J. Meteor. Res.  2019, Vol. 33 Issue (4): 609-626 PDF
http://dx.doi.org/10.1007/s13351-019-9009-x
The Chinese Meteorological Society
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#### Article Information

CHEN, Lijuan, Zhensong GONG, Jie WU, et al., 2019.
Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Niña and Intraseasonal Oscillation. 2019.
J. Meteor. Res., 33(4): 609-626
http://dx.doi.org/10.1007/s13351-019-9009-x

### Article History

in final form August 5, 2019
Extremely Active Tropical Cyclone Activities over the Western North Pacific and South China Sea in Summer 2018: Joint Effects of Decaying La Niña and Intraseasonal Oscillation
Lijuan CHEN1,2, Zhensong GONG1, Jie WU1, Weijing LI1,2
1. Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing 100081;
2. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044
ABSTRACT: In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China, ranking respectively the second and the first highest since 1951. Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of decaying La Niña and intraseasonal oscillation (ISO) on the extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Niña episode from October 2017 to March 2018 led to above-normal sea surface temperature (SST) over central–western Pacific, lower sea level pressure and 500-hPa geopotential height over WNP, and abnormally strong convective activities over the western Pacific in summer 2018. These preceding oceanic thermal conditions and their effects on circulation anomalies are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more eastward WNP subtropical high as well as the significant wave train with a " − + − +” height anomaly pattern over the midlatitude Eurasia–North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successively active periods of Madden–Julian Oscillation (MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation (BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks. The MJO was stagnant over the Maritime Continent to western Pacific, leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Niña, and the MJO and BSISO; their joint effects result in increased TC occurrences and the TC tracks being shifted more northwest to northward than normal.
Key words: western North Pacific (WNP)     tropical cyclone (TC)     La Niña event     Madden–Julian Oscillation (MJO)     boreal summer intraseasonal oscillation (BSISO)
1 Introduction

The western North Pacific (WNP) is one of the regions with most frequent tropical cyclone (TC) activities. On average, about 11 TCs form in this region each summer. Tropical cyclones are often accompanied by strong winds, heavy rains, and storm surges, leading to huge economic losses and casualties. China, Japan, and the other Southeast Asian countries are greatly affected by TCs. Therefore, study on the formation and activity pattern of TCs, especially the TC occurrence frequency, moving path, intensity, and level of influence, is important for the prevention and mitigation of TC-induced disasters.

Meteorologists have long been concerned of weather and climate conditions related to TCs over the WNP (Chen, 1965; Sadler, 1978; Chen and Ding, 1979; Ding and Reiter, 1981a, b; Wang, 1981). Further studies indicate that the TC activity in the WNP has significant intraseasonal, interannual, and interdecadal variabilities (Chen et al., 1998; Chan, 2005; Emanuel, 2005; Webster et al., 2005).

TC activity is closely associated with El Niño–Southern Oscillation (ENSO). ENSO affects TC activity through its impacts on the thermal and dynamic conditions over the WNP. Earlier studies suggested that in El Niño years, sea surface temperature (SST) over the WNP is lower than normal, sea level pressure (SLP) is higher than normal, and convection is weaker than normal; as a result, fewer TCs can form in WNP and the TC genesis positions are located to the east of their normal positions. In La Niña years, the situations are reversed (Chan, 1985; Wu and Lau, 1992; Lander, 1994). Further in-depth studies found that during different stages of the ENSO cycle, various combinations of the ENSO intensity, spatial pattern, and SST anomaly in the Indian Ocean would affect the TC genesis, TC track, TC intensity, and TC landfall in different seasons (Li C. Y., 1987; Chen et al., 1998, 2006; He et al., 1999; Chan, 2000; Wang and Chan, 2002; Kim et al., 2011; Zhan et al., 2011; Li T., 2012; Li and Zhou, 2012; Yu et al., 2016b; Xie at al., 2018). Note that results of various studies are different due to differences in data, definition, and research routines among these studies. Wang and Chan (2002) proposed that in El Niño developing years, most TCs form over the southeast quadrant of the WNP with relatively strong intensity; in La Niña developing years, TCs form largely over the northwest quadrant of the WNP with relatively weak intensity. From July to September, mean TC genesis position in strong El Niño years is further south than that in La Niña years. Xie et al. (2018) found that in the years of El Niño decaying, most of the TCs are originated in the South China Sea and move westward; in the years of La Niña decaying, TCs mostly originate over the WNP near the coastal water of China (including the Taiwan Island), and they often follow a recurvature path along the coast.

In addition, some studies suggested that the Antarctic Oscillation (AAO), Arctic sea ice, snow cover in the Tibetan Plateau, North Pacific Oscillation (NPO), SST over North Atlantic, etc., all have impacts on the interannual variability of TC activity (Wang and Fan, 2006; Fan, 2007; Wang et al., 2007; Yu et al., 2016a; Zhan et al., 2016).

