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Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer

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This research was supported by the National Natural Science Foundation of China (Grant No. 41721004)


doi:  10.1007/s00376-021-1171-2

  • The contribution of tropical cyclones (TCs) to the East Asia–Pacific (EAP) teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to 2018. The results showed that the TCs over the western North Pacific (WNP) correspond to a strengthened EAP pattern: During the summers of strong convection over the tropical WNP, TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere, an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere, and a more northward shift of the East Asian westerly jet in the upper troposphere. Further analyses indicated that two types of TCs with distinctly different tracks, i.e., westward-moving TCs and northward-moving TCs, both favor the EAP pattern. The present results imply that TCs over the WNP, as extreme weather, can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.
    摘要: 基于1979-2018年JTWC和NCEP-2再分析数据集,本文研究了夏季西北太平洋热带气旋对东亚-太平洋遥相关型的影响。结果表明,与整个夏季平均相比,在热带西北太平洋存在热带气旋活动时平均的环流表现为更强的东亚-太平洋遥相关型,即对流层低层西北太平洋气旋异常、中层位势高度异常跷跷板模态以及高层西风急流北移均更强。进一步研究结果表明,西行和北上热带气旋均会使东亚-太平洋遥相关型增强。这表明热带气旋作为一种强烈天气系统可以显著地影响西北太平洋和东亚的夏季气候异常。
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  • Figure 1.  Time series of the standardized WNPCI (bars) and the number of TC days (line) during 1979–2018. The black bars indicate the strong-convection years.

    Figure 2.  Composite maps of OLR anomalies for (a) TC days and (b) the JJA mean during the strong-convection summers. The contour interval is 5 W m−2. Zero contours are omitted. Shading indicates that the correlation coefficient between the WNPCI and OLR anomalies is significant at the 95% confidence level.

    Figure 3.  Composite maps of (a, b) 850-hPa horizontal wind anomalies (units: m s−1), (c, d) 500-hPa geopotential height anomalies (units: m), and (e, f) 200-hPa zonal wind anomalies (units: m s−1) during the strong-convection summers. The left-hand panels denote TC days, and the right-hand panels denote the JJA mean. Small values of horizontal wind (magnitude less than 0.7 m s−1) are omitted in (a, b). The contour interval is 4 m in (c, d) and 1 m s−1 in (e, f). Zero contours are omitted. The thick solid lines in (e, f) show the axis of climatological mean westerly jet. Shading in (b) indicates that the correlation coefficient between the WNPCI and zonal or meridional wind anomalies is significant at the 95% confidence level, and shading in (d, f) is the same as in (a) but for geopotential height and zonal wind anomalies.

    Figure 4.  Composite tracks of (a) westward-moving TCs and (b) northward-moving TCs during the strong-convection summers. The red (blue) dots represent TC genesis (extinction) locations. The numbers of the two types of TCs are included in the upper-right corners. The grey lines in (a) show the two TCs removed from the composite analysis.

    Figure 5.  As in Fig. 2 but for (a) westward-moving TC days and (b) the difference between westward-moving TC days and the JJA mean.

    Figure 6.  As in Fig. 3 but for westward-moving TC days (left) and the difference between westward-moving TC days and the JJA mean (right).

    Figure 7.  As in Fig. 4 but for extratropical TCs.

    Figure 8.  As in Fig. 2 but for (a) extratropical TC days and (b) the difference between extratropical TC days and the JJA mean.

    Figure 9.  As in Fig. 3 but for extratropical TC days (left) and the difference between extratropical TC days and the JJA mean (right).

    Table 1.  The number of TC days, westward-moving TC days, northward-moving TC days, and extratropical TC days during the strong-convection summers, and the average number of days for these six years.

    Strong-convection summers198119852001200420122018Mean
    TC46534863576956
    Westward-moving TC1561615161714
    Northward-moving TC14162744273928
    Extratropical TC891319102614
    DownLoad: CSV

    Table 2.  The WNPCI (units: W m−2), WNPSMI (units: m s−1), EAPI and EAJI (units: m s−1), for the JJA mean, TC days, westward-moving TC days, and extratropical TC days during the strong-convection summers. Values in parentheses are the changes in these values relative to the JJA mean.

