Time series of the genesis frequency of autumn TCs and all TCs over the WNP are shown in Fig. 1. In the 68 years, 11.3 TCs per year occurred in autumn over the WNP, with a maximum autumn frequency of 18 TCs in 1964 and a minimum of 6 TCs in 2010. The number of TCs in autumn has a weak decreasing trend over the period, but this trend is not as prominent as that for annual TC frequency, consistent with the findings of Hsu et al. (2014). There are clear wavelike characteristics in the time series at the interdecadal scale. TC frequency is generally below the climatic average before the 1960s, above the climatic average in the 1960s, below the climatic average from the 1970s to 1985, above the climate average from 1985 to 1995, and below the average after 1995 (Fig. 1). This variation pattern might be associated with the abrupt decrease in TC frequency in autumn over the WNP since 1998 (Hsu et al., 2017) and the decrease in overall TC activity since the mid-1990s (Choi et al., 2015; Lin and Chan, 2015). In 2013 and 2016, there are 15 and 14 autumn TCs, respectively, which are above the climatic average. These two positive autumn anomalies may be related to the atmospheric thermal and dynamic circulation anomalies caused by the autumn SST anomalies in the equatorial central Pacific (Liu and Chan, 2013; Fan et al., 2019).
Figure 1. Evolution of the genesis frequency of autumn TCs (red and yellow curves) and annual TCs (blue and green curves) over the western North Pacific (WNP) during 1949–2016.
Table 1 shows the seasonal variation of TC genesis frequency over the WNP during 1949–2016. TCs mainly occurred in summer and autumn. Annual average numbers of summer and autumn TCs over the period are 11.5 and 11.3, which equate to 42.7% and 42.1% of the total number of TCs in an average year, respectively. Combined summer and autumn TCs make up 84.8% of the total number of TCs.
Season Total TC number Annual average TC number Percentage (%) Summer (June–August) 781 11.5 42.7 Autumn (September–November) 770 11.3 42.1 Winter (December–February) 130 1.9 7.1 Spring (March–May) 146 2.1 8.0 Total 1827 26.9 100.0
Table 1. Seasonal variation of TC genesis frequency over the WNP during 1949–2016
Time series of the frequency of summer and autumn TCs as a percentage of the annual total during the period 1949–2016 (Fig. 2) show that in more than 70% (49 of 68) of years, the proportion of autumn TCs was inversely related to that of summer TCs. The annual proportion of autumn TCs reached its peak (64.3%) in 1998, when the annual proportion of summer TCs was only 28.6%. Thus, the proportion of autumn TCs in 1998 was 21.6% higher than the average of the period, while that of summer TCs was 14.6% lower than the time series average. A similar inverse pattern occurred in 1954 and 2007, when the anomaly proportions for autumn (summer) TCs were 17.8% (−10.8%) and 14.3% (−12.4%), respectively.
In order to understand the characteristics of autumn TC tracks, especially their similarities and differences with those of summer TCs, the track density in each season, along with the difference field, is shown in Fig. 3. The variability in TC track density reflects both the variation in the location of TC genesis and that in TC tracks (Mei et al., 2015). Figure 3 clearly shows that autumn TCs are more active at lower latitudes, and that the region to the west of 140°E, the South China Sea, and the region east of the Philippines have the highest frequency of autumn TCs. Contrasting with summer TCs, fewer autumn TCs affect Taiwan, Hainan, and the coastal areas of mainland China. Furthermore, the area affected by autumn landfalling TCs is not as large as that affected by summer landfalling TCs. Contrasting with the summer landfalling TCs, autumn landfalling TCs mainly affect the Southeast and South China coastal areas, and rarely affect areas of the China mainland north of 30°N (Figs. 3a, b). The difference in the climatological spatial distribution between summer and autumn TC track density (Fig. 3c) reveals that the tracks of autumn landfalling TCs are farther south than those of summer landfalling TCs, indicating that autumn landfalling TCs on average have more westward tracks and less northward tracks than summer landfalling TCs. These trends are closely related to differences in atmospheric circulation between the two seasons, especially the direction and intensity of the large-scale steering flows.
Figure 3. Spatial distributions of the climatological TC track density over the WNP during 1949–2016, calculated at each 1° × 1° grid cell in (a) summer (June–August), (b) autumn (September–November), and (c) the difference (autumn minus summer) field. Black crosses in (c) indicate the areas where the track density difference is statistically significant at the 95% confidence level based on the Student’s t-test.
