1. Introduction
The Tibetan Plateau (TP), often referred to as the “roof of the world,” exerts a significant influence on regional and global climate and weather, particularly during the summer season, owing to its remarkable thermal and mechanical forcing effects (Cuo et al. 2014; Zhao et al. 2016a, 2019a; You et al. 2020). Moreover, the TP holds the distinction of being recognized as the “Asian water tower” due to its unique ability to draw in moisture from the perimeter zone, resulting in a distinct region characterized by elevated humidity within the atmosphere (Xu et al. 2008; Boos and Kuang 2010; Wu et al. 2012; Yan et al. 2020).
TP, influenced by westerly winds and the southwest monsoon, serves as a crucial moisture transfer station that significantly impacts atmospheric circulation and precipitation patterns downstream (Liu et al. 2020; Zhao et al. 2019b, 2021; Zhu et al. 2021; Cheng et al. 2024). Son et al. (2020) applied the topographically forced barotropic Rossby wave theory, demonstrating that the subtropical zonal wind interacting with the TP plays a key role in generating downstream cyclonic and anticyclonic circulation anomalies, leading to the development of meridional winds carrying abundant moisture toward East Asia. Zhao et al. (2019c) investigated the relationship between extreme precipitation frequency and integrated moisture flux, revealing two distinct moisture and cyclonic vortex channels along the north and east of the TP, illustrating the TP’s mechanical forcing effect. These two channels could converge over the North China or the lower reaches of the Yangtze River associated with extreme precipitation events (Zhao et al. 2016b). In terms of the TP’s local thermal effect, Duan et al. (2020) employed composite analysis on multisource observational data and found that both positive surface sensible heating and condensation heating anomalies over the TP influence the onset, duration, and total precipitation of the rainy season in South China. Zhao et al. (2019d) used C-band radar observations in the central TP and demonstrated that anomalous convection activity can be transported to the lower reaches of the Yangtze River through the formation of a low-pressure vortex with abundant moisture supply and favorable upper-level divergence structure (Zhao et al. 2023). Despite these achievements, there is still a gap in understanding the impact of diabatic heating over the TP on farther-distance weather and climate, i.e., the atmospheric river (AR) activity in the North Pacific. Duan et al. (2022) highlighted the stronger influence of latent heating compared to sensible heating in summer over the TP. Therefore, it is important to study how an increased condensation latent heating generates over the TP by examining anomalous moisture contributions. In recent studies concerning moisture, Y. Li et al. (2022) categorized the TP into six main regions and identified that moisture contributions to the southern TP originate from westerlies and the Indian summer monsoon, while Zhang et al. (2024) further suggested that moisture travels along the southern side of the Himalaya Mountains and moves northward into the TP, accompanied by air currents from the Indian Ocean. From a dynamic perspective, several studies have illustrated the significant influence of Atlantic sea surface temperature (SST) heating on the TP’s heating (Z. Wang et al. 2018; Yao et al. 2021; Lu et al. 2023). They underscore how this heating process is linked to the attenuation of midlatitude westerlies in the upper troposphere over Eurasia, which in turn adjusts the export of water vapor from the TP (Zhou et al. 2019; Xu et al. 2020; Wang et al. 2024). This suggests a potential remote dynamic influence from the Atlantic Ocean is mediated by plateau heating.
AR, characterized by filamentary-shaped moisture transport bands, plays a critical role in delivering extreme precipitation when they make landfall over midlatitude regions (Waliser and Guan 2017; Kamae et al. 2021; Tian et al. 2023, 2024a,b; Pan et al. 2024; Song et al. 2024; Zhao et al. 2024). Pan et al. (2024) and Song et al. (2024) emphasize the significance of North Pacific AR activities, delineating their primary pathways and extensive hydrological impacts around the Pacific rim. Specifically, Pan et al. (2024) identify five major long-haul AR routes originating from East Asia or the western North Pacific, which terminate over western North America or Alaska, delivering heavy rainfall. These AR pathways are influenced by pressure dipoles in the North Pacific; however, the initial disturbances that establish these pressure dipole configurations remain unexplored. Conversely, Song et al. (2024) document that clustered extreme weather events—from South Asia to East Asia and onto North America—over subseasonal time scales are frequently linked to persistent ARs that connect these regions experiencing extreme conditions. Furthermore, Song et al. (2024) suggest that a significant source of these organized disturbances could be tropical, for instance, the dipole heating anomalies highlighted by B. Wang et al. (2022). Mahoney et al. (2016) highlighted that AR identification frameworks can be useful in forecasting extreme precipitation, particularly at medium- to longer-range forecast lead times. Western North America is one of the most prevalent regions for AR activity worldwide, responsible for bringing abundant moisture and inducing extreme precipitation (Mahoney et al. 2016; Michaelis et al. 2022). Moreover, Michaelis et al. (2022) emphasized that an increasingly volatile hydroclimate is heightening western North America’s reliance on precipitation from AR for water resources, particularly in the context of global warming. Regarding the global-scale impact, previous studies primarily placed greater emphasis on the summer intraseasonal oscillation in Southeast Asia, which serves as a trigger for the extratropical Rossby wave train characterized by a cyclonic circulation in the central North Pacific and an anticyclonic circulation in the eastern North Pacific (Guo et al. 2021; Lin et al. 2022; Mo et al. 2022). B. Wang et al. (2022) recently explored a new Asian teleconnection that links extreme precipitation events from India to North America, triggered by three consecutive extreme precipitation events that occurred in November 2021. They identified two cross-Pacific wave trains reinforced by diabatic heating anomalies, rapidly developing over the tropical Asian monsoon region. These wave trains steered ARs, influencing North America’s precipitation patterns. Zhang and Villarini (2018), using three AR activity clusters, highlighted a strong linkage between the East Asian subtropical jet and the AR frequency over the western United States, surpassing other potential climate modes. These previous studies have demonstrated the teleconnection between ARs in the Pacific and Asian regions. However, the relationship between the TP, a pivotal region in Asia, still remains unexplored. TP, acting as a massive thermal pump, is highly sensitive to climate change, experiencing surface warming twice as large as the global average warming (Duan and Xiao 2015; Cheng et al. 2024). Wu et al. (2023) recently summarized the last 10 years of TP research, confirming the crucial role of TP heating in influencing summer monsoon circulation from the perspectives of energy, potential vorticity, and angular momentum conservation theories. Given the above context, for a comprehensive understanding of AR activity in the North Pacific and its impact on extreme precipitation in western North America, it becomes intriguing to explore the teleconnection coupled with the TP diabatic heating.
