| Abstract: An analysis of 11 years of European Centre for Medium-Range
Weather Forecasts data focuses primarily on the vertically averaged high-frequency
transients. The conclusions are discussed in the context of (a) the winter
storm track, (b) monthly variability, and (c) interannual variability.
(a) Winter storm track: Results show that the pattern of the forcing by
the high-frequency eddies along the storm track is highly correlated with
the stationary circulation, and the forcing itself is primarily responsible
for the location of the trough-ridge system associated with the stationary
flow. The results also clarify the role of wind component covariance terms
u'v' and (v'2 - u'2) in the column-averaged vorticity forcing.
The simpler term u'v' has the well-known effect of intensifying
the anticyclonic (cyclonic) tendencies on the southern (northern side of
the jet, thereby producing an increase in the barotropic component of the
zonal jet. The (v'2 - u'2) term displays a quadrupole pattern, which
is also approximately in phase with the trough-ridge system associated
with the stationary flow. (b) Monthly variability: Eddy activity has been
shown to possess a seasonal life cycle, increasing during the early fall
and reaching a maximum around the month of November, then decaying for
most of the winter months. Month-to-month variations in eddy activity over
the Pacific Ocean show that energy levels increase up through November,
decreasing thereafter, at the same time the trough-ridge circulation pattern
is intensifying. By December, baroclinicity in the western Pacific has
increased substantially, and low-level eddies are found to break by the
middle of the ocean. Upper-level eddies start to break well before reaching
the west coast of North America, resulting in a displacement of the maximum
in (v'2 - u'2) westward from its November position and increasing
the trough-ridge forcing by the high-frequency eddies. (c) interannual
variability: Wintertime eddy kinetic energy is seen to extend further eastward
through the Pacific Ocean during the warm phase but displays an abrupt
termination during the cold phase. Anomalies in the eddy transient forcing
tend to be quite similar to that of the Pacific-North American pattern
itself. The extension of the storm track during the warm phase resembles
that of fall conditions and is present in the winter season because the
source of low-level baroclinicity is extended well into the eastern Pacific
for this El Niño-Southern Oscillation phase. |