    JJA meanTC daysWestward-moving TC daysExtratropical TC days
    WNPCI−9.23−12.17 (+32%)−15.01 (+63%)−7.44 (−19%)
    WNPSMI3.244.70 (+45%)6.00 (+85%)3.25 (0%)
    EAPI1.221.86 (+52%)2.07 (+70%)1.32 (+8%)
    EAJI3.234.40 (+36%)2.53 (−22%)5.66 (+75%)
    DownLoad: CSV
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Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer

    Corresponding author: Riyu LU, lr@mail.iap.ac.cn
  • 1. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 2. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The contribution of tropical cyclones (TCs) to the East Asia–Pacific (EAP) teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to 2018. The results showed that the TCs over the western North Pacific (WNP) correspond to a strengthened EAP pattern: During the summers of strong convection over the tropical WNP, TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere, an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere, and a more northward shift of the East Asian westerly jet in the upper troposphere. Further analyses indicated that two types of TCs with distinctly different tracks, i.e., westward-moving TCs and northward-moving TCs, both favor the EAP pattern. The present results imply that TCs over the WNP, as extreme weather, can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.

摘要: 基于1979-2018年JTWC和NCEP-2再分析数据集,本文研究了夏季西北太平洋热带气旋对东亚-太平洋遥相关型的影响。结果表明,与整个夏季平均相比,在热带西北太平洋存在热带气旋活动时平均的环流表现为更强的东亚-太平洋遥相关型,即对流层低层西北太平洋气旋异常、中层位势高度异常跷跷板模态以及高层西风急流北移均更强。进一步研究结果表明,西行和北上热带气旋均会使东亚-太平洋遥相关型增强。这表明热带气旋作为一种强烈天气系统可以显著地影响西北太平洋和东亚的夏季气候异常。

    • In the boreal summer, there is a meridional teleconnection pattern characterized by zonally elongated anomalies appearing alternately in the meridional direction over the western North Pacific and East Asia (WNP–EA) region, which is referred to as the Pacific–Japan (PJ) pattern (Nitta, 1987) or East Asia–Pacific (EAP) pattern (Huang and Sun, 1992). This teleconnection pattern is the most dominant mode of interannual variations over WNP–EA. In addition, the anticyclonic/cyclonic anomaly over the subtropical WNP in the lower troposphere and southward/northward shift of the East Asian westerly jet (EAJ) in the upper troposphere have been illustrated as the components of the meridional teleconnection (Lau et al., 2000; Wang et al., 2001; Lu, 2004; Lu and Lin, 2009; Lin et al., 2010). The lower-tropospheric anticyclonic (cyclonic) anomaly corresponds to a westward extension (eastward retreat) of the western North Pacific subtropical high (WNPSH). This meridional teleconnection plays a crucial role in linking the circulation between tropical and extratropical regions and significantly affects the variability of WNP–EA summer climate (Nitta, 1989; Nitta and Hu, 1996; Wakabayashi and Kawamura, 2004; Kosaka et al., 2011; Huang et al., 2012; Li et al., 2014).

      As is well known, the WNP is the most prolific tropical cyclone (TC) basin, accounting for about 30% of all TCs globally (Wu and Wang, 2004; Woodruff et al., 2013). The mean life span of TCs is about a week, with a minimum of a couple of days and a maximum of about one month. TCs over the WNP can remarkably affect atmospheric environments (Sobel and Camargo, 2005; Schenkel and Robert, 2015), air–sea interaction (Sobel and Camargo, 2005; Wang et al., 2019), and oceanic conditions (e.g., Zhang et al., 2021). Generally, the tracks of TCs can be categorized into two types: westward (or straight-ward) and northward (or recurving), and the recurving tracks include recurving landfall and recurving ocean tracks (Wu et al., 2005; Kim et al., 2011; Colbert et al., 2015; He et al., 2015; Zhang et al., 2018). The westward-moving TCs mainly affect Southeast Asia, and the northward-moving TCs affect East Asia.

