Precipitation and the North Atlantic Oscillation: A study of climatic variability in northern England

Variations in precipitation in Yorkshire, UK, are linked to large-scale atmospheric circulation using the North Atlantic Oscillation (NAO) index, objective Lamb weather types, and long-term seasonal and annual precipitation indices at seven sites. Since the 1960s, a significant decrease in summer precipitation has been observed across the region, with increasing winter precipitation at western locations since 1970. At western sites, precipitation is highly correlated to the NAO, with positive and negative winter-NAO indices producing anomalies from the long-term winter precipitation average of 112% and 77% respectively. The recent winter half-year precipitation increase is concentrated in March and related to a large increase in monthly NAO and the increased occurrence of westerly weather types. The decrease in summer precipitation, however, is caused by both declining receipt from westerly weather types and their decreased occurrence. This is significantly correlated to a decreasing summer index for the NAO. The NAO, the frequency of weather types and precipitation totals are all intimately linked on an annual and monthly time scale. Strong connections between winter precipitation and the NAO may prove very important, particularly in western Yorkshire where most surface water supplies are located. If the NAO can be predicted using sea-surface temperature, it may allow the estimation of winter reservoir replenishment in western Yorkshire a few months in advance. This may provide an important predictor of future hydrological drought episodes and facilitate forward planning and management of water resource systems located in climatologically similar regions of the UK. Copyright (C) 2002 Royal Meteorological Society.