Interannual Changes of Tropical Cyclone Intensity in the Western North Pacific

Journal of the Meteorological Society of Japan, Vol. 89, No. 3, pp. 243 253, 2011 DOI:10.2151/jmsj.2011-305 243 Interannual Changes of Tropical Cyclone Intensity in the Western North Pacific Haikun ZHAO, Liguang WU, and Weican ZHOU Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China (Manuscript received 27 October 2010, in final form 22 February 2011) Abstract The individual contributions of changes in sea surface temperature (SST), vertical wind shear and tropical cyclone (TC) tracks to the interannual TC intensity change in the western North Pacific (WNP) basin are examined based on the selected 7 warm years and 7 cold years during the period 1970 2007. The selected warm and cold years are defined by the Nino-3.4 SST anomalies index, and correspond to El Niño and La Niña events, respectively. The intensity model used in this study can simulate the spatial distribution and differences of TC intensity when the model is integrated along the observed TC tracks in the warm and cold years. It is found that the change of TC tracks plays a dominant role in the observed TC intensity difference between warm and cold years. During the warm years, TC formation is enhanced in the southeast quadrant, and more TCs take a northwestward track during the warm years than during the cold years because of the interannual change in the large-scale steering flows. As a result, TCs have a longer time for intensification and develop into intense TCs during the warm years when compared to the cold years. 1. Introduction Tropical cyclone (TC) activity in the western North Pacific (WNP) is closely associated with El Ni no- Southern Oscillation (ENSO) (Chan 1985; Dong 1988; Lander 1994; Chen et al. 1998; Wang and Chan 2002; Chia and Ropelewski 2002; Elsner and Liu 2003; Wu et al. 2004; Camargo and Sobel 2005; Camargo et al. 2007; Zhan et al. 2010). Previous studies have focused on its influence on TC annual frequency (Wu and Lau 1992; Chan and Shi 1996; Lander and Guard 1998) and displacement of TC formation locations (Chan 1985, 2000; Chia and Ropelewski 2002; Wang and Chan 2002). While the effect of ENSO on TC intensity in the WNP basin and the associated mechanism had not been well understood, a few studies suggested that ENSO also affects TC intensity (Pudov and Petrichenko 1998, Corresponding author: Liguang Wu, Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing 210044, China. E-mail: liguang@nuist.edu.cn 2011, c Meteorological Society of Japan 2000; Chia and Ropelewski 2002, Wang and Chan 2002; Camargo and Sobel 2005). A significant impact of ENSO on the displacement of TC formation locations in WNP basin was documented in literature (Chan 1985, 2000; Wang and Chan 2002; Chia and Ropelewski 2002). Dividing the WNP basin into four quadrants, Wang and Chan (2002) found that TC formation is significantly enhanced in the southeast quadrant during strong El Niño years, along with a decrease in the northwest quadrant, indicating a southeastward shift in TC formation locations during El Niño years when compared to La Ni na years. In addition, many studies investigated the ENSO influence on TC tracks and landfall patterns (Harr and Elsberry 1991, 1995; Saunders et al. 2000; Wang and Chan 2002; Wu et al. 2004; Fudeyasu et al. 2006; Camargo et al. 2007). For example, Saunders et al. (2000) and Wu et al. (2004) found that ENSO had a remarkable impact on the TC landfall patterns in Vietnam, the Philippines and China. Pudov and Petrichenko (1998, 2001) found an increase in the TC intensity during El Niño years. Chia and Popelewski (2002) showed that more intense TCs occurred in El Niño years than in La Niña years. The

244 Journal of the Meteorological Society of Japan Vol. 89, No. 3 intensity change may be due to changes in low-level cyclonic shear and sea surface temperature (SST) associated with the ENSO events (Chan and Liu 2004; Wang and Zhou 2008). Given the significant changes in the TC formation locations and subsequent tracks, Camargo and Sobel (2005) suggested that the change in TC lifetime appeared to be the main factor for the observed change in TC intensity. As indicated above, several relevant studies have examined the relationship between TC intensity and SST; shear and TC tracks, but their relative contributions were not quantified. Moreover, different procedures of measuring TC intensity in the WNP make it difficult to clarify the relationship between the ENSO and TC intensity (Wu et al. 2006; Yu et al. 2007; Song et al. 2010). The focus of this study is to quantify the effect of the changes of SST, shear, and TC track changes associated with ENSO events on TC intensity change. The rest of the paper is organized as follows. The data, selection of the warm and cold years, and methods are described in Section 2. In Section 3, the interannual observed TC intensity changes in the WNP basin are discussed. The individual contribution of changes in the SST, shear and TC tracks to TC intensity change are examined in Section 4, followed by a summary and discussion in Section 5. 