Association Between Northward-Moving Tropical Cyclones and Southwesterly Flows Modulated by Intraseasonal Oscillation

5072 JOURNAL OF CLIMATE VOLUME 25

Association between Northward-Moving Tropical Cyclones and Southwesterly Flows Modulated by Intraseasonal Oscillation

JAU-MING CHEN Institute of Maritime Information and Technology, National Kaohsiung Marine University, Kaohsiung, Taiwan

CHING-FENG SHIH Research and Development Center, Central Weather Bureau, Taipei, Taiwan

(Manuscript received 13 May 2011, in ﬁnal form 14 October 2011)

ABSTRACT

Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC–southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30–60- and 10–24-day ISOs make positive contributions to the TC–southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly–westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC–southwesterly flow association tends to occur during the minimum phase of the 30–60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10–24- day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.

1. Introduction mid-June, and August, respectively (e.g., Wu and Wang 2001; Wang and LinHo 2002; Wu 2002; Wang 2006). The summer monsoon and tropical cyclones (TCs) After onset, the monsoon system evolves with two major strongly inﬂuence weather and climate of East Asia and intraseasonal oscillations (ISOs): 30–60 and 10–24 days the western North Paciﬁc (WNP) (e.g., Gray 1968, 1975; (e.g., Chen and Chen 1995; Chen et al. 2000; Ding 2007). Chang and Krishnamurti 1987; McBride 1995; Chen and The 30–60-day ISO characterizes the northward progres- Chen 2003; Ding and Sikka 2006; Li and Wang 2005; sion of large-scale circulation associated with variability of Chen et al. 2007; Wang and Chen 2008; Chen et al. 2010b). the monsoon trough–ridge system in the 808–1308E region The summer monsoon in these regions consists of the (e.g., Yasunari 1979; Lau and Chan 1986; Li et al. 2005; South China Sea (SCS) monsoon (e.g., Chang and Chen Hsu 2005). This meridional progression may connect with 1995; Chen and Chen 1995), the East Asian (EA) mon- the eastward-propagating tropical Madden–Julian oscil- soon (e.g., Ding 1994; Chang 2004), and the WNP mon- lation (Madden and Julian 1971; Li and Wang 1994; Wang soon (e.g., Murakami and Matsumoto 1994; Wang et al. and Xie 1997; Lawrence and Webster 2002; Jiang et al. 2001) with different onset time approximate to mid-May, 2004). The 10–24-day ISO features the westward propagation of a low–high system from the WNP toward the SCS along the 158–208N latitudes (e.g., Chen et al. 2000, Corresponding author address: Jau-Ming Chen, Institute of Mar- itime Information and Technology, National Kaohsiung Marine 2009). Its spatial scale is much smaller than that of the University, No. 482, Jhongjhou 3rd Rd., Kaohsiung 805, Taiwan. 30–60-day ISO (e.g., Hartmann et al. 1992). In gen- E-mail: [email protected] eral, the monsoon activity is primarily controlled by the

