The relationship of frequent tropical cyclone activities over the western North Pacific and hot summer days in central-eastern China
Tropical cyclones (TCs) formed over the western North Pacific (WNP) often make landfalls over East Asia and Southeast Asia, causing strong winds and torrential rain in the coastal countries. However, they also bring cool weather on hot summer days and mitigate drought impacts. The present study demonstrates that TC activities over the WNP can strongly modulate extreme summer weather events in eastern mainland China, i.e., frequent TC activities would indirectly lead to more hot days in central-eastern China along the lower and middle reaches of Yangtze River, besides compensating the decreasing of hot days induced by the direct impact of TCs. Such indirect impact is largely determined by the feedback effect of TCs on pressure system, especially on the western Pacific subtropical high, resulting in an abnormal anticyclonic circulation band that dominates a large area from central-eastern China to the Pacific Ocean to the east of Japan in the middle and lower troposphere.
Tropical cyclones (TCs) over the western North Pacific (WNP) are among the most devastating weather events that affect East Asia and Southeast Asia. Landfalling TCs have substantial socio-economic impacts. Therefore, the TC occurrence frequency and TC tracks are a topic of profound societal significance and intense scientific interest (Mendelsohn et al. 2012).
Heat waves that occurred in recent years on both regional and global scales have attracted great attention (Easterling et al. 2000; Grumm 2011) since extreme high-temperature events are meteorological disasters. In the context of global warming, the occurrence frequency of extremely hot weather and heat waves demonstrates a rising trend (IPCC 2007; Coumou and Rahmstorf 2012; Hu et al. 2017). Previous studies indicated that high-temperature events often occur under the joint effect of large-scale circulation and local physical processes (Diffenbaugh et al. 2005; Miralles et al. 2014; Horton et al. 2015). It is found that high-temperature events largely occur in clear days when radiative heating is strong, thereby the number of rainy days in the summer is highly negatively correlated with the number of hot days on the interannual time scale (Ding et al. 2010).
The western Pacific subtropical high (WPSH) is one of the major synoptic and climate systems that affects temperature change and extreme high-temperature events in the summer over eastern China (Liang and Wu 2015). TC activities in the WNP have significant impacts on regional weather and climate systems like the WPSH over East Asia-WNP (EA-WNP) region (Nitta 1987; Zhong and Hu 2007; Sun et al. 2015; Chen et al. 2017). One good example is that TC-induced precipitation accounts for a large proportion of the total precipitation in most areas of southeastern China (Ren et al. 2002) and makes influence on the spatiotemporal variations in typhoon season rainfall in south China (Lee et al. 2010; Chen et al. 2012). In the coastal region of southeastern China, it can be more than 500 mm per year, which accounts for 20∼40% of the total annual precipitation there (Ren et al. 2006; Zhang et al. 2013). Those imply possible fewer hot days in southeastern China in the years when more TCs form over the WNP. However, whether this is true still remains an unanswered question, since it was found that the influence of TCs may affect heat waves in Southeastern Australia indirectly (Parker et al. 2013) and the persistent heat wave periods occurred in South China were related to the sinking motion of upper outflow of TC (Fang and Jian 2011). The current work aims to investigate the impact of TCs on extreme hot events in eastern China. Results presented here will help to reveal the abnormal pattern of hot days induced by TC activities and the possible mechanism.
The data and methods used in the present study are described in Section 2. Results of composite analysis and numerical experiments are presented in Section 3. Conclusions and discussion are given in Section 4.
2 Data and Methods
The TC best-track data is obtained from the Regional Specialized Meteorological Center (RSMC) of Japan Meteorological Agency (JMA) (http://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pub-eg/RSMC_HP.htm). The data includes TC name, TC position in latitude and longitude, TC center pressure, and maximum sustained winds at 6-h intervals. Daily surface air temperature, precipitation, and sunshine duration collected at 2474 weather stations in mainland China are provided by the National Meteorological Information Center of China Meteorological Administration. Atmospheric circulation regimes are derived from the NCEP/NCAR reanalysis dataset (Kalnay et al. 1996).
The 50 summers (June–August) from 1961 to 2010 are chosen for TC statistics and large-scale environment field analysis over EA-WNP in active and inactive TC years. Among the selected summers, according to the TC genesis frequency, the 10 most active TC years (AY) are determined to be 1965, 1966, 1967, 1971, 1972, 1978, 1981, 1992, 1994, and 2004, and the 10 least active TC years (IY) are identified to be 1969, 1975, 1977, 1979, 1980, 1983, 1998, 2007, 2008, and 2010.
The composite method is employed to highlight different features of sunshine duration (SSD), surface air temperature (SAT), precipitation (PRE), hot days (HDs), and regional atmospheric circulation between AY and IY. Here, a HD is defined as the day with daily maximum temperature is equal to or greater than 35 °C (Ding et al. 2010).
To verify the results of diagnostic analysis, numerical experiments for the simulation of the super typhoon Solik (2013) were conducted using the non-hydrostatic model WRFV3.4.
