Does the NAO index represent zonal flow? The influence of the NAO on North Atlantic surface temperature

The influence of zonal and meridional flow on surface temperature in the North Atlantic/European region is investigated. The degree to which the North Atlantic Oscillation (NAO) index reflects these two different flow types is considered, as is the relationship between the NAO index and surface temperature. Zonal and meridional circulation indices extending back to the early nineteenth or eighteenth centuries are based on surface pressure observations from the North Atlantic and Europe and on an empirical orthogonal function (EOF) analysis of European surface pressure from 1845–1995. The NAO index appears to integrate aspects of both zonal and meridional flow types. The pattern associated with the NAO index is composed of a quadrupole correlation pattern with surface temperature, showing positive correlations over Europe and the Sargasso Sea and negative correlations over northwest Africa and the Greenland/Labrador Sea region. Analysis indicates that the relationship between the NAO index and temperatures downstream of the Atlantic is associated with zonal flow, whereas the influence of the NAO on temperatures upstream is more closely linked to meridional flow patterns. Running correlations indicate that while there is no obvious link between the NAO index and the secular temperature trend, the second principal component of temperature is closely linked to atmospheric circulation, with a relationship which in winter has remained fairly steady through the twentieth century. Notwithstanding this, there have been changes in the strength of the correlation between temperature and circulation. These fluctuations in climate–circulation relationships should be further investigated and addressed in studies of climate change, especially in the calibration of paleoclimatic time series and downscaling models.