Thermal Impact of the Southern Tibetan Plateau on the Southeast Asian Summer Monsoon and Modulation by the Tropical Atlantic SST

Mengmeng Lu aState Key Laboratory of Severe Weather and Institute of Tibetan Plateau Meteorology, Chinese Academy of Meteorological Sciences, Beijing, China
bSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

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Song Yang bSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China
cSchool of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China
dGuangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Zhuhai, China

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Congwen Zhu eState Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China

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Junbin Wang cSchool of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China

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Shuheng Lin cSchool of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China

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Wei Wei bSouthern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China
cSchool of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China
dGuangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Zhuhai, China

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Hanjie Fan fState Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China

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Abstract

While it is commonly accepted that the thermal effect of the Tibetan Plateau (TP) strengthens the Asian summer monsoon, a recent analysis based mainly on idealized model experiments revealed that the TP effect weakened the Southeast Asian summer monsoon (SEASM). Based on both observational analyses and model experiments, the current study further deciphers the physical mechanism for the TP’s thermal impact on the SEASM and the modulation of this impact by the sea surface temperature (SST) in the tropical Atlantic. When diabatic heating is enhanced over the southern TP, the South Asian high (SAH) intensifies and extends eastward, leading to convergence over the southeastern flank of the anomalous upper-level anticyclone and sinking motion that cause downward advection of negative vorticity. Accompanied by this anomalous anticyclonic pattern, the western Pacific subtropical high (WPSH) extends westward and the monsoon over Southeast Asia is weakened. The TP–SEASM relationship is enhanced when SST and convection increase over the tropical Atlantic, which cause an anomalous barotropic wave train propagating southeastward from eastern North America to East Asia, leading to an eastward extension of the SAH and a westward extension of the WPSH. The anomalous heating over the tropical Atlantic also modulates the Walker circulation through two anomalous vertical cells, with ascending motions over the Maritime Continent and the eastern tropical Indian Ocean, inducing a lower-level anticyclone over Southeast Asia as a Gill-type response. Thus, a warming tropical Atlantic can intensify the TP’s thermal forcing, weaken the SEASM, and then modulate the TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation.

Significance Statement

The Southeast Asian summer monsoon (SEASM) exhibits significant interannual variability. This study is aimed at better understanding the thermal impact of the Tibetan Plateau (TP) on the interannual variability of SEASM intensity and the possible modulating effect of the tropical Atlantic on the TP–SEASM relationship. We find that enhanced heating over the southern TP weakens the SEASM circulation and its associated precipitation by extending the South Asian high eastward and expanding the western Pacific subtropical high westward. Tropical Atlantic warming enhances this TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation. Thus, seasonal prediction of the SEASM can be improved by further considering the synergistic impact of the TP’s thermal forcing and the tropical Atlantic surface temperature.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Song Yang, yangsong3@mail.sysu.edu.cn

Abstract

While it is commonly accepted that the thermal effect of the Tibetan Plateau (TP) strengthens the Asian summer monsoon, a recent analysis based mainly on idealized model experiments revealed that the TP effect weakened the Southeast Asian summer monsoon (SEASM). Based on both observational analyses and model experiments, the current study further deciphers the physical mechanism for the TP’s thermal impact on the SEASM and the modulation of this impact by the sea surface temperature (SST) in the tropical Atlantic. When diabatic heating is enhanced over the southern TP, the South Asian high (SAH) intensifies and extends eastward, leading to convergence over the southeastern flank of the anomalous upper-level anticyclone and sinking motion that cause downward advection of negative vorticity. Accompanied by this anomalous anticyclonic pattern, the western Pacific subtropical high (WPSH) extends westward and the monsoon over Southeast Asia is weakened. The TP–SEASM relationship is enhanced when SST and convection increase over the tropical Atlantic, which cause an anomalous barotropic wave train propagating southeastward from eastern North America to East Asia, leading to an eastward extension of the SAH and a westward extension of the WPSH. The anomalous heating over the tropical Atlantic also modulates the Walker circulation through two anomalous vertical cells, with ascending motions over the Maritime Continent and the eastern tropical Indian Ocean, inducing a lower-level anticyclone over Southeast Asia as a Gill-type response. Thus, a warming tropical Atlantic can intensify the TP’s thermal forcing, weaken the SEASM, and then modulate the TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation.

Significance Statement

The Southeast Asian summer monsoon (SEASM) exhibits significant interannual variability. This study is aimed at better understanding the thermal impact of the Tibetan Plateau (TP) on the interannual variability of SEASM intensity and the possible modulating effect of the tropical Atlantic on the TP–SEASM relationship. We find that enhanced heating over the southern TP weakens the SEASM circulation and its associated precipitation by extending the South Asian high eastward and expanding the western Pacific subtropical high westward. Tropical Atlantic warming enhances this TP–SEASM relationship through both the extratropical wave train and the tropical zonal circulation. Thus, seasonal prediction of the SEASM can be improved by further considering the synergistic impact of the TP’s thermal forcing and the tropical Atlantic surface temperature.

