Based on the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and the Global Precipitation Climatology Project (GPCP) data, this study investigates the meteorological anomalies tied to the interannual variability of the South China Sea summer monsoon onset (SCSSMO), and its relationship with the change of the subtropical westerly jet position (SWJP), and the potential influence of the precursory thermal forcing over the eastern Tibetan Plateau (TP). The results show that during the earlier (later) SCSSMO years, there exist significant cyclonic (anticyclonic) circulation anomalies over the South China Sea (SCS) and its adjoining areas, featuring enhanced (suppressed) precipitation. The earlier SCSSMO years correspond to the southward-shifted upper-level subtropical westerly jet position to the north of the SCS. This favors the occurrence of non-geostrophic southward winds in the upper troposphere, the occurrence of upper-level divergence (convergence) and low-level convergence ascending (divergence) over the SCS and its nearby areas (Yangtze River basin) on the south (north) side of the jet axis, and the strengthening of the meridional circulation anomaly with anomalous ascending over the low-latitude and descending over the middle-latitude East Asia. Further analysis suggests that the anomalous heating over the eastern TP is significantly related to the SCSSMO and the change of SWJP. When the previous first-two-pentad heating anomalies over the eastern TP are positive, an anomalous anticyclonic circulation is formed in the upper troposphere, stimulating an eastward-propagating wave train. This, in turn, generates an anomalous cyclonic circulation downstream of the TP in the upper troposphere. As a result, the subtropical upper-level westerly jet downstream of TP shifts southward, which further affected the variation of atmospheric circulation over East Asia, ultimately leading to the earlier SCSSMO.
Research Article
Impact of the East Asian subtropical westerly jet on the interannual variability of the South China Sea summer monsoon onset and associated precursory thermal forcing role of the Tibetan Plateau
https://doi.org/10.21203/rs.3.rs-4956723/v1
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Monsoon is a phenomenon in which the prevailing wind direction in a large range changes significantly with the seasons, among which Asia is a famous monsoon activity area in the world, especially in East Asia and South Asia (Zhu et al. 1990; Li et al. 1999, 2000, 2004). It is worth noting that the onset and withdrawal of Asian summer monsoon are closely related to the intensity and frequency of severe climatic events such as droughts and floods, which can further affect ecosystems, agricultural production and economic development in Asia (Webster et al. 2004; Chen et al. 2006; Liu et al. 2011; Ding et al. 2004, 2008, 2009, 2014, 2020; Wang et al. 2009, 2021). Asian summer monsoon is divided into the Indian (South Asian) summer monsoon and the East Asian summer monsoon. As an important part of the East Asian summer monsoon, the South China Sea (SCS) summer monsoon (SCSSM) has distinct effects on the atmospheric circulation and climate anomalies in East Asia and even the Northern Hemisphere (Zhu et al. 1990; Li et al. 1999, 2000). The onset of SCSSM marks the transition from winter circulation to summer circulation, and also indicates the coming of the East Asian summer monsoon and the beginning of the rainy season in China (Lau and Yang 1997; Wu and Wang 2001; Wang and LinHo 2002; Wang et al. 2004; Ding et al. 2015).
