3012 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 68

Observational Analysis of Sudden Track Changes in the Vicinity of the East Sea

LIGUANG WU,HUIJUN ZONG, AND JIA LIANG Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China

(Manuscript received 24 May 2010, in final form 16 December 2010)

ABSTRACT

An observational analysis of observed sudden track changes is conducted with a focus on the underlying mechanism and the possible role of slowly varying low-frequency flows. Four that took a generally northwestward track prior to sharply turning northeastward in the vicinity of the Sea are investigated. It is found that the sudden track changes occurred near the center of the Madden–Julian oscillation (MJO)- scale cyclonic circulation or at the bifurcation point of the steering flows at 700 hPa, and they were all associated with a well-developed quasi-biweekly oscillation (QBW)-scale gyre. Calculation of vorticity advection suggests that the peripheral ridging resulting from the interaction between the typhoons and the flows on the MJO and QBW scales can compress the typhoon circulation, leading to an area of high winds to the east or south of the typhoon center. The enhanced synoptic-scale winds shifted the typhoons northward and placed them in a northeastward orbit under the steering of the flows associated with the Pacific subtropical high. The sudden track change can be likened to the maneuvering of satellite orbit change in that the enhanced synoptic-scale winds act as a booster rocket to shift the typhoons northward to the southwesterly steering flows.

1. Introduction the Korean Peninsula and . Obviously the sudden track change represents a forecasting challenge for fore- A tropical cyclone that forms over the western North casters in the East Asia and thus it is important to un- Pacific generally takes one of three prevailing tracks: derstand the associated mechanisms. westward moving, northwestward moving, or northeast- A pioneering study on the sudden tropical cyclone track ward recurving (Wu and Wang 2004; Wu et al. 2005). change over the western North Pacific was conducted by While the southern provinces of China, including Guang- Carr and Elsberry (1995), who argued that many sudden dong, Guangxi, and Hainan, are primarily affected by west- track changes that typically consist of rapid slowing of the moving storms, tropical cyclones that make over westward movement and a substantial northward accel- southeastern coastal regions ( and prov- eration might be explained as the interaction between a inces) usually take a northwestward track. Recent studies tropical cyclone and a monsoon gyre. The latter is a spe- have revealed that Fujian and Zhejiang provinces expe- cific pattern of the evolution of the low-level monsoon rience the most economic loss from tropical cyclones circulation and can be identified as a nearly circular cy- among the coastal provinces of China because of strong clonic vortex with a radius of about 2500 km, which intensity at landfall (Zhang et al. 2009; Zhang et al. 2010). roughly occurs once per year and lasts 2–3 weeks over the A survey of historical typhoon tracks shows that a few western North Pacific during the summer (Lander 1994). typhoons would hit Fujian and Zhejiang provinces within Chen et al. (2004) indicated that nearly 70% of tropical 24 h, but they otherwise underwent a sudden northeast- cyclones over the northwestern Pacific are associated ward turn in the vicinity of the to affect with a monsoon gyre. Using a barotropic vorticity model, Carr and Elsberry Corresponding author address: Dr. Liguang Wu, Key Laboratory (1995) found that tropical cyclones can coalesce with of Meteorological Disaster of Ministry of Education, Nanjing Uni- versity of Information Science and Technology, Nanjing, Jiangsu monsoon gyres and then exhibit sudden northward track 210044, China. changes, which are very similar to the sudden poleward E-mail: [email protected] track changes observed in the western North Pacific.

DOI: 10.1175/2010JAS3559.1

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FIG. 1. 700-hPa wind fields associated with (2008) at 1800 UTC 10 Sep 2008 with closed dots indicating the typhoon center: (a) total FNL winds, (b) 20-day low-pass filtered winds, (c) 10–20-day bandpass filtered winds, and (d) synoptic-scale winds.

