Comparison of Different Blocking Indices and Analysis of Underlying Dynamics and Synoptic Situations

Atmospheric blockings are among the most frequently studied weather patterns. They not only cause extreme weather events and associated losses but also significantly influence general weather variability. A deeper understanding and more reliable prediction of these phenomena would therefore be of great value to both the scientific community and the public.

However, various definitions and identification methods for atmospheric blockings are currently applied, which can lead to inconsistent results and confusion. While all approaches are valid and justified, the precise differences between these definitions and their implications often remain unclear.

This study examines two widely used blocking algorithms: the Anomaly Index, which is based on vertically integrated potential vorticity (PV) anomalies (see Schwierz et al., 2004), and the Absolute Index, which identifies blockings through the reversal of the 500 hPa geopotential height gradient (see Davini et al., 2012).

The two indices differ substantially already with regard to climatological blocking frequencies: the Anomaly Index primarily detects blockings south of Greenland/Iceland, whereas the Absolute Index identifies a local maximum over southern Scandinavia. Our analyses have not indicated any systematic longitudinal, latitudinal, or temporal offset between the events captured by the two indices. A synoptic investigation suggests that the algorithms detect different types of blockings: the Absolute Index requires a Rossby wave breaking for identification, while the Anomaly Index considers an extended ridge sufficient.

Further research aims to clarify the differences in dynamical and synoptic conditions between these and other algorithms.