The SCSSM, located over the center of the Asian-Australian monsoon region, is a complex monsoon system, and its onset is influenced by many factors in the tropical and mid-high latitude climatic systems (Lau et al. 2000; Wang et al. 2009). In the tropical areas, the west Pacific subtropical high (WPSH) is one of the major influencing factors, and the eastward withdrawal of the WPSH is conducive to the establishment of the SCSSM (Deng et al. 2020). Studies suggested that during the warm (cold) ENSO year or the year after, the Northwest Pacific Sea SST becomes colder (warmer), thereby strengthening (weakening) the WPSH. As such, the SCSSM onset is later (earlier), and the monsoon intensity is weaker (stronger) (Zhou and Chan 2007; Liang and Wu 2003). In addition, the tropical intra-seasonal oscillation (e.g., quasi-biweekly oscillation and 30–60-day oscillation in the Indian Ocean (Chang and Chen 1995; Wang and Wu 1997; Zhou et al. 2005; Tong et al. 2009; Wu 2010), as well as the monsoon vortex in the Bay of Bengal (Ding et al 2004; Wu et al. 2005; Wu et al. 2011), also have certain impacts on the SCSSM onset. As for the influence of the systems in the mid-high latitude, the southward movement of the continental frontal system is conducive to enhancing the latent heating released by convection over the northern SCS; meanwhile, due to the equatorial convergence zone (Zhou et al. 2005), non-adiabatic cooling is formed in the south of SCS, thus forming a closed meridional circulation which is conducive to the SCSSM onset (Chang and Chen 1995; Huangfu et al. 2018). Some studies have also emphasized the thermal effect of the Tibetan Plateau (TP). When there is less snow cover in winter, the transition time from a cold source in winter to a heat source in summer over the TP is advanced; meanwhile, the warm advection over the TP enhances the temperature gradient between the Plateau and its eastern and southeastern areas, affecting the adaptation of wind pressure field over the southeast areas. Thus, the SCSSM onsets earlier (Zheng et al. 2012; Li et al. 2019). Moreover, Sun and Ding (2002) pointed out that the earlier (later) establishment of the sensible heat over the TP and the stronger (weaker) intensity of heat source in spring and summer can enhance (weaken) the meridional circulation of the monsoon, strengthening (weakening) the southwest summer monsoon over the south and southeast TP and thus resulting in the earlier (later) onset of the SCSSM.
Concerning the factors tied to the SCSSM onset, previous studies mainly focused on the direct influence of external forcing factors on the SCSSM onset. Less attention has been paid to the study of the internal evolution characteristics of atmospheric circulation affecting the SCSSM onset. Studies have shown that the subtropical westerly jet in the Northern Hemisphere is the hub of mass and energy exchange generated by the interaction of circulation systems of different longitudes and latitudes (Fang et al. 2016). The seasonal transition of atmospheric circulation in East Asia and the beginning and ending of rainy season in the monsoon region are closely related to the position of the westerly jet and its south-northward movement and intensity changes (Tao and Zhao 1958). Previous study indicated that the SCSSM onset is closely related to the apparently northward jump of the upper westerly jet (Li et al. 2000). It is also found that during the seasonal transition, the northward jump of the jet axis and the westward shift of the jet center can jointly affect the establishment and the advance of the Asian monsoon circulation. For instance, the east-west position movement and intensity change of the jet center are closely related to the beginning of Meiyu over the Yangtze-Huaihe River Basin (Zhang and Kuang 2008). Notably, as for the East Asian subtropical westerly jet (SWJ) movement, the surface heating overlying the TP domain is an important factor that leads to its displacement. Kuang and Zhang (2006) pointed out that the intra-seasonal variation of the position of SWJ is significantly affected by the heating of the TP. The significant cooling over the eastern TP resulted in the formation of anomalous cyclonic in the upper troposphere, which caused the southward shift of the SWJ (Huang et al. 2015).
According to the aforementioned studies, the following issues regarding the interrelationships among the SCSSM onset, the position of SWJ, and the TP thermal forcing are far less understood. What is the connection between the early/late onset of SCSSM and the variation of the position of SWJ? How does the north-south movement of SWJ modulate the early and late onset of SCSSM? What about the role of TP thermal forcing anomalies in causing the north-south movement of SWJ? This study aims to respond these questions, revealing the physical mechanisms tied to the precursory anomalous heat source over the TP, which could be helpful to enhancing the prediction of the interannual SCSSM onset variability.