The Madden–Julian Oscillation (MJO) is the most significant mode of tropical atmospheric circulation (Madden and Julian, 1971), which is characterized by eastward propagation of large-scale deep convection anomalies in the tropics. These convection anomalies initiate from the Indian Ocean, pass through Indonesia and the nearby regions, enter the western Pacific, and eventually disappear near the international dateline. TC activities and TC tracks are affected obviously by the MJO (Hu and Wang, 1992; Sobel and Maloney, 2000; Zhu et al., 2004; Huang and Chen, 2007; Kim et al., 2008; Chen and Huang, 2009; Liu G. et al., 2009; Sun et al., 2009; Pan et al., 2010; Tian et al., 2010a, b; Huang et al, 2011; Li and Zhou, 2013a, b; Liu Q. et al., 2018). Most studies conclude that the MJO would affect the atmospheric temperature, water vapor content, and sea level pressure during its eastward propagation. When the MJO is in its active phase, convection is significantly enhanced, a cyclonic vortex develops to the north of the equator at lower levels while an anticyclonic vortex develops in the upper troposphere, vertical wind shear decreases, and upper-level divergence intensifies. Such a large-scale circulation background favors the generation and development of TCs. In the westerly wind phase of the MJO, zonal convergence and meridional shear of zonal winds lead to energy transfer from low-frequency kinetic energy to high frequency kinetic energy, which subsequently intensifies high-frequency motions. As a result, more TCs are generated in this region. On the contrary, when the MJO is in the easterly wind phase over the western WNP, TC genesis is suppressed. In addition, different phases of the MJO and the quasi-biweekly oscillation (QBWO) over the WNP also affect the TC genesis, intensity, track, and landing. TCs are also under the influences of intraseasonal oscillation in the subtropics and circulation patterns over the mid–high latitude regions.

The above discussion indicates that TC activity is influenced by multiple factors. The TC genesis, track, landing, and TC-induced disasters are quite different in different years (Liu et al., 2007; Gong and Chen, 2013). TC activity can be significantly active or inactive in the summer and autumn seasons. In the summer of 2018, a total of 18 TCs formed in the WNP and South China Sea (SCS) and 8 TCs landed in China, both are much more than normal. The TC genesis frequency over the WNP and SCS in 2018 ranked the second highest since 1951, and the number of landfall TCs in China in 2018 ranked the highest since 1951. Apparently, TC activity is extremely active in summer 2018. Meanwhile, TC tracks are also complex in this season. Climatologically, the annual mean number of TCs that land along the coastal region of Zhejiang–Shanghai–Jiangsu is only 1, whereas this number rises to 4 in 2018, among which 3 landed in Shanghai, accounting for half of the total TCs that landed in Shanghai since 1951 (including those that landed for the second time in Shanghai). Typhoons Ampil (1810), Yagi (1814), and Rumbia (1818) travelled across central–northern parts of East China and entered inland China, affecting large areas and bringing in extremely heavy precipitation along their moving paths. In particular, Typhoon Ampil went all the way north to the southeastern Inner Mongolia. It was maintained for up to 62 h over land area, which seldom occurred in history. In addition, five TCs directly landed in or influenced Japan and two TCs landed in the Korean Peninsula, incurring severe disasters there. Among the five TCs, after landing in Honshu Island, Japan, Jongdari (1812) turned west and landed in Shanghai, China. In summer 2018, TCs were crowded together, mutually restrained, and twisted, turned, or zigzagged over offshore regions, showing com-plicated tracks. However, among the eight TCs that landed in China, only one reached the strong typhoon intensity [Maria (1808)] and the other seven TCs remained as tropical storms or strong tropical storms. The average wind speed of the eight landfalling TCs is 26 m s–1 (category 10), weaker than the climatological mean value of 32.8 m s–1 (category 12). Although the TCs that landed in China in 2018 were weak, the TC-induced precipitation was strong, and heavy rainfall occurred over large areas such as East China, eastern North China, southern Northeast China, and South China.

In this paper, the mechanisms for the extremely active TC season of summer 2018 and the complicated TC tracks are analyzed from perspectives of ocean thermal condition, large-scale circulation, and intraseasonal oscillation. Results of the present study are expected to reveal and provide precursor signals for TC prediction and improve the capability for prevention and mitigation of TC-induced disasters.

2 Data and method

Meteorological elements such as geopotential height, horizontal winds, vertical velocity, and so on are extracted from the NCEP/NCAR monthly reanalysis data (Kalnay et al., 1996; Kistler et al., 2001). The data cover the period of 1948–2018 with a horizontal resolution of 2.5° × 2.5°. The NOAA 1.0° × 1.0° Optimum Interpolation Sea Surface Temperature dataset (OISST-V2) is used (Reynolds et al., 2002). Daily atmospheric analysis data include horizontal winds at 850 and 200 hPa are extracted from the Global/Regional Assimilation and Prediction System (GRAPES) daily analysis data with a resolution of 25 km × 25 km, together with daily outgoing longwave radiation (OLR) on a resolution of 0.01° × 0.01°. All data are interpolated to 2.5° × 2.5°grids. The Niño3.4 SST index (5°N–5°S, 120°–170°W) is produced by the U.S. Climate Prediction Center (CPC) based on ERSST v5 (Huang et al., 2017) and can be downloaded from http://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php.

The historical information on TC occurrence frequency and genesis position is derived from the best track dataset provided by Shanghai Typhoon Institute of China Meteorological Administration (CMA) (Ying et al., 2014). The data can be downloaded from http://tcdata.typhoon.org.cn/zjljsjj_zlhq.html. In this dataset, TCs are defined as those with average wind speed of 17 m s–1 (category 8) or above that occur especially in the WNP and SCS, including tropical storms, strong tropical storms, typhoons, strong typhoons, and super typhoons. This best track dataset also provides the position and intensity of all TCs over the WNP (including the SCS, the Pacific Ocean to the north of the equator and to the east of 180°) since 1949 at 6-h intervals. Unless specifically stated, in this paper, the climatological mean refers to the 30-yr average over 1981–2010.