Figure 4 shows the mean large-scale steering flow and subtropical high at 500 hPa for summer and autumn during 1949–2016. In autumn, the mean subtropical high moves southward and westward compared with its location in summer, and strong easterly steering flows are noted in the tropical area over 0°–20°N, 90°E–180°. In summer, the easterly steering flows gradually weaken as they approach the Philippines and turn into a more northerly direction in the WNP. Steering flows in the South China Sea show significant differences to those in the same area in autumn. Consequently, the prevailing tracks of autumn landfalling TCs are more westward, and the prevailing tracks of summer landfalling TCs are more northward.
Table 2 shows the frequency distribution of TCs with different intensities occurring in summer, autumn, and the full year over the WNP from 1949 to 2016. On average, there are 4.1 TS, 6.3 STS, 5.8 TY, 4.6 STY, and 6.0 SupTY each year. The most common intensity category is STS, making up 23.5% of annual TCs, with SupTY and TY ranking second (22.4%) and third (21.6%), respectively. TS is the least frequent category, accounting for only 15.3% of annual TCs. The frequency distribution of TCs at different intensities in summer is basically the same as the annual results. The proportion of STS is the highest (26.1%), followed by TY (24.3%), and the proportion of SupTY is only 18.3%. In autumn, the highest number and proportion (28.1%) is for SupTY. Although TC genesis frequency in summer is higher than that in autumn, the number and proportion of STY and SupTY events are higher in autumn. Autumn SupTY accounts for almost half of the annual number. The reason is that the cold airflows in the Northern Hemisphere become stronger in autumn and can reach tropical regions, which is conducive to the development and intensification of TCs. This phenomenon has been shown in case studies (Dong et al., 2013; Wang et al., 2018) and was reviewed by Yu (2012).
Intensity scale Annual Summer Autumn Average Percentage (%) Average Percentage (%) Average Percentage (%) TS 4.1 15.3 1.9 16.5 1.5 13.2 STS 6.3 23.5 3.0 26.1 2.3 20.1 TY 5.8 21.6 2.8 24.3 2.3 20.2 STY 4.6 17.2 1.7 14.8 2.1 18.4 SupTY 6.0 22.4 2.1 18.3 3.2 28.1
Table 2. Average number of annual, summer, and autumn TCs at different intensities over the WNP during 1949–2016
The annual number of autumn TCs of different intensities during 1949–2016 is shown in Fig. 5. Despite the overall decreasing trend of autumn TCs over the period, the annual average genesis frequency of autumn SupTY increased after 2011 compared with the previous 40 years.
Figure 5. Annual frequencies of autumn TCs with different intensities over the WNP during 1949–2016.
Fourteen of the top 20 TCs with the lowest central pressure during 1949–2016 are autumn TCs. Only two of the top 10 TCs with the lowest central pressure occurred in summer, and the rest are all autumn TCs (Table 3). The strongest TC in the WNP is Typhoon Tip (1979), which has the lowest sea level pressure on record. The TC with the strongest wind speed is Typhoon Ida (1958), which formed on 20 September 1958, and the central maximum wind speed reached 110 m s−1 at 0800 BJT 24 September.
History ranking TC genesis time Minimum pressure (hPa) TC name (ID) Year Month Day Time (BJT) 1 1979 10 05 0200 870 Tip (1979) 2 1973 10 02 0800 875 Nora (1973) 3 1975 11 16 0800 875 June (1975) 4 1983 9 20 0200 876 Forrest (1983) 5 1958 9 20 2000 878 Ida (1958) 6 1978 10 17 1400 878 Rita (1978) 7 1984 10 22 2000 879 Vanessa (1984) 8 1971 11 08 0800 884 Irma (1971) 9 1959 8 25 0200 885 Joan (1959) 10 1951 8 10 2000 886 Marge (1951) 11 1966 6 20 1400 886 Kit (1966) 12 1961 9 08 0200 888 Nancy (1961) 13 1969 9 19 0200 888 Elsie (1969) 14 1983 8 04 1400 888 Abby (1983) 15 2014 7 10 0800 888 Rammasun (2014) 16 1954 8 24 0800 890 Ida (1954) 17 1962 10 01 2000 890 Emma (1962) 18 1980 10 03 0200 890 Wynne (1980) 19 2013 11 03 1400 890 Haiyan (2013) 20 2016 9 09 0800 890 Meranti (2016)
Table 3. Top 20 TCs with the lowest central minimum pressure in the WNP during 1949–2016
Based on the CMA-STI TC size dataset for 1980–2016, the statistical characteristics of the 34-knot radius (R34) for each category of summer and autumn TCs are shown in Fig. 6. The average R34 is 146 km for TS, 171 km for STS, 185 km for TY, 226 km for STY, and 255 km for SupTY for autumn TCs. The corresponding values for summer TCs are 157 km for TS, 177 km for STS, 205 km for TY, 226 km for STY, and 257 km for SupTY. The results show that R34 increases with TC intensity for both summer and autumn TCs. The SupTY category has the smallest variation of R34 of all the TC intensity categories, indicating that the strongest TCs have a consistent outer size.
