Large Increasing Trend of Tropical Cyclone Rainfall in Taiwan and the Roles of Terrain
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4138 JOURNAL OF CLIMATE VOLUME 26 Large Increasing Trend of Tropical Cyclone Rainfall in Taiwan and the Roles of Terrain CHIH-PEI CHANG Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan, and Department of Meteorology, Naval Postgraduate School, Monterey, California YI-TING YANG AND HUNG-CHI KUO Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan (Manuscript received 23 July 2012, in final form 7 November 2012) ABSTRACT Taiwan, which is in the middle of one of the most active of the western North Pacific Ocean’s tropical cyclone (TC) zones, experienced a dramatic increase in typhoon-related rainfall in the beginning of the twenty-first century. This record-breaking increase has led to suggestions that it is the manifestation of the effects of global warming. With rainfall significantly influenced by its steep terrain, Taiwan offers a natural laboratory to study the role that terrain effects may play in the climate change of TC rainfall. Here, it is shown that most of the recently observed large increases in typhoon-related rainfall are the result of slow-moving TCs and the location of their tracks relative to the meso-a-scale terrain. In addition, stronger interaction between the typhoon circulation and southwest monsoon wind surges after the typhoon center moves into the Taiwan Strait may cause a long-term trend of increasing typhoon rainfall intensity, which is not observed before the typhoon center exits Taiwan. The variation in the location of the track cannot be related to the effects of global warming on western North Pacific TC tracks reported in the literature. The weaker steering flow and the stronger monsoon–TC interaction are consistent with the recently discovered multidecadal trend of intensifying subtropical monsoon and tropical circulations, which is contrary to some theoretical and model projections of global warming. There is also no evidence of a positive feedback between global warming– related water vapor supply and TC intensity, as the number of strong landfalling TCs has decreased signifi- cantly since 1960 and the recent heavy rainfall typhoons are all of weak-to-medium intensity. 1. Introduction a series of TCs with extraordinary amounts of rainfall since the late 1990s. In addition to an increase in the number of The increased water vapor capacity and thermody- TCs, 9 of the top 12 typhoons in terms of total rainfall since namic energy potential associated with global warming hourly rainfall observations started in 1960 have occurred has led to considerable interest in its possible impacts on in the twenty-first century (Table 1). The most extreme tropical cyclone (TC) trends. Several observational studies case is the record-breaking Typhoon Morakot in 2009 (Ge (Kim et al. 2006; Lau et al. 2008; Lau and Zhou 2012) suggest et al. 2010; Lee et al. 2011; Chien and Kuo 2011; Hendricks an increasing trend in TC rainfall and intensity in various et al. 2011), which caused huge economic and life losses regions. Modeling studies also suggest that TC intensity and and became the first natural disaster in Taiwan that actu- precipitation may increase with global warming, although ally triggered a change of government. Whether the heavy the frequency may decrease and considerable variability rainfall associated with these heavy rainfall typhoons is an exists among different basins (Knutson et al. 2010). impact of global warming, and in particular whether global Taiwan, which is situated in one of the main paths warming–enhanced water vapor capacity caused a positive of western North Pacific Ocean’s TCs, has experienced interaction between water vapor supply and TC that led to the observed large increasing trend of heavy rainfall, is the Corresponding author address: Chih-Pei Chang, Department of At- subject of intense debate (Hsu et al. 2011; Chang et al. 2012). mospheric Sciences, National Taiwan University, Taipei 10617, Taiwan. The terrain of Taiwan is dominated by the Central E-mail: [email protected] Mountain Range, which has a significant effect on TC DOI: 10.1175/JCLI-D-12-00463.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 09/30/21 08:38 PM UTC 15 JUNE 2013 C H A N G E T A L . 