Saltzman B. (editor) Anomalous Atmospheric Flows and Blocking
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FOREWORD
It has been our custom occasionally to devote a single volume ofAdvances
in
Geophysics to a special topic of timely interest. In the present volume we
consider the rapidly growing subject of low-frequency weather variability.
This variability is exemplified by the persistence of certain large-scale
“anomalous” features of the atmospheric flow that are often related to the
so-called blocking weather patterns. It is widely acknowledged that if an
extension of the range of reliable weather prediction is to be achieved it will
depend on an understanding of the behavior of these persistent anomalous
flows.
A
notable feature of this collection of reviews and original articles
is its broad coverage of the observational, theoretical, and numerical-
experimental aspects of this subject by many of the leading investigators in
this field. Preliminary versions of these articles were presented at a workshop
meeting held in Rome in the summer of
1984
under the auspices of IBM.
The articles give an excellent overview of the subject, clearly elucidating
the full scope of the problems encountered in trying to understand low-fre-
quency weather phenomena. Perhaps the most significant new results to
emerge are the observational confirmations, reported here for the first time,
of the existence of a bimodality in large-scale wave structure of the atmo-
spheric flow. The inference from this bimodality is that an important long-
wave instability must
be
present that is probably associated with the nonho-
mogeneous topographic and/or thermal properties of the lower boundary. In
a
sense, these findings represent a triumph of a rapidly emerging new para-
digm of dynamical meteorology, pioneered by
E.
N.
Lorenz, based on exam-
ining the finite-amplitude nonlinear properties of the atmosphere from the
viewpoint of low-order dynamical systems analysis. Although the first (bar-
otropic) theoretical scenarios proposed along this line do not seem applicable
to the particular form of bimodality revealed by the data, some of the theo-
retical ideas proposed in this volume offer much promise that a successful
and illuminating low-order theory is in the offing. In spite of some continu-
ing reservations concerning these ideas, a few even voiced in this volume, it
appears that a major new opening has been achieved that, one hopes, will
lead to measurable improvement in longer range atmospheric predictability.
BARRY
SALTZMAN
xi
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PREFACE
In the last few years a good deal of scientific attention and work has been
devoted to trying to understand the physical causes and mechanism of
anomalous circulation in the atmosphere. As often happens in meteorology,
most of the effort has been addressed to posing the right questions to be
answered. At the end of August
1984,
a workshop was organized in Rome,
bringing together most of the leading experts in the field, with the purpose of
summarizing and clarifying the scientific results obtained
thus
far. The cen-
tral question for the workshop
was
as
follows: does there exist
a
possible
physically sound definition
of
anomalous circulation or blocking, and if
so
which processes are connected to the development and decay of anomalous
flows
so
defined?
Looking back at the scientific literature it is easy to recognize that the
starting point of recent interest in this subject was the winter of
1976,
when
extreme cold weather conditions were experienced in North America. Since
that year a number of papers were devoted to describing or simulating
blocking events in the atmosphere.
A
challenging problem was
also
posed
concerning the degree to which medium-range weather forecasting in gen-
eral could improve qualitatively and quantitatively,
by
improving numerical
forecasting of blocking events.
One
of the outcomes of the Rome meeting has been to restate the problem
in terms of a number of well-defined scientific questions to be answered.
From this point of view, this book represents the contribution by all the
speakers of the workshop with the common intention of underlining the
scientific strategy for the future. Here we shall briefly synthesize the major
results.
It has been known since the work of Charney
(1 947)
and Eady
(1 949)
that
the driving engine for the atmospheric motion
is
the baroclinic instability
acting on a scale
L
of about
4000
km. The discovery of this process had made
possible the development
of
operational numerical weather forecasts in
which cyclogenetic processes could be predicted. The characteristic range of
validity of numerical forecasts at present is estimated to be of the order of
4
-
5
days. This characteristic time corresponds quite well to the time scale
L/U
where U is the zonal wind speed typically of the order
of
10
m sec-I.
Simmons’ article offers a systematic review of the
ECMW
operational
model used for the last few years. It is pointed out that the loss
of predictabil-
ity in this time range is mainly due to inability to predict ultralong waves of
about
10,000
km. If we assume that a different physical process is acting on
the very large scale
of
atmospheric circulation, we could guess that the range
...