DOI: 10.1175/JCLI-D-11-00264.1

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30–60-day oscillation but modulated by the 10–24-day and postlandfall (2–3 July) periods, resulting in wide- oscillation (e.g., Mao and Chan 2005). ranging floods in central and southern parts of western TCs in the tropical WNP prefer to form within coun- Taiwan. Chien et al. (2008) found that Mindulle’s north- terclockwise mixed Rossby–gravity (MRG) gyres as they ward movement concurred with enhanced monsoon move off the equator (e.g., Frank and Roundy 2006). The southwesterly flows from the SCS to Taiwan, thereby MRG wave pattern recurves northwestward and pro- maintaining the cloud bands and prolonged heavy duces substantial variance in convection (e.g., Kiladis rainfall. On the other hand, some TCs with a northward et al. 2009). TCs in the tropical western Pacific exhibit track similar to Mindulle did not coincide with inten- two major tracks: northwestward toward Asia and the sified southwesterly flows or cloud bands, such as Alex SCS or northward toward the northwestern Pacific and (1987) and Ofelia (1990). The TC–southwesterly flow Japan (e.g., Ho et al. 2004; Chen et al. 2005). The TC association appears to be determined by a factor other tracks show apparent variability on intraseasonal, inter- than the TC track. Chen et al. (2009) demonstrated that annual, and interdecadal time scales and are regulated the monsoon ISO can closely regulate the intensity of by large-scale circulation anomalies of the respective southwesterly flows. One may infer that enhanced south- time scales (e.g., Harr and Elsberry 1991, 1995; Chan westerly flows and prolonged heavy rainfall associated 2000; Hall et al. 2001; Ho et al. 2004; Chen et al. 2005; Kim with Mindulle are likely results of the ISO modulation. et al. 2008; Tu et al. 2009; Chen et al. 2010b; Chen and However, such an intraseasonal modulation on TC rain- Chen 2011). Chen et al. (2009) demonstrated that the fall variability has thus far not been comprehensively northward progression of the 30–60-day oscillation of the studied. monsoon ridge–trough system results in intraseasonal The official Central Weather Bureau (CWB) of Tai- variability of monsoon westerlies and trade easterlies. wan has categorized TC tracks affecting Taiwan into An intensified Pacific subtropical high and enhanced nine major types (Fig. 1a). A TC with prolonged heavy trade easterlies tend to steer a straight-moving TC, while rainfall discussed above has a type-6 track: moving north- an enhanced monsoon trough and strong monsoon westward along the oceans east of Taiwan to pass over or near erlies are favorable to a TC recurving northward. Ko and northern Taiwan. Based upon the above discussions, Hsu (2006, 2009) pointed out that during the westerly some major questions are raised as follows: phase of the ISO, the deepened monsoon trough facilitates a TC to develop and propagate northward– d If a TC has a type-6 track, what is the likelihood that it northwestward toward the East China Sea. The above will also cause prolonged heavy rainfall in Taiwan? results suggest that the ISO modulates WNP TC activity. d What are the salient differences in rainfall and circula- The landfall of a TC often brings with it heavy rainfall tion patterns between the type-6 TCs associated and not via its own vigorous convective systems, causing signif- associated with enhanced southwesterly flows in Tai- icant property damage and casualties. TC-induced rain- wan? fall itself normally dissipates quickly with the departure d How do the ISOs affect the TC–southwesterly flow of the storm. However, one particular case appears association? What are the related large-scale pro- whereby heavy rainfall sometimes persists in Taiwan cesses? during the postlandfall period to cause severe flooding. When a TC moves northward from the oceans east of Previous studies have analyzed how the ISOs modu- Taiwan toward northern Taiwan, intense and elongated late TC tracks and genesis frequency (e.g., Hall et al. cloud bands may appear from the SCS into Taiwan to 2001; Kim et al. 2008; Chen et al. 2009), but not in the connect with the southern section of this TC system. aspect of rainfall variability. Analysis of this study should These cloud bands provide abundant moisture supply help to better predict TC-induced heavy rainfall in Tai- for enhanced rainfall on the western (windward) side of wan during the southwesterly flow season. Consequently, Taiwan in addition to rainfall directly caused by a TC’s possible flooding events can be better prepared for in own convective system. Later, the cloud bands may advance to mitigate any related damage. detach from the fast-moving TC and linger over Taiwan, producing continuous heavy rainfall after the departure 2. Data of the TC. Prolonged heavy rainfall across the landfall and postlandfall periods significantly increases the po- This study analyzes three datasets. The first set in- tential for severe flooding. For example, typhoon Mindulle cludes daily rainfall records from 10 major meteoro- (2004) itself and accompanying cloud bands over its logical stations in Taiwan (Fig. 1b). They are used to southern regions produced 600–1400 mm of rainfall delineate TC-related rainfall in Taiwan. These stations over Taiwan’s mountain areas during its landfall (1 July) are located in the coastal plain regions throughout the

