Journal of the Meteorological Society of Japan, Vol. 81, No. 2, pp. 251--257, 2003 251

Monsoon Variability in the under the Condition of Global Warming

Keqin DUAN

Cold and Arid Environmental and Engineering Research Institute, Chinese Academy Science, Lanzhou, China National Laboratory of Western China’s Environmental Systems, Lanzhou University, Lanzhou, China


Tandong YAO

Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy Science, Lanzhou, China

(Manuscript received 16 2002, in revised form 5 2002)


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- 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 are heavily dependent on long series of reliable data for large contiguous the performance of the ( 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 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 surface temperatures (Tourre China. E-mail: [email protected] and White 1995), variations in Eurasian ( 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 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- are associated with positive (negative) ability during the present 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 ’’. 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, , 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 and 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 . 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 of the ’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- 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 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 (Cl, thickness of an annual layer can be easily mea- 2 þ þ 2þ 2þ SO4 ,NO3 ,Na,K,Mg ,Ca ) analyses. sured throughout the Dasuopu ice core by Borehole temperatures were 14Cat10m counting the gaps between two peaks of d18O, depth and 13C 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, a1 in snow thickness (1000 mm w.e. a1), 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 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. 3 and Fig. 4) by the relationship between ice core is expected to provide a well-dated pre- accumulation and velocity (Bolzan 1985; Reeh cipitation history with high resolution in the 1988). Moreover, as mentioned above, the Himalayas. peaks of d18O and ions appear in the dry stage In the monsoon region the peaks of d18O and of (Wushiki 1977; Zhang 2001), so ac- ions in the rainfall appear in the dry stage of cumulation recovered in the core represents spring (Wushiki 1977; Zhang 2001; Muradkami precipitation from spring to spring. Most of the 1987; Luo and Yanai 1983). Thus the high an- annual precipitation on Dasuopu falls during nual accumulation allows preservation of dis- the summer monsoon (June through tinct seasonal cycles in d18O, and ions which August), suggesting accumulation from spring make it possible to reconstruct an annual rec- to spring is almost equal to precipitation in the ord for the upper part of the cores. Then the monsoon season. core is dated by counting annual peaks of d18O 3. Results and discussion with reference of peaks of Ca2þ (Fig. 2). A time- depth relation for the cores was established The 300-year accumulation record indicates using the well-preserved seasonal fluctuations fluctuations in the intensity of the South Asian of d18O and ion. The snow in one layer, (in Fig. Monsoon in the Himalayas that is interpreted 2 one layer is the thickness from one d18O peak as a good correlation between the record and to next d18O peak), falls in one year. It turned precipitation in India (Sontakke et al. 1996), 254 Journal of the Meteorological Society of Japan Vol. 81, No. 2

1600 40

1600 30 1200 2 7.9 26 5.1 9.1 20 1400 95% 800 1 India (mm) Ice Core (mm) 14.2 Rainfall of Northeast 10 Accumulation of Dasuopu Spectral power (Variance Unit)

400 1200 1850 1880 1910 1940 1970 2000 0 3132333 Year (A.D.) Period (Year) Fig. 3. Five-year running means of ac- cumulation (line 1) from the Dasuopu Fig. 4. Power spectral of accumulation glacier are significantly correlated with series of the Dasuopu ice core Northeast India monsoon precipitation (line 2) since 1860 the period 1930–1995 is characterized by drier years over the Himalayas and Northeast India, close to the Himalayas. Empirical orthogonal and by wetter years over central India (Duan et function (EOF) analysis was applied on the al. 2002a). This suggests that the precipitation accumulation record and the summer (June climatology of the Himalayas and its foothills through ) precipitation for all India, behave differently from the southern part of the and for Indian regions (Sontakke et al. 1996) to subcontinent, and the all-India average precip- identify the spatial feature difference between itation record does not provide a valid repre- precipitation in the central Himalayas and in sentation of the entire subcontinent, especially Indian regions (Duan et al. 2002a). The result of the Tibetan Plateau. showed in the EOF1 associations the accumu- Oscillatory characteristics in the accumula- lation of the Dasuopu ice core, and precipita- tion record of the Dasuopu ice core are detected tion in northeast Indian are positively corre- by spectral analyses. Figure 4 shows significant lated, and anti-correlated with that in central peaks at periods close to 5.1, 7.9, 9.1, 14.2 and India and all-India, reflecting the regional dis- 26 years in the accumulation time series during tribution of monsoon rainfall. A linear regres- 1700–1996. These periods are in agreement sion between accumulation in the Dasuopu with the global temperature peaks (Ghil and ice core, and precipitation in northeast India, Vautard 1991). This may suggest the accumu- yields a correlation coefficient 0.3 at the 95% lation variation in the Dasuopu ice core con- significant level during 1860–1995 A.D. The nects to temperature variability. Dasuopu ice core accumulation record repre- To identify common trends and features sents conditions at a single location, and a related to broad-scale climate variability, the dating error of just 1 year may affect the sta- accumulation record of the Dasuopu ice core tistical correlation. Therefore, a 5-year mean is compared with the 300-year northern was applied to both records, giving R ¼ 0:58 hemisphere temperature (Mann et al. 1999) (sig. ¼ 99%) (Fig. 3). (Fig. 5A). The comparison clearly shows that In the EOF2 associations accumulation in a negative correlation exists between the two the Dasuopu ice core is positively correlated records, with a correlation coefficient 0.22 with precipitation in northern India, and nega- at 95% confidence level (n ¼ 295). A 5-year tively correlated with that in southern India. unweighted running mean was applied to Based on these associations, distinct climatic both records, giving coefficient 0.41 at 99% regimes in monsoon precipitation with main confidence level. Thus, contemporary records changes around the year 1930 are identified demonstrate that decreasing accumulation in (Duan et al. 2002b). The period 1850–1930 the Dasuopu ice core connects to warming in is characterized by more wet years over the northern hemisphere and visa versa. For ex- Himalayas and Northeast India and by drier ample, in the last century, accumulation in the years over southern and central India, while Dasuopu ice core has decreased by 425 mm April 2003 K. DUAN and T. YAO 255