The RMM (Realtime Multi-variate MJO) index, which consists of zonal winds at 850 (U850) and 200 hPa (U200) and OLR, is used to monitor the MJO (Wheeler and Hendon, 2004). The RMM index can well represent the coupling structure of MJO between large-scale convection and circulation anomalies. The two components of the RMM index, named as RMM1 and RMM2, are used to produce a two-dimensional phase space diagram that can directly reflect the position and propagation characteristics of MJO convection in real time. This type of two-dimensional phase space diagram has been widely applied to the operational monitoring and scientific studies of MJO. To calculate the RMM index, climatic seasonal cycles (0–3 waves) and interannual variabilities (average of the previous 120 days) of U200, U850, and OLR are firstly removed from their daily values, and the meridional averages are then calculated over the tropics (15°S–15°N) and normalized by dividing their respective standard deviations. Finally, the results are projected to the first two EOF patterns to obtain the pair of RMM monitoring index. In order to ensure the consistency of results, the standard deviations and spatial modes used for normalization and projection are downloaded from http://poama.bom.gov.au/project/maproom/RMM/. In addition, the two pairs of indices defined by Lee et al. (2013), which consist of U850 and OLR and can be calculated by the similar method for RMM index and projected onto four EOF patterns, are used as monitoring index for the boreal summer intraseasonal oscillation (BSISO) (refer to http://iprc.soest.hawaii.edu/users/jylee/bsiso/).

3 Characteristics of TC activity over the WNP in summer 2018

The whole year of 2018 witnessed 29 TCs formed over the WNP and SCS, 3 more than the climatological average (26); 10 TCs landed in China, 3 more than the normal (7). The TC activity was active in summer and inactive in autumn (Fig. 1) with 18 TCs formed in summer, significantly more than the climatological mean (11). In June, July, and August, TC genesis frequency is 235%, 135%, and 156% that of the respective climatological monthly average. Eight TCs landed in China in summer 2018, more than the climatological mean (4.6). The number of TCs that landed in June, July, and August is 158%, 150%, and 206% that of the climatological mean in each corresponding month. In autumn (September–November) 2018, only 8 TCs formed, much fewer than the climatological mean (11). Among the eight TCs formed in autumn, two landed in China, similar to the climatological mean number. Statistics of TCs forming in the WNP and SCS and landing in China in summer during 1951–2018 (Fig. 2) suggests that the TC genesis frequency in summer 2018 is only smaller than that in summer 1994 (19), ranking the second highest together with that in 1967 and 1974. The number of TCs that landed in China in summer 2018 ranks the first with that in 1994, exceeding two standard deviations of its climatic value. The above results indicate that the TC activity is extremely active in summer 2018 and this abnormality could be found in every month of the summer. It is worth exploring the reasons for the abnormal large number of TCs that formed over the WNP and SCS and landed in China in summer 2018.

 Figure 1 Comparison of (a) the number of TCs that formed over the WNP and SCS (0°–30°N, 100°E–180°) and (b) the number of TCs that landed in China, in 2018 (column in light gray) and their climatological values (column in black).
 Figure 2 Temporal evolutions of the number of TCs generated over the WNP and SCS (solid line) and landfall TCs in China (dashed line) in summer during 1951–2018. Black and gray solid lines denote the climatological mean values, and dotted lines indicate one standard deviation, for total TCs and landfall TCs, respectively.

Based on the regions of TC genesis, we chose three regions: the SCS (west of 120°E), WNP1 (120°–145°E), and WNP2 (145°E–international dateline) to perform further analysis. TC genesis frequencies over the above three regions are listed in Table 1. It is shown that over the entire year, the TC genesis frequencies in 2018 are slightly higher than the climatological values in all the three regions. But in summer, only the TC genesis frequencies in WNP1 and WNP2 are significantly higher than their climatological values by 50% and 120%, respectively. This indicates that abnormally more TCs formed in the middle–eastern WNP in summer 2018.

Table 1 Numbers of TC genesis in three regions in 2018
 SCS (West of 120°E) 120°–145°E (WNP1) 145°E–180° (WNP2) Summer 2018 3.0 9.0 6.0 Summer (climate) 2.3 6.3 2.7 2018 total 6.0 14.0 9.0 Annual total (climate) 4.9 13.4 7.5

According to the TC track classification proposed by Tian et al. (2010b), TC tracks comprise westward-moving track, northwestward-moving track, track to the west of Japan, track for TCs landing in Japan, and track to the east of Japan. The above five types of TC track account for 27.5%, 20.1%, 20.1%, 22.8%, and 9.4% of the total TC tracks in summer. If we take the last three types of TC tracks as northward-moving recurving type, they account for 52.3% of the total TC tacks. For summer 2018, the number of TCs that follow the above five types of tracks is 2, 4, 2, 5, and 4 [Ewiniar (1804) is not counted since it formed in the SCS and landed in South China], accounting for 11.1%, 22.2%, 11.1%, 27.8%, and 22.2% of the total, respectively; and the last three types of TC tracks contribute 61.1% of the total number. That is, more TCs has followed the northwestward-moving track and the northward-moving recurving track (the large contribution tracks are those for TCs landing in Japan and the track to the east of Japan), while fewer TCs followed the westward-moving track (Fig. 3). The four TCs (1808, 1810, 1814, and 1818) that moved northwestward landed in Fujian, Zhejiang, and Shanghai, respectively (Table 2). The TC activity occurred for 5, 23, and 19 days in June, July, and August, respectively. In particular, the number of days with TC activity during July–August accounts for 68% of the total days. The four TCs that followed the northwestward track were active during 4–11 July, 18–25 July, 8–14 August, and 15–21 August. The five TCs that landed in Japan were active during 25–29 July, 3–10 August, 11–15 August, 18–24 August, and 28 August–5 September. Therefore, July–August is the period when TC impacts are concentrated in eastern China and Japan.

