Stable isotopes of summer monsoonal precipitation in southern China ...

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J. Geogr. Sci. (2008) 18: 155-165 DOI: 10.1007/s11442-008-0155-9 © 2008

Science in China Press

Springer-Verlag

Stable isotopes of summer monsoonal precipitation in southern China and the moisture sources evidence from δ18O signature LIU Jianrong1,2, SONG Xianfang1, YUAN Guofu3, SUN Xiaomin3, LIU Xin1,2, WANG Zhimin1, WANG Shiqin1,2 1. Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China; 2. Graduate University of Chinese Academy of Sciences, Beijing 100039, China; 3. CERN Sub-center of Water, Beijing 100101, China

Abstract: Summer monsoons (South Asian monsoon, South China Sea monsoon and Subtropical monsoon) are prominent features of summertime climate over southern China. Different monsoons carry different inflow moisture into China and control the temporal and spatial distributions of precipitation. Analyses of meteorological data, particularly wind, temperature and pressure anomalies are traditional methods of characterizing moisture sources and transport patterns. Here, we try to utilize the evidence from stable isotopes signatures to trace summer monsoons over southern China. Based on seven CHNIP (Chinese Network of Isotopes in Precipitation) observatory stations located in southern China, monthly composite precipitation samples have been collected and analyzed for the composition of δ18O during July, 2005. The results indicated that the spatial distributions of δ18O in precipitation could properly portray the moisture sources together with their transport pathways. Moreover, the amount effect, altitude effect, temperature effect and the correlation between δ18O vs. relative humidity were discussed. Keywords: summer monsoon; precipitation; δ18O; vapor inflow corridors; southern China

1

Introduction

Monsoonal circulation is an important carrier of moisture transport. Its temporal and spatial variations and consequent changes in air circulation patterns together with moisture sources determine the rainfall variability (Froehlich, 2003). Stable isotopes in precipitation are not only controlled by geographical and meteorological parameters, such as temperature, precipitation amount, latitude and altitude, but also, to a certain extent, by regional clima-

Received: 2007-08-28 Accepted: 2007-12-06 Foundation: National Natural Science Foundation of China, No. 40671034; Foundation of Isotopes in Precipitation of Chinese Ecosystem Research Network Author: Liu Jianrong (1982−), MS Graduate Student, specialized in isotopic hydrology and water cycle. E-mail: [email protected]

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tological background, e.g. monsoon (Wei et al., 1994; Pang et al., 2004). Furthermore, stable isotope oxygen (δ18O) and hydrogen (δD) in precipitation are powerful tracers for identifying the sources of water vapor generated during the monsoon. They can also be used to delineate the climatic parameters controlling their latitudinal distributions (Saikat et al., 2006). It shows that, isotopes in the water molecule are indispensable tools for understanding moisture sources, formation and transport at large scales (Kattan 1997; Athanassios et al., 2006; Polona et al., 2006; Yamanaka et al., 2007; Liu et al., 2008). Some domestic investigations on Dongtaigou catchment of Chaohe and Baihe river basins (Liu et al., 2005), Chabagou catchment of the Loess Plateau (Liu et al., 2007), etc. have also convinced that the stable isotope information of precipitation could provide convincing evidence for the catchment scale circulation patterns. China is one of the most famous monsoonal regions in the world, and its rainy season is well correlated with the onset and withdrawing of the monsoons (Shen et al., 1982). Southern China is the intersection of different moisture inflow corridors and the initial stage of summer monsoon transport. Therefore, to a certain extent, the isotopic composition of precipitation here could be considered as a reasonable representative of the background values of precipitation in other parts of China. In order to collect data on stable isotope content of precipitation nationwide systematically and, consequently, to provide basic isotope data for applying environmental isotope tools in hydrological investigations, in 2004, Chinese Network of Isotopes in Precipitation (CHNIP) (Song et al., 2004; Song et al., 2007) was set up. Its establishment was under a lot of successful experiences of GNIP (Global Network of Isotopes in Precipitation) and some other previously built national networks of isotopes in precipitation, such as ANIP (Austrian Network of Isotopes in Precipitation) (Kralik et al., 2003), CNIP (Canadian Network of Isotopes in Precipitation) (Fritz et al., 2003) and NISOT (Swiss National Networks) (Schürch et al., 2003). CHNIP was based on the field research stations of Chinese Ecosystem Research Network (CERN), which was initially established in 1988. CERN has played important roles in the dynamic observations, scientific research and production demonstrations of ecosystem and environment in China (Fu et al., 2007). The present study was performed during July 2005, at 7 CHNIP stations located in southern China (Figure 1a). The spatial distribution of δ18O in monthly composite precipitation samples, has confirmed that δ18O could be used as a tracer to properly portray the summer monsoonal moisture sources and their transport patterns. Finally, the amount effect, altitude effect, temperature effect and the relationship between δ18O and relative humidity were also discussed.

