Straight-moving tropical cyclones over the western North Pacific trigger the wave trains over the North Pacific during winter

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Corresponding to the straight-moving TCs, quasi-stationary wave trains appear as alternative geopotential height anomalies in the upper troposphere stretching from East Asia to the North Pacific. Specifically, the anomalous anticyclones are initially formed to the south of Japan and then lead to the subsequent anomalies over the Sea of Okhotsk and the Gulf of Alaska, respectively. The wave trains extend along an approximate great circle path and differ from those triggered by the recurving TCs in summer and autumn, which propagate eastward along the westerly jet. Further analysis reveals that the upper-level anticyclonic anomalies are excited by negative Rossby wave sources, which are mainly attributed to the poleward vorticity advection by anomalous divergence relevant to TCs. In addition, the diagnosis indicates that the generation of wave source is caused by the product of the TC-induced divergent flows and the prominent meridional vorticity gradient in association with East Asian upper-tropospheric westerly jet. These findings imply that the tropical disturbances over the WNP, such as straight-moving TCs, can remotely affect weather over the extratropics, and thus have implications for improving the weather forecast over the extratropics through improving tropical disturbance forecast. Tropical cyclone Rossby wave train East Asian westerly jet Western North Pacific Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1 Introduction The western North Pacific (WNP) is the most prolific tropical cyclone (TC) basin, accounting for about 30% of all TCs globally (Wu and Wang 2004 ). TCs over this region induce tremendous rainfall and destructive winds along their pathways due to their eyewall structure and spiral rain bands (e.g., Kunkel et al. 1999 ), and the precipitation induced by TCs accounts for a large proportion of the total annual amounts in the coastal regions, such as Japan (e.g., Kamahori and Arakawa 2018 ), southern China (e.g., Ren et al. 2002 ), and Southeast Asia (e.g., Nguyen-Thi et al. 2012 ). In addition, TCs can also provoke precipitation over the areas that are far away from the TC centers through the transportation of warm moisture (e.g., Galarneau et al. 2010 ). For instance, the unprecedented rainfall that occurred on 20 July 2021 in Zhengzhou (35°N, 113°E), an inland city in China, was attributed to Typhoon In-Fa and Cempaka which were active over the WNP and South China Sea, respectively (Deng et al. 2022 ). The tracks of TCs over the WNP can be generally categorized into straight-moving and recurving TCs (e.g., Kim et al. 2011 ). The recurving TCs, when they approach the extratropical upper-tropospheric westerly jets, can interact with the jets and exert pronounced impacts on the weather and climate of extratropical regions. There is a significant increase in Rossby wave amplitude and occurrence frequency in the North Pacific, documented by case studies (Agustí-Panareda et al. 2005 ; Harr and Dea 2009 ; Grams et al. 2013a ; Keller et al. 2014) and statistical studies (Archambault et al. 2013 ; Quinting and Jones 2016 ; Riboldi et al. 2018 ). Then, the westerly jets further serve as a waveguide and disperse the kinetic energy to downstream areas by promoting the subsequent cyclogenesis along the waveguide (Riemer and Jones 2010 ; Archambault et al. 2015 ). As a result, the interaction between the recurving TCs and westerly jets contributes to the amplification of synoptic transient eddy activity over the North Pacific (Ha et al. 2022 ) and modulates weather over North America (Bosart et al. 2017 ; Stuivenvolt-Allen et al. 2021; Stuivenvolt-Allen and Wang 2023), causing the enhanced forecast uncertainty in downstream regions (Aiyyer 2015 ; Torn 2017 ). The influence on downstream flows is associated with TCs recurving into midlatitudes and undergoing extratropical transition (Jones et al. 2003 ). It is reported that the diabatic processes play a key role in modifying the extratropical circulation, as the deep warm core of TC is replaced by a cold and asymmetric structure when it moves poleward into a baroclinic environment (Riemer and Jones 2010 ; Grams et al. 2011 ; Keller et al. 2019 ). Then, the latent heat release in ascending air of TCs yields a transport of low potential vorticity to the tropopause, which further amplifies the upper-level waves (e.g., Grams et al. 2013b ; Grams and Archambault 2016 ; Evans et al. 2017 ; Riboldi et al. 2018 ). The majority of previous studies focused on the impacts of recurving TCs on the extratropical circulation in summer and autumn, which are the main seasons of TC occurrences over the WNP. Moreover, relatively more TCs tend to be recurving in these seasons in comparison with the other two seasons, i.e., winter and spring (Archambault et al. 2013 ). However, the influence of straight-moving TCs on extratropical circulation has yet to be examined. It was documented that the straight-moving TCs in summer can hardly trigger organized wave anomalies, as they are far away from the upper-tropospheric westerly jet (Hirata and Kawamura 2014 ). However, the westerly jet is strongest over East Asia and locates most southward in winter (Zhang et al. 2006 ). Thus, it provides a probability of strong interaction between the jet and TCs, even the straight-moving TCs that do not approach the jet. To verify this hypothesis, we focus on the impacts of straight-moving TCs over the WNP on the extratropical circulation in winter. In the following parts, section 2 presents the data and methods. Section 3 demonstrates the evolutions of circulation anomalies relevant to the straight-moving TCs over the WNP. Section 4 further explores the corresponding mechanism of how the TCs induce extratropical anomalies. Conclusions and discussion are given in section 5. 2 Data and Methods The daily-mean atmospheric reanalysis data are obtained by the average of 6-hourly data derived from the European Center for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5; Hersbach et al. 2020 ), with a horizontal resolution of 0.75°×0.75°. The analysis period of this study is 1979–2020 and the daily anomalies are calculated by removing the 42-yr mean of the calendar day from the raw data. The TC best-track data are obtained from the Shanghai Typhoon Institute of the China Meteorological Administration (CMA) with a 6-hourly interval (Ying et al. 2014 ; Lu et al. 2021 ). In this study, TCs are analyzed when the maximum surface wind speed exceeds 17.2 m/s, i.e., at tropical storm intensity or above. The straight-moving TC is identified when its extinction point is located at the most western point of the entire TC track. Accordingly, 100 straight-moving TCs are identified, and all of them completely dissipate before entering the mid-latitude. In this study, the winter is defined as November–January (NDJ), which is one month earlier than the more widely used one, i.e., December–February. The modification is conducted as TCs are relatively active in early winter but rarely appear in February. The lead-lag composites are performed to analyze the large-scale circulation associated with TCs. The statistical significance is examined by using a two-tailed Student’s t -test and the comparison is conducted between the composite days and the remainder in winter (Wilks 2006 ). The effective degree of freedom ( \({N}^{\text{*}}\) ) is computed as (Zwiers and von Storch 1995): $${N}^{*}=N\frac{1-{r}_{1}}{1+{r}_{1}}, \left(1\right)$$ where \(N\) and \({r}_{1}\) are the original sample size and the lag-one autocorrelation coefficient, respectively. The Rossby wave source ( S ) is utilized to diagnose the Rossby wave generation excited by diabatic heating, and its perturbation can be written as (Sardeshmukh and Hoskins 1988 ): $${S}^{{\prime }}=-\left(f+\stackrel{-}{\zeta }\right)\nabla \cdot {\varvec{V}}_{\chi }^{{\prime }}-{\zeta }^{{\prime }}\nabla \cdot {\stackrel{-}{\varvec{V}}}_{\chi }-{\varvec{V}}_{\chi }^{{\prime }}\cdot \nabla \left(f+\stackrel{-}{\zeta }\right)-{\stackrel{-}{\varvec{V}}}_{\chi }\bullet \nabla {\zeta }^{{\prime }}, \left(2\right)$$ where \(\zeta\) , \(f\) and \({ \varvec{V}}_{\chi }\) are relative vorticity, planetary vorticity, and divergent wind, respectively; \(\nabla\) represents the horizontal gradient; overbar and prime denote the climatology and anomalies, respectively. Eq. (2) shows that the anomalous S consists of vortex stretching and advection (Lu and Kim 2004 ). Specifically, the first two terms on the right-hand side denote the vortex stretching forced by climatological vorticity and anomalous divergence, and by anomalous vorticity and climatological divergence, respectively. The other two terms represent the climatological vorticity advection by anomalous divergent flow, and the anomalous vorticity advection by climatological divergent wind, respectively. Hereafter, the above four terms are referred to as S1 to S4 by turns. In this study, the S1 is masked out where the anomalous divergence is weak (i.e., within ± 5×10 − 6 s − 1 ), as the disordered divergences are enlarged by strong planetary vorticity in the extratropics. 