- Email: [email protected]
interdecadal scales during the past 500 years
Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541 – 549 www.elsevier.com/locate/palaeo
Precipitation at Lake Qinghai, NE Qinghai–Tibet Plateau, and its relation to Asian summer monsoons on decadal/interdecadal scales during the past 500 years H. Xu ⁎, Z.H. Hou, L. Ai, L.C. Tan Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, Shaanxi Province, 710075, China Received 31 October 2006; received in revised form 7 July 2007; accepted 12 July 2007
Abstract Knowledge of the variability of precipitation at Lake Qinghai and its relation to the Asian summer monsoons is helpful in constraining global climatic dynamics. Based on the high-resolution precipitation indicators of δ13C of the organic matter (δ13Corg), C/N atomic ratio, and the detrended total organic carbon content (TOCdetrended), we found that the trend of precipitation at Lake Qinghai is inversely correlated to that of the Indian Summer Monsoon (ISM) inferred from layer thickness of a stalagmite (S3) in southern Oman on decadal/interdecadal scales. The Chinese Drought/Flood (D/F) indices, which can indicate the dryness/wetness over large geographic areas, are also used to indicate the intensity of the monsoon rainfall. The D/F index of Xining near Lake Qinghai is synchronous with those of the regions in northern China where the East Asian Summer Monsoon (EASM) dominates; while it is anti-phase with those of southwestern China where ISM prevails. These materials suggest that, during the past 500 years, the source of moisture to Lake Qinghai on decadal/interdecadal scales is controlled mainly by the EASM, but not by the ISM. It is also suggested that the intensity of EASM is inversely related to that of the ISM on decadal/interdecadal scales. The decadal/ interdecadal variability of ENSO may be responsible for the inverse relationship between the intensity of EASM and that of ISM. © 2007 Elsevier B.V. All rights reserved. Keywords: Lake Qinghai; Precipitation; EASM; ISM; ENSO
1. Introduction Lake Qinghai, the largest lake in China, is located in the arid/semi-arid area on the northeast Qinghai–Tibet Plateau. It is one of the most sensitive regions for studying the global climate, with four planetary-scale atmospheric circulations present there. These include the East Asian summer monsoon (EASM), Indian summer monsoon (ISM), East Asian winter monsoon, and the westerly jet stream. Knowledge of the variability in local ⁎ Corresponding author. E-mail address: [email protected] (H. Xu). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.07.007
precipitation is also crucial to the local economy, to landuse management, and to the evaluation and prediction of the sustainability of ecology and the environment. Great efforts have been devoted to exploring the dynamics of precipitation at Lake Qinghai (e.g. Lister et al., 1991; Yu and Kelts, 2002; Shen et al., 2005; Ji et al., 2005). However, because of the variable atmospheric circulations, the complex topography, etc., the nature of precipitation at Lake Qinghai is far from fully understood. The relatively short instrumental climate records and the scarcity of reliable high-resolution proxy records further limit our understanding. For example, it remains unclear what is the major source
542
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
of moisture to Lake Qinghai, and how it is related to the EASM and the ISM? On the other hand, in recent years, increasing evidence suggests that the intensity of the EASM is inversely correlated to that of the ISM (refer to Hong et al. (2006) and references therein). This is especially important for understanding the activities and the linkages of some key components in the monsoon system, such as the west Pacific subtropical high, the Intertropical Convergence Zone (ITCZ), the Mascarene high, and El Niño-Southern Oscillation (ENSO), etc. However, much more evidence is necessary to support such an important relationship, especially on different timescales. Moreover, if it is true that the EASM is inversely correlated to ISM, what mechanism is responsible and how does it modulate the variability of precipitation at Lake Qinghai? In this study, we examined the precipitation inferred from proxy indices of the recent sediments from Lake Qinghai. We discussed the mechanism of precipitation at Lake Qinghai and suggested an inverse relationship between the EASM and the ISM on decadal/interdecadal scales for the past 500 years. 2. Background and materials Mean annual precipitation at Lake Qinghai is about 300–400 mm (Fig. 1A). Mean annual temperature at Lake Qinghai is about − 0.7 °C (Fig. 1B). More than