The main objective of this study is to investigate the pivotal role of TP heating as a significant driver of AR activity in the North Pacific. To achieve this objective, we have designed a systematic approach consisting of three key steps. First, we aim to identify a sensitive TP heating region that exhibits a close relationship with the frequency of AR occurrences in the Pacific. Through this investigation, we can pinpoint the region most influential in AR activity. Second, we utilize the Water Accounting Model-2Layers (WAM-2Layers) to quantitatively study moisture sources and contributions, taking into account the impact of atmospheric circulation. This analysis will provide valuable insights into the underlying causes of TP heating anomalies with anomalous sea surface temperature influence of the Atlantic Ocean. Third, we use synoptic-scale diagnostic analysis to investigate how the anomalous circulation in the upper-level troposphere effected by the TP key region heating impacts AR activity in the North Pacific. In this respect, we employ the linear baroclinic model (LBM) in the analysis to validate the above theory. The outcomes of this study will significantly contribute to our understanding of the TP’s crucial role in modulating AR activity in the Pacific. Moreover, our research emphasizes the importance of TP heating as a potential factor for forecasting and highlights the need for further investigations in this area. Given the context of global warming, this study underscores the global effect of TP heating.
The paper is structured as follows: Section 2 provides a comprehensive overview of the data and methods utilized in this study. In section 3, we present the results, focusing on the relationship between TP heating and AR activity. This section also delves into the factors responsible for the anomalous TP heating, highlighting a quantitative assessment of moisture contribution with anomalous atmospheric circulation. Moreover, a diagnostic and verification process to examine the impact of the TP on AR activity is performed in the North Pacific. Last, in section 4, we summarize and discuss the key findings derived from our analysis.
2. Data and methods
a. Data
This study utilizes the ERA5 datasets, which is the fifth major global reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) (Hersbach et al. 2020). The ERA5 dataset provides essential variables with a spatial resolution of 1.0° × 1.0° and a temporal resolution of 6 h. These variables include horizontal wind fields (zonal and meridional components), specific humidity, surface pressure, total column water, vertical integrals of cloud-frozen water flux, cloud liquid water flux, water vapor flux, evaporation, and precipitation. The horizontal wind fields and specific humidity are obtained from 17 out of the 137 available model levels covering the entire vertical column, with a focus on the lower troposphere where moisture predominantly accumulates. The datasets cover the summer season from 1980 to 2018. It is important to note that all data, except for precipitation and evaporation, are available at 6-hourly time steps (0000:00/0600:00/1200:00/1800:00 UTC). Precipitation and evaporation data are obtained hourly. The summer season is defined as June–July–August in this study.
b. Methods
1) AR detection method
The AR frequency datasets utilized in this research were derived using Pan and Lu’s method (hereafter PanLu 2019), which employs an hourly temporal resolution defined with covering grids. The PanLu methodology enhances the spatial representation of the 85th percentile of integrated vapor transport by applying Gaussian kernel density smoothing, mitigating biased estimates that can arise from sparse sampling within individual grid cells. Furthermore, a baseline threshold—set at the 85th percentile of integrated vapor transport intensity across all grid cells—is established to filter out weaker moisture transport events. Moreover, the PanLu criteria adhere to the AR community’s agreed-upon geometric parameters for identifying an AR pathway, which stipulate that the length must exceed 2000 km and the width must be less than 1000 km (Zhou et al. 2021; Zhao et al. 2024). In addition to these criteria, PanLu introduces a constraint on the turning angle between trajectory vectors, requiring it to be less than 360° to distinguish ARs from cyclonic moisture movements. For a more comprehensive explanation of the procedure, readers are directed to Pan and Lu’s publications in Pan and Lu (2019, 2020). The PanLu algorithm, as implemented in the recent study by Zhang et al. (2024), is also available for consultation. Moreover, some studies excluded the grid point in the tropical region, whose purpose is to exclude a substantial proportion of tropical cyclones and tropical cyclone-like objects (McClenny et al. 2020; Guan et al. 2023; Wang et al. 2023). To be consistent with the previous studies, we also covered the tropical region as shown in Fig. 1.