3 Circulation anomalies relevant to the straight-moving TCs Figure 1 a shows the tracks of the 100 straight-moving TCs. In general, most TCs are formed over the WNP, then move westward to the South China Sea, and finally decay over the Indo-China Peninsula. Additionally, the spatial distribution of TC activity, counted as the numbers in each 2.5°×2.5° latitude–longitude grid, reveals that the majority of TCs are active over the Philippine Sea and South China Sea (Fig. 1 b). It should be noticed that some TCs are far away from the active region. Thus, the key area is selected as (10°–17.5°N, 110°–135°E; box in Fig. 1 b). The 87 TCs passing through the key area are selected (Fig. 1 c), while the other 13 TCs are excluded in the following analyses (Fig. 1 d). However, it should be mentioned that the composite results including these dispersed-track TCs, i.e., composite results for all the TCs, are very similar to the present ones, possibly due to the minority of dispersed-track TCs and the TC-relative framework that is to be introduced in Fig. 3 . The reference day (day 0) is designated when the TC is closest to the point (12.5°N, 125°E; star in Fig. 1 b), where the TC occurrence is the highest. The locations of selected TCs are illustrated from day − 2 to day + 4 (Fig. 2 ). It is found that the TCs generally display a westward track from the Philippine Sea to the South China Sea, but disperse away from their mean position on each day. For instance, the longitudinal extent of TC occurrence exceeds 25° on day 0, which spans from 110° to 135°E (Fig. 2 b). In addition, the deviations keep expanding when the mean location of TCs moves into the South China Sea (Fig. 2 c, d). The large spatial variability of these dispersed TCs could result in composite smearing when the analyses are conducted based on geographic grids. To demonstrate the possible deviation arising from conventional composite, TCs are grouped into two types according to their longitudes on day 0, i.e., 24 TCs are located to the west of 120°E, and 63 ones lie to the east of 120°E, respectively. The 200-hPa geopotential height and wind anomalies relevant to these two types of TCs are shown in Fig. 3 . Here, the results on day + 2 are illustrated when the wavelike anomalies are the clearest. Both types of TCs are associated with a wave train from East Asia to the North Pacific, but the wave train shifts when the mean location of TCs changes. Specifically, for TCs located to the west of 120°E, the alternative anomalies in the upper troposphere are found over the south of China, Japan, and the Bering Sea, respectively (Fig. 3 a). However, the upstream height anomalies shift eastward to the south of Japan when TCs lie over the Philippines (Fig. 3 b). The above result suggests that the wavelike anomalies in the extratropics would be weakened when the composite analyses are performed by geographic grids. Instead, composite analyses conducted in a TC-relative framework are able to maximize the synoptic features of extratropical anomalies induced by TCs (Archambault et al. 2015 ). That is, the fields for a given TC case are shifted such that the center of TC is collocated with the mean point of all TCs on day 0, shown as the star in Fig. 2 b. In the following analyses, the composites are constructed relative to the center of TC, rather than a fixed geographic grid. Figure 4 demonstrates the evolution of 200-hPa geopotential height and wind anomalies associated with all selected TCs based on the TC-relative framework. The result shows the process of how the wavelike anomalies develop in the upper troposphere. The initial disturbances can be traced back to days − 2 and − 1, characterized as positive height anomalies over the East China Sea when TCs are located over the Philippine Sea (Fig. 4 c). Along with the westward movement of TCs, the negative height anomalies are triggered over the Sea of Okhotsk in accompanied with the strengthened positive ones over East Asia on days 0 and + 1 (Fig. 4 d). Hereafter, another cell of positive height anomalies is triggered over the Gulf of Alaska, and the anomalies are manifested as a quasi-stationary wave train and become clearest on days + 2 and + 3 when TCs approach to the South China Sea (Fig. 4 e). The wave-like anomalies resemble the Pacific–Japan pattern induced by the recurving TCs during summer (e.g., Yamada and Kawamura 2007 ; their Fig. 3 ). However, the anomalies locate much southward here, i.e., the center of anomalies over East Asia is located at 40°N in summer but shifts to about 30°N in winter. The difference might be associated with the distinct locations of TCs which stay equatorward here. Afterward, the wave-like anomalies become weakened with the westward movement of TCs (Fig. 4 g–h). Actually, the wave trains can also be triggered by the recurving TCs in winter, but they tend to propagate eastward continuously accompanied by the northward movement of TCs (figure not shown). Figure 5 displays the corresponding evolution of 850-hPa geopotential height and wind anomalies. The result bears some similarity to the evolution of the upper-tropospheric anomalies (Fig. 4 ), but shows remarkable difference over East Asia. There are significant cyclonic anomalies over the Philippine Sea on days − 4 and − 3, which correspond well to the mean location of TCs (Fig. 5 b). After that, the cyclonic anomalies move westward and reach their maximum on days 0 and + 1 (Fig. 5 d). Meanwhile, the negative height anomalies are also found over the vicinity of the Bering Sea, which shift slightly northeastward than the ones at 200 hPa (Fig. 4 d). However, distinguished from the prominent anticyclones over East Asia in the upper troposphere, there is no obvious anomaly in the lower troposphere, which is continuously absent even when the upper-level wave train is well-established on days + 2 and 3 (Fig. 5 e). This is quite different to the Pacific–Japan pattern in summer, which is featured as a succession wave train from the Philippine to the eastern North Pacific in the low level (e.g., Nitta 1987 ; Kawamura and Ogasawara 2006 ). Moreover, the present results suggest that the TC-induced anomalies exhibit different vertical structures over East Asia and the North Pacific. The vertical cross-section of geopotential height anomalies are further examined along the pathway marked in Figs. 4 e and 5 e (Fig. 6 ). The result highlights the notable differences in vertical structures between East Asia and the North Pacific. The significant disturbance is found over East Asia in the upper troposphere on days − 2 and − 1 (Fig. 6 c), and further develops into a wave train that is clearest on days + 2 and + 3 (Fig. 6 e). In addition, the extratropical anomalies extend among the whole troposphere, but the ones in the lower latitude are confined to the upper troposphere. This reveals that the wave train exhibits a barotropic structure over the North Pacific, but a baroclinic one over East Asia. The earlier appearance of anomalies in the upper troposphere and weaker anomalies in the lower troposphere over East Asia suggest that the responses of extratropical circulations to tropical TCs are mainly through the upper troposphere. Moreover, it should be noticed that the downstream anomalies over the North Pacific reach their peak around 300 hPa, but the upstream ones over East Asia are centered at 200 hPa (Fig. 6 f). The difference corresponds well with the distinct heights of tropopause between the tropics and extratropics, where the potential vorticity is equal to 2 potential vorticity units (PVU; 1 PVU = 10 − 6 K kg − 1 m 2 s − 1 ). Hereafter, we focus on the results at 200 hPa to investigate the possible mechanisms responsible for the formation of wave trains induced by TCs. 4 The physical mechanism for the formation of wave train The results in the preceding section explore the wavelike anomalies induced by the straight-moving TCs during winter. In this section, the source of Rossby wave is diagnosed to investigate the possible mechanism and relevant processes responsible for the formation of wave train. Figure 7 demonstrates the evolution of S anomalies at 200 hPa associated with the selected TCs. The remarkable negative anomalies are found over East Asia, corresponding well to the anticyclonic anomalies in the upper troposphere (Fig. 4 ). The anomalous S appears to the east of Japan on days − 4 and − 3, indicating the origin of Rossby waves (Fig. 7 a). Afterward, the anomalies develop rapidly and become conspicuously strong from day − 2 to day + 2, with the minima value exceeding − 2.8×10 − 10 s − 2 (Fig. 7 b–d). The above process is in agreement with the evolution of wavelike anomalies in the upper level (Fig. 4 ), implying that the negative S anomalies can efficiently trigger the anticyclonic anomalies over East Asia and downstream wavelike anomalies. Besides, the other negative S anomalies are found over the WNP, which move westward in conjunction with the migration of TCs. Figure 8 shows the temporal evolution of the S anomalies and budget terms to measure their contributions quantitatively. Here, the results are averaged over the domain (25°–40°N, 115°–145°E; box in Fig. 7 c) where the anomalous S occurs prominently. Based on Eq. (2), the variations of S are determined by stretching ( S1 and S2 ) and advection ( S3 and S4 ) terms, respectively. In general, the wave source stays negative and becomes significant from day − 2 to + 2, with a peak value of − 0.64×10 − 10 s − 2 on day 0. Furthermore, the advection terms are much stronger than the stretching terms, indicating an overwhelming role in generating the wave train. In particular, the evolution of S3 is almost identical to that of the total S , accounting for about 93% of the magnitude during the prominent period from day − 2 to day + 2. Meanwhile, the S4 term also stays negative but is weak. The stretching term ( S1 and S2 ) is also negligible, which spans a range of ±0.1×10 − 10 s − 2 . Therefore, the advection of the climatological vorticity by anomalous divergence ( S3 ) is the dominant term that plays a crucial role in the formation of Rossby waves. In comparison, the result of recurving TCs shows that both S1 and S3 terms contribute to the development of Rossby waves (figure not shown). The different roles of S1 are reasonable as the anomalous divergences are found over East Asia with the northward movement of recurving TCs but stay in the tropics for the straight-moving TCs here. Besides, the other two non-linear terms ( \(-{\varvec{V}}_{\chi }^{{\prime }}\bullet \nabla {\zeta }^{{\prime }}\text{a}\text{n}\text{d} -{\zeta }^{{\prime }}\nabla \cdot {\varvec{V}}_{\chi }^{{\prime }}\) ) are also examined, and they tend to be out of phase, making the sum of them negligible (not shown). The spatial distributions of four budget terms averaged over the prominent period from day − 2 to day + 2 are displayed in Fig. 9 . In agreement with the results