Numerical model studies have suggested that the ongoing global warming will likely affect tropical cyclone activity. But so far little observed evidence has been detected to support the projected future changes. Using satellite-supported best-track data from 1965 to 2003, we show for the first time that over the past four decades the two prevailing typhoon tracks in the western North Pacific (WNP) have shifted westward significantly; the typhoon activity over the South China Sea has considerably decreased; and East Asia has experienced increasing typhoon influence. Our trajectory model simulation indicates that the long-term shifts in the typhoon tracks result primarily from the changes in the mean translation velocity of typhoons or the large-scale steering flow, which is associated with the westward expansion and strengthening of the WNP subtropical high

Geng, Shuqin

Wang, BIn

Wu, Liguang

NASA Technical Reports Server

Growing Typhoon Influence on East Asia 
Liguang Wu', Bin Wang2, Shuqin Geng3 
1. Goddard Earth and Technology Center, University of Maryland, Baltimore County, 
and Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland 
2. Department of Meteorology, School of Ocean and Earth Science and Technology, 
University of Hawaii at Manoa, Honolulu, Hawaii. USA 
3. National Marine Environmental Research and Forecast Center, Beijing, China 
Corresponding author address: Dr. Liguang Wu, NASA/GSFC, Code 613.1, Greenbelt, 
MD 20071. E-mail: Liguang@agnes.gsfc.nasa.gov 
ABSTRACT 
Numerical model studies have suggested that the ongoing global warming will likely 
affect tropical cyclone activity. But so far little observed evidence has been detected to support 
the projected future changes. Using satellite-supported best-track data from 1965 to 2003, we 
show for the first time that over the past four decades the two prevailing typhoon tracks in the 
western North Pacific (WNP) have shifted westward significantly; the typhoon activity over the 
South China Sea has considerably decreased; and East Asia has experienced increasing typhoon 
influence. Our trajectory model simulation indicates that the long-term shifts in the typhoon 
tracks result primarily from the changes in the mean translation velocity of typhoons or the large- 
scale steering flow, which is associated with the westward expansion and strengthening of the 
WNP subtropical high. 
1. Introduction 
The typhoon movement in 2004 is unusual. Unprecedented number (10) of typhoons hits 
Japan whereas South China was ravaged by the worst drought since 1951 due to lack of landfall 
https://ntrs.nasa.gov/search.jsp?R=20050180415 2019-08-29T20:17:54+00:00Z
typhoons (NCDC 2004). Does this track shift reflect a long-term change in the typhoon 
prevailing tracks? So far, many studies, mostly based on numerical model results of global 
warming experiments, have suggested that ongoing global climate change will likely increase 
typhoon intensity (Knutson et al. 1998; Knutson and Tuleya 2004), change frequency of 
occurrence (Bengtsson 1996; Henderson-Sellers 1998), and shift prevailing tracks (Walsh and 
Katzfey 2000; Wu and Wang 2004). However, little observed evidence has been shown to 
support these projected changes (Knutson and Tuleya 2004; Chan and Liu 2004). 
TC tracks are essentially controlled by large-scale atmospheric circulation patterns. 
Previous studies have demonstrated that the variability of seasonal typhoon activity is related to 
the El Nino Southern Oscillation (ENSO) (Chan 1985, 2000; Lander 1994; Chen et al. 1998; 
Wang and Chan 2002), the quasi-biennial oscillation (Chan 2000) and interdecadal variations 
(Chan and Shi 1996). The tropical sea surface temperatures (SSTs) have changed abruptly 
around 1976 (Nitta and Yamada 1989; Kumar et al. 2004), and the atmospheric circulation over 
the Pacific and the properties of El Nino have also shown concurrent changes in response to the 
interdecadal SST change (e.g., Trenberth and Hurrell 1994; Wang 1995). Can these changes in 
large-scale atmospheric circulation lead to changes in the prevailing typhoon tracks? The global 
temperature has been rapidly increased since late 1970s, which is likely due to anthropogenic 
influence. The present study is aimed to address a heated issue: Is there any evidence of changes 
in typhoon activity in the WNP and East Asia? 
