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Monsoon Variability in the Himalayas Under the Condition of Global Warming

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Monsoon Variability in the Himalayas Under the Condition of Global Warming Journal of the Meteorological Society of Japan, Vol. 81, No. 2, pp. 251--257, 2003 251 Monsoon Variability in the Himalayas under the Condition of Global Warming Keqin DUAN Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy Science, Lanzhou, China National Laboratory of Western China’s Environmental Systems, Lanzhou University, Lanzhou, China and Tandong YAO Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy Science, Lanzhou, China (Manuscript received 16 January 2002, in revised form 5 December 2002) Abstract An ice core-drilling program was carried out at the accumulation area of Dasuopu glacier (28230 N, 85430E, 7100 m a.s.l.) in the central Himalayas in 1997. The ice core was analyzed continuously for stable isotopes (d18O), and major ions throughout the core. Cycles indicated by d18O, cations were iden- tified and counted as seasonal fluctuations as annual increment from maximum to maximum values. Reconstructed 300-year annual net accumulation (water equivalent) from the core, with a good correla- tion to Indian monsoon, reflects a major precipitation trend in the central Himalayas. The accumula- tion trend, separated from the time series, shows a strong negative correlation to Northern Hemisphere temperature. Generally, as northern hemisphere temperature increases 0.1C, the accumulation de- creases about 80 mm, reflecting monsoon rainfall in the central Himalayas has decreased over the past decades in the condition of global warming. 1. Introduction forecast the monsoon have been made with only Agriculture, industry and hydroelectric moderate success (Webster and others 1998). A power in south Asia are heavily dependent on long series of reliable data for large contiguous the performance of the summer (June to Sep- spatial domains is not available except for tember) monsoon rainfall, which provides 75 to India, which limits our ability to determine the 90 per cent of the annual rainwater over most rainfall patterns in the monsoon regions. Pre- parts of the area. A weak monsoon year gener- viously published results suggest that variation ally corresponds to low crop yields. And strong of south Asian monsoon is closely linked to the monsoon usually produces abundant crops, al- greater heat capacity of the ocean relative to though too much rainfall may produce dev- the surrounding landmasses (Fu and Fletcher astating floods. However, modeling efforts to 1985; Li and Yanai 1996). On short time scales, variations in the strength of the South Asian Corresponding author: Cold and Arid Regions monsoon have been explained by changes in Environmental and Engineering Research Insti- internal boundary conditions, such as increas- tute, Chinese Academy Science, Lanzhou 730000, ing tropical sea surface temperatures (Tourre China. E-mail: [email protected] and White 1995), variations in Eurasian snow ( 2003, Meteorological Society of Japan cover (Sirocko et al. 1993; Barnett et al. 1988; 252 Journal of the Meteorological Society of Japan Vol. 81, No. 2 Hahn and Shukla 1976; Dicksonet et al. 1984), and linkages with the El Nino-Southern Oscil- lation (ENSO) (Charles et al. 1997; Webster et al. 1998; Cole et al. 2000). Some studies show that the Himalayas and the Tibetan plateau play an important role in the evolution of the boreal summer monsoon (Ye 1981; Yanai and others 1992, 1994). Flohn (1957) suggested that the seasonal heating of the elevated surface of the Tibetan plateau, and the consequent reversal of the meridional tem- perature and pressure gradients, trigger the Fig. 1. The location of the Dasuopu Gla- large-scale change of the general circulation cier over Asia and the monsoon burst over the In- dian subcontinent. The strong (weak) monsoon also demonstrate considerable natural vari- years are associated with positive (negative) ability during the present climate epoch, in- tropospheric temperature anomalies over Tibe- cluding a number of climate extremes such as tan plateau (Fu and Fletcher 1985). However, the so-called ‘‘Little Ice Age’’. analysis of how these relations vary, particu- In this paper we present a 300-year proxy larly on decadal and longer time scales, is record of precipitation from the central Hima- hampered by the limited instrumental precipi- layas, Tibet, which allows us to examine the tation record in the Tibetan plateau, especially long-term precipitation variability in the in the Himalayas, which rarely spans more Himalayas and its relationship with Northern than the past few decades. Therefore, it is sig- Hemisphere temperature. nificance to detect monsoon precipitation vari- 2. Sampling and analysis ability on secular time scales in the Hima- layas in order to improve the understanding of Three ice cores were