2.1 Reanalysis data
The atmospheric data used in this study are daily and monthly reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) (Kalnay et al. 1996), which has a horizontal resolution of 2.5° × 2.5°. The variables we utilize in this study include wind, air temperature, geopotential height, omega, surface pressure, relative humidity, and specific humidity. Monthly global precipitation data are taken from version 2.3 of the Global Precipitation Climatology Project (GPCP), with a horizontal resolution of 2.5° × 2.5° (Adler et al. 2003). All datasets span the period from 1979 to 2022.
2.2 Definition of the SCSSM onset date
The definition of the SCSSM onset date adopts the standard formulated by the National Climate Center (NCC) of China. The SCSSM onset is identified as the time when the zonal wind steadily changes from easterly to westerly, and the potential pseudo-equivalent temperature is consistently greater than 340 K in the key SCS region (10°-20°N, 110°-120°E). The temporal resolutions of these SCSSM onset data are pentad mean (5-day mean), with the time-varying SCSSM onset information showing in Fig. 1.
2.3 Statistical methods
Since we focus on the interannual variations, a Lanczos filter method (Duchon 1979) is used to extract interannual signals in variables. We use the 9-yr high-pass filtering approach, a widely employed filtering band to extract the corresponding interannual component (Chen et al. 2021; Chen and Song 2018; Chen and Wu 2017). Additionally, several statistical methods are utilized in this study, including correlation analysis, composite analysis, and regression analysis. To evaluate the statistical significance of our results, a two-tailed Student’s t test is employed. Here, all data are linearly detrended before analyses to exclude potential impacts of long-term trends in variables.
2.4 Water vapor transport and its divergence
The vertically integrated horizontal water vapor transport (WVT) and its divergence (WVT_div) are calculated based on the following equations:
Where 
denotes the horizontal divergence in pressure coordinates; g is the gravitational acceleration; Ps is the surface pressure; q is the specific humidity; and V = (u, v) is the horizontal wind vector (u and v represent zonal and meridional wind, respectively).
2.5 TP atmospheric heat source (Q1)
Q 1 is calculated by referring to the inverted algorithm of Yanai et al. (1992):
where T is the temperature; V is the horizontal wind vector; θ is the potential temperature; ω is the vertical velocity; p is the pressure; and variable p0 (= 1000 hPa) is the standard pressure. All these variables are in p coordinates; k = R/cp ≈ 0.286, where R and cp are the dry atmospheric constant and the isobaric specific heat capacity, respectively. From Eq. (3), Q1 consists of three items. When calculating the atmospheric heat source < Q1 > in the troposphere, we set ω = 0 at 100 hPa at the top of the troposphere according to Yanai et al. (1992).
3.1 Characteristics of meteorological condition anomalies tied to the SCSSM onset
Figure 1 shows the South China Sea summer monsoon onset (SCSSMO) dates for 1979–2022, showing prominent interannual variations. The mean date for the SCSSMO is 28.2 pentad, and the standard deviation (SD) is 1.85 pentad. The year in which the onset date is one standard deviation less than the mean date is defined as the earlier SCSSMO, and the year in which the onset date is one standard deviation more than the mean date is defined as the later SCSSMO. As such, the earlier SCSSMO years are 1979, 1986, 1990, 1994, 2001, 2004, 2008, 2011, 2013, 2019, and 2022; the later SCSSMO years are 1982, 1987, 1989, 1991, 1993, 1999, 2005, 2009, 2014, 2018, and 2021 (11 years).
Figure 2 shows the composite difference distributions of May 850-hPa pseudo-equivalent temperature and precipitation anomalies between earlier and later SCSSMO years. It can be observed that during the earlier SCSSMO years, the lower-level pseudo-equivalent potential temperature anomalies over the SCS and its nearby areas are dominated by significantly positive anomalies (Fig. 2a), indicating that the lower troposphere atmosphere is in a state of higher temperature/humidity, which is conducive to the enhancement of convective activities over the SCS. As such, there exist significantly above-normal precipitation over the SCS (Fig. 2b). From the above analysis, it can be concluded that the higher temperature/humidity conditions with positive pseudo-equivalent potential temperature anomalies over the SCS and the corresponding positive precipitation anomalies are closely related to the earlier SCSSMO. On the contrary, when the SCSSMO is later, the localized precipitation is suppressed with lower temperature/humidity conditions.
