The significance of tropical heat sources on higher latitude jet streams has been examined by numerous investigators. Hurrell and Vincent (1990) provide a summary of many of these investigations in their observational case study of the relationship between tropical heating and subtropical wind maxima in the Southern Hemisphere during SOP-1, FGGE. They showed that the divergent outflow from tropical heating associated with the South Pacific Convergence Zone (SPCZ), acted on by the coriolis force, was an important factor in maintaining the subtropical jet on the poleward side of the SPCZ during the period, 6-20 January 1979. They found a similar, but weaker relationship, over the southern Indian Ocean from 3-17 February 1979, a period when the SPCZ heating was greatly reduced and the jet was essentially non-existent. Since their findings were based on a case study and involved the use of the highly-specialized FGGE data set, the natural questions which arose were: (1) Is this relationship a regular feature of the circulation over the South Pacific? and, (2) If so, can it be detected with a routine data set? Another question posed by Hurrell and Vincent in their papers was:(3) How important was the intraseasonal oscillation in causing the enhanced heating and divergent outflow in the Pacific Ocean in January and southern Indian Ocean in February? The purpose of the present paper is to address the answer to these three questions. To accomplish this, some circulation features for an entire warm season in the Southern Hemisphere were examined. The year selected was 1984-85, and the warm season consisted of the 6-month period, 1 November 1984 - 30 April 1985. This period was chosen because there were numerous cases of the westerly wind maxima over the South Pacific and the intraseasonal oscillation was well documented

Funk, A.

Hurrell, James W.

Sperling, T.

Speth, P.

Vincent, Dayton G.

Zube, S.