During the coalescence phase, the large and relatively (QBW) (Murakami and Frydrych 1974; Murakami 1975; weak monsoon gyre undergoes a b-induced dispersion, Kikuchi and Wang 2009) to the Madden–Julian oscil- producing strong ridging to the east and southeast of the lation (MJO) (Madden and Julian 1971, 1972; Wang tropical cyclone and an intermediate region of high and Rui 1990). Both the QBW and MJO, called tropical southerly winds that resemble the observed monsoon intraseasonal oscillations (ISOs), are associated with the surge. Although Carr and Elsberry (1995) compared their active/break cycles of Asia summer monsoon (Yasunari simulations with several real cases in terms of the flow 1981; Goswami et al. 2003; Chen et al. 2000; Chan et al. pattern and the position of the monsoon gyre relative to a 2002). As an important component in the Asian summer typhoon, so far it is not clear whether and how the pro- monsoon, QBW convection is closely associated with posed mechanism works in observed sudden track changes. tropical wave disturbances that propagate northwestward Tropical cyclone activity in the western North Pacific (Kikuchi and Wang 2009). It can be identified as an alter- is always accompanied with prominent atmospheric nating cyclonic and anticyclonic wave train with a wave- variability ranging from the synoptic-scale tropical length of about 3500 km in the northwestern Pacific (Chen disturbance (Liebmann et al. 1994; Lau and Lau 1990; and Sui 2010). Little attention has been paid to its influence Chang et al. 1996) to the quasi-biweekly oscillation on tropical cyclone activity.

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With a global-scale wavenumber-1 or -2 feature, the MJO typically propagates eastward in the tropics and is dominated by large-scale regions of enhanced and suppressed deep convection and across the tropical Indian and Pacific Oceans (Wang and Rui 1990). Kemball-Cook and Wang (2001) identified the northwestward-moving of MJO anomalies across the western North Pacific, which may affect the activity of the monsoon trough. Liebmann et al. (1994) showed that the active phase of the MJO is associated with the in- crease in tropical cyclone activity. Studies revealed that tropical cyclone tracks alternate between clusters of straight and recurving paths with an MJO time scale (Harr and Elsberry 1991, 1995; Chen et al. 2009). At this time, however, little is known about how the slowly varying MJO affects sudden tropical cyclone track changes. Following Carr and Elsberry (1995), this study provides an observational analysis of the observed sudden ty- phoon track changes that occurred in the vicinity of the East China Sea with a focus on the associated mech- anism. Special attention is paid to the roles of the slowly varying ISOs in sudden tropical cyclone track changes. A description of the data and analysis method used in this study is given in section 2. The selected sudden track change cases associated with Typhoons Saomai (2000), Maemi (2003), Sinlaku (2008), and Jangmi (2008) are briefly described in section 3. The ISO time scale flow patterns associated with the sudden track changes and the influence of the synoptic-scale flows on the sudden track changes are discussed in sections 4 and 5, respec- tively. The results of vorticity advection analysis are pre- sented in section 6, followed by a summary in section 7.

2. Data and analysis method This study employs the National Centers for Envi- ronmental Prediction (NCEP) Final (FNL) Operational Global Analysis data on 1.0831.08 grids at every 6 h (http://dss.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. The data are available on the surface, at 26 pressure levels FIG. 2. Tracks of the selected typhoons and the associated from 1000 to 10 hPa, including surface pressure, sea level 700-hPa 20-day low-pass filtered wind fields at the occurrence of pressure, geopotential height, temperature, relative hu- the sudden track changes denoted by closed dots on the tracks: (a) midity, and wind. The tropical cyclone data in the western Saomai (0600 UTC 14 Sep 2000), (b) Maemi (0000 UTC 11 Sep 2003), (c) Sinlaku (1800 UTC 14 Sep 2008), and (d) Jangmi (0000 UTC North Pacific are the best-track dataset from the Joint 29 Sep 2008). Typhoon Warming Center (JTWC), including tropical cyclone position and intensity also at 6-h intervals. In previous studies three prominent categories of at- Frydrych 1974; Murakami 1975; Krishnamurti and mospheric variations were identified in the tropical Pacific. Ardanuy 1980; Chen and Chen 1993; Kiladis and Wheeler The first is the MJO on the time scale of 40–50 or 30–60 1995). The third category is synoptic-scale disturbances days (Madden and Julian 1971, 1972, 1994). The second is with periods between 3 and 10 days (Lau and Lau 1990; the QBW on the time scale of 10–20 days (Murakami and Chang et al. 1996), which may be associated with tropical