We further explore circulation anomalies responsible for the interannual SCSSM onset variations. After removing the linear trend and the corresponding interdecadal signals, the corresponding standardized SCSSMO time series is defined as the interannual SCSSMO index (SCSSMOI), with positive phase indicating the later SCSSMO and the negative phase indicating the earlier SCSSMO. To investigate the circulation anomalies that affect the timing of SCSSMO on an interannual scale, the regression patterns of the circulation situations in the upper (200-hPa), middle (500-hPa), and lower (850-hPa) troposphere in May against the negative SCSSMOI are further presented (Fig. 3). Corresponding to the negative phase of the SCSSMOI, i.e., during the earlier SCSSMO years, the SCS and its surrounding areas are dominated by an anomalous cyclonic circulation from the lower troposphere (Fig. 3a) to the middle troposphere (Fig. 3b), with significant negative anomalies in terms of the geopotential height. In contrast, the upper troposphere over the SCS and its nearby areas are controlled by an anomalous anticyclonic circulation (Fig. 3c), indicating a quasi-barotropic structure in the lower and middle troposphere and a more pronounced baroclinic structure from the middle to the upper troposphere. In this circumstance, the lower troposphere over the SCS is dominated by anomalous westerly winds (Fig. 3a), whereas the upper troposphere is dominated by anomalous easterly winds (Fig. 3c), resulting in strong vertical wind shear anomalies. Additionally, the aforementioned cyclonic anomaly leads to the convergence of anomalous northerly winds stemmed from northern China (indicating abnormally active cold air) and anomalous westerly winds stemmed from low latitudes (indicating abnormally active monsoonal winds) over the SCS. These environmental conditions are conducive to generating strong vertical upward motions (Chang and Chen 1995; Liu and Ding 2007). Meanwhile, influenced by the anomalous cyclonic circulation, water vapor transport from both the Indian Ocean and the Pacific Ocean converges over the SCS, ensuring abundant water vapor sources and facilitating the formation of convective instability, thereby leading to increased precipitation over the SCS (Fig. 2b). Furthermore, an anomalous cyclonic circulation can be observed to the northern of the SCS in the upper troposphere, with significant westerly and easterly anomalies located respectively to the south and north of the subtropical westerly jet axis. This indicates that during the earlier SCSSMO years, the SWJ to the northern of the SCS tends to shift more southward.
3.2 Relationship between the position change of SWJ and SCSSMO
With reference to the definition of SWJ position (SWJP) change index (SWJPI) proposed by Dong et al. (1999) and Liao et al. (2004), the SWJPI affecting the SCSSMO is defined as the difference between the May-averaged 200-hPa zonal winds between two magenta frames (45°–50°N, 110°–130°E) and (25°–30°N, 110°–130°E) that locate to the north of SCS, as shown in Fig. 3c. Note that a positive SWJPI indicates a northward-located SWJP compared against the climatological SWJ, whereas a negative difference indicates a southward-located SWJP (Fig. 3c). Further correlation analysis is performed on the standardized interannual time series between SCSSMOI and SWJPI (Fig. 4). We can clearly observe that the temporal variation between the two variables are similar, with a positive temporal correlation coefficient (TCC) of 0.37 (p < 0.05). This suggests that, when the SWJP to the north of the SCS shifts more southward, the outbreak of SCSSM tends to be earlier.