      Interaction between the meridional teleconnection and TCs has been indicated by previous studies. On the one hand, the EAP pattern has a significant influence on TC activities, including genesis frequency, track, intensity, landfall location, and recurvature location (Choi et al., 2010; Kim et al., 2011; Kubota et al., 2016). On the other hand, TCs can stimulate the EAP pattern through stationary Rossby waves (Kawamura and Ogasawara, 2006; Yamada and Kawamura, 2007). In particular, TC activities can affect the intensity and location of the WNPSH (Zhong and Hu, 2007; Chen et al., 2019). Moreover, frequent TC activities can lead to a northward shift of the EAJ through stimulating the EAP pattern and modulating the meridional gradient of tropospheric temperature, according to (Chen et al. 2017a) and Hu et al. (2019), who focused on the interannual variation of TC activities. However, it remains unknown whether there are any significant differences between TC days and other days within summer.

      The EAP pattern, which is generally represented by circulation anomalies, is closely related to convection anomalies over the tropical WNP, and the former is considered to be a result of the latter, i.e., the anomalous convection over the tropical WNP can trigger the EAP pattern (Kurihara and Tsuyuki, 1987; Nitta, 1987; Huang and Sun, 1992). Therefore, the interaction between the EAP pattern and TCs is a complex topic that involves the following two issues: First, as a strong vortex, TCs can induce positive low-level vorticity anomalies (Sobel and Camargo, 2005). TCs, especially northward-moving ones, might serve as noise for the EAP-related circulation anomalies, which are well organized and appear alternately in the meridional direction over WNP–EA. Therefore, northward-moving TCs, which usually span a large area in the meridional direction, can be expected to obscure the EAP pattern. However, TC activities are closely related to convection over the WNP, i.e., TCs induce strong convection and the enhanced convection favors TC occurrence (e.g., Camargo et al., 2009; Zhao et al., 2016; Chen et al., 2017b). Furthermore, westward-moving and northward-moving TCs may play distinctly different roles in affecting the EAP due to their different heating locations. In this study, we investigate the composite anomalies for TC days and compare them with summer-mean anomalies, and then we examine the different roles of westward-moving and northward-moving TCs.

      The remainder of this paper is organized as follows: Section 2 presents the datasets and indices used in this study. Section 3 presents the features of the EAP pattern associated with TCs. Section 4 discusses the effects of two types of TCs on the EAP pattern. Section 5 provides conclusions and some further discussion.

    2.   Datasets and indices
    • TC best track data at 6-hourly intervals were obtained from the Joint Typhoon Warning Center (JTWC). TCs with a maximum surface wind speed ≥ 34 kt (17 m s−1) were analyzed in this study. The tracks of TCs were classified into two categories: westward-moving and northward-moving. The westward-moving TCs were simply defined according to their extinction latitudes being lower than 25°N. The northward-moving TCs were defined based on their genesis and extinction positions: the genesis (extinction) latitude was lower (higher) than 20°N (30°N). We excluded TCs to the east of 160°E from our analysis and focused on those to the west of 160°E, which were consistent with the regions of the EAP pattern and thus likely had strong impacts upon it.

      Daily mean NCEP-2 data (Kanamitsu et al., 2002) were used to obtain the atmospheric circulation variables, including 500-hPa geopotential height, and 850-hPa and 200-hPa horizontal winds, at a horizontal resolution of 2.5° × 2.5°. Additionally, to measure tropical convection, we also used the daily outgoing longwave radiation (OLR) data (Liebmann and Smith, 1996) from NOAA satellites at the same resolution. A tropical WNP convection index (WNPCI) was defined as the OLR anomalies averaged over the region (10°–20°N, 110°–160°E), following Lu (2001) and Xue et al. (2017). The analyses were carried out for the summer season (June–August, JJA) during the period 1979–2018.

