Blocking variability and linkages with teleconnections

The authors further develop the index of Pelly and Hoskins (2003), which identified blocking as the result of a Rossby wave-breaking. The aim is to describe the wave-breaking characteristics, hence it is possible to classify blocking into different types according to its morphological characteristics. This is obtained via two new indices, which are called respectively the Direction of Breaking (DB) index and Relative Intensity (RI) index. In particular, these are calculated in a 2-dimensional form (i.e., latitude vs longitude), which is very useful to identify blocking at higher latitudes.

The application of such a methodology to the winter Northern Hemisphere (ERA-40 reanalysis) allows to differentiate between high-latitude, cyclonic blocking, typically occurring over the oceans, and mid-latitude, anticyclonic blocking, typically occurring over Europe and Asia. An opposite behaviour is observed during summer, with cyclonic and anticyclonic blocking mainly present over land and over the oceans, respectively. Blocking is further subdivided thanks to the RI index and new blocking types are identified, these are associated with some of the prominent teleconnection modes such as the North Atlantic Oscillation, The West Pacific and Pacific-North American pattern.