The above-mentioned statistical analysis indicates that the meridional variation of the SWJP to the north of the SCS has a significant TCC with the interannual variability of the SCSSMO. Then, it is natural to ask how does the meridional variation of the SWJP affect the interannual variability of the SCSSMO? Fig. 5 shows the regression patterns of 500-hPa geopotential height and winds at 200-hPa, 500-hPa, and 850-hPa onto the negative SWJPI, which are fairly similar to the regressed patterns of the negative SCSSMOI at interannual time scales (Fig. 3). Corresponding to the negative SWJPI, i.e., the southward shift of the SWJP, a large-scale anomalous cyclonic circulation dominates the SCS and the regions to the north in the lower and middle troposphere (Figs. 5a, b). Such circulation anomalies suggest a deeper East Asian trough and the resultant northerly wind anomalies over eastern China, which is conducive to the southward intrusion of cold air behind the trough at the lower and middle levels, whereas the anomalous southwesterlies occur over the SCS and its adjoining areas (Figs. 5a, b), thereby leading to the activation of convection activities in situ. Furthermore, an anomalous anticyclonic circulation dominates the SCS and its surroundings in the upper troposphere, with anomalous easterlies over the SCS (Fig. 5c). The lower-tropospheric westerly anomalies, in conjunction with the upper-tropospheric easterly anomalies, could result in strong local vertical wind shear anomalies, which is conducive to the enhancement of vertical updraft motions over the SCS (Fig. 6b) and the cyclonic anomaly in the lower troposphere, thus leading to the earlier SCSSMO. Meanwhile, the water vapor transport divergence over the SCS and its nearby areas shows significant negative anomalies (Fig. 6a), indicating prominent moisture convergence anomalies, which may induce enhanced precipitation anomalies over the SCS (Fig. 2b).
Previous studies suggested that upper-level jets could play vital roles in modulating the local vertical meridional overturning circulations (Uccellini and Johnson 1979; Brill et al. 1985; Uccellini and Kocin 1987). The south and north sides of the westerly jet entrance region in the lower troposphere generate convergence and divergence, respectively; whereas the south and north sides generate divergence and convergence in the upper troposphere, respectively. The westerly jet can stimulate an anomalous circulation with ascending motions on the south side and descending motions on the north side, together with non-geostrophic southerlies at the upper troposphere and non-geostrophic northerlies at the lower troposphere in the jet entrance region. With the southward shift of the SWJP, the anomalous westerly center shifts southward near 25°N. The SCS and the Yangtze River basin are located on the south and north sides of the upper-level jet entrance region, respectively. Anomalous northerlies occur in the lower and middle troposphere, which contribute to the occurrence of convection and the release of latent heat over the middle and northern SCS, stimulating striking ascending motion over the SCS (Fig. 6b) (Ding and Liu 2001). This leads to the enhancement of the anomalous cyclonic circulation over the SCS and its nearby areas, which is conducive to the earlier SCSSMO. Our results generally concur with previous studies focusing on the relationship between the SCSSMO and the meridional variation of the SWJP (e.g., Wen et al. 2016).