 Figure 3 Tracks of the 18 TCs formed in the WNP and SCS in summer 2018.
Table 2 Features of TCs that landed in China in summer 2018
 TC number Time of genesis Time of landing Position of landing Time of extinction 1804 5 June 6–7 June Xuwen of Guangdong, Haikou of Hainan, Yangjiang of Guangdong 9 June 1808 4 July 11 July Lianjiang of Fujian 11 July 1809 17 July 17 July Wanning of Hainan; Vietnam 19 July 1810 18 July 22 July Chongming of Shanghai 25 July 1812 25 July 3 August Jinshan of Shanghai; Onshu of Japan 3 August 1814 8 August 12 August Wenling of Zhejiang 14 August 1816 12 August 15 August Leizhou of Guangdong 17 August 1818 15 August 17 August Pudong of Shanghai 21 August
4 Large-scale circulation features favorable for summertime TC activities

As mentioned previously, TC activity is extremely active in summer 2018. Based on historical records shown in Fig. 2, an active (inactive) TC year is defined if the normalized TC genesis frequency in the summer of that year is one positive (negative) standard deviation larger (smaller) than its climatological value. Only the years after 1981 are considered because of the decadal variability of the atmospheric circulation (Li et al., 2016). Active TC years are identified to be 1981, 1989, 1992, 1994, 2002, 2004, and 2017; inactive TC years are 1983, 1998, 2007, 2008, 2010, and 2014. If the normalized TC gene-sis frequency in a specific year is within one standard deviation, that year is defined as a normal TC activity year. The composite large-scale circulation is obtained for active, inactive, and normal TC activity years and compared with that in 2018.

In normal TC activity years, the eastern part of the monsoon trough in the WNP is located around 140°E, the western Pacific subtropical high (WPSH) extends westward to near 130°E (Fig. 4a) with its ridge line over 25°–27°N, and a weak cyclonic circulation anomaly develops near the Philippines (Fig. 5a). In active TC years, the eastern part of the monsoon trough extends eastward till 150°E (Fig. 4c), and the western most point of WPSH withdraws to 140°E (to the east of its normal position) with a more northward ridge line at 27°–29°N. Westerly winds prevail in the low latitude of the WNP, low-level convergence is significantly stronger than normal over the warm pool region of the western Pacific, while a large-scale cyclonic circulation anomaly develops to the east of the Philippines, corresponding to large-scale divergence at the upper levels (Fig. 5c). The low-level convergence and high-level divergence are significant at the 0.1 level, resulting in a circulation pattern favorable for the formation and development of TC (Wang et al., 2006). In inactive TC years, the eastern part of the monsoon trough only reaches near 120°E (Fig. 4d) in the South China Sea with its position shifted westward, and easterly winds that are statistically significant at the 0.1 level prevail over the low-latitude area of the western Pacific. The WPSH extends further westward to 120°E, and its ridge line is retreated southward to near 25°N. An anticyclonic circulation anomaly prevails over the Pacific Ocean to the east of the Philippines. Convection in the lower troposphere is suppressed, and divergence at the upper levels is weak (Fig. 5d). Such a circulation composite passes the significance test in most areas, and is unfavorable for the formation of TC.

 Figure 4 Composites of 850-hPa divergence (contours; unit: 10−5 s−1; areas shaded from light to dark indicate values significant at 0.1, 0.05, and 0.01 levels), 850-hPa winds (vectors; green arrows are for values at/above the 0.1 significance level), and 5880-gpm contour (thin red line) in summer over the WNP for (a) normal TC activity years, (b) 2018, (c) active TC activity years, and (d) inactive TC activity years. Note that the black line denotes the climatological 5880-gpm contour, which coincides with the 5880-gpm contour for normal TC activity years in (a); and the thick red line indicates the monsoon trough.
 Figure 5 Composites of 200-hPa divergence anomaly (contours; unit: 10−5 s−1; areas shaded from light to dark indicate values significant at 0.1, 0.05, and 0.01 levels) and 850-hPa wind anomaly (vectors; green arrows indicate values significant at/above the 0.1 level) for (a) normal TC activity years, (b) 2018, (c) active TC activity years, and (d) inactive TC activity years.

In summer 2018, the eastern part of the monsoon trough reached 150°E (Fig. 4b), the WPSH extended westward and reached 140°E, and both were located to the east of their climatological positions. The WPSH ridge line was located at 29°–30°N, north of its climatological position. Westerly winds prevailed in the low-latitude area of the western Pacific, while cyclonic circulation occupied the area east of the Philippines. Convection was active in the lower troposphere, corresponding to large areas of abnormally strong divergence in the upper troposphere over the western Pacific between 5° and 35°N (Fig. 5b). This circulation pattern basically agrees well with the circulation characteristics in active TC years (Fig. 5c). Particularly, the divergence developed near the northern Philippines was statistically significant. In addition, the WPSH ridge line was located further north than the composite location for active TC years. In other words, the monsoon trough over the WNP in summer 2018 was stronger than normal and extended eastward, resulting in eastward shift of the TC genesis position; while the background circulation with convergence at low levels and divergence at upper levels was favorable for TC genesis. The above discussion explains why more TCs formed in summer 2018 and why the TC genesis position was located to the east of its climatological position as shown in Table 1. Next, the SST field is further analyzed to explore the oceanic thermal background for formation of such large-scale circulation features.

