Interactions Between the Cryosphere. Climate and Greenhouse Gases (Proceedings of IUGG 99 Symposium HS2, Birmingham, July 1999). IAHS Publ. no. 256, 1999. 245

The distributional features of ô O in modern-day precipitation on the Tibetan Plateau

ZHANG XINPING Department of Resources and Environment, Hunan Normal University, Changsha 41008!, China e-mail: [email protected]

YAO TANDONG & TIAN LIDE Lanzhou Institute of Glaciology and Geocryology, Chinese Academy of Sciences, Lanzhou 730000,Gansu, China

Abstract There are marked positive relations between 8180 in precipitation and temperature at Tuotuohe, Delingha and Xining located on the Tibetan Plateau. The differences of d5180/dr among the three stations are notable on the synoptic scale. However, after eliminating the synoptic fluctuation, the differences of d8l80/dr decrease. Contrary to current knowledge, the mean ô'sO in precipitation in summer gradually increases from south to north on the Plateau, especially to the north of the Tanggula Mountains. The pattern reflects the influence of the vapour origins of 8I80. The analytical results show that the contribution of vapour originating from evaporation on the inner Tibetan Plateau, to 81S0 in precipitation is significant during summer, in addition to that from the Bay of Bengal and the Arabian Sea.

INTRODUCTION

The relation between stable isotopes and temperature is important for palaeoclimatic study. One aim of ice core climatology is to reconstruct the processes of past climate change according to the relation between the two (Dansgaard, 1964; Jouzel, 1986). In order to establish the corresponding relationship between 8I80 and temperature on the Tibetan Plateau, Tuotuohe (34°13'N, 92°26'E; 4533 m a.m.s.l), Delingha (37°22'N, 97°22'E; 2981 m a.m.s.l.) and Xining (36°37'N, 101°46'E; 2261 m a.m.s.l.) were selected as sampling stations for the period September 1991-December 1992. In addition, precipitation was also sampled at the following 10 stations, from south to north on the Plateau, for spatial variation analysis from May to September, 1993: Nyalam (28°11'N, 85°58'E; 3810 m a.m.s.L), Xixiabangma (28°27'N, 85°45'E; 5680 m a.m.s.l.), Tingri (28°38'N, 87°05'E; 4300 m a.m.s.l.), Xigaze (29°15'N, 88°53'E; 3836 m a.m.s.l.), Lhasa (29°40'N, 91°08'E; 3649 m a.m.s.l.), Damxung (30°29'N, 91°06'E; 4200 m a.m.s.l.), Nagqu (31°29'N, 92°04'E; 4507 m a.m.s.l), Amdo (32°21'N, 91°06'E; 4800 m a.m.s.l), Tuotuohe and Wudaoliang (35°13'N, 93°05'E; 4612 m a.m.s.l.) (Fig. 1). Temperature varies during a precipitation event. In this study, the sampling temperature was taken at the end of each precipitation event, except at Xining where the sampling temperature was taken automatically throughout the event and the statistical mean calculated after necessary correction. 246 Zhang Xinping et al.

80°E 85°E 90°E 95°E 100°E

i : '. i i lu—i w si 1 80°E 85°E 90°E 95°E 100°E Fig. 1 The sampling stations on the Tibetan Plateau. 1: Nyalam. 2: Xixiabangma. 3: Tingri. 4: Xigaze. 5: Lhasa. 6: Damxung. 7: Nagqu. 8: Amdo. 9: Tuotuohe. 10: Wudaoliang. 11: Delingha. 12: Xining.

All collected samples were sealed in plastic bottles and kept at low temperature until they were sent to the laboratory. The measurement of 8180 was by mass spectrometry (MAT-252 mass spectrometer). The magnitude of the stable oxygen isotope ratio 8180 is expressed as the per mille deviation from Standard Mean Ocean Water (SMOW):

18n/16n _ 18n/'6n ô'80 (°/ ) = ^sample w/ wSMOW /J\ w/ SMOW The precision of the 8I80 measurement is ±0.05%o.