3.3 Influence of thermal forcing over the TP on the meridional SWJP
Studies have pointed out that the abnormal heating in the middle and upper troposphere over the TP plays a crucial role in the onset and maintenance of the SCSSM (Sun and Ding 2002; Zheng et al. 2012; Li et al. 2019). The thermal forcing of the TP can affect the onset and maintenance of the monsoon through inducing thermal direct circulation and the changes in the north-south temperature gradient in the middle and upper troposphere of the monsoon region (Jian and Luo 2002; Sun and Ding 2002). Then, a question arises here as to how does the thermal forcing of the TP affect the SCSSMO by influencing the meridional variation of the SWJP over the downstream of the TP? To explore the possible relationship between the abnormal heating conditions over the TP and the meridional variation of the SWJP over the downstream of the TP, we exhibit the spatial distributions of the TCCs between the interannual variations of the non-adiabatic heating averaged over the first two pentads before the climatological-mean time for the SCSSMO (Q1 for short) and the negative SCSSMOI (Fig. 7a)/SWJPI (Fig. 7b). It can be seen that the spatial distributions of TCCs between the negative SCSSMOI and the negative SWJPI with Q1 are basically consistent, and both show significant positive correlations with the interannual Q1 variations over the eastern TP (Figs. 7a, b). This indicates that when the Q1 over the eastern TP is significantly enhanced before the SCSSMO, the SWJP over the downstream of the TP shifts more southward and the SCSSMO is earlier; and vice versa. Meantime, it can be found that significant positive correlations with the Q1 variations over the longitudinal zone from the eastern Bay of Bengal to the Indochina Peninsula and then to the SCS, indicating that when the SWJP shifts southward and the SCSSMO is earlier, there is an enhancement of Q1 in this longitudinal zone, accompanied by positive precipitation anomalies locally (Fig. 2b). Based on the spatial distribution of TCCs over the TP, the Q1 over the key region (30°–35°N, 90°–105°E) of the TP averaged over the first two pentads before the monsoon onset is regionally averaged and then standardized to define the TP non-adiabatic heating index (TPQ1 for short). Further correlation analysis is performed between the interannual time series of SWJPI and TPQ1. It can be seen that there is an intimate negative correlation between them (Fig. 7c), with a TCC of − 0.39 (p < 0.05). This indicates that when the Q1 over the eastern TP is significantly enhanced, the SWJP over the downstream of TP shifts more southward.
So, how does the precursory thermal forcing of the TP specifically affect the meridional variation of the SWJP? We find that the evolution of circulation patterns is closely related to changes in the north-south temperature gradient in the middle and upper troposphere (Fig. 8). When the negative SWJPI and the positive TPQ1 are regressed against the May-mean temperature gradient within the middle and upper troposphere and the simultaneous horizontal winds at 200 hPa, the spatial modes of the regressed results are generally consistent (Fig. 8a vs. 8b), especially for winds and temperature gradient east of 110°E. Specifically, in years with southward-shifted SWJP, positive and negative anomalies of temperature gradients correspond to the south and north sides of the climatological-mean jet axis, respectively, forming an anomalous cyclonic circulation in the upper troposphere (Fig. 8a). Moreover, when the TPQ1 is in a positive phase, there indicate positive anomalies in Q1 over the eastern TP. As such, a Rossby wave train propagating eastward along the subtropical westerly jet is excited. There are negative and positive temperature gradients on the south and north sides of the jet over the main body of the TP, respectively, forming an in situ anomalous anticyclonic circulation in the upper troposphere. There are large-scale easterly anomalies on the south of the TP and northerly wind anomalies on the east of the TP. Also, positive and negative temperature gradients exist on the south and north sides of the jet over the downstream of the TP, respectively, forming an anomalous cyclonic circulation in the upper troposphere. Accordingly, there are easterly anomalies on the north of the jet and westerly anomalies on the south of the jet. As a result, the SWJ to the east of the TP shifts more southward (Fig. 8b), inducing earlier SCSSMO.
In summary, when the previous first-two-pentad heating anomalies over the eastern TP are positive, an anomalous anticyclonic circulation is formed in the upper troposphere, stimulating an eastward-propagating wave train (Fig. 9). This, in turn, generates an anomalous cyclonic circulation over the downstream of TP in the upper troposphere, causing a southward-shifted SWJ over the downstream of TP. The southward-positioned SWJ and the convective latent heat release related to the convergence of water vapor transport over the SCS (Fig. 6) could play vital contributions to the local meridional circulation anomaly in East Asia, especially the ascending motion anomalies over the SCS (Fig. 9). Aided by the anomalous cyclonic circulation in the middle troposphere, we can observe the deepening of the East Asian trough (Fig. 5), the earlier withdrawal of the WPSH from the SCS, the southward cold air in the lower troposphere, and the earlier advance of the tropical monsoon. Under these favorable environments, an anomalous convergence occurs over the central and northern SCS, linking the earlier SCSSMO (Fig. 9). As such, the precursory heating anomalies over the eastern TP can further regulate the interannual variation of the atmospheric circulations over East Asia through the change in the meridional position of the SWJ over the downstream of TP, ultimately affecting the interannual variability of the SCSSMO.