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3. Type-6 northward-moving TC cases In Taiwan, TCs are active from June to October, while the southwesterly flows prevail from May to August. The TC–southwesterly flow association is thus analyzed from the coherent active season of June–August. Ac- cording to the Typhoon Database of the CWB (http:// rdc28.cwb.gov.tw/data.php), 108 summer TCs affected Taiwan (15 in June, 42 in July, and 51 in August) during 1958–2009. Among them, 16 TCs (14.8%) have a type-6 track and are selected for analysis in this study. Their detailed information is listed in Table 1. They are separated into three types for comparison based upon their track and rainfall characteristics. Regarding the track characteristics, 12 TCs passed over or near northern Taiwan with a track to the west of 258N, 1228E (Figs. 2a,b), while four other TCs moved with a distant track to the east of 258N, 1238E (Fig. 2c). The four distant TCs are sorted as the ‘‘OTHER’’ type (see Table 1). The 12 TCs near Taiwan are further partitioned based upon rainfall characteristics in stations on the windward side of the southwesterly flows over western Taiwan (i.e., Hsinchu, Taichung, Tainan, and Kaohsiung). A TC with rainfall in at least one of the four western stations exceeding 1) 200 mm during the landfall–approaching day and the ensuing two days, and 2) 100 mm during the ensuing two days is considered to have prolonged rainfall in con- currence with enhanced southwesterly flows. Four TCs satisfy these two rainfall criteria and are sorted as the southwesterly flow (SW) type—equal to 25% of all 16 type-6 TCs. The remaining eight TCs are grouped as the nonsouthwesterly flow (NSW) type (see Table 1). No TC of the OTHER type matches the above two rainfall criteria. The CWB Typhoon Database also documents that the four SW-type TCs were followed by southwesterly flow-related rainfall in the postlandfall period.

FIG. 1. (a) The nine major track types of TCs affecting Taiwan 4. Rainfall and flow features in Taiwan defined by the CWB, Taiwan. (b) The topography of Taiwan and 10 major meteorological stations around the coastal plain areas of this One marked feature of the TC–southwesterly flow island. association is prolonged heavy rainfall during the landfall and postlandfall periods. To illustrate the extending- island, with an altitude ranging from 2 to 34 m. The rainfall feature, composite rainfall patterns accumulated second set is the National Centers for Environmental from the landfall day and the ensuing two days are Prediction–National Center for Atmospheric Research shown in Fig. 3. Generally speaking, composite rainfall (NCEP–NCAR) reanalysis (Kalnay et al. 1996). The patterns have the greatest value in the SW type and the daily reanalysis data are used to depict the large-scale smallest value in the OTHER type. The OTHER-type ISO regulatory processes on TC activity and local rain- TCs exert minor impacts on local rainfall because of the fall in Taiwan. The third set is the 6-h WNP TC best- distant tracks from Taiwan. It is of interest to examine track data from the Joint Typhoon Warning Center why TCs of the SW and NSW types exhibit similar tracks (JTWC). It is employed to delineate TC tracks and de- but distinct rainfall features. Our analyses hereafter fo- fine landfall and approaching periods in Taiwan. cus on the comparisons between the SW and NSW types.

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Table 1. Information of 16 summer TCs affecting Taiwan with a type-6 track deﬁned by the CWB of Taiwan. These TCs are separated into three types: SW, NSW, and OTHER.

Year Landfall/approaching day TC name Type 1962 22 Jul Kate SW 1987 21 Jul Vermon 1994 8 Aug Doug 2004 1 Jul Mindulle 1965 18 Jun Dinah NSW 1974 19 Jul Jean 1981 20 Jun June 1984 3 Jul Alex 1986 24 Jun Nancy 1987 27 Jul Alex 1990 23 Jun Ofelia 2000 9 Jul Kai-Tak 1979 15 Aug Irving OTHER 1982 9 Aug Cecil 1984 19 Aug Holly 2000 30 Aug Prapiroon

Composite rainfall patterns are much larger in western Taiwan than eastern Taiwan in the SW type (Fig. 3a) but somewhat larger in eastern Taiwan in the NSW type (Fig. 3b). The rainfall difference pattern (SW minus NSW; Fig. 3d) reveals that the SW type has more rainfall than the NSW type by 190–240 mm in western Taiwan over its central and southern parts. The 10-m wind (V10) difference patterns (Fig. 3e) characterize strong westerly– southwesterly anomalies from the northern SCS across Taiwan into the WNP. These wind anomalies reflect an intensification of mean southwesterly flows in the SW type, which is consistent with the findings of Chien et al. (2008).