5 Characteristics of TC activity over the WNP in La Niña decaying years

Tropical SST anomaly (SSTA) has pronounced impacts on atmospheric circulation in the WNP. Based on the monitoring by the National Climate Center of China, a weak eastern-type La Niña event occurred over central–eastern Pacific from October 2017 to March 2018 (Wang et al., 2018). The SSTA over the Indian Ocean gradually became weakly negative since December 2017, demonstrating a lagged response to the La Niña event. Starting from April 2018, negative SSTA over the equatorial central Pacific weakened rapidly. The Niño3.4 SST index issued by NOAA CPC was 0.1 in summer 2018, close to its normal value. Since 1981, there are 9 yr with the Niño3.4 SST index ≤ 0.2, among which 2001, 2017, and 2018 experience more TCs in summer, while 1995, 2003, and 2014 have fewer than normal TCs, and 1986, 2005, and 2006 are normal TC years. This indicates that the summer TC activity in WNP bears no linkage to Niño3.4 SSTA. However, in summer 2018, which is a La Niña decaying year, significant SSTA occurred in tropical western–central Pacific, which may induce atmospheric circulation anomalies and lead to anomalous TC activity in WNP.

Based on the ENSO identification criteria proposed by Ren et al. (2018), decadal variability is removed and 9 La Niña events after 1981 are identified in the present study, i.e., October 1984–June 1985, May 1988–May 1989, September 1995–March 1996, July 1998–June 2000, October 2000–February 2001, August 2007–May 2008, June 2010–May 2011, August 2011–March 2012, and October 2017–March 2018. All the peaks of the above events occurred in the winter months of December and January except that during September 1995–March 1996, whose peak appeared in November. Therefore, 1985, 1989, 1996, 2000, 2001, 2008, 2011, 2012, and 2018 are identified as La Niña decaying years in the present study. Among the above 9 years, TC genesis frequency was smaller than normal in 2008 and 2011, equivalent to the multi-year average value in 1996 and 2000, and larger than normal in the remaining 5 years (Fig. 2). Anomalies of SST, sea level pressure (SLP), 500-hPa geopotential height, and vertical velocity (averaged over 5°–25°N) in the first 8 La Niña decaying years are compared with those in 2018, and the results are displayed in Fig. 6.

 Figure 6 Composites of (a, b) SSTA, (c, d) SLP anomaly, (e, f) 500-hPa geopotential height anomaly, (g, h) vertical and zonal (averaged over 5°–25°N) wind anomaly during June–August, and (i, j) vertically (850–300 hPa) averaged horizontal winds (vectors) during July–August, for (a, c, e, g, i) La Niña decaying years and (b, d, f, h, j) the summer of 2018. Areas marked by black dots in (a, c, e), yellow shadings in (g), and red arrows in (i) indicate values significant at/above the 0.1 level. Blue lines in (e, f) denote the climatological (1981–2010 mean) 5880-gpm contour; and black dashed lines denote that for (e) La Niña decaying years and (f) summer 2018.

In the summers of La Niña decaying years, SSTA over the equatorial Pacific overall demonstrated a west warm–east cold pattern, SSTA was negative to the east of the international dateline and positive to the west of the dateline. However, the area with SSTA significant at the 0.1 level was not large and mainly located to the west of the international dateline near 150°E; in addition, SSTA was negative over the northern India Ocean, while the SSTA showed a “− + −” pattern from the tropical Atlantic to the north. Note that such an SSTA pattern failed to pass the significance test (Fig. 6a). Looking at the spatial pattern of SSTA in summer 2018, it is clear that although the Niño3.4 SST index was close to normal in the entire summer, significant positive SSTA prevailed from the area near the international dateline to its west at around 150°E with the SSTA above 0.6°C over the central warm area (Fig. 6b), which is consistent with the area of SSTA that passed the significance test in the summers of La Niña decaying years. This result suggests that the SSTA over the western Pacific in summer has more direct impacts on TC activity. In addition, the obvious negative SSTA in the northern Indian Ocean and the significant “− + −” SSTA pattern over North Atlantic are also favorable for TC activity (Yu et al., 2016a, b).

The sea level pressure is another environmental variable that reflects the degree of tropical TC activity (Li et al., 2012). The spatial pattern of SLP anomaly in the summers of La Niña decaying years shows that SLP is lower than normal from the SCS to the east of the Philippines and central Pacific, and the central area of negative SLP anomaly is located to the east of the Taiwan Island at around 140°E, where the SLP anomaly passes the significance test. Positive SLP anomaly appears in the mid- and high-latitude regions of eastern Asia (Fig. 6c). In summer 2018, SLP was distinctly lower than normal from the SCS to the east of the Philippines. The negative SLP anomaly center was located within 110°–130°E from the northern SCS to the east of Taiwan, which was further west than that in the summers of La Niña decaying years. Meanwhile, the area of negative SLP anomaly extended northwestward to the Asian continent (Fig. 6d), favorable for TC activity in northwestern Pacific.

In La Niña decaying years, 500-hPa geopotential height demonstrates a “− + − +” wave pattern from Eurasia to the midlatitude region of the northern Pacific Ocean, and the positive geopotential height anomalies to the south of Lake Baikal are statistically significant. A long wave trough appears at around 150°E to the east of Japan, while the EAP teleconnection wave train develops over the WNP with a “− + −” pattern from south to north along West Pacific. However, note that this pattern is not statistically significant (Fig. 6e). Basically, in summer 2018, the 500-hPa geopotential height showed typi-cal circulation features as those in La Niña decaying years (Fig. 6f), except that the positive anomaly center in the midlatitude Eurasia was even larger and extended southeastward from Lake Baikal to Northeast China. The positive geopotential height anomaly above Lake Baikal was coupled with the long wave trough near 150°E to the east of Japan. When the long wave trough intensified, the warm, high pressure ridge over North and Northeast China moved eastward, overlapping with the WPSH. As a result, the WPSH extended more westward, steering TCs to move westward and landed in China (Wang, 1981). In summer 2018, the circulation anomaly pattern in the midlatitude Asia (Fig. 6f) was highly favorable for the TCs to move westward and make landfall in China. Meanwhile, the WPSH abnormally shifted northward in summer 2018, subsequently the TC activity was also more northward and affected the further northward areas of China (Table 2).