in Fig. 8 , the S3 term is almost analogous to the total S anomalies, in terms of both spatial distribution and magnitude. As shown in Fig. 7 , the total S is featured as negative anomalies over East Asia, along with the weaker ones over the WNP. The dominant anomalies of total S in the midlatitude are mainly attributed to S3 ( \(-{\varvec{V}}_{\chi }^{{\prime }}\bullet \nabla \left(f+\stackrel{-}{\zeta }\right)\) ), extending from the east of China to the south of Japan (Fig. 9 c). These negative anomalies are located to the north of TCs, where the climatological westerly jet stream occurs. This implies that the interaction between the westerly jet and TCs might be crucial for generating the Rossby waves. Apart from that, the negative anomalies over East Asia can also be found in S4 ( \(-{\stackrel{-}{\varvec{V}}}_{\chi }\bullet \nabla {\zeta }^{{\prime }}\) ; Fig. 9 d), but are much weaker than that of S3 , consistent with the results shown in Fig. 8 . Besides, the tropical anomalies over the WNP of the total S are attributed to S1 ( \(-\left(f+\stackrel{-}{\zeta }\right)\nabla \cdot {\varvec{V}}_{\chi }^{{\prime }}\) ; Fig. 9 a). These anomalies are located over the region where the TCs reside in the tropics, and thus the vorticity stretching arises from the upper-level divergence anomalies of TCs. Figure 10 further illustrates the composite of 200-hPa divergent wind anomalies and meridional gradient of climatological absolute vorticity, which consists of the S3 term. The result reveals the interference between the climatological westerly jet and TCs. On the one hand, the climatological absolute vorticity is characterized as a strong and poleward gradient along the westerly jet, which is dominated by relative vorticity and modulated slightly by the planetary vorticity. The large gradient suggests that the westerly jet is important to trigger the Rossby wave. On the other hand, the prominent divergent anomalies induced by TCs are found to the south of the westerly jet. The prevailing southerly anomalies generally penetrate into the strong gradient of climatological vorticity, and thus result in negative S anomalies over East Asia (Fig. 9 c). These results suggest the unique feature of the westerly jet in winter, i.e., the prominently strong zonal winds over East Asia, provides a favorable condition to interact with TCs in the tropics. 5 Conclusions and discussion In this study, the extratropical circulation anomalies induced by the straight-moving TCs over the WNP in winter are investigated. It is found that there is a zonally oriented wave train emanating from East Asia to the Gulf of Alaska in the upper troposphere. The wave train can be traced back to the anticyclonic anomalies over East Asia in the upper troposphere. Here, the anomalous anticyclones almost coincide with the climatological westerly jet, suggesting a possible interaction between the mean flows and synoptic disturbances. Subsequently, the downstream anomalies are triggered and a clear wave train is formed, accompanied by the westward movement of TCs. Meanwhile, the lower-tropospheric wave anomalies are similar to their upper-level counterparts over the North Pacific, i.e., in a barotropic structure, but are much vaguer over East Asia. Further analyses are conducted to diagnose the physical mechanism whereby the TCs excite wave trains in the extratropics. There are remarkable negative Rossby wave sources over East Asia, which correspond well with the anomalous anticyclones in the upper troposphere. Moreover, the contributions of budget terms in the Rossby wave source are quantified. The results show that the advection terms play a vital role in the formation of wave trains, while the stretching terms are negligible. Additionally, the advection terms mainly result from the climatological vorticity advection by anomalous divergent winds ( \(-{\varvec{V}}_{\chi }^{{\prime }}\bullet \nabla \stackrel{-}{\zeta }\) ). Specifically, the climatological absolute vorticity shows a strong poleward gradient near the westerly jet over East Asia, where the prominent anomalous divergent winds induced by TCs are dominated. The above process suggests that the interaction between the TCs and climatological westerly jet results in negative wave source anomalies over East Asia, which is crucial for the formation of wave trains. The straight-moving TCs during winter are focused on in this study. The winter season is special or typical for straight-moving TCs to trigger the wavelike anomalies over the extratropics since the East Asian jet is strongest and located most equatorward in this season. In this regard, it provides a favorable condition for the straight-moving TCs to affect the extratropical flows without entering into the mid-latitude or transition to extratropical cyclones. In addition, the wave anomalies triggered by the straight-moving TCs are distinct from those accompanied by the recurving TCs. On the one hand, the wave train in the present work is quasi-stationary and extends across the North Pacific along the great circle path, while the one responding to recurving TCs propagates eastward along the westerly jet (Archambault et al. 2015 , their Figs. 3 and 4 ). On the other hand, the diagnosis implies different processes in the formation of Rossby wave source. Compared to the comparable roles of both stretching and advection terms for the recurving TCs, the generation of wave train is dominant by the advection term relevant to the anomalous divergent winds induced by the straight-moving TCs. In this study, the East Asian westerly jet is supposed to be the climatological mean when the Rossby wave source is calculated, to simplify the explanation of the Rossby wave source formation over East Asia. This is reasonable, at least in the climatological sense or for the composite features, considering that the terms ( \(-{\varvec{V}}_{\chi }^{{\prime }}\bullet \nabla {\zeta }^{{\prime }}\) and \(-{\zeta }^{{\prime }}\nabla \cdot {\varvec{V}}_{\chi }^{{\prime }}\) ) are weak and thus the feedback of TC-induced jet anomalies (indicated by \({\zeta }^{{\prime }}\) ) on the Rossby wave source is weak. However, in real or at the synoptic timescales, the East Asian westerly jet varies greatly in intensity, location, and shape (e.g., Yang et al. 2002 ). Thus, the TC-induced Rossby wave source over East Asia may depend significantly on the specific features of East Asian jet during the appearance of TCs in the WNP. This issue needs to be further investigated. Finally, the composite results suggest the extratropical wavelike anomalies are mainly over East Asia and the North Pacific, but weak over North America. This cannot exclude the possibility that in some cases, the straight-moving TCs in the WNP can trigger the wavelike anomalies that propagate into North America. In addition, the waves triggered by TCs may interact with short waves over the North Pacific, which may be induced by the internal atmospheric variability (e.g., Branstator 2002 ) or affected by El Niño–Southern Oscillation (e.g., Chen 2002 ). If this is true, the straight-moving TCs may play a non-negligible role in affecting the extended weather forecast over North America. Declarations Funding This work was supported by the National Natural Science Foundation of China (Grant No. 42005029). Author contributions RYL and BP contributed to the study conception and design. Data collection and analysis were performed by SQM. SQM and BP wrote the manuscript. RYL and XYZ provided intellectual contributions to the improvement of this manuscript. Data a vailability All the data used in this study are openly available. The ERA5 data are obtained from https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5. The TC best-track data of CMA are retrieved from https://tcdata.typhoon.org.cn/en/zjljsjj_zlhq.html. Conflict of interest The authors have no relevant financial or non-financial interests to disclose. References Agustí-Panareda A, Gray SL, Craig GC, Thorncroft C (2005) The extratropical transition of tropical cyclone Lili (1996) and its crucial contribution to a moderate extratropical development. Mon Weather Rev 133(6):1562–1573. https://doi.org/10.1175/MWR2935.1 Aiyyer A (2015) Recurving western North Pacific tropical cyclones and midlatitude predictability. 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Mon Weather Rev 147:1077–1106. https://doi.org/10.1175/MWR-D-17-0329.1 Kim H-S, Kim J-H, Ho C-H, Chu P-S (2011) Pattern classification of typhoon tracks using the fuzzy c -means clustering method. J Clim 24:488–508. https://doi.org/10.1175/2010JCLI3751.1 Kunkel KE, Pielke RA, Changnon SA (1999) Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: A review. Bull Amer Meteor Soc 80:1077–1098. https://doi.org/10.1175/1520-0477(1999)0802.0.Co;2 Lu R, Kim B-J (2004) The climatological Rossby wave source over the STCZs in the summer Northern Hemisphere. J Meteorol Soc Japan 82:657–669. https://doi.org/10.2151/jmsj.2004.657 Lu X, Yu H, Ying M, Zhao B, Zhang S, Lin L, Bai L, Wan R (2021) Western North Pacific tropical cyclone database created by the China Meteorological Administration. 