3.1 Composite analysis
The statistical analysis shows that there are large differences in TC genesis frequency and TC track between AY and IY. In total, there were 161 TCs for the 10 AYs while there were only 72 TCs for the 10 IYs, less than half of that in AYs. After their genesis, most TCs moved northwestward before they turned northward and affected the middle to high latitudes. In the 10 AYs, 41 TCs made landfall and 67 TCs reached north of 35° N. In contrast, only 27 TCs made landfall and 19 TCs reached north of 35° N in the 10 IYs.
Looking at the TC track density, two maximum centers can be found at the northern South China Sea and the Pacific Ocean to the east of Taiwan in AYs with the maximum values of 19 times/year and 21 times/year, respectively (Fig. 1d). In IYs, however, the largest track density was located at the Bashi Channel with the maximum value of 11 times/year (Fig. 1e). Over the entire EA-WNP region, the TC track density in AYs was always larger than that in IYs. Except for the ocean area to the east of the Bashi Channel, spatial distribution of the difference in TC track density between the active and inactive TC years was also consistent with that of the TC track density in the active TC years and the largest difference is about 9 times/year (Fig. 1f). Therefore, large differences can be found not only in the TC genesis frequency and location but also in statistical TC track density between AYs and IYs, which result in large differences in circulation and precipitation as well as summertime weather and climate features over EA-WNP.
The spatial distribution of summer PRE indicates that in both AYs and IYs, PRE in eastern mainland China gradually decreased from southeast to northwest (Fig. 2d, e), while the largest difference in PRE between AYs and IYs was still centered along the LMRYR (Fig. 2f) and PRE there in AYs was less than that in IYs. Therefore, although frequent TC activities in AYs could bring more precipitation to southeastern China during TC periods (Ren et al. 2002; Lee et al. 2010; Chen et al. 2012), the feedback of TC activities on the atmospheric circulation might lead to reduced precipitation in central-eastern China before and after the passing of TCs. In addition, precipitation decreases centered along the LMRYR in AYs were consistent with longer summertime SSD there (Fig. 2c). However, precipitation in the coastal region of southern China that was under the strongest influence of TCs in AYs was slightly more than that in IYs (Fig. 2f).
Longer SSD and less precipitation both can result in higher SAT in the summer, and thereby increase the occurrence frequency of high-temperature events (Ding et al. 2010; Soon et al. 2011; Qian et al. 2012). Figure 2 g and h exhibit clearly that summertime average SAT was higher than 26 °C over most areas to the south of the Huaihe River in eastern mainland China in both AYs and IYs. In AYs, the summertime average SAT ranged between 26–28 °C in southeastern China and higher than 28 °C in some inland provinces (Fig. 2 g). In IYs, however, the average SAT was lower than 26 °C in southeastern China but higher than 28 °C in parts of southern China (Fig. 2 h). Differences in summertime SAT between AYs and IYs suggest that the SAT in AYs was also significantly higher than that in IYs in central-eastern China along the LMRYR, and the largest difference could be more than 0.6 °C. In the coastal region of southern China, however, SAT in AYs was lower than that in IYs with the maximum difference of about − 0.4 °C (Fig. 2i).
It is expected that the average HD distribution is similar to that of SAT, since the occurrence frequency of extreme high-temperature events increases in response to the increase in SAT (Alexander et al. 2006; IPCC 2007). As shown in Fig. 2 j and k, the summertime average HD in AYs was more than 15 days in central and southeastern China, and HD longer than 30 days occurred to the south of LMRYR with the maximum of more than 35 days (Fig. 2j). In IYs, the areas with average summer HD longer than 15 days were largely found to the south of LMRYR, and the maximum HD was 32 days (Fig. 2 k). Again, the differences in summertime average HD between AYs and IYs were positive in central-eastern China along the LMRYR, with the maximum 14 days in central Anhui province. On the contrary, the negative maximum of − 3 days was found in the coastal region of southern China (Fig. 2 l).
In brief, frequent TC activities over the WNP in the summer could lead to increases in SSD as well as SAT and decreases in precipitation over central-eastern China along LMRYR, which prolonged HD there and gave rise to more high-temperature events, while SAT and HD both decreased in AYs only in the coastal region of southern China.
It is worth noting that, similar to the situation that the geopotential height differences between AYs and IYs are consistent in the middle and lower troposphere (Fig. 3c, f), the differences in the summer circulation between TC occurrence periods and TC-free periods at 850 hPa (Figure omitted) also are similar to that at 500 hPa (Fig. 5c). This indicates that the impacts of TC activities on regional circulation show a quasi-barotropic feature in the middle and lower troposphere. Therefore, the frequent TC activities indeed have feedback effects on large-scale pressure system. In response to such kind of feedback, the WPSH shifts northward, leading to the formation of abnormal divergence and anticyclonic circulation band over central-eastern China along the LMRYR, which is favorable for the maintenance of clear sky there and leading to net increasing in HD, besides compensating the decreasing in HD induced by the direct impact of TCs. In addition, the HDs in South Korea and southern Japan would also have the same experiences as in central-eastern China, since those areas are in the same abnormal anticyclonic band induced by frequent TC activities in summer.