© 2023 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Song Yang, yangsong3@mail.sysu.edu.cn

1. Introduction

The Asian summer monsoon is not spatially uniform; it consists of regional or even larger-scale characteristics. Compared with the South Asian summer monsoon, the East Asian summer monsoon, which refers to the monsoon phenomena east of the central Bay of Bengal (about 100°E), is composed of different atmospheric circulation systems (Tao and Chen 1987). It is also known that southern East Asia (e.g., 20°N and southward) is characterized by different monsoon features from its northern counterpart. Thus, the Southeast Asian summer monsoon (SEASM), sometimes called the western North Pacific summer monsoon, is also an important Asian monsoon component that has received much research interest (Chang and Chen 1995; Lau and Yang 1997; Wang and Fan 1999; Qian and Yang 2000; Wu and Wang 2000; Wang et al. 2001; Lee et al. 2011). The SEASM usually occurs around late spring and exists until October; its main domain, as addressed in this study, covers the Indo-China Peninsula, the Maritime Continent, the South China Sea, the Philippines, part of southern China, and the far western Pacific. The abnormal activity of the SEASM system cannot only cause disastrous weather and climate events involving floods and droughts, but also exert strong impacts on climate variations over East Asia and other remote regions through thermally driven vertical circulations and atmospheric wave trains (Nitta 1987; Huang 1990; Huang and Sun 1992; Lau et al. 2000; Lu 2001; Li et al. 2016; Yang et al. 2021). Moreover, the SEASM is an important bridge for tropical oceans to affect extratropical climate (Huang and Wu 1989; Shen and Lau 1995). Therefore, investigating the mechanisms behind SEASM variability will not only enhance our understanding of SEASM dynamics, but also provide valuable climate background information and theoretical basis for improving seasonal prediction of Southeast and East Asian climate.

Sea surface temperature (SST) and land surface process are the two crucial factors that affect the SEASM (Chen and Wang 1998; Chen and Hu 2003; Ding et al. 2004; Yang and Lau 2006; Yoo et al. 2006; Kajikawa and Wang 2012; He and Wu 2013; Zhao et al. 2015; Ma et al. 2018; Gao et al. 2019; Lin and Zhang 2020). As a huge heat source during the warm season, the Tibetan Plateau (TP) exerts a great influence on the Asian summer monsoon and even global climate (e.g., Yanai et al. 1992; Ye and Wu 1998; Wu et al. 2015; Liu et al. 2020). Previous studies have indicated that the TP’s thermal forcing can modulate the western Pacific subtropical high (WPSH), which is viewed as a key system that affects the regional monsoons in East and Southeast Asia (Chen et al. 2003; Jian et al. 2004; Liu and Wu 2004; Wang et al. 2014). Duan et al. (2017) found that the spring sensible heating over the TP could lead to intensification and westward extension of the WPSH. The eastward extension of the South Asian high (SAH), an atmospheric circulation system that is closely related to the thermal condition over the TP, can also be a precursor for predicting the onset of the South China Sea summer monsoon (Liu and Zhu 2016). On the interannual time scale, atmospheric convection becomes stronger over the eastern TP but weaker over the South China Sea region when the SAH shifts eastward (Wei et al. 2014). Recently, Lu et al. (2021) indicated that the TP heating could weaken the SEASM, which was different from its enhancing effect on the East and South Asian summer monsoons, whereas the conclusion was mainly drawn based on idealized model sensitivity experiments. Therefore, further improving our understanding of the TP’s thermal effect on the SEASM and the physical processes involved using observations and more numerical experiments is undoubtedly necessary.

It has been noticed recently that the warming of the tropical Atlantic can weaken the summer monsoon circulation and precipitation in the Southeast Asia–western Pacific regions by inducing an atmospheric teleconnection wave train in the extratropics, triggering coupled convection-circulation interaction, and changing the Walker circulation and the atmospheric Kelvin wave in the tropical/equatorial regions (Chen et al. 2015; Choi and Ahn 2019; Feng et al. 2020; Takaya et al. 2021). Several studies have indicated that the Atlantic apparently affects both the heat source over the TP and the East and Southeast Asian summer monsoons (Cui et al. 2015; Wang et al. 2018; Liu and Zhu 2019; Yu et al. 2021). However, the impacts of the TP’s thermal forcing and Atlantic SST on the SEASM were separately investigated previously. Therefore, a thorough investigation of the synergistic effects of both the TP’s thermal forcing and the Atlantic Ocean on the interannual variability of the SEASM is important for improving our understanding of the variation and even prediction of the SEASM.