English

NASA Technical Reports Server

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RELATIONSHIP BETWEEN INTRASEASONAL OSCILLATION AND
SUBTROPICAL WIND MAXIMA OVER THE SOUTH PACIFIC OCEAN
Reprinted from the Preprint Vnlume of the Fifth
Con(erence on Climlle VirtattonJ, October 14-I8 Fx_l.
Denver. (_olo. Published by the American
Meteorolugical Society, Boston, Ma_.
Dayton G. Vincent
Dept. Earth & Atmospheric Sciences
West Lafayette, IN 47907
James W. Hurrell
NCAR
Boulder, CO
P. Speth, T. Sperling,
A. Funk and S. Zube
Institute for Geophysics & Meteorology
Cologne, Germany
I. INTRODUCTION
The significance of tropical heat sources
on higher latitude jet streams has been examined
by numerous investigators. Hurrell and Vincent
(1990) provide a summary of many of these
investigations in their observational case study
of the relationship between tropical heating and
subtropical wind maxima in the Southern
Hemisphere during SOP-I, FGGE. Their paper,
together with subsequent observational and
modeling investigations of the same case study
(Hurrell and Vincent, 1991a, b), provided the
main impetus for the present study. They showed
that the divergent outflow from tropical heating
associated with the South Pacific convergence
zone (SPCZ), acted on by the coriolis force, was
an important factor in maintaining the sub-
tropical jet on the poleward side of the SPCZ
during the period, 6-20 January 1979. They found
a similar, but weaker relationship, over the
southern Indian Ocean from 3-17 February 1979, a
period when the SPCZ heating was greatly reduced
and the jet was essentially non-existent. Since
their findings were based on a case study and
involved the use of the highly-specialized FGGE
data set, the natural questions which arose were,
"Is this relationship a regular feature of the
circulation over the South Pacific?", and, "If
so, can it be detected with a routine data set?".
Another question posed by Hurrell and Vincent in
their papers was, "How important was the intra-
seasonal oscillation in causing the enhanced
heating and divergent outflow in the Pacific
Ocean in January and southern Indian Ocean in
February?".
The purpose of the present paper is to
address the answer to these three questions. To
accomplish this, some circulation features for an
entire warm season in the Southern Hemisphere
were examined. The year selected was 1984-85,
and the warm season consisted of the 6-month
period, I November 1984 - 30 April 1985. This
period was chosen because there were numerous
cases of the westerly wind maxima over the South
Pacific and the intraseasonal oscillation was
well documented (Vincent et al.., 1991).
The primary data set used in this study was
essentially the same as that used by Vincent
et al. (1991). Briefly, it consists of the ECMWF
analyses in spherical harmonics, with a
triangular truncation of T63, but these data were
transformed to a 3 ° latitude by 5 ° longitude
grid. The analyses were available at O0 UTC each
day at standard pressure levels up to 30 mb and
contained geopotential height, temperature,
relative vorticity, horizontal divergence,
vertical velocity, and relative humidity.
Horizontal wind components were computed from
vorticity and divergence, and velocity potential
from divergence. Since the analyses were global,
no boundary conditions were required for the
computation of velocity potential. The variables
used in the present study are the zonal (u)
component of the wind, velocity potential (x) and
the meridional component of velocity potential
(vv). The level used to depict the relationship
between episodes of enhanced tropical divergent
outflow and subtropical westerly wind maxima is
200 mb. Vincent et al. (1991) showed a high
correlation between tropical heat sources, as
indicated by outgoing longwave radiation (OLR),
and z at 200 mb.
2. RESULTS AND DISCUSSION
A Hovmoeller diagram of the zonal wind at
200 mb, averaged from 21-36°S, for the 6-month
period noted in Section I, is shown in Fig. I.
Values greater 30 m/s (in some cases, 25 m/s)
have been shaded to highlight regions and times
of local wind maxima. It is obvious that
numerous cases of wind maxima occurred within the
Southern Hemisphere subtropics during their
summer season. Particularly noticeable are those
over the western and central South Pacific Ocean
and South America. Since the present study is
interested in examining the potential relation-
ship between tropical heat sources in the South
Pacific and their corresponding subtropical wSnd
maxima, several cases of distinct episodes of the
latter were identified. Eight of these cases,
between early December 1984 and mid-April 1985,
are depicted in Fig. I. It is seen that the wind
maximum in each case persisted for i-2 weeks,
and, with one or two exceptions, tended to
propagate eastward. It is important to state
that in all 8 cases the subtropical wind maxima
were distinct from the higher latitude jet stream
which meandered between latitudes, 40-55°S.
Figure 2 shows a Hovmoeller diagram of the
meridional component of the divergent wind, vv,
at 200 mb, averaged between latitudes, 12-24°S.
Values of poleward-moving air greater than 2 m/s
have been shaded. It is seen in most cases
