1. Introduction
The Tibetan Plateau (TP) is located in Southwestern China. It is the highest plateau in the world. It is known as the third pole of the Earth and is also the source area of many rivers [
1,
2,
3,
4]. For the same troposphere height in summer, the moisture content over the plateau is much higher than that in the other surrounding areas. The sensible heating of the plateau is an important reason for the abrupt change in the East Asian circulation, which plays an important role in modulating the East Asian monsoon [
5,
6,
7,
8,
9,
10]. Ye [
11] pointed out that the southeastern part of the plateau is an exceedingly high humidity center in summer compared with the surrounding areas. The plateau serves the function of transferring the water vapor from the south to the east, and the strength of this effect directly affects the drought and flood conditions in the middle and lower reaches of the Yangtze River [
11,
12,
13,
14]. Previous studies have also shown that the southeastern part of the plateau is a high value center of the total water vapor in summer [
15,
16,
17,
18,
19]. Qu and Zhang [
19] studied the distribution of the summer water vapor flux field in East Asia and concluded that there are three water vapor transport channels in East Asia in July. The first is from the Bay of Bengal and the east coast of India to China; the second is from Southern and Southeastern China to Eastern China; and the third channel trends east-west, from East Asia to China [
19]. Therefore, it is crucial to study the surface flux and its influence on the water vapor transport in the southeastern part of the plateau to gain a better understanding of the land–atmosphere interactions and their influence on the high-increasing water vapor transport on the plateau.
In recent years, the Lagrangian method has been gradually applied to the study of water vapor transport. Massacand et al. inferred the mesospheric humidity source of heavy precipitation on the southern side of the Alpine area by examining the specific humidity along the back trajectories [
20]. Bertò et al. used the Lagrangian trajectory model (i.e., the HYSPLIT model) to analyze the water vapor source during a heavy precipitation event in Trentino, Italy, in 2002 [
21]. They found that the main water vapor channel was transported from subtropical Africa to Trentino through the Mediterranean. James et al. investigated the change in the net water along a large number of backward trajectories to identify the water source in the flooded areas of the Elbe River in August 2002 [
22]. Sodemann and Stohl [
23] employed the recently developed Lagrangian moisture source diagnostic of Sodemann et al. [
24] to determine the seasonality of moisture sources for all of Antarctica over a 5-year period. Previous studies indicate that the moisture source and transport path can change rapidly during a precipitation event [
25,
26]. Using the Lagrangian method, Jiang et al. (2013) studied the characteristics of the moisture contributions during the boreal summer over the Yangtze River valley (YRV) [
27]. Chu et al. (2021) focused on the effect of water vapor transport processes on the variations in the seasonal mean rainfall over East China [
28]. Chen and Luo (2018) used the Lagrangian model to explore the paths and sources of the water vapor carried to Southern China (SC) during the pre-flood season [
29]. Moreover, based on a Lagrangian model, Sun and Wang (2014) quantitatively calculated the water vapor transport from every water vapor source to Eastern China during 2000–2009 [
30].
The progress in meteorology research on the TP depends to a large extent on the development of various data about the plateau. With the launch of the three Field Observation Experiments of Atmospheric Science on the Tibetan Plateau, the research data about the plateau have been gradually improved [
31,
32,
33,
34]. In addition, it has been reported that even along the same latitudinal belt, the atmospheric circulation patterns [
35] and the surface heat fluxes [
36,
37,
38] regulating the moisture transport to the western TP are different from those of the eastern TP. The canyon area in southeastern Tibet is an important channel for water vapor transport from the Bay of Bengal to the south of the plateau to mainland China. The heat flux anomaly over the plateau affects the vertical movement and convergence and divergenceover the plateau, which leads to anomalies in the height field and wind field in East Asia [
18]. The changes in the surface flux cause the changes in the annular flow field over the region, and they affect the water vapor transport. To study the influence of the changes in the surface flux on the water vapor transport in the upper layer, the singular value decomposition (SVD) method was used to analyze the correlation between the water vapor flux divergence field and the surface heat fluxes fields and to separate multiple coupling modes from the two element fields to the greatest extent possible to reveal the temporal and spatial relationships between the water vapor flux divergence field and the surface heat flux fields.