Figure 6. Boxplots of the 34-knot radius (R34; km) for each category of (a) summer and (b) autumn TCs in the WNP during 1980–2016. For each box, the dot represents the median, the middle line represents the average, the upper (lower) end of the box represents the average plus (minus) the variance, and the whiskers represent the extreme values.
In order to analyze the regional characteristics of autumn TC genesis in the WNP, the WNP region was divided into 5° × 5° boxes and the genesis frequency was calculated in each box. The spatial distribution of autumn TC genesis frequency is shown in Fig. 7.
Figure 7. Spatial distribution of autumn TC genesis frequency (shading) and the genesis location of SupTYs in the WNP from 1949 to 2016. Black circles denote SupTYs that formed in September; green crosses denote SupTYs that formed in October; and red boxes denote SupTYs that formed in November. The black, green, and red solid lines indicate the average latitude and longitude of the SupTY genesis locations.
We find from Fig. 7 that there are three high value centers of autumn TC genesis; the first is located near (14°N, 115°E) over the northeastern South ChinaSea, and the other two are located in the vast oceanic area east of the Philippines around (14°N, 135°E) and (14°N, 145°E), respectively. SupTYs hardly occurred over the South China Sea, and SupTYs that formed in October and November are mostly located south of 15°N. From September to November, there is generally an equatorward shift of the genesis location of autumn TCs. In contrast, there is no obvious trend in meridional shift from September to November, for the average longitude during the period is 146.6°E, 144.8°E, and 149.1°E in September, October, and November, respectively.
From 1949 to 2016, 529 TCs made landfall in China, with an average of 7.8 each year. The maximum number of landfalling TCs in a given year was 12 (1952, 1961, 1971, 1989, and 1994), and the minimum number was 4 (1982). The frequency decreased on average by 0.5 every 10 yr, though this trend is not significant. Before the late 1990s, the annual landfalling TC frequency and its climatic anomaly were relatively large. However, after the late 1990s, the annual landfalling TC frequency and its climatic anomaly were relatively small. For instance, the mean annual number of landfalling TCs was 7.1 from 1996 to 2016, which is 0.7 less than the average from 1949 to 2016. In total, 164 TCs made landfall in China in autumn, with an average of 2.4 per year and a maximum of 5 (1952 and 1974). However, in the autumns of 1959, 1984, 1997, 2006, and 2012, no TC made landfall in China (Fig. 8).
Figure 8. Evolution of annual landfalling TC frequency and autumn landfalling TC frequency over the WNP during 1949–2016.
In terms of seasonal variations over the period (Table 4), TCs mainly made landfall in China in summer and autumn, but never in winter. Summer landfalling TCs, with an average number of 5.1 per year, account for the largest proportion (65.8%) of landfalling TCs in a given year, followed by autumn landfalling TCs, which account for more than 30%. The frequency of autumn landfalling TCs does not show a significant long-term trend. There has been a slight decrease in landfalling TCs since the late 1990s, with an average annual landfalling frequency of 2.1 in the recent 20 years. About 28.9% of the total TCs during the study period made landfall in China, including 44.6% in summer and 21.3% in autumn.
