Observational Analysis of Tropical Cyclone Formation Associated with Monsoon Gyres

 
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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-scale monsoon gyres and the involved tropical cyclone formation over the western North 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 on monsoon gyres during the period 2000–10,
             with a focus on their structures and the associated tropical cyclone formation.
               A total of 37 monsoon gyres are identified in May–October during 2000–10, among which 31 monsoon 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 composited monsoon trough with
             a peak occurrence in August–October. Extending about 1000 km outward from the center at lower levels, the
             cyclonic circulation of the composited monsoon gyre shrinks with height and is replaced with negative relative
             vorticity above 200 hPa. The maximum winds of the composited monsoon 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                                                      WNP and SCS (Carr and Elsberry 1995; Ritchie and
                                                                     Holland 1999; Chen et al. 2004; Wu et al. 2011a,b; Liang
   The summertime monsoon circulation over the trop-
                                                                     et al. 2011). Holland (1995) and Molinari et al. (2007)
ical western North Pacific (WNP) and South China Sea
                                                                     argued that equatorward-moving midlatitude distur-
(SCS) is usually characterized with a low-level monsoon
                                                                     bances play a role in the gyre formation by producing
trough, in which westerly monsoon winds lie in the
                                                                     a region of persistent diabatic heating near 158N and
equatorward portion while easterly trade winds exist on
                                                                     that a monsoon gyre forms to the west of the heating as a
the poleward side (Holland 1995). The monsoon trough
                                                                     result of the Gill-type response (Gill 1980). Recently,
is closely associated with a large fraction of tropical cy-
                                                                     Molinari and Vollaro (2012) suggested that such cyclonic
clone (TC) formation in the WNP and SCS (Gray 1968;
                                                                     gyres were related to the interactions of the Madden–
Ramage 1974; Briegel and Frank 1997; Ritchie and
                                                                     Julian oscillation (MJO) and the midlatitude jet.
Holland 1999). Sometimes the monsoon trough is re-
                                                                       Our current understanding of TC formation associ-
placed by a large-scale monsoon gyre, which is a nearly
                                                                     ated with monsoon gyres is mainly from a few obser-
circular cyclonic vortex with a diameter of about
                                                                     vational studies. Lander (1994) first conducted a case
2500 km (Lander 1994, 1996; Harr et al. 1996). Studies
                                                                     study on the TC formation associated with the monsoon
suggest that such a monsoon gyre has important impli-
                                                                     gyres in 1991 and 1993. In his study, the monsoon gyre of
cations for TC formation, structure, and motion in the
                                                                     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
  Corresponding author address: Dr. Liguang Wu, Pacific Typhoon      gyre of July 1993. He argued that two modes associated
Research Center, Key Laboratory of Meteorological Disaster of
Ministry of Education, Nanjing University of Information Science     with TC formation in a monsoon gyre are 1) small TCs
and Technology, Nanjing 210044, China.                               that form in the eastern periphery of a monsoon gyre and
E-mail: liguang@nuist.edu.cn                                         2) a giant TC that develops from the gyre itself. Molinari

