Saltzman B. (editor) Anomalous Atmospheric Flows and Blocking
Подождите немного. Документ загружается.
THE ORIGIN
OF
STATIONARY ROSSBY WAVES
435
Indonesia and equatorial Africa, and agrees well with the intuitive notion of
a nonlinear “local Hadley cell circulation.”
5.
“SUPPRESSED REGIONAL HEATING”
EXPERIMENTS
The experiment discussed in the previous section indicated that tropical
heating was an important contributor to the January waves over South
America, and, indeed, over all the Southern Hemisphere.
Kalnay
et
al.
(1 986) have indicated that instances of a strong SPCZ shifted
to the east tend to be associated with strong “South Atlantic Convergence
Zones” (SACZs), such as occurred during January 1979. This association is
also apparent in the correlation maps of outgoing long-wave radiation with
convection at the dateline computed by Lau and Chan (1983).
In order to determine whether local convection, convection originating in
the SPCZ, or both, were necessary to maintain the South American January
waves (and the corresponding SACZ), we performed two more experiments.
In the first one, which we will denote “suppressed Pacific heating,” we
reduced the latent heat
of
condensation to one-tenth of its value at all
latitudes and over a sector of 180” longitude centered on the dateline (Fig.
7b). In the second experiment, denoted “suppressed Atlantic heating,” the
latent heat release was similarly suppressed but in the hemisphere centered
on the Greenwich meridian.
The 15-day forecasts of
F200
mbar
and Hovmoller plots corresponding to the
“suppressed Pacific heating” are presented in Fig. 10a and b. The reductions
in amplitude and change of the South American waves are even stronger
than in the “reduced tropical heating” experiment.
Although the latent heat is suppressed in the complete hemisphere cen-
tered on the dateline, there are rather small changes in the Northern Hemi-
sphere, and in the Southern Hemisphere the largest changes are in the SACZ
region. In the Hovmoller diagram, it is apparent that in the Pacific, the
stationary waves are immediately weakened. In our region of interest, the
SACZ waves remain stationary for about 6 days, and then they propagate
eastward with a speed of about
5
’
per day.
When the Atlantic hemisphere heating is suppressed, the corresponding
stationary waves and Hovmoller plots are those of Fig.
1
la and b. The
South American waves weaken and start propagating immediately.
The fact that the weakening of the South American waves in these two
experiments is much stronger than in the “reduced tropical heating” experi-
ment suggests that subtropical convection in the SPCZ and SACZ plays a
role in their maintenance, and they are not just a tracer of a circulation
determined by equatorial convection over South America and Indonesia.
THE
ORIGIN
OF
STATIONARY ROSSBY
WAVES
437
-IBO
-140
-100
-So
-20
20
60
100
140
180
FIG.
lob.
-
6.
“EASTERLY
DECELERATION”
EXPERIMENT
We performed an experiment in which we decelerated the initial zonal
flow by adding a solid body rotation velocity corresponding to
-
5
m sec-* at
the equator. The sea level pressure was also modified
so
as to maintain
geostrophic balance with the change in zonal flow.
The purpose of this experiment was to determine whether the South
American waves were sensitive to a shift in the position of the critical surface
separating easterlies and westerlies, as argued by Kalnay and Paegle
(1983).
The result of this experiment was very unexpected, and is apparent in Fig.
12.
The forecast of the stationary waves in the Northern Hemisphere is not
very affected by the change in zonal wave, whereas the Southern Hemisphere
changes dramatically. The intensity of the short subtropical stationary waves
centered at
30”s
in the control January forecast has indeed decreased.
On
the
other hand, it is clear that the change in zonal flow has resulted in the very
abrupt generation of almost stationary waves of zonal wavenumber 5 cen-
tered at
45
“S.
This result is reminiscent ofthe “ubiquitous” pentagonal wave
THE ORIGIN
OF
STATIONARY ROSSBY WAVES
439
-180
-140
-100
-60
-20 20
60
100
140
IW
FIG.
1
lb.
in the Southern Hemisphere observed during certain periods of the southern
summer and discussed by Salby (1 982), Hamilton
(
1983), and Randel and
Stanford (1983).
7.
SUMMARY
AND
CONCLUSIONS
Through the use
of
a comprehensive general circulation model, we have
been able to determine the mechanisms that maintain the large-amplitude
short-wavelength stationary waves in the Southern Hemisphere. The model
control forecast represents well the maintenance of the waves for about 10
days, af‘ter which barotropic instability results in their sudden disappearance.