(a) Spatial distribution of AR frequency with hourly temporal resolution during the summer season of 1980–2018. (b) Correlation between spatially averaged heating across southern TP and AR frequency in summer. The purple rectangle (145°E–127°W, 40°–52°N) represents the region of high correlation with AR activity. (c) Correlation between average AR frequency within the purple rectangle in (b) and TP heating, highlighting a highly positively correlated key region in the southern TP (83°–93°E, 28°–33°N) indicated by the purple dashed rectangle. The green line depicts the outline of the TP. (d) As in (b), but for the spatially averaged heating across southern TP. The gray shading represents the tropical region.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
2) The TP heating calculation
3) Water accounting Model-2Layers
4) Linear baroclinic model
The LBM is employed in this study to validate the physical process by which TP heating influences the upper-level circulation. In LBM, the hydrostatic primitive equations are linearized about a mean state and the linear response to a prescribed forcing is simulated. The LBM, as described by Watanabe and Kimoto (2000), has a resolution of T42L20, where the horizontal resolution is based on triangular truncation with a wavenumber of 42, and the vertical resolution consists of 20 levels on the model’s sigma coordinate. The model in this study set a time scale of 31 days for integration using ERA5 datasets as the background field. The LBM is a simplified model but effectively captures the response of the circulation to diabatic heating, providing valuable insights into the underlying dynamic processes. In this model setup, the horizontal diffusion is subject to a damping time scale of 1 day for the smallest wave. Additionally, the Newtonian damping and Rayleigh friction time scales are set at 0.5 day for the lowest three levels and the highest two levels, while a longer time scale of 30 days is applied for the remaining levels. The temperature heating rate profile is imposed as a heat source by the regional average over the southern TP as introduced in Fig. 1.
3. Results
a. The relationship between the TP heating and AR frequency
Figure 1 primarily illustrates the correlation between column-integrated TP heating and AR frequency during the summer season. Figure 1a depicts the distribution of the AR frequency in summer, revealing a prominent band of high AR frequency stretching from East China across the North Pacific (dashed rectangle in Fig. 1a) to western North America, with the highest frequency centered over the North Pacific. To investigate the link between TP heating and AR frequency, Fig. 1b displays the correlation pattern, showing a positive east–west banded structure over the North Pacific. Given the complex nature of TP, identifying the key heating region on the plateau becomes essential. Therefore, the correlation between AR frequency in the North Pacific (purple rectangle in Fig. 1b) and TP heating is also reverse examined. The result exhibits that the southern TP is a key heating area (Fig. 1c). Consequently, Fig. 1d reveals a strong correlation between southern TP heating and AR frequency, indicating that southern TP heating serves as a robust signal for estimating AR activity in the North Pacific which may be the primary driver of extreme precipitation events in western North America. Significantly, there is a region of negative correlation over the eastern Pacific; however, this area exhibits limited AR activity, as illustrated in Figs. 1a and 2. Consequently, this region is not a focus of analysis in this study. In brief, these findings suggest that intensified southern TP heating corresponds to a higher occurrence of AR in the North Pacific.
(a) Standardized anomalies of detrended spatially averaged heating across southern TP (W m−2). The red and green solid lines represent the one positive and negative standard deviation, respectively. Red dots indicate southern TP heating high years (1980, 1991, 1998, 1999, and 2008), while green dots represent southern TP heating low years (1983, 1986, 1989, 1994, 1997, and 2015). (b),(c) Summer AR frequency (shading) based on the southern TP heating high (low) years in (a). The purple dashed rectangle represents the same high correlation region as in Fig. 1b. The gray shading is the outline of the TP main body.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
To further validate the influence of TP heating on AR activity, composite AR frequency is plotted as the response to southern TP heating anomalies (Fig. 2). Five years with high TP heating anomalies and 6 years with low TP heating anomalies are selected based on detrended southern TP heating standardization calculation (Fig. 2a). The results clearly demonstrate that AR occurrence frequency in the North Pacific is significantly greater in years with high southern TP heating anomalies compared to the low anomaly years (Figs. 2b,c). Figures 2b and 2c illustrate that in years with high southern TP heating anomalies, AR is predominantly located in the northeast Pacific region, whereas in years with low southern TP heating anomalies, the AR frequency center is positioned south of the North Pacific, with lower frequencies compared to climatology in the North Pacific (Fig. 1a). Interestingly, AR activity is higher over the Arabian Sea in TP heating low anomaly years, which may be attributed to the anomalous stronger westerlies as shown in Fig. 4c. Notably, AR route appears as relatively straight ribbons in climatology and during low southern TP heating anomaly years. However, the North Pacific AR route exhibits a more meandering pattern in the high years with strong southern TP heating anomalies than in the low years. This turning route although sheds some lights on the unanswered source of heating origins that stirs different main routes of cross-Pacific ARs as identified by Pan et al. (2024) warrants further detailed investigation, as explained in section c of the results.