2. Data and Methods 2.1 Data On average the WNP basin experiences 26 TCs each year, accounting for about 33% of the total TCs all over the globe. The TC data in the WNP basin are the best track dataset from Joint Typhoon Warning Center (JTWC), including positions and intensities of tropical storms and typhoons at six-hour intervals. TCs that reached minimum tropical storm intensity (maximum sustained winds greater than 17.3 ms 1 ) are considered in this study. Although TCs can occur all year-round in the WNP basin, we only use the data for the peak typhoon season, namely from July to September. During this time, the TC activity is more frequent and the environmental circulations during the peak typhoon season are relatively steady so the climatological mean flows during the peak typhoon season can be taken as the climatological steering flows (Wu and Wang 2004; Wu et al. 2005). In order to quantify the vertical shear of environmental winds, the monthly wind field data derived from NCEP/NCAR Center on a 2.5 *2.5 grid is used (Kalnay et al. 1996). The environmental wind vertical shear is computed as the magnitude of the vector difference shear between 200 hpa and 850 hpa. The monthly Fig. 1. Niño-3.4 SST anomalies (Y-axis) index averaged over the peak typhoon season (July-September) according to the method in Wang and Chan (2002). The selected warm (cold) years are shown in black (gray) with SST anomalies greater than a standard deviation (black line), with white bars for neutral years. Extended Reconstructed SST version 2 developed on a 2 *2 grid by Smith and Reynolds (2004) is used in the present study. 2.2 Selection of warm and cold years The interannual variations of TC activity in the WNP have been documented in previous studies (Chan 1998; Wang and Chan 2002; Wu et al. 2004; Camargo and Sobel 2005; Camargo et al. 2007), which are closely correlated with the Niño-3.4 (5 S 5 N; 170 W 120 W) sea surface temperature anomalies (SSTAs) (Chan 1984; Lander 1994; Wang and Chan 2002; Zhao et al. 2010). As indicated in Wang and Chan (2002), the relationship between TC activity in the WNP basin and ENSO strongly depend on the intensity of ENSO episodes. For this reason, we discuss only the impacts of the strong warm years and strong cold years. The Niño-3.4 SSTAs from the Climate Prediction Center are used to stratify the warm and cold years using a similar manner to Wang and Chan (2002) in this study. In this study, we focus on the period 1970 2007 because of poor data quality in the pre-satellite era and stratified peak typhoon seasons divide SSTA into two categories: strong warm (SSTA 1 standard deviation) and strong cold (SSTA -1 standard deviation). Thus 7 warm years (1972, 1982, 1987, 1991, 1997, 2002, and 2004) and 7 cold years (1970, 1971, 1973, 1975, 1988, 1998, and 1999) (Fig. 1)

June 2011 H. ZHAO et al. 245 are selected. The years selected correspond well to the ENSO events obtained by using more traditional definitions (Trenberth 1997; Goddard and Dilley 2005). It should be pointed out that the selected warm and cold years are only slightly different from Wang and Chan (2002) due to the inconsistent study period. For example, Wang and Chan (2002) classified 1991 and 1971 as moderate warm and cold years, respectively. 2.3 Indices for basin-wide TC intensity Four parameters defined in this study are used to measure the basin-wide TC intensity change: average intensity, peak intensity, power dissipation index (PDI), and the frequency of intense TCs (categories 3 5 i.e., the maximum wind speed with greater than 49 ms 1 ), which are based on previous studies (Webster et al. 2005; Emanuel 2005; Wu 2007; Wu et al. 2008). The annual peak intensity is obtained by averaging the peak intensity of all the TCs each year. The PDI, similar to the accumulated cyclone energy (ACE) index (Camargo and Sobel 2005), is defined as the sum of the cube of the maximum wind speed over the TC lifespan containing TC-force winds. Correlations among the time series are made to determine to whether two series are significantly correlated. The statistical significance of correlations and the difference in mean are tested with Student s t-test method (Wilks 1995). 