Atmospheric Blocking

Fig. 1 Cross-section of the January 14, 1981 18 UTC, blocking episode over the Atlantic ocean. The colors represent PV anomalies (blue=negative, red=positive)

Atmospheric blocking events are sustained, vertically coherent, and quasi-stationary synoptic-scale high-pressure systems in the extratropics whose amplitude is such that they disrupt the prevailing westerly circumpolar flow. Lifetimes last from several days up to weeks so that blocking can therefore significantly determine the in-situ and downstream weather, and hence monthly circulation index values.

Although several different interpretations of blocking indices exists (e.g. Tibaldi and Molteni, 1990, Sausen et al., 1995, Pelly and Hoskins, 2003), a dynamical interpretaion of the phenomenon remains still a big challenge. In principle the formulation of a dynamically-based index should be consistent with the phenomenon's salient features including their spatial-scale and structure, amplitude, life cycle and duration, movement and geographical location. Such an index would yield a comprehensive blocking climatology incorporating the forementioned features, and could in turn shed light on the instigation, maintenance, break-down, and categorization of blocks.

Here we propose a two-dimensional blocking index based upon the potential vorticity field at tropopause level (Schwierz et al., 2004):

APV* Index definition

• the base field is the 6-hourly three-dimensional PV anomaly field
• this field is averaged between 500-150hPa to capture the key features (anomalies) at tropopause level (Fig. 1) -> VAPV field
• a tracking routine follows strong negative VAPV anomalies which exist for more than 5 consecutive days (time persistence)