Saltzman B. (editor) Anomalous Atmospheric Flows and Blocking
Подождите немного. Документ загружается.
64
RANDALL
M. DOLE
down and the corresponding maps following development may suggest clues
to the cause of the breakdown. Comparison of the maps
2
days
before
breakdown with (for example) the maps
6
days after onset, however, reveals
few striking changes. For the positive
cases,
the most noticeable differences
are the reduction in the apparent wavelength downstream from the main
center and the intensification of the negative anomaly center in the northeast
Pacific. We suspect that the latter development, which is
also
reflected in an
asymmetry between the positive and negative case mean composites dis-
cussed in Dole (1986), is at least partly the result of orographic effects: in
particular, northerly flow over the Brooks Range associated with an anoma-
lousiy strong ridge over the Aleutians
(as
is characteristic of the positive
cases) is often followed by intense cyclogenesis in the northeast Pacific
(Winston, 1955). The negative cases, in contrast, show little change in the
location or intensity of the downstream center.
Another difference evident in both the positive and negative cases when
comparing postdevelopment with prebreakdown maps is the change in sign
of the anomaly centers near Japan. Whereas at and immediately following
development anomalies in this region have the same sign
as
in the key region,
immediately preceding breakdown anomalies in this area and in the key
region are of opposite sign; however, the anomalies are of sufficiently weak
intensity that not much confidence can presently be attached to this result.
8.
DISCUSSION
The time scales for the development and decay of persistent anomalies
appear to be short compared to the time scales associated with major changes
in the boundary conditions (e.g., sea surface temperature anomalies). In
addition, the sequence of development for the PAC cases suggests that the
initial rapid growth
of
the main center may be partly associated with a
propagating, intensifying, synoptic-scale disturbance which apparently orig-
inates in midlatitudes over eastern Asia. These observations suggest that the
patterns may often, and perhaps primarily, grow and decay while the exter-
nal forcing remains nearly fixed. General circulation modeling experiments
(Lau,
198
1
)
provide further evidence that anomalies in the external forcing
are not required to produce patterns similar to those described here.
Nevertheless, Horel and Wallace
(
198
1)
have recently presented convinc-
ing observational evidence of a modest relationship between interannual
variability in tropical Pacific
sea
surface temperature anomalies (related
to
the El Niiio- Southern Oscillation phenomenon) and the Pacific- North
American teieconnection pattern (closely resembling our PAC anomaly
pattern). They have interpreted the extratropical circulation anomalies as
LIFE CYCLES
OF
PERSISTENT ANOMALIES
65
essentially a forced stationary Rossby wave response to tropical heating
anomalies centered over the central equatorial Pacific. Our results, however,
provide no clear indication for Rossby wave trains propagating northward
from this region immediately prior to development. If a link exists between
tropical sea surface temperature anomalies and the Pacific pattern, then it
may be more subtle than indicated by this picture.
An intriguing clue toward a possible link is provided by the significant
pattern located upstream over Asia and the extreme western Pacific preced-
ing the development of the PAC cases. The structure of this pattern suggests
that the associated wind anomalies are primarily in the zonal flow over both
the Himalayas and the southwestern North Pacific. The anomalous flow
patterns we observe in this region may
be
a reflection in part of changes in
tropical forcing over the
far
southwest Pacific and Indonesian regions, areas
where interannual precipitation anomalies are known to be strongly linked
to different phases of the Southern Oscillation (e.g., Horel and Wallace,
1981). In addition, we note that recent modeling studies (Simmons, 1982;
Branstator, 1983; Simmons
et
al.,
1983) have displayed a marked sensitivity
in the extratropical response centered over the central North Pacific to
forcing changes located in southwestern Pacific.
Some aspects of the evolution observed in the 500-mbar analyses are
reminiscent of the evolutions seen in a study of the barotropic instability
of
the Northern Hemisphere 300-mbar time-mean flow (Simmons
et
al.,
1983;
cf. their
Figs.