2.4 Methods The intensity model used in this study is adopted from Emanuel et al. (2006, 2008). In the model, three environmental parameters (SST, environmental vertical wind shear and TC tracks) are considered to affect TC intensity. All of the observed TCs are allowed to move along the observed TC tracks during the seven warm years and the seven cold years, and their intensity evolution is simulated with the intensity model. The details about the model can be found in Emanuel et al. (2006, 2008). After a series of numerical experiments, the model was initialized using a synthetic warm-core vortex with a maximum wind speed of 21 ms 1 rather than 17.3 ms 1 because the model vortex weakens at the beginning of the simulation. The other parameters of the initial vortex are the same as those in Emanuel et al. (2008). The same initial model setup is used for all the simulations in this study. 3. Interannual changes of TC intensity On the time scale of climate change, a TC can be taken as a point vortex. The TC track is determined by the TC formation location and subsequent movement. The latter is primarily dominated by large-scale envi- Fig. 2. Spatial distributions of TC formation locations during the warm years and (a) cold years (b). The TC formation numbers and percentage are also shown in each quadrant. ronmental circulation plus the minor propagation (Holland 1983; Wu and Wang 2004). Recently, Wang and Chan (2002) and Wu and Wang (2004) examined how ENSO affects the TC activity over the WNP basin. They pointed out that TCs tend to recurve northward to the extratropics during October of El Ni ño years, while during La Niña years, TCs more frequently take a westward track. In addition, the formation locations shift eastward in El Ni no years compared to La Ni na years. To show this, following Wang and Chan (2002), we divide the WNP into four subregions in this study. As shown in Fig. 2, the TC formation in the northwest quadrant (17 30 N, 100 140 E) and the southeast quadrant (5 17 N, 140 180 E) is significantly different. The difference is consistent with Wang and Chan (2002). During El Niño years, the monsoon trough extends farther east allowing TCs to form farther east, whereas, during La Niña years, stronger easterly trades dominate the low latitudes of WNP and keep the monsoon trough and TC formation region to the west of its typical location (Lander 1994). Following Holland (1983) and Wu and Wang (2005), the large-scale steering flows are defined as the mean

246 Journal of the Meteorological Society of Japan Fig. 3. Tracks of intense TCs (categories 3 5) and large-scale steering flows (m s 1 ) in the peak typhoon season during (a) the warm years and (b) cold years. The differences of large-scale steering flows and the frequency of intense TCs between the warm years and cold years are shown in (c) with contour intervals of 2.0. Shading indicates a change of intense TC occurrence frequency statistically significant at the 95% confidence level. flows averaged between 850 hpa and 300 hpa with mass adjustment. Using the Mann-Kendall test (Kundzewicz and Robson 2000), a notable difference in the intense TC tracks exists between the warm years and the cold Vol. 89, No. 3 years (Fig. 3), which is consistent well with the changes in WNP subtropical highs and changes in TC formations. As shown in Figs. 3a, b, an increase in the number of intense TCs affect the region north of Taiwan including East Asia and Japan during the warm years, with a decrease in the number of intense TCs that affect the region south of Taiwan during the cold years. Huang and Chen (2007) reached a similar result based on the state of the warm pool in the WNP basin. The increasing number of the northward-turning intense TCs near Japan during the warm years is consistent with the increase northward steering flows. The decrease in the number of westward moving intense TCs from the Philippine Sea to the southeast Chinese coast during the warm years agrees well with the decrease of the easterly steering flows in the region. These may be because more intense TCs form east of 160 E with a tendency for TCs to recurve toward higher latitude when the equatorial Pacific SSTs are higher than normal during the warm years. When the equatorial Pacific SSTs are colder than normal during the cold years, there is a tendency for TCs to prevail toward westward and northwestward tracks. Figure 3c clearly shows the track change in terms of the frequency of occurrence, which indicates how many TCs enter a specific 2.5 latitudes * 2.5 longitudes grid box. During the warm years, there is a significant increase in the northward-recurving TCs, which also agrees well with the changes of large-scale steering flows and the enhanced