It controls the strength of westerly winds between the “Azores high”’ and the “Icelandic low” that bring a succession of weather systems to Western Europe. New research has looked at the influence of the NAO on orographic precipitation, mountainous weather systems, and river flow in upland Britain.
Orographic precipitation is when moist air rises and flows over a mountain. The side with the wind will have a much wetter climate than the other side. In the UK, the heaviest orographic precipitation has long been associated with strong southwest to westerly winds in the warm, moist sectors of frontal depression.
The study, by the Universities of Durham and Bristol, has calculated seasonal precipitation totals for 90 station records over the last 180 years. The team used precipitation data from the British Atmospheric Data Centre Met Office Integrated Data Archive Service database and the UK Meteorological Office historic station database, together with other available long records.
The study has found that the hydroclimatology of rainfall and river flow in upland areas is closely linked to the strength of atmospheric circulation, an effect which strengthens with increasing altitude. The identified effects are large enough to cause very high river flow during periods of highly positive NAO but may also lead to severe drought when the NAO is highly negative.
Professor Tim Burt in the Department of Geography at Durham University said: “Our results cast a new light on rainfall variability in upland Britain and have implications worldwide for any mountainous region where significant orographic precipitation is generated.”
Dr Nicholas Howden, Senior Lecturer in Water in the Department of Civil Engineering at the University of Bristol, added: “What is novel here is the identification of large-scale interannual variability in seasonal precipitation and river flow totals across the British uplands controlled by the varying strength of atmospheric circulation in the North Atlantic region.”
The researchers have found that NAO variations cause large differences in seasonal precipitation totals compared to NAO-neutral conditions, an effect increased with altitude - what is known as “double orographic enhancement.” For NAO conditions since 1825, this gives a maximum range of 150 per cent in precipitation totals at the wettest upland location compared to NAO-neutral conditions.
The team have shown in autumn, winter, and spring, there is a strong positive relationship between upland precipitation and NAO, which is not seen at low altitude except on northwest coasts. In summer, significant negative relationships are evident in the English lowlands. These precipitation patterns directly translate to seasonal run-off.
The study concluded that on an interannual timescale, ocean-atmosphere drivers like NAO and El Niño-Southern Oscillation will have more immediate effect on extreme precipitation. How these drivers behave in the longer term in response to anthropogenic climate change is an issue of some significance and further research is needed in this area.