Observational Analysis of Tropical Cyclone …...monsoon gyre activity during the period 2000–10...
Transcript of Observational Analysis of Tropical Cyclone …...monsoon gyre activity during the period 2000–10...
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Observational Analysis of Tropical Cyclone Formation Associated with Monsoon Gyres
LIGUANG WU, HUIJUN ZONG, AND JIA LIANG
Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology,
Nanjing, and State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China
(Manuscript received 6 April 2012, in final form 31 October 2012)
ABSTRACT
Large-scalemonsoon gyres and the involved tropical cyclone formation over thewesternNorth Pacific have
been documented in previous studies. The aim of this study is to understand how monsoon gyres affect
tropical cyclone formation.An observational study is conducted onmonsoon gyres during the period 2000–10,
with a focus on their structures and the associated tropical cyclone formation.
A total of 37monsoon gyres are identified inMay–October during 2000–10, amongwhich 31monsoon gyres
are accompanied with the formation of 42 tropical cyclones, accounting for 19.8%of the total tropical cyclone
formation.Monsoon gyres are generally located on the poleward side of the compositedmonsoon troughwith
a peak occurrence in August–October. Extending about 1000 km outward from the center at lower levels, the
cyclonic circulation of the compositedmonsoon gyre shrinks with height and is replacedwith negative relative
vorticity above 200 hPa. Themaximumwinds of the compositedmonsoon gyre appear 500–800 km away from
the gyre center with a magnitude of 6–10 m s21 at 850 hPa. In agreement with previous studies, the com-
posited monsoon gyre shows enhanced southwesterly flow and convection on the south-southeastern side.
Most of the tropical cyclones associated with monsoon gyres are found to form near the centers of monsoon
gyres and the northeastern end of the enhanced southwesterly flows, accompanying relatively weak vertical
wind shear.
1. Introduction
The summertime monsoon circulation over the trop-
ical western North Pacific (WNP) and South China Sea
(SCS) is usually characterized with a low-level monsoon
trough, in which westerly monsoon winds lie in the
equatorward portion while easterly trade winds exist on
the poleward side (Holland 1995). The monsoon trough
is closely associated with a large fraction of tropical cy-
clone (TC) formation in the WNP and SCS (Gray 1968;
Ramage 1974; Briegel and Frank 1997; Ritchie and
Holland 1999). Sometimes the monsoon trough is re-
placed by a large-scale monsoon gyre, which is a nearly
circular cyclonic vortex with a diameter of about
2500 km (Lander 1994, 1996; Harr et al. 1996). Studies
suggest that such a monsoon gyre has important impli-
cations for TC formation, structure, and motion in the
WNP and SCS (Carr and Elsberry 1995; Ritchie and
Holland 1999; Chen et al. 2004; Wu et al. 2011a,b; Liang
et al. 2011). Holland (1995) and Molinari et al. (2007)
argued that equatorward-moving midlatitude distur-
bances play a role in the gyre formation by producing
a region of persistent diabatic heating near 158N and
that a monsoon gyre forms to the west of the heating as a
result of the Gill-type response (Gill 1980). Recently,
Molinari and Vollaro (2012) suggested that such cyclonic
gyres were related to the interactions of the Madden–
Julian oscillation (MJO) and the midlatitude jet.
Our current understanding of TC formation associ-
ated with monsoon gyres is mainly from a few obser-
vational studies. Lander (1994) first conducted a case
study on the TC formation associated with the monsoon
gyres in 1991 and 1993. In his study, themonsoon gyre of
August 1991 was associated with the genesis of six TCs
and finally the monsoon gyre itself developed into a giant
typhoon, while three small TCs formed in the monsoon
gyre of July 1993. He argued that two modes associated
with TC formation in a monsoon gyre are 1) small TCs
that form in the eastern periphery of a monsoon gyre and
2) a giant TC that develops from the gyre itself. Molinari
Corresponding author address:Dr. LiguangWu, Pacific Typhoon
Research Center, Key Laboratory of Meteorological Disaster of
Ministry of Education, Nanjing University of Information Science
and Technology, Nanjing 210044, China.