RELATIONSHIP BETWEEN ô,!iO AND TEMPERATURE

Figure 2 shows the scatter diagram of 8180 against sampling temperature at Tuotuohe, Delingha and Xining. The large scatter in the figure illustrates that the precipitation arises from various synoptic systems which have different condensation levels and temperature of precipitation parcels, different vapour origins, and which have travelled different paths (Jouzel & Merlivate, 1984). The regression equations of 8180 against the sampling temperature at the three stations are: 8l80(%o) = 0.40r(°C) - 11.07 for Tuotuohe, r = 0.44, «=119 (2a) 8180(%o) = 0.617T°C) - 12.45 for Delingha, r = 0.76, n = 76 (2b) 8'80(%o) = 0.297T°C) - 8.52 for Xining, r = 0.37, n = 105 (2c) where r is the correlation coefficient and n the number of samples. 77?? distributional feature of S,S0 in modern-day precipitation on the Tibetan Plateau 247

-10

6180 (%o)

-20 0 a: Tuotuohe * b: Delingha ° c: Xining

-30 -20 -10 0 10 20

Tm(°C) Fig. 2 Scatter diagram of Sl!iO against the sampling temperature Tfor sampling sites at Tuotuohe Delingha and Xining, September 1991-December 1992.

For the three stations as a whole: 5l80(%o) = 0.46r(°C) - 10.94 r = 0.58, n = 228 (2d) Mean monthly ô180 and mean monthly temperature can be calculated from the monthly sampled S180 and temperature, which subsequently give intermonthly variations of the two parameters. Figure 3 gives the correlated scatter diagram of the 18 l8 mean monthly S 0 (mô 0) against the mean monthly temperature (Tm). The calculated regression lines are:

18 mS 0 (%o) = 0.69 Tm (°C) - 11.54 for Tuotuohe, r = 0.86, n = 12 (3a)

18 mo 0 (%o) = 0.71 Tm (°C) - 13.39 for Delingha, r = 0.91, n = 10 (3b)

18 mS 0 (%o) = 0.49 Tm(°C)- 10.41 for Xining, r = 0.76, « = 10 (3c) For the three stations as a whole:

l8 mS 0 (%o) = 0.64 Tm (°C) - 12.23 r = 0.86, n = 32 (3d) Equation (3) is different from equation (2). The former reflects the interdependent relationship between 8lgO and temperature on a seasonal scale, and the latter reveals the relationship on a synoptic scale. d8180/d7 on a synoptic scale differs between the three stations, but the differences are less after eliminating synoptic fluctuation. In 18 addition, all dm8 0/drm on a seasonal scale are greater than those on a synoptic scale for the three stations.

SPATIAL DISTRIBUTION OF ô,80

Based on the analyses for S,80 in precipitation sampled at the 10 stations (Nyalam etc.) from May to September 1993, it is found that the trends in temporal variation of ôl80 are very similar to each other. However, ranges in variation are different because of the different contribution of the summer monsoon from south to north in the Plateau. The ranges in variation of S180 at Nyalam and Xixiabangma, located in the southernmost region of the Plateau, were small because the water vapour had the same origin; the 8180 fluctuations at the six stations from Tingri to Amdo situated between the Himalayas and the Tanggula Mountains were large because of the strong interactions of water vapour having different origins. The variation ranges at Tuotuohe and Wudaoliang, located in the northern regions of the Tanggula Mountains, were less than those at the six stations in the southern regions of the Tanggula Mountains due to weak interaction of water vapour from different origins. The results show that the correlations between S180 and sampling temperature do not reach a notable confidence level in the sample period from May to September at the 10 stations, except at Nyalam. However, when the fluctuation of ôl80 with sampling temperature was divided into several periods particular to each of the stations, the positive correlation between 8I80 and temperature markedly improved (Table 1), which shows that the origin of the water vapour is probably from one source during these periods. Figure 4 shows the spatial distribution of mean 8180 and mean temperature T at all stations in the Plateau during the sample period. Because of the high elevation, low temperature and light 8180 measured at Xixiabangma, only the relationship between 8I80 and T at the other nine stations are considered for the present. The calculated results show that the following logarithmic correlation is significant: m§180 (%o)=- 16.8671ogr(°C) + 0.567 (4) The non-linear correlation coefficient and confidence level reach -0.75 and 0.02 respectively. If 8,80 and T of all the stations are corrected to an elevation of 4000 m, The distributional feature of Ô O in modern-day precipitation on the Tibetan Plateau