80% of the total annual precipitation occurs from May to September (Fig. 1C). Two cores (QH0407-C-1, QH0407-C-2) collected at the same site in the north basin of Lake Qinghai were studied (Xu et al., 2006a,b). Core QH0407-C-2 was dated by 210Pb and 137Cs dating methods (Xu et al., 2006a). The sedimentation rate, derived from 210Pb radioactivity, correlates quite well with that inferred from 137Cs radioactivity (Xu et al., 2006a). We calculated the dates of core QH0407-C-2 in terms of mass accumulation rate and verified the dates by comparison to the AMS 14C dating model of a surface core in the lake (refer to Xu et al., 2006a for details). Mass accumulation rate is about 0.018 g cm− 2 a− 1. The average depth sedimentation rate is about 0.1 cm/a for the upper 5 cm and about 0.05 cm/a for the lower parts respectively. Such an obvious difference in depth sedimentation rate has been ascribed to the compaction of the surface sediments during early diagenesis. As a result, the 210Pb dates and/or 137Cs dates of some surface cores in Lake Qinghai examined by some other workers (e.g. Henderson et al., 2003; Zhang et al., 2004), which were calculated in terms of depth sedimentation rate (e.g. cm/a), may have suffered from the compaction of the surface sediments and therefore are considerably different from our dates from core QH0407-C-2. Dates of the sediments of core QH0407-C-1 were based on the dating model of core QH0407-C-2. The resolution of the samples of core QH0407-C-1 is about
Fig. 1. Climates at Lake Qinghai and Xining (see locations in Fig. 3). A. Mean annual precipitation; B. Mean annual temperature; C. Monthly precipitation; D. Mean annual precipitation at Lake Qinghai and Drought/Flood (D/F) index of Xining during 1960 to 2000 (Zhang et al., 2003) (r = − 0.45, α b 0.01). Meteorological records for Lake Qinghai are collected from the nearest Gangcha station, which is about 10 km northwards to Lake Qinghai. Xining is about 150 km eastwards to Lake Qinghai.
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
543
Fig. 2. Precipitation at Lake Qinghai inferred from TOCdetrended, C/N, and δ13Corg (normalized) (refer to Xu et al., 2006b for details), precipitation at Delingha reconstructed from tree ring widths (thick green curve) (normalized; 31-year running mean; Shao et al., 2005), and layer thickness of a stalagmite (S3) in Southern Oman which indicates the intensity of ISM (thick blue curve) (31-year running mean; Burns et al., 2002). Delingha is about 260 km westwards to Lake Qinghai. Note that the δ13Corg was reversely plotted because it is negatively correlated with the precipitation.
3.7-year for the upper 5 cm and about 7.4-year for the lower part (see Xu et al., 2006b for details). Multi-proxy indices of core QH0407-C-1 were developed and their climatic significances were evaluated (Xu et al., 2006b). Precipitation is the growth-limiting factor of the terrestrial plants around Lake Qinghai because of the relatively dry climatic conditions. Higher precipitation
leads to higher biomass of the terrestrial plants and higher influx of organic matter into the lake, and consequently to higher TOC in the lake sediments (Xu et al., 2006b). Higher precipitation will also lead to an increase of the C/N atomic ratio because terrestrial plants generally have higher C/N ratios than the algae/ plankton in the lake (refer to Xu et al., 2006b for
Fig. 3. Correlation coefficients between D/F index of Xining and those of other sites in China (data from CAMS (1981)). Positive coefficients (triangles) are centralized at northern China (grey shaded area) where the EASM prevails; while negative coefficients (circles) are centralized at southwestern China where ISM prevails (blackish shaded area). Also shown are the locations of Lake Qinghai and Xining, and the general directions of EASM, ISM, East Asian winter monsoon, and the Westerly Jet Stream.