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Mon Weather Rev 146:1283–1301. https://doi.org/10.1175/MWR-D-17-0219.1 Riemer M, Jones SC (2010) The downstream impact of tropical cyclones on a developing baroclinic wave in idealized scenarios of extratropical transition. Q J R Meteorol Soc 136:617–637. https://doi.org/10.1002/qj.605 Sardeshmukh PD, Hoskins BJ (1988) The generation of global rotational flow by steady idealized tropical divergence. J Atmos Sci 45:1228–1251. https://doi.org/10.1175/1520-0469(1988)0452.0.CO;2 Stuivenvolt Allen J, Wang S-YS (2023) North American fire weather catalyzed by the extratropical transition of tropical cyclones. Clim Dyn 61:65–78. https://doi.org/10.1007/s00382-022-06561-1 Stuivenvolt Allen J, Wang S-YS, LaPlante MD, Yoon J-H (2021) Three western pacific typhoons strengthened fire weather in the recent northwest U.S. conflagration. Geophys Res Lett 48(3):e2020GL091430. https://doi.org/10.1029/2020GL091430 Torn RD (2017) A comparison of the downstream predictability associated with ET and baroclinic cyclones. Mon Weather Rev 145:4651–4672. https://doi.org/10.1175/MWR-D-17-0083.1 Wilks DS (2006) Statistical methods in the atmospheric sciences, 2nd edn. Elsevier Publishers, New York Wu L, Wang B (2004) Assessing impacts of global warming on tropical cyclone tracks . J Clim 17 :1686–1698. https://doi.org/10.1175/1520-0442(2004)0172.0.CO;2 Yamada K, Kawamura R (2007) Dynamical link between typhoon activity and the PJ teleconnection pattern from early summer to autumn as revealed by the JRA-25 reanalysis. Sci Online Lett Atmos 3:65–68. https://doi.org/10.2151/sola.2007-017 Yang S, Lau KM, Kim KM (2002) Variations of the East Asian jet stream and Asian–Pacific–American winter climate anomalies. J Clim 15:306–325. https://doi.org/10.1175/1520-0442(2002)0152.0.CO;2 Ying M, Zhang W, Yu H, Lu X, Feng J, Fan Y, Zhu Y, Chen D (2014) An overview of the China Meteorological Administration tropical cyclone database. J Atmos Oceanic Technol 31:287–301. https://doi.org/10.1175/JTECH-D-12-00119.1 Zhang Y, Kuang X, Guo W, Zhou T (2006) Seasonal evolution of the upper-tropospheric westerly jet core over East Asia. Geophys Res Lett 33:L11708. https://doi.org/10.1029/2006gl026377 Zwiers FW, Vonstorch H (1995) Taking serial-correlation into account in tests of the mean. J Clim 8:336–351. https://doi.org/10.1175/1520-0442(1995)0082.0.Co;2 Cite Share Download PDF Status: Published Journal Publication published 29 Oct, 2024 Read the published version in Climate Dynamics → Version 1 posted Editorial decision: Major Revision 12 May, 2024 Reviewers agreed at journal 08 Apr, 2024 Reviewers invited by journal 06 Apr, 2024 Editor assigned by journal 28 Mar, 2024 First submitted to journal 28 Mar, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4181050","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":288300590,"identity":"6ee81a0a-76d0-49b7-8242-0129495bbb2b","order_by":0,"name":"Shuaiqiong Ma","email":"","orcid":"","institution":"Institute of Atmospheric Physics Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Shuaiqiong","middleName":"","lastName":"Ma","suffix":""},{"id":288300591,"identity":"6fafb897-c26e-4c5c-9aab-793126dec835","order_by":1,"name":"Bo Pang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6ElEQVRIiWNgGAWjYJACZgYGGyDJ3MCQQIKWNCDJSJqWw0AKqIUoYM5+9vDrgorz0fztQC0Pau4w8LcfYPxcgEeLZU9emvWMM7dzZxwGOezYMwaJMwnM0jPwaDE4kGNmzNt2O7cBrIUN6MIbDGzMPPi0nH8D0nIudz5Yy7/DDPIEtdzIMX7M23YgdwNIS2LbYaAIAS2WM96YMc84k5y7EajlQGLfMx7DM4nN0vi0mPPnGH8uqLDLnXf+8MGHP77dkZM7fvjgZ7wOY2Bgk4BxDgARD8EIAmph/oDEP4BX9SgYBaNgFIxMAAAmj080SGoWuwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-3502-9797","institution":"Institute of Atmospheric Physics Chinese Academy of Sciences","correspondingAuthor":true,"prefix":"","firstName":"Bo","middleName":"","lastName":"Pang","suffix":""},{"id":288300592,"identity":"777c1390-e60d-4457-88ed-599129301052","order_by":2,"name":"Riyu Lu","email":"","orcid":"","institution":"Institute of Atmospheric Physics Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Riyu","middleName":"","lastName":"Lu","suffix":""},{"id":288300593,"identity":"ccffd86f-7b54-48fb-8194-f31cb364954d","order_by":3,"name":"Xingyan Zhou","email":"","orcid":"","institution":"China Meteorological Administration","correspondingAuthor":false,"prefix":"","firstName":"Xingyan","middleName":"","lastName":"Zhou","suffix":""}],"badges":[],"createdAt":"2024-03-28 09:19:00","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4181050/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4181050/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00382-024-07462-1","type":"published","date":"2024-10-29T16:12:51+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":54511198,"identity":"13d3ea1b-1d85-48c9-8f56-f1813b47d541","added_by":"auto","created_at":"2024-04-11 15:31:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":107953,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Tracks and \u003cstrong\u003eb\u003c/strong\u003eoccurrence (interval: 20) of all straight-moving tropical cyclones (TCs) during winter. \u003cstrong\u003ec\u003c/strong\u003e–\u003cstrong\u003ed\u003c/strong\u003e as \u003cstrong\u003ea\u003c/strong\u003e, but for the TCs passing and not passing through the key area. The blue (red) dots represent TC genesis (extinction) locations. The numbers of TCs are included in the upper-right corners. The box and star in \u003cstrong\u003eb\u003c/strong\u003e refer to the key area and point of the highest TC occurrence, respectively\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/2a2b576d08c5527ea5608ed5.png"},{"id":54511200,"identity":"cb38db4b-cf64-458d-8058-2b95a7552b09","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":65316,"visible":true,"origin":"","legend":"\u003cp\u003eThe locations of selected TCs from day −2 to day +4. The stars denote the mean locations of TCs. The reference day (day 0) is designated when the TC is closest to the point (12.5°N, 125°E; star in Fig. 1b)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/7f3506e72c045e2cd6e88e63.png"},{"id":54511202,"identity":"a1d2bdb0-8735-42e1-a76a-895a0a774ff7","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":90656,"visible":true,"origin":"","legend":"\u003cp\u003eComposites of 200-hPa geopotential height (\u003cem\u003eZ\u003c/em\u003e; shading; units: m) and horizontal wind (vectors; units: m s\u003csup\u003e−1\u003c/sup\u003e) anomalies on day +2 for the TCs located to the \u003cstrong\u003ea\u003c/strong\u003e west and \u003cstrong\u003eb\u003c/strong\u003e east of 120°E shown in Fig. 2b. Shadings significant at the 95% confidence level are stippled and vectors are shown as thick and black when they are significant in at least one direction. The stars denote the mean locations of the TCs and the numbers of cases are shown in the upper-right corners\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/e262093cabc23b841ddd4c4c.png"},{"id":54511203,"identity":"9b5dd557-3fa8-4fe8-a4c1-6bc1a7b23fef","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":264009,"visible":true,"origin":"","legend":"\u003cp\u003eComposites of 200-hPa \u003cem\u003eZ \u003c/em\u003e(shading; units: m) and horizontal wind (vectors; units: m s\u003csup\u003e−1\u003c/sup\u003e) anomalies relative to TC centers. Shadings significant at the 95% confidence level are stippled and vectors are shown as thick and black when they are significant in at least one direction. The stars indicate the mean locations of TCs. The red curve in \u003cstrong\u003ee\u003c/strong\u003e represents the pathway of wave trains\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/0682109bad5458184e46491e.png"},{"id":54511207,"identity":"23960dbe-a874-4f8d-a242-4d537951bf96","added_by":"auto","created_at":"2024-04-11 15:31:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":225052,"visible":true,"origin":"","legend":"\u003cp\u003eAs Fig. 4, but for the results at 850 hPa\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/11b30a6c093dec5ea70bad17.png"},{"id":54511206,"identity":"4985dfa2-166c-49bc-9efb-a23053b434ed","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":153202,"visible":true,"origin":"","legend":"\u003cp\u003eComposite vertical profiles of\u003cem\u003e Z\u003c/em\u003e anomalies (units: m) along the pathway shown in Figs. 4e and 5e. Values significant at the 95% confidence level are stippled. The green line in \u003cstrong\u003ee\u003c/strong\u003e indicates the winter-mean dynamical tropopause identified as the level of 2 potential vorticity units (PVU; 1 PVU=10\u003csup\u003e−6\u003c/sup\u003e K kg\u003csup\u003e−1\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e−1\u003c/sup\u003e)\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/24546dbd9e48dd21e44ac250.png"},{"id":54511199,"identity":"6aa68589-2952-45d9-8671-8d80de112584","added_by":"auto","created_at":"2024-04-11 15:31:52","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":130074,"visible":true,"origin":"","legend":"\u003cp\u003eComposites of 200-hPa Rossby wave source (\u003cem\u003eS\u003c/em\u003e; units: 10\u003csup\u003e−10\u003c/sup\u003e s\u003csup\u003e−2\u003c/sup\u003e) anomalies. Values significant at the 95% confidence level are stippled. The stars indicate the mean locations of TCs. The box in \u003cstrong\u003ec\u003c/strong\u003e indicates the area (25°–40°N, 115°–145°E)\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/b5c9bb8ec9fe210bc2500b31.png"},{"id":54511204,"identity":"bda9b477-ec28-4a54-bfdf-71082d9fbe36","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":16371,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal evolution of area-averaged \u003cem\u003eS\u003c/em\u003e anomalies\u003cem\u003e \u003c/em\u003e(black) and budget terms (colored) in Eq. (2) (units: 10\u003csup\u003e−10\u003c/sup\u003e s\u003csup\u003e−2\u003c/sup\u003e). Thick segments represent values significant at the 95% confidence level\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/4ad0f4e6c588c92a4bd3b268.png"},{"id":54511205,"identity":"f631e568-e245-4837-a6f2-5c896df5ebb4","added_by":"auto","created_at":"2024-04-11 15:31:53","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":84773,"visible":true,"origin":"","legend":"\u003cp\u003eComposites of 200-hPa \u003cstrong\u003ea\u003c/strong\u003e \u003cem\u003eS1\u003c/em\u003e, \u003cstrong\u003eb\u003c/strong\u003e \u003cem\u003eS2\u003c/em\u003e, \u003cstrong\u003ec\u003c/strong\u003e \u003cem\u003eS3\u003c/em\u003e, and \u003cstrong\u003ed\u003c/strong\u003e \u003cem\u003eS4\u003c/em\u003e (unit: 10\u003csup\u003e−10\u003c/sup\u003e s\u003csup\u003e−2\u003c/sup\u003e) averaged from day −2 to day +2. Values significant at the 95% confidence level are stippled\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/797e35e9d3616f9af74cd2db.png"},{"id":54511631,"identity":"c60fa9b3-48b7-4c4f-963b-5d07127f7032","added_by":"auto","created_at":"2024-04-11 15:39:54","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":57098,"visible":true,"origin":"","legend":"\u003cp\u003eComposite of 200-hPa divergent