In the summers of La Niña decaying years, zonally averaged convection over 5°–25°N distinctly intensifies over 110°–150°E and such intensification is statistically significant at around 150°E (Fig. 6g). Characteristics of atmospheric circulation in the middle and lower troposphere over the WNP and tropical convective activities in summer 2018 were consistent with those in La Niña decaying years, and convective activities were abnormally strong, leading to active TC activities (Fig. 6h). The comparative analysis shown in Fig. 6 suggests that although SST over the tropical central–eastern Pacific in summer 2018 was close to its climatological value, SSTA to the west of the international dateline was significant, and the atmospheric circulations over the tropical and subtropical WNP and the mid to high latitude regions were consistent with those in La Niña decaying years. These circulation characteristics were typical responses to the decaying of La Niña. In particular, the “− + − +” pattern of circulation from Eurasia to the midlatitude area of northern Pacific and the “− + −” EAP wave train in East Asia provided large-scale circulation favorable for TC genesis in northern part of the WNP. The above circulation pattern was also favorable for the northwestward movement of TCs and their ensuing impacts in northern China. Comparing wind fields averaged over 850–300 hPa in July–August between La Niña decaying years and 2018 (Figs. 6i and 6j) finds that there appear clearly steering flows for TCs to move northwestward and northward in the WNP in both cases; particularly, the steering flow for TCs to move northwestward into the Yellow Sea and East Sea along coastal China in summer 2018 is distinct, which is advantageous for TCs to land in eastern China.

Further comparison of TC tracks in the summers of La Niña decaying years and 2018 (Fig. 7) shows that in the summers of La Niña decaying years, significant positive anomalies appear over zonally extending belts in northern SCS and from east of the Taiwan Island to the international dateline with the positive center located to the east of Taiwan Island and over 20°–25°N, 140°–150°E, indicating that more TCs move through this area; and significant negative anomalies appear from central–southern of SCS to low latitude area near 170°E, where fewer TCs move over (Fig. 7a). This result is consistent with the spatial distribution of TC track density in June–November during La Niña decaying years reported by Xie et al. (2018). In La Niña decaying years, the main body of the monsoon trough is located over 10°–25°N, 120°–145°E, which is more northward than that in El Niño developing/decaying years and La Niña developing years. In La Niña decaying years, the WPSH weakened and retreated eastward, with the western most ridge point located at 140°E (Fig. 6e). Under the influence of easterly winds to the northeast of the monsoon trough and the steering flow on the western flank of the WPSH, most of the TCs move northwestward and/or recurve.

 Figure 7 (a) The difference (unit: times grid−1) between the TC track frequency in La Niña decaying years and that in 1981–2010, and (b) the difference between the TC track frequency in 2018 and that in 1981–2010, averaged for June–August in the 2.5° × 2.5° grids over WNP. In (a), areas marked by black dots are for values significant at/above the 0.1 level. In (b), black contours are for 1981–2010 climatological values.

In summer 2018, the main body of the monsoon trough was located over 15°–25°N, 120°–150°E (Fig. 4b), which was further north and east than that in La Niña decaying years, and thus was more favorable for TCs to form in areas further north and east of WNP (Table 1). Among the 18 TCs in summer 2018, 11 formed within 10°–20°N, and 7 formed over 20°–30°N. Meanwhile, although the WPSH weakened to a certain degree, its western most ridge point reached near 140°E (Fig. 6f). Steered by the flow to the west of the WPSH, TCs were more likely to follow the northward track and then recurved in offshore areas.

Figure 7 displays TC tracks in La Niña decaying years and in summer 2018. The approach proposed by Xie et al. (2018) is used to calculate the TC tracks. The TC activity area is divided into 2.5 latitude × 2.5 longitude grids, and the TC occurrence frequency at each grid is calculated. The larger the TC occurrence frequency at a specific grid, the more TCs will pass through this grid. Spatial distribution of TC track density is obtained by this method. In Fig. 7b, positive anomalies of TC track density occurred over a northwest–southeast oriented belt extending from the north of Taiwan Island to near (15°N, 160°E), while negative anomalies occurred over the low latitude area from the SCS to near 170°E. This result indicates that the TC tracks in summer 2018 are overall similar to those in La Niña decaying years (Fig. 7a); however, the TC genesis positions and TC tracks are located further north, with the high value center located from the offshore region to the east of China and the ocean to the southeast of Japan. This feature is consistent with the results shown in Section 3 that more TCs in summer 2018 followed the northwestward-moving track and the northward-moving-recurving track (especially the track for TCs landing in Japan and the track for TCs to the east of Japan).

It is clear that the TC tracks in 2018 demonstrated general features in La Niña decaying years. However, the primary TC track in 2018 was distributed along the northwest–southeast direction, with more TCs moving northwestward and recurving to the north. These features were not completely similar to those in La Niña decaying years due to the fact that the main body of the monsoon trough shifted northward and eastward in 2018. As a result, the TC track density was significantly higher than normal over the offshore region to the east of China and the ocean to the southeast of Japan.