544
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
details). Carbon isotopic fractionation of terrestrial C3 plants during the physiological processes has been well studied. Numerous studies have shown that the relative humidity, as well as the amount of precipitation, is generally negatively related to the δ13C of terrestrial plants. Therefore, higher amount of precipitation will lead to a decrease of δ13C of the organic matter in the lake sediment because of both a physiological decrease of δ13Cinflowing and an increase of the influx of terrestrial organic matter into the lake. As a result, variations in δ13C of the organic matter (δ13Corg), C/N atomic ratios, as well as the total organic carbon content (after removal of the geographic trend), can be mainly ascribed to local precipitation. These inferences have been tested by comparison with instrumental climatic records and by comparison with the reconstructed climates based on tree rings at nearby sites (Xu et al., 2006b). In this study, we use these proxy indices to study the dynamics of precipitation at Lake Qinghai. Several data sets from the literature were used to perform comparisons in a broad context. (1) We collected data of precipitation reconstructed from tree ring widths at Delingha which is about 260 km west of Lake Qinghai (Shao et al., 2005). (2) The layer thickness, δ13C, and δ18O of a stalagmite (S3) in southern Oman show similar trends and have been used to assess precipitation (Burns et al., 2002). Since the time series of the layer thickness is annually scaled,
while those of the δ13C and the δ18O are unevenly scaled, we selected the layer thickness in this study and used it as an indicator of the intensity of ISM. (3) The sea surface temperature anomaly of the Niño3 region (Mann et al., 2000) was used as an indicator of the intensity of ENSO. (4) The instrumental climatic records are less than 50 years for most stations in China, which are too short to study decadal/interdecadal climatic variations, especially in areas with complex topography and variable atmospheric circulations. Instead, we use a Drought/Flood (D/F) index constructed successfully by Chinese scientists to reflect the dry/wet climatic conditions from abundant historical records (CAMS, 1981). The descriptions of climatic changes in the literature were categorized into five grades varying from 1 to 5 (CAMS, 1981). The lowest index indicates the wettest conditions while the highest index indicates the driest conditions. This index reflects changes of precipitation both on annual scales and over large geographic areas (CAMS, 1981). As shown in Fig. 1A, B, temperature and precipitation levels at Lake Qinghai are similar in trends with the corresponding parameter in Xining, suggesting similar climatic patterns between these two sites. As a result, precipitation at Lake Qinghai can be generally reflected by the D/F index of Xining (Fig. 1D). We use the D/F index of Xining during the past 500 years as a surrogate of precipitation at Lake Qinghai in this study (see below).
Fig. 4. Relation between D/F index of Xining and that of northern China (A) (r = 0.39, α b 0.01). Relation between D/F index of Xining and that of southwestern China (B) (r = − 0.42, α b 0.01). All the D/F indices are after 10-year running means.
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
3. Results and discussion 3.1. Inverse relation between precipitation around Lake Qinghai/Delingha and that at southern Oman As shown in Fig. 2, precipitation at Lake Qinghai inferred from δ 13 Corg, the C/N atomic ratio, and TOCdetrended correlates quite well with that at Delingha reconstructed from tree ring widths (Shao et al., 2005). The long-term trends of precipitation both at Lake Qinghai and at Delingha are inversely correlated to that recorded in the layer thickness of a stalagmite (S3) in southern Oman (Burns et al., 2002; Fig. 2). The intervals around 1500, 1650–1715, 1800–1830 are relatively dry both in indices of Lake Qinghai and in tree ring widths of Delingha. However, the ISM is relatively strong as inferred from the layer thickness of S3. On the other hand, during the period 1500–1650, the ISM decreased slightly; while precipitation at Lake Qinghai and Delingha increased. A remarkable decline of ISM occurred during the period of 1760–1800, with a most notable rainfall failure between 1790–1800, which led to about 600,000 deaths in India and low discharge of the Nile River (Grove, 1998; Fleitmann et al., 2004). However, during this period of time, precipitation at Lake Qinghai and Delingha was higher. As shown in Fig. 2, another striking decrease of ISM occurred between 1870–1890. The strong “Indian Famine” occurred around 1877 due to a catastrophic failure of ISM (Pant and Kumar, 1997). However, precipitation at Lake Qinghai and Delingha were relatively high during this interval. Over the last few decades, both the multiproxy indices in S3, and the meteorological records in India and Sahel zone in Africa show that monsoon rainfall decreased (Burns et al., 2002; Fleitmann et al., 2004). Again, the precipitation at Lake Qinghai and
545
Delingha increased slightly (Fig. 2). This inverse relationship possibly suggests that the ISM is not the main source of moisture to Lake Qinghai during the past 500 years. 3.2. Inverse relation between precipitation in the EASM-area and that in the ISM-area inferred from the Chinese Drought/Flood index We investigated the correlation coefficients between the D/F index of Xining and those of other sites for the past 500 years (Fig. 3). The results show that the D/F index (10-year running mean) in Xining is positively correlated with those in northern China along the Yellow River, and in northeast China. On the contrary, it is negatively correlated with those in southern China, especially southwestern China. The higher and more significant coefficients are generally centered on two regions: one in northern China and another in southwestern China (see the shaded areas in Fig. 3). D/F indices of the sites in these two regions were averaged to provide a general measure of D/F variations in northern China and southwestern China, respectively. As shown in Fig. 4A, the D/F index of Xining is positively correlated with the averaged D/F index of northern China; correlation coefficient is 0.39 (a b 0.01) after application of a 10-year running mean. Conversely it is negatively correlated with the averaged D/F index of southwestern China; the correlation coefficient is − 0.42 (a b 0.01) after application of a 10-year running mean (Fig. 4B). The positive relationship between the D/F index of Xining and that of northern China may suggest a common driving force of precipitation in the past 500 years. According to the monsoon divisions suggested by Gao et al. (1962) and by Wang et al. (2003), the grey shaded area in northern China should be
Fig. 5. Relationship between D/F index of Xining and the sea surface temperature anomaly in Niño3 region (SSTANiño3; Mann et al., 2000). Data after 10-year running means. Correlation coefficient is − 0.357 (α b 0.01).