4139 TABLE 1. The 12 typhoons in 1960–2011 with total rainfall over Taiwan exceeding 3500 mm during the three phases, ranked according to total rainfall amount. The eight since 2004 are highlighted in boldface. Rank Year Typhoon name PR (h) OL (h) EX (h) Total duration (h) Rainfall (mm) Type of track 1 2009 Morakot 12 15 18 45 8996 CWB 3 (C) 2 2001 Nari 10 51 14 75 8108 CWB Special 3 2008 Sinlaku 16 10 22 48 8105 CWB 2 (N) 4 2005 Haitang 11 9 12 32 5589 CWB 3 (C) 5 1996 Herb 5 7 4 16 4836 CWB 2 (N) 6 1989 Sarah 5 20 13 38 4655 CWB 3 (C) 7 1960 Shirley 3 11 10 24 4637 CWB 2 (N) 8 2007 Krosa 12 1 10 23 3936 CWB 2 (N) 9 2004 Mindulle 16 18 7 41 3856 CWB 6 10 2008 Jangmi 4 13 8 25 3800 CWB 2 (N) 11 2008 Kalmaegi 8 10 5 23 3763 CWB 2 (N) 12 2005 Talim 4 9 4 17 3526 CWB 3 (C) structure and rainfall (Chang et al. 1993; Ge et al. 2010). Whereas the Taiwan Central Weather Bureau (CWB) In this paper, the role that terrain plays in the TC rainfall categorizes all typhoons into nine types of tracks, the is investigated by analyzing typhoons that made landfall focus here is on the three leading types that directly during 1960–2011. The objective is to gain insight into cross the island and whose maximum width is 144 km: the mechanisms involved in the apparent large increase the northern type (N; CWB category 2; 26 cases; Fig. 1b), of Taiwan TC rainfall in the recent decades. central type (C; CWB category 3; 23 cases; Fig. 1c), and southern type (S; CWB category 4; 14 cases; Fig. 1d). While all three tracks have a southeast–northwest ori- 2. Data and method entation, the N and C types have a more northward The database includes 84 typhoons that made landfall component than the S type, which has a more westward in Taiwan during 1960–2011. Hourly rainfall records component. The average separation between tracks of from 21 rain gauge stations (Fig. 1a) are averaged to the northern and central types and between those of the produce one value each hour. All stations are manually central and southern types is about 100 km, which is operated with no missing observations. Three of the about one-half of the length of the Central Mountain stations, Taipei, Hsinchu, and Tainan, were relocated in Range. The remaining 21 cases are spread over six less the 1990s. The temporary relocation between 1992 and frequent track categories. Examples of the 925-hPa 1997 of the Taipei station within the same street block streamlines for each of the three types based on the and nearly identical physical environment should have European Centre for Medium-Range Weather Fore- negligible effects under typical heavy rainfall situations, casts (ECMWF)–Tropical Ocean and Global Atmo- so only the relocations of Hsinchu and Tainan may cause sphere (TOGA) Global Advanced Analysis are shown concern. To ensure that these relocations do not materi- in Fig. 1e. ally affect the conclusions of this study, we conducted two Typhoon tracks analyzed postoperationally are used experiments to estimate the probable range of errors in to estimate the starting and ending times of each ty- our results. One experiment used only the other 19 sta- phoon as it passes through three track phases. The first is tions for the island-wide average rainfall. The other ex- the prelanding phase (PR) that starts when the typhoon periment used a larger set of data, 25 stations that were center moves to within 100 km of the nearest coastline available since 1977, to compute the island-wide average point and ends when the center makes landfall. The after 1977 after a validation procedure. The procedure overland phase (OL) starts at the end of the PR phase compared the 21- and 25-station averages during an and ends when the typhoon center exits the coast. Fi- overlapping period of 1977–90, before the relocation of nally, the exit phase (EX) starts at the end of the OL the three stations started. The correlation between the phase and ends when the typhoon center reaches two averages exceeds 0.99 in all cases. The 25-station 100 km away from the nearest coastline. These phases average is then adjusted to give the same mean averages are defined from the postoperational tracks produced by and standard deviations. For both the 19-station and the the Typhoon Reanalysis Project using aircraft, satellite, mixed 21-/25-station experiments, all numerical values and radar fixes by CWB until 1997, and then continued vary by 11% or less from the 21-station results shown in by National Taiwan University starting in 1998 with Figs. 2–4 below. satellite and radar fixes. The sums of the hourly rainfall Unauthenticated | Downloaded 09/30/21 08:38 PM UTC 4140 JOURNAL OF CLIMATE VOLUME 26 FIG. 1. (a) Taiwan’s topography and rainfall stations. (b) The northern (N)-type tracks. (c) The central (C)-type tracks. (d) The southern (S)-type tracks. (e) Examples of 925-hPa streamlines for the three types of tracks: (top) N—Typhoon Kalmaegi (2006), (middle) C—Typhoon Morakot (2009), and (bottom) S—Typhoon Morakot (2003). during each of the three phases and the total rainfall for center location and size of a typhoon (Dr.