1 Introduction

Precipitation on the Tibetan Plateau (TP), an essential constituent of global water balance, is of critical significance for water resource supply, the stability of terrestrial ecosystems, and hydrological disasters (Immerzeel et al., 2010, 2020, Pang et al., 2017; Q. Zhang et al., 2022). As a sensitive area to global warming, TP has experienced rapid and uneven temperature rises in recent decades (Li et al., 2010; Meng et al., 2023; Yu et al., 2024). Against this backdrop, extreme precipitation in the TP has generally increased (Ge et al., 2017; Xiong et al., 2019), with extreme daily precipitation in the central and eastern regions showing a significant increasing trend (Cao et al., 2019; Ding et al., 2025; Yao et al., 2023) and large-scale extreme precipitation events rising markedly (S. Li et al., 2024). However, due to differences in precipitation data sources, study periods, and definitions, conclusions on variations in extreme precipitation vary to some extent, which reflects the complexity of precipitation changes over TP.

Critically, compared with daily extremes, extreme hourly precipitation (EHP) responds more sensitively to warming, often exceeding Clausius-Clapeyron scaling (Förster & Thiele, 2020; Haerter et al., 2010; Lenderink & van Meijgaard, 2008; H. Wang et al., 2018). As the primary trigger for flash floods, landslides, and mudslides on TP (G. Zhao et al., 2022), EHP poses escalating risks to vulnerable ecosystems and infrastructure. Despite its significance, sub-daily extreme precipitation events have received less research attention than daily extremes. Limited observational studies have reported increasing EHP occurrences in the southern (Y. Zhang et al., 2022) and southeastern TP (Li & Yao, 2024). But these studies primarily focus on localized temporal changes or spatial distributions, failing to provide a comprehensive understanding of the spatiotemporal heterogeneity of EHP.

The changes in EHP are primarily driven by the combined effects of enhanced atmospheric water-holding capacity under global warming and alterations in large-scale circulation (Fowler et al., 2021). In eastern China, increased EHP frequency over recent decades can be linked to prolonged Meiyu frontal systems (Ng et al., 2021), with contributions from atmospheric instability and elevated convective available potential energy (Qi et al., 2024). In southwestern and northern China, the overall rise in EHP is linked to the westward extension of the Western Pacific Subtropical High (WPSH) and significant increases in local temperature and humidity (Jiang et al., 2023; Pei et al., 2025). On TP, the South Asian High (SAH) and WPSH are the important large-scale atmospheric systems associated with extreme precipitation (Xu et al., 2023), whose abnormal position can modulate regional moisture transport, convective instability, and ascending motion (Chen et al., 2023; Ning et al., 2017; Peng et al., 2024). However, their composite and individual contributions, as well as the impact of related dynamic and thermodynamic processes on the EHP of the TP, have not been well explored.

60%–70% of precipitation on the Qinghai Tibet Plateau occurs in summer (X. Wang et al., 2018), with frequent rainy and heavy rainfall centers mainly located in the eastern part of TP (Lu et al., 2023; Ma & Yao, 2023), which is also a frequent area of secondary disasters. To ensure a longer observation period, this study focuses on the eastern edge of the Qinghai Tibet Plateau, aims to utilize the latest hourly precipitation data and ERA5 reanalysis data to explore the spatiotemporal variations of summer EHP over TP in summer, and investigate regulating roles of SAH and WPSH in affecting EHP change. These findings are expected to deepen our understanding of the mechanisms driving EHP change in the region.

2 Data and Methods

3 Results

The spatial distribution of the EHP indices in summer over the eastern edge of TP from 1988 to 2023 is shown in Figure 1. EHPA ranges from 40 to 150 mm/summer and decreases from south to north (Figure 1a). Based on this spatial difference and the meridional extent of the TP, we use 33°N as the boundary to separate the northern and southern regions for comparison. Both sub-regions have similar EHPI values ranging from 4 to 10 mm/hr, with higher values in the southernmost part (Figure 1b). The frequency of EHP directly leads to spatial differences in the total amount of that. In the southeastern edge of TP, EHP occurs more frequently (13–25 hr/summer, Figure 1c), with EHPA exceeding 80 mm (Figure 1a). In contrast, in the northeastern edge of TP, EHPF is lower (<14 hr/summer), and EHPA is less than 100 mm.