0.8 tion between the precipitation and the temper- A 4 ature with variation of temperature lead that 0.3 C) of precipitation around 15 years. These char- o acteristics, visible in Fig. 5, are supported -0.2 by cross-correlation between precipitation and 1.5 temperature, suggesting that precipitation in

Temperature( -0.7 of Dasuopu Ice Core

Northern Hemisphere the Himalayas will continuously decrease in Normalized acumulation future decades due to the Global warming in the -1.2 -1 past decades. 1700 1800 1900 2000 Figure 5B also shows clearly the variability Year (A.D.) trend of the accumulation record in the Da- 0.4 0.4 suopu ice core in the last 300 years, suggest- B ing South Asian Monsoon in the Himalayas th 0.2 0.2 had weakened in the 18 century, then C) o strengthened strongly during 1795–1850, again 0 0 moderatly weakened during 1850–1875 and strengthened during 1875–1920, after that it

Temperature( -0.2 -0.2 has weakened from early 1920 to the present. of Dasuopu Ice Core Northern Hemisphere Normalized acumulation Model studies suggest an increase (5–15%) in -0.4 -0.4 monsoon precipitation with global temperature 1700 1800 1900 2000 increases (Meehl et al. 1993, 1996). This is con- Year (A.D.) sistent with the basic principle of the monsoon Fig. 5. (A) Variations of the Dasuopu ac- increase in the strengthening land-ocean ther- cumulation (black line), and the mal contrast as a result of global warming hemisphere temperature (Mann et al. (Meehl 1994). It is the case that the average 1999) (gray line) over the past 300 Indian monsoon rainfall has increased slightly years. (B) The trends, separated from over the past decades. However, rainfall in two signals, show a negative relation- some parts of India such as the Northwest ship between accumulation in the Da- Peninsular India and Northeast India has de- suopu glacier (black line), and north creased over the past decades (Sontakke 1996). hemisphere temperature (gray line) Monsoon trough is assumed to result in trans- port of moisture to the Himalayas. Generally as from early 20th to late 20th century, while north monsoon trough locates in central Indian, the hemisphere temperature has increased about rainfall is above normal over central India and 0.5C. The corresponding regression coefficient below normal over northern India and foothills is about 800 G 100 mm(C)1, indicating that of the Himalayas. While as monsoon trough on average a 0.1C change in north hemisphere shifts to the Himalayas side, the rainfall is temperature is associated with about 80 mm above normal over northern India and foothills change in accumulation in the Dasuopu ice of the Himalayas and below normal over cen- core. tral India. A recent study concluded that in- To reveal the secular signals, low-passed fil- crease in temperature over the last few decades tered (>60 years) is applied to the two records. have led to changes in atmospheric circulation The extracted secular signals are illustrated that have resulted in a decrease in moisture in Figure 5B. The low-passed components cap- flux to the Tibetan plateau (Qin et al. 2000). ture 31% of the accumulation variance, and We assume increased temperature makes the 78% of the temperature. The accumulation low- monsoon trough easier to stay over central passed time sequences displays fluctuations Indian. This assumption is consistent with the with peaks near 1854 and 1915, with trough monsoon rainfall pattern over the last decades. near 1890, while the temperature low-passed The past decades is characterized by drier time sequences displays fluctuations with years over the Himalayas and Northeast India, peaks near 1875, with trough near 1840 and and by wetter years over central India. The 1900. It seems that there is a negative correla- results presented here are probably hardly 256 Journal of the Meteorological Society of Japan Vol. 81, No. 2 conclusive. There seems to need additional Acknowledgement research in this area for the Himalayas is This work was supported by the National the headwaters of such important rivers as the Natural Science Foundation of China (Grant Indus and the Ganges. According to IPCC re- No. 90102005 and No. 40101006), the In- ports (Houghton 1992), a global temperature novation Fund of Chinese Academy of Sci- will increase of 0.1–0.2C/10a, which will lead ences (Grant No. 210506) and the Ministry of to decreasing rainfall in the Himalayas. If this Science and Technology of China (Grant No. is the case, there will be not enough precipita- G1998040800). tion to match the melting volume of glaciers on the Himalayas. Then the glaciers on the Hima- References layas, including our studying site, will have been continual retreating, decreasing accumu- Barnett, T.P., L. Du¨ menil, U. Schlese and E. Roeck- lation and negative mass balance. The Hima- ner, 1988: The effect of Eurasian snow cover on Global climate. Science, 239, 504–507. layan glaciers continue to dissolve in the fu- Bolzan, J.F., 1985: Ice flow at the dome-c ice divide ture. The rivers originated in the Himalayas based on a deep temperature profile. J. 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