Do European blocking events precede North Atlantic Oscillation events?

Abstract

Using a two-dimensional blocking index, the cause and effect relationship between European blocking (EB) events and North Atlantic Oscillation (NAO) events is investigated. It is shown that the EB event frequency is enhanced over Northern (Southern) Europe for negative (positive) phases of the NAO. Enhanced EB events over Northern Europe precede the establishment of negative phase NAO (NAO−) events, while the enhanced frequency of EB events over Southern Europe lags positive phase NAO (NAO+) events. The physical explanation for why enhanced EB events over Northern (Southern) Europe lead (lag) NAO− (NAO+) events is also provided. It is found that the lead–lag relationship between EB events in different regions and the phase of NAO events can be explained in terms of the different latitudinal distribution of zonal wind associated with the different phases of NAO events. For NAO+ events, the self-maintained eastward displacement of intensified midlatitude positive height anomalies owing to the intensified zonal wind can enhance the frequency of EB events over Southern Europe, thus supporting a standpoint that EB events over Southern Europe lag NAO+ events. Over Northern Europe, EB events lead NAO− events because NAO- events arise from the self-maintained westward migration of intensified blocking anticyclones due to the weakened zonal wind in higher latitudes.