Saltzman B. (editor) Anomalous Atmospheric Flows and Blocking
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24
A.
C.
WIN-NIELSEN
1.5
DO
1.5
I
I
rn
0.2
0.4
0.6
0.8
1.0
u/uo
FIG.
10.
A
time integration
of
an initially
very
sharp and narrow
jet
leading
to
two
branches
afterabout
I
day.
cases (Fig.
10)
it is possible to reproduce the northward and southward
movement of the two maxima. Under the assumption that the splitting
of
the
jetstream appears first and the development
of
the blocking pattern
is
a
secondary effect, this theory is relevant to the phenomenon of blocking.
5.
SOME GENERAL CONSIDERATIONS
It
is
my hope that
I
succeeded in giving a reasonable introductory lecture
to the workshop on anomalous circulations including blocking. The partici-
pants undoubtedly provided many more original contributions. Before the
end
of
the lecture it seemed appropriate to discuss the state of affairs in this
important, but specialized field
1.
It
is
an important field of research because anomalous circulations are
connected with equally anomalous weather situations. The duration
of
these
anomalies is also such that the time scale reaches into the lower end
of
the
climatic time scale, which deals with the problems of long-range predictions
and interannual variation. It is perhaps here that the anomalous circulations
are particularly important from a practical point
of
view.
2.
The present studies of anomalous circulations are concerned with the
causes for their creation, maintenance, and dissipation. The physical factors
being investigated at the moment are the interactions between the atmo-
sphere and the Oceans and continents on the one hand and the mountain
effect on the other. The distinct possibility exists that the long-lastinganoma-
GLOBAL SCALE CIRCULATIONS-
A
REVIEW
25
lies cannot
be
understood without a coupled ocean
-
atmosphere model.
Further progress may depend on how successful we are in creating the proper
models for
a
two-way interaction with the oceans.
3. Even if the coupled models may not be necessary for the understanding
of
the anomalies of shorter duration, it is nevertheless of importance to know
how dependent simulations or predictions of anomalous circulations are on
the specifications of the forcing.
4.
As
you will have seen from the workshop lecture, there has been consid-
erable emphasis on the use
of
simple, although nonlinear, models in many
studies, with emphasis on multiple steady states
as
a possible explanation
of
the existence
of
anomalous circulations. Several questions arise in this re-
gard.
A
weakness in the theory is that it does not provide
an
answer to the
question of how the models (or the atmosphere) select a given steady stable
state.
Is
this an initial value problem? One may think
so,
because the selected
stable steady state could
be
determined by whether or not the initial state is
within the attractor basin of the given asymptotic state. On the other hand,
how does the atmosphere move from one attractor basin to another?
5.
A
property of the low-order, nonlinear systems is the existence of
multiple steady states, of which at least one corresponds to a stable anoma-
lous circulation.
Is
this picture, which is
so
clear for the simple models, a
good background for a study
of
the real atmosphere? Does the real atmo-
sphere possess such multiple, quasi-stationary states?
-
or are we dealing
with illusions or “barking up the wrong tree”?
6. There seems to be no doubt that the atmosphere from time to time can
operate in rather different regimes, as we can see from blocking situations or
a comparison between very anomalous and more normal winters and sum-
mers. An area
of
research for the future
is
to use our global, high-resolution
models to simulate such anomalous situations, while the object of the
GCM
studies in the past seems to have been to simulate the normal or long-term
average situation as well
as
possible.
6.
A
CONCLUDING
REMARK
This paper is written in a personal style, and it is an attempt to review only
some aspects
of
anomalous circulations. It has not been the purpose to write
a
complete review and, consequently, many important contributions from
the distant and near past remain unmentioned. With the recent greatly
increased interest in anomalous circulations it is certainly too early to at-
tempt a final account of the truly important contributions or those which, in
retrospect, looked interesting and important, but turned out to be a sidetrack
or
a
move in a wrong direction.
26
A.
C.
WIN-NIELSEN
ACKNOWLEDGMENT
I
want to thank
Ms.
Charlotte Wiin-Nielsen, who contributed to the paper and its calculations
and, furthermore, brought it into final form.
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Part
II
OBSERVATIONS
This Page Intentionally Left Blank
THE LIFE CYCLES
OF
PERSISTENT
ANOMALIES AND BLOCKING OVER THE
NORTH PACIFIC
RANDALL
M.
DOLE
Center for Meteorology and Physical Oceanography
Massachusetts Institute of Technology
Cambridge, Massachusetts
02139-4301
1.
INTRODUCTION
Recently, Dole and Gordon
(1
983)
studied the geographical variations in
the persistence characteristics of wintertime Northern Hemisphere height
anomalies. They focused particular attention on the behavior of anomalies
that persisted at a given location beyond the durations associated with syn-
optic-scale variability (“persistent anomalies”), without regard to the char-
acter of the concurrent large-scale flow patterns. They identified three key
regions characterized by relatively high numbers of persistent anomalies
(durations of
10
days or longer): the North Pacific to the south of the Aleu-
tians (PAC), the North Atlantic to the southeast of Greenland (ATL), and
from the northern Soviet Union northeastward to over the Arctic Ocean
(NSU).
Subsequently, Dole
(1
986)
described the typical time-average structures of
flow patterns occurring with persistent anomalies in these regions. He found
that, for each region, the majority of persistent anomaly cases were asso-
ciated with the amplification
of
a single basic anomaly pattern, with one
phase of the pattern frequently associated with blocking. For the PAC and
ATL regions, the primary patterns also resembled certain prominent tele-
connection patterns described by Wallace and Gutzler
(198
1)
[the “Pacific-
North American” (PNA) pattern, and the “Eastern Atlantic” (EA) pattern,
respectively]. Analyses of variance and lag autocorrelation characteristics
suggested that most of the temporal variability of the primary patterns was
contributed by low-frequency, intraseasonal fluctuations (predominantly by
periods between
10
and 90 days).