The complex topography of the elevated TP, along with its pronounced radiation effects, substantial sensible heating, and abundant remote moisture transport and local evaporation with latent heating release from phase transition, presents challenges in determining the dominant factors responsible for the southern TP heating. Notably, the southern TP is influenced by monsoon circulation (J. Wang et al. 2022), and the evapotranspiration and total water mass in lakes exhibit a clear increase with the warming of the TP (Yang et al. 2011; Yao et al. 2022; Cheng et al. 2024), implying enhanced evaporation. Consequently, latent heating may play a more significant role in driving anomalous diabatic heating in the southern TP. To address this, we calculate Q1 and Q2 to examine the role of latent heating (Fig. 3). Figure 3a demonstrates a strong positive correlation between column-integrated Q1 and Q2 in the southern TP heating after removing the linear trend. Additionally, the height–latitude cross section of Q1 and Q2 exhibits similar patterns during the southern TP heating anomalous high years and low years, as shown in Figs. 3b–e. Furthermore, moisture anomalies are significantly pronounced with strong upward vertical motion during years with anomalous high southern TP heating (Figs. 3b,d) and vice versa (Figs. 3c,e). These findings suggest that latent heating primarily contributes to southern TP diabatic heating. Therefore, further investigation is required to quantitatively explore the moisture sources responsible for the anomalous southern TP diabatic heating.
(a) The correlative relationship between the standardized 〈Q1〉 and 〈Q2〉. The latitudinal cross distribution of anomalous Q1 (shading; 100 × Q1; J kg−1 s−1) with vertical velocity (contours; 100 × ω from −2 to 2 with 0.2 interval; Pa s−1) along the meridional average (83°–93°E) of the southern TP region for TP heating (b) high years and (c) low years. (d),(e) As in (b) and (c), but for Q2 (shading; 100 × Q2; J kg−1 s−1) and specific humidity (contours; 104 × q from −6 to 6 with 0.8 interval; kg kg−1). The black shading is the TP topography.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
b. The causes of the anomalous southern TP diabatic heating
Based on the above analysis, we have confirmed that saturated moisture released latent heating plays a crucial role in the anomalous heating of the southern TP. However, given the presence of over 1200 lakes and increased water content due to solid water melt supplement in the TP as well as the strong local recycling (Y. Li et al. 2022; L. Wang et al. 2022; Zou et al. 2022; Cheng et al. 2024), local evaporation may not be neglected as a contributing factor to the southern TP heating. Zhang et al. (2017) also studied that the moisture sources change with the varied seasonally. The westerlies are the main moisture contribution source in May and June. However, the westerlies, the Indian summer monsoon, and the East Asian summer monsoon (Dai et al. 2021, 2023) all made significant contributions to the precipitation in west-central TP from July to August. To gain a comprehensive understanding of the atmospheric moisture’s internal recycle and outside circulation impact on anomalous TP heating, we conduct moisture backward tracking, dividing them into different regions: the local moisture contribution region (Pan TP) and remote moisture contribution regions, including Eurasia (EA), Arabian Sea (ABS), Bay of Bengal (BOB), and Southern Hemisphere (SH). These regions are identified based on the climatological transport features, which are the main moisture contributions to the southern TP (Fig. 4a). Figure 4a demonstrates that besides the local Pan TP contribution, there are two moisture channels that significantly contribute: the EA channel and the Southern Ocean contribution composed of the SH, ABS, and BOB regions. Quantitative climatological results in Fig. 4d reveal that the local contribution is the most dominant among the five main source regions, while the SH region exhibits the smallest contribution. The other three regions contribute almost equally to the moisture supply. During years with high southern TP heating, both local and remote moisture supplements show positive contributions (Figs. 4b,d). Conversely, in years with low southern TP heating, moisture contributions display negative anomalies in both local and remote sources (Figs. 4c,d). However, despite the local contribution remaining the largest in terms of moisture supply, its relative changes are the smallest among the five regions, regardless of abnormally southern TP heating high or low years. This indicates that remote moisture transport plays a more critical role in the anomalous southern TP heating (Figs. 4d,e). Additionally, Fig. 4e highlights that the moisture change in the Southern Ocean passage of SH, ABS, and BOB is 55.74%, which is 2.8 times larger than the 17.95% contribution from the EA passage during years with high southern TP heating. This suggests that the Southern Oceans have a more significant impact on anomalous TP diabatic heating, with the BOB region acting as the key moisture pathway region as shown in Figs. 4a–c. However, although moisture contribution is clearly increased or decreased in Figs. 4b and 4c, wind fields do not show significant changes in the Southern Ocean moisture channel which is different from our traditional view that enhanced Somali jet. Therefore, further investigation is required to understand how more moisture is transported and uplifted to the TP.
