North Atlantic Oscillation
The North Atlantic Oscillation is the principal mode of natural climate variability across a large area of the Northern Hemisphere (mainly western and central Europe, Scandinavia, Siberia, Greenland and eastern North America). It is typically defined as the difference in sea-level pressure (SLP) between the Azores and Iceland. The index serves as an indicator of the general strength and direction of the mid-latitude westerlies and is related to the position of the jet stream.
The index can be positive or negative:
- Positive – There is a strong meridional pressure gradient between the Azores High and the Icelandic Low. Under NAO+ conditions, the westerly (as in – coming from the west) storm tracks are focused across NW Europe, so regions such as the UK and Scandinavia will experience milder and wetter conditions and southern Europe (Iberia, Mediterranean etc) will experience drier conditions. There may be a higher frequency of storms in general during NAO+ conditions.
- Negative – The pressure gradient between the Azores and Iceland is reduced, as such, the storm tracks will be located further southward, focused generally around Iberia and northwestern Europe will experience drier conditions. When the pressure difference between the Azores and Iceland is much weaker than usual, (north-)easterly winds can bring in cold arctic air that can impact northwestern Europe (the winters of 2009, 2010 and March 2013 are recent examples of this).
The NAO is loosely defined, and as such, several different versions of the index are in common use. The one illustrated above is the 'station-based' index, defined as the difference in normalised SLP between the Azores and Iceland. The index is available up to March 2014 (it is updated when I need it/ on request, current work I am involved in has been done on further refining the early SLP values, so expect a significant update in the future).
12/04/2014: Updated CH14 NAOI. (If you downloaded a previous version, please replace with this).
The uploaded file contains the CH14 NAO in two different formats on separate sheets, the second sheet is probably more user friendly.
Below are correlation maps between the NAO and temperature, precipitation and upper level zonal winds from the ERA-Interim reanalysis (grey lines = approximate (p < 0.05), the period analysed is 1979-2013). These provide a picture of the broad influence of the oscillation throughout the year.
The reference and documentation for this version of the NAO is covered here. This is an Azores-Iceland station-based NAO. The normalisation period used was 1865-2012. Caution is advised when using the station-based index across the high summer months (JA) as it may not fully capture the seasonal migration of the oscillations' centers of action.