Several mechanistic experiments are performed. The
“No
Andes” fore-
cast shows that the waves exist independently of the orographic forcing of the
Andes. Several experiments modifying the coefficient of latent heat lead to
the conclusion that tropical heating is important in the maintenance of the
waves. However, the convection in the subtropical waves themselves is im-
THE ORIGIN OF STATIONARY ROSSBY WAVES
44
1
portant in sustaining their amplitude and phase, and the Walker-type circu-
lation associated with the SPCZ is also a contributor to the maintenance of
the South American waves. These results confirm the existence of a relation-
ship between the occurrence of a strong SPCZ, somewhat eastward from its
climatological position, and the strong “South Atlantic Convergence Zone”
observed in outgoing long-wave radiation maps.
A “pentagonal” stationary wave was the unexpected result of an experi-
ment in which the zonal flow was decelerated
by
a solid rotation of
-
5
m
sec-’ at the equator. This phenomenon, probably related to the observed
presence of waves with zonal wavenumber
5
in the Southern Hemisphere
summer, will
be
studied in further detail.
ACKNOWLEDGMENTS
This work is a continuation of the research started in collaboration with Jan Paegle, whose
interest in the problem is very gratefully acknowledged. We have had useful discussions with R.
Lindzen,
M.
Halem, W.
E.
Baker and
S.
Bloom.
The experiments performed in this study were made possible because of the collaboration of
J.
Pfaendtner, L. L. Takacs, and R. Balgovind, who contributed significantly to the develop
ment of the GLAS fourth-order
GCM.
R. Dlouhy and
M.
Almeida helped in setting up the
experiments. Most ofthe computer plots were developed and computed by R. Rosenberg and F.
Tahmasebi. We are very grateful to all of them for their enthusiastic help.
The manuscript was typed by B. Richardson and L. Wright, and the figures were prepared
with the collaboration of L. Rumburg and B. Sherbs.
REFERENCES
Atlas, R. (1984). The effect of physical parameterizations and initial data on the numerical
prediction of the President’s day cyclone.
Proc. Conf Weather Forecast. Anal., IOth,
Clearwater Beach, Fla., June 25- 29.
Baker, W. E. (1 983). Objective analysis and assimilation of observational data from FGGE.
Mon.
Weather Rev.
111,328-342.
Baker, W. E., Atlas, R., Halem, M., and Susskind, J. (1984). A
case
study of forecast sensitivity
to data and
data
analysis technique.
Mon.
Weather
Rev.
112,
1544
-
156
1.
Bengtsson, L. (1984). Operational medium range weather prediction at ECMWF.
Proc. ConJ
Weather Forecast. Anal., 10th. Clearwater Beach, Fla., June 25- 29.
Gill, A.
E.
(1980). Some simple solutions for heat induced tropical circulation.
Q.
J.
R. Me-
teorol. Soc.
106,441-462.
Halem,
M.,
Kalnay,
E.,
Baker, W. E., and Atlas, R. (1982). An assessment ofthe FGGE satellite
observing system during SOP-1.
Bull.
Am. Meteorol. SOC.
63,407-426.
Hamilton,
K.
(1983). Aspects of wave behavior in the mid and upper troposphere
of
the
southern hemisphere.
Atmos. Ocean.
21,40-54.
Kalnay, E., and Halem, M. (198 1). Large amplitude stationary Rossby waves in the southern
hemisphere.
Proc. Int. Conf Early Results FGGE Large Scale Aspects
Monsoon
Exp.,
Tallahasse Fla.. Jan.
12-
17.
442
EUGENIA
KALNAY AND KINGTSE C. MO
Kalnay,
E.,
and Merkine,
L.-0.
(
198
I).
A
simple mechanism for blocking.
J.
Atmos.
Sci.
38,
Kalnay,
E.,
and Paegle,
J.
(1983).
Large amplitude stationary waves in thesouthern hemisphere
observations and theory.
Proc.
Int.
Con/:
Southern Hemisphere Meteorol.,
1st.
Kalnay,
E.,
Balgovind,
R.,
Chao,
W.,
Edelmann,
D.,
Pfaendtner, J., Takacs, L., andTakano, K.
(1
983).
Documentation ofthe
GLAS
fourthorder general circulation model, Volumes
1,2,
and
3.
NASA Tech. Memo.
86064.
Kalnay,
E.,
Mo,
K.
C., and Paegle, J.
(I
986).
Large-amplitude, short-scale stationary Rossby
waves in the Southern Hemisphere: Observations and experiments to determine their
origin.
J.
Atmos.
