Multiannual Atmospheric Controls on Drought Stationarity: What the NAO can tell us about past behaviours and future climate change projections?

Atmospheric variability in the North Atlantic region is known to modulate hydrometeorological variables across Europe. In this context, oscillatory systems, such as the NAO, may be used to indicate future water resource behaviours, such as hydrological droughts. Existing hydroclimate studies have identified a sensitivity of certain water resources to multiannual periodicities in systems such as the NAO and have highlighted that these long-term behaviours may be valuable to existing drought forecasting systems; for instance, by indicating multi-year periods of increased drought risk. However, the importance of multiannual NAO periodicities for driving water resource behaviour, and the feasibility of this relationship for indicating future droughts, has yet to be assessed in the context of known non-stationarities that are internal to the NAO and its influence on European meteorological processes. Here, we explore the role of NAO periodicities in defining water resource and drought behaviours over the past 90 years using a large dataset of 136 groundwater level records and 767 streamflow gauges in the UK. We identify significant relationships between the NAO and a calculated index of wide-spread water resource drought and find several abrupt shifts in drought frequency driven by non-stationarities in multiannual NAO behaviour. This includes a 7.5-year periodicity that has predominated water resource behaviour (and extremes) since the 1970s but has been weakening over recent years, suggesting a new shift in drought frequency may soon impact water resources. Furthermore, we show that the degree to which these periodicities have influenced recorded water resource anomalies is comparable to the projected effects of a worst-case climate change scenario. We discuss the potential origins for these modes of non-stationarity and their implications for existing water resource forecasting and projection systems, as well as the utility of these periodic behaviours as an indicator of future water resource drought in Europe.