(a) The quantitative moisture contribution for the southern TP heating in climatology (shading; mm) and column-integrated water vapor flux (vectors; g m−1 s−1); the anomalous quantitative moisture contribution (shading; mm) and column-integrated moisture flux (gray vectors; g m−1 s−1) with their anomaly (black vectors; g m−1 s−1) at the 95% confidence level for the southern TP heating (b) high years and (c) low years. The purple rectangle is the southern TP heating region, and the black rectangles represent the local (Pan TP) and remote (EA, SH, ABS, and BOB) moisture supplement regions; (d) average moisture contribution in different regions for climatology and southern TP heating high and low years; (e) as in (d), but for the relative change in moisture contribution.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
The mid- to lower-tropospheric synoptic-scale circulation anomalies in southern TP heating high years and low years are shown in Fig. 5. The figure shows that there are two obvious anomalous SST warm centers in the BOB region and the North Atlantic Ocean of the EA region associated with the enhanced evaporation and the anticyclonic circulation over these two SST warm regions in the southern TP heating high years (Fig. 5a). First, for the north channel of the EA region, the result implies that the enhanced SST will make the lower-level wind fields stronger over the west of the EA and these anomalous wind fields make the moisture contribution increase over the EA terrestrial region. Second, for the primary moisture channel in the Southern Oceans, except for the anomalous SST heating, the western North Pacific subtropical high (WNPSH) obvious western extent brings the stronger easterly winds along the south of the WNPSH. The synergistic effect of the SST heating and WNPSH will make the moisture from the Southern Ocean turn toward the TP region as shown in Figs. 4b and 5a. In reverse, the SST in the BOB region and the North Atlantic Ocean are anomalous cold and the WNPSH is slightly weak in the southern TP heating low years compared with the climatology (Fig. 5b). Meanwhile, when the Atlantic exhibits anomalous SST condition (warmer minus colder condition), it typically fosters anomalous convection over this region, triggering a Rossby wave train in the extratropics that propagates toward the TP, culminating in a positive anticyclone over the southern TP (Fig. 5c). This result is similar to previous studies (Cui et al. 2015; Z. Wang et al. 2018; Yu et al. 2021; Lu et al. 2023), which provide a favorable dynamic condition to attract moisture lifted associated with the TP heating. As described in Figs. 4b and 5a, the BOB is the key moisture transfer area which makes the southern TP anomalous heating with more moisture transport from its west boundary and stronger easterly wind from its east boundary. To verify this finding, we quantitatively calculate the anomalous moisture flux by decomposing it into dynamic contribution and thermodynamic contribution in western boundary and eastern boundary of the BOB region as shown in Fig. 5d with details about the moisture equation in J. Li et al. (2022). The zonal wind velocity and specific humidity in each summer are expressed as
(a) Anomalous distributions of detrended SST (shading; K) and horizontal wind fields at 850 hPa (vectors; m s−1) with subtropical high (5860 gpm) at 500 hPa during the southern TP heating high years. (b) As in (a), but for the southern TP heating low years. Black contour is for the climatological subtropical high, and purple contour is the subtropical high for the southern TP heating high/low years. The white stippling represents the positive or negative SST anomaly for more than 3 years. (c) The difference in the 200-hPa wind fields (m s−1) with blue and gray vectors between the southern TP heating high years and low years. Composite wind field anomalies that are statistically significant at the 95% confidence level are indicated by blue vectors. (d) The decomposed moisture flux convergence for the eastern (105°E) and western (78°E) boundaries of the BOB region. The term 〈⋅〉 is the vertically integrated result, and the zonal winds point to the BOB region is defined as the positive contribution.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
c. The explanation for the connections between the southern TP heating and AR frequency
Based on the related phenomenon between the southern TP heating and AR activity in the North Pacific as well as the cause of the anomalous southern TP heating, the storyline in the following will solve the physical process of how the southern TP heating affects the AR activity in the North Pacific. In Fig. 6, we present a diagnostic and numerical experiment using the LBM to elucidate the impact of anomalous southern TP heating on synoptic-scale circulation anomalies downstream. The result in Fig. 6a demonstrates a clear wave train structure at the upper-level troposphere, featuring anticyclone and cyclone alternate circulation patterns. This structure exhibits from the southern TP region, passes over North China, and eventually reaches the east of Japan, highlighting the teleconnection between southern TP heating and the downstream atmospheric circulation. To mimic the TP warming locally due to the latent heating in the southern TP region and further validate the results from Fig. 6a, we impose heat over the southern TP centered at (88.5°E, 30.5°N) based on the southern TP heating rate profile on average of the TP heating high years and low years. Although the location and region of influence of the upper-level Rossby waves have some differences between Figs. 6a and 6b, the wave train signal simulated by the LBM in Fig. 6b does not extend into the North Pacific. This limitation may stem from several factors: the design of the heating profile, the omission of remote dynamic effects such as Rossby waves originating from the Atlantic Ocean, and the counteract in thermal intensity between years of high and low TP heating. Nevertheless, the simulation results reveal a consistent transport direction and pathway for the Rossby wave train, which supports our hypothesis regarding the impact of southern TP heating on northern Pacific circulation. Our results are also consistent with the findings of Wang et al. (2008), which although highlighted the whole TP heating but likewise indicated that the TP warming can make a downstream Rossby wave train whose center is along the core or south of the upper-level westerly jet stream in East Asia, leading to enhanced anticyclone circulation to the east of Japan. With the synoptic-scale vortex circulation affected by the southern TP heating propagating downstream, in the following, we focus on exploring how this system would affect the AR activity in the North Pacific.