546
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
mainly controlled by the EASM. As a result, the precipitation of Xining is most likely controlled by the EASM on the decadal scale over the past 500 years. Conversely, precipitation in southwestern China is generally controlled by the ISM (Gao et al., 1962; Hong et al., 2003; Wang et al., 2005). For instance, the trend of precipitation inferred from δ13C of peat cellulose at Hongyuan, southwestern China (Hong et al., 2003) correlates well with that recorded in sediments of the Arabian Sea (Gupta et al., 2003). Precipitation recorded in a stalagmite of Dongge cave in southwestern China (Wang et al., 2005) is also similar in trends with that registered in a stalagmite (Q5) in southern Oman (Fleitmann et al., 2003). Therefore, the inverse relationship between the D/F index of Xining and that of southwestern China possibly suggests that precipitation at Xining is inversely related to the intensity of the ISM, similar to the inverse relationship between precipitation at Lake Qinghai and that recorded in S3 in southern Oman (Fig. 2). This suggests again that the main factor controlling precipitation at Lake Qinghai is not the ISM, but maybe the EASM. 3.3. Inverse relationship between the EASM and the ISM The inverse relationship between precipitation around Lake Qinghai and that recorded in S3 in southern Oman on decadal/interdecadal scales (Fig. 2), together with the correlations between the D/F index of Xining, that of northern China, and that of southwestern China (Figs. 3 and 4), suggests that the intensity of EASM is inversely related to that of ISM on decadal/interdecadal scales. Studies based on instrumental records also suggest an inverse relationship between intensity of the EASM and that of the ISM. For example, Sun and Ying (1999), using the 500 hPa geopotential heights and the northern Hemisphere sea surface temperature (SST) between 1951 and 1994, as well as outgoing longwave radiation (OLR) data for 1974–1993, found that when the west Pacific subtropical high (WPSH for short) is strong, the ITCZ and the convective activity in the west Pacific is strengthened. Chao and Chen (2001) showed that the summer monsoons share almost all features with the ITCZ, and suggested further that “the summer monsoon is the off-equator ITCZ and its associated circulations” (Chao and Chen, 2001). Although this viewpoint may be debatable, it supports another very important inference that, on relatively longer timescales, such as interannual to interdecadal scales, an intensified ITCZ would correlate to an intensified EASM, while a weakened ITCZ would correlated to a decreased EASM. As a
result, a strong WPSH would be synchronous with an intensified ITCZ and an intensified EASM; and vice versa. Conversely, when the WPSH is strong, the intensity of ISM is weak inferred from the OLR data from the Indian Ocean to the Indian subcontinent (Sun and Ying, 1999), suggesting an inverse relationship between the EASM and the ISM. Based on the reanalysis data of monthly rainfall from NCEP-NCAR, monthly rainfall data from the China Meteorological Administration, and monthly OLR data from the National Ocean and Atmospheric Administration, Zhang (2001) showed that stronger moisture transport by the ISM correlates with weaker WPSH, which leads to less moisture transport to East Asia, and thus less rainfall in the middle and lower reaches of the Yangtze River (Zhang, 2001), suggesting again that the intensity of ISM is inversely related to that of the EASM. More and more proxy indices seem to support an inverse relationship between the intensity of EASM and that of ISM on centennial to millennial scales during the Holocene (Hong et al., 2005; Hong et al., 2006; Maher and Hu, 2006). For instance, Hong et al. (2005) found that monsoon precipitation at Hongyuan, southwestern China is clearly inversely correlated to that at Hani, northeastern China, providing very important evidence for the inverse relationship between the ISM and the EASM on centennial to millennial scales during the Holocene. By comparison