Journal of the Meteorological Society of Japan, Vol. 80, No. 2, pp. 311--331, 2002

311

Tropical–Extratropical Interaction Associated with the 10–25-day
Oscillation over the Western Pacific during the Northern Summer
Yoshiki FUKUTOMI1 and Tetsuzo YASUNARI2
Institute of Geoscience, University of Tsukuba, Tsukuba, Japan(Manuscript received August 2000, in revised form 11 January 2002)

Abstract
Ten years (1986–1995) of ECMWF analyses and NOAA outgoing longwave radiation (OLR) data wereused to investigate intraseasonal tropical–extratropical interactions on timescales of 10–25 days in thewestern Pacific region during Northern Hemisphere (NH) summer. Composite analyses were performed
separately for the early summer (June–July) and mid-summer (August). The analyses were based on the
OLR index in the base region (the South China Sea), and help describe the seasonal characteristics ofcirculation associated with tropical convection. Localized Eliassen–Palm (E–P) fluxes were used todiagnose eddy–mean flow interactions.
Composite circulation patterns associated with enhanced convection on a 10–25-day timescale forJune–July were significantly different from those for August. June–July composites show a significant
Rossby wave train in both the lower and upper troposphere, extending from the convective region into
the North Pacific along the mean westerly flow. In the lower troposphere, subtropical circulation anomalies as part of this wavetrain propagate southwestward along the subtropical duct. Tropical–extratropicallinkage is less pronounced during August. Circulation structures in August in both the lower and upper
troposphere are different from those of June–July; Rossby waves do not emanate from the South China
Sea and low-level subtropical circulation anomalies show no southwestward phase propagation.
Energy conversion between eddy and seasonal mean flow for each season was computed to investigate
the role of the time-mean flow in maintaining the 10–25-day transient eddies. In the lower troposphere,barotropic processes dominate the conversion from the time-mean flow to eddies in the exit region oftropical monsoon westerlies for both June–July and August. In the subtropics, baroclinic processes sustain the eddies. Moist baroclinic processes associated with eddy horizontal latent heat transport are important in increasing the potential energy available to eddies, especially during June–July. In the upper
troposphere, barotropic conversion of kinetic energy from eddies into the time-mean flow is most notablein the East Asian subtropics to the north of the South China Sea. This suggests that increased eddy
activity due to tropical convection is an important source of energy for the mean flow.

1.

1

2

Present affiliation: Frontier Research System for
Global Change, Institute for Global Change Research, JAMSTEC Yokohama Institute for Earth
Sciences, Yokohama 236-0001, Japan.
Corresponding author: Tetsuzo Yasunari, Institute of Geoscience, University of Tshukuba, Tsukuba, 305-8571, Japan.
E-mail: yasunari@atm.geo.tsukuba.ac.jp
Additional affiliation: Frontier Research System
for Global Change.
( 2002, Meteorological Society of Japan

Introduction

The characteristics of higher-frequency intraseasonal waves linked to tropical convection,and their associated tropical–extratropical interactions, have been the subject of considerable interest in the past decade. Intraseasonal
oscillations on so-called submonthly (about 6–30-day) timescales (e.g., Meehl et al. 1996) arerecognized as an important component of thesubseasonal atmospheric variability as sameas the Madden–Julian Oscillation (MJO: about

312

Journal of the Meteorological Society of Japan

30–60 days) (e.g., Madden and Julian 1972,
1994). Vincent et al. (1998) presented a comprehensive review of observational work on
submonthly scale variability. After Kiladis
and Weickmann (1992a, b) identified significant
linkages between tropical deep convection andlarge-scale circulation, a series of studies clarified the nature of tropical convective disturbances and the associated tropical–extratropicalinteractions on submonthly timescales. For example, Kiladis and Weickmann (1992b), Kiladis(1998), and Matthews and Kiladis (1999a) investigated midlatitude Rossby-wave forcingof tropical convection over the eastern Pacific.
Meehl et al. (1996) showed that this type of
Rossby-wave propagation into the tropics could
excite intertropical convergence zone (ITCZ)
convection and generate equatorial Rossby
waves (e.g., Kiladis and Wheeler 1995; Numaguti 1995; Wheeler and Kiladis 1999). Kiladis
et al. (1994) and Meehl et al. (1996) showedthat these equatorial Rossby waves propagate
westward along the equator into the western
Pacific, and contribute to tropical westerly windbursts accompanied by flare-ups of deep convection. Kiladis and his collaborators used theregression composite method to elucidate therole of submonthly timescale waves during NH
winter. In addition, Mo (1999, 2000) reported
that midlatitude waves over the Pacific, which
are responsible for North American precipitation variability on a similar timescale, link towestward-propagating convection in the tropical Pacific. In particular, Mo (2000) examined
the teleconnection between the Pacific intraseasonal mode and North American rainfall
during the NH summer. Relationships between
extratropical circulations on submonthly timescales and tropical and subtropical convectionin the South Pacific convergence zone (SPCZ)(e.g., Kiladis and Weickmann 1992b; Schrageand Vincent 1996; Vincent et al. 1997) and theSouth Atlantic convergence zone (SACZ) (e.g.,
Kiladis and Weickmann 1992a, 1997; Liebmann et al. 1999) have also been studied.
The relationship between tropical deep convection and extratropical Rossby waves at submonthly timescales is two-way. Tropical convection forces downstream extratropical waves,and the propagation of waves from midlatitudes
toward the tropics induces tropical or subtropical convection. Tropical–extratropical interac-Vol. 80, No. 2

tions on submonthly timescales and the rolesof the interactions in tropical and subtropicalvariability during the NH summer over the
Pacific basin have not been studied in depth. A
summertime extreme of convective activity inthis frequency range occurs over the western
tropical Pacific region including the South
China Sea (e.g., Chen and Chen 1995; Vincent
et al. 1998; Kiladis 1998). Our study examines
how this convection is triggered and the impact that it has on large-scale circulation. It
seems reasonable to expect that a tropical–
extratropical interaction similar to those mentioned above plays an important role in tropicaland subtropical monsoon activities.
In an earlier study, Fukutomi and Yasunari(1999, hereafter FY99) suggested that a twoway tropical–extratropical interaction on a 10–
25-day timescale exists over the western Pacificduring the early summer (June–July). They examined the relationships between tropical convection over the South China Sea and largescale circulation. They showed a lower- andupper-tropospheric wavetrain with zonal wavenumber 5@6 scale arcing into North Pacificmidlatitudes and suggested that the wavetrain
is amplified as a Rossby-wave response following enhanced (suppressed) convection over theSouth China Sea. A subtropical ridge (trough)that is part of this wavetrain modulates thesubtropical cloud activities over the Baiu frontal zone (BFZ). In the lower troposphere, asubtropical anticyclonic (cyclonic) circulation
anomaly propagates southwestward into theSouth China Sea region where the anomaly can
suppress (enhance) subsequent convection.
They proposed that this two-way interaction
between tropics and extratropics is the dominant feature in the western Pacific during thisseason. Furthermore, seasonal (June–July)mean westerlies continuously extend from thetropical monsoon region to the North Pacific,
acting as a duct that allows southwestward
phase propagation of lower tropospheric waves
from the subtropics to the tropics.
FY99, however, considered data only for four
early summer seasons, from 1991 through 1994,
and ignored the behavior of 10–25-day disturbances during the mid-summer (August). Most
previous studies of summertime intraseasonal
disturbances considered a standard season(JJA). As will be shown in later sections, how-