(a) The difference in the 200-hPa wind fields (m s−1) with blue and gray vectors between the southern TP heating high years and low years. Composite wind field anomalies that are statistically significant at the 95% confidence level are indicated by blue vectors. (b) The verification result of wind fields and geopotential height (m) at 200 hPa in our LBM experiment in which a southern TP average temperature heating rate with all selected anomalous summer years is imposed.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
Figure 7a presents the correlation between southern TP heating and horizontal wind fields at 200 hPa. The results reveal a significant high correlation anticyclone pattern located on the east of Japan (this anticyclone is referred to as the eastward-propagating anticyclonic vortex), consistent with the findings in Fig. 6. These results not only provide further confirmation of the TP’s impact on the eastward-propagating anticyclonic vortex but also establish a close relationship between southern TP heating and midlatitude westerlies and easterlies in the Pacific. The eastward-propagating anticyclone is positioned between the westerlies and the easterlies, leading to enhanced westward flow at its north and eastward flow at its south. This configuration is illustrated in Fig. 7b, which proposes a possible circulation pattern. According to the model proposed in Fig. 7b, the upper-level anticyclonic vortex induces two circulation branches at its north and south. The northern branch of the circulation is suggested to be linked to increased AR activity with stronger westward winds in the North Pacific, while the south branch circulation may be associated with the enhancement and western extension of the WNPSH and stronger eastward winds in the lower levels along the south of the WNPSH as depicted in Fig. 5a. To further verify Fig. 7b and explore atmospheric circulation during strong southern TP heating years, it presents an actual meridional cross section in Fig. 7c. The results show the similar circulation features to the schematic illustration in Fig. 7b, with upper-level divergence, low-level convergence, and strong vertical upward motion in the east of Japan. Notably, an obvious enhancement of westerly winds is observed to the north of the upper-level divergence, while an apparent enhancement of easterly winds is observed to the south of this divergence structure. There are two descending branches in the north of the upper-level westerlies and south of the upper-level easterlies, which build a close circulation with the eastern-propagating anticyclone in the east of Japan. Overall, the findings from Fig. 7 provide crucial insights into the dynamic relationships between southern TP heating, atmospheric circulation patterns in the east of Japan, and the behavior of midlatitude westerlies and easterlies. The circulation pattern will enforce the cyclonic development in the low-level transpose in the east of Japan and increase the strength of the WNPSH.
(a) Correlation between the southern TP heating and wind fields at 200 hPa with 95% confidence level (arrows). The shadings represent the zonal wind correlative relationship at the confidence level. (b) Schematic illustration of the meridional overturning circulation features along the 170°E. (c) The height–latitude cross section of the circulations averaged over 165°–175°E average during the southern TP heating anomalous high years. The shadings are the divergence fields (10−6 s−1), and the black contours present the vertical upward motion (solid lines; Pa s−1) and vertical downward motion (dashed lines; Pa s−1). The stippling and hatching areas stand for the anomalous westerly and easterly with greater than 2 m s−1, respectively.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
The circulation patterns in different heights during southern TP heating high and low years are depicted in Fig. 8, building upon the insights gained from Fig. 7. During the southern TP heating high years, the eastward-propagating anticyclonic vortex (as shown in Fig. 6) with intensified westerlies and easterlies envelops AR activity in the Pacific (Fig. 8a). This outcome aligns consistently with the observed correlation between zonal wind and southern TP heating in Fig. 7a. These amplified westerlies and easterlies contribute to the development of an anomalous high pressure system in the central–eastern Pacific in the midlevel (Fig. 8c), leading to a northward shift in the moisture channel and impacting the western North American region, which can explain the AR turning feature illustrated in Fig. 2b, as previously mentioned. Furthermore, a noticeable upper-level divergence structure extends from East China to the east of Japan and across the middle Pacific in Fig. 8a. In the midlevel to lower level (Fig. 8c), a banded cyclonic vortex structure forms from East China to the western North American region, attracting abundant moisture transport from the Southern Oceans into the North Pacific. The combined presence of the upper-level divergence and low-level cyclonic vortex creates favorable conditions for moisture lifting, resulting in increased AR activity in the North Pacific. Therefore, the southern TP heating high years exhibit a coherent banded structure characterized by the features of upper-level divergence, low-level cyclonic vortex, and then the AR activity. In contrast, during southern TP heating low years, although there is still cyclonic vortex activity in the Pacific (Fig. 8d), AR activity is noticeably diminished. The primary distinguishing factor between southern TP heating high and low years is the upper-level circulation features. Specifically, there is a lack of a prominent divergence structure in the Pacific during southern TP heating low years (Fig. 8b), which is unfavorable for moisture lifting, resulting in relatively weak AR activity.
The upper-level divergence field (shading; 10−6 s−1) at the 95% confidence level shown by white stippling, anomalous westerly wind fields (solid contours; m s−1), and anomalous easterly wind fields (dashed contours; m s−1) during the southern TP heating (a) high years and (b) low years. The zonal wind anomalies are contoured from −5 to 5 with 1 m s−1 interval. (c),(d) As in (a) and (b), but for the vorticity field at 700 hPa (shading; 10−6 s−1) at the 95% confidence level with white stippling and anomalous positive geopotential height at 500 hPa (contours; m).