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FIG. 3. (left) Zonal and (right) meridional components of the typhoon translation speed (red) and the corre- sponding steering (green) for (a) Saomai (2000), (b) Maemi (2003), (c) Sinlaku (2008), and (d) Jangmi (2008). The vertical lines denote the time for the occurrence of the sudden track changes. cyclogenesis over the western North Pacific (Dickinson point (Duchon 1979). A low-pass filter with a 20-day cutoff and Molinari 2002). To examine the influences of the period is used to isolate the MJO and the background ISO-scale environmental flows on sudden tropical cyclone state, which for convenience is called the MJO component track change, we use Lanczos filters in time at each grid in this study. A bandpass filter with a 10–20-day period is

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FIG. 4. 10–20-day bandpass 500-hPa wind fields for Typhoon Saomai (2000) with closed dots indicating the typhoon center at (a) 0600 UTC 11 Sep, (b) 0600 UTC 12 Sep, (c) 0600 UTC 13 Sep, and (d) 0600 UTC 14 Sep 2000.

used for the QBW component. The synoptic-scale dis- decomposition of the wind fields in Fig. 1 indicates that turbances including tropical cyclones are the difference tropical cyclones over the western North Pacific can be between the unfiltered field and the field from a 10-day a combination of the flows with a time scale ranging from low-pass filter. MJO to the synoptic time scale. As an example, Fig. 1 shows the 700-hPa total and fil- tered wind fields associated with Typhoon Sinlaku (2008) 3. Overview of the selected sudden track change cases at 1800 UTC 10 September 2008, when the intensity of Sinlaku was 125 kt (64.3 m s21). It is clear that the in- Four typhoons that underwent sudden track changes in tensity was underrepresented in the FNL data (Fig. 1a). the vicinity of the East China Sea are selected in the study. In fact, the cyclonic circulation of Sinlaku includes three Except for Sinlaku (2008), with a peak intensity of 125 kt components: an MJO cyclone centered north of the ty- (64.3 m s21), Saomai (2000), Maemi (2003), and Jangmi phoon center (Fig. 1b), a QBW cyclone roughly collo- (2008) all reached supertyphoon intensity (130 kt or cated with the typhoon (Fig. 1c), and a synoptic-scale 66.9 m s21) or equivalent category-5 hurricane intensity. cyclone that mainly represents the outer strength of These typhoons all occurred in September and took Sinlaku (Fig. 1d). Note that the MJO and QBW cyclones a generally northwestward track prior to making a sharp have a larger size than the synoptic-scale cyclone. The northeastward turn (Fig. 2). Over a 24-h period, the sharp

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FIG. 5. 10–20-day bandpass 500-hPa wind field for (2003) with closed dots indicating the typhoon center at (a) 0000 UTC 8 Sep, (b) 0000 UTC 9 Sep, (c) 0000 UTC 10 Sep, and (d) 0000 UTC 11 Sep 2003. turn exceeded 608 and the northwestward movement was Sea on 11 September (Fig. 2b). Maemi became a typhoon replaced by the northeastward one with a translation on 6 September and a supertyphoon (category 5) on speed less than 4.4 m s21 (16 km h21). 10 September. As one of the worst typhoons ever to hit Supertyphoons Saomai (2000) and Maemi (2003) made the Korean Peninsula, it made landfall on the southern landfall on the Korean Peninsula. Saomai developed into coast of Korean Peninsula at typhoon intensity on a tropical storm on 2 September 2000 and a typhoon 12 September. 2 days later. After 6 September, as shown in Fig. 2a, it Sinlaku (2008) and Jangmi (2008) occurred in the took a generally northwestward track before it suddenly field experiment called Tropical Cyclone Structure changed its track and moved northeastward over the East 2008 (TCS08), which was conducted during August and China Sea on 14 September. Saomai became a super- September 2008 and sponsored by the Office of U.S. typhoon (category 5) on 10 September and made land- Naval Research and the Naval Research Laboratory as fall on the southern coast of the Korean Peninsula on part of the U.S. Air Force and the Observing-System Re- 15 September. After forming as a tropical depression near search and Predictability Experiment (THORPEX) on 4 September 2003, Maemi moved to the north- Pacific Asian Campaign (T-PARC). Both of the typhoons west and took a fairly straight track until it sharply changed made landfall on Island and took a northeastward its direction to the north-northeast over the East China track toward Japan after their sudden track changes.