wind anomaly (vectors; unit: m s\u003csup\u003e−1\u003c/sup\u003e) and meridional gradients of climatological absolute vorticity (shading; unit: 10\u003csup\u003e−11\u003c/sup\u003e m\u003csup\u003e−1\u003c/sup\u003e s\u003csup\u003e−1\u003c/sup\u003e) averaged over day −2 to day +2. Contours denote the climatological westerly jet (interval: 10 m s\u003csup\u003e−1\u003c/sup\u003e). Vectors are shown as thick and black when they are significant at the 95% confidence level in at least one direction\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/42eea61539fd3d5256f6b861.png"},{"id":68206320,"identity":"8071b5e2-12c7-4d28-9901-d8e9d4448a9f","added_by":"auto","created_at":"2024-11-04 16:30:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1527418,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4181050/v1/a803e23b-3fb2-4718-99cb-3ac36d32f918.pdf"}],"financialInterests":"","formattedTitle":"Straight-moving tropical cyclones over the western North Pacific trigger the wave trains over the North Pacific during winter","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe western North Pacific (WNP) is the most prolific tropical cyclone (TC) basin, accounting for about 30% of all TCs globally (Wu and Wang \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). TCs over this region induce tremendous rainfall and destructive winds along their pathways due to their eyewall structure and spiral rain bands (e.g., Kunkel et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1999\u003c/span\u003e), and the precipitation induced by TCs accounts for a large proportion of the total annual amounts in the coastal regions, such as Japan (e.g., Kamahori and Arakawa \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), southern China (e.g., Ren et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), and Southeast Asia (e.g., Nguyen-Thi et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In addition, TCs can also provoke precipitation over the areas that are far away from the TC centers through the transportation of warm moisture (e.g., Galarneau et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). For instance, the unprecedented rainfall that occurred on 20 July 2021 in Zhengzhou (35\u0026deg;N, 113\u0026deg;E), an inland city in China, was attributed to Typhoon In-Fa and Cempaka which were active over the WNP and South China Sea, respectively (Deng et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe tracks of TCs over the WNP can be generally categorized into straight-moving and recurving TCs (e.g., Kim et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The recurving TCs, when they approach the extratropical upper-tropospheric westerly jets, can interact with the jets and exert pronounced impacts on the weather and climate of extratropical regions. There is a significant increase in Rossby wave amplitude and occurrence frequency in the North Pacific, documented by case studies (Agust\u0026iacute;-Panareda et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Harr and Dea \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Grams et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2013a\u003c/span\u003e; Keller et al. 2014) and statistical studies (Archambault et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Quinting and Jones \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Riboldi et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Then, the westerly jets further serve as a waveguide and disperse the kinetic energy to downstream areas by promoting the subsequent cyclogenesis along the waveguide (Riemer and Jones \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Archambault et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). As a result, the interaction between the recurving TCs and westerly jets contributes to the amplification of synoptic transient eddy activity over the North Pacific (Ha et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) and modulates weather over North America (Bosart et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Stuivenvolt-Allen et al. 2021; Stuivenvolt-Allen and Wang 2023), causing the enhanced forecast uncertainty in downstream regions (Aiyyer \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Torn \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe influence on downstream flows is associated with TCs recurving into midlatitudes and undergoing extratropical transition (Jones et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). It is reported that the diabatic processes play a key role in modifying the extratropical circulation, as the deep warm core of TC is replaced by a cold and asymmetric structure when it moves poleward into a baroclinic environment (Riemer and Jones \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Grams et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Keller et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Then, the latent heat release in ascending air of TCs yields a transport of low potential vorticity to the tropopause, which further amplifies the upper-level waves (e.g., Grams et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2013b\u003c/span\u003e; Grams and Archambault \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Evans et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Riboldi et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe majority of previous studies focused on the impacts of recurving TCs on the extratropical circulation in summer and autumn, which are the main seasons of TC occurrences over the WNP. Moreover, relatively more TCs tend to be recurving in these seasons in comparison with the other two seasons, i.e., winter and spring (Archambault et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). However, the influence of straight-moving TCs on extratropical circulation has yet to be examined. It was documented that the straight-moving TCs in summer can hardly trigger organized wave anomalies, as they are far away from the upper-tropospheric westerly jet (Hirata and Kawamura \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). However, the westerly jet is strongest over East Asia and locates most southward in winter (Zhang et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Thus, it provides a probability of strong interaction between the jet and TCs, even the straight-moving TCs that do not approach the jet. To verify this hypothesis, we focus on the impacts of straight-moving TCs over the WNP on the extratropical circulation in winter.\u003c/p\u003e \u003cp\u003eIn the following parts, section 2 presents the data and methods. Section 3 demonstrates the evolutions of circulation anomalies relevant to the straight-moving TCs over the WNP. Section 4 further explores the corresponding mechanism of how the TCs induce extratropical anomalies. Conclusions and discussion are given in section 5.\u003c/p\u003e"},{"header":"2 Data and Methods","content":"\u003cp\u003eThe daily-mean atmospheric reanalysis data are obtained by the average of 6-hourly data derived from the European Center for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5; Hersbach et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), with a horizontal resolution of 0.75\u0026deg;\u0026times;0.75\u0026deg;. The analysis period of this study is 1979\u0026ndash;2020 and the daily anomalies are calculated by removing the 42-yr mean of the calendar day from the raw data.\u003c/p\u003e \u003cp\u003eThe TC best-track data are obtained from the Shanghai Typhoon Institute of the China Meteorological Administration (CMA) with a 6-hourly interval (Ying et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Lu et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In this study, TCs are analyzed when the maximum surface wind speed exceeds 17.2 m/s, i.e., at tropical storm intensity or above. The straight-moving TC is identified when its extinction point is located at the most western point of the entire TC track. Accordingly, 100 straight-moving TCs are identified, and all of them completely dissipate before entering the mid-latitude. In this study, the winter is defined as November\u0026ndash;January (NDJ), which is one month earlier than the more widely used one, i.e., December\u0026ndash;February. The modification is conducted as TCs are relatively active in early winter but rarely appear in February.\u003c/p\u003e \u003cp\u003eThe lead-lag composites are performed to analyze the large-scale circulation associated with TCs. The statistical significance is examined by using a two-tailed Student\u0026rsquo;s \u003cem\u003et\u003c/em\u003e-test and the comparison is conducted between the composite days and the remainder in winter (Wilks \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The effective degree of freedom (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({N}^{\\text{*}}\\)\u003c/span\u003e\u003c/span\u003e) is computed as (Zwiers and von Storch 1995):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${N}^{*}=N\\frac{1-{r}_{1}}{1+{r}_{1}}, \\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(N\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{1}\\)\u003c/span\u003e\u003c/span\u003e are the original sample size and the lag-one autocorrelation coefficient, respectively.\u003c/p\u003e \u003cp\u003eThe Rossby wave source (\u003cem\u003eS\u003c/em\u003e) is utilized to diagnose the Rossby wave generation excited by diabatic heating, and its perturbation can be written as (Sardeshmukh and Hoskins \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1988\u003c/span\u003e):\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$${S}^{{\\prime }}=-\\left(f+\\stackrel{-}{\\zeta }\\right)\\nabla \\cdot {\\varvec{V}}_{\\chi }^{{\\prime }}-{\\zeta }^{{\\prime }}\\nabla \\cdot {\\stackrel{-}{\\varvec{V}}}_{\\chi }-{\\varvec{V}}_{\\chi }^{{\\prime }}\\cdot \\nabla \\left(f+\\stackrel{-}{\\zeta }\\right)-{\\stackrel{-}{\\varvec{V}}}_{\\chi }\\bullet \\nabla {\\zeta }^{{\\prime }}, \\left(2\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\zeta\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(f\\)\u003c/span\u003e\u003c/span\u003eand\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({ \\varvec{V}}_{\\chi }\\)\u003c/span\u003e\u003c/span\u003e are relative vorticity, planetary vorticity, and divergent wind, respectively;\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\nabla\\)\u003c/span\u003e\u003c/span\u003erepresents the horizontal gradient; overbar and prime denote the climatology and anomalies, respectively. Eq.