2. Data 
The satellite-supported best track data from the Joint Typhoon Warning Center (JTWC) 
were used to calculate two critical parameters that measure the seasonal mean TC motion (Wu 
and Wang 2004). The first, the frequency of TC occurrence, indicates how many TCs enter a 
2 
specific grid box of 2.5" latitudes by 2.5" longitudes. The higher the frequency in a given box, 
the more TCs affect it. The other is the mean translation velocity of the TCs. Both of the 
parameters are calculated annually from 1965 to 2003 for each box. The year of 1965 was 
chosen as the starting year because the satellite monitoring of weather events first became 
routine so that no TC would be missed. 
The wind data reanalyzed by the National Centers for Environmental Prediction/National 
Center for Atmospheric Research (NCEP/NCAR) and the European Center for Medium-Range 
Weather Forecasts (ECMWF) are used to calculate the climatological mean steering flow. The 
large-scale steering flow is defined in this study as the pressure-weighted mean flow from 850 to 
300 hPa (Holland 1993). In order to quantify the changes in the WNP subtropical high, two 
subtropical-high indices defined by the Chinese National Forecast Center were used in this study. 
The intensity index is defined as the sum of the grid points of geopotential height covered by 
contour 5880 m between 1lO"E and 180". For each 5"-latitude by 5O-longitude box, the point 
increases by one from zero when the maximum height in the box increases by each 10 m from 
5870 m. For example, the point is 2 if the height is between 5890 to 5899 m. The second index 
defined by the west-most longitude of the 5880 m contour measures the westward extension of 
the WNP subtropical high. 
3. Shifts in the prevailing tracks 
Figure l a  displays the spatial distribution of the 39-year mean frequency of TC 
occurrence for the typhoon season (June-October). TCs occur most frequently over northern 
South China Sea (SCS) and an adjacent region to the waters southeast of Taiwan, indicating a 
prevailing track of the westward-moving typhoons (Track I). The high frequency occurrence also 
extends from Philippine Sea to Korea and Japan, suggesting another prevailing track that 
influences the coastal region of East Asia (Track 11). In addition, some typhoons tend to recurve 
northeastward east of 130"E (Track 111) often during WNP subtropical ridge splits. As shown in 
Fig. lb, the temporal evolution of the seasonal mean frequency of TC occurrence in the activity 
center (17.5"N, 115"E) is characterized by interannual variations and a significant downward 
trend, indicating that the TC activity over the central South China Sea has persistently decreased 
since 1965, in particular in the last seven years. 
Two approaches were used to detect the trends in the frequency of TC occurrence and the 
mean translation velocity. First, we fit the frequency and mean velocity on each grid box by 
linear regression: A=ui+bit, where t is time and A is the frequency or velocity components on the 
i" grid. The first term (a,) represents the base state at t=O (1965) and the second term (b,t) 
represents the changes associated with linear trends. The significance of the linear trend on each 
grid box was tested with the Mann-Kendall method (Kundzewicz and Robson 2000). Second, we 
simply divide the 39-year data into t w o  epochs (1965-1983 and 1984-2003) and contrast their 
means. The significance of the epoch mean differences was tested with the Student t-test. 
The patterns of significant linear trend and epochal change detected by the above two 
approaches resemble each other in both of the TC occurrence frequency and the mean translation 
velocity (Figs. 2a and b). The robust spatial consistency among individual grids adds confidence 
to the results. Figure 2a and b indicate systematic shifts in the prevailing tracks during the past 
39 years. The negative anomalies over the central South China Sea mean a sharp decrease in the 
number of the TCs that follow track I, while the positive anomalies extending from Philippine 
Sea to the eastern coast of China and in the eastern part of the basin indicate westward shifts of 
prevailing tracks I1 and 111, respectively. 