DOI: 10.1175/JAS-D-12-0117.1

Ó 2013 American Meteorological Society
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et al. (2007) also examined the TC formation associated        favorable locations of the associated TC formation. The
with the monsoon gyre of August 1991 and found that            rest of the paper is organized as follows. In section 2, the
the monsoon gyre was actually associated with an               observational data and identification of monsoon gyres
equatorial Rossby wave packet with a period of 22 days         are described, followed by the climatological features of
and a wavelength of 3600 km. Ritchie and Holland               monsoon gyre activity during the period 2000–10 in
(1999) examined the relationship between monsoon               section 3. The composite structure of monsoon gyres
gyre activity and TC formation during 8 yr (1984–92, but       and their relationship with TC formation are discussed
not 1989) and found that 3% of the TC formation events         in sections 4 and 5, respectively. A summary of the ob-
were associated with monsoon gyres over the WNP.               servational analysis presents in section 6.
Based on the 24-yr data from 1979 to 2002, however,
Chen et al. (2004) examined the interannual variations
                                                               2. Data and identification of monsoon gyres
of TC formation associated monsoon gyres and found
that about 70% of TC formation events were linked to              Three main types of datasets are used in this study.
monsoon gyres. This difference may be due to their             The TC information in the WNP basin is from the Joint
different lifespans in selecting monsoon gyres. Although       Typhoon Warning Center (JTWC) best-track dataset,
the associated mechanisms are not well known, these            which includes the TC center position (latitude and
studies suggested that monsoon gyres play an important         longitude), the maximum sustained wind speed, and the
role in TC formation in the WNP basin.                         minimum sea level pressure. TC formation in this study
   Monsoon gyres are subjected to Rossby wave (b in-           is defined when the maximum sustained wind of a TC
duced) energy dispersion. The energy dispersion associ-        first exceeded 17 m s21 in the JTWC dataset. The wind
ated with a barotropic vortex was extensively investigated     fields are based on the National Centers for Environ-
in the presence of the planetary vorticity gradient or the     mental Prediction (NCEP) final (FNL) operational
beta effect (e.g., Anthes 1982; Flierl 1984; Chan and          global analysis data on 1.08 3 1.08 grids at every 6 h
Williams 1987; Luo 1994; McDonald 1998; Shapiro and            (http://rda.ucar.edu/datasets/ds083.2/). This product is
Ooyama 1990). While the TC formation associated with           from the Global Forecast System (GFS) that is opera-
the energy dispersion of preexisting TCs has been re-          tionally run 4 times a day in near–real time at NCEP. To
cently investigated (Li and Fu 2006; Ge et al. 2008), little   compare the identified monsoon gyres with those in
is known about how the energy dispersion of monsoon            previous studies (Lander 1994; Chen et al. 2004), we also
gyres plays a role in TC formation. Using a barotropic         use the NCEP–National Center for Atmospheric Re-
vorticity model, Carr and Elsberry (1995) demonstrated         search (NCAR) reanalysis data (Kalnay et al. 1996). The
that monsoon gyres underwent the b-induced energy              data for deep convective activity associated with mon-
dispersion, producing strong ridging to the east and           soon gyres are from the National Oceanic and Atmo-
southeast and an intermediate region of high southerly         spheric Administration (NOAA) outgoing longwave
winds. According to Holland (1995), the region where           radiation (OLR) dataset, which is available once a day
westerlies meet easterlies may be important for trapping       on 2.58 3 2.58 grids. The May–October activity of
tropical waves, accumulating wave energy, sustaining           monsoon gyres during the period 2000–10 is the focus in
the long-lived mesoscale convective system (MCS), and          this study.
providing a favorable environment for TC formation.               According to the American Meteorological Society
Sobel and Bretherton (1999) and Kuo et al. (2001) also         (AMS) Glossary of Meteorology (http://glossary.ametsoc.
suggested that nondivergent barotropic Rossby waves            org/wiki/Main_Page), a monsoon gyre over the WNP is
could grow in a region where westerlies meet easterlies,       characterized by 1) a very large nearly circular low-level
providing the seedlings for TCs. Thus it is conceivable        cyclonic vortex that has an outermost closed isobar with
that the Rossby wave (b induced) energy dispersion can         a diameter on the order of 2500 km, 2) a relatively long
play an important role in TC formation in the presence         (;2 weeks) life span, and 3) a cloud band bordering the
of monsoon gyres.                                              southern through eastern periphery of the vortex/surface
   The main objective of this study is to advance our          low. It is obvious that monsoon gyres are large-scale,
understanding on the role of monsoon gyres in TC for-          low-frequency phenomena.
mation. We identify all of the occurrences of monsoon             In this study, a monsoon gyre is selected if its diameter
gyres and the associated TC formation events during            is at least 2500 km with a band or a large area of deep
the period 2000–10. In particular, composite analysis is       convection in its southern and southeastern periphery.
performed to reveal climatological features of mon-            The identification of monsoon gyres in this study is
soon gyre activity and the associated TC formation, the        based on the low-pass filtered wind fields at 850 hPa. To
three-dimensional structure of monsoon gyres, and the          reduce the bias of TC circulation in filtering, we first use
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           FIG. 1. (a),(b) The monsoon gyre detected with unfiltered sea level pressure (contours, interval 5 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.