E-mail: [email protected]
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DOI: 10.1175/JAS-D-12-0117.1
� 2013 American Meteorological Society
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et al. (2007) also examined the TC formation associated
with the monsoon gyre of August 1991 and found that
the monsoon gyre was actually associated with an
equatorial Rossby wave packet with a period of 22 days
and a wavelength of 3600 km. Ritchie and Holland
(1999) examined the relationship between monsoon
gyre activity and TC formation during 8 yr (1984–92, but
not 1989) and found that 3% of the TC formation events
were associated with monsoon gyres over the WNP.
Based on the 24-yr data from 1979 to 2002, however,
Chen et al. (2004) examined the interannual variations
of TC formation associated monsoon gyres and found
that about 70% of TC formation events were linked to
monsoon gyres. This difference may be due to their
different lifespans in selecting monsoon gyres. Although
the associated mechanisms are not well known, these
studies suggested that monsoon gyres play an important
role in TC formation in the WNP basin.
Monsoon gyres are subjected to Rossby wave (b in-
duced) energy dispersion. The energy dispersion associ-
ated with a barotropic vortex was extensively investigated
in the presence of the planetary vorticity gradient or the
beta effect (e.g., Anthes 1982; Flierl 1984; Chan and
Williams 1987; Luo 1994; McDonald 1998; Shapiro and
Ooyama 1990). While the TC formation associated with
the energy dispersion of preexisting TCs has been re-
cently investigated (Li and Fu 2006; Ge et al. 2008), little
is known about how the energy dispersion of monsoon
gyres plays a role in TC formation. Using a barotropic
vorticity model, Carr and Elsberry (1995) demonstrated
that monsoon gyres underwent the b-induced energy
dispersion, producing strong ridging to the east and
southeast and an intermediate region of high southerly
winds. According to Holland (1995), the region where
westerlies meet easterlies may be important for trapping
tropical waves, accumulating wave energy, sustaining
the long-lived mesoscale convective system (MCS), and
providing a favorable environment for TC formation.
Sobel and Bretherton (1999) and Kuo et al. (2001) also
suggested that nondivergent barotropic Rossby waves
could grow in a region where westerlies meet easterlies,
providing the seedlings for TCs. Thus it is conceivable
that the Rossby wave (b induced) energy dispersion can
play an important role in TC formation in the presence
of monsoon gyres.
The main objective of this study is to advance our
understanding on the role of monsoon gyres in TC for-
mation. We identify all of the occurrences of monsoon
gyres and the associated TC formation events during
the period 2000–10. In particular, composite analysis is
performed to reveal climatological features of mon-
soon gyre activity and the associated TC formation, the
three-dimensional structure of monsoon gyres, and the
favorable locations of the associated TC formation. The
rest of the paper is organized as follows. In section 2, the
observational data and identification of monsoon gyres
are described, followed by the climatological features of
monsoon gyre activity during the period 2000–10 in
section 3. The composite structure of monsoon gyres
and their relationship with TC formation are discussed
in sections 4 and 5, respectively. A summary of the ob-
servational analysis presents in section 6.
2. Data and identification of monsoon gyres
Three main types of datasets are used in this study.
The TC information in the WNP basin is from the Joint
Typhoon Warning Center (JTWC) best-track dataset,
which includes the TC center position (latitude and
longitude), the maximum sustained wind speed, and the
minimum sea level pressure. TC formation in this study
is defined when the maximum sustained wind of a TC
first exceeded 17 m s21 in the JTWC dataset. The wind
fields are based on the National Centers for Environ-
mental Prediction (NCEP) final (FNL) operational
global analysis data on 1.08 3 1.08 grids at every 6 h
(http://rda.ucar.edu/datasets/ds083.2/). This product is
from the Global Forecast System (GFS) that is opera-
tionally run 4 times a day in near–real time at NCEP. To
compare the identified monsoon gyres with those in
previous studies (Lander 1994; Chen et al. 2004), we also
use the NCEP–National Center for Atmospheric Re-
search (NCAR) reanalysis data (Kalnay et al. 1996). The
data for deep convective activity associated with mon-
soon gyres are from the National Oceanic and Atmo-
spheric Administration (NOAA) outgoing longwave
radiation (OLR) dataset, which is available once a day
on 2.58 3 2.58 grids. The May–October activity of
monsoon gyres during the period 2000–10 is the focus in
this study.