5. Comparisons in large-scale processes Rainfall in Taiwan has been found to be closely regulated by the low-level circulation and associated moisture processes (e.g., Chen et al. 2008, 2010a). In this study, the low-level circulation is represented by 850-hPa streamfunction (S850).ThemoistureprocessesaredepictedÐ p0 by vertically integrated moisture flux (VQ 5 p Vqdp, where V is the horizontal wind vector, q is the specific humidity, and the vertical integral is from a given pres- sure level to p0 5 1000 hPa) and moisture flux divergence ($ Á VQ). To specifically examine the variability features of large-scale environments during the TC- affecting period, daily anomalies with respect to the June– August mean in each year are extracted. Composite anomaly patterns of the above fields averaged from landfall and the ensuing two days are displayed in Fig. 4. FIG. 2. Tracks of summer type-6 TCs affecting Taiwan: (a) the Composite S850 anomalies of the SW type (Fig. 4a) fea- SW type associated with enhanced southwesterly flows, (b) the NSW ture a cyclonic anomaly centering to the north of Taiwan type with no enhanced southwesterly flows, and (c) the OTHER type with a distant path to the east of 258N, 1238E.

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FIG. 3. Composite anomaly patterns averaged from landfall and the ensuing two days: rainfall in Taiwan for (a) the SW type, (b) the NSW type, and (c) the OTHER type, and differences between the SW and NSW types (SW minus NSW) for (d) rainfall and (e) 10-m winds. Unit of rainfall is mm.

and an anticyclonic anomaly to the south. This meridi- induce heavy rainfall. For the NSW type, S850 anomalies onal S850 pair accompanies with westerly–southwesterly (Fig. 4b) exhibit an east–west pair with a weak cyclonic VQ anomalies from the northern SCS toward Taiwan anomaly to the west of Taiwan and a strong anticyclonic (Fig. 4d). Along this path, convergent moisture ﬂux anomaly to the east. Anomalous southerly VQ anomalies

(negative $ Á VQ) anomalies occur in Taiwan (Fig. 4g) to (Fig. 4e) stretch from the SCS across Taiwan toward

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FIG. 4. As in Fig. 3, except for (left) composite anomaly patterns of low-level circulation (represented by 850-hPa streamfunction, S850), 5 2 21 (center) vertically-integrated moisture ﬂux (VQ), and (right) moisture ﬂux divergence ($ Á VQ). Contour intervals are 4 3 10 m s for 24 21 S850 and 3 3 10 kg s for $ Á VQ. Positive values of S850 and $ Á VQ anomalies are shaded.

Japan.Divergentmoistureﬂux(positive$ Á VQ)anom- SW-type TC event is associated with enhanced south- alies extend from the Philippine Sea into Taiwan (Fig. westerly ﬂows in company with a meridional pair of

4h), indicating unfavorable conditions for heavy rainfall. S850 anomalies sandwiching Taiwan. How monsoon ISOs The major difference between these two types is the affect this meridional circulation pair is thus investigated. orientation of the circulation pair. The S850 difference patterns (SW minus NSW) are notable by a meridional 6. Regulatory processes of the 30–60-day pair of cyclonic and anticyclonic anomalies straddling oscillation Taiwan to the north and south (Fig. 4c). Meanwhile, stronger anomalies of westerly–southwesterly moisture The ISOs in the SCS–EA–WNP regions exhibit two ﬂux (Fig. 4f) and moisture convergence (Fig. 4i) appear dominant components: 30–60 and 10–24 days (e.g., in the northern SCS–Taiwan region. The above dif- Hartmann et al. 1992; Mao and Chan 2005; Ding 2007; ferences indicate that prolonged heavy rainfall in a Hoyos and Webster 2007). How these two ISOs regulate

