Saltzman B. (editor) Anomalous Atmospheric Flows and Blocking
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54
Phbor)
700
50011
RANDALL
M.
DOLE
I
I
I
I
40
50
60
70
Lot.
(ON)
roool
30
Ub
(130'E
,
DAY-5
1
(m/sec)
FIG.
1
1.
Cross
section
of
the
geostrophic
Wind anomalies
(in
meters)
at
longitude
130"E
at
day
-
5.
displays the vertical structure of the geostrophic zonal wind anomalies over
this region
5
days before development. The anomalous vertical shear along
this cross-section suggests that, through most of the troposphere, there are
pronounced cold temperature anomalies to the south of about
40"
N,
with
relatively warm anomalies further north. The structure is also indicative of
an upward increase in positive vorticity, with maximum positive vorticity
anomalies located near the tropopause at about
40"N.
In the presence of
mean westerly winds, we anticipate through quasi-geostrophic theory (e.g.,
Holton,
1979)
that this structure should also
be
associated with anomalous
ascent and low-level convergence downstream of the vorticity maxima, with
an enhanced likelihood for strong cyclogenesis in this region.
6.
SYNOPTIC CHARACTERISTICS
OF
DEVELOPMENT
The analyses
so
far have illustrated the evolution of the height anomaly
fields. We will now briefly examine the synoptic characteristics
of
the evolu-
tion
of
the height and thermal patterns associated with the persistent anom-
aly development. For brevity, we will focus mainly on the development
of
LIFE
CYCLES
OF
PERSISTENT ANOMALIES
55
the Pacific negative cases, which are often associated with blocking over the
eastern Pacific and western North America. All of the analyses to be dis-
cussed subsequently are derived from the unfiltered series from the
1
1
winter
seasons.
Figure 12 displays the evolution of the composite 500-mbar height pat-
terns for the PAC negative cases at 2-day intervals from
5
days before onset
until
5
days following onset. Several aspects of this evolution are familiar to
synoptic meteorologists. At day
-
5,
the Aleutian low is abnormally weak,
with a center displaced
far
to the west of its climatological mean position. At
this time, a weak ridge is centered over the central North Pacific. As will be
illustrated later, this pattern resembles the final stages of the PAC positive
pattern.
Between day
-
3
and day
-
1, the jet initially confined to the far western
Pacific intensifies and begins to extend eastward over the western Pacific,
while the ridge over the central Pacific weakens, with the remnants
of
the
anticyclonic center drifting northwestward toward Siberia. From day
+
1
onward, a major upper level low center becomes established over the Aleu-
tians, with downstream amplification ofboth the ridge near the west coast of
North America and, subsequently, the trough over eastern North America
clearly evident. By day
+
5,
the highly amplified 500-mbar height pattern
characteristic of the time-average structure of the PAC negative cases (Dole,
1986) is well established.
Figure 13 displays the corresponding evolution of the 1000-mbar height
fields. We see that several days before development, there are two distinct
lows with centers over the western and eastern North Pacific, with a weak
high-pressure ridge over the Aleutians. This tendency for a split of the Aleu-
tian low center into two centers along the continental margins is typical
of
the Pacific positive pattern (Dole, 1986), and reflects
a
tendency for the
Aleutian low to vary between a single intense center located over the central
North Pacific in the negative cases and two centers located in the northwest
and northeast Pacific (Dole, 1986) for the positive cases. Similarly, persistent
anomaly cases in the Atlantic are typically associated with a tendency for
either a single abnormally intense Icelandic low centered in the central
North Atlantic in the negative anomaly cases or a split into two relatively
weak centers in the northwest and northeast Atlantic with a ridge over the
central Atlantic in the positive cases (Dole, 1986).
Between day
-
3
and
day
-
1,
the low centered to the northeast of Japan
propagates eastward and intensifies,
so
that, by day
+
1, a single major center
is located over the central North Pacific. This center continues to intensify
while remaining nearly stationary through day
+
3.
The circulation around
the low at this time essentially spans the entire North Pacific north
of
35' N.
The relationship between the surface low and the evolving thermal struc-
I
FIG.
12.
Composite time evolution
ofthe
unfiltered 500-mbar height fields(in meters)
for
the
PAC negative anomaly
cases
for
days
(a)
-
5,
(b)
-
3,
(c)
-
1,
(d)
+
1,
(e)
+
3,
and
(f)
+
5.
56
d
a
b
e
C
I
FIG.