April 2002

Y. FUKUTOMI and T. YASUNARI

ever, the background basic state on which theintraseasonal disturbances are superimposed
differs considerably between early and midsummer over the western Pacific. The changes
in the mean background flow that occur as thesummer evolves may affect the development of
tropical convective disturbance and their associated large-scale circulation over the western
Pacific. Therefore, the structures of tropical 10–
25-day disturbances and associated large-scale
circulation and their differences should be investigated for each phase of the summer season
to understand the character of subseasonal circulation systems in this region.
FY99 also speculated that dynamical interactions with the time-mean flow helped amplify
and maintain 10–25-day waves. Few studies
have assessed this interaction between submonthly scale eddies and the time-mean flowin the western Pacific during NH summer.
Kawamura and Murakami (1995) examined theinteraction between MJO-timescale eddies andthe mean monsoon flow in the lower troposphere. They suggested that eddies are maintained mostly by barotropic instabilities in thetropics and by baroclinic instabilities in midlatitudes. It is natural to question whether similar mechanisms exist to maintain 10–25-dayeddies.
This study extends FY99. Space-time structures of convection and large-scale circulation
anomalies at the 10–25-day time scale over thewestern Pacific are detailed to identify featuresin two seasons. Composite relationships between tropical convection and large-scale circulations on the 10–25-day timescale during earlysummer (June–July) and mid-summer (August)are used to isolate the dominant circulation
patterns associated with tropical convection forearly and mid-summer. Composites are constructed using data from 1986 through 1995.
The structure and behavior of 10–25-day convective disturbances and associated waves inthe subtropical and extratropical latitudes canbe discerned. The target convection amplifies
over the South China Sea. We will identify theseasonal dependence of the large-scale circulations associated with tropical convection over
the South China Sea. Localized Eliassen–Palm(E–P) flux diagnostics (Trenberth 1986) andtheir application (Kawamura and Murakami
1995) are then used to assess eddy–mean flow

313

interactions and to quantify the role of the mean
flow in maintaining 10–25-day eddies. Eddy
forcing of the mean flow is also investigated.
This paper is organized as follows. Section 2
describes data sources and processing. In Section 3, characteristics of disturbances and of themean flow in the two seasons (June–July andAugust) are given. In Sections 4 and 5, composite relationships between the 10–25-day convective events and large-scale circulation over aselected base region are presented. The structure and characteristics of disturbances are also
outlined in these sections. Section 6 includes
diagnostics of the eddy–mean flow interactions
and an evaluation of the dynamical interaction
between the eddy and time mean flow. The
paper concludes with a summary and discussion in Section 7.
2.

Data sources and processing

This study uses the same datasets as FY99.
Meteorological data (zonal and meridional wind,temperature, geopotential height, and relative
humidity) at ten standard pressure levels (1000,
850, 700, 500, 400, 300, 250, 200, 150, 100 hPa)
were extracted from the European Center for
Medium-Range Weather Forecasts (ECMWF)/
World Climate Research Programme (WCRP)/
Tropical Ocean-Global Atmosphere (TOGA)
BASIC LEVEL III operational analyses from1986 through 1995. We use uninitialized data
defined on a 2.5 lat-long grid. The original temporal resolution was twice daily (0000 and 1200
UTC). Humidity data were only available up to
300 hPa. Further details on the ECMWF data
are provided in Trenberth and Olson (1988).
Specific humidity was computed from relative
humidity and temperature. A daily mean thateliminated the diurnal cycle was computed
using an average of the twice-daily data.
Streamfunctions were computed from the horizontal winds using R25 truncation.
This study also uses outgoing longwave radiation (OLR) data as a proxy for convection. Theinterpolated OLR dataset for times overlapping
the ECMWF data was obtained from the National Oceanic and Atmospheric Administration (NOAA)/Climate Diagnostics Center (CDC)
and has global coverage on a 2.5 lat-long grid.
The first step in the data processing was toremove the seasonal trend from time series of allelements for the five-month period May to Sep-

314

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

tember. The seasonal trend was computed by
the least squares method as a quadratic curvefor the five months. A Butterworth band-passfilter (Kaylor 1977) with six poles was then applied to the time series of each perturbation
quantity to filter temporally into the 10–25-dayband. The 10–25-day band was chosen because
FY99 and Fukutomi (2000) showed that thisfrequency was prominent. In addition, the 10day low-pass cut off and 25-day high-pass cut
off eliminate synoptic- and MJO-timescale fluctuations, respectively. The 10–25-day timescaleis similar to those used by Kiladis (6–30-day)and by Vincent et al. (1998) (6–25-day).
3.

Differences in transient disturbance
activity between June–July andAugustThe 10–25-day disturbance activity is computed from the OLR variance and the perturbation kinetic energy (PKE). The PKE isderived from band-pass filtered meteorological data and is expressed as in Matthews andKiladis (1999a), i.e.,
PKE ¼

1 02
ðu þ v 02 Þ;
2

ð3:1Þ

where u and v are the zonal and meridional
wind components, ðÞ denotes the time mean,and ðÞ 0 is the time deviation with 10–25-dayband-pass filtering.
PKE and OLR variance distributions andbackground mean fields are displayed in Figures 1 and 2, respectively. From these figures,
we can determine where transient disturbances
are most common in June–July and August in
the western Pacific. The PKE fields at 850 hPa(Fig. 1a) show a maximum oriented southwest–
northeast from the South China Sea to theNorth Pacific during June–July. This maximum PKE band parallels the mean low-level
westerlies (as seen in Fig. 1c). In the tropics,
the maximum PKE is over the northern South
China Sea. The OLR variance field for June–
July (Fig. 1b) shows maximum convective activity on the 10–25-day timescale over the South
China Sea and Bay of Bengal, in agreementwith FY99. Past studies have shown that thepeak OLR variance for the 10–25-day timescaleoccurs over these regions during the summer(JJA) (e.g., Kiladis and Weickmann 1997; Vincent et al. 1998). Here, mean fields for June–

Fig. 1. Distributions of mean fields inJune–July 1986–1995. (a) The 10–25day perturbation kinetic energy (PKE)
at 850 hPa. The contour interval is
1 m 2 s2 . Shading indicates PKE lessthan 3 m 2 s2 . (b) Variance and percentage variance of 10–25-day filtered
OLR. The contour interval is 100 W 2m4 . (c) Mean OLR and 850-hPa windvectors and isotachs of zonal wind. Negative contours are dashed. The regionsof OLR values less than 240 W m2 areshaded.

July (Fig. 1c) are consistent with the maturephase of the south and southeast Asian monsoons and the Baiu front, which is marked byquasi-stationary subtropical cloud activity inEast Asia. The monsoon westerlies and thetropical easterlies on average meet over thePhilippines, where the highest disturbance
activity in the tropics is found.

April 2002

Y. FUKUTOMI and T. YASUNARI

315

mean fields (Fig. 2c) change dramatically inAugust, as noted by many previous studies(e.g., Matsumoto 1992; Murakami and Matsumoto 1994). The distribution of the transient
eddies also changes in association with thisstructural change in the mean flow. Disturbance activity in the tropics is closely related tothe spatial distribution of the mean flow, asshown by Webster and Yang (1989).
4.

Fig. 2. As in Fig. 1, but for August 1986–
1995.

