The Tibetan Plateau (TP) significantly influences regional and even global climate mainly due to its high altitude and unique topography. Therefore, the planetary boundary layer (PBL) scheme is important for regional climate simulations in the TP. In this study, the impact of the PBL schemes (PBLSs; i.e., the YSU, MYJ, QNSE, and ACM2 schemes) on simulated surface air temperature (SAT) in the TP is investigated using the Weather Research and Forecasting model (WRF) for the summer of 2003. Overall, the PBLSs can generally reproduce the SAT spatial distributions in the TP. Significant differences are found between local closure schemes (MYJ and QNSE) and nonlocal ones (YSU and ACM2). For example, QNSE produces the most significant cold bias of -4.9 °C, with the largest difference of 1.8 °C between QNSE and ACM2, which simulate the lowest and the highest SAT values, respectively. Based on the temperature change equation, the adiabatic (i.e. convection) and diabatic terms are dominant in the PBLS choice-induced SAT change, whereas the advection is less important. The PBLSs can also greatly change land surface fluxes and PBL parameters, thereby influencing SAT. Additionally, the PBLSs tend to produce an unrealistically long super-adiabatic state, which alleviates the convergence of near-surface sensible heat and decreases the SAT. Due to overestimation of surface albedo, all these schemes underestimate the SAT, and the simulated higher SATs are consistent with higher downward longwave flux and lower albedo values. All these help understand the SAT simulation for the TP by regional climate models. This graphical summary illustrates how different planetary boundary layer parameterization schemes (PBLSs) influence the simulation of summer surface air temperature (SAT) over the Tibetan Plateau, based on the Weather Research and Forecasting (WRF) model with a 12-member ensemble simulation. All four evaluated PBLSs produce notable cold biases in simulated SAT. The local PBLSs (i.e. MYJ and QNSE) exhibit average cold biases of 4–5 °C, which are approximately 3 °C larger than those of the non‑local PBLSs (i.e. YSU and ACM2). Compared with non‑local PBLSs, the local PBLSs tend to simulate a thicker and more unstable planetary boundary layer, accompanied by stronger adiabatic (or convective) heating and diabatic cooling processes near the surface. These processes lead to enhanced surface sensible and latent heat fluxes in the local‑scheme simulations, which dominate the SAT variation. Furthermore, owing to the high‑altitude environment of the Tibetan Plateau, the lower SAT produced by local schemes results in expanded snow and ice coverage and consequently higher surface albedo. This albedo feedback further amplifies the cold biases in local PBL simulations. Impact of the PBLS choice on simulated SAT was quantified. PBLS results of temperature change equation indicate the dominant role of adiabatic and diabatic processes on the seasonal scale. PBLS cold SAT biases in the TP were induced by an unrealistically long super-adiabatic state, high albedo and low downward longwave radiation.

Understanding runoff changes in the Yarlung Zangbo (YZ) basin is crucial for water resource management on the Tibetan Plateau (TP), but has been hindered by sparse data. This study provides a comprehensive hydrometeorological dataset for the YZ basin spanning 1961-2024, including both daily meteorological variables and hydrological components. The dataset integrates observations from 26 meteorological stations, 15 hydrological stations, and hydrological model simulations. Specifically, it includes: (1) daily meteorological variables (precipitation, mean, maximum, and minimum temperature, and wind speed) at both 26 meteorological stations and a 10-km gridded scale; and (2) daily total runoff and its components (snowmelt, glacier runoff, and rainfall runoff) at the 10-km gridded scale, derived from a physically based hydrological model. The meteorological fields were extended using a random forest-based machine learning approach combined with ERA5 reanalysis data, while the hydrological variables were generated from physically based hydrological model simulations. To facilitate direct application of hydrological station data, the gridded daily hydrological data for 1961-2024 were further routed to 15 hydrological stations. The generated dataset was validated against in situ meteorological at different scales and hydrological observations and compared with other meteorological products, demonstrating good applicability in the YZ. This dataset is available at the National Tibetan Plateau Data Center. The dataset enables basin-wide simulations of hydrological processes across multiple scales and provides a valuable resource for assessing water cycle dynamics and their responses to climate and cryospheric changes in the YZ basin.