      Our main focus in this study was the contribution of TCs to the EAP pattern. Accordingly, we adopted three indices to facilitate discussion on the change of the EAP pattern. First, the western North Pacific monsoon index (WNPSMI) was defined as the difference in the averaged 850-hPa zonal wind anomalies between (5°–15°N, 90°–130°E) and (22.5°–32.5°N, 110°–140°E), following Wang and Fan (1999). A positive (negative) WNPSMI indicates a cyclonic (anticyclonic) anomaly over the subtropical WNP. Second, an EAP index (EAPI) was measured as the standardized 500-hPa geopotential height anomaly at (40°N, 130°E) minus that at (20°N, 120°E), following Huang (2004), but with a modification according to the result in section 3. Finally, to measure the meridional displacement of the EAJ, an EAJ index (EAJI) was defined as the difference between the 200-hPa zonal wind anomalies averaged over (40°–50°N, 120°–150°E) and (30°–40°N, 120°–150°E), following Lu (2004). A positive EAJI indicates that the EAJ axis shifts northward, and vice versa.

    3.   Features of meridional teleconnection on TC days
    • Figure 1 shows the time series of the standardized WNPCI (bars) and TC days (line). A TC day was counted if there was at least one TC appearing to the west of 160°E over the WNP on a certain day during summer. Both time series show clear interannual variations and have no distinct decadal variations or long-term trends. The correlation coefficient between the WNPCI and TC days is −0.41 during 1979–2018, significant at the 99% confidence level. That is, when convection is enhanced in the tropical WNP, there are more TCs over the WNP.

      Figure 1.  Time series of the standardized WNPCI (bars) and the number of TC days (line) during 1979–2018. The black bars indicate the strong-convection years.

      Enhanced-convection years were selected based on the WNPCI, and six years were chosen according to the criterion of the WNPCI being lower than −1.0 standard deviation (1981, 1985, 2001, 2004, 2012, and 2018). The number of TC days during these six summers is 46, 53, 48, 63, 57, and 69, respectively (Table 1), and the ratios of TC days to total summer days range from 50% to 75%, suggesting that TCs appear frequently over the WNP, at least for these summers of strong convection. The following analyses are mainly concentrated on these six summers, on the basis that their relatively greater frequencies of TCs should make the contribution of TCs to the EAP pattern clearer. The contribution of TCs was estimated by comparing the results for TC days and for total days in summer.

      Strong-convection summers198119852001200420122018Mean
      TC46534863576956
      Westward-moving TC1561615161714
      Northward-moving TC14162744273928
      Extratropical TC891319102614

      Table 1.  The number of TC days, westward-moving TC days, northward-moving TC days, and extratropical TC days during the strong-convection summers, and the average number of days for these six years.

      Figure 2 shows the composite OLR anomalies for TC days (Fig. 2a) and total days (Fig. 2b) for the strong-convection summers. Negative OLR anomalies appear over the tropical WNP for both TC days and total days (JJA), indicating strong convection over that region. In addition, stronger convection occurs on TC days, as compared to the JJA mean. The WNPCI for TC days and total days is −12.17 and −9.23 W m−2, respectively (Table 2), indicating that convection is enhanced significantly over the tropical WNP during TC days. The WNPCI on TC days decreases by 32% relative to the JJA mean. In addition, there are positive OLR anomalies over the rainy belt of subtropical East Asia, as a component of the well-known seesaw pattern of rainfall between the tropics and subtropics in summer.

      Figure 2.  Composite maps of OLR anomalies for (a) TC days and (b) the JJA mean during the strong-convection summers. The contour interval is 5 W m−2. Zero contours are omitted. Shading indicates that the correlation coefficient between the WNPCI and OLR anomalies is significant at the 95% confidence level.

      JJA meanTC daysWestward-moving TC daysExtratropical TC days
      WNPCI−9.23−12.17 (+32%)−15.01 (+63%)−7.44 (−19%)
      WNPSMI3.244.70 (+45%)6.00 (+85%)3.25 (0%)
      EAPI1.221.86 (+52%)2.07 (+70%)1.32 (+8%)
      EAJI3.234.40 (+36%)2.53 (−22%)5.66 (+75%)

      Table 2.  The WNPCI (units: W m−2), WNPSMI (units: m s−1), EAPI and EAJI (units: m s−1), for the JJA mean, TC days, westward-moving TC days, and extratropical TC days during the strong-convection summers. Values in parentheses are the changes in these values relative to the JJA mean.