A striking feature during August is the eastward extension of the PKE band from theSouth China Sea to the western Pacific in thetropics and subtropics (Fig. 2a). The tropicalportion of this PKE maximum exists in conjunction with a large OLR variance (Fig. 2b) at
130 –150 E. Active regions of convection expand eastward from the South China Sea to thetropical western Pacific as the mean tropicalmonsoon westerlies penetrate into the tropicalwestern Pacific near 150 E (Fig. 2c). This eastward shift from June–July through August issimilar to the shift for MJO-scale eddies demonstrated by Kawamura and Murakami (1995).
In summary, enhanced disturbance activity
in the lower troposphere on the 10–25-daytimescale is concentrated in the exit region of
monsoon westerlies, i.e., where the zonal gradient of the mean westerlies is negative. The

Composite structures during
June–July

Compositing was used to determine thespace–time structures of disturbances associated with tropical convection on 10–25-daytimescales. The composite procedure used wasnearly identical to that of FY99. The reference
OLR time series in the base region 10 –20 N,
110 –120 E (the South China Sea) was adopted
as a convective index. As noted in Section 3,
this is the region of highest convective activity
at the 10–25-day timescale as shown in the
OLR variance (Figs. 1 and 2). Composite maps
include eight states (Categories 1 through 8)that correspond to individual phases of the convection lifecycle over the base region. When thenegative (positive) extreme of the OLR index
averaged over the base region exceeded 1.0
(1.0) standard deviations, that extreme was assigned to Category 3 (Category 7). This procedure holds in all composites in this analysis.
The mean time span between two subsequent
phases is approximately 1.5@2 days. Composite
fields were prepared for both June–July andAugust to reveal the structures and characteristics of disturbances during these times.
The composite patterns are significant at the
95% confidence level as revealed by a standard
t-test.
4.1 Horizontal structures
Two-way interactions between tropics andextratropics over the western Pacific in June–
July were outlined in Section 1. The interactions are present in the space-time structuresof the 10–25-day wave disturbances and therelated tropical–extratropical interactions during June–July as discussed in FY99. Composites in FY99, however, were based on a relatively small number of June–July cases during
1991–94. In the present analysis, we produce
composite fields from a total of 32 cases over 10

316

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

Fig. 3. The composite OLR, 850-hPa streamfunction and vector wind anomalies for Categories 2, 3, and
4 for June–July based on the OLR index in region 10 –20 N, 110 –120 E. The contour interval is
3.0 10 5 m 2 s1 . OLR anomalies less than 4 W m2 are shaded. Only locally statistically significant
wind vectors are shown.

years (1986–1995). June–July composites are
based on the OLR index for the South China
Sea because the OLR variance is largest in thisregion during June–July. The composites arecomparable to those for August, as discussed in
Section 5.
This section focuses only on phases during
enhanced convection (Categories 2–4), as characteristics of the entire composite lifecycle (fromCategory 1 through 8) are described in detail by
FY99. Figure 3 shows horizontal composites of

OLR, 850-hPa wind vector, and streamfunction
anomalies for Categories 2, 3, and 4, bracketingthe peak in convection over the base region.
The relationship between the convection represented by negative OLR anomalies in theSouth China Sea and the large-scale circulation anomalies is present, as in FY99. Following the convective peak (Category 3), a wavetrain clearly emanates from the convective
region into the North Pacific. This wave structure shows good agreement with FY99, but theApril 2002

Y. FUKUTOMI and T. YASUNARI

Fig. 4. As in Fig. 3, but for 200-hPa circulation fields. The contour interval is 5.0

anomalies described here are smoother than in
FY99, reflecting the larger number of samples
used to construct the composites. The cyclonicor anticyclonic circulation anomalies that arepart of this wavetrain are amplified along the
low-level mean westerlies that extend from thetropics to midlatitudes. Patterns of circulation
and OLR anomalies for Categories 6, 7, and 8(not shown) are reversed from Fig. 3. These twocategories represent the phases during whichconvection is suppressed over the South China
Sea. During the composite cycle (Category 1
through 8) provided by FY99, rotational wind
anomalies propagate southwestward from the317

10 5 m 2 s1 .

subtropics to the tropics, and precede the subsequent suppression or enhancement of convection over the base region. These features arealso clear in the present composite life cycle.
We made composites based on the region 10 –20 N, 130 –140 E (not shown); a downstream
wave structure similar to that in Fig. 3 resulted.
Figure 4 is similar to Fig. 3, but shows the
200-hPa wind fields. The composite map showsa wavetrain developing away from the north ofthe convection center, crossing the midlatitude
Pacific. These composite wave anomalies resemble those in FY99 and Mo (2000). The crossequatorial outflow anomalies into the Southern

318

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

Fig. 5. Category–pressure diagrams in the region 10 –20 N, 110 –120 E of the composite values of
the 10–25-day filtered (a) geopotential height (m), (b) zonal wind (m s1 ), (c) temperature (K), and(d) specific humidity (g kg1 ). Dark (light) shading represents positive (negative) significant values.

Hemisphere and the southwesterly outflow
anomalies into midlatitudes are strongest overthe convection in Categories 3 and 4. These
outflows are the divergent response to the convection. Fukutomi (2000) produced compositesof velocity potential and the divergent component of wind anomalies at 200 hPa. These composites show that the upper-level divergent outflow is displaced from the convection center,
which confirms the presence of an immediate
response to the convectively produced diabatic
heating over the South China Sea in the form of
an atmospheric circulation.
4.2 Vertical structures
We examined the vertical structure of the
10–25-day disturbance using cross sections(category–pressure) of temperature, specific hu-

midity, geopotential height, and zonal wind
perturbations averaged along 10 –20 N, 110 –120 E, at pressure levels from 1000 to 100 hPa
in the vertical. The vertical section of geopotential height (Fig. 5a) shows an equivalentbarotropic structure with negative anomalies
extending to about 300 hPa around Category 3.
This suggests that a deep cyclonic vortex in the
low and middle troposphere dominates over thebase region. A sign change occurs between 300
and 200 hPa, and positive anomalies (an anticyclone) are pronounced in the layers above
250 hPa. A similar pattern of anomalies, butof opposite sign, occurs around Category 7. The
zonal wind anomalies (Fig. 5b) show deep westerlies with a maximum at 700 hPa at a position
corresponding to the southern part of the lowlevel monsoon trough in Fig. 3. These deep

April 2002

Y. FUKUTOMI and T. YASUNARI

westerlies are similar to those accompanying
wintertime submonthly-scale waves coupledwith convection in the tropical central Pacific,
as described by Kiladis et al. (1994) and Meehl
et al. (1996). The upper-level easterlies thatstrengthen in the 200–100-hPa layer, capping
the deep westerly flow, show the increase of thevertical easterly shear in the middle to upper
troposphere.
The structures of composite variables over thebase region have also been examined for Category 3 using longitude–pressure and latitude–pressure diagrams (Fukutomi 2000). The phasestructures of the anomalous geopotential heightand zonal winds in the zonal direction show
little tilt in the vertical. Pronounced poleward
phase tilts with height are observed in latitude–pressure diagrams in the 300–150-hPa layer.
Phase nodes in the upper troposphere near the
tropopause imply the predominance of firstbaroclinic modes associated with the strong
convective disturbance. Although Lau and Lau(1990) showed a northwestward phase tilt withheight over the western Pacific of the geopotential height and vorticity anomalies associated with synoptic-scale disturbances withperiod of less than 10 days, our composites do
not have such a structure. This is one of thedifferences between 10–25-day intraseasonal
disturbances and shorter-period disturbances.
Temperature anomalies (Fig. 5c) have a large
maximum between 500 and 200 hPa, above thelow-level cyclonic circulation associated withthe peak convection (Category 3) in Figure 3.
This upper-level warming is probably caused bycondensation. Negative temperature anomalies
in the lower troposphere under the peak in theupper-layer warming may arise from adiabatic
cooling associated with ascent. A deep layer ofmoist air embedded in the low-level cyclonic
circulation during the former half cycle appears
in the specific humidity field (Fig. 5d). Thesemoist conditions are probably organized by cyclonic and convergent inflow in the lower troposphere (see Fig. 3). An increase in moisturein the lowest (1000–850-hPa) layer slightly precedes the peak in convection. Temperature andspecific humidity perturbations create upper
warm- (cool-) and lower moist- (dry-) core structures within cyclonic (anticyclonic) regions
during times of enhanced (suppressed) convection.

5.

319

Composite structure during August

Composites are prepared for OLR and windfields in August for comparison to those ofJune–July. August composites use the samebase region as June–July composites do. Thisregion occurs where the relative maxima of
OLR variance (Fig. 2a) and PKE (Fig. 2b) arelocated. Figure 6a shows horizontal composites
for Category 3 OLR and winds at 850 hPa
using the South China Sea base region. Tencases are averaged to produce this composite.
The disturbance does have a structure similar
to that of June–July. However, a wavetrain
emanating from the convection downstream into
midlatitudes, as shown in Fig. 3, is not clearlyseen associated with this peak in convection.
Furthermore, southwestward phase propagation of circulation anomalies from the subtropics
into the base region does not dominate from
Category 1 through 8 (not shown). In the upper
troposphere (Fig. 6b), weak northerly outflow
away from the base region appears, but the
midlatitude trough /ridge signals are weaker
than in June–July. Statistically significant circulations over the Pacific are confined to thetropics and subtropics near the convection. Effects on large-scale circulations are weaker thanduring June–July.
6.