TC formation in the southeast quadrant of the basin (Wang and Chan 2002). The track change is also indicated in the mean TC duration. For each individual year, the mean duration is defined by the average duration of all TCs that occurred in the peak typhoon season. The total number of days of TC occurrence for the warm years (95.5 days) is nearly twice that for the cold years (55 days). The mean duration is 6.2 days for the warm years versus 4 days for the cold years. In addition, the mean PDI is 1.91*107 m3 s 2 and 2.86 *107 m3 s 2 for the cold and warm years respectively, and the significant difference in mean PDI can be found below (Table 1). As shown in Table 1, there are remarkable differences in the TC intensity in the WNP basin as measured by the four parameters (average intensity, peak intensity, frequency of intense TCs and PDI), indicating the impact of ENSO signal on TC intensity change. 4. Contributions of changes in SST, shear and TC tracks The influences of SST and vertical wind shear on the climate change of TC intensity were discussed in many previous studies (Goldenberg et al. 2001; Emanuel

June 2011 H. ZHAO et al. 247 Table 1. Mean observed and simulated average intensity, peak intensity, the frequency of intense TCs (categories 3 5), and PDI for the warm and cold years. All of the differences of the observed and simulated average intensity, peak intensity, frequency of intense TCs (categories 3 5) and PDI between the warm and cold years are significant at the 95% confidence level. Average (ms -1 ) Peak (ms -1 ) Cat.3-5 PDI (10 7 m 3 s -2 ) Observation Warm (Cold) 36.4 (32.7) 44.6(39.2) 7.0(3.6) 2.86(1.91) Simulation WWW(CCC) 41.7(31.4) 48.2(37.0) 8.7(3.6) 4.31(1.08) WWC(CCW) 43.1(30.7) 49.3(36.2) 8.7(3.4) 4.80(1.00) WCW(CWC) 40.7(32.1) 46.6(38.0) 8.4(4.0) 3.86(1.19) CWW(WCC) 42.0(31.3) 47.8(37.1) 8.6(3.6) 4.31(1.10) Table 2. Descriptions of the designed experiments. Here W and C indicate that the tracks, vertical wind shear and SST are taken from for the warm (cold) years, respectively. Experiments Tracks Shear SST WWW Warm Warm Warm WWC Warm Warm Cold WCW Warm Cold Warm WCC Warm Cold Cold CCC Cold Cold Cold CCW Cold Cold Warm CWC Cold Warm Cold CWW Cold Warm Warm 1987, 2005, 2008; Webster et al. 2005; Wu et al. 2008). In order to evaluate the contributions of vertical wind shear, SST and track changes to basin-wide TC intensity change, we first examine the capability of the model in simulating TC intensity along the observed TC tracks during the warm years and the cold years by designing two experiments. As shown in Table 2, the observed TC tracks, climatological mean SST and climatological mean environmental wind vertical shear in the peak typhoon season for the warm (cold) years are used in the first (second) experiment, which is referred to experiment WWW (CCC). As shown in Fig. 4, although the simulated TC intensity is stronger (weaker) than the observation for the warm (cold) years, the spatial distribution of TC intensity is well simulated when compared to the observed spatial distribution of TC intensity. Figure 4f shows the simulated contrast of TC intensity between the warm and cold years in terms of spatial pattern, although the simulated difference is relatively large compared to the observation (Fig. 4c). As indicated in the observation, the intensity difference between WWW and CCC is significant in terms of the number of intense TC, average intensity, peak intensity, and PDI at the 95% confidence level (Table 1). In order to demonstrate the individual contributions of the TC tracks, SST and vertical shear, six additional experiments are preformed (Table 2). As shown in Table 2, the first, second and third capital letters in the experiment names indicate that the TC tracks, vertical shear and SST are taken from the warm (cold) years. For example, the tracks, vertical shear and SST are taken from the warm, cold and warm years in experiment WCW. First we compare two sets of experiments WWW and WCW or CWC and CCC, in which the observed TC tracks and SST remain unchanged, respectively. Figure 5a shows the significant difference of vertical shear in the peak typhoon season between the warm and the cold years. Compared to the cold years, the remarkable decrease in vertical shear occurred over the tropical WNP between 28 N and 18 N during the warm years, while the increase occurred mainly south of 15 N. Figures 6a, d indicate the shear effect on TC intensity. In general, decreasing (increasing) shear is related to the increase (decrease) in the TC intensity, which is in agreement with previous studies (Goldenberg et al. 2001; Emanuel 1987, 2005, 2008; Webster et al. 2005; Wu et al. 2008).

