The Asian summer monsoon system brings strong moisture advection and results in heavy precipitation in the summer rainy season [1
]. As one of the world’s most densely populated and economically fast-growing regions [4
], the Middle Yangtze River Basin (MYRB) and how it is influenced by the monsoon have been investigated in many studies [3
]. At the same latitude as the MYRB and influenced by the Indian summer monsoon, and as the richest region in terms of precipitation over the Tibetan Plateau (TP) [7
], the Southeast of the TP (SETP) experiences much less precipitation than the MYRB [1
]. How is this the case? Is it because of the distance from the ocean? In addition, precipitation has decreased for both the SETP and the MYRB. Moreover, the Asian summer monsoon has been reported to show a decreasing trend. It is queried whether the precipitation has decreased due to the weakening of the Asian summer monsoon and, if so, how did the large-scale weakening of the monsoon influence the precipitation in different longitudinal regions? All of these questions pose interesting research objectives.
Besides the moisture from the ocean, terrestrial evaporation is also a source for moisture in the atmosphere, which eventually precipitates. The evaporative source is the evaporation that contributes as a moisture source in the atmosphere for precipitation [14
]. The local evaporative source over the precipitation in a given area is named as precipitation recycling [17
]. Contributions of evaporation or evapotranspiration from the land and ocean to precipitation in the continents have been named as terrestrial and oceanic evaporative sources [14
]. The average altitude is 4000 m for the SETP and 400 m for the MYRB. Besides the topography, the heterogeneity of the terrain and the land surface characteristics in the SETP are very different from those in the MYRB. How does the land surface contribute to the precipitation difference in the two regions? What is the role of the Himalayas in the lower level of precipitation in the SETP compared to the MYRB?
Terrestrial and oceanic evaporative sources in the SETP and MYRB have been investigated separately in previous studies. Zhang et al. found more evaporative sources from land than ocean in the Southern TP, and found that 69% of the precipitation over the TP comes from terrestrial evaporation [22
]. Curio et al. claimed that around 63% of the precipitation comes from the local terrestrial source in the TP [24
]. However, Chen et al. and Drumond et al. concluded that, instead of land, oceans are the major moisture source for precipitation over the TP [25
]. Chen et al. found that the dominant origin of the moisture supplied to the TP is from the Arabian Sea and Bay of Bengal [25
]. Drumond et al. also claimed the same result of a dominant oceanic evaporative source of precipitation in the Southeastern TP [26
For the MYRB, many studies stated that the largest source was from the oceans. However, a dominant contribution of either a land or ocean source is still under debate. For instance, Wang et al. showed that the Indian Ocean and the Pacific Ocean together contributes to 60% of the precipitation in the MYRB, using the 2D dynamic recycling model (DRM) [27
]. The sources from the Indian Ocean and the Pacific Ocean, separately, account for 30% each. Using a FLEXible PARTicle dispersion model (FLEXPART), Chen et al. assessed that the oceanic source is even larger than 60% [28
]. The East China Sea, South China Sea, Arabian Sea, and the Bay of Bengal contribute around 70% of the precipitation in the Yangtze River Basin. On the contrary, Wei et al. found that local and land moisture contributions are the major source of precipitation in Yangtze River Valley; ocean is important only in initiating the moisture transfer.
From the above literature survey, we found that arguments differ with regard to the dominant evaporative sources for the two regions. Moreover, none of the previous literature compared the evaporative sources of precipitation between the SETP and the MYRB. Quantifying and comparing the terrestrial and oceanic sources for the SETP and MYRB can help our understanding of the difference in precipitation between these two same-latitude regions, and help us to explore the abovementioned unsolved questions.
