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

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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 conver- gence 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 me- ridional 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 propa- gation 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 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 10/11/21 04:33 AM UTC 15 JULY 2012 C H E N A N D S H I H 5073 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 west- ward 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 fa- cilitates 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 Unauthenticated | Downloaded 10/11/21 04:33 AM UTC 5074 JOURNAL OF CLIMATE VOLUME 25 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.
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