ROTATIONAL EVOLUTION OF POTENTIAL VORTICITY ASSOCIATED WITH A STRONG BLOCKING FLOW CONFIGURATION OVER EUROPE

Time evolution of a prominent tropospheric blocking flow configuration over Europe is examined based upon composites for the 15 strongest events observed during the last 27 winter seasons, from which the fluctuations associated with synoptic-scale eddies have been filtered out. The blocking, initiated in the exit region of the Atlantic jet stream, forms into a domain of the very weak westerlies. The tropopause-level potential vorticity (PV) exhibits a distinct, clockwise evolution within this domain, so that the negative correlation between PV and the meridional wind velocity associated with the amplifying blocking becomes positive as it decays, which signifies the convergence of the wave activity density flux and its subsequent divergence. Indeed, a quasi-stationary wavetrain across the Atlantic is evident during the blocking amplification, and then another wavetrain becomes apparent to the east of the decaying ridge. It appears that the wave activity propagating across the Atlantic is obstructed and accumulated in the weak-westerly domain associated with the developing blocking ridge until it is saturated, followed by the release of the activity to the east. The result suggests that the low-frequency dynamics of the atmospheric flow plays an important role in the formation of a certain type of blocking.