In this study, we apply both observational analyses and model simulations to investigate the thermal impact of the TP on the interannual variability of the SEASM, and explore the possible modulating effect of Atlantic SST anomalies on the above the TP–SEASM relationship. A conceptual model of the synergistic impact of both the TP’s thermal forcing and the Atlantic SST on the SEASM variability is further proposed.

The data and model used in this study, along with model experimental design, are introduced in section 2. In section 3, we depict the observational relationship between the TP’s thermal forcing and the SEASM, and represent the characteristics of the associated circulation systems. In section 4, we illustrate the responses of the SEASM circulation and precipitation to the TP heating in model sensitivity experiments and associated physical mechanisms. We discuss the modulating effect of Atlantic SST anomalies on the TP–SEASM relationship revealed by observational analyses and model sensitivity experiments in section 5. Conclusions and discussion are given in section 6.

2. Data, model, and experimental design

a. Observational data

The monthly precipitation data are from the Global Precipitation Climatology Project (GPCP) version 2.3 (Adler et al. 2003), with a resolution of 2.5° × 2.5°. Monthly atmospheric variables are from the latest European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5), with a resolution of 0.25° × 0.25° (Hersbach et al. 2020). The Extended Reconstructed Sea Surface Temperature (ERSST) version 5 is from the NOAA PSL (Huang et al. 2017), with a resolution of 2° × 2°. Our study period is from January 1979 to December 2020. Summer is represented by the average of July and August (JA). The observed results for June–August are also analyzed, which are generally similar to those for JA. However, we found that the atmospheric circulation patterns related to the diabatic heating over the southern TP in June are weaker and different from those in July and August (figure not shown), as reported in Jiang et al. (2016; their Fig. 4). Hence, we analyze the JA-averaged results in the paper.

The various indices used in this study are defined below:

  1. The SEASM index, which measures the local intensity of low-level relative vorticity in the SEASM region, is calculated by using the horizontal shear of 850-hPa zonal wind between 90°–130°E, 5°–15°N and 110°–140°E, 22.5°–32.5°N (Wang and Fan 1999), as in Yoo et al. (2006), Li and Yang (2017), and Lu et al. (2021), among others.

  2. The summer precipitation over the southern TP (STPP) index, which represents the variability of the regional atmospheric latent heating, is calculated by using JA precipitation averaged over 62°–105°E, 23°–35°N, where the topography is above 1500 m. We found that both apparent heat source Q1 (based on the ERA5) and summer rainfall (based on the GPCP) over the entire TP are highly related with the STPP index (with correlation coefficients of 0.85 and 0.97, respectively), indicating that the STPP index can well represent the variability of the TP’s thermal forcing.

  3. The Niño-3.4 index, which measures the SST change in the central and eastern equatorial Pacific, is calculated by using the area-averaged SST in the region of 170°–120°W, 5°S–5°N.

  4. The TA_SST index, measuring the SST change in the tropical Atlantic, is calculated by using the area-averaged SST in the tropical Atlantic (75°W–0°, 20°S–20°N), with its linear trend removed first.

  5. The EA_SST index, depicting the SST change in the extratropical North Atlantic, is calculated by the area-averaged SST in the North Atlantic (60°–10°W, 40–60°N), with its linear trend removed first.

b. Model and experimental design

The model used is the Community Earth System Model version 1.2.2 (CESM 1.2.2) from the National Center for Atmospheric Research (NCAR), which is a state-of-the-art, fully coupled Earth system model (Hurrell et al. 2013). It comprises interactive component models that simulate Earth’s atmosphere, land, ocean, and sea ice. The atmospheric component of the CESM 1.2.2 is the CAM4, with a horizontal resolution of 0.9° × 1.25° and 26 levels (Neale et al. 2013). All atmospheric model experiments are implemented with the F2000 component sets, indicating that carbon dioxide, aerosol, solar forcing, and ozone concentration are all fixed at their levels of year 2000.

Table 1 introduces model experimental design. The atmospheric model of the CESM 1.2.2 is driven by the monthly mean SST of the model default dataset, a merged product based on the monthly mean SST from the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST; Rayner et al. 2003) and the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation (OI) SST analysis (NOAA OISST; Reynolds et al. 2002). An integration of 30 years is taken for the control experiment (CTRL). To understand the impact of the TP’s thermal forcing on SEASM circulation and precipitation, a sensitivity experiment (TP_Noheat) is integrated for 30 years, by shutting down the JA diabatic heating over the southern TP (62°–105°E, 23°–45°N where the topography is above 1500 m and JA climatological precipitation in the CTRL is larger than 3 mm day−1).