According to the American Meteorological Society
(AMS)Glossary ofMeteorology (http://glossary.ametsoc.
org/wiki/Main_Page), a monsoon gyre over the WNP is
characterized by 1) a very large nearly circular low-level
cyclonic vortex that has an outermost closed isobar with
a diameter on the order of 2500 km, 2) a relatively long
(;2 weeks) life span, and 3) a cloud band bordering the
southern through eastern periphery of the vortex/surface
low. It is obvious that monsoon gyres are large-scale,
low-frequency phenomena.
In this study, a monsoon gyre is selected if its diameter
is at least 2500 km with a band or a large area of deep
convection in its southern and southeastern periphery.
The identification of monsoon gyres in this study is
based on the low-pass filtered wind fields at 850 hPa. To
reduce the bias of TC circulation in filtering, we first use
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the procedure proposed by Kurihara et al. (1993, 1995)
to subtract TC circulation from the FNL wind field. This
procedure has been used for removing TC circulation
from the analysis data in TC simulation and forecast
(Kurihara et al. 1993, 1995; Wu et al. 2002), as well as in
the study of the influence of TCs on low-frequency vari-
ability (Hsu et al. 2008). Readers are referred toKurihara
et al. (1993, 1995) for details. Then a low-pass Lanczos
filter with a 10-day cutoff period is applied to the wind
fields to obtain the low-frequency flows (Duchon 1979).
The synoptic-scale disturbances including TCs are
obtained as the difference between the unfiltered and
filtered fields. The center, size, and strength of a mon-
soon gyre are determined from the filtered wind fields
based on a combination of relative circulation calcula-
tion and visual examination. First, the relative circula-
tion on each grid within a radius of 660 km (68 in latitudeand longitude, which is nearly half of the radius of
a candidate monsoon gyre) is calculated for the filtered
850-hPa wind field at 6-h intervals. One or two circula-
tion maxima are selected as the initial gyre centers over
the whole tropical western Pacific region. Once a can-
didate gyre center is selected, the gyre size is visually
determined, which is the minimum diameter of the
outermost wind vectors that constitute a closed vortex.
The gyre center is adjusted visually to be the circulation
center. Considering the gyre size that is at least 2500 km
in diameter, the monsoon gyre strength is measured by
the circulation within a radius of 1250 km from the gyre
center.
To verify the identification method, the two monsoon
gyres that were investigated by Lander (1994) and Chen
et al. (2004), respectively, are first examined with the
2.58 3 2.58NCEP reanalysis data. The two cases are also
identified as monsoon gyres with our method. In Lander
(1994), the size of the monsoon gyre that occurred in
1991 was measured with the outermost closed isobar.
Here we can take the contour of 1005 hPa as the out-
most closed isobar of the monsoon gyre. Figure 1 com-
pares the surface pressure and the 850-hPa low-pass
filtered wind fields, suggesting that the monsoon gyre
indicated by the outermost closed isobar can be identi-
fied in the filtered wind field. At 0000 UTC 12 August
1991 (Figs. 1a,c), the monsoon gyre was associated with
two TCs (Ellie and Fred) and a tropical depression
(138W). Seven days later Ellie moved with the north-
westerly flows of the gyre and became a tropical de-
pression, while Typhoon Gladys was nearly collocated
FIG. 1. (a),(b) Themonsoon gyre detected with unfiltered sea level pressure (contours, interval5 2.5 hPa) in Lander
(1994) and (c),(d) 10-day low-pass filtered 850-hPa wind fields at (a),(c) 0000 UTC 12 Aug and (b),(d) 0000 UTC
19 Aug 1991. Triangles, small dots, and large dots indicate the locations of tropical disturbances, tropical cyclones,
and the centers of monsoon gyres, respectively.
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with the gyre (Figs. 1b,d) and finally transformed the
monsoon gyre into a giant typhoon (Lander 1994). In
Chen et al. (2004), the circulation of themonsoon gyre in
1989 was identified with unfiltered 850-hPa streamlines
(Fig. 2). As shown in this figure, the identified structure
of the monsoon gyre with the 850-hPa low-pass filtered
winds agrees well with the streamlines. At 1200 UTC
28 July 1989 (Figs. 2a,c), Typhoon Judy and Tropical
Depression 12W were associated with the monsoon
gyre. At 0000 UTC 31 July 1989 (Figs. 2c,d), the tropical
depression moved southwestward and Tropical Storm
Ken–Lola and Typhoon Mac appeared to the north and
southeast of the gyre center, respectively. Thus, the
identification method in this study is consistent with
those used in Lander (1994) and Chen et al. (2004), al-
though low-pass filtered wind fields are used in this
study.