Quantifying long-term historical changes in river runoff from the vulnerable high-mountain Third Pole is critical for Asia's water resources planning, but still unresolved from a coherent, regional perspective in the climate change context. Here we show that the mountain-outlet runoff generally experienced significant increases for the westerlies-dominated rivers (Indus, Amu Darya, Syr Darya, Tarim, Heihe, and, Shule) and insignificant declines for the monsoon-dominated rivers (Ganges, Brahmaputra, Mekong, and Salween) in the past half-century, largely driven by the enhanced westerlies and weakened Indian summer monsoon. Although the changing rates of runoff can be mostly explained by the varying precipitation minus evapotranspiration, the total water storage changes (e.g., regional glacier melting, groundwater depletion) cannot be neglected. After the year 1997, the contrasting changes in the westerlies- and monsoon-dominated regions have been remarkably accelerated, necessitating proactive adaptations to sustain regional water, ecology, and food security.

不同气候、冰雪下垫面分布及地理位置直接影响着第三极源区流域径流对气候变化的响应特征.然而,当前对第三极西风与季风主导源区径流变化及其对气候变化的响应差异尚未得到系统探讨.本研究基于西风主导源区(阿克苏河源、锡尔河源、叶尔羌河源、和田河源、阿姆河源及印度河源)与季风主导源区(长江源、黄河源、澜沧江源、怒江源及雅鲁藏布江源)的径流数据,全面分析了1961～2015年各源区年径流变化特征;利用多源气象数据与大尺度环流因子,探讨了西风与季风主导源区径流对气候变化的响应差异以及径流变化的大尺度环流机制.结果表明:(1) 1961～2015年间,第三极绝大多数源区(除了黄河源与雅鲁藏布江源)径流均呈现增加趋势,其中西风主导的阿克苏河源、叶尔羌河源及印度河源径流增加趋势显著.除了黄河源外,第三极其他源区年径流在1980s～2000s均发生了由减少向增加的突变.(2)过去55年间季风主导源区径流变化受降水主导,而西风主导源区径流对气候变化的长期响应存在气温主导(叶尔羌河源与和田河源)、降水主导(锡尔河源与阿姆河源)、降水与气温共同影响(阿克苏河源与印度河源)三类模式.自20世纪60年代以来,西风主导的多数源区径流对暖季气温的敏感性减弱,其径流对降水变化的响应增强.(3)本研究也揭示了大尺度环流-气候-第三极源区径流之间的联系.北大西洋年代际振荡(AMO)、西风指数(WI)及厄尔尼诺-南方涛动(ENSO)主要通过影响第三极源区降水或气温,进而作用于西风或季风主导源区的年径流变化.本研究可为第三极西风与季风主导源区水资源管理及气候变化应对提供科学参考.

Mountainous areas are of special hydrological concern because topography and atmospheric conditions can result in large and sudden floods, posing serious risks to water-related safety in neighbouring countries. The Yarlung Zangbo (YZ) River basin is the largest river basin on the Tibetan Plateau (TP), but how floods will discharge in this basin and how the role of glacier melt in floods will change throughout the 21st-century under shared socioeconomic pathways scenarios (SSP2-4.5 and SSP5-8.5) remain unclear. Here, we comprehensively address this scientific question based on a well-validated large-scale glacier-hydrology model. The results indicate that extreme floods was projected to increase in the YZ basin, and was mainly reflected in increased duration (4- 10 d per decade) and intensity (153- 985 m 3 s- 1 per decade). Glacier runoff was projected to increase (2- 30 mm per decade) throughout the 21st-century, but there was also a noticeable decrease or deceleration in glacier runoff growth in the late first half of the century under the SSP2-4.5, and in the latter half of the century under the SSP5-8.5. Glacier melt was projected to enhance the duration (12%- 23%) and intensity (15%- 21%) of extreme floods under both SSPs, which would aggravate the impact of future floods on the socioeconomics of the YZ basin. This effect was gradually overwhelmed by precipitation-induced floods from glacier areas to YZ outlet. This study takes the YZ basin as a projection framework example to help enrich the understanding of future flood hazards in basins affected by rainfall- or meltwater across the TP, and to help policy-makers and water managers develop future plans.

