October1966
Stratospheric
Notes
and Correspondence
Wave
Disturbances
over
the
Equatorial
291
Propagating
Pacific*
M. Yanai and T. Maruyama
GeophysicalInstitute, Tokyo University, Tokyo
(Manuscript received 19 July 1966)
1.
Introduction
Since the exciting discovery of quasi-annual
wind reversal (Ebdon,1960 ; McCreary, 1961;
Reed et al, 1961), extensive works have been
made on the long-period zonal wind oscillation
in the equatorial stratosphere.
The 26-month
(or quasi-biennial)
oscillation has been fully
documented (see Reed, 1965). Several authors
have suggested to relate the time change of
the stratospheric
mean zonal wind to the
horizontal convergence of the eddy transport
of momentum into the equatorial belt (Reed,
1962; Tucker, 1964). However, one still lacks
observational
evidence of large-scale
eddies
in the tropical stratosphere,
although Riehl
and Higgs (1960) noted a large shear line
moving across the Caribbean Sea area during
January 1960 at 110,000 f t (33.5 km) height.
From March to July 1958, a dense upperair observation network in the central tropical
Pacific was in operation in support of nuclear
tests (see Fig. 1). The present authors have
started to look into the data taken during the
test period (Joint Task Force Seven Meteoro-
Fig.
*
1. Upper-air
Pacific during
Division
stations
in the central
March-July
1958.
of Meteorology
, Contribution
equatorial
No. 149.
logical Center, 1958) to detect eddy disturbancs
in the lower stratospheric
winds and have
indeed found interesting facts.
The following
is a short tentative report of our findings.
2.
Short-period
wind oscillation
In March 1958, the general winds in the
concerned area were from the west at the
heights
between
60, 000 ft
(18.3 km)
and
70, 000 f t (21.3 km).
Overlying above, strong
easterly winds were blowing.
Towards the
end of the special observation (July 1958), the
depth of the layer of westerly winds gradually
became thin.
The horizontal width of the belt
of the westerly winds gradually became narrow. The westerly winds were seen only at
60, 000 ft height
and only above
several
stations very close to the equator at the end
of the observation period.
Within this gradually
weakening
westerly
flow, we find rather regular shortperiod oscillation at stations located close to the equator.
Fig. 2a shows the 70, 000 f t wind data at
Kapingamarangi
(1*N, 155*E) during April
1958. One may recognize repeating
changes
of wind direction between
240* and 300*.
Average
period of this wind oscillation is
somewhere
around 5 days.
Wind data at
Ponape (7*N, 158*E) during the same period
show very similar wind oscillation (Fig. 2b).
The observed phases of the wind oscillation
at these two stations are nearly identical.
The existence
of wind oscillation of this
nature in other areas and other periods have
been confirmed.
Figs. 2c and 2d respectively
illustrate
similar wind oscillation at Canton
Island (3*S, 172*W) and Balboa, Canal Zone
(9*N, 80*W) for February
and early March
1958.
A detailed vertical time section at Canton
Island for later half of April 1958 is reproduced
in Fig. 3 to show the vertical extent of the
292
Journal
Fig. 2.
Fig.
3. Vertical
Winds
with
of the Meteorological
Society
of Japan
Vol.44,No.5
Short-period wind oscillation found at
(a) Kapingamarangi,
April 1958 (70, 000 ft),
(b) Ponape,
April 1958 (70, 000 ft),
(c) Canton. Island, February-March
1958 (21 km),
(d) Balboa, Canal Zone, February-March
1958 (70, 000 ft).
time section
of the
southerly
components
lower
are
stratospheric
shaded.
winds
at Canton
Island
for 15-30 April
1958.
October1966
Notes
and Correspondence
293
short-period wind oscillation. At around 20 km
height
southerly
and northerly
wind components appear alternatively.
The wind shift
first occurs at about 23 km height and progresses downward at a speed of roughly
day to the tropopause level.
3.
Westward
propagation
1 km/
of the disturbance
After several preliminary analyses, we have
further found that the observed wind oscillation is, in fact, a reflection of retrogressive
waves moving against the general westerly
flow. In Fig. 4, the stations Kapingamarangi,
Nauru
(1*S, 167*E), Tarawa
(1*N, 173*E)
and Canton Island are placed
along the
abscissa which is nearly equal to the equator.
The date is taken as the ordinate.
When we
plot 12 hourly wind observations
taken at
70, 000 f t height (21 km for Canton), one can
recognize
large-scale
waves
propagating
westward.
Because of the limitted east-west
coverage of the stations along the equator,
we can estimate the phase velocity of the
waves only crudely from the slope of the
lines dividing southerly wind components and
northerly
components.
The estimated
propagation speed is about -2, 000 km/day or
-23 m/sec . When we use the relationship
L (wave length) =c (phase velocity)
x T
(period) and put T=5 days, we get L=
10, 000 km.
Although we cannot conclude the existence
of waves around the global equatorial belt at
this stage, the wind oscillation
found at
Balboa is very indicative.
It may be noteworthy that the estimated
phase velocity is
not too apart from the figure simply expected
from the formula
c= U- *L2/4 *2=13 m/sec
-58 m/sec = - 45 m/sec . Similar rapid propagation of large-scale vortices has been observed
in the upper troposphere
in the same area
(Yanai, 1963). Further
extensive
study of
the disturbance is now in progress and will
be reported in the near future.
Acknowledgments
The authors wish to express their thanks
to Mr. Y. Fujiki and Miss Masako Onozuka
for their assistance.
This research has been
partially
supported
through
a grant from
the Japan Society for the Promotion of Science
as part of the Japan-U. S. Cooperative Science
Fig. 4. Time series of 70, 000 ft (or 21 km)
winds at Kapingamarangi,
Nauru, Tarawa
and Canton Island for 15-30 April 1958.
Winds
with southerly
components
are
shaded.
294
Journal
of the Meteorological
Program.
References
Ebdon, R.A., 1960: Notes on the wind flow at
50mb in tropical and sub-tropical
regions in
January 1957 and January 1958. Quart. J. Roy.
meteor. Soc., 86, 540-542.
Joint Task Force Seven Meteorological Center, 1958:
Meteorological
Report on Operation Hardtack,
1958.
JTFMC TP-8, Vols. 1-6.
McCreary, F. E., 1961:
Variation
of the zonal
winds in the equatorial stratosphere.
Joint Task
Force Seven Meteorological
Center,
JTFMC
TP-20, 15pp.
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Meteor.
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Freien
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Berlin, 36, 245-257.
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Bull. Amer. meteor. Soc., 46, 374387.
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and D. G. Rogers,
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Evidence
of a
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Riehl, H., and R. Higgs, 1960:
Unrest in the
upper stratosphere over the Caribbean Sea during January 1960. J. Meteor., 17, 555-561.
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赤道 太平 洋上 を伝 播す る成 層圏 の波 動
柳
井
迫
Vol .44,No.5
雄・丸
山
健
東京大学理学部地球物理学教室
人