Based on the variation of “station-mean” EHP indices, all indices show increasing trends in 1988–2023 (Figures 2a–2c). Using PLFIM and RSD t-tests, a significant trend reversal (p < 0.1) from decline to increase is detected in EHPA around 2004, with consistent shifts in EHPI and EHPF. The spatial distribution of trends in EHP indices during the sub-periods (i.e., 1988–2003 and 2004–2023) further confirms the phenomenon of trend reversal. EHPA, EHPI, and EHPF exhibited trend shifts in 69% (18), 50% (13), and 73% (19) of 26 stations respectively (negative-to-positive sign reversal or trend difference >0.01 units; Figures 2d–2i). During 2004–2023, over 80% and 73% of stations show increasing trends in EHPA and EHPF, respectively (Figures 2g and 2i). Although most individual station trends lack statistical significance, the “station-mean” EHPA and EHPF have significantly increased during 2004–2023 (13.41 mm/10a and 1.78 hr/10a, respectively). These trends exceed those in 1988–2023, highlighting a robust increase of EHP over the eastern edge of TP, particularly during the recent two decades. Moreover, sensitivity tests using 97th/99th percentile thresholds confirm the robustness of the trend reversal and significant increases in EHPA/EHPF post-2004 (not shown), and Global Precipitation Measurement Integrated Multi-satellite Retrievals data further supports the significant increase during 2004–2023 (Figure S2 in Supporting Information S1).

Large-scale circulation systems configuration—specifically the SAH and the WPSH—are key drivers of heavy rainfall in TP (Chen et al., 2023; Sun et al., 2021; Xu et al., 2023). Existing research has found an intensification and movement of SAH and WPSH before extreme precipitation over TP (S. Li et al., 2024; Xu et al., 2023). On the eastern edge of TP, correlation analysis (Table S1 in Supporting Information S1) reveals that EHP indices exhibit significant correlation with circulation indices of SAH and WPSH. Specifically, the EHP indices exhibit positive correlation with the area, central intensity, and eastward ridge point of the SAH. For WPSH, EHP indices show a positive correlation with the area and central intensity, but a negative correlation with the westward ridge point.

Next, temporal changes of the SAH and WPSH indices during summers of 1988–2023 are examined (Figure 3). The area, central intensity, and eastward ridge point of SAH show opposite trends in the two sub-periods (Figures 3a, 3c and 3g), consistent with the trend shift of EHP indices from weakening to strengthening (Figures 2a–2c). Similarly, the area of WPSH demonstrates a comparable trend reversal (Figure 3b), with the westward ridge point shifting from eastward to westward extension (Figure 3h). Thus, it is inferred that changes in SAH and WPSH may directly regulate the EHP in the eastern TP. Especially during 2004–2023, the significant strengthening and expansion of SAH and WPSH led to the rapid increase of EHPA in the eastern edge of TP.

To investigate how the enhanced and expanded SAH and WPSH benefit EHP, we use their area as the key index to select strong and weak years for composite analysis, given the significant correlations among area, central intensity, and eastward/westward ridge points (Table S2 in Supporting Information S1). Strong (weak) years are defined as those with the area exceeding +0.8 (−0.8) standard deviations relative to the mean. Based on this criterion, we identify co-strong years (1998, 2010, 2016, 2020, 2021, 2022, 2023) and co-weak years (1989, 2000, 2002, 2004) for both SAH and WPSH. During co-strong years, the SAH shifts eastward and the WPSH extends westward (Figure 4a). Under this configuration, a favorable dynamical condition emerges over eastern edge of TP: enhanced lower-tropospheric convergence below 300 hPa and upper-tropospheric divergence in 300–150 hPa (Figure 4b), together with a deep column of strengthened upward motion (negative vertical velocity anomaly, Figure 4c). Thermodynamically, 500-hPa pseudo-equivalent potential temperature (θ_se) significantly increases (Figure 4d), and the 400–500 hPa θ_se difference becomes more negative (Figure 4e), indicating intensified convective instability promoting convective activities. Regarding moisture conditions, the TP's major moisture sources are the Arabian Sea, Bay of Bengal, South China Sea, and midlatitude westerlies, with net influx at southern and western boundaries (Simmonds et al., 1999; Wang et al., 2017). In co-strong years, specific humidity at 500 hPa increases significantly over the eastern edge region (Figure 4f), accompanied by enhanced westward water vapor transport from midlatitude westerlies and overall moisture flux convergence (Figure 4g). Collectively, these favorable thermodynamic and dynamic conditions drive the increase in EHP. Meanwhile, the majority of co-strong years occur in the recent two decades, while co-weak years occur up to 2004, matching the trend turning period of EHP. Thus, it can be inferred that the significant intensification and expansion of SAH and WPSH have created a favorable local thermodynamic and dynamic field over the eastern edge of TP for EHP, thereby driving its rapid increase in the recent two decades.

The scenarios of individual intensification of the SAH and WPSH are further explored. Based on the area index, years with SAH above the mean and WPSH below the mean are defined as SAH-strong years (1988, 2006, 2007, 2013, 2015, 2018), while years with WPSH above the mean and SAH below the mean (1995, 2003) are classified as WPSH-strong years. Results highlight distinct regional impacts.