The diverse climates, distribution of snow and glaciers, and geographic locations directly affect the runoff response to climate change in the upper basins of the Third Pole. At present, a comprehensive analysis of runoff variations and their distinct responses to climate change in the westerlies- and monsoon-dominated upper basins is still lacking. This study comprehensively analyzed annual runoff variations in westerlies-dominated basins (the upper basins of the Aksu (UAKS), Syr Darya (USRD), Yarkant (UYK), Hotan (UHT), Amu Darya (UAMD), and Indus (UI)) and monsoon-dominated basins (the upper basins of the Yangtze (UYA), Yellow (UYE), Lancang (ULC), Nujiang (UNJ), and Yarlung Zangbo (UYZ)) of the Third Pole from 1961 to 2015. Using multi-source meteorological data and large-scale circulation factors, this study investigated the divergent responses of runoff in the upper basins to climate change, and explored the large-scale circulation mechanisms underlying runoff variations in these upper basins. The results showed that: (1) The annual runoff in the majority of upper basins (except for the UYE and UYZ) exhibited an increasing trend, and the annual runoff in the UAKS, UYK, and UI showed a significant increasing trend from 1961 to 2015. The annual runoff in the upper basins of the Third Pole changed abruptly from decreasing to increasing between the 1980s and 2000s, with the exception of the UYE. (2) The runoff in the monsoon-dominated upper basins has been controlled primarily by changes in precipitation over the past 55 years. In contrast, the runoff in the westerlies-dominated upper basins exhibited three distinct long-term responses to climate change: temperature-dominated (UYK and UHT), precipitation-dominated (USRD and UAMD), and the combined influence of precipitation and temperature (UAKS and UI). Since the 1960s, the sensitivity of runoff to warm season temperature changes in the most westerlies-dominated upper basins has decreased, while the response of runoff to precipitation changes has intensified. (3) The study revealed the connection between large-scale circulation, climate, and runoff in the upper basins of the Third Pole. The Atlantic Multidecadal Oscillation, the Westerly Index, and the El Niño-Southern Oscillation predominantly impact the precipitation or temperature in the upper basins of the Third Pole, which in turn affect the runoff variations in the upper basins dominated by either the westerlies or the monsoon. This study will be a valuable scientific reference for water resource management and climate change adaptation for both the westerlies- and monsoon-dominated upper basins in the Third Pole.

A comprehensive understanding of spatiotemporal runoff changes in the Yarlung Zangbo (YZ) basin in the southern Tibetan Plateau (TP) at a sub-basin scale, amidst varying climatic and cryospheric conditions, is imperative for effective water resources management. However, spatiotemporal differences of runoff composition and change and their attribution within the YZ basin have not been extensively explored, primarily due to the lack of hydrometeorological observations, especially in the downstream region. In this study, we investigated historical and future evolution of annual and seasonal total water availability, as well as glacier runoff and snowmelt contributions across six sub-basins of the YZ, with a particular focus on the comparison between the upstream Nuxia (NX) basin and the downstream Nuxia-Pasighat (NX-BXK) basin, based on a newly generated precipitation dataset and a well-validated model with streamflow, glacier mass, and snow cover observations. Our findings revealed that large spatiotemporal differences in changes exist within the YZ basin for 1971-2020. Firstly, runoff generation was dominated by rainfall runoff throughout the YZ basin, with glacier runoff playing a more important role in the annual total runoff (19 %) in the NX-BXK sub-basin compared to other sub-basins. Notably, glacier runoff contributed 52 % of the total runoff at the Pasighat outlet of the YZ basin. Secondly, annual runoff exhibited an increasing trend in the NX basin but a decreasing trend in the NX-BXK, primarily attributed to rainfall runoff changes influenced by atmospheric moisture. Glacier runoff enhanced water supply by offsetting the decreasing contribution from rainfall. Total runoff will consistently increase (27-100 mm (10 yr)-1) across the sub-basins through the 21st century, resulting from increased rainfall runoff and a minor effect of increased snowmelt and glacier runoff.