11
and 19). In particular, that study indicates that zonally
elongated eddies (having
u’*
greater than
v’~)
located in regions where the
basic-state zonal flow is decreasing downstream are capable of growing baro-
tropically by extracting kinetic energy from the basic flow. The growing
disturbances that we have described have this general structure and are
primarily located in the jet exit region over the central North Pacific, sug-
gesting that this mechanism may contribute positively to their growth. Baro-
tropic instability of the zonally varying time-mean flow therefore provides
one possible source for the initial developments in the PAC region. In addi-
tion, the evolution of the 500-mbar anomaly pattern downstream from the
central North Pacific appears qualitatively similar to that seen in simple
linear barotopic models of horizontal energy dispersion on a sphere away
from a quasi-stationary localized source of vorticity (Hoskins
et
al.,
1977),
suggesting that this process may account for important
aspects
of the down-
stream development.
The vertical structures of the growing disturbances over the central North
Pacific, however, display considerable tilts with height
as
they amplify, indi-
cating that their developments may
be
significantly influenced by baroclinic
processes or by the possibility of vertical energy propagation. Indeed, in
several aspects the structures resemble those
of
amplifying baroclinic waves,
66
RANDALL
M.
DOLE
although their spatial scales are considerably larger and propagation speeds
considerably smaller than for typical disturbances. In addition, for the nega-
tive anomaly cases, the associated deformation fields produce horizontal
temperature advections that are highly favorable for concentrating tempera-
ture gradients along the evolving jet axis. These observations raise the dis-
tinct possibility that barotropic or equivalent barotropic models will be
inadequate for modeling important aspects of the initial developments.
Few attempts have been made to analyze the possible influences of baro-
clinicity on the development of persistent anomalies. White and Clark
(1975)
have suggested that baroclinic instability modified by sensible heat
exchange is responsible for the development of blocking over the central
North Pacific. For theoretical support they apply the results
of
Haltiner’s
(
1967)
analysis of the effects of diabatic heating on baroclinic instability in a
two-layer quasi-geostrophic model. Geisler
(
1977),
however, criticizes the
use of Haltiner’s model, since Geisler and Garcia
(1977)
find that in
a
continuously stratified model Newtonian cooling acts to reduce the growth
rates at all wavelengths, with the greatest reductions at long wavelengths.
Further, all unstable modes propagate eastward at a rate slightly larger than
the basic-state surface wind, and therefore would not remain geographically
stationary in the presence of a westerly surface flow.
More recently, Frederiksen
(1
983)
has attempted to analyze the instability
characteristics of the three-dimensional Northern Hemisphere wintertime
mean flow using a two-layer spherical quasi-geostrophic model. With this
model, he identifies certain unstable modes whose structures and evolutions
resemble in many gross
aspects
the features that we have described. Frede-
riksen interprets the development of the
PAC
pattern as essentially a two-
stage process involving both baroclinic and barotropic processes (where the
stages are essentially determined by variations in the basic-state stability).
During the stage in the development where baroclinic conversions domi-
nate, the unstable mode is eastward propagating; the larger scale, quasi-sta-
tionary mode is associated mainly with barotropic conversions. Further
investigation will be required, however, to ascertain the extent to which
Fredenksen’s model agrees with observed developments and, in particular,
whether it can provide any predictive information for the onset and evolu-
tion of the Pacific patterns.
9.
CONCLUSIONS
Our results suggest a number
of
typical characteristics in the evolution of
persistent anomaly patterns:
LIFE CYCLES
OF
PERSISTENT ANOMALIES
67
1.
Development rates are often rapid (full establishment in less than a
week).
2.
Over the key region, there is little evidence of an atmospheric precursor
until just prior to onset.
3. Following onset, anomaly centers develop and intensify in sequence,
forming a quasi-stationary wave train downstream from the main center.
Intensification occurs with little indication of phase propagation. This leads
to the establishment of the persistent anomaly pattern.
4.
Vertical cross-sections indicate that the PAC anomaly centers have
substantial tilts with height during development. Corresponding 1000-mbar
height and 1000- to 300-mbar thickness analyses for the PAC negative cases
provide further evidence that the developments may have a markedly baro-
clinic character, with horizontal temperature advections tending to increase
the temperature gradients to the south and southeast of the intensifying
surface low near the latitude of the jet maximum. The blocking ridge devel-
oping downstream over the eastern Pacific also amplifies in a region of strong
thermal advection and is characterized by a westward tilt with height during
development.