Dynamical interaction between
transient eddies and the mean flow

This study will show that the time-mean flow
over the western Pacific creates a favorable environment for generating and maintaining highfrequency intraseasonal wave activity. Particular emphasis is placed on processes causing
interactions between eddy activities with timescales of 10–25 days and the background timemean flow in June–July and August.
As shown above and in FY99, tropical convection centered over the South China Sea islikely an important cause of the amplificationof subtropical and midlatitude waves on 10–25day timescales. The tropical–extratropical wave
structure in the composites is representative ofstructures during the early summer. Wave amplification can be triggered by Rossby wave energy dispersion from the localized and periodic
heat sources in the region. Li and Nathan(1994, 1997) and Hsu (1996) show that radiation of wave energy into the extratropics by en-

320

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

Fig. 6. (a) As in Fig. 3, but for Category 3 in August. (b) As in Fig. 4, but for Category 3 in August.

ergy advection and extraction of energy fromthe mean flow must both be considered when
explaining the rapid growth and subsequent
maintenance of tropically forced wave anomalies in the subtropics and midlatitudes. Li andNathan used a spherical nondivergent barotropic model, and showed that the extratropicalresponse to intraseasonal tropical forcing isaffected by the background flow. If the extratropical response is manifest as disturbanceson the locally unstable basic flow, the disturbances will amplify as they extract energy fromthe unstable base state. It is interesting that the
disturbances in Li and Nathan have a timescale
similar to those in this study, although theirnumerical experiment is for a different season.
Wave anomalies that are part of the downstream wave train retrograde (travel southwestward) from the subtropics equatorward to
the tropics over the western Pacific, as shownin the low-level composite for June–July (Fig.
3). The anomalies then enhance or suppress
convection over the South China Sea. These cyclonic or anticyclonic anomalies maintain theirstrength in the tropical and subtropical west-

erly duct region after the extreme in convective
activity, as described by FY99. Maintenance ofthe 10–25-day waves could be linked to interactions between the mean flow and the 10–25day transients within this westerly duct. Thisequatorward-propagating wave signature is oneof the conditions necessary for exciting tropicalconvective disturbances.
The impact of the mean flow on the amplification of lower-tropospheric tropical disturbances over the South China Sea must also be
considered. Kawamura and Murakami (1995)
investigated the role of the deformed mean
tropical and extratropical westerlies on eddy
growth for MJO-scale tropical disturbances.
They showed that stretching and shearing deformation in the exit region of the monsoon
westerlies (in the tropics) supplies barotropic
energy to the MJO-scale eddies. They further
emphasized that baroclinic instability plays animportant role in eddy maintenance in extratropical regions. In the next section, diagnostic
tools based on dynamical theory are used toquantify the substantial contribution of themean flow to the 10–25-day eddy activity. Tran-

April 2002

Y. FUKUTOMI and T. YASUNARI

sient eddy activity and its relationship to themean flow in both the lower and upper troposphere for individual seasons are given.
6.1

Transient wave activities: E–P flux
diagnostics
The net effect of the time-mean flow on transient eddies can be analyzed in terms of theEliassen–Palm (E–P) flux, which was originallyformulated by Hoskins et al. (1983) and subsequently modified by Hendon and Hartmann(1985), Plumb (1986), and Trenberth (1986,
1991). E–P flux vectors are a useful measure of
intraseasonal or high-frequency (synoptic scale)transient eddies. The vectors describe the shapeand propagation characteristics of eddies andtheir feedback onto the mean flow (Matthews
and Kiladis 1999a). Recently, Kiladis (1998) andMatthews and Kiladis (1999a) used Hoskins’s
E vector to investigate submonthly scale (6–30day) wave activities and tropical–extratropicalinteractions over the Pacific in NH winter. They
showed extratropical forcing into the tropics
over the eastern Pacific.
Hurrell and Vincent (1991) and Magaña andYanai (1991) use E u represented by Trenberth(1991) to examine the eddy–mean flow interaction. This formulation is defined by transformed
equations derived from the primitive momentum, thermodynamic, and continuity equations.
Hurrell and Vincent (1991) used the threedimensional vectors to investigate the impact
of upper-level subtropical westerly maxima ontransient eddies over the South Pacific. Magañaand Yanai (1991) used two-dimensional vectors
to examine the tropical–midlatitude interaction
on 30–60-day (MJO) time scales in the uppertroposphere around the mid-Pacific trough.
Kawamura and Murakami (1995) extended
Trenberth’s version and derived a modified E u
for which the vertical (baroclinic) component includes a horizontal eddy latent heat flux effect.
They used this modified E–P flux to assess
the net effects of eddy moisture transport andorganized latent heat release by eddies, andshowed that the MJO-scale eddies are maintained through interactions with the low-levelsummer monsoon. Sobel and Bretherton (1999)
applied the barotropic component of wave activity flux (Plumb 1986) to an investigation of synoptic-scale Rossby waves in the low-level flow
over the western Pacific. The radiative part of321

the Plumb flux is the same as that developed by
Trenberth (1986), except for a pressure weighting. E–P flux diagnostics are a convenient way
to examine eddy–mean flow interactions.
In this analysis, we use the two-dimensional
E vector as in Trenberth (1991). In sphericalgeometry,
1 02ðv  u 02 Þ; u 0 v 0 cos f;
Eu ¼
ð6:1Þ
2
where f denotes latitude. The horizontal (barotropic) components of E u consist of eddy momentum flux, and the horizontal vectors areparallel to the eddy group velocity relative tothe mean flow (Trenberth 1986; Magaña andYanai 1991). Horizontal vectors point in the direction of eddy energy propagation relative tothe background flow. The vertical (baroclinic)
components of E u include the meridional eddy
sensible heat flux and vertical momentum flux
(not shown).
The stationary Rossby wavenumber Ks isused diagnostically to represent the mean background state on which transients are superimposed. Following Hoskins and Ambrizzi(1993), this wavenumber is derived from thenondivergent perturbation vorticity equation
for time-mean flow and is defined asKs ¼

b  U yy
U

1=2
;

ð6:2Þ

where U is the temporal mean of the zonal
wind, b  U yy is the temporal mean of themeridional absolute vorticity gradient, and Ks
is the wavenumber at which a barotropic Rossby
wave is stationary at a particular location in agiven background zonal flow. The wavenumber
acts as a refractive index for stationary Rossby
waves (Kiladis 1998; Matthews and Kiladis
1999a). Originally, Ks predicted the location of
waveguides for stationary Rossby waves (e.g.,
Karoly and Hoskins 1982; Hoskins and Ambrizzi 1993). However, Ambrizzi et al. (1995),
Kiladis (1998), Matthews and Kiladis (1999a)show how Ks can qualitatively describe theeffects of the base state on the propagationof high-frequency transient waves in the upper
troposphere. In this study, Ks is also used todetect westerly ducts that act as waveguides
for 10–25-day waves in both the lower andupper troposphere. Note that Ks is a complex

322

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

Fig. 7. Mean spatial distribution of E u (m 2 s2 ) and zonal wind at (a) 850 hPa during June–July,(b) 850 hPa during August, and (c) 200 hPa during June–August. The contour interval is (a),(b) 2 m s1 , and (c) 5 m s1 . Regions of westerlies are shaded.