Several recycling models have been used to quantify the evaporative source. For instance, the early-generation box models [16
], DRM [21
], water accounting model (WAM) [14
] and FLEXPART [34
] have been built and developed. The early one-dimensional box models were designed based on the equation of balance of atmospheric water. Huge uncertainties were found in these models due to three assumptions: time-averaged data are used to estimate moisture flux, the change in the atmosphere storage term is neglected, and finally, there is the well-mixed assumption [32
]. Monthly mean datasets were used due to these assumptions in early-generation models. Later on, two- and three-dimensional models were developed. Some of these box models were improved to run at a daily time scale. For instance, the moisture storage term and time-accumulated data were taken into account in WAM for moisture tracing [14
]. The model was able to quantify the source and sink of evaporation and precipitation [22
]. The change in moisture storage was incorporated into DRM [21
], which can be used to analyze moisture transport from daily to monthly and longer temporal scales. In DRM, the balancing equation of water vapor is solved in a Lagrangian framework. Apart from the box models, numerical water vapor tracing models are also applied in moisture tracing. These models make use of tagged tracers to trace water vapor by applying a Eulerian or Lagrangian framework. FLEXPART, a Lagrangian particle dispersion model, simulates the long-range mesoscale transport, diffusion, and dry and wet deposition of tracers [34
]. Its application fields have been extended to global and regional water cycles [25
]. The quasi-isentropic backward trajectory (QIBT) model was developed and used in moisture tracing for exploring the sink of irrigated fields and the source of monsoon precipitation, in which a back-trajectory Lagrangian algorithm was implemented [39
]. QIBT was designed to run on high temporal and spatial resolutions. Therefore, it has the advantage of tracing a real-time source of precipitation [42
]. The results of the QIBT model have been validated in many previous studies. Van der Ent et al. have made a validation of QIBT in their studies by comparing three different moisture tracing models [36
]. In the Yangtze River Bain, Wei et al. estimated the water vapor sources using QIBT [41
Our aims are three-fold: (1) To quantify the terrestrial and oceanic evaporative sources of precipitation for the SETP and MYRB by using the evaporative source tracing model QIBT; (2) to ascertain the dominant evaporative source for each of the two regions; (3) to explore the evaporative source contributing to precipitation change over the period of 1982–2011, for the two regions. The paper proceeds as follows. Section 2
introduces the study and source regions, the model, and the datasets used, and the methodology is also described. Section 3
illustrates the climatology, seasonal variability, and inter-annual change of evaporative source for the two regions. Section 4
presents the discussions and conclusions.
4. Discussions and Conclusions
As mentioned above, early-generation recycling models suffer from certain assumptions which may overestimate or underestimate the land–atmosphere interaction strength. The QIBT method also assumes a well-mixed atmosphere [36
]. Van der Ent et al. pointed out that the vertical layer of the launch of the precipitation parcels is the key to the overestimation [36
]. They discussed that the results are close to those of the evaporative source obtained by a realistic model when vertical locations of the launch points are chosen near the land surface and evaporative source contribution is determined based on the cloud fraction. Another assumption is that water vapor within the tropospheric column is equally likely to precipitate [15
]. This is a proper assumption for enhancing the efficiency of the model calculation but leaves certain processes in the atmosphere unsettled, especially in the boundary layer.
It must be noted that there are some differences between our results and previous studies. For instance, the Indian Ocean and Pacific Ocean together contribute 14% to precipitation in the MYRB as the oceanic source in our study. Bangladesh and Northeast India make positive contributions as evaporative sources of precipitation to the SETP. However, Wang et al. showed that the source from the Indian Ocean is equivalent to the source from the Pacific Ocean, at around 30% for the Mid-Lower Yangtze River Basin [27
]. The moisture source from either the South China Sea, Arabian Sea or Bay of Bengal contributes over 15% of the precipitation in the Yangtze River Basin [28
]. Chen et al. claimed that the southern neighboring lands of the SETP are a negative evaporation−precipitation (E−P) contributor to the SETP, based on using the FLEXPART model [25
]. These differences are related to three aspects. Firstly, the moisture includes not only evaporation from the land surface but also water vapor in the atmospheric column. The QIBT model focuses on evaporation from the land surface as the evaporative source. Meanwhile, the FLEXPART model unifies the evaporation and precipitation together (E−P) and uses change in specific humidity as the change in the moisture. This is the reason for the difference between our findings, with a high terrestrial evaporative source with the QIBT model used in our study and previous moisture advection studies indicating more moisture transportation from the ocean, based on FLEXPART. In the DRM model, however, the contribution of evaporation to moisture is based on an exponential solution [21
]. Secondly, the vertical layer of models and the initial release height of parcels might cause the differing results. We used 60 model layers, whereas no more than two vertical layers are used in previous studies. Van der Ent et al. claimed that the number of vertical layers and the initial release height of tagged water in the model are found to have the most significant influences on the results [36
]. Differences between our results and previous studies highlight that the evaporated moisture and water vapor in the atmospheric column are not well-mixed. Thirdly, the calculating technique differs among models. The QIBT and DRM models both use a simple iterative technique by Merril et al. [44
]. The FLEXPART model uses a “zero acceleration” scheme and one iteration of the Petterssen scheme [52
]. Although with different techniques, all of them belong to the Lagrangian method; in contrast, the WAM model uses the Eulerian method for moisture tracing.