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FIG. 6. 10–20-day bandpass 500-hPa wind field for Typhoon Sinlaku (2008) with closed dots indicating the typhoon center at (a) 1800 UTC 11 Sep, (b) 1800 UTC 12 Sep, (c) 1800 UTC 13 Sep, and (d) 1800 UTC 14 Sep 2008.

Sinlaku strengthened to a tropical storm on 8 September the tropical cyclone and its environment (Holland 1983; 2008 and reached peak intensity with maximum sustained Carr and Williams 1989; Fiorino and Elsberry 1989; Wu winds of 125 kt (64.3 m s21; category 3) on 10 September. and Wang 2000, 2001). The zonal and meridional speeds On 13 September it made landfall on Taiwan with a of the steering flows associated with the four typhoons of 90 kt (46.3 m s21). As are calculated and compared with the typhoon translation moved through Taiwan Island, Sinlaku turned sharply to speeds (Fig. 3). In this study the steering flow is calculated the northeast on 14 September and moved toward Japan. as the mass-weighted mean wind averaged within a radius Jangmi became a tropical storm on 24 September 2008 of 440 km between 850 and 300 hPa. As shown in Fig. 3, and intensified in to a super typhoon with winds of 140 kt the calculated steering is considerably consistent with the (72.0 m s21; category 5) on 27 September. It took a gen- typhoon translation speeds. erally northwestward track before 28 September, but it The sudden track changes were accompanied with an turned sharply to the northeast over the East China Sea eastward acceleration of the typhoon movement, which on 29 September and weakened its intensity rapidly after started at least 24 h prior to the occurrence of the it hit Taiwan on 28 September. northeastward recurving (left panels of Fig. 3). The ac- It has been long known that a tropical cyclone is largely celeration can be the westward (eastward) speed de- steered by the large-scale environmental flow and the crease (increase). Unlike the typhoon tracks shown ventilation flow that results from the interaction between in Fig. 2, the zonal translation speed did not show an

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FIG. 7. 10–20-day bandpass 500-hPa wind field for Typhoon Jangmi (2008) with closed dots indicating the typhoon center at (a) 1200 UTC 26 Sep, (b) 1200 UTC 27 Sep, (c) 1200 UTC 28 Sep, and (d) 1200 UTC 29 Sep 2008. abrupt change around the occurrence of the sudden that the outer asymmetric structure is important to tropi- track changes. In the meridional direction, Maemi and cal cyclone motion (Holland 1983; Chan and Williams Sinlaku started to accelerate northward about 24 h prior 1987; Wu and Wang 2000, 2001). to the recurving, while Saomai and Jangmi started to accelerate northward at the recurving. The sudden track 4. The ISO-scale flows associated with the sudden changes can be characterized with rapid slowing of the track changes westward movement and then a northeastward accel- eration, as described by Carr and Elsberry (1995). Although the sudden track changes all occurred during Notice that the deviation of the steering from the ty- late summer over the East China Sea, the low-frequency phoon translation in Fig. 3 may be due to uncertainties in flow patterns were very different. Figure 2 also shows the the FNL data, the typhoon track information, and the 20-day low-pass filtered 700-hPa wind fields, primarily definition of steering in this study. In addition, physical including the MJO component and the background flow processes other than the steering may also affect the at the occurrence of the sudden track changes. Since typhoon motion (Chan and Williams 1987; Wu and 8 September 2000, Saomai generally took a track along the Wang 2000, 2001). Figure 3 also suggests that, although axis of the monsoon trough, which was oriented from the typhoon intensity is underestimated, the outer asymmet- northwest over the East China Sea to the southeast over ric structure of the typhoons is reasonably captured in the the subtropical northwestern Pacific (Fig. 2a). The mon- FNL dataset since previous studies have demonstrated soon trough had basically evolved into a large MJO-scale