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
A significant observation is the pronounced enhancement of the anomalous WNPSH during southern TP heating high years, as depicted in Fig. 8c. This WNPSH exhibits a clear anticyclonic structure along the east coast of China. This structure not only facilitates increased moisture transport to the North Pacific but also induces enhanced easterlies at the south of the WNPSH, which channel zonal airflow toward the BOB region, reinforcing Southern Ocean moisture transport to the southern TP, as illustrated in Figs. 5a and 5c. This interplay results in a positive feedback mechanism between southern TP heating and the amplified easterly airflow by the western extension of the WNPSH, further influencing the anomalous anticyclonic vortex in the east of Japan triggered by the southern TP’s anomalous heating.
4. Discussion
The findings of this study provide valuable insights into the intricate ways in which TP heating impacts global weather and climate patterns. As the TP experiences warming and increased humidity, the intensified occurrence of ARs can significantly influence weather patterns, particularly in West North America, potentially leading to more frequent and devastating extreme precipitation events.
Although we highlighted that the variations in AR over the North Pacific are influenced by heating over TP, the related causal analysis requires further exploration, especially for the causes of TP heating. It is worth affirming that the pivotal role of the TP plays a bridge role in influencing extratropical climate and weather. Nevertheless, the quantitative assessment of various potential causes on the TP heating mechanism remains elusive. Numerous studies have demonstrated that heating of the Atlantic SST significantly affects the heat source of the TP, underscoring the vital role of Atlantic SST warming in intensifying TP heating (Wang et al. 2013; Cui et al. 2015; Z. Wang et al. 2018; Yao et al. 2021; Yu et al. 2021; Lu et al. 2023). We also affirm this conclusion in this study. However, the comparative effects of Atlantic SST on AR activity and of TP heating on AR need to be clarified. Moreover, the quantitative interactions between heating of the TP and Rossby waves originating from the Atlantic Ocean on the AR activity are still not well understood. Additionally, Lu et al. (2023) observed that beyond the effect of eastward-transported waves, anomalous heating in the Atlantic also alters the Walker circulation, forming two anomalous equatorial cells that amplify upward motions over the Maritime Continent. This results in a Gill-type response, where an anomalous lower-level anticyclone forms over the northeastern side of the heating center, the Maritime Continent, and the eastern tropical Indian Ocean, extending the WNPSH westward. They emphasized that Atlantic SST warming strengthens the TP’s thermal forcing through both extratropical wave trains and tropical zonal circulation, affecting the zonal dynamics of the WNPSH. Our study, in contrast, emphasizes the meridional influences on the WNPSH due to the enhanced upper-level jet stream. The increase in convective activity over the North Pacific Ocean induces upper-level divergence and lower-level cyclonic circulation. This sequence initiates a Rossby wave that affects the TP through low-level convergence and upper-level divergence. It also influences eastern China by inducing upper-level cyclonic and lower-level anticyclonic circulations. These findings underscore the complexity of the underlying mechanisms involved.
In addition to these implications, a study by Zhang et al. (2023) has shed light on the relationship between Arctic sea ice recovery and AR activity. Their research revealed that the moisture carried by more frequent ARs intensifies surface downward longwave radiation and precipitation, leading to the stronger melting of thin, fragile ice cover in the Arctic and a slower seasonal recovery of sea ice. Hence, the flooding events in the northwest American region could be attributed to the AR effect, not only because of the direct extreme precipitation but also from warm advection associated with the AR resulting in a large buildup of ice and snowmelt (Mo et al. 2022). Guo et al. (2021) highlighted the northward-propagating boreal summer intraseasonal oscillation (BSISO) convection over the Philippines (phases 7–8) induces a low-pressure anomaly and the corresponding anomalous cyclonic circulation leading to the enhanced poleward moisture transport and more frequent AR activity over the Pacific. Hu et al. (2022) revealed that TP directly influences the surrounding circulation and water vapor distribution via its dynamic and thermal effects and then modulates the northward propagation of the BSISO in the northern Bay of Bengal. Therefore, the role of the other phases of BSISO associated with TP heating indirectly affecting the AR activity is also worth studying. Especially, against the backdrop of ongoing global warming, the TP is experiencing warming and increased moisture content, a trend supported by the study by Kuang and Jiao (2016). Concurrently, AR intensity and frequency are also rising in the North Pacific, with shifts in their tracks toward higher latitudes, and causing more frequent extremes when making landfall as documented by Shearer et al. (2020), Sousa et al. (2020), O’Brien et al. (2022), Pan et al. (2024), Song et al. (2024), and Zhang et al. (2024). The results from our study indicate a significant northward shift in AR activity during high southern TP heating years. This raises intriguing questions about whether TP heating plays a more prominent role in this northward turning, particularly in the context of ongoing global warming. This topic presents a captivating avenue for future exploration, and finding answers to this question will contribute to a deeper understanding of the complex interactions between regional and global climate dynamics.
