APRIL 2013 W U E T A L . 1023 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 2 the gyre center with a magnitude of 6–10 m s 1 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: [email protected] 2) a giant TC that develops from the gyre itself. Molinari DOI: 10.1175/JAS-D-12-0117.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/04/21 05:50 AM UTC 1024 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 70 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 2 duced) energy dispersion. The energy dispersion associ- first exceeded 17 m s 1 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 1.0831.08 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 2.5832.58 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.