Blocking (1) and cut-off anti-cyclone (2)

Blocking and cut-off anticyclones can form in the final phase of anticyclonic wave-breaking (see filaments). Blocking is characterized by large and often circular intrusions of tropospheric low potential vorticity into the ‘red sea’ of stratospheric high potential vorticity. Such intrusion ends as a separated anti-cylcone moving slowly through the lower stratosphere before merging again with the troposphere. The processes of separation and merging are very abrupt and difficult to predict. Both processes are strong perturbations of the PV front.

A blocking event is a long living stable weather regime which, in summer, can be connected to droughts and heat waves.