Besides the choice of model, input datasets also result in uncertainties. The ERA-Interim reanalysis outperforms the other three reanalyses over the TP with regard to the surface air temperature and hydrological cycle. Gao et al. found that ERA-Interim better depicted the pattern of the observed precipitation; however, it overestimates the precipitation as well as the evapotranspiration in both regions, especially in the SETP [47
]. These overestimations will leave uncertainties in the evaporative source estimation. More accurate datasets are expected in future works.
The SETP has a higher altitude and more mountainous terrain than the MYRB. These terrestrial characteristic differences induce different quantities and dominant directions of evaporative sources for the two target regions. WE and RTP to the west of the SETP are listed in the top six evaporative source sub-regions for the SETP, which indicates a strong evaporative source from the west. For the MYRB, SCS and PAC are listed in the top six sub-regions, which indicates strong oceanic sources from the south. In addition, oceanic evaporative sources contribute 2% to the precipitation in the SETP and 14% in the MYRB. The smaller oceanic source portion but stronger terrestrial source from the west in the SETP implies the interference of the Himalayas in the northward moisture transport from the Indian Ocean.
Evaporative sources are quantified by using the advanced QIBT tracing model to illuminate the precipitation and its changes in the SETP and MYRB. The study region is divided into 16 sub-regions. Evaporative sources in the 16 sub-regions are combined into the terrestrial and oceanic portions. Dominant evaporative sources for the two target regions are revealed. The seasonal and inter-annual changes of the evaporative sources are investigated. Relationships between evaporative source changes and monsoon system evolutions are analyzed. The following findings are obtained.
The terrestrial evaporative sources dominate the precipitation in both regions, with 98% for the SETP and 86% for the MYRB. Oceanic evaporative sources contribute only 2% to the precipitation in the SETP and 14% in the MYRB. The Indian Ocean contributes 2.09%, and the Pacific Ocean contributes only 0.24% to the precipitation in the SETP. However, in the MYRB, sources from the Pacific Ocean and South China Sea contribute up to 11%, whereas the Indian Ocean contributes 3%. This difference in the oceanic source is highly related to the distance from the oceans. In addition, the smaller oceanic source portion in the SETP implies the interference of the Himalayas in the northward moisture transport from the Indian Ocean.
The top six evaporative sources of precipitation in the two target regions are found through two ranking approaches. The whole TP contributes 63% to the precipitation in the SETP, with 35% from a local source. The MYRB shows a local evaporative source of 29%, and 29% from its neighboring sub-region of Southern China. The recycling ratio reaches the highest in August for both regions. The higher local evaporative source in the SETP compared to the MYRB implies a stronger land–atmosphere interaction.
Monthly variations of the evaporative source pattern show that the SETP and the MYRB are controlled by the transition of the Indian monsoon and East Asian summer monsoon systems. In the early monsoon season (April and May), the common evaporative source area for the SETP and MYRB is west–east oriented. In the mid-monsoon season (June and July), it is southwest–northeast oriented. In the later monsoon season (August and September), it is south–north oriented. This seasonal variation implies the transition of the climate system over China from the westerly to the monsoon system. July is the turning point of the evaporative source from being Indian monsoon dominated to East Asian summer monsoon dominated.
The annual mean precipitation decreases from 1982 to 2011, in both target regions. In the SETP, the evaporative source decreases slightly in the terrestrial portion and heavily in the oceanic portion. However, in the MYRB, the evaporative source decreases heavily in the terrestrial portion and slightly in the oceanic portion. The precipitation recycling ratio increases in both regions despite the decrease in the terrestrial and oceanic sources. This indicates that the decreased advection from outside terrestrial sources and oceans is responsible for the decrease in precipitation in both regions. Temporally, the decrease in precipitation reaches its greatest point in September. Spatially, the evaporative source in the Bay of Bengal decreases the most among the 16 sub-regions.