5.
Breakdown rates are
also
often rapid. Until immediately prior to
breakdown, the patterns resemble the corresponding patterns obtained fol-
lowing development.
6.
From development through decay, corresponding positive and nega-
tive patterns display striking similarities in their evolutions.
The evolution of the 500-mbar anomaly pattern downstream from the
central North Pacific appears qualitatively similar to that seen in simple
time-dependent models of horizontal energy dispersion on a sphere away
from a quasi-stationary localized source of vorticity
,
suggesting that this
process may account for important aspects of the downstream development.
For the PAC cases, the most systematic precursors are related to variations in
the jet intensity and structure over eastern Asia and the southwestern North
Pacific and to an eastward-propagating, intensifying anomaly center that
appears to originate at midlatitudes.
Some aspects of the initial development are reminiscent of the evolutions
seen in studies of the barotropic instability of the time-mean flow; however,
the growing disturbance over the Pacific is associated with pronounced
temperature advections and, in the negative anomaly cases, a deformation
field favorable for concentrating temperature gradients along the jet axis.
The structure of this disturbance bears considerable resemblance to that of a
classic amplifying baroclinic wave, although its spatial scale is considerably
larger and its propagation speed considerably smaller than for typical baro-
clinic disturbances. Thus, the observations cast some doubt on whether
68
RANDALL M. DOLE
barotropic or equivalent barotropic models
will
be
sufficient
for
modeling
important
aspects
of
the developments, or whether models that take into
account the
full
three-dimensional structure
of
the
flow
will
be required to
adequately describe the development
of
many persistent anomaly cases.
ACKNOWLEDGMENTS
I thank
Ms.
Isabelle Kole for her help in
drafting
several of the
figures.
Some of the calcula-
tions were performed on the
Goddard
Space
Flight Center computer system
located
at Green-
belt, Maryland; the remainder were performed on the National Center for Atmospheric Re-
search computer system. NCAR
is
supported by the National Science Foundation. Support for
the research
was
provided by NASA Grant NASA-g NAGw-525.
REFERENCES
Austin,
J.
F.
(
1980).
The blocking of middle latitude westerly winds by planetary scale waves.
Q.
J.
R.
Meteorol.
Soc.
106,327-350.
Blackmon, M.
L.
(1976).
A
climatological
spectral
study of the geopotential height
of
the
Northern Hemisphere.
J.
Atmos.
Sci.
33,
1607- 1623.
Branstator,
G.
(1983).
Horizontal energy propagation in a bamtropic atmosphere with meri-
dional and zonal structure.
J.
Atmos.
Sci.
40,
1689
-
1708.
Colucci,
S.
J.,
Loesch, A., and
Bosart,
L.
(
198
1).
Spectral
evolution of
a
blocking episode and
a
comparison with wave interaction theory.
J.
Atmos.
Sci.
38,2092 -2
1
1
1.
Dole,
R.
M.
(
1982).
Persistent anomalies of the extratropical Northern Hemisphere wintertime
circulation. Ph.D. thesis, Massachusetts Institute of Technology, Cambridge,
Mass.
02 139.
Thesis available from the author
on
request.
Dole,
R.
M.
(1986).
Persistent anomalies ofthe extratropical Northern Hemisphere wintertime
circulation: Structure.
Mon.
Weather
Rev.
114, 178 -207.
Dole,
R.
M., and Gordon, N. D.
(1983).
Persistent anomalies of the extratropical Northern
Hemisphere wintertime circulation: Geographical distribution and regional persistence
characteristics.
Mon.
Weather
Rev.
111, 1567- 1586.
Frederiksen,
J.
S.
(1983).
A unified three-dimensional instability theory ofthe onset ofblocking
and cyclogenesis. 11. Teleconnection patterns.
J.
Amos.
Sci.
40,2593-2609.
Gall,
R.
(1976).
Structural changes in growing baroclinic waves.
J.
Atmos.
Sci.
33,374-390.
Geisler,
J.
E.
(1
977).
On the application of baroclinic instability and sensible heat exchange to
Geisler,
J.
E.,
and Garcia,
R.
P.
(1977).