4 
The shifts in the prevailing tracks can be seen clearly by comparing linear components of 
the frequency of TC occurrence in the beginning (1965) and the end (2003) of the period 
examined (Fig. 3). In 1965 three TC active centers between 110-120"E (South China Sea), 
between 120-130"E (east of Taiwan), and between 140-145"E can be identified. By 2003 the 
most active region over the SCS shifted northeastward and merged with the center to the east of 
Taiwan. The resulting new center is located to the north of Philippines. Meanwhile, the third 
active center shifted westward by about 10 degrees of longitude. As a result, East Asia tends to 
experience growing typhoon influence. 
4. Cause of the prevailing track shift 
For a given TC, its track depends on its formation location and subsequent movement. 
The TC movement is primarily determined by large-scale steering plus a minor propagation 
(beta-drift) component. The steering component is advection of TC potential vorticity by large- 
scale environmental flows. The propagation component arises from nonlinear interactions among 
the environmental flow, planetary vorticity gradient, and TC circulation (Holland 1983; Carr and 
Elsberry 1990; Wang and Li 1992). Recently Wu and Wang (2004) put forward a trajectory 
model with which the spatial distribution of TC occurrence frequency can be determined given 
the TC formation locations and the climatological mean TC translation velocity at each grid box. 
The latter is also composed of the mean large-scale environmental steering and beta drift. 
It is hypothesized that the track changes are mainly due to changes in the large-scale 
mean flow field whereas the beta drift does not change. Two numerical simulations using the 
trajectory model were performed, in which all TCs that formed during the period of June- 
October are assumed to move with the mean translation velocity fields deduced from the epoch 
means of 1965-1983 and 1984-2003, respectively. The trajectory model simulations capture 
faithfully the observed frequency changes west of 140"E (Fig. 4), suggesting that the changes in 
the mean steering flows (translation velocity) are a dominant factor responsible for the prevailing 
track shift. We also evaluated the influence of the changes in the formation locations on the 
prevailing track shift. Using the same mean TC translation velocity averaged over the period of 
1965-2003 the trajectory model is run with the formation location data over the periods of 1965- 
1983 and 1984-2003, respectively. We find that the changes in the formation locations play a 
minor role in terms of the magnitudes. 
The changes in the mean TC translation velocity are closely associated with the large- 
scale steering flow. The mean large-scale steering flows computed by using NCEPlNCAR and 
ECMWF reanalysis data show very similar patterns of the large-scale steering flow changes 
between the periods of 1965-1983 and 1984-2003, especially to the west of 130"E. Here we 
only show that derived from the NCEP/NCAR data (Fig. 4a). The change in large-scale steering 
flows is characterized by a cyclonic circulation centered over the eastern China. The changes in 
the large-scale steering flows can well explain the changes in the mean translation velocity of 
TCs (Fig. 2). Further examination reveals that the enhanced cyclonic steering flow in the last two 
decades results mainly from the upper-level circulation change (500 hPa and above). 
5. Discussion 
The 39-year typhoon data are relatively short for full determination of the climatic trend 
in typhoon tracks. However, the identified prevailing track shift shows physically meaningful 
consistency between the changes in the mean translation velocity and the changes in the large- 
scale circulation. That is, the prevailing typhoon track shift occurred over the past four decades is 
a primary result of changes in the large-scale steering flow. Since the track shift is intermingled 
with strong interannual variations, one feels more evident track shift in some extreme years like 
2004. Regardless of whether it is a consequence of anthropogenic impacts or is due to a long- 
term natural variability, the demonstrated shift in the prevailing typhoon tracks may have a 
profound influence on the countries of East Asia. The increasing trend in the typhoon affecting 
East Asia appears to be concurrent with the on-going global warming trend. 