      Figure 3 shows the composite circulation anomalies for TC days and total days for the strong-convection summers. These circulation anomalies, including 850- and 200-hPa wind anomalies and 500-hPa geopotential height anomalies, can be used to illustrate the meridional teleconnection pattern over WNP–EA. In the lower troposphere, there is a cyclonic anomaly over the subtropical WNP, and a relatively weak anticyclonic anomaly centered over southern Japan, for both TC days and total days (Figs. 3a and 3b). However, the cyclonic anomaly on TC days (Fig. 3a) is obviously stronger than that in the JJA mean (Fig. 3b), which is confirmed by the WNPSMI being larger for TC days than in the JJA mean (4.70 m s−1 versus 3.24 m s−1, equivalent to a 45% increase; Table 2).

      Figure 3.  Composite maps of (a, b) 850-hPa horizontal wind anomalies (units: m s−1), (c, d) 500-hPa geopotential height anomalies (units: m), and (e, f) 200-hPa zonal wind anomalies (units: m s−1) during the strong-convection summers. The left-hand panels denote TC days, and the right-hand panels denote the JJA mean. Small values of horizontal wind (magnitude less than 0.7 m s−1) are omitted in (a, b). The contour interval is 4 m in (c, d) and 1 m s−1 in (e, f). Zero contours are omitted. The thick solid lines in (e, f) show the axis of climatological mean westerly jet. Shading in (b) indicates that the correlation coefficient between the WNPCI and zonal or meridional wind anomalies is significant at the 95% confidence level, and shading in (d, f) is the same as in (a) but for geopotential height and zonal wind anomalies.

      There are negative and positive 500-hPa geopotential height anomalies over the subtropical WNP and midlatitude East Asia, respectively, for both TC days and JJA (Figs. 3c and 3d). There are also negative anomalies over the Okhotsk Sea and adjacent Far East, but with a moderate statistical significance, as indicated by Fig. 3d. Thus, we defined an EAP index by the subtropical and midlatitude components of this pattern, as mentioned in section 2, without considering the high-latitude anomalies. It was found that both the subtropical and midlatitude anomalies are remarkably stronger on TC days (Fig. 3c) than in the JJA mean (Fig. 3d). The EAPI for TC days is 1.86, stronger than that for the JJA mean (1.22). Compared with the JJA mean, the EAPI on TC days increases by 52% (Table 2).

      In the upper troposphere, zonal wind anomalies are zonally elongated and alternately negative and positive from the tropical western Pacific to East Asia, for both TC days and JJA (Figs. 3e and 3f). In particular, there are positive (negative) anomalies to the north (south) of the climatological jet axis, which is defined by where the first derivative of the 200-hPa zonal winds is zero and represented by the thick solid lines, suggesting a northward displacement of the EAJ. On the other hand, both the positive and negative anomalies to the north and south of the jet axis are stronger on TC days than in JJA, further indicating a more remarkable northward displacement of the EAJ on TC days. This can be confirmed by the composite EAJI results, i.e., 4.40 m s−1 and 3.23 m s−1 on TC days and in JJA, respectively. This difference is equivalent to a 36% increase (Table 2).

      The results presented in this section indicate that this meridional teleconnection, illustrated by the cyclonic anomaly in the lower troposphere, EAP pattern in the middle troposphere, and EAJ meridional displacement in the upper troposphere, is stronger on TC days than on a JJA-mean basis. Therefore, it can be concluded that TCs strengthen rather than weaken this meridional teleconnection.