value; Rossby waves are evanescent in theimaginary Ks region.
Figure 7 shows the 10–25-day horizontal E uvectors and the mean zonal winds at 850 hPa
in June–July (Fig. 7a), August (Fig. 7b) and at
200 hPa in June–August (Fig. 7c) for 1986–
1995 data over Asia and the Pacific. At 850-hPa
in June–July (Fig. 7a), E u vectors point westward and southwestward along the subtropicaland midlatitude westerlies over the western
Pacific. In addition, increased southward fluxes
are present at the low-level jet core over thecentral Pacific (30 –50 N, 170 E–170 W). E u
vectors point southwestward in the westerly
exit region of the tropical monsoon westerlies,
and point westward to the east of Southeast
Asia. These vectors suggest westward andsouthwestward eddy energy propagation rela-April 2002

Y. FUKUTOMI and T. YASUNARI

323

Fig. 8. Mean spatial distribution of Ks at (a) 850 hPa during June–July, (b) 850 hPa during August,and (c) 200 hPa during June–August. Contours are drawn at zonal wavenumbers 0, 4, 5, 6, 7, 8, 10,
15, 20. Regions of imaginary Ks are shaded.

tive to the mean flow, which is consistent witheastward and northeastward transient momentum transport within the tropical and subtropical westerly duct. These stronger componentsof E u , which are directed up the gradient of themean westerly flow, may be associated with theseasonal ensemble disturbance amplification
shown in Fig. 3. These flux vectors highlightthe predominance of barotropic eddy–mean flowinteractions in this area. Divergence of E u vec-

tors can be interpreted as a forcing on the westerly mean flow (Trenberth 1986). Such divergence occurs locally over the South China Sea
and western Pacific north of 10 N. Convergenceof E u implies mean easterly acceleration andoccurs over southern China, near the Equator
over the South China Sea and western Pacific(south of 10 N), and in the subtropics southeastof Japan. Figure 8a shows the distribution of Ks
at 850 hPa in the same season. Regions of real

324

Journal of the Meteorological Society of JapanKs values are superimposed over climatologicalvalues for June–July tropical monsoon westerlies in South and Southeast Asia, and subtropical and midlatitude westerlies over the Pacific. The westerlies are a preferred region for
Rossby wave energy propagation. Low-levelcomposite wave anomalies (Fig. 3) are locatedalong the distribution of real Ks values.
The distributions of E u and the mean zonal
wind change significantly in the lower troposphere in August (Fig. 7b). One big change
from the mean state of June–July is the eastward penetration of tropical monsoon westerlies into the western tropical Pacific near150 E. Another change is the disappearance ofa continuous duct (present in June–July) as
the mean westerly component over the subtropics to the south of the East China Sea andJapan weakens. The E u vectors are characterized by a westward component directed upstream within the mean monsoon westerlies tothe west of the South China Sea. The vectors
have the same magnitude as in June–July.
Near the exit region of this westerly current
over the western tropical Pacific, where higher
amplitude PKE is found (Fig. 2b), the southwestward pointing E u vectors are of larger
magnitude than in June–July. Large westward
and northwestward vectors are observed in thesubtropics near Japan, where weak mean easterlies dominate. This region of subtropicaleasterlies corresponds to a region of imaginary
Ks (Fig. 8b).
A waveguide at 200-hPa accompanying thewesterlies and the real Ks values is present
from June through August (Fig. 7c and Fig. 8c).
A second duct exists over the tropical central
and eastern Pacific. A region of imaginary Ks
to the north of the midlatitude waveguide is a
result of the negative meridional gradient ofmean absolute vorticity. The E u vectors point
southward, which indicates northward transport of zonal momentum due to eddies in the
midlatitude waveguide. The largest vectors arenear the core of the mean westerly flow over
China and the East Asian coast (20 –50 N,
100 –140 E). These southward and southwestward vectors converge over the East China Sea
and north of the South China Sea. Eddy energy
propagates southwestward from subtropics totropics in the central Pacific westerly duct(10 –25 N, 160 E–140 W). This energy propa-

Vol. 80, No. 2

gation may amplify the mid-Pacific trough as
discussed by Magaña and Yanai (1991). Another feature to note is the group of southwardand southeastward vectors in the exit regionof the westerly jet (35 –50 N, 180 E–150 W).
Enhanced signals in E u vectors are indicativeof the eddy–mean flow interaction at these locations. A separate examination of the upperlevel distribution of Ks and E u for June–Julyand for August similar to that done for Fig. 8(not shown) reveals little change in the structure of the upper-level waveguide from June
through August.
6.2

Interaction between transients and themean flow
The barotropic E u vector signatures associated with 10–25-day eddies along the lowerand upper-level waveguide implies transientmean interactions in the westerly flow. A
simple energetics analysis is performed to describe maintenance processes for the 10–25-day
transient wave activities forced by eddy–meaninteractions in barotropically and baroclinically unstable background flows. The formulation is obtained from the correlation between
the three-dimensional localized E–P flux components and the horizontal and vertical shearsof the mean zonal and meridional flows (Kawamura and Murakami 1995). It includes bothbarotropic and baroclinic eddy–mean interactions. An advantage of the formulation is thatthe effects associated with eddy moisture transport and the release of latent heat by eddies areincluded (see e.g., Salustri and Stone 1983).
Following Kawamura and Murakami (1995),
the local barotropic interaction is given by1ququ cos f
CK ¼  ðv 02  u 02 Þ þ u 0 v 02qx
cos fqy
qv 1
qv cos f
þ u 0 v 0 þ ðv 02  u 02 Þcos f:
qx 2
cos fqyð6:3Þ

Here qx ¼ a cos fql, qy ¼ aqf, a is radius of
the earth, and l refers to longitude. Negativevalues of CK occur when kinetic energy is converted from the mean flow into eddies associated with the eddy momentum transport
within the horizontal shear of the time-mean
flow. The first (fourth) term of CK is the interaction between anisotropic eddies and zonal

April 2002

Y. FUKUTOMI and T. YASUNARI(meridional) shear of the mean westerly (northerly) flow. For example, if zonally elongatededdies (u 02 > v 02 ) are embedded in a region
where qu=qx < 0, the first term becomes negative, which implies a conversion of perturbation
kinetic energy from the mean. The second andthird terms are typical forms of barotropic
instability. If the eddy momentum transport(u 0 v 0 ) is positive in the region of qv=qx < 0 and
qu=qy < 0, eddies can gain kinetic energy fromthe mean flow. In this analysis, barotropic interactions between the 10–25-day eddies andthe time mean flow at both the 850-hPa and200-hPa levels are estimated.
The eddy–mean interactions due to dry andmoist baroclinic processes are calculated to
examine the role of local baroclinic processes
in maintaining eddies associated with the lowlevel 10–25-day waves. These interactions aredefined as


f
qu
qv
CAS ¼
 u 0T 0
v 0T 0cos f;
ð6:4Þ
s
qp
qp


L f
qu
qv
0
0
0
0
u q
vq
CAL ¼
cos f:
cp s
qp
qp

ð6:5Þ

Where f is the Coriolis parameter, p is thepressure, T is the air temperature, q is the
specific humidity, L is the latent heat of condensation, and cp is the heat capacity at a constant pressure. The static stability parameter(s) is calculated using s ¼ RT=cp p  ðqT=qpÞ,where R is the gas constant. These are scalar
products between the horizontal eddy sensible
and latent heat fluxes (the horizontal part ofthree-dimensional E vectors) and the vertical
shear of the mean flow.
Negative CAS and CAL represent local baroclinic conversions from the mean available
potential energy to eddy available potential
energy and are proportional to the amount ofhorizontal sensible or latent heat transport andthe vertical shear of the mean flows. The scalar
product CAS is similar to the form used by Cai
and Mak (1990) and Robinson (1993). The effectof latent heating induced by transient eddies
can be isolated in the CAL estimation. In thesubtropical and midlatitude lower tropospheres,
northward eddy sensible heat and moisture
fluxes (v 0 T 0 > 0; v 0 q 0 > 0) can exist in regions ofwesterly vertical shear (qu=qp < 0), which isconsistent with dry and moist baroclinic insta-