Baroclinic instability at long wavelengths on
a
beta
Haltiner,
G.
J.
(1967).
The effects of sensible heat exchange on the dynamics
of
baroclinic
Holton,
J.
R.
(
1979).
“An Introduction to Dynamic Meteorology.” Academic
Press,
New York.
Horel,
J. D.,
and Wallace,
J.
M.
(
198
1
).
Planetary scale atmospheric phenomena associated with
Hoskins,
B.
J.,
and
KaroIy,
D.
(
198
1
).
The steady linear response
of
a
spherical atmosphere to
explain blocking ridge development.
J.
Afmos.
Sci.
34,
3
I
1
-
32
1.
plane.
J.
Atmos.
Sci.
34,
31 1-321.
waves.
Tellw
19,
183- 198.
the Southern Oscillation.
Mon.
Weather
Rev.
109,8
13
-
829.
thermal and orographic forcing.
J.
Amos.
Sci.
38,
1
179-
I
196.
LIFE CYCLES
OF
PERSISTENT ANOMALIES
69
Hoskins,
B.
J.,
Simmons,
A.
J., and Andrews,
D.
G.
(1977).
Energy dispersion in a barotropic
atmosphere.
Q.
J.
R.
Meteorol.
SOC.
103,553-567.
Joung, C.
H.,
and Hitchman,
M.
H.
(1982).
On the role of successive downstream development
in East Asia cold
air
outbreaks.
Mon.
Weather
Rev.
110, 1224- 1237.
Lau, N. C.
(198
1).
A
diagnostic study of recurrent meteorological anomalies appearing in a
15-year simulation
with
a GFDL general circulation model.
Mon.
Weather
Rev.
109,
Reinhold, B. B., and Pierrehumbert,
R.
T.
(1982).
Dynamics ofweather regimes: Quasi-station-
ary waves and blocking.
Mon.
Weather
Rev.
110,
1105-
1145.
Simmons, A.
J.
(
1982).
The forcing
of
stationary wave motion by tropical diabatic heating.
Q.
J.
R.
Meteorol.
Sot.
108,503-534.
Simmons, A.
J.,
and Hoskins, B.
J.
(1978).
Thelife-cycles ofsome nonlinear baroclinic waves.
J.
Atmos.
Sci.
35,414-432.
Simmons, A. J., Wallace,
J.,
and Branstator,
G.
(1983).
Barotropic wave propagation and
instability and atmospheric teleconnection patterns.
J.
Atmos.
Sci.
40,1363
-
1392.
Wallace,
J.
M.,
and Gutzler,
D.
S.
(1981).
Teleconnections
in
the geopotential height field
during the Northern Hemisphere Winter.
Mon.
Weather
Rev.
109,784-812.
White, W. B., and
Clark,
N.
E.
(1975).
On the development
of
blocking ridge activity over the
central North Pacific.
J.
Atmos.
Sci.
32,489-502.
Winston,
J.
S.
(1955).
Physical
aspects
of rapid cyclogenesis in the Gulf of Alaska.
Tellus
7,
2287-231
1.
481-500.
This Page Intentionally Left Blank
ON ATMOSPHERIC BLOCKING TYPES
AND
BLOCKING NUMBERS
HEINZ-DIETER
SCHILLING
Meteorology Institute
University
of
Munich
0-8000
Munich
2
Federal Republic
of
Germany
1.
INTRODUCTION
During the past few years work on blocking in the westerlies has grown
rapidly. Many studies have suggested a variety of possible blocking mecha-
nisms. Whereas each proposal is by itself interesting and suggestive, it must
be proved to be an essential feature of the formation or maintenance of at
least some real blocking cases. On the other hand, the study ofblocking cases
with either observational or model data raises certain difficulties. This article
addresses two of them.
The first problem
is
whether the study should be made circumpolar or not.