\u0026nbsp;(2) shows that the anomalous \u003cem\u003eS\u003c/em\u003e consists of vortex stretching and advection (Lu and Kim \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Specifically, the first two terms on the right-hand side denote the vortex stretching forced by climatological vorticity and anomalous divergence, and by anomalous vorticity and climatological divergence, respectively. The other two terms represent the climatological vorticity advection by anomalous divergent flow, and the anomalous vorticity advection by climatological divergent wind, respectively. Hereafter, the above four terms are referred to as \u003cem\u003eS1\u003c/em\u003e to \u003cem\u003eS4\u003c/em\u003e by turns. In this study, the \u003cem\u003eS1\u003c/em\u003e is masked out where the anomalous divergence is weak (i.e., within \u0026plusmn;\u0026thinsp;5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), as the disordered divergences are enlarged by strong planetary vorticity in the extratropics.\u003c/p\u003e"},{"header":"3 Circulation anomalies relevant to the straight-moving TCs","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the tracks of the 100 straight-moving TCs. In general, most TCs are formed over the WNP, then move westward to the South China Sea, and finally decay over the Indo-China Peninsula. Additionally, the spatial distribution of TC activity, counted as the numbers in each 2.5\u0026deg;\u0026times;2.5\u0026deg; latitude\u0026ndash;longitude grid, reveals that the majority of TCs are active over the Philippine Sea and South China Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). It should be noticed that some TCs are far away from the active region. Thus, the key area is selected as (10\u0026deg;\u0026ndash;17.5\u0026deg;N, 110\u0026deg;\u0026ndash;135\u0026deg;E; box in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The 87 TCs passing through the key area are selected (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), while the other 13 TCs are excluded in the following analyses (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). However, it should be mentioned that the composite results including these dispersed-track TCs, i.e., composite results for all the TCs, are very similar to the present ones, possibly due to the minority of dispersed-track TCs and the TC-relative framework that is to be introduced in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The reference day (day 0) is designated when the TC is closest to the point (12.5\u0026deg;N, 125\u0026deg;E; star in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), where the TC occurrence is the highest.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe locations of selected TCs are illustrated from day \u0026minus;\u0026thinsp;2 to day\u0026thinsp;+\u0026thinsp;4 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). It is found that the TCs generally display a westward track from the Philippine Sea to the South China Sea, but disperse away from their mean position on each day. For instance, the longitudinal extent of TC occurrence exceeds 25\u0026deg; on day 0, which spans from 110\u0026deg; to 135\u0026deg;E (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). In addition, the deviations keep expanding when the mean location of TCs moves into the South China Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, d). The large spatial variability of these dispersed TCs could result in composite smearing when the analyses are conducted based on geographic grids. To demonstrate the possible deviation arising from conventional composite, TCs are grouped into two types according to their longitudes on day 0, i.e., 24 TCs are located to the west of 120\u0026deg;E, and 63 ones lie to the east of 120\u0026deg;E, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe 200-hPa geopotential height and wind anomalies relevant to these two types of TCs are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Here, the results on day\u0026thinsp;+\u0026thinsp;2 are illustrated when the wavelike anomalies are the clearest. Both types of TCs are associated with a wave train from East Asia to the North Pacific, but the wave train shifts when the mean location of TCs changes. Specifically, for TCs located to the west of 120\u0026deg;E, the alternative anomalies in the upper troposphere are found over the south of China, Japan, and the Bering Sea, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). However, the upstream height anomalies shift eastward to the south of Japan when TCs lie over the Philippines (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The above result suggests that the wavelike anomalies in the extratropics would be weakened when the composite analyses are performed by geographic grids. Instead, composite analyses conducted in a TC-relative framework are able to maximize the synoptic features of extratropical anomalies induced by TCs (Archambault et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). That is, the fields for a given TC case are shifted such that the center of TC is collocated with the mean point of all TCs on day 0, shown as the star in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. In the following analyses, the composites are constructed relative to the center of TC, rather than a fixed geographic grid.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e demonstrates the evolution of 200-hPa geopotential height and wind anomalies associated with all selected TCs based on the TC-relative framework. The result shows the process of how the wavelike anomalies develop in the upper troposphere. The initial disturbances can be traced back to days \u0026minus;\u0026thinsp;2 and \u0026minus;\u0026thinsp;1, characterized as positive height anomalies over the East China Sea when TCs are located over the Philippine Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Along with the westward movement of TCs, the negative height anomalies are triggered over the Sea of Okhotsk in accompanied with the strengthened positive ones over East Asia on days 0 and +\u0026thinsp;1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Hereafter, another cell of positive height anomalies is triggered over the Gulf of Alaska, and the anomalies are manifested as a quasi-stationary wave train and become clearest on days\u0026thinsp;+\u0026thinsp;2 and +\u0026thinsp;3 when TCs approach to the South China Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). The wave-like anomalies resemble the Pacific\u0026ndash;Japan pattern induced by the recurving TCs during summer (e.g., Yamada and Kawamura \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; their Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the anomalies locate much southward here, i.e., the center of anomalies over East Asia is located at 40\u0026deg;N in summer but shifts to about 30\u0026deg;N in winter. The difference might be associated with the distinct locations of TCs which stay equatorward here. Afterward, the wave-like anomalies become weakened with the westward movement of TCs (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg\u0026ndash;h). Actually, the wave trains can also be triggered by the recurving TCs in winter, but they tend to propagate eastward continuously accompanied by the northward movement of TCs (figure not shown).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e displays the corresponding evolution of 850-hPa geopotential height and wind anomalies. The result bears some similarity to the evolution of the upper-tropospheric anomalies (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), but shows remarkable difference over East Asia. There are significant cyclonic anomalies over the Philippine Sea on days \u0026minus;\u0026thinsp;4 and \u0026minus;\u0026thinsp;3, which correspond well to the mean location of TCs (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). After that, the cyclonic anomalies move westward and reach their maximum on days 0 and +\u0026thinsp;1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Meanwhile, the negative height anomalies are also found over the vicinity of the Bering Sea, which shift slightly northeastward than the ones at 200 hPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). However, distinguished from the prominent anticyclones over East Asia in the upper troposphere, there is no obvious anomaly in the lower troposphere, which is continuously absent even when the upper-level wave train is well-established on days\u0026thinsp;+\u0026thinsp;2 and 3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee). This is quite different to the Pacific\u0026ndash;Japan pattern in summer, which is featured as a succession wave train from the Philippine to the eastern North Pacific in the low level (e.g., Nitta \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Kawamura and Ogasawara \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Moreover, the present results suggest that the TC-induced anomalies exhibit different vertical structures over East Asia and the North Pacific.