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As indicated by dark shading, strong moisture convergence anomalies occur over Taiwan and its northern regions. The NSW type exhibits anomalous easterly flows over the Philippine Sea along the southern periph- ery of an anomalous anticyclonic circulation to the east of Taiwan (Fig. 5b). These anomalous flows later turn northwestward to cross Taiwan and the northern SCS. They act to weaken moisture supplies of the mean southwesterly flows from the SCS into Taiwan, leading to a lack of strong moisture convergence and heavy rainfall in Taiwan. Salient features of the 30–60-day anomaly patterns shown above resemble well the daily anomaly patterns shown in Fig. 3. This resemblance suggests the important role of the 30–60-day ISO in the regulation of TC–southwesterly flow association in Taiwan. Detailed regulatory processes of the 30–60-day ISO are delineated by the evolution patterns. The landfall day is defined as day 0, while the day before (after) the landfall day by x days is defined as day 2x (x). Com- ~ posite evolution patterns of S850 anomalies from day 212 to day 12 for the SW type are displayed in Fig. 6. ~ From day 212 to day 26, S850 anomalies exhibit the meridional pairing characteristics of an elongated cyclonic anomaly in the north and an anticyclonic anomaly in the south with a steady northward progression from the tropics toward Taiwan. Taiwan is located under- neath the center of a cyclonic anomaly on day 23, and on the southern boundary from day 0 to day 3. The cyclonic anomaly affecting Taiwan decays from day 6 to day 12. The 30–60-day ISO of the SW type features a significant and systematic northward progression. Tai- wan is influenced by anomalous southwesterly–westerly flows on the southern boundary of a cyclonic anomaly FIG. 5. Composite anomaly patterns of 30–60-day 10-m winds from day 0 to day 3. The mean southwesterly flows are ~ ~ (V10; vectors) and moisture flux divergence ($ Á VQ; shading) intensified by the 30–60-day ISO by which persistent averaged from landfall and the ensuing two days for (a) the SW ~ moisture supplies from the northern SCS maintain pro- type and (b) the NSW type. Negative $ Á VQ anomalies smaller than 24 3 1024 (between 22 3 1024 and 24 3 1024)kgs21 are longed rainfall over western Taiwan during the landfall indicated by dark (light) shading. and postlandfall periods. ~ Composite evolution patterns of S850 anomalies for the NSW type are shown in Fig. 7. From day 212 to day the TC–southwesterly flow association is examined in 23, a cyclonic anomaly overlying Taiwan moves north- this section. The 30–60- and 10–24-day components of westward with a decaying magnitude. Meanwhile, an an- field A are extracted by a fourth-order Butterworth ticyclonic anomaly near 1608E moves westward toward bandpass filter scheme (e.g., Murakami 1979) and are Taiwan along 208N. As the anticyclonic anomaly con- hereafter denoted as A~ and A^, respectively. tinues to move westward, Taiwan is influenced by its The composite 30–60-day anomalies of 10-m winds western boundary from day 0 to day 3 and by its center ~ ~ (V10) and moisture flux divergence ($ Á VQ) averaged from day 6 to day 12. From day 0 to day 3, anomalous from landfall and the ensuing two days are computed for southerly flows appear over Taiwan (see Fig. 5b). This the SW and NSW types. The major 30–60-day feature of indicates that the 30–60-day ISO does not intensify the SW type (Fig. 5a) is strong anomalous southwesterly southwesterly flows during the passage of a NSW-type flows from the northern SCS into Taiwan in company TC. Hsu and Weng (2001) found that the 30–60-day with a meridional pair of anomalous cyclonic and anti- ISO in summer may propagate from the Philippine Sea cyclonic circulations to the north and south of Taiwan. toward Asia along the 208N latitude. The 30–60-day