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FIG. 8. 10-day high-pass 700-hPa wind vectors and wind speeds (contour) for Typhoon Saomai (2000) with closed dots indicating the typhoon center at (a) 0600 UTC 11 Sep, (b) 0600 UTC 12 Sep, (c) 0600 UTC 13 Sep, and (d) 0600 UTC 14 Sep 2000. Contour intervals are 5 m s21 with wind speeds less than 10 m s21 suppressed. monsoon gyre.1 The sudden track change occurred when primarily influenced by the northeasterly flow prior to Saomai was located near the center of the cyclonic cir- and during the sudden turning. culation. On the other hand, the sudden track changes were all The sudden track changes of Maemi and Jangmi did not associated with a QBW cyclonic gyre. Figures 4–7 show occur in a monsoon trough, which was oriented from the the 3-day evolution of the 10–20-day bandpass filtered west over the South China to the east over the Philippine 500-hPa wind fields immediately before the occurrence Sea (Figs. 2b,d). The sudden track changes occurred when of the sudden track changes. As shown in Fig. 4, Saomai Maemi and Jangmi were located at a location where the was coalescing with a QBW cyclonic gyre during 11–14 southeasterly flow bifurcated into easterly and south- September 2000. The center of the QBW cyclonic gyre easterly winds. Sinlaku was associated with a reversed- moved northwestward from the northwestern Pacific to oriented monsoon trough, which was oriented from the the East China Sea. During 11–12 September, Saomai southwest over the South China Sea to the northeast over was located slightly to the north of the center of the QBW the subtropical western Pacific (Fig. 2c). The typhoon was gyre (Figs. 4a,b) and became nearly concentric with the QBW gyre about 24 h prior to the sudden track change (Figs. 4c,d). Unlike Typhoon Saomai, Maemi did not 1 MJO- and QBW-scale gyres in this study are the cyclonic cir- become concentric with the QBW gyre (Fig. 5). It was culation identified from the 20-day low-pass and 10–20-day band- steered by the westerly flow during 8–10 September and pass filtered wind fields, respectively. by the southerly flow on 11 September. Sinlaku moved

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FIG. 9. 10-day high-pass 700-hPa wind vectors and wind speeds (contour) for Typhoon Maemi (2003) with closed dots indicating the typhoon center at (a) 0000 UTC 8 Sep, (b) 0000 UTC 9 Sep, (c) 0000 UTC 10 Sep, and (d) 0000 UTC 11 Sep 2000. Contour intervals are 5 m s21 with wind speeds less than 5 m s21 suppressed. northwestward with a well-developed QBW gyre (Fig. 6). 1995). With a relatively small maximum wind and a rela-

The center of Sinlaku was generally located near the tively large Rm, the energy of the QBW gyre can be more center of the QBW gyre during 11–14 September 2008. easily lost to the b-induced dispersion (or Rossby wave This approach to the QBW gyre center also occurred for dispersion) than that of a tropical cyclone. In agreement Jangmi, which was located to the southeast of the gyre with Carr and Elsberry (1995), as shown in Figs. 4–7, the center during 26–27 September and became concentric well-defined anticyclonic gyres on the QBW time scale with the gyre on September 28, 24 h prior to the sudden can be clearly identified for the cases of Saomai and track change (Fig. 7). Sinlaku (Figs. 4 and 6). Since both Saomai and Sinlaku Carr and Elsberry (1995) argued that the b-induced were closely associated with a monsoon trough (Figs. 2a,c), dispersion of a cyclonic vortex is associated with the it is likely that the background flows may be an im- 2 quantity Vm/Rm, where Vm and Rm are the maximum portant factor for the formation of the Rossby wave tangential wind and the radius of Vm, respectively. The train observed on the QBW time scale. In a shallow water quantity measures the radial shear of the angular tangen- model, Aiyyer and Molinari (2003) showed that the MJO tial wind and the stability of the vortex to the b-induced enhanced monsoon trough can make the westward-moving dispersion (Carr and Williams 1989; Carr and Elsberry equatorially trapped mixed Rossby–gravity waves excite