3. Monsoon gyre activity over 2000–10
Using the identification method described above,
a total of 37 monsoon gyres are identified during the
period 2000–10, with an average of 3.4 monsoon gyres
per year. The annual occurrence frequency is much
higher than that in Lander (1994), in which monsoon
gyres are fairly uncommon, on average occurring once
every 2 yr. Chen et al. (2004) identified the monsoon
gyres during the period 1979–2002 with a life span of at
least 5 days, suggesting a much higher occurrence rate
(6 monsoon gyres each year) than that in our analysis.
The difference may result from the different lifetime
requirements in identifying monsoon gyres. In this study,
the lifespan of a monsoon gyre is the period that the
gyre can be identified as a closed cyclone with a size of
at least 2500 km. In our study, two cases have a lifespan
of at least 14 days, comparable to the result of Ritchie
and Holland (1999). However, the 31 cases that have a
lifespan of at least 5 days indicate a lower occurrence
rate than that in Chen et al. (2004). We speculate that
the result in Chen et al. (2004) may include large TCs
since they used unfiltered wind fields. In our selected
37 cases, the lifespans of monsoon gyres range from 4 to
17 days, with an average of 8.0 days.
Figure 3 shows the monthly frequency of monsoon
gyres during the period 2000–10. The peak occurs in
August and 75% monsoon gyres are observed in
FIG. 2. (a),(b) The monsoon gyre detected with unfiltered streamlines in Chen et al. (2004) and (c),(d) 10-day low-
pass filtered 850-hPawind fields (m s21) at (a),(c) 1200UTC 28 Jul and (b),(d) 0000UTC 31 Jul 1989. Triangles, small
dots, and large dots indicate the locations of tropical disturbances, tropical cyclones, and the centers of monsoon
gyres, respectively.
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August–October. After examination of geostationary
satellite imagery, Lander (1994) found that monsoon
gyres usually occur during late July–early September.
Figure 4 shows the locations of the monsoon gyres when
they reached their maximum strength during May–July
and August–October, respectively. For comparison, the
composited 850- and 200-hPa wind fields are also plotted
in the figure. Figure 4 indicates that monsoon gyres oc-
curred mostly on the poleward side of the composited
monsoon trough. Note that monsoon gyres in May–July
were identified only over the WNP. As the 850-hPa
easterly winds extend westward, the monsoon gyres also
occurred over the SCS in August–October. Northwest-
ward movement can be seen for some monsoon gyres
(figure not shown).
Recently, Ding et al. (2011) found that the develo-
pment of the summertime midlatitude jet over the
northeastern Asian coast was associated with a positive
seasonal rainfall anomaly over India. Molinari and
Vollaro (2012) examined the development of the mon-
soon gyre during July 1988 and argued that its de-
velopment was associated with the interactions of the
MJO and the midlatitude jet. Diabatic heating in the
MJO leads to the enhancement of the upper-tropospheric
westerly jet and repeated equatorward wave-breaking
events downwind of the jet exit region over the north-
western Pacific. The 200-hPa wind field (Fig. 4) shows
that the monsoon gyres were located in the eastern part
of the South Asian anticyclone centered over the Tibet
FIG. 3. Monthly frequency of monsoon gyres during 2000–10.
FIG. 4. The centers of 11-yrmonsoon gyres (gray dots) during the periods (a),(c)May–July and (b),(d)August–October
and the composited 10-day low-pass filtered winds (arrows) at (a),(b) 200 and (c),(d) 850 hPa. Shading in (a),(b)
indicates wind speeds exceeding 30 m s21 and thick lines in (c),(d) indicate the monsoon trough lines.
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Plateau. In agreement withMolinari andVollaro (2012),
the monsoon gyres generally developed south of the
200-hPa westerly trough and comparison of Fig. 4b with
Fig. 4a shows that the enhanced monsoon gyre activity
in August–October was accompanied with increasing
wind speed of the midlatitude upper-level jet over the
northwestern Pacific.