the procedure proposed by Kurihara et al. (1993, 1995)                outermost wind vectors that constitute a closed vortex.
to subtract TC circulation from the FNL wind field. This              The gyre center is adjusted visually to be the circulation
procedure has been used for removing TC circulation                   center. Considering the gyre size that is at least 2500 km
from the analysis data in TC simulation and forecast                  in diameter, the monsoon gyre strength is measured by
(Kurihara et al. 1993, 1995; Wu et al. 2002), as well as in           the circulation within a radius of 1250 km from the gyre
the study of the influence of TCs on low-frequency vari-              center.
ability (Hsu et al. 2008). Readers are referred to Kurihara              To verify the identification method, the two monsoon
et al. (1993, 1995) for details. Then a low-pass Lanczos              gyres that were investigated by Lander (1994) and Chen
filter with a 10-day cutoff period is applied to the wind             et al. (2004), respectively, are first examined with the
fields to obtain the low-frequency flows (Duchon 1979).               2.58 3 2.58 NCEP reanalysis data. The two cases are also
   The synoptic-scale disturbances including TCs are                  identified as monsoon gyres with our method. In Lander
obtained as the difference between the unfiltered and                 (1994), the size of the monsoon gyre that occurred in
filtered fields. The center, size, and strength of a mon-             1991 was measured with the outermost closed isobar.
soon gyre are determined from the filtered wind fields                Here we can take the contour of 1005 hPa as the out-
based on a combination of relative circulation calcula-               most closed isobar of the monsoon gyre. Figure 1 com-
tion and visual examination. First, the relative circula-             pares the surface pressure and the 850-hPa low-pass
tion on each grid within a radius of 660 km (68 in latitude           filtered wind fields, suggesting that the monsoon gyre
and longitude, which is nearly half of the radius of                  indicated by the outermost closed isobar can be identi-
a candidate monsoon gyre) is calculated for the filtered              fied in the filtered wind field. At 0000 UTC 12 August
850-hPa wind field at 6-h intervals. One or two circula-              1991 (Figs. 1a,c), the monsoon gyre was associated with
tion maxima are selected as the initial gyre centers over             two TCs (Ellie and Fred) and a tropical depression
the whole tropical western Pacific region. Once a can-                (138W). Seven days later Ellie moved with the north-
didate gyre center is selected, the gyre size is visually             westerly flows of the gyre and became a tropical de-
determined, which is the minimum diameter of the                      pression, while Typhoon Gladys was nearly collocated
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           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-hPa wind fields (m s21) at (a),(c) 1200 UTC 28 Jul and (b),(d) 0000 UTC 31 Jul 1989. Triangles, small
         dots, and large dots indicate the locations of tropical disturbances, tropical cyclones, and the centers of monsoon
         gyres, respectively.

with the gyre (Figs. 1b,d) and finally transformed the                per year. The annual occurrence frequency is much
monsoon gyre into a giant typhoon (Lander 1994). In                   higher than that in Lander (1994), in which monsoon
Chen et al. (2004), the circulation of the monsoon gyre in            gyres are fairly uncommon, on average occurring once
1989 was identified with unfiltered 850-hPa streamlines               every 2 yr. Chen et al. (2004) identified the monsoon
(Fig. 2). As shown in this figure, the identified structure           gyres during the period 1979–2002 with a life span of at
of the monsoon gyre with the 850-hPa low-pass filtered                least 5 days, suggesting a much higher occurrence rate
winds agrees well with the streamlines. At 1200 UTC                   (6 monsoon gyres each year) than that in our analysis.
28 July 1989 (Figs. 2a,c), Typhoon Judy and Tropical                  The difference may result from the different lifetime
Depression 12W were associated with the monsoon                       requirements in identifying monsoon gyres. In this study,
gyre. At 0000 UTC 31 July 1989 (Figs. 2c,d), the tropical             the lifespan of a monsoon gyre is the period that the
depression moved southwestward and Tropical Storm                     gyre can be identified as a closed cyclone with a size of
Ken–Lola and Typhoon Mac appeared to the north and                    at least 2500 km. In our study, two cases have a lifespan
southeast of the gyre center, respectively. Thus, the                 of at least 14 days, comparable to the result of Ritchie
identification method in this study is consistent with                and Holland (1999). However, the 31 cases that have a
those used in Lander (1994) and Chen et al. (2004), al-               lifespan of at least 5 days indicate a lower occurrence
though low-pass filtered wind fields are used in this                 rate than that in Chen et al. (2004). We speculate that
study.                                                                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
3. Monsoon gyre activity over 2000–10
                                                                      17 days, with an average of 8.0 days.
  Using the identification method described above,                       Figure 3 shows the monthly frequency of monsoon
a total of 37 monsoon gyres are identified during the                 gyres during the period 2000–10. The peak occurs in
period 2000–10, with an average of 3.4 monsoon gyres                  August and 75% monsoon gyres are observed in
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                                                                     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-
 FIG. 3. Monthly frequency of monsoon gyres during 2000–10.          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
August–October. After examination of geostationary                   MJO leads to the enhancement of the upper-tropospheric
satellite imagery, Lander (1994) found that monsoon                  westerly jet and repeated equatorward wave-breaking
gyres usually occur during late July–early September.                events downwind of the jet exit region over the north-
Figure 4 shows the locations of the monsoon gyres when               western Pacific. The 200-hPa wind field (Fig. 4) shows
they reached their maximum strength during May–July                  that the monsoon gyres were located in the eastern part
and August–October, respectively. For comparison, the                of the South Asian anticyclone centered over the Tibet

          FIG. 4. The centers of 11-yr monsoon 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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          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) day 24 (21 monsoon 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).