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FIG. 10. 10-day high-pass 700-hPa wind vectors and wind speeds (contour) for Typhoon Sinlaku (2008) with closed dots indicating the typhoon center at (a) 1800 UTC 11 Sep, (b) 1800 UTC 12 Sep, (c) 1800 UTC 13 Sep, and (d) 1800 UTC 14 Sep 2008. Contour intervals are 5 m s21 with wind speeds less than 10 m s21 suppressed. a secondary wave train that propagates along the mon- 5. Influence of synoptic-scale flows on the sudden soon trough axis. Further study on how the background track changes flows affect the development of a Rossby wave train is required. In their barotropic experiments, Carr and Elsberry Previous studies suggest that wave trains associated (1995) found that an area of enhanced winds forms ap- with tropical cyclones have a wavelength of 2000–2500 km proximately 500–600 km primarily to the east of the tropi- (Li and Fu 2006; Ge et al. 2008). In the barotropic simu- cal cyclone that is coalescing with a monsoon gyre. They lation, Carr and Elsberry (1995) found that an argued that the high-wind region corresponds to the large is located approximately 1200 km to the east or southeast streamfunction gradient between the combined tropical of the monsoon gyre. The QBW-scale anticyclone that cyclone and monsoon gyre and the dispersion-induced remains about 1200 km to the southeast of Sinlaku during anticyclone. Such a high-wind region can also be found in 13–14 September 2008 is consistent with the prediction in the observed sudden track changes. Figures 8–11 display the previous studies, but the wavelength is larger for the the 700-hPa synoptic-scale wind fields for the four Saomai case, in which the anticyclone remains about typhoons, which mainly represent 72-h evolution of the 2000 km to the southeast of the typhoon during 13–14 synoptic-scale flows prior to the sudden track changes. September 2000. Note that no well-developed wave trains on the synoptic

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FIG. 11. 10-day high-pass 700-hPa wind vectors and wind speeds (contour) for Typhoon Jangmi (2008) with closed dots indicating the typhoon center at (a) 1200 UTC 26 Sep, (b) 1200 UTC 27 Sep, (c) 1200 UTC 28 Sep, and (d) 1200 UTC 29 Sep 2008. Contour intervals are 5 m s21 with wind speeds less than 10 m s21 suppressed. time scale can be found, in agreement with the argument similar to Sinlaku in that the high winds were dominant that a more intense tropical cyclone tends to be more about 24 h prior to the sudden track change although the stable with the Rossby wave energy dispersion. high-wind areas to the east of the typhoon center were The enhanced winds to the east of the typhoon center observed during 26–28 September 2008 (Fig. 11). In are very clear in the cases of Saomai, Sinlaku, and agreement with Carr and Elsberry (1995), these cases Jangmi. As shown in Fig. 8a, a secondary high-wind re- suggest that the coalescing of the typhoon and the QBW gion appeared to the east of the center of Saomai at gyre is accompanied with enhanced synoptic-scale winds 0600 UTC 11 September 2000, while the synoptic winds to the east of the typhoon center. were mainly enhanced to the west of the center. The two Compared to the other three typhoons, the synoptic- high-wind regions were combined as the western part of scale winds associated with Maemi (2003) were relatively the strong winds enhanced and shifted cyclonically at weak on 8 and 9 September 2003 (Figs. 9a,b). The en- 0600 UTC 12 September (Fig. 8b). As Saomai and the hanced northeasterly winds tended to shift the typhoon QBW gyre were concentric on 13 and 14 September southward on 10 September. However, the winds to the (Figs. 4c,d), the high winds to the east of the typhoon south of the typhoon center were enhanced significantly center enhanced and became dominant (Figs. 8c,d). In on 11 September 2003 (Fig. 9d). The possible mechanism the case of Sinlaku (Fig. 10), the high winds to the east of for the enhanced synoptic-scale winds will be discussed in the typhoon center can be seen during 11–14 September the next section. 2008, but they were also dominant on 13 and 14 September, To identify the influence of the enhanced synoptic- about 24 h prior to the sudden track change. Jangmi is scale winds on the sudden track changes, following the

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FIG. 12. (left) Zonal and (right) meridional steering components calculated from total wind (black), 20-day low- pass filtered winds (green), 10–20-day bandpass filtered winds (blue), and 10-day high-pass filtered winds (red) for (a) Saomai (2000), (b) Maemi (2003), (c) Sinlaku (2008), and (d) Jangmi (2008). The vertical lines denote the time for the occurrence of the sudden track change. calculation in Fig. 3, we calculate the steering flows on after the sudden track changes (left panels of Fig. 12). For individual time scales (Fig. 12). Except for Typhoon Saomai, although its zonal movement resulted from the Saomai, the zonal movement of the three other typhoons combined effect of the three components, the increase in was dominated by the 20-day low-pass filtered winds, even the zonal translation speed since 11 September 2000 was