What causes the enhanced cyclonic steering flows in the East Asia during the last two 
decades (Fig. 4a) is an important issue that deserves further investigation. Yu et a1 (2004) found 
that in contrast to the global warming trend over the past 50 years, a distinctive tropospheric 
cooling trend is found in the middle latitude East Asia. Accompanying this cooling, the upper- 
level westerly jet stream shifts southward. We suggest that this tropospheric cooling in the last 
two decades may be responsible for the large-scale circulation change shown by Fig. 4a. The 
East Asian tropospheric cooling is expected to generate a cyclonic circulation anomaly that 
increases with height, resulting in a vertically averaged cyclonic steering flow anomaly due 
primarily to the lowering of the upper tropospheric geopotential height. On the other hand, this 
tropospheric cooling induces an anomalous surface anticyclone, which is associated with the 
westward expansion of the strengthening subtropical high over the WNP (Figs. 4b and c), 
resisting the northward advance of the southwest monsoon. Consequently, the Y angtze River 
Valley tends to have more frequent flooding (Gong and Ho 2002, Yu et al. 2004), whereas the 
northern SCS tends to be drier and less tropical cyclone activity due to large-scale subsidence 
induced by enhanced western Pacific subtropical High (Figs. 4b and c). 
REFERENCES 
Bengtsson, L., M. Botzet, and M. Esch: Will greenhouse gas induced warming over the next 50 
7 
years lead to higher frequency and greater intensity of hurricanes? Tellus, 48A, 57-73, 
1996. 
Carr, L. E., and R. L. Elsberry: Observational evidence for predictions of tropical cyclone 
propagation relative to steering. J.  Atmos. Sci. 47,542-546, 1990. 
Chan, J. C. L. and K. S. Liu: Global warming and western north Pacific typhoon activity from an 
observational perspective. J .  Clim. 17,4590-4602,2004. 
Chan, J. C. L., Tropical cyclone activity in the northwest Pacific in relation to the El 
Nino/Southern Oscillation phenomenon. Mon. Weu. Rev. 113,599-606, 1985. 
Chan, J. C. L.: Tropical cyclone activity over the western North Pacific associated with El Nino 
and La Nina events. J. Clim. 13,2960-2972,2000. 
Chan, J. C. L., and J. Shi: Long-term trends and interannual variability in tropical cyclone 
activity over the western north Pacific, Geo. Res. Lett. 23,2765-2767, 1996. 
Gong, D.-Y., and C,-H. Ho, Shift in the summer rainfall over the Yangtze River valley. Geophys. 
Res. Lett., 29,78-82,2002. 
Henderson-Sellers, A., and coauthors: Tropical cyclones and global climate change: A post-IPPC 
assessment. Bull. Amer. Meteor. SOC. 79, 19-36, 1998. 
Holland, G. J., Ed., Tropical cyclone motion. Global Guide to Tropical Cyclone Forecasting, 
World Meteorological Organization Tech. Document WMO/TD 560, Tropical Cyclone 
Programme Rep. TCP-3 1, Geneva, Switzerland ,1993. 
Holland, G. J. : Tropical cyclone motion: Environmental interaction plus a beta effect. J.  Atmos. 
Sci. 40,328-342, 1983. 
Knutson, T. R., and R. E. Tuieya: Impacts of COZinduced warming on simulated hurricane 
8 
intensity and precipitation: Sensitivity to the choice of climate Model and Convective 
Parameterization. J .  Clim. 17,3477-3495,2004. 
Knutson, T. R., R. E. Tuleya, and Y. Kurihara, The recent increase in Atlantic hurricane activity: 
Causes and implications. Science 279, 1018, 1998. 
Kumar, A., F. Yang, and L. Goddard, S. Schubert: Differing trends in the tropical surface 
temperatures and precipitation over land and oceans, J. Clim. 17,653-664,2004. 
Kundzewicz, Z. W. and A. Robson, Ed., Detecting trend and other changes in hydrological data. 
World Climate Program-Water, WMO/UNESCO, WCDMP-45, WMO-TD 1013, 
Geneva, 157 pp, 2000. 
Lander, M. A., An exploratory analysis of the relationship between tropical storm formation in 
the western North Pacific and ENSO. Mon. Wea. Rev. 122,636-651, 1994. 
National Climatic Data CentedNational Oceanic and Atmospheric Administration 
(NCDCINOAA), http://w w w .ncdc.noaa. gov/oa/climate/research/2004/ann/events. html, 
2004. 
Nitta, T., and S. Yamada: Recent warming of tropical sea surface temperature and its relationship 
to the Northern Hemisphere circulation. J. Meteorol. SOC. Japan 67,375-383, 1989. 