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~ FIG. 6. Taking the landfall day as the reference (day 0), composite evolution patterns of 30–60-day S850 anomalies for four TCs of the SW type. The evolution is from 12 days before the landfall (day 212) to 12 days after (day 12). Contour intervals are 4 3 105 m2 s21 and positive values are shaded. propagating feature shown in Fig. 7 is largely consistent TC–southwesterly flow association. For the NSW type with their finding. (Fig. 8b), only one TC, Dinah (1965), has a landfall day To depict the temporal relationship between a TC and near the minimum 30–60-day phase, while the other ~ the 30–60-day ISO, S850 time series averaged from the seven TCs occur in a phase with near-zero or positive ~ cyclonic anomaly center north of Taiwan shown on day S850 anomalies. A detailed investigation shows that 0 of the SW type (i.e., 258–308N, 1158–1258E; see Fig. 6e) Dinah concurs with an anomalous cyclone extending are illustrated in Fig. 8. The landfall day of each TC is southwest–northeasterly across Taiwan. Anomalous west- indicated by a dark solid circle. For the SW type (Fig. 8a), erly flows stretch from the SCS into the open oceans south each TC tends to make landfall in Taiwan near the min- of Taiwan. No intensified southwesterly flows occur imum phase of the 30–60-day oscillation, characterizing over Taiwan. the establishment of a strong cyclonic anomaly center- The major 30–60-day features favorable to the TC– ing to the north of Taiwan. This pattern facilitates an southwesterly flow association include a northward pro- intensification of mean southwesterly flows and the gression of a meridional pair of cyclonic and anticyclonic

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~ FIG. 7. As in Fig. 6, except for composite evolution patterns of 30–60-day S850 anomalies associated with eight TCs of the NSW type. anomalies and the establishment of a cyclonic anomaly anomaly to the south during day 0 and day 1. From day 2 and the arrival of a minimum 30–60-day phase to the to day 4, the cyclonic anomaly decays quickly off the north of Taiwan during the passage of a TC. eastern coasts of China, while the anticyclonic anomaly continues to move northward toward Taiwan with a decaying magnitude. Ko and Hsu (2006) found that 7. Roles of the 10–24-day oscillation the submonthly perturbation may propagate north– Roles of the 10–24-day ISO in the TC–southwesterly northwestward from the Philippine Sea toward the ^ flow association are examined from composite S850 open oceans between Taiwan and Japan. The SW-type ^ patterns from day 24 to day 4. For the SW type (Fig. 9), S850 anomalies exhibit a propagating feature coherent salient features on day 24 include a cyclonic anomaly with their finding. Since Taiwan is influenced by anom- centering over the Philippine Sea and an elongated an- alous southwesterly flows from day 0 to day 2, the 10– ticyclonic anomaly over the equator. This meridional 24-day ISO acts to enhance the mean southwesterly pair moves northwestward toward Taiwan and eastern flows during the passage of a SW-type TC. It is a positive China from day 23 to day 1. Taiwan is surrounded by contributor in the facilitation of the TC–southwesterly a cyclonic anomaly to the north and an anticyclonic flow association.

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~ FIG. 8. Time series of 30–60-day S850 anomalies averaged from the anomalous cyclone center located to the north of Taiwan (258–308N, 1158–1258E; see Fig. 6e) for summers with the (a) SW- and (b) NSW-type TC. The landfall day of each TC is marked by a dark solid circle.

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^ FIG. 9. Taking the landfall day as the reference (day 0), composite evolution patterns of 10–24-day S850 anomalies in association with four TCs of the SW type. The evolution is from 4 days before the landfall (day 24) to 4 days after (day 4). Contour intervals are 3 3 105 m2 s21 and positive values are shaded.

^ For the NSW type (Fig. 10), composite S850 anomalies 10–24-day ISO is manifested by the westward propaga- on day 24 show a cyclonic anomaly over the Philippine tion from the western Pacific toward the SCS, which is Sea and an anticyclonic anomaly to the east in the WNP. coherent with the above analyses of the NSW type. This east–west circulation pair moves westward toward To specifically examine the 10–24-day processes dur- ^ Asia along 208N and decays quickly after it reaches ing the landfall period, composite patterns of V10 and ^ the landmass. Along this propagation course, Taiwan is $ Á VQ anomalies averaged from landfall and the ensu- influenced by a cyclonic anomaly from day 23 to day 1 ing two days are computed for the SW and NSW types. and by an anticyclonic anomaly afterward. From day 0 to In the SW type (Fig. 11a), anomalous southwesterly day 2, Taiwan is surrounded by a cyclonic anomaly to the flows extend from the SCS into Taiwan to provide large west and an anticyclonic anomaly to the east. Anoma- moisture supplies for strong moisture convergence (in- lous southerly flows occur over Taiwan, indicating no dicated by dark shading) over Taiwan and the East China intensification of the mean southwesterly flows. Chen Sea. In the NSW type (Fig. 11b), anomalous southerly et al. (2000) and Mao and Chan (2005) reported that the flows stretch from the northern SCS across Taiwan