Plateau. In agreement with Molinari and Vollaro (2012),           (Lander 1994; Ritchie and Holland 1999; Chen et al.
the monsoon gyres generally developed south of the                2004), their general structure and evolution have not
200-hPa westerly trough and comparison of Fig. 4b with            been documented in the literature. Here a composite
Fig. 4a shows that the enhanced monsoon gyre activity             analysis is conducted with the 37 monsoon gyres iden-
in August–October was accompanied with increasing                 tified during the period 2000–10. The wind fields are
wind speed of the midlatitude upper-level jet over the            composited relative to the maximum strength of mon-
northwestern Pacific.                                             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%)
4. The composited structure of monsoon gyres
                                                                  monsoon gyres available.
  Although monsoon gyres and their association with                  Figure 5 indicates the evolution of the composited
tropical cyclogenesis were discussed in previous studies          850-hPa wind and OLR fields 4 days before and 3 days
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                                                                       FIG. 7. Vertical profiles of the composited 10-day low-pass fil-
  FIG. 6. Vertical profiles of the composited 10-day low-pass fil-   tered vorticity (contours, 1025 s21) and temperature anomalies
tered (a) zonal and (b) meridional wind components (m s21) when      (shading, 8C) along the (a) south–north and (b) east–west di-
monsoon gyres reached their maximum strength.                        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.
after the monsoon gyres reached their maximum strength,
respectively. The composited monsoon gyre is charac-
terized by a large-scale nearly circular cyclone with the            interesting to note the change in the shape of the com-
enhancement of southwesterly winds on the southeast-                 posited monsoon gyre. The horizontal scale in the east–
ern side. The asymmetric structure of the composited                 west direction shrinks on days 3 and 4 when the monsoon
monsoon gyre is very clear in the convective activity.               gyre weakens.
On days 24 and 23, the enhanced convection is mostly                   The enhanced southwesterly winds can be clearly
observed to the south-southeast of the gyre center. The              seen in the vertical profiles of the zonal and meridional
enhanced band of convective activity, which is accom-                wind components of the composited monsoon gyre on
panied with strong southwesterly winds, is located about             day 0 (Fig. 6). The maximum westerly (easterly) com-
800 km southeast of the gyre center. The enhanced deep               ponent of 10 (6) m s21 occurs around 850 hPa, about
convection maintains until the day of maximum strength               800 (500) km away from the monsoon gyre center, re-
(day 0) and day 1, but moves close to the central region             spectively. The cyclonic circulation of the composited
of the monsoon gyre. The enhanced convection further                 monsoon gyre decreases with height and disappears
moves to the north of the gyre while the deep convection             above 300 hPa. At 200 hPa, the westerly (easterly) jet can
band to the southeast of the center still can be identified.         be observed about 208 latitude away from the monsoon
Since the unfiltered OLR data are used in this study, the            gyre center, with a maximum speed of 26 (16) m s21.
movement of the enhanced deep convection toward the                  In the west–east vertical profile of the meridional wind
gyre center may be a manifestation of the associated TC              (Fig. 6b), the maximum of the southerly (northerly)
activity. Relatively weak anticyclonic circulation can be            wind component around 900 hPa occurs with stronger
seen to the south-southeast of the monsoon gyre. It is               southerly winds on the eastern side.
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          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.

  Figure 7 further shows the vertical profiles of the              5. Tropical cyclone formation associated with
relative vorticity and temperature anomaly of the com-                monsoon gyres
posited monsoon gyre at the time of maximum strength
(day 0). The temperature anomaly is based on the mean                 We first select two cases to illustrate the TC formation
temperature averaged over a radius of 2500 km. The                 associated with monsoon gyres. The first monsoon gyre
cyclonic circulation ranges about 1000 km away from                that occurred in September 2000 was associated with
the center at 900 hPa and the positive vorticity shrinks           the formation of three TCs (Fig. 8). Note that Typhoon
with height and is replaced by the negative vorticity              Saomai formed before the monsoon gyre can be iden-
above 200 hPa. The composited monsoon gyre has a                   tified in the low-pass filtered wind field (Fig. 8a). In this
warm core with the maximum around 300 hPa in the                   study, the formation of Saomai is not identified as a
south–north and west–east vertical profiles.                       TC associated with the monsoon gyre. At 1800 UTC
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           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.