    4.   Meridional teleconnection features in two types of TC tracks
    • As stated in the introduction, we hypothesized that the two types of TC tracks, i.e., westward-moving and northward-moving, will be associated with different meridional teleconnection features. Figure 4 displays the TC tracks for these two types of TCs during the strong-convection years. The composite TC tracks have good coherence with these definitions. There are 24 westward-moving TCs and 25 northward-moving TCs during the six summers. Among the 24 westward-moving TCs, two appear only to the east of about 140°E, while the remaining 22 show roughly similar routes (Fig. 4a). In the composite analyses upon which the following discussion of results is based, we excluded these two TCs and used only the remaining 22. Although including them did not greatly change the results (not shown), this decision was made because they may have been only weakly related to the meridional teleconnection during their life span. Additionally, the composite analyses were performed on both the westward-moving and northward-moving TC days.

      Figure 4.  Composite tracks of (a) westward-moving TCs and (b) northward-moving TCs during the strong-convection summers. The red (blue) dots represent TC genesis (extinction) locations. The numbers of the two types of TCs are included in the upper-right corners. The grey lines in (a) show the two TCs removed from the composite analysis.

    • Figure 5a shows the composite OLR anomalies on westward-moving TC days. Zonally elongated negative anomalies cover the tropical WNP, indicating stronger convection there. Compared with the JJA mean, the negative anomalies are relatively westward-shifted and stronger, corresponding to the negative difference concentrated over the South China Sea and western Philippine Sea in Fig. 5b. The change of convection anomalies is consistent with the tracks of westward-moving TCs (Fig. 4a). The WNPCI is −15.01 W m−2 on westward-moving TC days, which not only is less than that in the JJA mean (−9.23 W m−2), but also less than that on TC days (−12.17 W m−2; Table 2). The relative change corresponding to the JJA mean is 63%.

      Figure 5.  As in Fig. 2 but for (a) westward-moving TC days and (b) the difference between westward-moving TC days and the JJA mean.

      Figure 6 shows the meridional teleconnection pattern on westward-moving TC days and the difference between westward-moving TC days and the JJA mean, analogously illustrated by the 850- and 200-hPa wind anomalies and 500-hPa geopotential height anomalies. There is a clear cyclonic anomaly over the South China Sea and western Philippine Sea at 850 hPa (Fig. 6a). The composite differences also correspond to a cyclonic anomaly over the South China Sea with a similar pattern, indicating a stronger cyclonic anomaly on westward-moving TC days (Fig. 6b). This can be confirmed by the WNPSMI value, which is 6.00 m s−1 on westward-moving TC days—greater than the 3.24 m s−1 and 4.70 m s−1 in the JJA mean and for all TC days, respectively (Table 2). This change in WNPSMI on westward-moving TC days is equivalent to an 85% increase relative to the JJA mean. In addition, this cyclonic anomaly tends to shift westward, which is consistent with a similar shift of negative OLR anomalies in the tropical WNP (Fig. 5a). The wind anomalies tend to show a wave-like structure over the extratropical North Pacific and Northeast Asia, but detailed discussion on them is beyond the scope of this study.

      Figure 6.  As in Fig. 3 but for westward-moving TC days (left) and the difference between westward-moving TC days and the JJA mean (right).

      The composite 500-hPa geopotential height anomalies are negative in the subtropical WNP and positive in midlatitude East Asia (Fig. 6c). The composite differences between westward-moving TC days and the JJA mean display a similar but weaker pattern of anomalies (Fig. 6d). This means that both the negative and positive anomalies are obviously stronger, as compared to the JJA mean. Therefore, the EAPI is also greater, being 2.07 (70%) higher than that in the JJA mean (Table 2). The regions of negative and positive anomalies are also slightly westward-shifted in comparison with the JJA mean. Similar results were found when compared to all TC days (not shown). We do not discuss other anomalies in the middle and high latitudes, for a similar reason as the above-mentioned 850-hPa wind anomalies.