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^ FIG. 10. As in Fig. 9, except for composite evolution patterns of 10–24-day S850 anomalies for the NSW type. toward Japan. Without the enhancement of mean south- Vernon even in a maximum phase. The corresponding ~ westerly flows, only weak moisture convergence occurs in S850 time series in Fig. 8a reveal that the landfall of the open oceans between Taiwan and Japan rather than in Kate and Mindulle concurs with a strong 30–60-day Taiwan. minimum phase, while weak minimum phases exist for It is clear that the 10–24-day ISO makes positive the landfalls of Vernon and Doug. By considering effects contributions to the TC–southwesterly flow association from both the 30–60-day and 10–24-day ISOs, the TC– and heavy rainfall over western Taiwan during the pas- southwesterly flow association should be strong for Kate sage of a SW-type TC. The temporal relationship be- and Mindulle, moderate for Doug, and weak for Vernon. tween a SW-type TC and the 10–24-day ISO is depicted To substantiate this argument, total rainfall averaged ^ by S850 time series averaged from the cyclonic anomaly from three western Taiwan stations (Taichung, Tainan, center north of Taiwan shown in the day-0 pattern in Fig. and Kaohsiung) for landfall and the ensuing two days is 9e (i.e., 258–308N, 1158–1258E). As shown in Fig. 12, both computed. The averaged values for Kate, Mindulle, Kate and Mindulle make landfall in a strong minimum Doug, and Vernon are 433, 379, 198, and 170 mm, re- phase, while Doug does so in a weak minimum phase and spectively. It is clear that that the TC–southwesterly flow

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severe floods in western Taiwan. The aforementioned association between a type-6 northward-moving TC and the southwesterly flows is investigated in this study. How large-scale ISOs regulate this association is also studied. According to the CWB Typhoon Database, 16 out of 108 summer TCs affecting Taiwan during 1958–2009 had a type-6 track. Among them, four TCs moved with a distant path to the east of 258N, 1238E and caused minor rainfall in Taiwan. The remaining 12 TCs passed over or near northern Taiwan with a track to the west of 258N, 1228E. For these 12 TCs, a TC with rainfall in at least one of the four stations over western Taiwan (i.e., Hsinchu, Taichung, Tainan, and Kaohsiung) exceeding 1) 200 mm during landfall and the ensuing two days and 2) 100 mm during the ensuing two days is considered as a prolonged heavy rainfall case in company with evident TC– southwesterly flow association. Four TCs satisfy these two rainfall criteria and are grouped as the southwesterly flow (SW) type. It is equal to 25% of all 16 type-6 TCs. The other eight TCs are sorted as the nonsouthwesterly flow (NSW) type. Processes regulating the TC–southwesterly flow association are examined from the comparisons between the SW and NSW types. Overall, rainfall is much larger in western Taiwan than eastern Taiwan in the SW type, but somewhat larger in eastern Taiwan in the NSW type. Salient circulation features of the SW type during the landfall period are a meridional pair of cyclonic and anticyclonic anomalies straddling Taiwan to the north and south and accompanying intensified southwesterly flows. For the NSW type, Taiwan is surrounded by an east–west circulation pair and affected by anomalous southerly flows, showing no TC–southwesterly flow association. FIG. 11. Composite anomaly patterns of 10–24-day 10-m winds ^ ^ Regarding the regulatory processes of the ISOs, both (V10; vector) and moisture flux divergence ($ Á VQ; shading) averaged from landfall and the ensuing two days for (a) the SW 30–60- and 10–24-day oscillations make positive contri- ^ 24 and (b) the NSW types. Negative $ Á VQ anomalies ,24 3 10 butions to the TC–southwesterly flow association. For 24 24 21 (between 22 3 10 and 24 3 10 )kgs are indicated by the SW type, both the 30–60- and 10–24-day ISOs ex- dark (light) shading. hibit a meridional pair of anomalous cyclonic and anticyclonic circulations progressing northward from the association and accompanying heavy rainfall are jointly tropics toward Taiwan. Their common features are il- regulated by the intensity of both the 30–60- and 10– lustrated by the schematic diagram in Fig. 13. During 24-day ISOs. landfall and the ensuing few days, Taiwan is embed- ded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. Both 8. Concluding remarks anomaly pairs are accompanied by anomalous westerly– In summer, a TC may move northward along the open southwesterly flows or enhanced mean southwesterly oceans to the east of Taiwan and later pass over or near flows. This TC–southwesterly flow association intensifies northern Taiwan. This path is categorized as the type-6 moisture supplies from the SCS into Taiwan, leading to track by the Central Weather Bureau (CWB) of Taiwan. prolonged heavy rainfall over western Taiwan across the Under certain conditions, a type-6 TC may be associated landfall and postlandfall periods. The landfall of a SW- with intensified monsoon southwesterly flows from the type TC coincides with the arrival of a minimum phase northern SCS into Taiwan, resulting in prolonged heavy of the 30–60-day circulation in the vicinity of Taiwan but rainfall across the landfall and postlandfall periods and with various phases of the 10–24-day circulation. Heavy

