70°N) from 1500 to 1999. All axes are projected in one plane, so some axes are plotted on top of each other. The axes of the last 50 years, which is the calibration period of this specific reconstruction (using NCEP/NCAR reanalysis data) are shown in red. Note that the axes connect the centers of action within a particular teleconnection pattern, that is, for the classical NAO, the Iceland Low and the Azores High. Additional to the axis the black dots show the grid points of the reconstruction.

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An example of the location of proxy records used to reconstruct the NAO index: in blue the precipitation sensitive proxy locations used in Trouet et al.,6 in yellow to dark red the proxy locations (only pressure sensitive) used in the MSLP reconstruction of Kuettel et al.79 Note that the proxy records of Trouet et al.6 span the period 1000–1995, whereas the proxy records of Kuettel span different periods; the red boxes are 8° × 8° grid boxes sampling wind speed and direction derived from ship log books from 1662 to 1855,81 where 99.6% span the period from 1750 to 1855.79

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(a) Low‐frequency variability of different NAO reconstructions5,6,16,73–79 for the past millennium and of observations from 1864 to 200932 (Figure 2). To focus on low‐frequency variations a 31‐year time filter is applied to the data except for the NAO reconstructions by Trouet and et al.,6 which only reconstructed the low‐frequency variations (also using a slightly different 31‐year time filter). Note that the indices of Trouet et al.,6 Hurrell,32 Cook et al.74 represent the season DJFM, whereas the others are estimated for the season DJF. Because of the reconstructions methods the variance is underestimated. To make the indices comparable the low‐frequency time series are normalized. The Kuettel index79 is based on selected grid points near Iceland and Lisbon of the sea level pressure fields provided and then the index is calculated similar to the method of Hurrel.32 (b) As (a) but only showing the period starting 1800.

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Time series of the station‐based NAO index from 1864 to 2009 for the season DJFM (updated data from the web page http://www.cgd.ucar.edu/cas/jhurrell/indices.html).32 In red the data represent unfiltered data from year to year; in black a 31‐year filter is applied to illustrate the low‐frequency behavior.

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