      The 200-hPa zonal wind anomalies are positive and negative to the north and south of the EAJ axis (Fig. 6e), consistent with the results shown in Figs. 3e and 3f. However, the positive anomalies are not well organized, and the negative anomalies are relatively westward-shifted. A similar asymmetric pattern is shown in the difference between the westward-moving TC days and the JJA mean (Fig. 6f). The EAJI on westward-moving TC days is 2.53 m s−1, which is lower than that in the JJA mean and on TC days (Table 2). However, it should be mentioned that the EAJI was defined through the JJA mean result, and thus may not be appropriate to be used to estimate the northward displacement of the jet for westward-moving TCs.

    • Northward-moving TCs span a large meridional scope from the tropical WNP to the mid-latitudes. During the days when they remain in the tropics, northward-moving TCs may play a similar role in the EAP pattern as westward-moving TCs. However, after the northward-moving TCs enter the extratropical WNP and East Asia, they might play a quite different role in the EAP pattern, due to completely different locations of diabatic heating. Therefore, in the following analysis, we focus on the periods when northward-moving TCs appear to the north of 25°N and refer to them as extratropical TC days. Figure 7 shows the tracks of these northward-moving TCs, which are the same as those to the north of 25°N in Fig. 4b. The yearly-averaged number of extratropical TC days is 14, indicating that northward-moving TCs spend half of their entire lifetime (yearly-averaged days being 28 days) over the extratropics.

      Figure 7.  As in Fig. 4 but for extratropical TCs.

      Figure 8a shows the composite OLR anomalies on extratropical TC days. Negative anomalies appear over the tropical WNP, similar to in the JJA-mean result (Fig. 2b). There are, however, distinct differences between them: Stronger convection covers the east of Taiwan to Japan (Fig. 8b), corresponding to the tracks of extratropical TCs in Fig. 7. The WNPCI for the extratropical TC days is −7.44 W m−2, which is only slightly weaker than that for the JJA mean (−9.23 W m−2; Table 2).

      Figure 8.  As in Fig. 2 but for (a) extratropical TC days and (b) the difference between extratropical TC days and the JJA mean.

      Figure 9 shows the composite circulation anomalies on extratropical TC days and the difference between extratropical TC days and the JJA mean. The 850-hPa wind anomalies are characterized by a cyclonic anomaly over the subtropical WNP (Fig. 9a). The composite differences indicate that this cyclonic anomaly is northward-shifted and stronger than that for the JJA mean (Fig. 9b), which is in agreement with the shift of enhanced convection. The WNPSMI on extratropical TC days is 3.25 m s−1, which is equivalent to that for the JJA mean (3.24 m s−1; Table 2).

      Figure 9.  As in Fig. 3 but for extratropical TC days (left) and the difference between extratropical TC days and the JJA mean (right).

      In the middle troposphere, there are negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia, respectively (Fig. 9c). The composite differences manifest as meridionally extended negative anomalies (Fig. 9d), due to the strengthened and northward-shifted negative geopotential height anomalies over the subtropical WNP and the southeastward-shifted positive anomalies over midlatitude East Asia. In addition, the positive difference to the east of Japan corresponds to the enhanced positive anomaly center. The EAPI is 1.32 on extratropical TC days, which is higher than that for the JJA mean (1.22; Table 2), even though the index may underestimate the anomalies on extratropical TC days because of the displacement of anomaly centers.

      The 200-hPa zonal wind anomalies on extratropical TC days are characterized by easterly and westerly anomalies to the south and north of the jet axis (Fig. 9e), which are similar to those for the JJA mean in distribution, but clearly stronger. The differences between extratropical TC days and the JJA mean also manifest as alternately positive and negative anomalies over midlatitude East Asia, but the westerly anomalies contribute most to the farther northward displacement of the EAJ during extratropical TC days (Fig. 9f). The EAJI for the extratropical TC days is 5.66 m s−1, which is much stronger than that for the JJA mean (3.23 m s−1; Table 2).

      The composite circulation anomalies on extratropical TC days shown in Fig. 9 are better organized and more similar to those for the JJA mean, as compared with the westward-moving TC days shown in Fig. 7. On the one hand, the sample size for westward-moving TCs is smaller because of relatively fewer TCs with this track in the six selected years, and this might induce uncertainty in these extratropical anomalies. On the other hand, this implies that the large-scale circulation anomalies related to the EAP pattern may favor northward-moving TCs.