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^ FIG. 12. Time series of 10–24-day S850 anomalies averaged from the anomalous cyclone center located to the north of Taiwan (258–308N, 1158–1258E; see Fig. 9e) for summers with the SW-type TC. The landfall day of each TC is marked by a dark solid circle. rainfall in western Taiwan is jointly regulated by the in- Moreover, a 30–60-day cyclonic anomaly tends to oc- tensity of these two ISOs—that is, a stronger cyclonic cur in the Taiwan region several days before the ap- anomaly for heavier rainfall and vice versa. For the NSW proaching TC. The appearance of a cyclonic anomaly type, both 30–60- and 10–24-day ISOs exhibit an east– indicates a weakening in the Pacific subtropical high. A west pair of circulation anomalies surrounding Taiwan weakened subtropical high tends to retreat eastward so during the landfall period, and thus cannot facilitate the TC–southwesterly flow association. This study demonstrates that both 30–60- and 10– 24-day ISOs effectively regulate the TC–southwesterly flow association for the type-6 TCs and accompanying prolonged heavy rainfall over western Taiwan. Com- parisons between the SW and NSW types indicate that the ISO patterns are the key factor for the prolonged heavy rainfall in Taiwan after the departure of a TC. Thus, a flooding event due to the TC–southwesterly flow association can be considered as chance combinations of heavy TC rainfall with ISO patterns that extend the total rainfall. The northward progression of the 30–60- and 10– 24-day circulations may be adopted as a useful index in typhoon forecasting. Figure 6 shows that the center of a cyclonic anomaly is located to the south of Taiwan at 208N on day 26 and to the north of Taiwan at 268N FIG. 13. Schematic diagrams for the major regulating processes imposed by the 30–60- and 10–24-day ISOs on the TC–southwesterly on day 0, with a northward-propagating speed about flow association. These two ISOs tend to exhibit a cyclonic anomaly 21 18 day . Similarly, Fig. 9 shows that a 10–24-day cy- (C) centering to the north of Taiwan and an anticyclonic anomaly clonic anomaly to the south of Taiwan moves northward (AC) to the south. This circulation pair intensifies monsoon south- from 208Nonday23to308N on day 1, with a speed about westerly flow (gray arrow) from the northern SCS toward Taiwan. It 2.58 day21. The above progression paths and speeds also sets up favorable conditions for a TC to move northward along the oceans to the east of Taiwan toward northern Taiwan (1 sym- of these two ISOs may combine with TC forecasting bol). A northward-moving TC concurs with intensified southwesterly to predict whether there will be a TC–southwesterly flows to maintain prolonged heavy rainfall in western Taiwan during flow association in Taiwan during the landfall period. the landfall and postlandfall periods.

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