Table 1

Model experimental design.

Table 1

As reviewed in the introduction, it can be reasonably assumed that TP diabatic heating and Atlantic SST variation are two of the most important boundary forcing fields for SEASM variability. To quantify the effect of tropical Atlantic SST anomalies, another sensitivity experiment (TA_Warm) is integrated for 30 years, in which the atmospheric model has a resolution of 1.9° × 2.5°, forced by twice the composite difference in observed JA SST anomalies over the tropical Atlantic (20°S–20°N) between the high and low years of southern TP precipitation. The outputs for the last 20 years in all experiments are analyzed, and the differences between the sensitivity and control experiments are compared to understand the individual and synergistic effects of TP diabatic heating and tropical Atlantic SST on SEASM variability.

3. Observed relationship between TP thermal forcing and the SEASM

According to its definition, the SEASM index can measure the intensity of low-level vorticity, and thus the atmospheric circulation patterns over Southeast Asia (including the South China Sea, the Indo-China Peninsula, and the Maritime Continent), parts of southern East Asia, and far western North Pacific (Fig. 1a). It can also portray the regional rainfall patterns and decipher the dynamics of WPSH variability. Associated with the SEASM index, a meridional tripole distribution of summer precipitation appears with more rainfall over the SEASM region but less rainfall over the southern TP, East Asia, and the Maritime Continent. A cyclonic circulation dominates over the SEASM region, and an anticyclonic circulation appears over East Asia. Figure 1b displays the regression patterns of summer precipitation and 850-hPa winds against the STPP index, exhibiting similar patterns to those shown in Fig. 1a but with opposite signs. It indicates that a significantly negative relationship exists between the summer precipitations over the southern TP and the SEASM region. Similar feature can also be found in Fig. 1c, which provides the standardized SEASM and STPP indices, both showing large interannual variabilities. The correlation coefficient between the two indices is −0.54, significantly exceeding the 99.9% confidence level, based on the Student’s t test (and likewise for the results in the rest of the paper).

Fig. 1.
Fig. 1.

Regressed summer precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1) against the (a) SEASM index and (b) STPP index. The black boxes indicate the two domains (90°–130°E, 5°–15°N and 110°–140°E, 22.5°–32.5°N) used for defining the SEASM index. White stippling and blue vectors indicate the regression coefficients that are significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contours. (c) Standardized time series of the SEASM and STPP indices. The three asterisks following the correlation coefficient indicate the significant values above the 99.9% confidence level.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

The regressed patterns of summer geopotential height fields at 200 and 500 hPa against the STPP index are shown in Fig. 2. When the TP heating is enhanced, the 200-hPa SAH intensifies and its center shifts eastward, with an anomalous cyclonic circulation over the northern TP and northern East Asia, but an anomalous anticyclonic circulation mainly over the Maritime Continent (Fig. 2a). At 500 hPa (Fig. 2b), geopotential height decreases over the TP and to its west and east, but increases over most of the latitudinal band of 15°–30°N in the study domain. The most noteworthy feature is that an anomalous anticyclonic circulation forms over the SEASM region, especially over southern China, the northern-central South China Sea, and the western subtropical Pacific, implying that the WPSH intensifies and shifts southwestward. Apparently, this strengthened and westward-extended WPSH results in anomalous negative vorticity over Southeast Asia, and weakens the SEASM circulation and precipitation.

Fig. 2.
Fig. 2.

Regressed summer geopotential heights (shading; m) at (a) 200 and (b) 500 hPa against the STPP index. Black contours represent the summer climatology of the South Asian high in (a) and the summer climatology of the western Pacific subtropical high in (b), where the blue and green contours, respectively, reveal the locations of the subtropical high in the high and low STPP years (based on the values greater than +1 and less than −1 standard deviations). White stippling indicates the regression being significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contours.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

To clarify the possible connection between the SAH and the WPSH, and between the TP heating and SEASM circulation, we show the cross sections of vorticity and vertical circulation from 35°N, 80°E to 10°N, 130°E in Fig. 3. Climatologically, upward motions and upper-level negative vorticity dominate over almost the entire study domain (Fig. 3a). Positive lower-level vorticity exists over the TP. When the STPP increases, lower-level positive vorticity, upper-level negative vorticity, and upward motions over the TP all intensify correspondingly (Fig. 3b). Consistent with the study of Wei et al. (2019), when the SAH extends southeastward, convergence forms over the southeastern flank of the anomalous upper-level anticyclone, inducing descending motions that cause downward advection of negative vorticity. Consequently, an anomalous anticyclone appears over southern China and the northern South China Sea, leading to a westward extension of the WPSH.

Fig. 3.
Fig. 3.