2

The study area and methodology

The present study was performed at 7 CHNIP stations of Banna, Ailaoshan, Huitong, Dinghushan, Taoyuan, Qianyanzhou and Yingtan located in southern China (21°N−29°N), ranging from 45 to 2481 m in elevation. The annual average surface air temperature varies from 11℃ to 22℃ (Figure 1b), and the annual precipitation amount is about 1200−1931 mm (Figure 1c). The first four stations were forest ecosystem stations, mainly focusing on the studies of structure, function, dynamics and ecological technologies of the tropical and sub-tropical forests. The latter three ones were agricultural ecosystem stations,

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157

Figure 1 Location of the selected CHNIP stations in southern China (a) and spatial distributions of yearly surface temperature (b), precipitation amount (c) (Meteorological data source: National Meteorological Information Center, China Meteorological Administration)

mostly targeted at the structural and functional optimization of agro-ecosystem, the protection and highly efficient and sustainable utilization of agricultural resources in the red-soil hilly areas. Monthly composite precipitation samples have been collected during July, 2005, following the procedure mentioned in the IAEA guideline. They were transferred to 50 ml polyethylene bottles and analyzed in the Environmental Isotope Laboratory of Institute of Geographic Sciences and Natural Resources Research, the Chinese Academy of Sciences. Water samples have been measured by Finnigan MAT253 mass spectrometer, using the TC/EA method, for δ18O and δD contents. Results were expressed by convention as parts per thousand deviations from the Vienna Standard Mean Ocean Water (V-SMOW) as follows: ⎡ Rsample − RVSMOW ⎤ δsample = ⎢ (1) ⎥ × 1000(‰) RVSMOW ⎣ ⎦ where R is the ration of D/H or 18O/16O in sampled water (Rsample) or in the Vienna Standard Mean Ocean Water, V-SMOW (RVSMOW). The precision of ~±0.3‰ and ±2‰ were obtained for δ18O and δD respectively.

3

Results and discussion

Meteorological parameters (surface air temperature, precipitation amount and relative humidity) have been monitored at the same time (Table 1).

158 Table 1

Journal of Geographical Sciences Isotopic composition in precipitation and meteorological data of the CHNIP stations

Station name Taoyuan

Longitude (°)

Latitude (°)

δ18O(‰)

δD(‰)

T (℃)

P (mm)

111.44

28.93

−6.88

−56.1

29.77

110.2

RH (%) 74

Yingtan

116.56

28.12

−3.33

−29.5

30.33

67.6

67

Qianyanzhou

115.03

26.44

−5.26

−44.5

29.14

50.2

71

Huitong

109.61

26.85

−9.54

−61.6

27.06

94.6

72

Ailaoshan

101.03

24.55

−9.22

−62.8

15.72

357.8

93

Dinghushan

112.55

23.16

−9.47

−65.2

29.27

175.2

71

Bannan

101.26

21.93

−7.97

−54.8

26.05

262.4

83

(Meteorological data source: Synthesis Research Centre of CERN)

Given by some previous studies (Tian et al., 2004; Sun et al., 2006), in summer, there were three main low latitudinal vapor inflow corridors into southern China, namely, southwestern corridor, South China Sea corridor, and southeastern corridor, standing for South Asian monsoon, South China Sea monsoon, and subtropical monsoon, respectively (Figure 2).