4. The composited structure of monsoon gyres
Although monsoon gyres and their association with
tropical cyclogenesis were discussed in previous studies
(Lander 1994; Ritchie and Holland 1999; Chen et al.
2004), their general structure and evolution have not
been documented in the literature. Here a composite
analysis is conducted with the 37 monsoon gyres iden-
tified during the period 2000–10. The wind fields are
composited relative to the maximum strength of mon-
soon gyres in time and their centers in space. Because of
differences in the life span of these monsoon gyres, here
the wind fields are composited with at least 19 (.50%)
monsoon gyres available.
Figure 5 indicates the evolution of the composited
850-hPa wind and OLR fields 4 days before and 3 days
FIG. 5. Ten-day low-pass filtered 850-hPa winds (arrows, m s21) and unfiltered OLR fields (shading, W m22)
composited relative to the time that monsoon gyres reached their maximum strength: (a) day24 (21monsoon gyres),
(b) day 23 (28 monsoon gyres), (c) day 22 (32 monsoon gyres), (d) day 0 (37 monsoon gyres), (e) day 11 (37
monsoon gyres), and (f) day 13 (19 monsoon gyres).
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after themonsoon gyres reached their maximum strength,
respectively. The composited monsoon gyre is charac-
terized by a large-scale nearly circular cyclone with the
enhancement of southwesterly winds on the southeast-
ern side. The asymmetric structure of the composited
monsoon gyre is very clear in the convective activity.
On days 24 and 23, the enhanced convection is mostly
observed to the south-southeast of the gyre center. The
enhanced band of convective activity, which is accom-
panied with strong southwesterly winds, is located about
800 km southeast of the gyre center. The enhanced deep
convectionmaintains until the day of maximum strength
(day 0) and day 1, but moves close to the central region
of the monsoon gyre. The enhanced convection further
moves to the north of the gyre while the deep convection
band to the southeast of the center still can be identified.
Since the unfiltered OLR data are used in this study, the
movement of the enhanced deep convection toward the
gyre center may be a manifestation of the associated TC
activity. Relatively weak anticyclonic circulation can be
seen to the south-southeast of the monsoon gyre. It is
interesting to note the change in the shape of the com-
posited monsoon gyre. The horizontal scale in the east–
west direction shrinks on days 3 and 4 when the monsoon
gyre weakens.
The enhanced southwesterly winds can be clearly
seen in the vertical profiles of the zonal and meridional
wind components of the composited monsoon gyre on
day 0 (Fig. 6). The maximum westerly (easterly) com-
ponent of 10 (6) m s21 occurs around 850 hPa, about
800 (500) km away from the monsoon gyre center, re-
spectively. The cyclonic circulation of the composited
monsoon gyre decreases with height and disappears
above 300 hPa. At 200 hPa, the westerly (easterly) jet can
be observed about 208 latitude away from the monsoon
gyre center, with a maximum speed of 26 (16) m s21.
In the west–east vertical profile of the meridional wind
(Fig. 6b), the maximum of the southerly (northerly)
wind component around 900 hPa occurs with stronger
southerly winds on the eastern side.
FIG. 6. Vertical profiles of the composited 10-day low-pass fil-
tered (a) zonal and (b) meridional wind components (m s21) when
monsoon gyres reached their maximum strength.
FIG. 7. Vertical profiles of the composited 10-day low-pass fil-
tered vorticity (contours, 1025 s21) and temperature anomalies
(shading, 8C) along the (a) south–north and (b) east–west di-
rections from the mean temperature over an area with a radius of
2750 km (;258 latitude and longitude) when monsoon gyres
reached their maximum strength.
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Figure 7 further shows the vertical profiles of the
relative vorticity and temperature anomaly of the com-
posited monsoon gyre at the time of maximum strength
(day 0). The temperature anomaly is based on the mean
temperature averaged over a radius of 2500 km. The
cyclonic circulation ranges about 1000 km away from
the center at 900 hPa and the positive vorticity shrinks
with height and is replaced by the negative vorticity
above 200 hPa. The composited monsoon gyre has a
warm core with the maximum around 300 hPa in the
south–north and west–east vertical profiles.
