The North Atlantic Oscillation and the UK

The NAO and UK climate variability

The relationship between the NAO and seasonal climate in the UK is illustrated by the diagrams below, with seasonal average temperatures and rainfall amount for the UK as a whole plotted against NAO indices for winter, and then for summer (SNAO indices), over the past 60 years.

In winter (upper panels) higher temperatures (left column) and rainfall (right column) tend to occur when the NAO index is positive, while negative NAO values are associated with lower temperatures and rainfall. In summer (lower panels), positive values of the SNAO are usually associated with higher temperatures but lower rainfall, while negative SNAO values are associated with lower temperatures and higher rainfall. The relationship is strongest for summer rainfall.

Data sources:

Precipitation and temperature data from the Met Office National Climate Information Centre.

The winter NAO index values are from the Hurrell Dec-Jan-Feb seasonal station-based data provided by the Climate Analysis Section, National Center for Atmospheric Research, USA (note that these differ in scale from values derived from monthly data).

The summer NAO index data are provided by the Met Office.

Winter NAO and UK districts

There is substantial regional variability in the relationships of temperature and precipitation with the NAO, particularly in winter. To provide an indication of the variations within the UK, some statistics regarding such relationships in winter (Dec-Jan-Feb) are provided  in the table below for regions from the UK climate districts map . These are based on observations in the one hundred year period 1914-2013.

The central double column headed 'Average' provides the seasonal mean temperature (°C) and precipitation amount (mm) for each district, averaged over all 100 years. The double column headed 'NAO bottom 25' shows the seasonal temperature change (in °C), and precipitation change (as a percentage of the 100-year average) averaged over the winters with the lowest 25 NAO index values. Entries that are not significant at the 99% level are marked 'ns'. Likewise, the double column headed 'NAO top 25' shows the change in seasonal temperature and precipitation averaged over the winters with the highest 25 NAO index values.

In each district temperatures are significantly higher when averaged over the 'NAO top 25' cases and lower when averaged for the 'NAO bottom 25'. In districts with significant change, the precipitation is on average increased when the NAO index is in the top 25, and decreased for the bottom 25. The precipitation changes are more robust in northern districts.