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe vertical cross-section of geopotential height anomalies are further examined along the pathway marked in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The result highlights the notable differences in vertical structures between East Asia and the North Pacific. The significant disturbance is found over East Asia in the upper troposphere on days \u0026minus;\u0026thinsp;2 and \u0026minus;\u0026thinsp;1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), and further develops into a wave train that is clearest on days\u0026thinsp;+\u0026thinsp;2 and +\u0026thinsp;3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee). In addition, the extratropical anomalies extend among the whole troposphere, but the ones in the lower latitude are confined to the upper troposphere. This reveals that the wave train exhibits a barotropic structure over the North Pacific, but a baroclinic one over East Asia. The earlier appearance of anomalies in the upper troposphere and weaker anomalies in the lower troposphere over East Asia suggest that the responses of extratropical circulations to tropical TCs are mainly through the upper troposphere. Moreover, it should be noticed that the downstream anomalies over the North Pacific reach their peak around 300 hPa, but the upstream ones over East Asia are centered at 200 hPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). The difference corresponds well with the distinct heights of tropopause between the tropics and extratropics, where the potential vorticity is equal to 2 potential vorticity units (PVU; 1 PVU\u0026thinsp;=\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e K kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Hereafter, we focus on the results at 200 hPa to investigate the possible mechanisms responsible for the formation of wave trains induced by TCs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4 The physical mechanism for the formation of wave train","content":"\u003cp\u003eThe results in the preceding section explore the wavelike anomalies induced by the straight-moving TCs during winter. In this section, the source of Rossby wave is diagnosed to investigate the possible mechanism and relevant processes responsible for the formation of wave train. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e demonstrates the evolution of \u003cem\u003eS\u003c/em\u003e anomalies at 200 hPa associated with the selected TCs. The remarkable negative anomalies are found over East Asia, corresponding well to the anticyclonic anomalies in the upper troposphere (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The anomalous \u003cem\u003eS\u003c/em\u003e appears to the east of Japan on days \u0026minus;\u0026thinsp;4 and \u0026minus;\u0026thinsp;3, indicating the origin of Rossby waves (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). Afterward, the anomalies develop rapidly and become conspicuously strong from day \u0026minus;\u0026thinsp;2 to day\u0026thinsp;+\u0026thinsp;2, with the minima value exceeding \u0026minus;\u0026thinsp;2.8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb\u0026ndash;d). The above process is in agreement with the evolution of wavelike anomalies in the upper level (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), implying that the negative \u003cem\u003eS\u003c/em\u003e anomalies can efficiently trigger the anticyclonic anomalies over East Asia and downstream wavelike anomalies. Besides, the other negative \u003cem\u003eS\u003c/em\u003e anomalies are found over the WNP, which move westward in conjunction with the migration of TCs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the temporal evolution of the \u003cem\u003eS\u003c/em\u003e anomalies and budget terms to measure their contributions quantitatively. Here, the results are averaged over the domain (25\u0026deg;\u0026ndash;40\u0026deg;N, 115\u0026deg;\u0026ndash;145\u0026deg;E; box in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec) where the anomalous \u003cem\u003eS\u003c/em\u003e occurs prominently. Based on Eq.\u0026nbsp;(2), the variations of \u003cem\u003eS\u003c/em\u003e are determined by stretching (\u003cem\u003eS1\u003c/em\u003e and \u003cem\u003eS2\u003c/em\u003e) and advection (\u003cem\u003eS3\u003c/em\u003e and \u003cem\u003eS4\u003c/em\u003e) terms, respectively. In general, the wave source stays negative and becomes significant from day \u0026minus;\u0026thinsp;2 to +\u0026thinsp;2, with a peak value of \u0026minus;\u0026thinsp;0.64\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e on day 0. Furthermore, the advection terms are much stronger than the stretching terms, indicating an overwhelming role in generating the wave train. In particular, the evolution of \u003cem\u003eS3\u003c/em\u003e is almost identical to that of the total \u003cem\u003eS\u003c/em\u003e, accounting for about 93% of the magnitude during the prominent period from day \u0026minus;\u0026thinsp;2 to day\u0026thinsp;+\u0026thinsp;2. Meanwhile, the \u003cem\u003eS4\u003c/em\u003e term also stays negative but is weak. The stretching term (\u003cem\u003eS1\u003c/em\u003e and \u003cem\u003eS2\u003c/em\u003e) is also negligible, which spans a range of \u0026plusmn;0.1\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. Therefore, the advection of the climatological vorticity by anomalous divergence (\u003cem\u003eS3\u003c/em\u003e) is the dominant term that plays a crucial role in the formation of Rossby waves. In comparison, the result of recurving TCs shows that both \u003cem\u003eS1\u003c/em\u003e and \u003cem\u003eS3\u003c/em\u003e terms contribute to the development of Rossby waves (figure not shown). The different roles of \u003cem\u003eS1\u003c/em\u003e are reasonable as the anomalous divergences are found over East Asia with the northward movement of recurving TCs but stay in the tropics for the straight-moving TCs here. Besides, the other two non-linear terms (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\varvec{V}}_{\\chi }^{{\\prime }}\\bullet \\nabla {\\zeta }^{{\\prime }}\\text{a}\\text{n}\\text{d} -{\\zeta }^{{\\prime }}\\nabla \\cdot {\\varvec{V}}_{\\chi }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e) are also examined, and they tend to be out of phase, making the sum of them negligible (not shown).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe spatial distributions of four budget terms averaged over the prominent period from day \u0026minus;\u0026thinsp;2 to day\u0026thinsp;+\u0026thinsp;2 are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. In agreement with the results in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the \u003cem\u003eS3\u003c/em\u003e term is almost analogous to the total \u003cem\u003eS\u003c/em\u003e anomalies, in terms of both spatial distribution and magnitude. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, the total \u003cem\u003eS\u003c/em\u003e is featured as negative anomalies over East Asia, along with the weaker ones over the WNP. The dominant anomalies of total \u003cem\u003eS\u003c/em\u003e in the midlatitude are mainly attributed to \u003cem\u003eS3\u003c/em\u003e (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\varvec{V}}_{\\chi }^{{\\prime }}\\bullet \\nabla \\left(f+\\stackrel{-}{\\zeta }\\right)\\)\u003c/span\u003e\u003c/span\u003e), extending from the east of China to the south of Japan (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec). These negative anomalies are located to the north of TCs, where the climatological westerly jet stream occurs. This implies that the interaction between the westerly jet and TCs might be crucial for generating the Rossby waves. Apart from that, the negative anomalies over East Asia can also be found in \u003cem\u003eS4\u003c/em\u003e (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\stackrel{-}{\\varvec{V}}}_{\\chi }\\bullet \\nabla {\\zeta }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ed), but are much weaker than that of \u003cem\u003eS3\u003c/em\u003e, consistent with the results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Besides, the tropical anomalies over the WNP of the total \u003cem\u003eS\u003c/em\u003e are attributed to \u003cem\u003eS1\u003c/em\u003e (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-\\left(f+\\stackrel{-}{\\zeta }\\right)\\nabla \\cdot {\\varvec{V}}_{\\chi }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea). These anomalies are located over the region where the TCs reside in the tropics, and thus the vorticity stretching arises from the upper-level divergence anomalies of TCs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e further illustrates the composite of 200-hPa divergent wind anomalies and meridional gradient of climatological absolute vorticity, which consists of the \u003cem\u003eS3\u003c/em\u003e term. The result reveals the interference between the climatological westerly jet and TCs. On the one hand, the climatological absolute vorticity is characterized as a strong and poleward gradient along the westerly jet, which is dominated by relative vorticity and modulated slightly by the planetary vorticity. The large gradient suggests that the westerly jet is important to trigger the Rossby wave. On the other hand, the prominent divergent anomalies induced by TCs are found to the south of the westerly jet. The prevailing southerly anomalies generally penetrate into the strong gradient of climatological vorticity, and thus result in negative \u003cem\u003eS\u003c/em\u003e anomalies over East Asia (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec). These results suggest the unique feature of the westerly jet in winter, i.e., the prominently strong zonal winds over East Asia, provides a favorable condition to interact with TCs in the tropics.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"5 Conclusions and discussion","content":"\u003cp\u003eIn this study, the extratropical circulation anomalies induced by the straight-moving TCs over the WNP in winter are investigated. It is found that there is a zonally oriented wave train emanating from East Asia to the Gulf of Alaska in the upper troposphere. The wave train can be traced back to the anticyclonic anomalies over East Asia in the upper troposphere. Here, the anomalous anticyclones almost coincide with the climatological westerly jet, suggesting a possible interaction between the mean flows and synoptic disturbances. Subsequently, the downstream anomalies are triggered and a clear wave train is formed, accompanied by the westward movement of TCs. Meanwhile, the lower-tropospheric wave anomalies are similar to their upper-level counterparts over the North Pacific, i.e., in a barotropic structure, but are much vaguer over East Asia.