Dynamics of blocked weather regimes in the Atlantic-European region: a combined PV and local wave activity approach

Atmospheric blocking events are high-pressure weather events which block the mid-latitude westerly flow and can last from days to weeks. Owing to their persistence they can lead to extreme surface weather in the affected regions. Until now the largest forecast busts are associated with the onset of atmospheric blocking due to their complex dynamical nature. 

In this work we investigate atmospheric blocking dynamics through the lens of four year-round blocked weather regimes in the Atlantic-European region. It is of our interest to compare the mid-latitude dynamics of these weather regimes, namely Atlantic Ridge, Greenland Blocking, Scandinavian Blocking and European Blocking by a PV framework and a local wave activity framework. The PV framework is applied on isentropes intersecting the tropopause. The PV dynamics quantify downstream dispersion of wave energy, PV eddy fluxes, baroclinic growth, and divergent outflow. The latter is assumed to communicate the impact of mid-tropospheric latent heat release to tropopause height. We complement the diagnostic by the finite-amplitude local wave activity budget.

We discuss the different dynamics for the four weather regimes by time-lagged composites from the last 40 years in the ERA5-reanalysis and -reforecast datasets. Our analysis reveals among others an intriguing variability in formation: Scandinavian blocking appears as part of an Rossby wave packet (RWP) propagating into northern Asia, whereas European blocking terminates the RWP.  We further find that moist-baroclinic growth has an important but mostly ‘’indirect’ effect on the formation of the blocked regimes: ”indirect” in the sense that the associated ridge amplification occurs upstream of the region over that the regime is subsequently established.