Figure 2 Three vapor inflow corridors of summer monsoonal precipitation (South Asian monsoon, South China Sea monsoon, subtropical monsoon) (redrawing according to Tian et al., 2004; Sun et al., 2006) and four typhoon (tropical storms) pathways occurred in the west Pacific Ocean during July, 2005. Among four of them, typhoon Haitang (No. 0505) and tropical storm Washi (No. 0508) landed and brought plenty of rainfalls to southern China

After the moisture carried by the three corridors landed on southern China, they commenced their journey over China. Therefore, a full examination of the isotopic composition and the accordingly meteorological parameters here were essential for a comprehensive understanding of moisture sources related to specific monsoon dynamics.

LIU Jianrong et al.: Stable isotopes of summer monsoonal precipitation in southern China

3.1

159

Tracing summer monsoonal corridors using δ18O

According to the analysis results, the isolines of δ18O have been plotted (Figure 3). It showed that the δ18O values peaked in the eastern part of the study area but at neap when it reached Guizhou and Guangxi provinces. To the western part of southern China, the δ18O value declined eastward until it reaches Guizhou and Guangxi provinces. This distribution pattern has been conformed to the three origins of the summer monsoons. Hereafter, the study area would be divided into three parts and discussed respectively.

Figure 3 Isolines of δ18O in precipitation and the three deducted water vapor corridors which affected the summer monsoonal precipitation during July, in southern China, 2005

3.1.1

Western part—Yunnan Province

Rayleigh fractionation theory pointed out that δ-values of precipitation under the equilibrium fractionation could be expressed as follows (Dansgaard, 1964): δrain = δ0 × f 18

(α −1)

(2)

where δrain is the δ O or δD of precipitation, δ0 is the initial isotope value, f is the proportion of remaining water vapor, and α is the fractionation coefficient at the condensation temperature t (℃). Thus it was clear that, the δ-values were determined not only by the condensation temperature, but also by the residual percentage of water vapor in precipitable clouds. Yunnan Province is located in the southwestern border of China, and its precipitation was the result of water vapor jointly from southwest and South China Sea corridors. The spatial distribution of δ18O showed a negative gradient from west to east in Yunnan Province. Firstly, it was because, after the water vapor of western corridor evaporating above the Arabian Sea, it started the movement across the Indian continent. It rained successively along the vapor transport route, and the proportion of moisture preserved in the cloud, f, decreased

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continuously. From equation (2), the δ-value decreased accordingly. Secondly, the water vapor evaporated from Bay of Bengal, and the heavy isotopes would also deplete throughout the rain-out process across the South Asian continent. Therefore, the moisture led to the depletion of the heavy isotopes when they arrived at the western part of Yunnan Province. There was a degressive trend of δ18O towards east in the province. The isolines reflected the monsoonal inflow direction of the corridors. 3.1.2

Eastern part—Zhejiang, Fujian, Jiangxi and Hunan provinces 18

The δ O isolines declined in longitudinal direction from east to west. Due to the water vapor carried by southwest and South China Sea vapor corridors progressively raining out when it moved eastwards, its influence on the precipitable water vapor decreased. Water vapor from the west Pacific Ocean played a more dominant role in this part. West Pacific Ocean is a region where typhoons and tropical storms frequently break out. During July, 2005, there occurred four, and among which, the tropical storm Nalgae and Banyan have not land on the continent, therefore they have had little effect on the precipitation in southern China (Figure 2). However, typhoon Haitang (No.0505) which landed on Fujian Province, contributed a lot to the precipitation amount in the provinces of Fujian, Jiangxi, Hunan and Guangdong during 19–21, July (Figure 4).