325

bility. Calculation of these values for the layer
between 850 and 700-hPa levels follows methods in Kawamura and Murakami (1995).
Figure 9 shows CK at 850 and 200 hPa. At
850 hPa in June–July (Fig. 9a), large negativevalues (shaded region) extend from the South
China Sea to the tropical western Pacific, theexit region of the tropical monsoon westerlies.
There is a local minimum over southern China.
This suggests that barotropic interaction withthe mean flow is important in the growth andmaintenance of 10–25-day waves in these regions. Estimation of each CK term indicatesthat the first term is of primary importance inthis interaction (not shown). Cyclonic or anticyclonic circulations are zonally elongated inthis tropical jet exit region (Fig. 3); this distribution allows mean to eddy energy conversions
by the first term in CK. MJO-scale eddies aremaintained in the same way in the same region,
as discussed by Kawamura and Murakami(1995). In the subtropics near Japan, however,a band of positive CK values is located along
the low-level mean subtropical westerly flow.
An energy transfer from eddy to mean flow occurs at this low-level subtropical jet entrance.
In August (Fig. 9b), the barotropic energy
conversion from the mean flow to the 10–25-day
eddy increases over the western tropical Pacificfrom 130 –160 E, but decreases slightly over
the South China Sea. As identified in the distributions of PKE and E u , the center of dynamical interaction between the mean flow andtransient eddies in the tropics shifts from theSouth China Sea into the western Pacific. A
weak mean to eddy energy transfer appears inthe subtropics southwest of Japan.
At 200-hPa (Fig. 9c), large positive CK values
persist throughout JJA over East Asia and thesubtropical western Pacific. This maximum coincides with a large area of southward pointing
E u vectors (Fig. 7c) located to the north of theregion of convection (as in Fig. 3). This agrees
with eddies supplying kinetic energy to theupper-level mean flow in the region. The second
term in the CK equation is dominant south ofthe mean westerly core. Conversely, negativevalues lie north of the upper-level jet that extends from central Asia to eastern Pacific during
JJA. The 10–25-day eddies receive kinetic energy from the mean within this region of negative CK values.

326

Journal of the Meteorological Society of Japan

Vol. 80, No. 2

Fig. 9. Mean spatial distribution of barotropic eddy–mean flow interactions (CK) at (a) 850 hPa
during June–July, (b) 850 hPa during August, and (c) 200 hPa during June–August. The contour
interval is (a), (b) 5.0 106 m 2 s3 , and (c) 5.0 105 m 2 s3 . Negative values are shaded.

Baroclinic energy processes for June–July
are shown in Figure 10. These processes dominate in the maintenance of 10–25-day perturbations in extratropical regions just as for thecase of MJO-scale perturbations (Kawamura
and Murakami 1995). The dry baroclinic process (CAS) (Fig. 10a) shows large negativevalues over the midlatitude North Pacific andeastern Asia, with comparatively smaller negative values over the subtropics near the East

Asian coast. Energy conversion from the mean
available potential energy to the 10–25-day
eddy available potential energy is associatedwith transient sensible heat fluxes in these regions. However, the small negative values of
CAS over the subtropical western Pacific suggest that CAS is not important in maintaining
10–25-day waves in this region. In addition, aband of negative CAS values lies to the north ofthe composite wavetrain (Fig. 3). Significant

April 2002

Y. FUKUTOMI and T. YASUNARI

327

Fig. 10. Mean spatial distribution of (a) dry baroclinic (CAS) and (b) moist baroclinic (CAL) eddy–
mean flow interactions. Contour intervals are 2.0 105 m 2 s3 . Negative values are shaded.

values of CAS are not observed in the tropics,
suggesting that CAS is unimportant for 10–25day waves in the tropical lower troposphere.
The field of moist baroclinic processes (CAL)
for June–July (Fig. 10b) is characterized by azonally large area of negative values in the extratropical regions of Asia and the Pacific. The
CAL contribution exceeds that of CAS in thesubtropics and midlatitudes over the entire
Pacific. The band of negative values is more
pronounced over the North Pacific, and the areal
extent of negative values of CAL is greaterthan that of CAS. This suggests that moist
baroclinic processes are more important thandry baroclinic processes in maintaining 10–25day eddies in the subtropics and midlatitudes
during June–July. The prominence of baroclinic
processes in the subtropics and midlatitudes is
associated with the northward eddy sensible
and latent heat transport within the mean

westerly vertical wind shear in the lower troposphere. This physical process is the same asfor extratropical 40-day eddies (Kawamura andMurakami 1995). There is a region of negative
CAL over the South China Sea, indicating thatmoist baroclinic eddy–mean flow interactions
occurs locally within this very narrowly defined
region.
The August distribution of baroclinicallyforced eddy–mean flow interactions showsslight differences to that of June–July. The bandof negative CAS values along the midlatitude
westerlies shifts slightly to higher latitudes.
This poleward shift is also observed in the negative CAL to the east of Japan. These shifts
might be caused by a decrease in westerly vertical shear of the mean zonal wind in the subtropics and lower-midlatitudes, which is attributed to the northward shift of mean westerlies
and the disappearance of a mean westerly com-

328

Journal of the Meteorological Society of Japanponent in the subtopics. A negative CAL region
overspreads the subtropical Pacific east of140 E.
7.

Summary and discussion

The structure and maintenance of 10–25-day
disturbances over the western Pacific during
summer were investigated. The disturbanceswere examined for two different seasons,namely June–July and August. The characteristics and behavior of anomalies in composites
are identified for individual seasons and compared between seasons. E–P flux diagnostics
and an energetics analysis reveal the roles ofbarotropic and baroclinic interactions between
eddy and time-mean flow in maintaining transient wave activities in the lower troposphere.
In particular, the forcing of transient eddies on
the time-mean flow in the upper troposphere isalso described.
Composites for June–July suggest significant
relationships between tropical convection andextratropical circulations with timescales of 10–
25 days. FY99 and this paper both suggest thattropical convection over the South China Sea
can force subtropical and midlatitude circulations through a downstream Rossby wave response. Subsequent to the maximum enhancement (suppression) of convection, a downstream
wave train is well developed toward the North
Pacific along the subtropical and midlatitude
westerlies in both the lower and upper troposphere. In the lower troposphere, a subtropicaltrough (ridge) is strengthened as part of thiswavetrain. Subtropical anomalies in this wavetrain then move southwestward into the tropics,
and initiate subsequent suppression (enhancement) of convection in the South China Sea
region (cf., FY99). This two-way interaction
between the tropics and the extratropics is adominant feature of the convectively coupled
10–25-day waves during June–July.
August composites show the large-scale
structure and behavior of circulation anomalies
in the lower-troposphere to be considerably different from June–July. The downstream wavetrain emanating from the South China Sea ispoorly defined in the August composites. Moreover, there is no evidence of southwestward
anomaly propagation around this base region.
The composite circulation anomalies have westward and northwestward tracking character-Vol. 80, No. 2

istics. A two-way interaction mechanism does
not become established in the lower troposphere
during August.
The difference in the structure of the mean
background flow between seasons may be responsible for the differences in the composite
structures in the lower troposphere between
June–July and August. During June–July, continuous mean westerlies extend from the tropics to midlatitudes in the lower troposphere.
FY99 and this study suggest that this region ofwesterlies is a waveguide for the Rossby wavetrain and its southwestward phase propagationfrom the subtropics into the tropics over thewestern Pacific. During August, however, thestructure of the low-level mean flow changes
and the subtropical portion of the duct vanishes. This inhibits subtropic to tropic phasepropagation. In addition, the absence of thearc-like downstream wavetrain may be related
to the large-scale basic state, which includes a
discontinuous waveguide between tropics andextratropics. The sensitivity of the extratropicalresponse to transient convection acting on abasic state, and the phase propagation of forced
waves can be checked using GCMs, as done by
Jin and Hoskins (1995), Newman and Sardeshmukh (1998), and Matthews and Kiladis(1999b).
Eddy–mean flow interactions that maintainthe 10–25-day waves are investigated. In thelower troposphere, barotropic processes maintain the 10–25-day eddies in the exit region of
the monsoon westerly flow in the tropical western Pacific. Convectively coupled rotational disturbances are amplified in this region, not only
by diabatic effects such as external heating(or cooling) associated with convection, but also
by barotropic eddy–mean flow interactions. In
contrast, the 10–25-day eddies supply kinetic
energy to the mean flow over the subtropicalwestern Pacific in June–July. Dry and moist
baroclinic processes maintain the 10–25-dayeddies in the subtropics and midlatitudes. The
main contributor to the growth of the southwestward propagating waves during June–Julyis condensational heating produced by eddies
embedded within a westerly vertical shear, especially in the subtropics. This subtropical ductwith westerly vertical shear can support
favorable conditions for wave maintenance.
Baroclinic growth of the 10–25-day eddies