3.2 Numerical experiment
Two experiments for the simulation of Typhoon Solik (2013) that covered its lifetime from 0000 UTC 8 July to 1200 UTC 14 July 2013 were conducted with the WRF model. For the control run (hereafter CR) of Solik simulation, the center of the model domain is located at (37° N, 132° E) with a grid spacing of 20 km and 435 × 335 horizontal grid points. The top of the model is set to 50 hPa with 35 vertical levels. The initial and lateral boundary conditions are extracted from the NCEP/NCAR reanalysis data on global 1° × 1° grids ( https://doi.org/10.5065/D6M043C6) at 6-h intervals. Important model physical schemes include the WSM 5-class microphysics scheme, the Kain-Fritsch (New Eta) cumulus parameterization scheme, the RRTM long-wave radiation scheme, the (old) Goddard short-wave radiation scheme, the YSU planetary boundary scheme, and the Unified Noah land surface scheme, and the Monin-Obukhov similarity theory is applied to describe surface layer physics. Detailed descriptions of the WRF dynamics and physical schemes can be found in Skamarock et al. (2008).
The sensitivity run (hereafter SR) was conducted by removing the vortex of TC Solik from the first-guess large-scale field at the initial time, which is the first step of the so-called bogus technique (Christopher and Simon 2001), which is usually applied to investigate interactions between the TC activity and surrounding large-scale circulation (Zhong and Hu 2007; Tang et al. 2013). Except for the TC vortex removal at initial time of the simulation, all other options in the SR are identical to those in CR.
4 Conclusions and discussion
In the present study, composite analysis has been conducted to explore the large-scale circulation differences in the summers between TC active years and TC inactive years over the WNP. It is found that TC activities over the WNP can strongly influence extreme summer climate in eastern mainland China, and frequent TC activities would indirectly lead to more hot days in central-eastern China centered along the lower and middle reaches of the Yangtze River, and make up for the decrease in hot days induced by the direct impact of TCs. Moreover, this study reveals that the feedback effect of TCs on the WPSH plays an important role in the formation of an abnormal anticyclonic circulation band that extends from central-eastern China to Pacific Ocean to the east of Japan in the middle and lower troposphere, which is favorable for more hot days in central-eastern China.
Generally, TCs bring huge amounts of heat and water vapor to the middle and high latitudes as they move northwestward, and thus transport and disperse energy to extratropical areas. TC activities are an important factor that affects the transportation, distribution, and budget of global energy (Korty et al. 2008; Pasquero and Emanuel 2008; Jansen and Ferrari 2009; Ha et al. 2013). Moreover, it has been found that TCs have great feedbacks not only on the atmosphere and ocean but also on regional/global climate and climate variability (Sobel and Camargo 2005; Zhong and Hu 2007; Hsu et al. 2008; Sriver 2013). The direct impact of TCs, no matter whether they make landfalls or sweep over the coastal region of eastern China, would bring abundant precipitation and lower temperature in southeastern China. However, the present study emphasizes the “indirect” TC impact on precipitation and hot days by exploring TC feedbacks on the WPSH in the summer, as the influence of TCs on heat waves in Southeastern Australia (Parker et al. 2013).
In addition, it is well known that the meridional displacement of the East Asian circulation in the summer is closely related to the evolution of the Pacific-Japan (PJ) teleconnection pattern on interannual timescale (Lu 2004; Lu and Lin 2009; Zhong et al. 2015), which is attributed to the northward propagation of Rossby waves triggered by anomalous convective activities over the tropical WNP (Nitta 1987; Huang and Sun 1992). Furthermore, it is also found that stationary Rossby waves induced by typhoons over the WNP could stimulate the PJ teleconnection pattern (Kawamura and Ogasawara 2006) and vice versa (Choi et al. 2010; Kubota et al. 2016). This explains why strong abnormal meridional circulation develops and displays wave train characteristics in AYs, accompanied by northward propagation of Rossby waves. Therefore, the abnormal meridional circulation can influence the atmospheric circulation in the mid-latitudes (Yamada and Kawamura 2007; Kosaka and Nakamura 2010). In particular, strong convections including the contribution of TCs in the tropical WNP will lead to abnormal descending flows along the latitudes at southern Japan, which will intensify the pressure system there and cause a poleward shift of the WPSH as pointed out by Wang et al. (2019). In other words, while TC activities can directly increase positive vorticity and intensify ascending motions in the middle and lower troposphere along their tracks, frequent TC activities can indirectly increase negative vorticity and intensify descending motions in the mid-latitudes from central-eastern China to the Pacific Ocean to the east of Japan. Such indirect TC impacts can partly make up for the decrease in geopotential height in that region, resulting in a relatively small decrease in geopotential height and an abnormal anticyclonic band in the middle and lower troposphere (Fig. 2c, f).
This work is financially supported by the National Natural Science Foundation of China (41430426, 41605072, 41505058) and the R&D Special Fund for Public Welfare Industry (Meteorology) (GYHY201306025).
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