North Atlantic Oscillation drives double orographic enhancement of 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 affects 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 autumn, winter and spring, there is a strong positive relationship between upland precipitation and NAO; this is not seen at low altitude except on north-west coasts. In summer, significant negative relationships are evident in the English lowlands. These precipitation patterns directly translate to seasonal runoff. Our findings significantly revise our understanding of the hydroclimatology of rainfall and river flow in upland areas; we argue that the effects observed are of global significance, not merely a local result. Identified effects are large enough to have major implications for water resources and flood risk.