The upper Indus basin (UIB) supplies water for the largest irrigation system in Asia and for the livelihoods of millions of people downstream. Yet a large gap still remains in the current knowledge of the hydrological pro-cesses in this region due to data scarcity, especially at sub-basin scales. In this work, we evaluated eight pre-cipitation datasets and screened out an optimized (MERRA-2) to be inversely corrected through the Variable Infiltration Capacity (VIC)-glacier hydrological model across the UIB. Divergent runoff regimes, changes, and their attributions were quantified at sub-basin scales based on corrected MERRA-2 precipitation and the well-constrained VIC-glacier model against observed streamflow and glacier/snow changes. Spatial variations exist in the corrected precipitation over the UIB with the lowest mean annual average of 414 mm in the Kharmong and the highest of 1318 mm in the Astore, and a mean of 646 mm over the entire basin in 1980-2013. Three runoff regimes were identified across the UIB basins, including the glacial regime of Shigar, Hunza, and Shyok with glacier runoff dominating (41.6%-53.4%), nival-pluvial regime of Astore and Gilgit with rainfall (46.9%-51.9%) and snowmelt (33.1%-40.7%) runoff dominating, and pluvial regime of UIB-M and Kharmong with rainfall runoff (56.7%-60.9%) dominating. Total runoff generally exhibited stable or insignificant positive tendencies over the UIB basins during 1980-2013. Runoff increases in the glacial regime basins were mainly controlled by the increased glacier runoff, while runoff changes in the non-glacial regime basins were mostly attributed to the changes of precipitation-induced runoff from non-glacierized areas. The increased glacier runoff served as a buffer against hydrological drought by offsetting the non-glacierized runoff decrease caused by the overall decreased precipitation. This study is expected to further the understanding of divergent hydrological regimes in the UIB, and provide essential information for policy decisions on sustainable water resource management in this water-stressed area.

Abstract. An improved understanding of runoff regimes and flow changes in the Yarlung Zangbo (YZ) river basin in the southern Tibetan Plateau (TP) is crucial for water resources management. However, regional characteristics in runoff regimes and changes are not comprehensively investigated in the YZ mostly due to the lack of hydrometeorological observations. Here, we comprehensively investigated runoff regimes and changes across six sub-basins in the YZ for 1971–2020 with a particular focus on the comparison between the upstream of the Nuxia (NX) basin and the downstream NX-Pasighat (NX-BXK), based on a newly generated precipitation dataset and a well-validated model with streamflow, glacier mass and snow cover observations. Our results reveal that large regional differences in runoff regimes and changes exist in the YZ basin. Firstly, runoff generation is dominated by rainfall in the entire YZ, and glacier runoff plays more important role in annual total runoff (19 %) in the NX-BXK than other sub-basins. Secondly, annual runoff shows an increasing trend in the NX basin but a decreasing trend in the NX-BXK due to rain-induced runoff changes, resulting in a weak increasing trend (3.1 mm/10 yr) in the YZ basin. Thirdly, total runoff increases of 5 %–22 % in the NX but decreases of 3 %–20 % in the NX-BXK in all seasons in 1998–2020 relative to the period 1971–1997. Finally, the NX basin faces a considerably hazard from extreme flood, but the NX-BXK basin faces more severe hydrological droughts. Glacier runoff shows limited roles in mitigating water shortages caused by drought in dry seasons, but it intensifies the flood frequency and severity among the basins in wet season. Our study offers a basic framework for clarifying the runoff regimes and flow changes in the TP basins.

The Hindu Kush–Karakoram–Himalayan system, named the Third Pole because it is the largest global store of frozen water after the polar regions, provides a reliable water supply to almost 2 billion people. Marked atmospheric warming has changed the balance of this so-called Asian water tower and altered water resources in downstream countries. In this Review, we synthesize observational evidence and model projections that describe an imbalance in the Asian water tower caused by accelerated transformation of ice and snow into liquid water. This phase change is associated with a south–north disparity due to the spatio-temporal interaction between the westerlies and the Indian monsoon. A corresponding spatial imbalance is exhibited by alterations in freshwater resources in endorheic or exorheic basins. Global warming is expected to amplify this imbalance, alleviating water scarcity in the Yellow and Yangtze River basins and increasing scarcity in the Indus and Amu Darya River basins. However, the future of the Asian water tower remains highly uncertain. Accurate predictions of future water supply require the establishment of comprehensive monitoring stations in data-scarce regions and the development of advanced coupled atmosphere–cryosphere–hydrology models. Such models are needed to inform the development of actionable policies for sustainable water resource management.

