| Abstract: Time evolutions of prominent blocking flow configurations
over the North Pacific and Europe are compared based upon composites for
the 30 strongest events observed during 27 recent winter seasons. Fluctuations
associated with synoptic-scale migratory eddies have been filtered out
before the compositing. A quasi-stationary wave train across the Atlantic
is evident during the blocking amplification over Europe, while no counterpart
is found to the west of the amplifying blocking over the North Pacific.
Correlation between the tropopause-level potential vorticity (PV) and meridional
wind velocity associated with the amplifying blocking is found to be negative
over Europe in association with the anticyclonic evolution of the low-PV
center, but it is almost zero over the North Pacific. Feedback from the
synoptic-scale eddies, as evaluated in the form of 250-mb geopotential
height tendency due to the eddy vorticity flux convergence, accounts for
more than 75% of the observed amplification for the Pacific blocking and
less than 45% for the European blocking. This difference is highlighted
in two types of "contour advection with surgery" experiments.
In one of them PV contours observed four days before the peak blocking
time were advected by composite time series of the low-pass-filtered observational
wind, and in the other experiment they were advected by the low-pass-filtered
wind from which the transient eddy feedback evaluated as above had been
removed at every time step. Hence, the latter data should be dominated
by low-frequency dynamics. For the European blocking both experiments can
reproduce the anticyclonic evolution of low-PV air within a blocking ridge
as observed. For the Pacific blocking, in contrast, the observed intrusion
of low-PV air into the higher latitudes cannot be reproduced without the
transient feedback. Furthermore, in a barotropic model initialized with
the composite 250-mb flow observed three days before the peak time, a simulated
blocking development over the North Pacific is more sensitive to the insertion
of the observed transient feedback than that over Europe. These results
suggest that the incoming wave activity flux associated with a quasi-stationary
Rossby wave train is of primary importance in the blocking formation over
Europe, whereas the forcing by the synoptic-scale transients is indispensable
to that over the North Pacific. |