Using local methods the consequences of the blocking event on other regions
are not taken into account (see double blocking structure and the monthly
700-mbar anomaly charts published by the
Monthly Weather Review
for
months with strong, long-lasting blocking; e.g., Stark, 1965; Dickson, 1967,
1969; Taubensee, 1975). Using the circumpolar method we could possibly
mask some special features
of
the blocking case by averaging with other far
downstream or upstream events. Therefore it is desirable to develop a crite-
rion to characterize the longitudinal extent of the influence of the blocked
zone. Our proposed solution
is
to introduce a blocking number defined in
terms
of
circumpolar parameters which therefore responds to circumpolar
characteristics of the streamfield. Moreover, this single number
is
given at
any instant. In this way, it is possible to study the response
of
distant regions
to blocking activity in certain other locations. This is done here by comput-
ing the temporal correlation between the 500-mbar topography and the
blocking number. This response, in turn, helps to answer the question as to
whether or not the anomaly studies of the type of Charney
et al.
(198
1)
are
representative for the circumpolar type of blocking.
The second problem in data studies is that the dynamics and energetics
of
blocking events seem to vary widely from case to case.
As
a
first step, case
studies can contribute to the understanding of different mechanisms. How-
71
Copyright
0
1986
by
Academic
Press,
Inc.
All
rights
of
reproduction
in
any
form
reserved.
ADVANCES
IN
GEOPHYSICS,
VOLUME
29
72
HEINZ-DIETER SCHILLING
ever, we believe, a statistical approach is the next step to obtain insight into
the cooperation of the main proposed mechanisms of blocking formation.
This raises the question
as
to how we
can
group blocking events together to
perform sample statistics. Since there are many proposed or observed block-
ing mechanisms, the definition of types of blocking is by itself useful to bring
order into the somewhat confusing collection of previous results. Clearly the
question as it stands is very complicated and needs some simplification.
In the following treatment one simplification will be to concentrate on
dominant waves (or wave groups) and on the energy fluxes feeding them. We
expect that statistics based on
this
approach will not necessarily weaken
signals from other dynamical processes that trigger the block formation or
stabilize the formed block rather than feed the blocking waves. If their
influence on blocking is systematic it
will
show up more clearly in such
sample statistics,
if
calculated for each
type
of block separately.
As
to the blocking number, we note another potential advantage of it.
Being an averaged quantity, the blocking number should have an extended
predictability. Therefore, correctly predicting this parameter’s tendency
some days in advance will be the equivalent to predicting the atmosphere’s
tendency toward blocking states. In practice, however, the predicted abso-
lute value ofthe blocking number is not
as
important
as
the increase above a
certain threshold value. That critical value should be chosen
so
that it indi-
cates high possibility of occurrence of blocking states.
2.
ENERGY
PARAMETERS
AND
DATA
According to present knowledge, there are too many aspects of observed
blocking for a proper treatment by only one method. However, we decided
to concentrate on circumpolar energetical aspects of the transformation of
the flow under blocking conditions.
A
considerable amount of evidence has
been given, indicating that even simple circumpolar energy parameters of
the blocked latitude belt show
a
distinct behavior during blocking (Schilling,
1982; Chen and ShuMa, 1983; Fischer, 1984; Hansen and Chen, 1982; Speth
and Meyer, 1984). We choose here the following parameters (for defining
relations see Appendix
A):
KE,
the kinetic energy of pertubations
of
zonal-mean flow
K,,
the kinetic energy of the vertical mean of zonal-averaged flow
KT,
the kinetic energy
of
the vertical shear of zonal-mean flow, roughly
representing
A,,
the available potential energy
(APE)
of the zonal mean
These and other energetic parameters were computed from oncedaily
height field data at three levels (300,
500,
and
1000
mbar) for the period
ON ATMOSPHERIC BLOCKING TYPES AND NUMBERS
73
1967-1976. These data were prepared by the German Weather Service
(DWD).
Figure
1
shows how these parameters develop during
20
cases of Atlantic/
European blocking. The cases are taken from the catalog
of
Schilling (198
1).
This catalog was recently revised on the basis of the following definition
(after Schilling, 1982, and Egger, 1978): A blocking is defined as (a) a
500-
FIG.
1.
KE, K,,
and
K,
averaged over several stages of Atlantic blocking
(20
cases).
For
comparison,
two
nonblocking cases
are
provided.
See
text for definitions.
VB
-
5,
Average over
5
days before
VB
NB
+
5,
average over
5
days
after NB
I
-
5,
I11
+
5
analogous
to
definitions
above.