North Atlantic Oscillation amplifies orographic precipitation and river flow in upland Britain

Large-scale climatic variability in the North Atlantic region modulates seasonal rainfall and river flow across the British Isles. We show how the North Atlantic Oscillation (NAO) dramatically increases orographic enhancement of upland precipitation. NAO variations cause large differences in seasonal precipitation totals compared to NAO-neutral conditions, an effect amplified with altitude-what we term "double orographic enhancement.'' For NAO conditions since 1825, this gives a maximum range of 150% in precipitation totals at the wettest (upland) location compared to NAO-neutral conditions. In autumn, winter, and spring, there is a strong positive relationship between upland precipitation and NAO; this 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 runoff. Our findings show that the hydroclimatology of rainfall and river flow in upland areas is closely coupled to the strength of atmospheric circulation, an effect which strengthens with increasing altitude. 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 NAO is highly negative.