Figure 4

Daily temperature, relative humidity and precipitation of Dinghushan and Yingtan stations

Usually, the vapor moistures carried by the frontier of typhoons were originally formed by rapid evaporation of seawaters (Shinji et al., 1996). Sugimoto and Higuchi (2003) showed that strong ascending currents prevented small precipitation particles of relatively high isotope ratios from falling down to the ground. However, after the uplift became weak, such small particles could start to fall (Sugimoto et al., 1989). That is, the δ-values presented relatively higher in the beginning of a typhoon while lower in the end of an event. However,

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this conclusion was gained at a fixed observatory site and under a high-temporal resolution condition. Here, although the precipitation samples were monthly composite ones and were mainly a reflection of monthly averaged conditions, a similar phenomenon would also appear. We considered there might be two reasonable explanations: the precipitation amount brought by the typhoon Haitang accounted for a large part of the total monthly precipitation amount, thus, accordingly, the isotope values of the typhoon precipitation accounted for a large part of those of the monthly composite ones. Furthermore, from the spatial distributions of observatory stations, Yingtan and Qianyanzhou stations were closer to the landing place of the typhoons, therefore, their precipitation might be induced by the forward parts of the typhoons, while the precipitation in Taoyuan and Huitong station might be induced by the rear parts of the typhoons approximately. 3.1.3

Central part—Guangxi and Guizhou provinces

There was a northwestwards curve of δ18O isoline in this region, which was the result of another strong severe tropical storm, Washi (No.0508). Washi has been formed on 29th, July, and landed on Hainan Province the next day. It brought plenty of rainfall to Hainan, Guangxi and Guangdong provinces. According to the record of Dinghushan observatory station, during 29th to 31st, the rainfall accounted for 51% of the monthly precipitation amount. The curve of δ18O isoline was similar to the water vapor transport pathway after Washi landing on southern China. And this again validated the indication function of the precipitation isotope in tracing the pathway of the typhoons. 3.2

LMWL of southern China

Concerned about evaporation and condensation process, hydrogen isotopes are fractionated in relation to oxygen isotopes, and Craig firstly defined this relation in Global Meteoric Water Line (GMWL): (3) δD=8δ18O+10 As a matter of fact, GMWL is a global average level. Therefore, local meteoric water lines (LMWL) usually differ from GMWL in both slopes and intercepts according to the regional climatic and geographic parameters. The relationship between δD and δ18O at all stations of southern China during the observatory period could be expressed as (Figure 5): (4) δD = 5.15δ18O – 15.5 (r = 0.97)

Figure 5 δD-δ18O relationship from CHNIP stations precipitation samples with regression line (solid line) and regression equation. The global meteoric water line (GMWL; dashed line) was also given for reference.

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Craig pointed out that, the slope of the δD/δ18O close to 8 suggested that precipitation took place under near-equilibrium conditions, while lower values indicated the presence of kinetic fractionations. Therefore, the slope of the LMWL, 5.15, may be attributed to the non-equilibrium conditions in the precipitation during the summer month. The point of intersection (δ18O = −8.95‰, δD = −61.6‰) was an approximate reflection of the original composition of the water vapor. 3.3 3.3.1

Isotopic effects Amount effect of δ18O

The amount effect refers to the negative correlation between isotopic composition and precipitation amount. For some low latitudinal regions, especially those coastal regions, the amount effect is considerably distinct. In southern China, there have appeared an expected negative correlation between δ18O and monthly precipitation amount (with statistically 95% significant level, Figure 6a). δ18O (‰) = −2.27ln (P)(mm)+3.64 (r = 0.68)

(5)

Figure 6 Amount (a), altitude (b), temperature effect (c) of δ18O and relationship between relative humidity and δ18O in precipitation (d)