(a) Climatology of summer vorticity (shading; s−1, multiplied by 106) and vertical circulation (vectors; horizontal wind in m s−1; vertical velocity is multiplied by 100 and is in Pa s−1) along a section from 35°N, 80°E to 10°N, 130°E. (b) Regressions of summer vorticity and vertical circulation along a section from 35°N, 80°E to 10°N, 130°E against the STPP index. White stippling and blue vectors in (b) indicate the values significantly above the 95% confidence level. Topography is marked by the dark gray shading.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

4. Impact of the TP’s thermal forcing on the SEASM in climate model simulations

To understand the thermal impact of the TP on the SEASM, a control experiment CTRL with the atmospheric model of the CESM is conducted, and a corresponding sensitivity experiment TP_Noheat is carried out by turning off the diabatic heating over the southern TP. Figure 4 presents the differences in summer precipitation, 850-hPa winds, and 200- and 500-hPa geopotential heights between CTRL and TP_Noheat. When the southern TP heating increases, precipitation decreases over southern China, the northern South China Sea, the Indo-China Peninsula, the northern Bay of Bengal, and northern India, corresponding with an anomalous lower-level anticyclonic circulation (Fig. 4a). Indeed, the negative relationship between the precipitation over the southern TP and that over northern India was noticed previously (Jiang et al. 2016; Wu et al. 2016; Lu et al. 2018, 2021). Moreover, the SAH intensifies and extends southeastward (Fig. 4b), and the WPSH extends southwestward accordingly (Fig. 4c). These features of anomalous circulation and precipitation and the physical processes involved are consistent with those observed but with stronger magnitudes.

Fig. 4.
Fig. 4.

Differences in summer (a) precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1), (b) 200-hPa geopotential height (shading; m), and (c) 500-hPa geopotential height (shading; m) between CTRL and TP_Noheat. Black contours in (b) represent the South Asian high in CTRL; blue and black contours in (c) reveal the locations of the western Pacific subtropical high in CTRL and TP_Noheat, respectively. White stippling in each panel and blue vectors in (a) indicate the values significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contour.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

The changes in vorticity and vertical circulation are shown in Fig. 5. When the southern TP heating increases, lower-level positive vorticity, upper-level negative vorticity, and rising motions intensify as in observations. Associated with the eastward-extended SAH, sinking motions and downward advection of negative vorticity form over the SEASM region. To summarize, the above features well match the main characteristics of weak SEASM, and are quite similar to the observed patterns shown in Figs. 13. Nevertheless, the patterns in the model experiments slightly shift northward compared with the observed ones, probably associated with the northward shift of mean circulation including the WPSH and the SAH in CTRL.

Fig. 5.
Fig. 5.

(a) Summer climatology of vorticity (shading; s−1, multiplied by 106) and vertical circulation (vectors; horizontal wind in m s−1; vertical velocity multiplied by 100 in Pa s−1) along a section from 35°N, 80°E to 10°N, 130°E in CTRL. (b) As in (a), but for the differences between CTRL and TP_Noheat. White stippling and blue vectors in (b) indicate the values significantly above the 95% confidence level. The topography is marked by the dark gray shading.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

5. Modulating effect of Atlantic SST on the TP–SEASM relationship

Previous studies indicated that SST anomalies especially those in the North Atlantic, as well as the SST anomaly related to the North Atlantic Oscillation, could affect the TP’s thermal forcing and the Asian summer monsoon (Cui et al. 2015; Jiang et al. 2015, 2016; Wang et al. 2018; Zhao et al. 2018; Yu et al. 2021). Hence, we further investigate the possible modulation of SST on the TP–SEASM relationship. Figure 6 shows the composite difference in SST between the high and low STPP years, which are selected using the criterion of ±1 standard deviation. It can be seen that the STPP-related SST distribution presents a La Niña–like pattern and a warm Atlantic pattern, indicating that El Niño–Southern Oscillation (ENSO) and the Atlantic Ocean are two important factors for modulating the diabatic heating over the southern TP and the TP–SEASM relationship (Fig. 6a). A warming in the extratropical North Atlantic is evident. However, the changes in precipitation and 850-hPa winds, which are related to the extratropical Atlantic SST index (EA_SST), are smaller than those related to the TA_SST index, suggesting that the contribution of extratropical Atlantic SST to the SEASM variability is relatively small (figure not shown). Therefore, the impact of extratropical Atlantic SST on the SEASM is not considered in the current study. Moreover, the associated SST pattern can also be found in the previous spring, but with a weaker magnitude, over the tropical Pacific (Fig. 6b), further suggesting that the warm SST in the tropical Atlantic is an important precursor for modulating the TP heating.

Fig. 6.
Fig. 6.