Unauthenticated | Downloaded 09/30/21 11:32 PM UTC DECEMBER 2011 W U E T A L . 3025 primarily a result of the change in the 20-day low-pass Although the peripheral ridging was not always suf- filtered winds. This suggests that the key to turning the ficiently strong to form the anticyclonic circulation, the typhoons with a northeast heading is to make them be can be identified 500–800 km to the south- steered by the flows associated with the subtropical high east of the typhoon center in Figs. 8b, 10d, and 11c, sug- (Fig. 2). gesting that the synoptic-scale peripheral ridging plays As shown in the right panels of Fig. 12, such a north- a role in producing the high-wind area to the east and ward shift was fulfilled by an abrupt increase in the south of the typhoons. Carr and Elsberry (1995) sug- meridional steering of the synoptic-scale winds prior to gested that nonlinear vorticity advection can induce pe- the sudden track changes. This contribution was very ripheral ridging (troughing) that compresses (stretches) small or southward before the abrupt increase but star- the axisymmetric structure of a tropical cyclone, leading ted to intensify at least 24 h prior to the occurrence of to asymmetries in the tropical cyclone circulation. the sudden track change. It is clear that the abrupt in- To examine the influence of the peripheral ridging creases in the steering of the synoptic-scale winds were as- due to the interaction between the typhoon circulation sociated with the asymmetric flows that were enhanced and the ISO flows, we impose an idealized vortex on the to the east or south of the typhoon center (Figs. 8–11). ISO flows at the observed typhoon center with the ob- For example, the westward and southward steering as- served maximum sustained wind speed. The radial profile sociated with Maemi started to decrease on 10 September of tangential wind speed is the one used by Carr and 2003, when the enhanced winds to the south of the ty- Elsberry (1995) with a radius of maximum wind at phoon center started to replace those to the northwest. 100 km. The interaction is indicated by the advection of Based on the above analysis, the sudden track changes the MJO and the QBW vorticity by the symmetric ty- may be likened to the maneuvering of satellite orbit phoons and the advection of the typhoon vorticity by the change, and the enhanced synoptic-scale winds act as background and MJO and the QBW flows. These ad- a booster rocket to place the typhoons in a northeast- vection terms associated with 700-hPa QBW and the ward orbit. The enhanced synoptic-scale winds can ef- MJO flows are shown in the left and middle panels of fectively push the typhoons into the northeastward orbit Figs. 13–16. Notice that the vorticity advection that is under the steering flows associated with the subtropical associated with the typhoon movement does not con- high because the sudden track changes occur near the tribute to the peripheral ridging and troughing. This center of the MJO-scale cyclonic circulation or at the part in this study is estimated with the advection of the bifurcation point of southeasterly and westerly winds typhoon vorticity by the MJO and the QBW steering (Fig. 2). flows, which is calculated as the flows averaged within a radius of 440 km between 850 and 300 hPa. The re- sulting ridging (troughing) is shown in the right panels 6. Peripheral ridging due to interaction between of Figs. 13–16. typhoons and ISO flows As shown in these figures, the vorticity advection as- Note that the area of the enhanced synoptic-scale sociated with the MJO and the QBW components mainly winds in Figs. 8–11 formed about 200 km from the ty- show a wavenumber-1 pattern with the maxima (min- phoon center, much closer than the barotropic simula- ima) at about 200 km away from the typhoon center. tions conducted by Carr and Elsberry (1995), who found Further examination indicates that the advections are that an area of enhanced winds (monsoon surge) was dominated by the advection of the typhoon vorticity approximately 500–600 km from the tropical cyclone by the MJO and the QBW flows. In the case of Saomai, center, midway between the tropical cyclone and the the maximum positive (negative) vorticity advection resulting anticyclone. As discussed in section 4, the associated with the QBW component occurred to the QBW-scale anticyclones associated with Sinlaku and northwest (southeast) of the typhoon center on 11 and Saomai were about 1200 and 2000 km apart from the 12 September 2003, when the typhoon was steered by typhoons, respectively, while no well-defined wave train the southeasterly flow of the QBW gyre (Figs. 4a,b). In can be found in the cases of Maemi and Jangmi. That is, the meantime, maximum positive (negative) vorticity in the cases of Maemi and Jangmi, the enhanced pres- advection associated with the MJO component re- sure gradient cannot account for the enhanced synoptic- mained to the northwest (southeast) of the typhoon scale winds specifically to the east of the typhoon center. center during 11–13 September 2003 since Saomai It is suggested that the enhanced synoptic-scale winds was also steered by the southeasterly flow associated that formed about 200 km from the typhoon center are with the monsoon trough. Consistent with the enhanced not due directly to the resulting streamfunction gradient synoptic-scale winds, the peripheral ridging was en- between the typhoon and the QBW-scale anticyclone. hanced prior to the sudden track changes and remained