5. Tropical cyclone formation associated withmonsoon gyres
We first select two cases to illustrate the TC formation
associated with monsoon gyres. The first monsoon gyre
that occurred in September 2000 was associated with
the formation of three TCs (Fig. 8). Note that Typhoon
Saomai formed before the monsoon gyre can be iden-
tified in the low-pass filtered wind field (Fig. 8a). In this
study, the formation of Saomai is not identified as a
TC associated with the monsoon gyre. At 1800 UTC
FIG. 8. Ten-day low-pass filtered 850-hPa wind field (arrows, m s21) and daily OLR (shading, W m22) for the
monsoon gyre at (a) 0000 UTC 5 Sep, (b) 1800 UTC 5 Sep, (c) 0000 UTC 8 Sep, (d) 1800 UTC 10 Sep, (e) 0000 UTC
14 Sep, and (f) 0600 UTC 15 Sep 2000. Closed dots, triangles, and typhoon symbols indicate the locations of monsoon
gyres, tropical disturbances, and tropical cyclones, respectively.
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5 September (Fig. 8b), two tropical disturbances were
within the monsoon gyre although the monsoon gyre
was just identified in the low-pass filtered wind field.
At this time, Saomai was located to the southeast of
the gyre. At 0000 UTC 8 September (Fig. 8c), the two
tropical disturbances became Typhoons Bopha and
Wukong and were located to the northwest and west
of the gyre center, respectively. Meanwhile, Saomai
merged with a band of the enhanced convection and was
nearly collocated with the monsoon gyre at 1800 UTC
10 September (Fig. 8d). Four days later a tropical dis-
turbance emerged and became Typhoon Sonamu (Figs.
8e,f). It is evident that the monsoon gyre moved north-
westward and anticyclonic circulation can be found to its
southeast.
The second monsoon gyre occurred in October 2004
with the formationof two typhoons (Tokage andNock-ten).
Figure 9 shows the 10-day low-pass filtered wind fields
associated with themonsoon gyre. A tropical disturbance
to the southeast of the gyre center can be found at
0600 UTC 10 October (Fig. 9a), and it reached tropical
storm strength at 1800 UTC 12 October (Fig. 9b). When
Tokage approached the center of the monsoon gyre,
another tropical disturbance can be identifiedmore than
2000 km southeast of the gyre center (Fig. 9c). It be-
came Tropical StromNock-ten at 0600 UTC 16October
(Fig. 9d). The formation of the two TCs was associated
with the southwesterly winds of the monsoon gyre and
enhanced convective activity. As shown in Fig. 9, the
monsoon gyre moved northwestward and its size ex-
panded as Tokage formed and approached the gyre
center. The anticyclonic circulation can also be seen to
the southeast of the monsoon gyre.
In this study, an observed TC formation event was
associated with a monsoon gyre if it formed as a named
tropical storm or stronger in the JTWC dataset within
the cyclonic circulation of the gyre or the confluence
zone between the westerly winds and easterly trade
FIG. 9. Ten-day low-pass filtered 850-hPa wind field (arrows, m s21) and daily OLR (shading, W m22) for the
monsoon gyre at (a) 0600 UTC 10 Oct, (b) 1800 UTC 12 Oct, (c) 1800 UTC 13 Oct, and (d) 0600 UTC 16 Oct 2004.
Closed dots, triangles, and typhoon symbols indicate the locations of monsoon gyres, tropical disturbances, and
tropical cyclones, respectively.
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winds to the east of the monsoon gyre during its life
span. As shown in the first example, Typhoon Saomai
(2000) is not counted because it formed before the
monsoon gyre can be identified. Accounting for 19.8%
of the total TC formation events inMay–October during
2000–10, 42 TCs were linked to 31 monsoon gyres while
the activity of the other 6 monsoon gyres was not ac-
companied directly with any TC formation. Figure 10
shows the composited 850- and 200-hPa wind fields at
the TC formation time and the locations of the TCs with
respect to the gyre center. The time that a TC first rea-
ches the tropical storm intensity is taken as the TC for-
mation time.