240 <
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_"i-_-__ _ _--C_"_'_:_'_ ....
.... .....
Fig. I. Hovmoeller plot of the daily zonal
wind at 200 mb in m/s averaged from 21-36S for i
Nov 84 - 30 Apr 85. Values _ 25 or 30 are shaded
and 8 cases of wind maxima are identified.
3E
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Vx (rns-q 200rob _2-24S i984-85
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Fig. 2. As in Fig. I, except for the
divergent component of the meridional wind in m/s
averaged from 12-24S. Values _ -2 are shaded.
)_(IOmmm$ _) 200m_b 0-15S #984-8_
BAN0 P_SS FLTEI:E0 40-80 0_YS
io
Fig. 3. As in Fig. I, except for bandpass
filtered (40-_Oodays) values of velocity
potential in 10_m:/s averaged from 0-15S.
(those selected, as well as those which were not)
that the divergent outflow from the tropics tends
to maximize near the entrance region of the
individual subtropical wind maxima. This
suggests that the outflow from the tropics, under
the influence of the coriolis force, is a factor
in acting to enhance and maintain subtropical
"jets" in the Southern Hemisphere. It should be
noted that in all 8 cases, as well as others,
enhanced values of velocity potential in the
tropics were associated with the maxima of v and
u (not shown). It's also important to emphasize
that divergent out-flow from the tropics may not
be the sole source for the development and
maintenance of the observed wind maxima. Hurrell
and Vincent (1991a) and others (e.g., Trenberth,
1986, 1987) have used the E-P flux equations to
show that transient eddies can also play an
important role. More will be said concerning
this later.
Next, we examine the role of the intra-
seasonal oscillation as a stimulator of divergent
outflow from the tropics during periods of
enhanced subtropical wind maxima. Figure 3 shows
a Hovmoeller diagram of the velocity potential at
200 mb which has been zonally-averaged from O-
15°S and band passed filtered for a period of
40-80 days using the recursive filter technique
described by Murakami (1979). Before the data
were filtered, they were statistically tested and
it was determined that thesignificant spectral
peaks were between 50 and 67 days. For details
regarding these results, the reader is referred
to Vincent et al. (1991). Also shown in Fig. 3
are the boxes which outline the locations and
times of the 8 cases referred to in Fig. I. It
appears that the tropical oscillation might be
important in 4 of the 8 cases. Table I gives the
details concerning each case and shows, as stated
earlier, that each wind maximum lasts from 1-2
weeks. It appears that the subtropical winds in
cases 2, 3, 6 and 8 might have been enhanced by
divergent outflow from the tropics due to the
passage of the intraseasonal oscillation. The
remaining cases occurred during the non-
convective stage of the oscillation, as indicated
by the positive anomalies seen in the filtered
velocity potential.
To conserve space, only 2 of the 8 cases
are illustrated and discussed; however, it should
be noted that the results which follow were
common to all 8 cases. Maps of total x (not
Table I. Dates and longitudinal extent of 8 cases of
subtropical wind maxima located between latitudes 21-36S.
Also indicated is whether or not a case appears to be
influenced by the intraseasonal oscillation.
Longitudinal
Case No. Date_____ss Extent Oscillation
I 5 Dec - 14 Dec 160W - 120W No
2 24 Dec - ] Jan 155E - ]lOW Yes
3 4 Jan - 15 Jan ]30E - ]70W Yes
4 16 Jan - 23 Jan 165E - 150W No
5 3! Jan - 7 Feb 160E - 170W No
6 ]l Feb - 26 Feb 140E - 175W Yes
l 12 Mar - 25 Mar I75E - 140W No
8 8 Apr - 16 Apr 145E - ]65W Yes
241
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Fig. 4. Time averaged horizontal distribu-
tions of x, v and u at 200 mb for Case 5, 31 Jan
85 - 7 Feb 85_
band-passed), vv and u at 200 mb, which have been
time-averaged _ver the life cycle of the sub-
tropical wind maximum for case 5, are shown in
Fig. 4. Recall that this case is one where the
intraseasonal oscillation is not considered to be
an important factor in causing divergent outflow
from the tropics. It is _nm_:.a _ the minimum
value of x (about -11 x ] ) is located
over Borneo. Maximum poleward divergent flow
occurs over northeastern Australia and the axis
of maximum (negative) values stretches from there
to the northern tip of New Zealand. Furthermore,
this axis is located in the entrance region of
the subtropical jet which is centered n_ar the
Dateline with a maximum value of 30-35 ms" . For
comparison, Fig. 5 shows the same three variables
for case 6, a period when the intraseasonal
oscillation was suspected of being an important
factor in causing the divergent outflow
associated with initiating and maintaining the
observed subtropical wind maximum. The patterns
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12Q' C ;80 _ -_ :80 = C :80 = . :2.C=
Fig. 5. As in Fig. 4, except for Case 6, 17
Feb 85 - 26 Feb 85.
of x and v are similar to those in Fig. 4, but
the magnitudes of both are much greater in Fig.
5. This is not surprising since case 6 involves
the oscillation. Also, as in case 5, case 6
shows that the axis of maximum poleward divergent