In this study, the water vapor transport characteristics were analyzed using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)_v4 backward trajectory model at Danka and Motuo stations in the canyons in the southeastern TP from November 2018 to October 2019. The contribution rates of the different water vapor paths were quantitatively analyzed to further deepen our scientific understanding of the water vapor transport paths on the TP. Then, using ERA-5 reanalysis data and the characteristics of the high-altitude water vapor transport, the impact of the changes in the surface fluxes on the water vapor transport was analyzed using SVD. The results help reveal the source of the water vapor and the mechanism by which the Earth-atmosphere interactions influence the water vapor transport on the TP.
4. Summary and Conclusions
In this study, the 11-day backward trajectories of two observation stations located in the southeastern Tibetan Canyon from November 2018 to October 2019 were analyzed using the HYSPLIT_v4 backward trajectory model. Then the SVD method was used to analyze the relationships between the sensible heat and latent heat and the water vapor flux divergence in the southeastern Tibet gorge region. Due to the effects of the atmospheric circulation patterns and seasonal heat fluxes, the patterns of the moisture sources for southeastern Tibet exhibited significant seasonal differences. The main conclusions of this study are as follows.
- (1)
The sources of the water vapor were different during the Asian monsoon and non-Asian monsoon seasons, and the main sources of the water vapor in the study area during the non-AMS were from the west and southwest. During the AMS, there was mainly southwest air flow and a small amount of southeast air flow in the lower layer. The westerly flow and northwesterly flow were the main sources of water vapor in winter. During the AMS, the southwestern water vapor transport accounted for more than half of the total. There was a certain correlation between the transportation height of each station and the source of water vapor. The height of the water vapor transportation channel of the western air flow was higher than 3000 m, and the height of the water vapor transportation channel of the southwestern and southeastern air flows was about 2000 m.
- (2)
The sensible heat and latent heat in the northern part of the southeastern Tibet Canyon during the non-AMS were directly proportional to the change in the northward water vapor flux in the central and eastern parts of the plateau. During the AMS, the sensible heat and latent heat were directly proportional to the northward water vapor flux. When the sensible heat and latent heat decreased during the non-AMS, the eastward water vapor flux increased. The sensible heat and latent heat were negatively correlated with the eastward water vapor flux, while the sensible heat and latent heat were positively correlated with the eastward water vapor flux during the AMS.
- (3)
There was a negative correlation between the surface fluxes and the water vapor flux divergence in this area. The southwest boundary of southeast Tibet was the key area affecting the water vapor flux divergence. When the surface sensible and latent heat fluxes increased in southeastern Tibet, the divergence of the water vapor flux decreased, that is, the water vapor transport to the region was weakened. When the sensible and latent heat fluxes decreased, the divergence of water vapor flux increased and the water vapor transport increased.
- (4)
During the non-AMS, when the sensible heat in the canyon area of southeastern Tibet decreased, the eastward water vapor flux increased. Additionally, the northward water vapor fluxes on the plateau east of 75° E increased, while they decreased on the western part. When the latent heat increased, the eastward water vapor flux decreased, and the sensible heat and latent heat were negatively correlated with the northward water vapor flux during the non-AMS. That is, when the surface flux increased in southeastern Tibet, the water vapor transport from west to east increased. During the AMS, when the sensible heat and latent heat in southeastern Tibet increased as a whole, the eastward water vapor flux in the total-column in southeastern Tibet increased. This indicates that when the surface flux in southeastern Tibet increased during the AMS, the water vapor transport increased from west to east.
Our results show that the source of water vapor in the study area is different in different seasons, which will provide a certain theoretical basis for further research on the extreme precipitation of the Tibetan Plateau in the future. It is of great value to further study the different source areas and density of water vapor sources to improve extreme precipitation forecasts. In addition, some questions remain to be addressed. For example, this article only analyzes the seasonal characteristics of water vapor sources in southeastern Tibet from 2018 to 2019, only the data of the past 30 years is selected for analysis. Why were there large fluctuations in 2009 and 2013, whether the VIMDF is mainly influenced by the surface thermal effect on the TP, and whether the relationship is regulated by other external forcing factors, such as sea surface temperature (SST). Cui et al. (2015) pointed out that during the positive phase of the North Atlantic Oscillation (NAO) in winter, it can inspire a stable downstream Rossby wave train, inducing the Asian subtropical westerly jet to intensify and the India-Burma trough to deepen, and it also increases the snow depth on the TP in winter, followed by a positive SSHF anomaly in spring in most areas of the TP [
41]. What are the synergetic effect and contribution rates of the NAO and the SSHF on the TP? These issues need further study.