Based on NCEP/NCAR reanalysis data and GPCP data, the meteorological anomalies tied to the interannual variability of SCSSMO and its relationship with the concurrent SWJP and precursory TP heating anomalies are analyzed. We also propose the associated physical mechanisms, which are depicted schematically (Fig. 9). The results of this study show that the anomalous heat source over the eastern TP can further modulate the SCSSMO by influencing the meridional position change of SWJ over the downstream of TP, which will provide a new reference for predicting the time of SCSSMO.
The major conclusions can be summarized as follows.
1) The SCSSMO features salient interannual variations. During the earlier SCSSMO years, there is a consistent anomalous cyclonic circulation from the lower troposphere to the middle troposphere over the SCS. The lower troposphere over the SCS is dominated by westerly anomalies, whereas the upper troposphere is dominated easterly anomalies, which is conducive to the development of convective instability and strong ascending motion, resulting in enhanced precipitation in situ. Such meteorological anomalies facilitate earlier SCSSMO, and vice versa.
2) The interannual variability of the SCSSMO is closely related to the meridional position variation of the SWJ to the north of SCS in the upper troposphere. When the position of SWJ is significantly southward, the non-geostrophic southward wind appears in the upper troposphere, and the SCS and its nearby areas (Yangtze River Basin) on the south (north) side of the jet axis have divergence (convergence) at upper troposphere and convergence ascending (divergence descending) at low troposphere, thus strengthening the meridional circulation anomaly of ascending over the low-latitude and descending over the mid-latitude East Asia. Aided by the anomalous cyclonic circulation in the middle troposphere, the deepening of the East Asian trough, the earlier withdrawal of the WPSH from the SCS, the southward cold air in the lower troposphere, and the earlier advance of the tropical monsoon can be stimulated, thereby forming an anomalous convergence over the central and northern SCS. As such, the SCSSMO is earlier.
3) The previous (first two pentads before the climatological-mean SCSSMO onset date) heating anomalies over the eastern TP can further influence the change of atmospheric circulation in East Asia through the modulation of the change of the meridional position of the SWJ over the downstream of TP, ultimately affecting the interannual variability of SCSSMO. When the previous heating over the eastern TP is a positive anomaly, an anomalous anticyclonic circulation is formed in the upper troposphere, stimulating an eastward-propagating wave train. This, in turn, generates an anomalous cyclonic circulation over the downstream of TP in the upper troposphere, causing a southward-shifted SWJ over the downstream of TP, and further affecting the atmospheric circulations in East Asia, which can ultimately induce the earlier SCSSMO.
Acknowledgements This work was jointly supported by the Guangdong Major Project of Basic and Applied Basic Research (Grant No. 2020B0301030004) and the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant No. 2019QZKK010204-02). Yanju Liu acknowledges the support by the Key Innovation Team of China Meteorological Administration "Climate Change Detection and Response".
Author contribution Yanju Liu designed the study. Chengyu Song wrote the main manuscript text. Jing Wang and Yanju Liu revised and reviewed the manuscript. All authors read and approved the final manuscript.
Funding This work was funded by the Guangdong Major Project of Basic and Applied Basic Research (Grant No. 2020B0301030004) and the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant No. 2019QZKK010204-02).
Data availability Daily and monthly NCEP/NCAR data are openly available at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. Monthly GPCP precipitation data are openly available at https://psl.noaa.gov/data/gridded/data.gpcp.html.
Consent for publication Not applicable.
Competing interest The authors declare no competing interests.
Ethics approval Not applicable.
Consent to participate Not applicable.
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No competing interests reported.