recovered by an electro- monsoon variability. This data gap can be ad- mechanical drill in the accumulation area of dressed with high-fidelity paleoclimate records Dasuopu glacier (28230N, 85430E) (Fig. 1), from long-record ice cores recovered from care- Himalayas, between August and October 1997. fully selected, high elevation glaciers in the The first core (C1) was 159.9 m long, and was Himalayas. drilled at 7000 m above sea level (a.s.l.) down The Himalayas contain the largest glacier the flow line from the top of the col, and two mass outside the polar region. On the high cores (C2 and C3), 149.2 and 167.7 m long, re- mountains in the Tibetan Plateau, snow accu- spectively, were drilled to bedrock 100 m apart mulates year by year and glaciers form. So gla- on the col at 7200 m a.s.l. Visible stratigraphy ciers continuously record the chemical and showed no hiatus features in any of the cores. physical nature of the Earth’s atmosphere. Ice The sites are influenced seasonally by sum- cores drilled from carefully selected sites often mer monsoon and westerly winds, suggesting provide the records of the past climate change most of the annual precipitation on Dasuopu with seasonal, annual, decadal and centennial falls during the summer, and the greatest resolutions. Over the last two decades high- aerosol entrainment occurs from mid-February quality ice core records have been obtained through late May (Thompson et al. 2000). from the Tibetan Plateau and records of accu- The C2 was brought (in a frozen state) to mulation and d18O in the cores have been dem- the Lanzhou Institute of Glaciology and Geo- onstrated as good indexes for precipitation and cryology. C3 was brought (also frozen) to the temperature on the Plateau (Yao et al. 1991, Byrd Polar Research Center in America, and 1992, 1997, 1999; Thompson et al. 1997). All C1 was split between the two institutes. All these ice core records have shown local, re- cores were analyzed over their entire lengths gional and large-scale climate variations. Data for oxygen isotopic ratio (d18O), chemical com- from these cores have indicated a long perspec- position, and dust concentration. The results tive of the Tibetan Plateau’s climate. These presented here are from C2, which was cut into April 2003 K. DUAN and T. YAO 253 5 1.40 out the depth of 114 m of the core extended back to 1700 A.D. for obvious annual variation. -5 1990 1989 1988 1987 1986 1985 1984 1.05 ) ) Below this horizon, annual layers are thinned o -1 -15 0.70 and stretched as new snow accumulates con- O(% 18 (ng. g tinually and the ice flows outward from the 2+ -25 0.35 Ca center, which made annual resolution of the records impossible. The annual layer count- -35 0.00 ing was verified at 35.5 m and 42.2 m by the 46 8101214 Depth (m) location of a 1963 and 1953 beta radioactivity horizon produced by the 1962 and 1952 atmo- 18 Fig. 2. Variations of d O (black line) and spheric thermonuclear tests (Picciotto and Wil- 2þ Ca (gray line) with depth in the Da- gain 1963). Due to high accumulation, low tem- suopu ice core and ice core dating perature and strong seasonality, these time estimates carry little uncertainty. Although the 4110 samples for d18O and major ions (ClÀ, thickness of an annual layer can be easily mea- 2À À þ þ 2þ 2þ SO4 ,NO3 ,Na,K,Mg ,Ca ) analyses. sured throughout the Dasuopu ice core by Borehole temperatures were À14Cat10m counting the gaps between two peaks of d18O, depth and À13C at the ice-bedrock contact, the layer thickness do not directly represent demonstrating that the Dasuopu glacier is fro- the snowfall thickness originally deposited at zen to its bed. the glacier surface. Because these layers are At the col of Dasuopu, the annual snowfall thinned and stretched as new snow accumu- between 1996 and 1997 is as high as 2500 mm lates and the ice flows outward from the center, aÀ1 in snow thickness (1000 mm w.e. aÀ1), the layer thickness must be rectified in order to as determined by snow pit and shallow core recover original snowfall deposited on the gla- studies, and by the measurement of a 12-stake cier surface. The vertical velocity of the glacier accumulation network established during a re- is proportional to the accumulation rate and connaissance survey in July 1996. Meteorology the vertical velocity can be easily obtained from observation had also been making at the core the change in layer thickness (Bolzan 1985; site (7100 m a.s.l.) while the cores had been Reeh 1988). Then the depth-age relation for the drilling during August 15 to October 3 in 1997. upper 114 m establishes an annually precise The maximum temperature was below 0C and age model (Thompson and others 1982). Fol- no surface snow melting was detected during lowing the model, annual accumulation (water the observation period at the site. Therefore the equivalent) of the core can be reconstructed annual net accumulation record in the Dasuopu (Fig.
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