Table 1 dô^O/aTin the Tibetan Plateau in different sample periods. Station Period d5l80/dr r n Nyalam 10 July to 5 September 0.95 0.64 41 Xixiabangma 14 July to 25 August 0.88 0.33 22 Tingri 5 July to 14 August 0.61 0.38 19 Xigaze 14 July to 9 September 0.60 0.37 28 9 September to 16 September 0.57 0.40 24 Lhasa 27 July to 28 August 0.91 0.50 28 Damxung 18 June to 27 August 0.99 0.59 56 Nagqu 31 July to 20 September 0.69 0.51 70 Amdo 19 July to 15 September 1.12 0.50 64 Tuotuohe 4 May to 11 July 0.35 0.29 49 27 July to 13 September 0.60 0.36 45 Wudaoliang 21 May to 13 July 0.52 0.37 29 assuming dÔ,80/d# =-0.239fo/100 m and dT/àH = -0.58°C/100 m (Zhang et al, 1995), where H is elevation, the linear correlation coefficient between the 8180 and T of the 10 stations including Xixiabangma is -0.54. As shown in Fig. 4, T decreases gradually and 8180 increases basically from south to north in the Plateau, especially at Tuotuohe and Wudaoliang located in the north of the Tanggula Mountains where 8180 increases greatly. This is contrary to the normal significant "continental effect" that suggests the stable isotopic ratio decreases with the distance to the ocean. It is thought that this is related to the origin of the water vapour. The vapour of summer precipitation in the southern Plateau mainly originates from the Bay of Bengal and the Arabian Sea. A vast amount of precipitation is produced after surmounting the Himalayas. Subsequently, 8180 in vapour is depleted greatly, resulting in more negative 8I80 values in precipitation in the southern Plateau. On the other

J°° ô 18O=-16.867lgr+0.567

\ 9

18 1 5 0 (%o)

2 • l I 0 2 4 6 8 10 12 14

Tm(°C) Fig. 4 Spatial distribution of mean ô'^O against mean temperature T in the Tibetan Plateau. 250 Zhang Xinping et al. hand, there are less negative values of S180 in the vapour in the northern Plateau. This is caused by evaporation in the inner Plateau. Ô180 in the water of many lakes is heavy due to strong evaporation in the Plateau where the rate of evaporation is greater than the rate of supply. The mean S180 in water of the Qinghai Lake is about +1.97%o and that of the Haiyanwan about +2.89%o (Zhang, 1994). Consequently, 8I80 values should increase in the precipitation supplied from the evaporation of lake water in the inner Plateau. It is notable that the minimum 5180 occurs at Xigaze and Lhasa along the Yarlungzangbo River instead of at the southernmost parts of the Plateau, owing to a water vapour pathway that exists in the basin between the Gangdise Mountains and the Himalayas (Gao et al, 1985). S180 decreases greatly as a result of long-distance transportation, while the warm and moist air flows upward along the pathway in summer.

CONCLUSIONS

(a) There is marked positive correlation between 8I80 in precipitation and sampling temperature at Tuotuohe, Delingha and Xining, located in the northern Tibetan Plateau. The d8,80/dr is 0.40, 0.61 and 0.29 %o/°C at the three stations

respectively, and the composite gradient of the three stations is 0.46%o/°C. l8 (b) After eliminating the influence of synoptic fluctuation, the dm8 0/drm on the seasonal scale is 0.69, 0.71 and 0.49%o/°C at Tuotuohe, Delingha and Xining

respectively, and the composite gradient of the three stations is 0.64%0/°C. (c) The mean 8I80 in precipitation increases gradually from south to north of the Plateau during summer, especially in the northern regions of the Tanggula Mountains. This result is related to the various origins of the water vapour.

REFERENCES

Dansgaard, W. ( 1964) Stable isotopes in precipitation. Tel/us 16,436-468. Gao, D. Y., Zou, H. & Wang, W. (1985) Influence of water vapour pass along the Yarlungzangbo River on precipitation (in Chinese with English abstract). Mountain Res. 3,239-249. Jouzel, J. & Merlivate, L. (1984) Deuterium and oxygen-18 in precipitation: modeling of the isotopic effects. J. Geophys. Res. 89, 11 749-11 757. Jouzel, J. (1986) Isotopes in cloud physics: multiphase and multistage condensation processes. In: Handbook of Environmental Geochemistry (ed. by P. Fritz & J.-Ch. Fontes), vol. 2, 61-112. Elsevier, New York. Zhang, B. Z. (1994) Distribution characteristics of stable isotopes of waters in the Qinghai Lake area and their evolutional law (in Chinese with English abstract). In: Evolution of Recent Environment in Qinghai Lake and its Prediction (ed. by Shi Yafeng el at), 29-40. Science Press, Beijing. Zhang, X. P., Shi, Y. F. & Yao, T. D. (1995) Relation between 8lsO in atmospheric precipitation and temperature and precipitation. Chinese Geographical Sci. 5, 289-299.