Given the increasing importance of understanding the impacts of TP heating on AR activity in a changing climate, it is both necessary and urgent to develop a prediction model. Such a model should encompass various aspects of AR behavior, including frequency, intensity, and location, under the influence of TP heating in future climate projections. This research would provide valuable insights for climate scientists, meteorologists, and policymakers, enabling better preparation for potential shifts in weather patterns and the associated impacts on different regions.
5. Summary
In this study, our primary objective is to investigate the influence of TP heating on the frequency of AR in the North Pacific, with direct relevance to extreme precipitation occurrences in the West North America region. To gain a comprehensive understanding of the underlying physical processes linking TP heating and AR activity, we employed a combination of synoptic-scale circulation diagnosis analysis and quantitative assessment of moisture contributions. We used a backward tracking model within the Eulerian framework to track moisture sources, and additionally, we utilized a linear baroclinic model to validate the propagation of Rossby waves affected by the heating of the southern TP, ensuring that these waves can indeed be transported to the east of Japan. The specific research work is expanded through three steps:
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The initial task involves establishing the correlation between TP heating and AR frequency. Utilizing correlative analysis, our findings identify the southern TP as a crucial heating region influencing AR activity in the North Pacific. Remarkably, AR frequency in the North Pacific is notably higher during periods of intense heating in the southern TP and vice versa. Furthermore, a compelling similarity emerges when comparing the distributions of atmospheric apparent heat source and apparent moisture sink. The patterns of them exhibit a strong correlation, implying that the primary contributor to southern TP heating is moisture condensation.
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Our second endeavor is to delve into the role of moisture in driving anomalous southern TP heating. Building on the findings from the previous step, we aim to differentiate the impact of local versus remote moisture contributions to southern TP heating. Meanwhile, the Atlantic fosters anomalous convection triggering a Rossby wave train in the extratropics that propagates toward the TP, culminating in a positive anticyclone over the southern TP, which provides a favorable dynamic condition to make moisture lift. Furthermore, we quantitatively analyze moisture sources and channels during anomalous high and low southern TP heating years using the Water Accounting Model-2Layers. Our results indicate that remote moisture plays a more significant role in anomalous TP heating, largely due to its substantial moisture contribution. Furthermore, during southern TP heating high years, two prominent moisture channels emerge: the contribution from the Southern Oceans, encompassing the SH, the ABS, and the BOB regions, and the EA channel. The BOB region, a primary moisture source within the Southern Oceans, acts as a key transfer station affected not only by anomalous warm SST but also by thermodynamic effects on its western boundary with more moisture supplement and dynamic effects on its eastern boundary influenced by the western extension of the WNPSH. The more moisture in EA as the secondary channel is mainly attributed to the anomalous warm SST in the North Atlantic Ocean with enhancing westerlies circulation. The final objective is to comprehend how the Rossby waves triggered by the anomalous large southern TP heating impact AR activity in the North Pacific. Through diagnostic analysis and validation using a linear baroclinic model, we observe the eastward transport of upper-level anticyclone and cyclone alternate circulation patterns to the east of Japan. This structure intensifies both the westerlies and the easterlies in the region, strengthening the midlevel anticyclonic vortex in the central–eastern Pacific. Moreover, the anticyclonic vortex in the east of Japan develops a low-level cyclonic vortex, attracting moisture from lower latitudes and transporting it to higher latitudes, where it turns northward due to the influence of the anticyclone in the central–eastern Pacific developed by the enhanced westerlies and easterlies from the eastward-propagating anticyclonic vortex. Consequently, this leads to an increase in AR activity in the North Pacific. Additionally, the enhanced easterlies in the south of the eastward-propagating anticyclonic vortex reinforce the WNPSH, extending it westward, which in turn strengthen the easterlies in the lower levels along the south of the WNPSH, facilitating more westward transport toward the BOB region. This outcome reinforces the transport of moisture from the Southern Oceans to the southern TP, thereby enhancing diabatic heating. In conclusion, a positive feedback interaction mechanism emerges between southern TP heating and the eastward-propagating anticyclonic vortex, facilitated by the westward extension of the WNPSH.
An explicit illustration of the main physical processes that elucidate the influence of southern TP heating on AR activity in the North Pacific is presented in Fig. 9. This visual representation greatly enhances the understanding of the pivotal role played by TP heating and its broader impact on global atmospheric dynamics. The outcomes of this study emphasize the significance of the southern TP as a robust signal region, establishing the foundation for establishing valuable relationships with AR activity in the North Pacific.
The physical image of the comprehensive mechanism of the southern TP heating affecting the AR activity in the North Pacific.
Citation: Journal of Climate 38, 1; 10.1175/JCLI-D-23-0706.1
Acknowledgments.
This research contributes to and is financially supported by the Hong Kong Research Grants Council (RGC)-funded projects (16200920 and C6032-21G). Y. Z. is supported by the Taishan Scholar Foundation of Shandong Province (TSQN202408080). T. F. C. acknowledges the support from the RGC Postdoctoral Fellowship Scheme 2023/24 (Ref.: PDFS2324-6S05, supervisor: M. Q. Lu). The authors are grateful to the anonymous reviewers for their insightful and constructive comments that significantly improved the manuscript.
Data availability statement.
The ERA5 reanalysis data are available from the European Centre for Medium-Range Weather Forecasts (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels?tab=form).
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