3.3.2

Altitude effect of δ18O

When air masses are orographically uplifted, they cool and precipitate preferentially the heavier isotopes. Usually, the effect is used to estimate the altitude of groundwater recharge areas. There was a distinct altitude diversity (45–2481 m), but not a distinct latitude diversity among the seven selected observatory stations (21°N–29°N), it could evenly shield the influence of the latitude effect, therefore, it is a suitable place for the altitude effect investigation. The logarithmic negative relation can express the effect (Figure 6b): δ18O (‰) = −1.13ln(H)(m) −1.3 (r = 0.69)

(6)

LIU Jianrong et al.: Stable isotopes of summer monsoonal precipitation in southern China

3.3.3

163

Temperature effect of δ18O

According to the investigations at North Atlantic Ocean, Dansgaard (1964) pointed out that, there was a distinct positive correlation between isotopic composition in precipitation and temperature. And this correlativity was most significant in the interior of the continent. However, at the moist tropical island stations, this correlativity might be masked by the rain-out, evaporation and the water vapor interaction above the ocean surface. Therefore, where data showed a lack of correlation in δ18O/T gradient, it did not invalidate the isotope approach. It might be the thermometer that has measured precipitation, volume, source, or continentality instead (Araguás-Araguás et al., 1998). Seen from what we have obtained, the simple liner relation between δ18O and monthly averaged temperature was virtually non-existent in southern China. Besides the factors above, there may be another reasonable explanation. Taking Dinghushan and Yingtan stations for example, during the period of typhoons Haitang and Washi, the daily temperature was obviously lower than the monthly averaged temperature. And this observatory result was consisted with former results obtained in Asian monsoonal region (Figure 6c). The quadratic (5) showed the temperature effect of δ18O: δ18O (‰) = 0.07T2(℃)−2.75T(℃)+17.9 (r = 0.68) (7) 3.3.4

Relationship between δ18O and relative humidity

Relative humidity is an indicator of dry/humid condition of the atmosphere. Therefore, different relative humidity is expected to produce different isotopic signatures of rainwater. The relationship between δ18O and monthly relative humidity was expressed as below (Figure 6d): δ 18 O (‰) = 0.01RH 2 (%) − 2.27RH(%) + 87.31 (r = 0.61)

4

(8)

Conclusions

The stable isotopes of deuterium and oxygen-18 in precipitation have been investigated in southern China during July, 2005. The main results and conclusions are summarized as follows: According to the δ18O isolines, the δ18O values peaked in the eastern part while at neap in the central part of southern China. In the western part, the degressive trend of δ18O towards eastern direction reflected the inflow corridor of South Asian Monsoon. In the eastern part, precipitable moistures were mostly formed at the west Pacific Ocean and carried by the subtropical monsoon. The degressive δ18O value at longitudinal direction was the indication of this water vapor corridor. In the central part, the northwestwards curve of δ18O isoline was the result of a tropical storm landed on southern China. All these distribution patterns reconstructed the three origins of the monsoonal pathways, and validated the indication function of the isotopes in precipitation in tracing the origin and pathways of summer monsoons during July, 2005. Slopes of linear relationship between δD and δ18O in precipitation were lower than 8, the average global level, indicating that the precipitation process has undergone kinetic conditions.

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The observed isotope distribution in space and time could be related to a number of environmental parameters which characterized not only by the source regions but also the existing sampling sites. Precipitation amount, temperature, altitude, relative humidity, together with the source specific fractionation between oxygen-18 and deuterium, all these effects could contribute to the isotope content of a sample of precipitation. In southern China, the amount effect and altitude effect were significant, and could be expressed in logarithmic form. However, the temperature effect was virtually non-existent, which might be masked by the rain-out, evaporation and the water vapor interactions above the ocean surface. Quadratics could describe the temperature effect and the correlation between δ18O vs. relative humidity.

Acknowledgements The authors would like to acknowledge Synthesis Center of Chinese Ecosystem Research Center and National Meteorological Information Center, China Meteorological Administration for providing the meteorological data. We also thank Yang Jinrong and Yuan Jingjing for sample analysis. At the same time, sincerely appreciation is given to all the observatory field stations for the collection of precipitation samples and a tour of their facilities.

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