13. Composite time evolution
of the
unfiltered 1OOO-mbar height fields (in meters)
for
the
PAC
negative anomaly
cases
for
days
(a)
-5,
(b)
-3,
(c)
-
1,
(d)
4-
1,
(e)
+3,
and
(f)
+5.
57
58
RANDALL
M.
DOLE
ture (as reflected in the
IOOO-
to 300-mbar mean thickness fields) during the
period
of
most rapid intensification is displayed in Fig.
14.
In
many respects,
the gross aspects
of
the evolution resemble that of an amplifying baroclinic
wave, although on a scale that appears considerably larger than for typical
baroclinic developments. The low to the northeast
of
Japan on day
-3
is
developing
on
the cyclonic shear side
of
the jet in a region
of
pronounced
baroclinicity.
On
its western flank, there
is
enhanced cold advection
off
the
Asian
continent over the western North Pacific
on
the north side
of
the jet
a
C
b
d
FIG.
14.
Composite 1000-mbar heights(so1id lines, meters)and
1000-
to
300-mbarthickness
(dashedlines,dekameters)forthePACnegativeanomalyfordays(a)-3,
(b)-
1,
(c)+
1,
and(d)
+
3.
LIFE CYCLES
OF
PERSISTENT ANOMALIES
59
axis, somewhat reminiscent of an eastern Asian “cold surge” (e.g., Joung and
Hitchman, 1982); the associated deformation field tends to support increas-
ing temperature gradients over this region near the latitude of the intensify-
ing jet.
As the developing low propagates eastward, the region ofcold advection to
the north of the jet axis extends eastward to the central Pacific, again tending
to
increase temperature gradients over this region centered at latitudes near
40”
N.
By day
+
1, the developing southerly flow to the east of the major low
center produces strong warm advection over the northeastern North Pacific,
northwestern North America, and Alaska under and to the west of the
developing upper level ridge. The thermal advections at day
+
1
and day
+
3
tend to bring relatively cold air southward and eastward from the Asian
continent and relatively warm air northward and westward over the north-
east Pacific and Alaska; we might anticipate that heat flux statistics obtained
through this period would show considerable correlations in the zonal
(u’T’)
as well as meridional(v’7”) fluxes. Following day
+
3 (not shown), there are
reduced advections as the low assumes a roughly cold-core structure with
little tilt with height.
7.
BREAKDOWN
The previous analyses suggest that persistent anomaly patterns often de-
velop rapidly, with corresponding positive and negative cases displaying
several parallel features. We now briefly examine the subsequent break-
downs of the patterns for further evidence
of systematic behaviors. Compos-
ites for this section are constructed relative to the time when the anomaly
first falls below the threshold value at the key point. The unfiltered analyses
provided no obvious indication that small-scale, mobile disturbances were
systematically involved in the breakdowns; for this reason, only low-pass
composites are presented.
Figures
15
and
16
present, respectively, composite low-pass analyses for
the PAC positive and negative cases at 2-day intervals from
4
days prior to
breakdown (day
-
4)
to
6
days following breakdown (day
+
6).
We see that
during breakdown the evolutions also display a number of striking similar-
ities. Up until day -2, the patterns strongly resemble the PAC composite
patterns described previously. Breakdown then proceeds rapidly. By
day
0,
the main centers have moved northwestward to the Bering Sea and have
weakened considerably. These features then remain nearly quasi-stationary
and continue to decay. Anomalies over the key region are not significantly
different from zero beyond day
+
2.
Differences between the composite maps immediately preceding break-
FIG.
1
5.
Composite
mean time evolution
of
low-pass
filtered
500-mbar
height
anomalies
(in
meters) obtained
from
15
PAC
positive
anomaly
cases
for
days
(a)
-
4,
(b)
-
2,
(c)
0,
(d)
+
2,
(e)
+
4,
and
(f)
+
6, relative tothe breakdown time.
Areas
where
the composite mean anomaliesare
greater
or
less
than
zero
at
varying
confidence levels
are
shown
for
the
same
times
in a’-f’.
Dashed
lines
indicate negative anomaly values.
60
3
FIG.
15d-f'.
61
FIG.
16.
As
in
Fig.
15,
for
the composite mean time evolution
of
low-pass
filtered 500-mbar
height anomalies
(in
meters)
obtained
from
I3
PAC
negative anomaly
cases
for
days
(a)
-
4,
(b) -2,
(c)
0,
(d) +2,
(e)
+4,
and
(f)
+6, relative to
the
breakdown time.
62
3
FIG.
16d-f’.
63