248 Journal of the Meteorological Society of Japan Vol. 89, No. 3 Fig. 4. Spatial distribution of the observed TC intensity in the peak typhoon season for (a) the warm years, (b) cold years, and (c) the difference of the observed TC intensity between the warm and cold years. (d), (e) and (f) are the same as in (a), (b) and (c), but for the simulated TC intensity, respectively. As shown in Table 1, however, the simulated difference of the mean average intensity, peak intensity, frequency of intense TCs and PDI between WWW and WCW and between CWC and CCC is statistically insignificant, suggesting that the shear change between the warm and cold years has little influence on the basin-wide TC intensity change. Similar results also can be obtained for the two sets of experiments WWC and WCC and CWW and CCW. The difference of SST over the peak typhoon season between the warm and cold years is shown in Fig. 5b, which shows a remarkable increase in WNP east of 160 N and south of 20 N and a decrease in other areas. In association with the difference of SST distri- bution between the warm and cold years, the impacts of change in SST on TC intensity are examined based on the simulated difference between the two sets of experiments: WWW and WWC and between experiments CCW and CCC. As expected, Figs. 6b, e show that the increase (decrease) in SST corresponds to an increase (decrease) in TC intensity. As also shown in Table 1, the increasing SST leads to a small increase in the average intensity, peak intensity, frequency of intense TCs, and PDI, suggesting that the observed basin-wide intensity change does not result mainly from the SST difference between the warm and cold years. In addition, the combined effect of SST and vertical shear changes is also examined by comparing the simulated intensity differ-

June 2011 H. ZHAO et al. 249 Fig. 5. The observed differences of (a) environmental wind vertical shear and (b) sea surface temperature (SST) over the peak typhoon season between the warm and the cold years. Shading indicates the difference of SST and shear averaged the peak typhoon season between the warm and cold years statistically significant at the 95% confidence level. ence between WWW and WCC and between CCC and CWW. It is found that the intensity difference is basically the sum of their individual contributions of vertical shear and SST (Figure not shown). Then we focus on the contribution of changes in TC tracks to the TC intensity change by comparing the simulated intensity in WWW (WCC) with that in CWW (CCC). The simulated intensity difference in Figs. 6c, f is comparable with that in Fig. 4f, indicating that the change of TC tracks plays a key role in the observed TC intensity difference between the warm and cold years. This result confirms that the change in TC duration is the main factor for the observed change in TC intensity (Camargo and Sobel 2005) because TC duration

Journal of the Meteorological Society of Japan 250 Vol. 89, No. 3 Fig. 6. Simulated difference of TC intensity for two sets of experiments WWW and WCW (a) and CWC and CCC (d), in which the environmental vertical wind shear changed while the shear unchanged with the TC tracks from the warm years (a) or from the cold years (d), all indicating the effect of vertical wind shear change on the TC intensity (m s 1 ). Similarly, (b) and (d) indicate the effect of the SST change on the TC intensity. (c) and (f) indicate the effect of track changes on the TC intensity. See the detail in the text. change is closely associated with changes in TC tracks. Moreover, TC duration is intimately associated with the TC formation location. During the warm years, the enhanced TC formation in the southeast quadrant leads to longer TC tracks and thus longer duration. As a result, these TCs tend to have longer time for intensification and develop into more intense TCs during the warm years than during the cold years. 5. Summary and discussions In this study, based on the selected 7 warm years and 7 cold years during the period 1970 2007, the individ- ual contributions of changes in SST, vertical shear and TC tracks to TC intensity change on the interannual time scale are examined with a downscaling hurricane intensity model from Emanuel et al. (2006). The intensity model can simulate the spatial distribution and differences of TC intensity when the model is integrated along the observed TC tracks in the warm and cold years. In agreement with previous studies (Wang and Chan 2002; Camargo and Sobel 2005), it is confirmed that the TC track change is the most important factor for the basin-wide TC intensity (average intensity, peak intensity, frequency of intense TCs and PDI) change on