6 Impacts of tropical intraseasonal oscillation on TC activity in summer

In summer 2018, intraseasonal oscillation was active over the WNP. By using the amplitude of RMM index ( $\sqrt {{\rm (RMM}{1)^2} + {\rm (RMM}{2)^2}}$ ) greater than 1 as the criterion (Wheeler and Hendon, 2004), two consecutive MJO active periods, 8–28 July and 2–15 August 2018, can be identified (Fig. 8). The two periods correspond to the concentration periods of TC genesis and TC activity, during which they severely affected China and Japan (Table 2). The first MJO event started at Phase 4, propagated eastward and ended at Phase 6, when convection was active over the Maritime Continent (MC) and western Pacific (WP); the second event lingered at Phase 6, when convection was stably residing over the WP.

 Figure 8 (a) The RMM index phase space diagram in JJA 2018. (b) Time series of RMM1 and RMM2, and (c) amplitude of the RMM index, from January to October 2018.

Further analysis of the zonal (Figs. 9a, b) and meridional (Figs. 9c, d) propagations of U850 and OLR anomalies reveals the characteristics of circulation and convection anomalies corresponding to the MJO. Note that the contours in Fig. 9 indicate the anomalies that was reconstructed by the RMM index. Details can be found in Wu et al. (2016). As shown in Fig. 9, corresponding to the two MJO episodes, two distinct activities related to abnormal westerly winds can be found in the U850 field (Fig. 9a) in July and early August, respectively. In July, the eastward propagation of westerly wind anomaly was significant; in early August, however, the westerly wind anomaly largely remained over the area from the MC to WP, which promoted the transition of SSTA from negative to positive and was also favorable for the formation of Walker circulation anomaly. Low-level convergence and high-level divergence developed near the Philippines with intensified ascending motion (Tian et al., 2010a). The MJO was also reflected in convective activity, although the scale was slightly smaller than that in the circulation field (Fig. 9b). The relatively active MJO to the east of the Philippines increased the low-level convergence and mid-level moisture in this area, and thus provided favorable thermal and dynamic conditions for TC genesis. The position of TC genesis was further east than normal, which made the TCs prone to the northward moving track and recurvature (Zhao and Li, 2019). In addition, the time–latitude cross-section along 115°–130°E (Figs. 9c, d) indicates that intraseasonal convection and circulation anomalies in summer 2018 over the WP were characterized by northward propagation, which could partly be explained by the MJO impact. Convection propagated northward from the equator to 20°N near the Philippines, and triggered meridional “− + −” wave trains similar to the EAP teleconnection pattern (Huang and Li, 1987; Nitta, 1987; Huang, 1992). As a result, the WPSH shifted northward. The above circulation patterns are well reflected in the seasonal averaged 500-hPa geopotential height anomaly (Fig. 6f), which also explains why the WPSH ridge line in 2018 (see Fig. 4b) was located further north than that in La Niña decaying years. Due to the abnormal northward shift of the WPSH ridge line, TC became more active in the WNP area (Hu et al., 2005).

 Figure 9 (a, b) Time–longitude (averaged over 15°S –15°N) and (c, d) time–latitude (averaged over 115°–130°E) cross-sections for anomalies of (a, c) U850 (m s−1) and (b, d) OLR (W m−2) during June–August 2018. The shadings and contours represent the original and reconstructed ano-malies by using the RMM index

Wave activities of OLR in the tropics are analyzed based on zonal wavenumber–frequency spectrum decomposition following Wheeler and Kiladis (1999). Figure 10a displays characteristics of activities related to the MJO and the equatorial Kelvin waves and Rossby waves. Due to the westward dispersion of MJO energy, the dry event in late June was a precursor of the MJO convective activity in July. However, the MJO event in early August identified by the RMM index was mainly reflected in the circulation field (Fig. 9a), and the convective anomalies largely occurred to the north of the equator and were relatively weak near the equator. In addition to the slowly eastward propagating MJO, the Kelvin waves that rapidly propagated eastward and the Rossby waves that propagated westward were also active in the tropics. Figure 10b presents the time–longitude cross-section of velocity potential at 200 hPa, which can roughly reflect large-scale divergence and convergence. It is shown that the large-scale MJO signals in July and August were more significant at upper levels, and the convection-active area generally corresponded to large areas of divergence in the upper troposphere. In other words, the MJO activities in July–August further promoted convective activities in the lower troposphere and divergence in the upper troposphere over the tropical and southern subtropical regions of the WNP. The MJO effects are basically consistent with the influence of large-scale circulation in La Niña decaying years.

 Figure 10 Hovmöller diagram of (a) OLR (shaded; unit: W m−2; averaged over 10°S–10°N) and (b) 200-hPa velocity potential (shaded; unit: 106 m−2 s−1; averaged over 15°S–15°N) anomalies during June–August 2018. The MJO, Kelvin wave, and Rossby wave are shown by black, red, and blue contours in (a) respectively, and the contours in (b) represent anomalies of reconstructed RMM index.

The number of days for MJO in various phases is calculated. According to Pan et al. (2010), when the MJO is in Phases 5–7, more TCs will generate, with the highest frequency in Phase 6, followed by that in Phase 5. The correlation coefficient between the number of days when the MJO is in Phases 5–6 and TC genesis frequency is 0.46 (for 1981–2010), which is statistically significant at the 0.01 level. In summer 2018, the number of days when the MJO was in Phases 4–6 (Table 3) was much higher than the climatological value. Particularly, the number of days when the MJO was in Phases 5–6 was almost 150% that of its normal value, indicating that the longer period of MJO staying in Phases 5–6 in summer 2018 played a vital role for the extremely active TC activities.