In the lower troposphere (850 hPa), as shown in Fig. 10,
a large gyre with a radius of about 2000 km is accom-
panied with a relatively weak anticyclone to the south-
east of the gyre. As suggested by Carr and Elsberry
(1995), the anticyclone is related to the Rossby wave
energy dispersion of monsoon gyres that have relatively
weak maximum winds at a relatively large radius. Most
TCs formed near the center or the northeastern end of
the enhanced southwesterly flows between the monsoon
gyre and the anticyclone. TCs also formed on the west-
ern and northern sides of the monsoon gyre. On the
southern side, TC formation rarely occurred because of
strong vertical wind shear (Figs. 10a,b). With a westerly
jet to the north at the upper level (200 hPa), an anticy-
clone is located about 600 km northeast of the center
of the monsoon gyre. Figure 11 shows the total vertical
wind shear between 200 and 850 hPa, which is com-
posited 2 days before TC formation and on the forma-
tion day, indicating that most TCs formed in the area of
relatively weak vertical wind shear.
6. Summary
Previous studies suggested that monsoon gyres played
an important role in TC formation in the WNP basin
(Lander 1994; Ritchie and Holland 1999; Chen et al.
2004). However, the general structure of monsoon gyres
and the associated mechanisms for TC formation have
FIG. 10. Formation locations (typhoon symbols) of tropical cy-
clones relative to monsoon gyres (dots) during the period 2000–10
and the composited 10-day low-pass filtered (a) 200- and (b) 850-hPa
wind fields at the time of tropical cyclone formation, with contours
indicating wind speeds. Units for wind are m s21.
FIG. 11. Ten-day low-pass filtered total vertical wind shear (m s21)
between 200 and 850 hPa, which is composited relative to the
tropical cyclone formation time: (a) 2 days before the formation
and (b) at the formation time. Dots and typhoon symbols indicate
the locations of monsoon gyre centers and tropical cyclone centers,
respectively.
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not been documented. Using the NCEP FNL opera-
tional global analysis and the JTWC best-track dataset,
an observational study is conducted on the monsoon
gyre activity, the composited structure, and the associ-
ated TC formation during the period 2000–10.
During the period 2000–10, 37 monsoon gyres are
identified in May–October. Monsoon gyres formed on
the poleward side of the composited monsoon trough
with a peak inAugust–October. Extending about 1000 km
outward from the center at lower levels, the cyclonic
circulation of the compositedmonsoon gyre shrinks with
height and is replaced with negative vorticity above
200 hPa. The maximum winds of the composited mon-
soon gyre appear 500 (800) km away from the center
with a magnitude of 10 (6) m s21 at 850 hPa. In agree-
ment with previous studies, the composited monsoon
gyre shows an asymmetric structure with enhanced
southwesterly flow and convection on the southern and
southeastern side.
It is found that 42 TCs were linked to 31 monsoon
gyres while the activity of the other 5 monsoon gyres
was not accompanied directly with any TC formation.
These TCs account for 19.8% of the total TCs that
formed inMay–October during 2000–10. Among the 31
monsoon gyres in which TC formation occurs, 22 (9) of
them are associated with single (multiple) TC forma-
tion. In relatively weak vertical wind shears, most of
the TCs associated with monsoon gyres form near the
centers of monsoon gyres and the northeastern end of
the enhanced southwesterly flows. In other words, TC
formation prefers to the eastern portion of monsoon
gyres.
As mentioned in the introduction, our analysis agrees
with previous studies in that TCs prefer to form in the
region where westerly monsoon flowsmeet with easterly
trade winds (Holland 1995; Ritchie and Holland 1999).
It is argued that the confluence zones can trap westward-
traveling long-wave disturbances that are shifted to
higher frequency on encountering westerly flow, leading
to an accumulation of wave energy in the region (Chang
andWebster 1990; Sobel andBretherton 1999; Kuo et al.
2001). Further, the accumulated wave energy can escape
to higher latitudes (Webster and Holton 1982; Zhang
and Webster 1989). Since the Rossby wave energy dis-
persion of monsoon gyres can enhance the confluence
zone, further study is needed to understand the roles of
monsoon gyres in TC formation.
Acknowledgments.This researchwas jointly supported
by the Typhoon Research Project (2009CB421503) of
the National Basic Research Program of China, the Na-
tional Natural Science Foundation of China (NSFCGrant
40875038), the social commonweal research program of
the Ministry of Science and Technology of the People’s
Republic of China (GYHY200806009), and the Priority
Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
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