April 2002

Y. FUKUTOMI and T. YASUNARI

decreases over the subtropics south of Japan
during August reflecting the weaker vertical
shear in that season.
An energetics analysis shows that baroclinic
processes associated with eddy sensible and latent heat transport are minor contributors to
the maintenance or amplification of the lowlevel 10–25-day waves in the tropical domain.
Barotropic processes dominate eddy–mean flowinteractions in the tropical lower troposphere.
Our results emphasize that barotropic processes
and diabatic processes forced by convection areimportant in amplifying low-level waves in thisregion. A complete three-dimensional energetics
analysis using composite anomalies and theirlife cycles, as done by Lau and Lau (1992), could
clarify the relative roles of barotropic and baroclinic processes.
E–P flux diagnostics and energetics for the
lower troposphere suggest that eddy energy accumulates in the exit region of tropical monsoon westerlies. This energy accumulation may
be related to phase propagation of wave anomalies from other regions, as shown in the composites, and the energy can be enhanced throughbarotropic interactions with the mean flow.
Negative longitudinal stretching shear of thezonal wind (qu=qx < 0) is effective for barotropic energy conversions from the mean flow toeddies in the exit region of the westerly monsoon. In particular, as a Rossby response to theconvective heating, a wavetrain emanates fromthis energy-accumulation region into the extratropics during June–July. Webster and Chang(1988) proposed a similar energy accumulation–
emanation mechanism.
Eddy–mean flow interactions in the upper
troposphere have been analyzed. The 10–25day eddies act as an energy source for the mean
flow in the subtropics north of the tropical convection from 100 E to 150 E in the summer.
There are large southward pointing E u vectors
and barotropic conversion from eddy to mean
kinetic energy over the subtropics in this longitudinal sector, and its location roughly corresponds to that of the amplified waves in thecomposites (Fig. 4). Enhanced eddies centeredover the South China Sea produced by convection at this timescale may be an important energy source for the upper-level subtropical mean
flow, especially in June–July.
We have focused on the structure and main-329

tenance of the 10–25-day disturbances over thewestern pacific domain in Northern Hemispheresummer, leaving other subjects to be examinedlater. As discussed by Hartmann et al. (1992),
Schnadt et al. (1998), Heta (1990), and Chen
and Weng (1999), the relationship between theoccurrence of tropical cyclones and this higherfrequency intraseasonal oscillation in the tropics is an important subject for future research.
Furthermore, the interaction between high frequency eddies and the MJO will be demonstrated in the future, as was done with theNorthern Hemisphere winter case by Matthews
and Kiladis (1999a). Scale interactions associated with the low-frequency waves, as demonstrated by Higgins and Schubert (1993, 1994)
in the summer over Asia and the Pacific, require further study. Since our focus was on thecharacteristics of the 10–25-day disturbance itself, scale interactions were not considered here.
The interannual variability of convection andthe activity of transient eddies on this timescale
and their relation to ENSO and the troposphericbiennial oscillation is also an interesting topic
for future research.
Acknowledgments
We wish to acknowledge the insightful comments and suggestions from anonymous reviewers and Dr. N. Iwasaka. They were extremely valuable in revising an earlier versionof this manuscript. We would also like to thankDr. R. Kawamura for encouragement and useful discussion. This paper is part of the Ph.D.
thesis of Y. Fukutomi of the Institute of Geoscience, University of Tsukuba. ECMWF analyses were kindly provided by Dr. H.L. Tanaka
and are the same as those used by Tanaka andKimura (1997). The OLR data were obtained
from the NOAA Climate Diagnostics Center(http://www.cdc.noaa.gov). Figures were madewith the Generic Mapping Tools (GMT) graphics system developed by Wessel and Smith(1995). This study was supported in part by aGrant-in-Aid for Scientific Research on Priority
Areas (B) from the Japanese Ministry of Education, Science, Sports, and Culture, No. 11201101(Energy and Water Cycle in Asian Monsoon
Region), and was completed under the auspicesof the Hydrological Cycle Research Program of
the FRSGC.

330

Journal of the Meteorological Society of Japan

References
Ambrizzi, T., B.J. Hoskins, and H.-H. Hsu, 1995:
Rossby wave propagation and teleconnection
patterns in the austral winter. J. Atmos. Sci.,
52, 3661–3672.
Cai, M. and M. Mak, 1990: On the basic dynamics ofregional cyclogenesis. J. Atmos. Sci., 47, 1417–
1442.
Chen, T.-C. and J.-M. Chen, 1995: An observational
study of the South China Sea monsoon during
the 1979 summer: Onset and life cycle. Mon.
Wea. Rev., 123, 2295–2318.
——— and S.-P. Weng, 1999: Interannual and intraseasonal variations in monsoon depressions
and their westward-propagating predecessors.
Mon. Wea. Rev., 127, 1005–1020.
Fukutomi, Y. and T. Yasunari, 1999: 10–25-day intraseasonal variations of convection and circulation over East Asia and western North
Pacific during early summer. J. Meteor. Soc.
Japan, 77, 753–769.
———, 2000: Tropical–extratropical interactions on
the intraseasonal timescales over the western
Pacific during early summer. Ph.D. thesis, Institute of Geoscience, University of Tsukuba,
162 pp.
Hartmann, D.L., M.L. Michelsen, and S.A. Klein,
1992: Seasonal variations of tropical intraseasonal oscillations: A 20–25-day oscillationin the western Pacific. J. Atmos. Sci., 49, 1277–
1289.
Hendon, H.H. and D.L. Hartmann, 1985: Variability
in a nonlinear model of the atmosphere withzonally symmetric forcing. J. Atmos. Sci., 42,2783–2797.
Heta, Y., 1990: An analysis of tropical wind fields inrelation to typhoon formation over the western
Pacific. J. Meteor. Soc. Japan, 68, 65–77.
Higgins, R.W. and S.D. Schubert, 1993: Low-frequency synoptic-eddy activity in the Pacificstorm track. J. Atmos. Sci., 50, 1672–1690.
——— and ———, 1994: Simulated life cycles of
persistent anticyclonic anomalies over the
North Pacific: role of synoptic-scale eddies. J.
Atmos. Sci., 51, 3238–3260.
Hoskins, B.J., I.N. James, and G.H. White, 1983: The
shape, propagation and mean-flow interactionof large-scale weather systems. J. Atmos. Sci.,
40, 1595–1612.
——— and T. Ambrizzi, 1993: Rossby wave propagation on a realistic longitudinally varying
flow. J. Atmos. Sci., 50, 1661–1671.
Hsu, H.-H., 1996: Global view of the intraseasonal
oscillation during northern winter. J. Climate,96, 2386–2406.