Table 3 Number of days for various MJO phases in summer 2018
 MJO phase 1 2 3 4 5 6 7 8 2018 5 13 2 16 23 32 1 0 Climatological value 16.2 15.2 9.7 10 9.9 12.4 9.6 9.1

The boreal summer intraseasonal oscillation (BSISO), which is composed of BSISO1 and BSISO2, was also active during the summer of 2018 (Fig. 11). BSISO1 represents the northward propagating BSISO over the Asian summer monsoon region with a 30–60-day quasi-oscillating period; and BSISO2 mainly captures the northward and northwestward propagating BSISO with periods of both around 30 days and 10–20 days (Lee et al., 2013). Figure 11 shows that the BSISO1 mode is mainly active in Phases 6–8, while the BSISO2 mode is active in Phases 2–4. As shown in Fig. 12, during the period of concentrated TC genesis in early and middle August of 2018, abnormally active convection persisted from the SCS to the WNP. Note that although convective activities during this period were to a certain degree related to the RMM index, they were actually more consistent with convection and circulation corresponding to BSISO1. The convection anomalies slowly propagated northward instead of eastward (Fig. 9d), corresponding to the activities of BSISO1 mode in Phases 6–7. In the lower troposphere, large-scale cyclonic vorticity and water vapor anomalies persisted over the SCS and WNP, which was favorable for TC genesis in these areas. This result and the long-term statistics from climatological data can be mutually confirmed (Yoshida et al., 2014; You et al., 2019).

 Figure 11 Phase space diagrams of the (a) BSISO1 and (b) BSISO2 indices from July to August of 2018
 Figure 12 Anomalies of 850-hPa wind (vectors; m s−1), OLR (contours; W m−2), and precipitation (shaded; mm day−1) in the first four pentads of August 2018. The left column shows the original anomalies, the middle column is for the anomalies reconstructed from BSISO1, and the right column is the reconstruction from BSISO2.
7 Conclusions and discussion

In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China. The number of total TCs and landfall TCs ranked the first and second highest respectively since 1951. In addition, five TCs directly landed in Japan or significantly affected Japan, and two TCs landed in the Korean Peninsula. Overall, 2018 is an extremely active TC year. Although the intensity of TCs landed in China was generally weak, precipitation induced by TCs was intense with heavy rainstorms and disastrous weather on their paths. TC genesis was concentrated in summer 2018, multiple TCs were mutually restrained, and TCs twisted, turned, or zigzagged over offshore regions, showing complicated tracks. The spatial pattern of TC track density indicates that within 120°–160°E in the WNP, more TCs occurred to the north and fewer TCs occurred to the south of 20°N. That is, positive anomalies of TC track density occurred over a northwest–southeast oriented belt extending from the north of Taiwan Island to Japan and its southeast ocean, while negative anomalies occurred over the low latitude area from the SCS to near 170°E. The present study analyzes impacts of tropical SST anomaly and ISO activity on TC genesis and TC track.

From October 2017 to March 2018, an eastern-type La Niña event occurred over the central–eastern Pacific. It is a relatively weak event and the Niño3.4 SST index was already close to its normal value in summer. However, SST was higher than normal in the central western Pacific with positive anomaly of above 0.6°C in the central area of anomaly. Atmospheric circulation in tropical and subtropical regions of the WNP demonstrated characteristics as that in La Niña decaying years. SLP was abnormally low from the SCS to the east of the Philippines; correspondingly, 500-hPa geopotential height was also abnormally low. Convective activities averaged over 5°–25°N were abnormally strong within 110°–150°E, which was favorable for the intensification and eastward extension of the monsoon trough. As a result, more TCs formed and the TC genesis position shifted further north than its normal position. Meanwhile, the “− + − +” pattern of wave trains prevailed from Asia to the midlatitude region of northern Pacific. Warm high-pressure ridge dominated a large area from Lake Baikal to Northeast China. A long wave trough was located near 150°E to the east of northern Japan, which provided large-scale circulation condition for westward extension of the WPSH. This circulation pattern is also favorable for TCs to move northwestward before recurving to the north. Apparently the configuration of large-scale circulation in the tropics, subtropics, and midlatitudes was favorable for TCs to move northwestward and recurve northward.

Two consecutive MJO active periods occurred in summer 2018, which played a critical role in promoting TC activities. The number of days when the MJO was located from the MC to the WP (Phases 5–6) was almost 150% that of its normal value. Corresponding to these two MJO events, westerly wind anomalies developed, which were favorable for the formation of Walker circulation anomaly. As a result, low-level convergence, high-level divergence, and intensified ascending motion developed near the Philippines. The relatively more active MJO to the east of the Philippines in the western Pacific provided favorable dynamic condition for TC genesis, leading to much more TCs formed in the east of 120°E over WNP (Table 1). In addition, intraseasonal convection and circulation anomalies in summer 2018 over the WP were characterized by northward propagation, and the BSISO was also active. The corresponding BSISO1 mode persisted in Phase 6–7, while large-scale cyclonic vorticity and water vapor anomalies maintained from the SCS to the WNP. Such a pattern was favorable for TC genesis. Convective activities triggered the EAP teleconnection pattern, which forced the WPSH to shift northward and provided favorable large-scale conditions for TCs to move northward. In other words, the MJO and BSISO activities during July–August of 2018 promoted low-level convective activities and upper-level divergence over tropical and southern subtropical regions of the WNP. Their impacts on large-scale circulation are consistent with the impacts in La Niña decaying years.

Therefore, TC activity in summer 2018 was under the joint influences of decaying La Niña and abnormal ISO activities. The wave trains from Asia to the midlatitude region of northern Pacific provided favorable large-scale circulation conditions for TC activity in the WNP. The mechanism for the midlatitude circulation anomalies is discussed in another paper (Chen et al., 2019). The present study provides a diagnostic base for exploring the precursor signals of TC activities and establishing predictive models.

Acknowledgments. We thank the anonymous reviewers for their constructive comments that led to improvements to this paper.

References