When only SAH intensifies and extends eastward (Figure 5a), anomalous low-level convergence below 350 hPa over the northeastern edge of TP enhances lower-tropospheric ascent (Figures 5b and 5c), while the southeastern TP exhibits low-level divergence coupled with high-level convergence with suppressed ascent. Concurrently, on the northeastern edge of TP, positive θ_se and specific humidity anomalies at 500 hPa (Figures 5d and 5f), along with significant convective instability (negative θ_se difference in 400–500 hPa, Figure 5e), promote moist convection here. Strengthened southeasterly wind transports more moisture from the Bay of Bengal toward eastern TP (Figure 5g), and convergence occurs in its northern part, further supporting EHP development. Thus, SAH strengthening mainly promotes EHP in the northeastern TP via thermodynamic modulation, particularly convective instability.

Conversely, when only WPSH intensifies and extends westward (Figure 5h), deep ascending motion dominates the eastern TP (p < 0.1 below 150 hPa, Figure 5j), driven by low-level convergence and upper-level divergence (Figure 5i). Thermodynamic influence is weaker, as shown in positive θ_se difference in 400–500 hPa (Figure 5l). In the southeastern TP, positive θ_se and specific humidity anomalies at 500 hPa can be observed, but not significantly (Figures 5k and 5m). Moisture transport shows that, influenced by anomalous southeast and westerly flow in the southern TP and anomalous northeasterlies in the northern TP, more water vapor from both the Bay of Bengal and mid-latitude westerlies is transported to the TP and converges near 33°N (Figure 5n). Overall, the vertical motion triggered by convergence-divergence coupling is the primary driver of EHP in the southeastern edge of TP when WPSH individual intensifies.

4 Conclusions and Discussions

This study analyzes the spatiotemporal characteristics of EHP and its large-scale circulation driving mechanisms over the eastern edge of TP from the summer of 1988–2023. Results show that a trend reversal is identified in all EHP indices: a downward trend from 1988 to 2003 followed by an upward trend from 2004 to 2023. Especially between 2004 and 2023, EHPA and EHPF have experienced significant rapid increase. This variation coincides with the evolution of SAH and WPSH. Composite analysis shows that the expansion and strengthening of these two high-pressure systems can create local thermal and dynamic conditions conducive to the EHP over the eastern edge of TP, such as the enhancement in high-level divergence and low-level convergence, deep ascending motion, low-level convective instability, 500 hPa specific humidity, and convergence of column-integrated moisture flux. Moreover, there is a spatial difference in the contribution of the two high-voltage systems. The expansion of the SAH primarily enhances EHP over the northeastern edge of the TP through thermodynamic processes, such as increased convective instability, whereas the expansion of the WPSH mainly intensifies EHP over the southeastern edge of the TP via dynamic mechanisms, particularly through convergence–divergence coupling.

The post-2004 SAH/WPSH amplification may be linked to tropical ocean-atmosphere interactions. Tropical Indian Ocean warming strengthens SAH via tropospheric heating while exciting Kelvin waves that reinforce WPSH (Cao et al., 2022; Xie et al., 2009, 2016). Anomalous warming of the tropical Atlantic sea surface temperature (SST) may enhance the zonally overturning atmospheric circulation anomalies and strengthen subsidence over the tropical central Pacific, intensifying the WPSH (Chang et al., 2016; Chen et al., 2015; Hong et al., 2015). Anthropogenic forcing may also potentially modulate these processes. For instance, increasing greenhouse gas concentrations contribute to Indian Ocean warming (L. Dong & Zhou, 2014; Yang et al., 2022), which indirectly influences SAH and WPSH. Additionally, Asian anthropogenic aerosols can induce western North Pacific anticyclonic anomalies (B. Dong et al., 2019), thereby enhancing WPSH.

Additionally, while this study focuses on large-scale circulations, these are not the only drivers of EHP over the TP. Mesoscale systems such as shear lines (Sun et al., 2021), plateau vortices (Cao et al., 2025; Lin et al., 2022), and local convective systems also play significant roles, yet their contributions to EHP change require further investigation. Previous studies also highlight the critical role of human activities in EHP variations (Deng et al., 2024; Zheng et al., 2021). Over the TP, anthropogenic warming can accelerate local evapotranspiration, glacial melt (Song et al., 2014), and lake expansion (Yang et al., 2018; Zhang et al., 2020), intensifying hydrological cycles, which may contribute to changes in EHP over TP. Besides, SST anomalies in the North Pacific and Arabian Sea correlate with TP precipitation variability (Wang et al., 2023; J. Zhang et al., 2022). The North Atlantic Oscillation (NAO) (Hu et al., 2022), El Niño-Southern Oscillation (ENSO) (Hu et al., 2021; Q. Li et al., 2024; Y. Wang & Xu, 2018), and the Indian Ocean Dipole (IOD) (He et al., 2022; Luo et al., 2020) also exert significant influences. In the future, it's necessary to explore how these aspects can drive changes in EHP over TP.