5 September (Fig. 8b), two tropical disturbances were            to the southeast of the gyre center can be found at
within the monsoon gyre although the monsoon gyre                0600 UTC 10 October (Fig. 9a), and it reached tropical
was just identified in the low-pass filtered wind field.         storm strength at 1800 UTC 12 October (Fig. 9b). When
At this time, Saomai was located to the southeast of             Tokage approached the center of the monsoon gyre,
the gyre. At 0000 UTC 8 September (Fig. 8c), the two             another tropical disturbance can be identified more than
tropical disturbances became Typhoons Bopha and                  2000 km southeast of the gyre center (Fig. 9c). It be-
Wukong and were located to the northwest and west                came Tropical Strom Nock-ten at 0600 UTC 16 October
of the gyre center, respectively. Meanwhile, Saomai              (Fig. 9d). The formation of the two TCs was associated
merged with a band of the enhanced convection and was            with the southwesterly winds of the monsoon gyre and
nearly collocated with the monsoon gyre at 1800 UTC              enhanced convective activity. As shown in Fig. 9, the
10 September (Fig. 8d). Four days later a tropical dis-          monsoon gyre moved northwestward and its size ex-
turbance emerged and became Typhoon Sonamu (Figs.                panded as Tokage formed and approached the gyre
8e,f). It is evident that the monsoon gyre moved north-          center. The anticyclonic circulation can also be seen to
westward and anticyclonic circulation can be found to its        the southeast of the monsoon gyre.
southeast.                                                         In this study, an observed TC formation event was
  The second monsoon gyre occurred in October 2004               associated with a monsoon gyre if it formed as a named
with the formation of two typhoons (Tokage and Nock-ten).        tropical storm or stronger in the JTWC dataset within
Figure 9 shows the 10-day low-pass filtered wind fields          the cyclonic circulation of the gyre or the confluence
associated with the monsoon gyre. A tropical disturbance         zone between the westerly winds and easterly trade
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                                                                          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,
  FIG. 10. Formation locations (typhoon symbols) of tropical cy-
                                                                       respectively.
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.
                                                                       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
winds to the east of the monsoon gyre during its life                  gyre and the anticyclone. TCs also formed on the west-
span. As shown in the first example, Typhoon Saomai                    ern and northern sides of the monsoon gyre. On the
(2000) is not counted because it formed before the                     southern side, TC formation rarely occurred because of
monsoon gyre can be identified. Accounting for 19.8%                   strong vertical wind shear (Figs. 10a,b). With a westerly
of the total TC formation events in May–October during                 jet to the north at the upper level (200 hPa), an anticy-
2000–10, 42 TCs were linked to 31 monsoon gyres while                  clone is located about 600 km northeast of the center
the activity of the other 6 monsoon gyres was not ac-                  of the monsoon gyre. Figure 11 shows the total vertical
companied directly with any TC formation. Figure 10                    wind shear between 200 and 850 hPa, which is com-
shows the composited 850- and 200-hPa wind fields at                   posited 2 days before TC formation and on the forma-
the TC formation time and the locations of the TCs with                tion day, indicating that most TCs formed in the area of
respect to the gyre center. The time that a TC first rea-              relatively weak vertical wind shear.
ches the tropical storm intensity is taken as the TC for-
mation time.
                                                                       6. Summary
  In the lower troposphere (850 hPa), as shown in Fig. 10,
a large gyre with a radius of about 2000 km is accom-                    Previous studies suggested that monsoon gyres played
panied with a relatively weak anticyclone to the south-                an important role in TC formation in the WNP basin
east of the gyre. As suggested by Carr and Elsberry                    (Lander 1994; Ritchie and Holland 1999; Chen et al.
(1995), the anticyclone is related to the Rossby wave                  2004). However, the general structure of monsoon gyres
energy dispersion of monsoon gyres that have relatively                and the associated mechanisms for TC formation have
APRIL 2013                                         WU ET AL.                                                                1033

not been documented. Using the NCEP FNL opera-              the Ministry of Science and Technology of the People’s
tional global analysis and the JTWC best-track dataset,     Republic of China (GYHY200806009), and the Priority
an observational study is conducted on the monsoon          Academic Program Development of Jiangsu Higher
gyre activity, the composited structure, and the associ-    Education Institutions (PAPD).
ated TC formation during the period 2000–10.
   During the period 2000–10, 37 monsoon gyres are
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