Decadal relationship between European blocking and the North Atlantic Oscillation during 1978-2011. Part I: Atlantic conditions

This study examines their relationship from a decadal change perspective. Since the winter-mean NAO index is defined as a time average of instantaneous NAO indices over the whole winter, it is unclear how the activity of European blocking (EB) events can be related to the variation of the positive mean NAO index.

Here, this question is examined by dividing the winter period 1978-2011 into two decadal epochs: 1978-94 (P1) with an increasing and high NAO index and 1995-2011 (P2) with a decreasing and low NAO index. Using atmospheric reanalysis data, it is shown that there are more intense and persistent EB events in eastern Europe during P1 than during P2, while the opposite is true for western Europe. It is further shown that there are more NAO⁺ (NAO⁻) events during P1 (P2).