Sci
43,252-275.
Lau, K.
M.,
and Chan, P. H.
(
1983).
Short term climate variability and atmospheric teleconnec-
tion from satellite observed outgoing longwave radiation, Part
I:
Simultaneous relation-
ships.
J.
Amos.
Sci.
40,2735-2750.
Matsuno, T.
(
1966).
Quasi-geostrophic motions in the equatorial area.
J.
Meteorol.
SOC.
Jpn.
44,
25-43.
Newell. R.
E.,
Kidson, J.
W.,
Vincent, D.
G.,
and Baer,
G.
J.
(1972).
“The General Circulations
of the Tropical Atmosphere,” Vol.
I.
p.
1
16.
MIT
Press, Cambridge, Massachusetts.
Paegle,
J., and Baker,
W.
E.
(1983).
The influence
of
the tropics
on
the prediction of ultra long
waves, Part
11:
Latent heating.
Mon.
Weather Rev.
I1
1,
I356
-
137
1.
Randel,
W.
J.,
and Sanford,
J.
L.
(1984).
Structure of medium-scale atmospheric waves in the
southern hemisphere summer
1983.
J.
Amos.
Sci.
40,2312-2318.
Salby,
M.
L.
(1982).
A
ubiquitious wave
5
anomaly in the southern hemisphere during
FGGE.
Mon. Weather Rev.
110,
1712- 1720.
Trenberth, K.
E.
(1979).
Planetary waves at
500
mb in the Southern Hemisphere.
Mon.
Weather Rev.
108,
1378- 1389.
Taljaard, J.
J.,
van
Loon,
H., Crutcher, H. L., and Jenne,
R.
L.
(1969).
Climate ofthe upper air:
Southern Hemisphere, Vol.
1.
Temperatures, dew points and heights at selected pressure
levels.
NAVAIR
I
50-1C-3-55, Chief Naval
Oper.,
Washington,
DC.
van
Loon.
H.,
and Jenne,
R.
L.
(1972).
The zonal harmonic standing waves
in
the Southem
Hemisphere.
J.
Geophvs.
Res.
77,
992- 1003.
van
Loon.
H.,
Jenne, R.
L.,
and Lakitzke,
K.
(1973).
Zonal harmonic standing waves.
J.
Geoph-v.y. Res.
78,4463 -447
I.
Webster, P.
J.
(1972).
Response
of the tropical atmosphere to local steady forcing.
Mon.
Weather Rev.
100,
5
I8
-
54
I.
2077
-
209
I.
SST
ANOMALIES
AND
BLOCKING
J.
SHUKLA
Center for Ocean
-
Land- Atmosphere Interactions
Department of Meteorology
University of Maryland
College Park, Maryland
20742
1.
INTRODUCTION
There is a large body of published literature on the possible role of sea
surface temperature (SST) anomalies in producing quasi-stationary circula-
tion anomalies in midlatitudes, to
be
referred to, loosely, as blocking. We
present here only a summary
of
what we consider to
be
important physical
concepts that have appeared during the past decade in connection with the
problem of the influence of SST anomalies on extratropical circulation
anomalies.
Let us first ask a basic question.
How
does a SST anomaly influence the
atmospheric circulation? The immediate effect of a
SST
anomaly is to alter
the sensible heat flux and evaporation over the region of the
SST
anomaly.
This will alter the temperature and humidity of the overlying air. If that was
all the SST anomaly did, there would be little change in the atmospheric
circulation. However, if the sensible heating
of
the air produces gradients of
surface pressure, then it can change the convergence of the moist air and
precipitation in the region of the
SST
anomaly. The magnitude and the
structure of the convergence pattern will depend upon the magnitude and
structure of the SST pattern itself. Warmer and moister surface air might
lead to deeper moist convection and heating of the atmospheric column.
This might lead to further enhancement of the surface pressure gradients, an
increase in moisture convergence in the boundary layer, and an increase in
precipitation. This Convective Instability of the Second Kind, or CISK-type
positive feedback, among heating of the vertical column, convergence in the
boundary layer, and precipitation can transform the initial surface heat flux
anomaly into a three-dimensional heating anomaly which can produce
changes in large-scale atmospheric circulation. Based on these arguments it
is possible to present the following simple conceptual framework to under-
stand the effects of
SST
anomalies on atmospheric circulation.
aT-,
aQ-+
ac
443
Copyright
0
1986
by Academic
Press,
Inc.
All
rights
of
reproduction in any
form
reserved.
ADVANCES
IN
GEOPHYSICS,
VOLUME
29