Tao, S., and L. Chen: A review of recent research on the east Asian summer monsoon in China. 
Monsoon Meteorology, C.-P Chang and T. N. Krishnamurti, Eds., Oxford University 
Press, 60-92, 1987. 
Trenberth, K., and J. W. Hurrell: Decadal atmosphere-ocean variations in the Pacific. Climate 
Dynamics 9, 303, 1994. 
Walsh, K. J. E. and J. Katzfey: The impacts of climate change on the polaeward movement of 
tropical cyclone-like vortices in a regional climate model. J.  Clim. 13, 11  16-1 132,2000. 
9 
Wang, B., and J. C. L. Chan: How strong ENS0 events affect tropical storm activity over the 
western North Pacific. J.  Clim. 15, 1643-1658,2002. 
Wang, B.: Interdecadal changes in El Nino onset in the last four decades J. Clim. ,8,267-285, 
1995. 
Wang, B., and X. Li: The beta drift of three-dimensional vortices: A numerical study. Mon. Wea. 
Rev., 120,579-593, 1992. 
Wu, L., and B. Wang: Assessing impacts of Global warming on tropical cyclone tracks, J .  Clim. 
17, 1686-1698,2004. 
Yu, R., B. wang, and T. Zhou, 2004: Distinct Tropospheric Cooling Trend in East Asia: Does it 
come fkom stratosphere? In press (update?) 
Figure Captions 
Figure 1: (a) June-October mean frequency of TC occurrence (unit per year-' ) derived from the 
JTWC best-track data from 1965 to 2003 and (b) the time series of the seasonal mean frequency 
of TC occurrence at the most active center A (17.5"N, 115"E) with a linear fit indicated by the 
straight line. The thick solid lines with arrows highlight schematically three prevailing typhoon 
tracks. 
Figure 2: (a) Changes of the June-October mean frequency of TC occurrence and the motion 
vectors based on the derived linear trend change and (b) the difference in TC occurrence 
frequency between the periods 1965 - 1983 and 1984 - 2003. The areas with confidence level 
10 
exceeding 95% for the identified changes are shaded. The contour interval is 0.3 year.' and the 
unit of the vectors is ms-'. The thick solid lines with arrows denote the prevailing typhoon tracks. 
Figure 3: (a) The June-October mean frequency of TC occurrence in 1965 and (b) the same as in 
(a) except that a linear trend has been added so that the plot can be viewed as June-October mean 
frequency of TC occurrence in 2003 obtained by linear regression. The contour intervals are 0.5. 
Figure 4 (a) The changes of the June-October mean frequency of TC occurrence resulting from 
the changes in the mean translation velocity. Superimposed are the climatological prevailing 
typhoon tracks (thick solid lines with arrows). The vectors are the differences of the large-scale 
steering flows between the periods of 1965-1983 and 1984-2003 derived from the NCEP/NCAR 
reanalysis data. The contour intervals are 0.3 yea?' and the unit of the vectors is m s-'. (b and c) 
Time series of the intensity and west-most longitude 'of the July-September mean WNP 
subtropical high, respectively 
5N 
IOOE 1lOE lZOE l3OE 140E 150E 160E 170E 
10 - 
0-  
Year 
Figure 1 
12 
35N - 
3QN - 
25N - 
20N 
15N 
ION- 
(4 -4.3 
110E 120E 130E 140E 150E 1 
40N 
35N 
JON 
25N 
20N 
15N 
1 ON 
5Y1 
1 
E 110E 1 ZQE 1 &)E l h E  15bE 1( - 
2 
110E 120E 13OE 140E 15M 
40N I 
U.3’ 
110E 120E 130E 140E 1 WE 
Faure 3 
14 
"'iOOE 1 IO€ 1 ZOE 13OE 1 &E 1 SO€ 
" t970 1975 lQB0 1985 I990 1995 2doo 
I970 1975 1960 1985 I990 1995 2doo ' 
Year 
IS 


Growing Typhoon Influence on East Asia

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server