XI11
xiv
PREFACE
of numerical weather prediction, at least for the slow component of the
system, can
be
extended to time scale of the order of
10,000
km/U, namely,
10-
15
days. This speculation, underlying most of the meeting discussion, is
directly linked with the observational studies presented by many authors
(Dole, Schilling, Sutera, Hansen). In particular Dole and Schilling, analyz-
ing Pacific and Atlantic blocking events, found that the characteristic time
scale for such anomalous flows is of the order of
10
days. Moreover, the
results presented by Egger
et
al.
and Sutera show that the probability distri-
bution of the duration
in
time of anomalies is with good accuracy a Poisson
distribution with average time of about
10
days. During the workshop dis-
cussion it was clarified that a Poisson distribution is not the consequence of
an absence of a characteristic time scale but, on the contrary, it is indicative
that an instability mechanism is acting on the system. In agreement with this
are the results of Frederiksen, who studied the set of normal mode instabili-
ties acting on the climatological state. Stationary waves dominated the
growing unstable field after
5
days, with a pattern resembling North Pacific
and Atlantic anomalies. Assuming that an instability different from ordinary
baroclinic instability
is
acting on the ultralong planetary waves in middle
latitude circulation, we are faced with the question whether this process is
triggered by changes in the boundary conditions or in the forcing terms (such
as tropical forcing or sea surface temperature anomalies) or, alternatively, is
mainly due to the internal variability ofthe atmospheric system. Kalnay and
Mo’s
article addresses directly this question for the January 1979 Southern
Hemisphere anomaly using the general circulation model (GCM) of GLAS.
In that case tropical forcing seems to
be
particularly important
to
explain
both the duration and the location of the anomaly. ShuMa discusses explic-
itly the Northern Hemisphere response to an “El Nifio” year by GCM
simulations. The correlations between
sea
surface temperature (SST) anom-
alies and blocking events is indicative of a nonnegligible response of North-
ern Hemisphere circulation to tropical forcing. It is in general very difficult
to use
GCMs
to distinguish between internal and external variability
of
atmospheric circulations. In the article by Mechoso
et
al.
the effects of high
and low spatial resolutions (both vertical and horizontal) on the simulation
of Northern Hemisphere anomalies using the UCLA GCM model are dis-
cussed. Special attention is focused on the ability to forecast stratospheric
sudden warming. Resolution problems
for
the stationary forcing due to
orography are also discussed in Tibaldi’s article. The numerical experiments
performed by Tibaldi show that systematic corrections of the zonal
flow
probably have the most important effect in improving the predicted behav-
ior of the ultralong planetary waves. By this brief discussion of GCM simula-
tions we conclude that the representation of the anomalous behavior in
numerical models is not yet perfected; much more work is therefore needed.
Nevertheless, as
GCM
simulations are the analogs in meteorology of labora-
PREFACE
xv
tory experiments in physics, we can expect a systematic use of GCMs in the
future to test different theories proposed to explain anomaly events.
The question of the relative importance of the external forcing and the
internal variability for the atmospheric circulation is discussed theoretically
in several articles in this book. Three main different aspects are studied,
namely, the theory
of
multiple equilibria, the role of synoptic forcing con-
nected with the regional character ofblocking events, and the possible effects
of
ultralong two-dimensional traveling Rossby waves. The introductory ar-
ticle by Wiin-Nielsen gives an extensive review of the present state of
the art in both theoretical and observational studies. The theory
of
multiple
equilibria was independently proposed in
1979
by Charney and DeVore and
Wiin-Nielsen. The basic idea of the theory is to study the coarse-grained
structure of the phase space for the atmospheric
“slow”
variables, namely,
the ultralong planetary waves. In its first formulation, the theory raised many
doubts because of its apparent inconsistency with the observed atmospheric
flows. However, the effect of nonlinear dynamics and a reinterpretation of
the form drag equation are able to improve significantly the quality of the
theory, as discussed in Speranza’s contribution. A systematic check of multi-
ple equilibria theory is presented by Sutera, who estimates the probability
distribution of the amplitude of ultralong planetary waves (wave numbers 2,
3,4)
in the Northern Hemisphere winter circulation. An extensive analysis
of a 20-year record gives strong evidence
of
a bimodal distribution as pre-
dicted by the theory of multiple equilibria. The persistence in time around
each equilibrium has a Poisson distribution in agreement with the above-
mentioned observations. The energetics of the two modes is presented in
Hansen’s article, in which the relevance of the stationary waves in both
modes and the baroclinic source
of
energy in maintaining the high-ampli-
tude mode are discussed.