\u003c/p\u003e \u003cp\u003eFurther analyses are conducted to diagnose the physical mechanism whereby the TCs excite wave trains in the extratropics. There are remarkable negative Rossby wave sources over East Asia, which correspond well with the anomalous anticyclones in the upper troposphere. Moreover, the contributions of budget terms in the Rossby wave source are quantified. The results show that the advection terms play a vital role in the formation of wave trains, while the stretching terms are negligible. Additionally, the advection terms mainly result from the climatological vorticity advection by anomalous divergent winds (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\varvec{V}}_{\\chi }^{{\\prime }}\\bullet \\nabla \\stackrel{-}{\\zeta }\\)\u003c/span\u003e\u003c/span\u003e). Specifically, the climatological absolute vorticity shows a strong poleward gradient near the westerly jet over East Asia, where the prominent anomalous divergent winds induced by TCs are dominated. The above process suggests that the interaction between the TCs and climatological westerly jet results in negative wave source anomalies over East Asia, which is crucial for the formation of wave trains.\u003c/p\u003e \u003cp\u003eThe straight-moving TCs during winter are focused on in this study. The winter season is special or typical for straight-moving TCs to trigger the wavelike anomalies over the extratropics since the East Asian jet is strongest and located most equatorward in this season. In this regard, it provides a favorable condition for the straight-moving TCs to affect the extratropical flows without entering into the mid-latitude or transition to extratropical cyclones. In addition, the wave anomalies triggered by the straight-moving TCs are distinct from those accompanied by the recurving TCs. On the one hand, the wave train in the present work is quasi-stationary and extends across the North Pacific along the great circle path, while the one responding to recurving TCs propagates eastward along the westerly jet (Archambault et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, their Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). On the other hand, the diagnosis implies different processes in the formation of Rossby wave source. Compared to the comparable roles of both stretching and advection terms for the recurving TCs, the generation of wave train is dominant by the advection term relevant to the anomalous divergent winds induced by the straight-moving TCs.\u003c/p\u003e \u003cp\u003eIn this study, the East Asian westerly jet is supposed to be the climatological mean when the Rossby wave source is calculated, to simplify the explanation of the Rossby wave source formation over East Asia. This is reasonable, at least in the climatological sense or for the composite features, considering that the terms (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\varvec{V}}_{\\chi }^{{\\prime }}\\bullet \\nabla {\\zeta }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(-{\\zeta }^{{\\prime }}\\nabla \\cdot {\\varvec{V}}_{\\chi }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e) are weak and thus the feedback of TC-induced jet anomalies (indicated by \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\zeta }^{{\\prime }}\\)\u003c/span\u003e\u003c/span\u003e) on the Rossby wave source is weak. However, in real or at the synoptic timescales, the East Asian westerly jet varies greatly in intensity, location, and shape (e.g., Yang et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). Thus, the TC-induced Rossby wave source over East Asia may depend significantly on the specific features of East Asian jet during the appearance of TCs in the WNP. This issue needs to be further investigated.\u003c/p\u003e \u003cp\u003eFinally, the composite results suggest the extratropical wavelike anomalies are mainly over East Asia and the North Pacific, but weak over North America. This cannot exclude the possibility that in some cases, the straight-moving TCs in the WNP can trigger the wavelike anomalies that propagate into North America. In addition, the waves triggered by TCs may interact with short waves over the North Pacific, which may be induced by the internal atmospheric variability (e.g., Branstator \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) or affected by El Ni\u0026ntilde;o\u0026ndash;Southern Oscillation (e.g., Chen \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). If this is true, the straight-moving TCs may play a non-negligible role in affecting the extended weather forecast over North America.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003eThis work was supported by\u0026nbsp;the\u0026nbsp;National Natural Science Foundation of China (Grant No.\u0026nbsp;42005029).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e RYL\u0026nbsp;and\u0026nbsp;BP\u0026nbsp;contributed to the study conception and design. Data collection and analysis were performed\u0026nbsp;by SQM. SQM and BP wrote the manuscript. RYL\u0026nbsp;and XYZ provided intellectual contributions to the improvement of this manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ea\u003c/strong\u003e\u003cstrong\u003evailability\u003c/strong\u003eAll the data used in this study are openly available. The ERA5 data are obtained from https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5. The TC best-track data of CMA are retrieved from https://tcdata.typhoon.org.cn/en/zjljsjj_zlhq.html.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u0026nbsp;\u003c/strong\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAgust\u0026iacute;-Panareda A, Gray SL, Craig GC, Thorncroft C (2005) The extratropical transition of tropical cyclone Lili (1996) and its crucial contribution to a moderate extratropical development. Mon Weather Rev 133(6):1562\u0026ndash;1573. https://doi.org/10.1175/MWR2935.1\u003c/li\u003e\n \u003cli\u003eAiyyer A (2015) Recurving western North Pacific tropical cyclones and midlatitude predictability. 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J Clim 8:336\u0026ndash;351. https://doi.org/10.1175/1520-0442(1995)008\u0026lt;0336:Tsciai\u0026gt;2.0.Co;2\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"climate-dynamics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cldy","sideBox":"Learn more about [Climate Dynamics](https://www.springer.com/journal/382)","snPcode":"382","submissionUrl":"https://submission.nature.com/new-submission/382/3","title":"Climate Dynamics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Tropical cyclone, Rossby wave train, East Asian westerly jet, Western North Pacific ","lastPublishedDoi":"10.21203/rs.3.rs-4181050/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4181050/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates the large-scale circulation anomalies induced by straight-moving tropical cyclones (TCs) over the western North Pacific (WNP) during winter. Corresponding to the straight-moving TCs, quasi-stationary wave trains appear as alternative geopotential height anomalies in the upper troposphere stretching from East Asia to the North Pacific. Specifically, the anomalous anticyclones are initially formed to the south of Japan and then lead to the subsequent anomalies over the Sea of Okhotsk and the Gulf of Alaska, respectively. The wave trains extend along an approximate great circle path and differ from those triggered by the recurving TCs in summer and autumn, which propagate eastward along the westerly jet. Further analysis reveals that the upper-level anticyclonic anomalies are excited by negative Rossby wave sources, which are mainly attributed to the poleward vorticity advection by anomalous divergence relevant to TCs. In addition, the diagnosis indicates that the generation of wave source is caused by the product of the TC-induced divergent flows and the prominent meridional vorticity gradient in association with East Asian upper-tropospheric westerly jet. These findings imply that the tropical disturbances over the WNP, such as straight-moving TCs, can remotely affect weather over the extratropics, and thus have implications for improving the weather forecast over the extratropics through improving tropical disturbance forecast.\u003c/p\u003e","manuscriptTitle":"Straight-moving tropical cyclones over the western North Pacific trigger the wave trains over the North Pacific during winter","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-11 15:31:44","doi":"10.21203/rs.3.rs-4181050/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2024-05-12T14:23:12+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-04-08T07:41:56+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-04-06T20:40:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-03-28T13:07:38+00:00","index":"","fulltext":""},{"type":"submitted","content":"Climate Dynamics","date":"2024-03-28T05:18:46+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"climate-dynamics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cldy","sideBox":"Learn more about [Climate Dynamics](https://www.springer.com/journal/382)","snPcode":"382","submissionUrl":"https://submission.nature.com/new-submission/382/3","title":"Climate Dynamics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"f1da8032-815f-4657-bcea-f4263a650c6c","owner":[],"postedDate":"April 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-04T16:20:58+00:00","versionOfRecord":{"articleIdentity":"rs-4181050","link":"https://doi.org/10.1007/s00382-024-07462-1","journal":{"identity":"climate-dynamics","isVorOnly":false,"title":"Climate Dynamics"},"publishedOn":"2024-10-29 16:12:51","publishedOnDateReadable":"October 29th, 2024"},"versionCreatedAt":"2024-04-11 15:31:44","video":"","vorDoi":"10.1007/s00382-024-07462-1","vorDoiUrl":"https://doi.org/10.1007/s00382-024-07462-1","workflowStages":[]},"version":"v1","identity":"rs-4181050","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4181050","identity":"rs-4181050","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}