Composite difference in SST (shading; °C) between high and low STPP years for (a) July–August and (b) March–May, respectively. White stippling indicates the values significantly above the 95% confidence level. The two black boxes in (a) and (b) represent the Niño-3.4 region and the tropical Atlantic, respectively.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

To evaluate the possible impacts of the ENSO and the tropical Atlantic SST on the TP–SEASM relationship, we obtain regressed patterns of precipitation and 850-hPa winds against the STPP index with the signals of the ENSO and/or tropical Atlantic SST removed (Fig. 7). The changes in SEASM circulation and precipitation, especially those in its southern regions, become smaller after the ENSO signal is removed (cf. Figs. 7a and 1b). Compared to the changes in SEASM circulation and precipitation with ENSO signal removed, those without the tropical Atlantic SST variation are less significant (cf. Figs. 7a and 7b), indicating that the impact of the tropical Atlantic SST on the TP–SEASM relationship is possibly larger than that of ENSO-related SST. Moreover, the correlation coefficient between the STPP index in July–August and the Niño-3.4 SST in the following December–February, the following September–November, and the preceding March–May are −0.591, −0.513, and −0.125, respectively. It turns out that the STPP index is more likely followed by rather than led by a La Niña event, indicating that it may be a factor that affects the ENSO event. Hence, the role of ENSO-related SST variation is not considered in the current study. In addition, significant changes in SEASM circulation and precipitation are still found when both ENSO and tropical Atlantic SST signals are removed (Fig. 7c), suggesting that tropical oceans only modulate, but not determine, the TP–SEASM relationship on the interannual time scale.

Fig. 7.
Fig. 7.

Regressed summer precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1) against the STPP index without the linear signals of (a) the Niño-3.4 index, (b) the TA_SST index, and (c) both the Niño-3.4 index and the TA_SST index. White stippling and blue vectors indicate the regression coefficients significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contours.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

The above results seem to indicate that the TP-related tropical Atlantic SST anomalies can affect the TP–SEASM relationship. To further discuss the possible modulation of those SST anomalies on the TP–SEASM relationship, we present the regressed patterns of precipitation, atmospheric circulation, and wave activity flux against the TA_SST index in Fig. 8. When the tropical Atlantic SST increases, precipitation is enhanced, and anomalous cyclonic circulation prevails locally with westerlies on the southern flank of the cyclone (Fig. 8a). Precipitation decreases over the tropical Pacific and the SEASM region, corresponding to the intensified trade wind over the central-western equatorial Pacific and the westward-extended WPSH, respectively. Meanwhile, the precipitation over the southern TP, East Asia, and the Maritime Continent increases.

Fig. 8.
Fig. 8.

Regressed summer (a) precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1), (b) 200-hPa geopotential height (shading; m) and wave activity flux (vector; m2 s−2), (c) 500-hPa geopotential height (shading; m), and (d) longitude–height vertical circulation (vectors; zonal wind in m s−1; vertical velocity multiplied by 50 in Pa s−1) and vertical motions (shading; Pa s−1, multiplied by 50) averaged along 0°–10°N against the TA_SST index. White stippling and blue vectors in (a) and (d) indicate the regression coefficients significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contour.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

As a response to the anomalous convection over the tropical Atlantic and its vicinity, an anomalous lower-level cyclone and upper-level anticyclone form locally. On the northeastern side of the heating-induced cyclone, anomalous quasi-barotropic anticyclonic circulation occurs over eastern North America as a Rossby response (Figs. 8b,c). An anomalous stationary wave train with a quasi-barotropic structure appears in the extratropical Northern Hemisphere at 200 and 500 hPa, propagating southeastward to East Asia with positive centers over eastern North America, the extratropical North Atlantic, the eastern Mediterranean Sea, and the southeastern TP/East Asia, but negative centers over western Europe and West Asia. Clearly, this wave train shifts the center of the SAH eastward, leading to the formation of convergence over the southeastern flank of the anomalous upper-level anticyclone and inducing descending motions that cause downward advection of negative vorticity. Consequently, an anomalous anticyclone appears over the SEASM region, leading to a westward extension of the WPSH (Fig. 8c). Besides, the anomalous heating over the tropical Atlantic also modulates the Walker circulation through two anomalous equatorial vertical cells (Fig. 8d). Heating-induced upward motions form over the tropical Atlantic. Correspondingly, anomalous subsidence and decreased precipitation occur over the tropical Pacific, while ascending motions and increased precipitation appear over the Maritime Continent and the tropical eastern Indian Ocean. As a Gill-type response (Gill 1980), an anomalous lower-level anticyclone forms over the northeastern side of the heating center over the Maritime Continent and the eastern tropical Indian Ocean, also favoring a westward extension of the WPSH. Thus, the tropical Atlantic SST anomalies can modulate both the TP’s thermal forcing and the SEASM, and then affect the TP–SEASM relationship through the above-discussed two processes.