The upper Syr Darya (USD) and Amu Darya (UAD) basins are the two biggest flow formation zones in Central Asia and the only water supply sources for the Aral Sea. Upstream snow and ice reserves of those two basins, important in sustaining seasonal water availability, are highly sensitive and prone to climate change, but their importance and changes are still uncertain and poorly understood due to data scarcity, inaccessibility, harsh climate, and even geopolitics. Here, an improved forcing dataset of precipitation and temperature was developed and used to drive a physically-based hydrological model, which was thoroughly calibrated and validated to quantify the contributions of different runoff components to total flow and the controlling factors for total runoff variations for 1961-2016. Our analysis reveals divergent flow regimes exist across the USD and UAD and an ongoing transition from nival-pluvial toward a volatile pluvial regime along with rising temperatures. Annual total runoff has weakly increased from 1961 to 2016 for the entire USD and UAD, while the subbasins displayed divergent flow changes. Spring runoff significantly increased in all the USD and UAD basins primarily due to increased rainfall and early snow melting, tending to shift the peak flow from June-July to April-May. In contrast, distinct runoff changes were presented in the summer months among the basins primarily due to the trade-off between the increase in rainfall and the decrease in snowmelt and glacier runoff. These findings are expected to provide essential information for policy-makers to adopt strategies and leave us better poised to project future runoff changes in ongoing climate change.

追踪并量化青藏高原不同流域的降水水汽来源,有助于揭示西风与季风协同作用下高原流域尺度的大气水循环基本特征,理解气候变化背景下“亚洲水塔”的水资源时空变化机制.本研究基于欧拉水汽扩散模式（WAM-2）和3套大气再分析资料（ERA-I、MERRA-2和JRA-55）,追踪了近35年（1979/1980～2015年）青藏高原6个主要流域的降水水汽来源.结果表明:印度河上游、塔里木河（塔河）上游和柴达木盆地的水汽源主要随着中纬度西风带向欧亚大陆西部延伸;雅鲁藏布江流域（雅江）、高原内流区以及长江、澜沧江和怒江源区（三河源）的水汽源向西、南均有延伸,但以随印度季风向南延伸为主.冬、春季,各流域主要受西部水汽源影响;夏季,西风北移使得各流域水汽源向北推移,而印度季风爆发使得雅江、内流区和三河源的水汽源向印度洋大面积扩张;秋季,印度河上游、塔河上游和柴达木盆地的水汽源收缩到西部源区,而雅江、内流区和三河源的水汽源则收缩到高原中南及印度次大陆.通过量化多源水汽贡献发现:高原主要流域均以陆源水汽主导,尤其在塔河上游和柴达木盆地可达62～73%;而海源水汽贡献率最高出现在印度河上游（38～42%）和雅江（38～41%）.冬季,由西部源区大面积水体（地中海、红海、波斯湾等）蒸发的水汽明显高于周边陆地,使得各流域在该季节表现为海（陆）源水汽贡献率峰（谷）值;夏季,印度河上游、塔河上游和柴达木盆地表现为陆源水汽贡献率峰值,而季风带来的大量印度洋水汽使得雅江、内流区和三河源的陆源水汽峰值不明显,其中,雅江的海、陆源水汽贡献在夏季几乎持平.

Upper Brahmaputra (UB) is the largest (similar to 240,000 km(2)) river basin of the Tibetan Plateau, where hydrological processes are highly sensitive to climate change. However, constrained by difficult access and sparse in situ observations, the variations in precipitation, glaciers, frozen ground, and vegetation across the UB basin remain largely unknown, and consequently the impacts of climate change on streamflow cannot be accurately assessed. To fill this gap, this project aims to establish a basinwide, large-scale observational network (that includes hydrometeorology, glacier, frozen ground, and vegetation observations), which helps quantify the UB runoff processes under climate-cryosphere-vegetation changes. At present, a multisphere observational network has been established throughout the catchment: 1) 12 stations with custom-built weighing automatic rain/snow meters and temperature probes to obtain elevation-dependent gradients; 2) 9 stations with soil moisture/temperature observations at four layers (10, 40, 80, 120 cm) covering Alpine meadow, grasslands, shrub, and forest to measure vegetation (biomass and vegetation types) and soil (physical properties) simultaneously; 3) 34 sets of probes to monitor frozen ground temperatures from 4,500 to 5,200 m elevation (100-m intervals), and two observation systems to monitor water and heat transfer processes in frozen ground at Xuegela (5,278 m) and Mayoumula (5,256 m) Mountains, for improved mapping of permafrost and active layer characteristics; 4) 5 sets of altimetry discharge observations along ungauged cross sections to supplement existing operational gauges; 5) high-precision glacier boundary and ice-surface elevation observations at Namunani Mountain with differential GPS, to supplement existing glacier observations for validating satellite imagery. This network provides an excellent opportunity to monitor UB catchment processes in great detail.