The kind
of
instability assumed in the theory of multiple equilibria has a
global nature, typically involving wave number
3
in the atmosphere. Super-
imposed on this scenario there is the possibility that some of the anomalous
flows have a local character. It is implied that synoptic scale forcing plays a
leading role in this case. The article by Egger
et
al.
discusses such a role
of
synoptic scale forcing, as computed from observational data, on planetary
waves using the barotropic vorticity equation.
A
good resemblance is found
between computed and observed blocking events. Shutts describes the role of
eddy vorticity forcing by which some Atlantic blockings are initiated and
maintained. Finally Pierrehumbert discusses the concept of local multiple
equilibria arising from absolute instability in both barotropic and baroclinic
flows. It is worthwhile to remark that local theories of blocking are neither
inconsistent nor at variance with global theories even if
a
unified approach
has never been undertaken.
At variance with the interpretation of anomalous events
as
being the
xvi
PREFACE
results of global
or
local
dynamical instabilities, Lindzen proposed a theory
depending on the behavior of two-dimensional traveling neutral Rossby
waves. The theory is supported by observational analysis of
FGGE
data over
the whole hemisphere, using Hough’s modes. The characteristic period of
such ultralong waves
is
about
20
days, and adjustment processes are sup
posed to
be
the triggering mechanism of these waves. It is pointed out that
interference between waves of different scale length can produce patterns
which synoptically are similar to anomaly flows.
We are not able at this stage
to
distinguish between the different interpre-
tations discussed at the meeting, and future work
is
needed to solve some
of
the more puzzling problems. We feel confident, however, that we are point-
ing in the right direction and making fundamental strides toward improved
understanding of large-scale anomalous states of the atmosphere with the
hope
that the range of weather forecasting can
be
extended.
ROBERTO BENZI
AKSEL
C.
WIIN-NIELSEN
Part
I
HISTORICAL INTRODUCTION
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GLOBAL SCALE CIRCULATIONS-A REVIEW
A. C. WIIN-NIELSEN
Danish Meteorological Institute
DK-2100 Copenhagen
Denmark
1.
INTRODUCTION
It has been a pleasure for me that the organizers of the workshop on Global
Scale Anomalous Circulation in the Atmosphere and Blocking (Rome, Italy,
1984)
asked me to serve as a chairman
of
that interesting event. At the same
time they were kind in inviting me to present an introductory lecture at the
opening of the workshop. I selected to prepare a review of our understanding
of the global circulation, and it was of course the most recent events which
were the most interesting. In this regard I faced a dilemma. It was my
privilege to know some of the studies which would be presented later on in
the program, but I tried as far as possible to avoid stealing thunder from
subsequent presentations.
The IBM Scientific Center in Rome, Italy, deserves great praise for its
support of the workshop. On behalf of all the participants I should like
to
express appreciation to IBM for inviting us to the eternal city of Rome to
discuss some of the most exciting meteorological problems facing our com-
munity. It is most welcome that a private corporation shows such an interest
in our meteorological field. We want naturally to include our colleague, Dr.
Roberto Benzi, who took all of the trouble in taking care of the scientific
organization of the workshop.
My main task was to try to set the stage with the introductory lecture and
otherwise to help create a constructive discussion of the problems which had
been selected as main topics for the workshop. What follows herein
is
a
version of the lecture presented at the workshop.
2.
THE DISTANT PAST
When I was a young student at the newly created International Meteoro-
logical Institute in Stockholm and worked under Professor Carl-Gustaf
Rossby, and we had the problem ofbuilding a small library, it was the general
story that Rossby should have said that you can safely disregard all literature
3
Copyright
0
1986
by Academic Press, Inc.
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rights
of
reproduction in any
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