Two scatter diagrams are provided to further illustrate the relationship between the TP’s thermal forcing and the SEASM, and that between the TP forcing and the tropical Atlantic SST (Fig. 9). Standard deviations of ±0.5 for each index are chosen for identifying the significant cases in the four quadrants. It can be seen that almost all significant cases in Fig. 9a are located in the second and fourth quadrants, indicating a robust negative correlation between the precipitation over the southern TP and the SEASM. In Fig. 9b, significant cases are located in the first and third quadrants, presenting an evident positive relationship between the precipitation over the southern TP and the tropical Atlantic SST. To further evaluate the synergistic impact of TP heating and tropical Atlantic SST, we conduct composite analyses for the eight years with both positive STPP and TA_SST anomalies and for the eight years with both negative STPP and TA_SST anomalies, and for the seven years with positive STPP only and for the seven years with negative STPP only (Fig. 10). The monsoon circulation and precipitation over the SEASM region decrease more significantly when the STPP and TA_SST indices are both positive (Fig. 10a). However, features are less significant in the cases of positive STPP only (Fig. 10c), suggesting that the warm tropical Atlantic SST plays an important role in enhancing the TP–SEASM relationship. However, the features in the negative STPP years with and without the influence of the TA_SST do not show evident distinction (cf. Figs. 10b and 10d), implying an asymmetry between the impacts of warm and cold tropical Atlantic SST anomalies on the TP–SEASM relationship.

Fig. 9.
Fig. 9.

Scatter diagrams of the standardized (a) SEASM and STPP indices and (b) TA_SST and STPP indices. The three asterisks and one asterisk following the correlation coefficients indicate significant values exceeding the 99.9% and 95% confidence levels, respectively.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

Fig. 10.
Fig. 10.

Composite differences in summer precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1) (a) between high STPP/warm TA_SST years and climatology, (b) between low STPP/cold TA_SST years and climatology, (c) between high STPP years and climatology, and (d) between low STPP years and climatology. White stippling and blue vectors indicate the values significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contour.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

To further confirm the observed modulating impact of tropical Atlantic SST on the TP–SEASM relationship, a sensitivity experiment with forcing twice the observed composite SST anomalies in the tropical Atlantic (TA_Warm) is conducted. Responses of summer precipitation and atmospheric circulation to the tropical Atlantic SST anomalies in TA_Warm are presented in Fig. 11. When the tropical Atlantic SST increases, precipitation increases, and anomalous cyclonic circulation prevails locally with westerlies on the southern flank of the cyclone (Fig. 11a). Meanwhile, the precipitation over the southern TP, East Asia, and the Maritime Continent increases, whereas the precipitation over the SEASM region decreases, corresponding to the intensified trade wind over the central-western equatorial Pacific and the westward extension of the WPSH, respectively. In response to the anomalous heating over the tropical Atlantic and its vicinity, a significant extratropical wave train appears in the Northern Hemisphere and propagates southeastward from eastern North America to the southeastern TP/East Asia, leading to an eastward extension of the SAH, a westward extension of the WPSH, and weakened SEASM (Figs. 11b,c). The reason for choosing the doubled SST anomalies here is to magnify the model responses of precipitation and atmospheric circulation to the SST forcing. An experiment with the observed SST anomalies was also conducted, in which the distributions of precipitation and circulation were similar but the magnitude was reduced compared with the experiment of doubled SST anomalies (figure not shown). The atmospheric wave response over North America and the North Atlantic in TA_Warm is consistent with the observed wave pattern, although the anomalous cyclonic circulation over the central North Atlantic is stronger than the observed. However, a few differences exist between the observed and model-simulated patterns. Compared to the observed patterns (Figs. 8b,c), the anticyclonic circulation over the eastern mid-Atlantic and the Mediterranean Sea extends northeastward, and the downstream cyclonic center over the northwest of the TP is stronger. This could be caused by the air–sea decoupling in the sensitivity experiment, model bias, and/or by other confounding factors in observations. Nevertheless, the main features around the TP–SEASM regions in the model sensitivity experiments are consistent with those in observations.

Fig. 11.
Fig. 11.