Precipitation is one of the most important atmospheric inputs to hydrological models. However, existing precipitation datasets for the Third Pole (TP) basins show large discrepancies in precipitation magnitudes and spatiotemporal patterns, which poses a great challenge to hydrological simulations in the TP basins. In this study, a gridded (10 km x 10 km) daily precipitation dataset is constructed through a random-forest-based machine learning algorithm (RF algorithm) correction of the ERA5 precipitation estimates based on 940 gauges in 11 upper basins of TP for 1951-2020. The dataset is evaluated by gauge observations at point scale and is inversely evaluated by the Variable Infiltration Capacity (VIC) hydrological model linked with a glacier melt algorithm (VIC-Glacier). The corrected ERA5 (ERA5_cor) agrees well with gauge observations after eliminating the severe overestimation in the original ERA5 precipitation. The corrections greatly reduce the original ERA5 precipitation estimates by 10%-50% in 11 basins of the TP and present more details on precipitation spatial variability. The inverse hydrological model evaluation demonstrates the accuracy and rationality, and we provide an updated estimate of runoff components contribution to total runoff in seven upper basins in the TP based on the VIC-Glacier model simulations with the ERA5_cor precipitation. This study provides good precipitation estimates with high spatiotemporal resolution for 11 upper basins in the TP, which are expected to facilitate the hydrological modeling and prediction studies in this high mountainous region. Significance StatementThe Third Pole (TP) is the source of water to the people living in the areas downstream. Precipitation is the key driver of the terrestrial hydrological cycle and the most important atmospheric input to land surface hydrological models. However, none of the current precipitation data are equally good for all the TP basins because of high variabilities in their magnitudes and spatiotemporal patterns, posing a great challenge to the hydrological simulation. Therefore, in this study, a gridded daily precipitation dataset (10 km x 10 km) is reconstructed through a random-forest-based machine learning algorithm correction of ERA5 precipitation estimates based on 940 gauges in 11 TP basins for 1951-2020. The data eliminate the severe overestimation of original ERA5 precipitation estimates and present more reasonable spatial variability, and also exhibit a high potential for hydrological application in the TP basins. This study provides long-term precipitation data for climate and hydrological studies and a reference for deriving precipitation in high mountainous regions with complex terrain and limited observations.

Seasonal melting of glaciers and snow from the western Third Pole (TP) plays important role in sustaining water supplies downstream. However, the future water availability of the region, and even today's runoff regime, are both hotly debated and inadequately quantified. Here, we characterize the contemporary flow regimes and systematically assess the future evolution of total water availability, seasonal shifts, and dry and wet discharge extremes in four most meltwater‐dominated basins in the western TP, by using a process‐based, well‐established glacier‐hydrology model, well‐constrained historical reference climate data, and the ensemble of 22 global climate models with an advanced statistical downscaling and bias correction technique. We show that these basins face sharply diverging water futures under 21st century climate change. In RCP scenarios 4.5 and 8.5, increased precipitation and glacier runoff in the Upper Indus and Yarkant basins more than compensate for decreased winter snow accumulation, boosting annual and summer water availability through the end of the century. In contrast, the Amu and Syr Darya basins will become more reliant on rainfall runoff as glacier ice and seasonal snow decline. Syr Darya summer river‐flows, already low, will fall by 16%–30% by end‐of‐century, and striking increases in peak flood discharge (by >60%), drought duration (by >1 month) and drought intensity (by factor 4.6) will compound the considerable water‐sharing challenges on this major transboundary river.