June 2011 H. ZHAO et al. 251 shown in Figs. 7a, b, the monsoon trough extends southeastward over the WNP to about 170 E (135 E) in the warm (cold) years. Chen et al. (1998) argued that interannual variations of the monsoon trough were attributed to an anomalous wave train resulting from SST anomalies in the tropical Pacific. In this sense, we argue that the interannual variability of SST may play an indirect role in the difference of TC intensity between the warm and cold years. Acknowledgements The authors thank Prof. Kerry Emanuel, who allowed us to use his TC intensity model. This research was jointly supported by the typhoon research project (2009CB421503) of the national basic research program (the 973 Program) of China, the National Science Foundation of China (NSFC grant No. 40875038), the social commonwealth research program of Ministry of Science and Technology of the People s Republic of China (GYHY200806009), and the research project funded by the Colleges and Universities in Jiangsu Province graduate study innovation plan (CX09B 224Z). Fig. 7. 850 hpa wind fields in the peak typhoon season, which are averaged from (a) the warm years and (b) the cold years. Monsoon troughs are shown with dashed lines. the interannual time scale, while the changes in vertical shear and SST do not directly contribute to the interannual TC intensity change. Numerical experiments are conducted to examine the effects of interannual changes in SST, vertical wind shear and TC tracks. It is found that the change of TC tracks plays a dominant role in the observed TC intensity difference between the warm and cold years. During the warm years, TC formation is enhanced in the southeast quadrant and more TCs take a northwestward track during the warm years than during the cold years because of the interannual change in the largescale steering flows. As a result, more TCs have longer time for intensification and develop into intense TCs during the warm years than during the cold years. The enhanced TC formation in the warm years may result from the difference of the monsoon trough over the WNP between the warm and cold years. Ritchie and Holland (1999) found that more than 75% of TCs in the WNP basin are associated with the monsoon trough. Figure 7 shows the mean monsoon trough averaged over the 7 warm years and 7 cold years, respectively. As References Camargo, S. J., A. W. Robertson, S. J. Gaffney, P. Smyth, and M. Ghil, 2007: Cluster analysis of typhoon tracks, Part II: Large-scale circulation and ENSO. J. Climate, 20, 3654 3676. Camargo, S. J., and A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO. J. Climate, 18, 2996 3006. Chan, J. C. L., 1985: Tropical cyclones activity in the northwest Pacific in relation to the El Ni ño /Southern Oscillation phenomenon. Mon. Wea. Rev., 113, 599 606. Chan, J. C. L., 2000: Tropical cyclones activity over the western North Pacific associated with El Ni ño and La Niña events. J. Climate, 13, 2960 2972. Chan, J. C. L., and J. E. Shi, 1996: Long-term trends and interannual variability in tropical cyclone activity over the western North Pacific. Geophys. Res. Lett., 23, 2765 2767. Chan, J. C. L., and K. S. Liu, 2004: Global warming and western North Pacific typhoon activity from an observational perspective. J. Climate, 17, 4590 4602. Chen, T. C., and S. P. Weng, 1998: Interannual Variation of the Summer Synoptic-Scale Disturbances Activity in the Western Tropical Pacific. Mon. Wea. Rev., 126, 1725 1733. Chia, H. H., and C. F. Ropelewski, 2002: The interannual variability in the genesis location of tropical cyclones in the northwest Pacific. J. Climate, 15, 2934 2944. Dong, K., 1998: El Niño and tropical cyclone frequency in

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