flow is located in the entrance region of the
subtropical wind maximum. Perhaps the mos_t
surprising fact in comparing Figs. 4 and 5 is
that the maximum value of u is essentially the
same in both cases. This implies that other
factors may be responsible for decelerating the
wind in the entrance region of maximum u,
particularly in case 6. Hurrell and Vincent
(19gla), using the localized E-P flux equations
of Trenberth (1986), found that transports of
heat and momentum (both horizontal and vertical)
were responsible for decelerating the subtropical
wind maximum associated with the SPCZ in its
entrance region. However, the effect of the eddy
fluxes was not as great as the Hadley-type
forcing discussed in this paper, thus the net
effect of these two mechanisms was to enhance the
subtropical jet.
242
A comparison of the averages for some of
the key features between the four oscillation and
four non-oscillation cases is given in Table 2.
As expected, the average maximum negative value
of x for the four oscillation cases was greater
than that for the cases when the oscillation was
not present. Also, as expected, the average
speed of the poleward divergent flow was greater
for the oscillation cases. There is no apparent
difference in the duration of wind maxima between
oscillation and non-oscillation cases, and it
appears that the mean distances in latitude among
the three variables being considered are not
significantly different; however, the average
locations of each variable is farther poleward
for the non-oscillation cases. A somewhat
surprising result, however, is that the average
speed of the maximum zonal wind for the
oscillation cases was less than that for the
non-oscillation cases. Of course, the 8 cases
examined do not represent a sufficient number
from which to draw any definite conclusions
regarding this "unexpected" result. Furthermore,
as suggested earlier, it's possible that middle
latitude wave dynamics (i.e., E-P fluxes) are
more important (e.g., to decelerate the wind)
when forcing from the tropical oscillation is
occurring. Additional cases need to be compiled,
and perhaps composited, in order to address the
question concerning the significance of tropical
versus middle latitude forcing during episodes of
subtropical wind maxima in the Southern
Hemisphere.
Finally, what the present study has shown,
in response to the three questions posed in the
Introduction, is: (I) poleward divergent outflow
from tropical heat sources was an important
factor in causing and maintaining subtropical
wind maxima over the South Pacific Ocean; (2)
this relationship occurs quite frequently and can
be detected from routine analyses; and (3) the
intraseasonal oscillation was an important factor
in perturbing the divergent outflow, but
perturbations in the tropical heating field on
other temporal scales appear to be equallyimportant.
Table 2. Averages of some key variables at 200 mb For
the 8 cases of subtropicalwind maxima.
Variable 4 oscillationcases 4 non-oscillationcases
x (max) -16 x I06 m2 s"! -IO x 106 m2 s"]
vx (max) -5 ms-! -3 ms"I
u (max) 36 ms-I 38 ms-I
fat z (max) 8°S 13:S
fat vx (max) 20°S 22°S
fat u (max) 28:S 30oS
duration lO days lO days
ACKNOWLEDGMENTS
The authors thank Mr. Douglas Miller for
drafting the figures and Ms. Helen Henry for
typing the paper. The research was supported by:
I) the Minister of Research of the Federal
Republic of Germany within the Climate Research
Program under Grant 07KF2121 and 2) the National
Aeronautics and Space Administration under
Contract NAS8-37127 and Grant NAG8-836 issued to
Dr. D.G. Vincent, Purdue University. Some of the
research was completed while Dr. Vincent was on
sabbatical leave from Purdue at the Institute for
Geophysics and Meteorology, University of
Cologne, Germany.
REFERENCES
Hurrell, J.W. and D.G. Vincent, 1990: Relation-
ship between tropical heating and sub-
tropical westerly maxima in the Southern
Hemisphere during SOP-I, FGGE. J. Clim.,
_, 751-768.
_, 1991a: The maintenance of short-term sub-
tropical wind maxima in the Southern
Hemisphere During SOP-I, FGGE. J. Clim., 4
(to appear in the September issue).
_, 1991b: Observational and modeling study of
the maintenance of short-term sub-tropical
wind maxima in the Southern Hemisphere
during SOP-I, FGGE. Preprint from the 19th
Conference on Hurricanes and Tropical
Meteorology, Amer. Meteor. Soc., Boston,
MA, May 6-10, 1991, Miami, FL, 422-427.
Murakami, M., Ig7g: Large-scale aspects of deep
convection activity over the GATE area.
Mon. Wea. Rev., 10__Z,994-1013.
Trenberth, K.E., 1986: An assessment of the
impact of transient eddies on the zonal
flow during a blocking episode using
localized Eliassen-Palm flux diagnostics.
J. Atmos. Sci., 43, 2070-2087.
_, 1987: The role of eddies in maintaining
the westerlies in the Southern Hemisphere
winter. J. Atmos. Sci., 4__44,1498-1508.
Vincent, D.G., T. Sperling, A. Fink, S. Zube _d
P. Speth, 1991: Intraseasonal oscillation
of convective activity in the tropical
Southern Hemisphere: May 1984 - April
1986. J. Clim., 4, 40-53.
243


Relationship Between Intraseasonal Oscillation and Subtropical Wind Maxima Over the South Pacific Ocean

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