Tracking and quantifying the moisture sources of precipitation in different drainage basins in the Tibetan Plateau (TP) help to reveal basin-scale hydrological cycle characteristics under the interactions between the westerlies and Indian summer monsoon (ISM) systems and to improve our understanding on the mechanisms of water resource changes in the ‘Asian Water Tower’ under climate changes. Based on a Eulerian moisture tracking model (WAM-2) and three atmospheric reanalysis products (ERA-I, MERRA-2, and JRA-55), the contributions of moisture sources to the precipitation in six major sub-basins in the TP were tracked during an approximately 35-year period (1979/1980–2015). The results showed that in the upper Indus (UI), upper Tarim River (UT), and Qaidam Basin (QB), the moisture sources mainly extended westward along the mid-latitude westerlies to the western part of the Eurasian continent. In contrast, in the Yarlung Zangbo River Basin (YB), inner TP (ITP), and the source area of three eastern rivers (TER, including the Nujiang River, Lancang River, and Yangtze River), the moisture sources extended both westward and southward, but mainly southward along the ISM. In winter and spring, all of the sub-basins were dominated by western moisture sources. In summer, the western sources migrated northward with the zonal movement of the westerlies, and simultaneously the southern sources of the YB, ITP, and TER expanded largely toward the Indian Ocean along the ISM. In autumn, the moisture sources of the UI, UT, and QB shrank to the western sources, and the moisture sources of the YB, ITP, and TER shrank to the central-southern TP and the Indian subcontinent. By quantifying the moisture contributions from multiple sources, we found that the terrestrial moisture dominated in all of the sub-basins, particularly in the UT and QB (62–73%). The oceanic contributions were relatively high in the UI (38–42%) and YB (38–41%). In winter, evaporation from the large western water bodies (such as the Mediterranean, Red Sea, and Persian Gulf) was significantly higher than that from the continental areas. This contributed to the peak (valley) values of the oceanic (terrestrial) moisture contributions to all of the sub-basins. In summer, the terrestrial moisture contributions to the UI, UT, and QB reached their annual maximum, but the abundant oceanic moisture transported by the ISM restrained the appearance of land source contribution peaks in the YB, ITP, and TER, resulting in almost equal moisture contributions in the YB from the ocean and land.

Reliable simulations of historical and future climate are critical to assessing ecological and hydrological responses over the Third Pole (TP). In this study, we evaluate the historical and future temperature and precipitation simulations of 18 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in southeastern TP (SETP) and the upstream of the Amu Darya and Syr Darya (UAS) regions, two typical TP subregions dominated by the Indian summer monsoon system and westerlies, respectively. Comparison against station observations suggests that CMIP6 models generally capture the intra-annual variability and spatial pattern of historical climate over both subregions. However, the wetting and cold biases observed in CMIP5 still persist in CMIP6; annual temperature is underestimated by most models and annual precipitation is overestimated by all models. Multi-model average cold biases in SETP and UAS are 1.18°C and 0.32°C, respectively, and wet biases in SETP and UAS are 119% and 46%, respectively. We further analyze climate projections under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. Both SETP and UAS subregions are projected to experience significant warming in 2015–2100, with warming trends 34%–42% and 40%–50% higher than the global trend, respectively. Model projections suggest that the warming trend will slow down under SSP1-2.6 and SSP2-4.5 but further intensify under SSP5-8.5 in 2050–2100. Monsoon-dominated SETP is projected to experience a significant wetting trend stronger than UAS over the entire future period, especially in summer (cf. winter in westerlies-dominated UAS). Concurrently, a significant drying trend in summer is found in UAS during 2050–2100 under SSP5-8.5, suggesting the intensified uneven distributions of seasonal precipitation based on projections.

In this study, two sets of precipitation estimates based on the regional Weather Research and Forecasting model (WRF) –the high Asia refined analysis (HAR) and outputs with a 9 km resolution from WRF (WRF-9km) are evaluated at both basin and point scales, and their potential hydrological utilities are investigated by driving the Variable Infiltration Capacity (VIC) large-scale land surface hydrological model in seven Third Pole (TP) basins. The regional climate model (RCM) tends to overestimate the gauge-based estimates by 20–95% in annual means among the selected basins. Relative to the gauge observations, the RCM precipitation estimates can accurately detect daily precipitation events of varying intensities (with absolute bias < 3 mm). The WRF-9km exhibits a high potential for hydrological application in the monsoon-dominated basins in the southeastern TP (with NSE of 0.7–0.9 and bias of -11% to 3%), while the HAR performs well in the upper Indus (UI) and upper Brahmaputra (UB) basins (with NSE of 0.6 and bias of -15% to -9%). Both the RCM precipitation estimates can accurately capture the magnitudes of low and moderate daily streamflow, but show limited capabilities in flood prediction in most of the TP basins. This study provides a comprehensive evaluation of the strength and limitation of RCMs precipitation in hydrological modeling in the TP with complex terrains and sparse gauge observations.