Although the previous studies described certain statistical aspects of the
temporal variability of persistent anomaly patterns, they did not examine
specific temporal relationships associated with the development and decay
of the patterns. In the present investigation, we will extend the previous
studies by attempting to identify systematic aspects of the temporal evolu-
31
Copyright
0
1986
by Academic
Press,
Inc.
AU
rights
of
reproduction
in
any
fonn
reserved.
ADVANCES
IN
GEOPHYSICS,
VOLUME
29
32
RANDALL
M.
DOLE
tion of persistent anomalies, concentrating on periods around the develop
ment and decay of the anomaly patterns. For brevity, we will focus here
mainly on a description of the evolution of the anomaly and flow patterns
associated with persistent height anomalies over the central North Pacific,
but we will also attempt to summarize systematic aspects of the evolution of
persistent anomalies in the other regions.
2. DATA
The data base for the 500-mbar height analyses is that employed pre-
viously by Dole and Gordon (1983) and Dole (1986). It consists of twice-
daily
(oo00
and 1200 GMT) National Meteorological Center (NMC) final
analyses of the Northern Hemisphere 500-mbar heights for the 14 120-day
winter seasons (beginning
I
5
November) from 1963/ 1964 through
1976/1977. The data base for the vertical structure analyses consists of the
twice-daily NMC final analyses of geopotential heights at five levels (1000,
700,500,300, and
I00
mbar) for the
1
1
120-day winter seasons (beginning
15 November) from 1965/ 1966 through 1975/ 1976. Details of the NMC
analyses are described in Dole and Gordon (1983). Missing analyses were
linearly interpolated in time.
All
analyses were performed on a
5"
by
5"
latitude- longitude grid. Data were spatially interpolated from the NMC grid
to the latitude-longitude grid
by
the procedure described in Dole and Gor-
don
(
1983).
Raw anomalies are defined
as
the departures of the analyzed values from
the corresponding long-term seasonal trend values. For the 500-mbar height
anomaly analyses, the seasonal trend time series at a point is determined by a
least-squares quadratic fit to the 14-winter mean time series for that point
(the first value of the 14-winter mean time series is the average of the 14
different
OOOO
GMT
15
November values, the second value is the average of
the 1200 GMT
15
November values, etc.). Anomalies in the vertical struc-
ture analyses are similarly defined
as
departures from the corresponding long
term
(1
1-winter) seasonal trend values.
In addition, for the 50qmbar analyses, the raw height anomalies
h'
have
been further normalized
(as
in Dole and Gordon) by a scale factor which
varies
inversely with the sine of latitude:
zh
=
[(sin 45")/sin
@]h'
This scaling is motivated by a recent study on atmospheric energy dispersion
(Hoskins
et
a).,
1977) showing that,
as
a result
of
the latitudinal variation
of
the Coriolis parameter, height field analyses provide a poor indication of the
meridional component
of
energy propagation. Hoskins
et
a!.
suggest that
LIFE CYCLES
OF
PERSISTENT ANOMALIES
33
quantities such as streamfunction or vorticity provide better indicators
of
horizontal energy propagation. Note that this normalization is similar to
that used in obtaining a geostrophic streamfunction from height
data.
3.
PROCEDURE
Most of the results to be described subsequently are determined from
composite time evolution analyses, where the compositing is performed
relative to objectively defined case onset (or termination) times. In the
present study, we will focus on the evolutions of persistent anomaly cases
that occurred in the PAC key region; characteristics of the evolutions for the
other two regions are described elsewhere (Dole, 1982). Our procedure for
identifying persistent anomaly cases follows that of Dole and Gordon
(1
983)
and
so
will only be briefly summarized here. As in Dole and Gordon, we
define the occurrence of a persistent positive (negative) anomaly at a point if
an anomaly at that point remains equal to or greater (less) than a specified
magnitude thresholdMfor at least Tdays. The total duration
D
of the case is
defined as the time from which the anomaly first crosses
M
(onset) to the
next crossing of
M
(termination). In this study, we will focus on results
obtained for one set of criteria values
[(+
100 m,
10
days) and
(-
100 my 10
days)] that appear broadly representative of a large range
of
values.
For all analyses, case onset and termination dates are determined by
applying the selection criteria to 500-mbar height anomaly time series ob-
tained at certain “key” points that were previously identified (Dole and
Gordon) as the locations of the regional maxima in the total numbers of
(positive and negative) persistent anomaly cases. In order to remove possible
effects of brief transient fluctuations associated with passing synoptic scale
disturbances on the starting (and ending) times, the time series were first
lightly low-pass filtered before applying the selection criteria. The specific
filter used removes periods of less than approximately 10 days; its character-
istics are described in detail by Blackmon
(
1976). The “key point” identified
for the PAC cases,
45”N
170”
W,
had 15 positive and 13 negative cases,
about double the numbers obtained from the unfiltered data (Dole and
Gordon, 1983). Dole (1986) provides
a
list of the case dates and durations
along with a description of the composite time-average three-dimensional
structures of these cases.
Composite time evolution fields for cases of a given region and sign (for
example, the PAC positive cases) are obtained by averaging over all of the
cases relative to the time when the low-pass filtered anomaly crosses the
threshold value at the key point (relative to the onset time for the develop-
ment analyses and termination time for the breakdown analyses); this.