Vol. 80, No. 2

Hurrell, J.W. and D.G. Vincent, 1991: On the maintenance of short-term subtropical wind maxima in the Southern Hemisphere during SOP1, FGGE. J. Climate, 4, 1009–1022.
Jin, F. and B.J. Hoskins, 1995: The direct responseto tropical heating in a baroclinic atmosphere.
J. Atmos. Sci., 52, 307–319.
Karoly, D.J. and B.J. Hoskins, 1982: Threedimensional propagation of planetary waves. J.
Meteor. Soc. Japan, 60, 109–123.
Kawamura, R. and T. Murakami, 1995: Interaction
between the mean summer monsoon flow and
45-day transient perturbations. J. Meteor. Soc.
Japan, 73, 1087–1114.
Kaylor, R.E., 1977: Filtering and decimation of digital time series. Tech. Note BN 850, Instituteof Physical Science Technology, University of
Maryland, College Park, 42 pp.
Kiladis, G.N. and K.M. Weickmann, 1992a: Circulation anomalies associated with tropical convection during northern winter. Mon. Wea. Rev.,
120, 1900–1923.
——— and ———, 1992b: Extratropical forcing of
tropical convection during northern winter.
Mon. Wea. Rev., 120, 1924–1938.
———, G.A. Meehl, and K.M. Weickmann, 1994:
Large-scale circulation associated with westerly wind bursts and deep convection over thewestern equatorial Pacific. J. Geophys. Res.,99(D9), 18527–18544.
——— and M. Wheeler, 1995: Horizontal and vertical structure of observed tropospheric equatorial Rossby waves. J. Geophys. Res., 100(D11),22981–22997.
——— and K.M. Weickmann, 1997: Horizontal
structure and seasonality of large-scale circulations associated with submonthly tropicalconvection. Mon. Wea. Rev., 125, 1997–2013.
———, 1998: Observations of Rossby waves linked toconvection over the eastern tropical Pacific. J.
Atmos. Sci., 55, 321–339.
Lau, K.-H. and N.-C. Lau, 1990: Observed structure
and propagation characteristics of tropicalsummertime synoptic scale disturbances. Mon.
Wea. Rev., 118, 1888–1913.
——— and ———, 1992: The energetics and propagation dynamics of tropical summertime synoptic-scale disturbances. Mon. Wea. Rev., 120,
2523–2539.
Li, L. and T.R. Nathan, 1994: The global atmospheric response to low-frequency tropical forcing: Zonally averaged basic states. J. Atmos.
Sci., 51, 3412–3426.
——— and ———, 1997: Effects of low-frequency
Tropical Forcing on intraseasonal tropical–
extratropical interactions. J. Atmos. Sci., 54,
332–346.

April 2002

Y. FUKUTOMI and T. YASUNARI

Liebmann, B., G.N. Kiladis, J.A. Marengo, T. Ambrizzi, and J.D. Glick, 1999: Submonthly convective variability over South America and thesouth Atlantic convergence zone. J. Climate,12, 1877–1891.
Madden, R.A. and P.R. Julian, 1972: Description of
global scale circulation cells in the tropics with a
40–50 day period. J. Atmos. Sci., 29, 1109–1123.
——— and ———, 1994: Observations of the 40–50day tropical oscillation: A review. Mon. Wea.
Rev., 122, 814–837.
Magaña, V. and M. Yanai, 1991: Tropical–
midlatitude interaction on the time scale of 30
to 60 days during the northern summer of
1979. J. Climate, 4, 180–201.
Matsumoto, J., 1992: The seasonal changes in Asianand Australian monsoon regions. J. Meteor.
Soc. Japan, 70, 257–273.
Matthews, A.J. and G.N. Kiladis, 1999a: The
tropical–extratropical interaction between
high-frequency transients and the Madden–
Julian oscillation. Mon. Wea. Rev., 127, 661–
677.
——— and ———, 1999b: Interactions between
ENSO, transient circulation, and tropical convection over the Pacific. J. Climate, 12, 3062–
3086.
Meehl, G.A., G.N. Kiladis, K.M. Weickmann, M.
Wheeler, D.S. Gutzler, and G.P. Compo, 1996:
Modulation of equatorial subseasonal convective episodes by tropical–extratropical interaction in the Indian and Pacific Ocean regions. J.
Geophys. Res., 101(D10), 15033–15049.
Mo, K.C., 1999: Alternating wet and dry episodes
over California and intraseasonal oscillations.
Mon. Wea. Rev., 127, 2759–2776.
———, 2000: Intraseasonal modulations of summer
precipitation over North America. Mon. Wea.
Rev., 128, 1490–1505.
Murakami, T. and J. Matsumoto, 1994: Summer
monsoon over the Asian continent and Western Pacific. J. Meteor. Soc. Japan, 72, 719–
745.
Newman, M. and P.D. Sardeshmukh, 1998: The impact of the annual cycle on the North Pacific/
North American response to remote lowfrequency forcing. J. Climate, 11, 2491–2504.
Numaguti, A., 1995: Characteristics of 4-to-20-dayperiod disturbances observed in the equatorial
Pacific during the TOGA COARE IOP. J.
Meteor. Soc. Japan, 73, 353–377.
Plumb, R.A., 1986: Three-dimension propagation oftransient quasi-geostrophic eddies and its relationship with the eddy forcing of the timemean flow. J. Atmos. Sci., 43, 1657–1678.
Robinson, W.A., 1993: Mechanisms of low-frequency

331

variability in a simple model with orography.
J. Atmos. Sci., 50, 878–888.
Salustri, G. and P.H. Stone, 1983: A diagnostic studyof the forcing of the Ferrel cell by eddies withlatent heat effects included. J. Atmos. Sci., 40,
1101–1109.
Schrage, J.M. and D.G. Vincent, 1996: Tropical convection on 7–21 day timescales over the Western Pacific. J. Climate, 9, 587–607.
Schnadt, C., A. Fink, D.G. Vincent, J.M. Schrage,
and P. Speth, 1998: Tropical cyclones, 6–25
day oscillations, and tropical–extratropical interaction over the northwestern Pacific. Meteor. Atmos. Phys., 68, 151–169.
Sobel, A.H. and C.S. Bretherton, 1999: Developmentof synoptic-scale disturbances over the summertime tropical northwest Pacific. J. Atmos.
Sci., 56, 3106–3127.
Tanaka, H.L. and K. Kimura, 1998: Time series
analysis of natural variability in barotropic
energy of the atmosphere with ECMWF global
analysis. J. Meteor. Soc. Japan, 76, 207–274.
Trenberth, K.E., 1986: An assessment of the impactof transient eddies on zonal flow during a
blocking episode using localized Eliassen–
Palm flux diagnostics. J. Atmos. Sci., 43, 2070–
2087.
———, 1991: Storm tracks in the Southern Hemisphere. J. Atmos. Sci., 48, 2159–2178.
——— and J.G. Olson, 1988: An evaluation and intercomparison of global analyses from the National Meteorological Center and the European
Center for Medium Range Weather Forecasts.
Bull. Amer. Meteor. Soc., 69, 1047–1057.
Vincent, D.G., K.-C. Ko, and J.M. Schrage, 1997:
Subtropical jet streaks over the South Pacific.
Mon. Wea. Rev., 125, 438–447.
———, A. Fink, J.M. Schrage, and P. Speth, 1998:
High- and low-frequency intraseasonal variance of OLR on annual and ENSO timescales.
J. Climate, 11, 968–986.
Webster, P.J. and H.-R. Chang, 1988: Equatorial energy accumulation and emanation regions: Impacts of a zonally varying basic state. J. Atmos.
Sci., 45, 803–829.
——— and S. Yang, 1989: The three-dimensional
structure of perturbation kinetic energy and its
relationship to the zonal wind field. J. Climate,2, 1210–1222.
Wheeler, M. and G.N. Kiladis, 1999: Convectively
coupled equatorial waves: Analysis of clouds
and temperature in the wavenumber-frequency
domain. J. Atmos. Sci., 56, 374–399.
Wessel, P. and W.H.F. Smith, 1995: New version of
the Generic Mapping Tools released. EOS
Trans. Amer. Geophys. U., 76, 329.