      The results presented in this section indicate that both westward- and northward-moving TCs correspond to strengthened circulation anomalies related to the meridional teleconnection pattern over WNP–EA. The only exception is the weaker northward displacement of the EAJ for westward-moving TCs, which is somewhat uncertain due to the small sample size and unreasonable definition. In addition, the almost identical convection anomalies on extratropical TC days, as compared with the JJA mean, correspond to stronger circulation anomalies, particularly over the extratropics, suggesting again that the extratropical circulation anomalies related to the EAP pattern may favor northward-moving TCs.

    5.   Conclusions and discussion
    • In this study we investigated whether the occurrence of TCs over the WNP strengthens or weakens the EAP pattern, which is a dominant meridional teleconnection over the WNP and East Asia in summer, using the best track data of JTWC and NCEP-2 reanalysis datasets from 1979 to 2018. Because of the close relationship of TC occurrence and tracks to the EAP pattern, as a first-order approximation, we analyzed the composite anomalies of simultaneous convection and circulation for TC days, i.e., the days when at least one TC occurs over the WNP, without considering the lead–lag relationship between them or detailed physical processes. Then, the possible contribution of TCs to the EAP pattern was estimated by comparing these TC-related anomalies with the anomalies for all summer days. Furthermore, we selected six summers of strong convection over the tropical WNP, during which more TCs appeared over the WNP, and performed composite analyses to highlight the role of TCs.

      The results clearly indicated that TCs over the WNP, regardless of their locations, correspond to a strengthened EAP pattern. The strengthening is presented in the relevant anomalies over both the tropics and extratropics, and in the lower, middle, and upper troposphere. Specifically, during TC days, the atmospheric convection is generally enhanced over the tropical WNP, the cyclonic anomaly over the subtropical WNP is stronger in the lower troposphere, the seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia is strengthened in the middle troposphere, and the northward displacement of the EAJ is more remarkable in the upper troposphere. These aspects of convection and circulation anomalies are typical features of the EAP pattern.

      In particular, we analyzed the anomalies associated with two quite different locations of TCs, i.e., westward-moving TCs and northward-moving TCs. The results showed that both types of TCs correspond to a strengthened EAP pattern. The only exception is a weaker northward displacement of the EAJ for westward-moving TCs, which is possibly due to the small sample size for this kind of TC.

      In this study, we did not investigate the physical processes through which the TC–EAP relationship is established. The TCs over the WNP and the EAP pattern may be interactive. On the one hand, the EAP pattern, supposed at the phase shown in this paper, can result in more northward-moving TCs through modulating the steering flows (Choi et al., 2010). On the other hand, TCs, with their diabatic heating, can trigger the EAP pattern. If we presume TCs can trigger the EAP pattern through Rossby waves, it would take time for the waves to propagate from the lower latitudes to midlatitude East Asia, especially for westward-moving TCs. Therefore, midlatitude circulation anomalies would be somewhat delayed, rather than simultaneous as they were in this study. However, as mentioned, the relationship between TCs and the EAP pattern might be interactive, and thus it would be hard to precisely determine the lead–lag periods. The present results should be considered as a first-order approximation of the contribution of TCs to the EAP pattern.

      The present conclusion that TCs over the WNP favor the EAP pattern might provide an encouraging base for seasonal forecasting of summer climate over WNP–EA. Although current climate models cannot capture the activities of TCs, they are capable of reliably forecasting the convection over the tropical WNP (e.g., Li et al., 2012). On the other hand, the present results imply that models may tend to underestimate the intensity of the EAP pattern, owing to their shortcomings in capturing TCs.

      Recent studies have suggested that the EAP pattern experienced obvious interdecadal changes in the late 1990s or early 2000s (Xu et al., 2019; He et al., 2020; Li and Lu, 2020). It would be interesting to examine whether TCs show different contributions to the EAP pattern in different interdecadal epochs.

      Acknowledgements. This research was supported by the National Natural Science Foundation of China (Grant No. 41721004).

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