Differences in summer (a) precipitation (shading; mm day−1) and 850-hPa winds (vectors; m s−1), (b) 200-hPa geopotential height (shading; m), and (c) 500-hPa geopotential height (shading; m) between TA_Warm and CTRL. White stippling and blue vectors in (a) indicate the values significantly above the 95% confidence level. The topography of the TP above 1500 m is marked by the dashed gray contour.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

6. Conclusions and discussion

We investigate the thermal impact of the TP on the SEASM and the possible modulating effect of tropical Atlantic SST on the TP–SEASM relationship. Both observational analyses and model sensitivity experiments indicate that a robust negative correlation exists between the precipitations over the southern TP and the SEASM. The associated physical mechanisms are summarized in Fig. 12. When diabatic heating intensifies over the southern TP, the upper-level SAH enhances and extends eastward. Correspondingly, convergence forms over the southeastern flank of the anomalous upper-level anticyclone, inducing sinking motions that causes downward advection of negative vorticity. As a result, an anomalous lower-level anticyclone appears over southern China and the northern South China Sea, leading to a westward extension of the WPSH and weakening of the SEASM circulation.

Fig. 12.
Fig. 12.

Schematic diagram showing physical mechanisms for the thermal impact of the TP on SEASM and the modulation of tropical Atlantic SST anomalies on the TP–SEASM relationship.

Citation: Journal of Climate 36, 5; 10.1175/JCLI-D-22-0493.1

Compared with the years of positive STPP anomalies only, the SEASM precipitation and circulation weaken more significantly in the years when the STPP and tropical Atlantic SST anomalies are both positive, suggesting that tropical Atlantic SST warming plays an important role in enhancing the TP–SEASM relationship. When the tropical Atlantic is warmer than normal, precipitation increases and an anomalous cyclonic circulation prevails locally with westerlies on the southern flank of the cyclone. As a response to the anomalous convection over the tropical Atlantic, an anomalous barotropic wave train occurs in the extratropics, propagating southeastward from eastern North America to East Asia, with a positive geopotential height center over southeastern TP and East Asia, favoring an eastward-extended SAH and a westward-extended WPSH. In the meantime, the anomalous heating over the tropical Atlantic modulates the Walker circulation through two anomalous equatorial cells, with rising motions and increased precipitation over the Maritime Continent and the eastern tropical Indian Ocean. As a Gill-type response, an anomalous lower-level anticyclone forms over the northeastern side of the heating center over the Maritime Continent and the eastern tropical Indian Ocean, causing a westward extension of the WPSH. Hence, the warming of tropical Atlantic SST intensifies the TP’s thermal forcing and weakens the SEASM, which enhances the TP–SEASM relationship through both extratropical wave train and tropical zonal circulation.

It should be pointed out that the tropical Atlantic SSTs in the boreal spring and summer also play an important role in influencing ENSO events (Ding et al. 2012; Ham et al. 2013a,b; Wang 2019; Jiang and Li 2021; Wang and Wang 2021; Jiang et al. 2022). While the current study has addressed the modulating impact of tropical Atlantic SST on the TP heating and the SEASM through the atmospheric bridge effect, the importance of interaction between the Atlantic and the Pacific on the TP–SEASM relationship has not been fully considered. How the Atlantic SST affects the TP–SEASM relationship through its interaction with the Pacific SST remains a question, which may require a fully coupled model experiment to address this issue. Furthermore, here we only discussed the TP–SEASM relationship on the interannual time scale. A recent study revealed an interdecadal change in the relationship between TP temperature and South China Sea summer monsoon precipitation (Liang et al. 2020). Whether and how interdecadal variation exists in the TP–SEASM relationship between the different mean states of the atmosphere and ocean are not clear. Therefore, further investigations are needed to better demonstrate the instability of the TP–SEASM relationship, the impacting factors, and the responsible physical mechanisms by applying both observational analyses and numerical experiments.

Acknowledgments.

The authors thank Dr. Boqi Liu of the Chinese Academy of Meteorological Sciences for constructive discussions. The three anonymous reviewers provided valuable comments and suggestions for improving the overall quality of the paper. This research was jointly supported by the National Natural Science Foundation of China (Grant 42205042), the Basic Research Fund of the Chinese Academy of Meteorological Sciences (2022Y010), the Second Tibetan Plateau Scientific Expedition and Research (STEP) program (Grant 2019QZKK0105), the Guangdong Major Project of Basic and Applied Basic Research (Grant 2020B0301030004), the Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies (Grant 2020B1212060025), and the Jiangsu Collaborative Innovation Center for Climate Change. Model experiments were conducted at the supercomputing system Tianhe-2 in Guangzhou, China.

Data availability statement.

GPCP monthly precipitation data are provided by the NOAA PSL, Boulder, Colorado, United States, from their website at https://psl.noaa.gov/data/gridded/data.gpcp.html. ERA5 monthly reanalysis data can be obtained from https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels-monthly-means?tab=form. ERSST V5 monthly data are from the NOAA PSL, Boulder, Colorado, United States, from their website at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html. The Niño3.4 index is obtained from https://psl.noaa.gov/data/correlation/nina34.anom.data.

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