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FIG. 13. 700-hPa vorticity advection associated with (left) the QBW flows and (middle) the background and MJO flows and (right) their contribution to ridging/troughing associated with Saomai (2000) at (a) 0000 UTC 11 Sep, (b) 0000 UTC 12 Sep, (c) 0000 UTC 13 Sep, and (d) 0000 UTC 14 Sep 2000. Contour intervals are 50 3 10210 s22 for the left and middle columns and 10 3 10210 s22 for the right column with the typhoon center denoted by a closed dot in the center of the 590 km 3 590 km domain.

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FIG. 14. As in Fig. 13, but associated with Maemi (2003) at (a) 0000 UTC 8 Sep, (b) 0000 UTC 9 Sep, (c) 0000 UTC 10 Sep, and (d) 1800 UTC 10 Sep 2003.

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FIG. 15. As in Fig. 13, but associated with Sinlaku (2008) at (a) 1800 UTC 9 Sep, (b) 1800 UTC 10 Sep, (c) 1800 UTC 11 Sep, and (d) 1800 UTC 12 Sep 2008.

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FIG. 16. As in Fig. 13, but associated with Jangmi (2008) at (a) 1200 UTC 26 Sep, (b) 1200 UTC 27 Sep, (c) 1200 UTC 28 Sep, and (d) 1200 UTC 29 Sep 2008.

Unauthenticated | Downloaded 09/30/21 11:32 PM UTC 3030 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 68 to the east or southeast of the typhoon centers in the The enhanced synoptic-scale winds provide a signifi- cases of Saomai, Sinlaku, and Jangmi and to the south cant northward acceleration and placed the typhoons in of the center of Maemi, suggesting that the peripheral the northeastward orbit under the steering of the flows ridging plays an important role in producing the high- associated with the subtropical high. The sudden track wind areas that shifted the typhoons into the northeast- changes are similar to the maneuvering of satellite orbit ward orbit under the steering flows associated with the change in that the enhanced synoptic-scale winds act subtropical high. However, it is difficult to distinguish as a booster rocket to shift the typhoons northward to how much the vorticity advections associated with the the southwesterly steering flows. It seems that both of the QBW and MJO components contribute to the periph- MJO and QBW components play a role in producing the eral ridging. Figures 13–16 suggest that both of the MJO high-wind area associated with these observed sudden and QBW components play a role in the peripheral track changes. ridging. For example, it is clear that the MJO contributes to the peripheral ridging to the southeast of the center of Acknowledgments. This research was supported by Maemi during 9–10 September 2003 (Figs. 14b,c). the Typhoon Research Project (2009CB421503) of the National Basic Research Program (the 973 Program) of 7. Summary China, the National Science Foundation of China (NSFC Grant 40875038), and the Social Commonweal Research In this study an observational analysis of the observed Program of the Ministry of Science and Technology of sudden typhoon track changes is conducted with a focus the People’s Republic of China (GYHY200806009). on the mechanism responsible for the observed sudden track changes and the roles of the slowly varying ISOs in sudden tropical cyclone track changes. Four typhoons REFERENCES that underwent sudden track changes in the vicinity of Aiyyer, A., and J. Molinari, 2003: Evolution of mixed Rossby– the East China Sea are investigated. These typhoons all gravity waves in idealized MJO environments. J. Atmos. Sci., occurred in September and took a generally northwest- 60, 2837–2855. Carr, L. E., and R. Williams, 1989: Barotropic vortex stability ward track prior to turning sharply northeastward. Ex- to perturbations from axisymmetry. J. Atmos. 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