Season Total number Annual average landfalling TC number Season ratio (%) Landfalling ratio (%) Summer (June–August) 348 5.1 65.8 44.6 Autumn (September–November) 164 2.4 31.0 21.3 Winter (December–February) 0 0.0 0.0 0.0 Spring (March–May) 17 0.3 3.2 11.6 Total 529 7.8 100.0 28.9
Table 4. Seasonal number of landfalling TCs in the WNP during 1949–2016
The variation in the proportion of landfalling TCs in autumn is consistent with that for the year as a whole. The proportion of annual landfalling TCs increased by around 0.8 per decade over the period. However, the increasing trend in autumn is not as significant as that for the year as a whole, with an average increase in the landfalling frequency of about 0.3 per decade. The landfalling TC ratio in the autumns of 1973, 2010, 1999, 2008, and 1985 reached 50.0%, 50.0%, 44.4%, 44.4%, and 40.0%, respectively, which are the top five highest values during 1949–2016 (Fig. 9).
Table 5 presents the frequency distribution of landfalling TCs at different intensities in summer and autumn over the WNP during 1949–2016. The annual frequency of autumn landfalling TCs is 0.2 for TS, 0.4 for STS, 0.6 for TY, 0.4 for STY, and 0.7 for SupTY. By comparing with the summer landfalling TC frequencies, we find that although the frequency of every TC intensity category in summer is higher than that in autumn, the landfalling ratios for SupTY and STY in autumn are higher than those in summer.
Intensity scale Summer Autumn Total number Annual average number Percentage (%) Total number Annual average number Percentage (%) TS 46 0.7 13.2 15 0.2 9.1 STS 92 1.4 26.4 30 0.4 18.3 TY 90 1.3 25.9 40 0.6 24.4 STY 51 0.8 14.7 30 0.4 18.3 SupTY 69 1.0 19.8 49 0.7 29.9
Table 5. Frequency distribution of landfalling TCs at different intensities in summer and autumn over the WNP during 1949–2016
The annual number of autumn landfalling TCs in each intensity category during 1949–2016 is shown in Fig. 10. On average, autumn landfalling TCs with the largest numbers are SupTYs (29.9%), followed by TYs (24.4%). Since the early 2000s, the number of autumn landfalling SupTYs has increased significantly. For example, there were four autumn landfalling SupTYs in 2016 and two autumn landfalling SupTYs in 2015, exceeding the climatological average (0.7). Overall, there is no significant annual linear trend of autumn landfalling TCs with different intensities over the 68-yr period (figure omitted).
Figure 11 shows the frequency distribution of the genesis location of autumn TCs making landfall in China during 1949–2016. Most of the autumn TCs that made landfall in China were generated over the ocean west of 150°E, with high value centers over the eastern South China Sea and at around 130°E. Compared with the genesis distribution of all autumn TCs (Fig. 7), the genesis locations of landfalling autumn TCs are farther westward, with autumn TCs formed over the ocean close to 170°E rarely making landfall in China. The genesis characteristics of SupTYs making landfall in autumn are similar to those of lower intensity landfalling TCs. SupTY genesis hardly ever occurred in the South China Sea, and most of the SupTYs formed in October and November were generated south of 15°N.
Figure 11. Spatial distribution of autumn landfalling TC genesis frequency (shading) and the genesis locations of SupTYs making landfall in China during 1949–2016 (markers). Black circles denote the SupTYs in September; blue crosses denote those in October; and red squares denote those in November.
In autumn, as the subtropical high retreats eastward and southward, the genesis locations and tracks of TCs are farther southward than in any other seasons. The southward moving cold air in autumn is more active, which prevents the subtropical high from moving northward and further makes it harder for the autumn TCs to move northward (Fig. 4). Therefore, the TCs mostly make landfall in the southern coastlands of China (Fig. 12). Autumn TCs made landfall in Guangdong Province most frequently; from 1949 to 2016, there were 68 autumn TCs in total making landfalling in Guangdong, equating to an average of 1.0 landfalling autumn TC per year and accounting for almost 1/3 of all landfalling TCs. The second most frequent landfalling location was Hai-nan Province with 51 autumn TCs making landfall over the period, giving a frequency of 0.8 per year and accounting for almost 1/4 of all landfalling TCs. Taiwan ranks the third, with an average of 0.6 autumn landfalling TCs per year. Fujian Province, which is affected by Taiwan, ranks the fourth with an average of 0.4 autumn landfalling TCs per year. During 1949–2016, the total number of autumn landfalling TCs in Zhejiang, Guangxi, Shanghai, and Hong Kong was 10 or less in each province, while other northern coastal provinces had no record of autumn landfalling TCs. In terms of intensity, the strongest typhoons that made landfall in Guangdong, Hainan, Fujian, or Hong Kong all occurred in September.
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