between the intensity of southeast summer monsoon at Duowa and that of ISM at the west African coast, Maher and Hu (2006) showed that at 6 ka BP, the southeast summer monsoon was weakened, while the northwest African/southwest Asian monsoons were strengthened; After that, from ∼ 5 ka BP, the EASM was intensified, while northwest Africa/ southwest Asia became dry. The ISM was strong between 7–5 ka BP, which can be illustrated by the precipitation recorded in a stalagmite (Q5) in southern Oman (Fleitmann et al., 2003), in the sediments of the Arabian Sea (Gupta et al., 2003), and in peat sediments at Hongyuan (Hong et al., 2003), etc. However, it was obviously dry on the Alashan Plateau, northwestern China, during this period of time (Chen et al., 2003). Chen et al. (2003) pointed out that the EASM may have decreased between 7000 and 5000 years BP; dry conditions spread throughout a quite large region of the southern part of the Inner Mongolian Plateau. 3.4. Possible mechanism for the inverse relationship between EASM and ISM However, little previous work has been undertaken on the inverse relation between the EASM and the ISM
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
on decadal/interdecadal scales as in this study. Much more evidence is necessary to demonstrate this relationship. If true, the next concern is: what is the reason behind the inverse relationship and how does it modulate precipitation at Lake Qinghai? The EASM and the ISM are two interlinked but independent monsoon subsystems (Tao and Chen, 1987). Intensity of EASM is influenced by the west Pacific subtropical high, ITCZ, Indian summer monsoon, the Tibet anticyclone, Eurasian snow cover, etc; while intensity of the Indian summer monsoon is affected by the Mascarene high, the Tibet low, Eurasian snow cover, etc. Solar activities (Agnihotri et al., 2002), thermohaline circulation (Hong et al., 2005), as well as the ENSO (Krishnamurthy and Goswami, 2000; Cane, 2005), may also play important roles in modulating the variability of Asian summer monsoons. Hong et al. (2005) suggested “on the centennial to millennial timescales the inverse phase relationships between EAM and ISM could reflect the more frequent and perhaps more severe ENSO activities that occurred during the period”. They further pointed out that “Each of the El Niño-like patterns seems to be accompanied by the clear change of atmospheric circulation over the tropical Pacific, led to abnormally decreasing or increasing of precipitation in the ISM and EAM regions, respectively”. As pointed out by Wang et al. (2003), ENSO has been recognized as a major cause of the variability of Asian Summer monsoons. Therefore, despite the variable elements behind the EASM and the ISM, we simply focus on the role of ENSO in this study and suggest that ENSO may be one of the key elements sustaining the inverse relationship between the EASM and the ISM. Numerous works have demonstrated that ENSO can influence heat transport through ocean currents and atmospheric circulations (Enfield, 1989; Graham, 1995; Cane, 2005, and references therein). It has been suggested that ENSO is closely linked to the west Pacific subtropical high (Huang and Chen, 2002); higher SSTNiño3 correlates to a higher index of the west Pacific subtropical high, and vice versa (Wang and Zhou, 2004). Since higher pressure in the west Pacific would correspond to a stronger EASM on long-term scales (as mentioned above), the intensity of ENSO would be expected to be synchronous with the intensity of EASM. As a result, precipitation at Lake Qinghai should also be synchronous with the intensity of ENSO since it may be mainly controlled by the EASM, as suggested above. The negative relationship between SSTNiño3 and the D/F index of Xining strongly supports such an inference (Fig. 5).
547
On the other hand, numerous studies have revealed that the intensity of ENSO is negatively correlated with that of the ISM (Webster et al., 1998; Kumar et al., 1999; Cane, 2005), with the exception after 1980 when this inverse relation was broken, as pointed out by Kumar et al. (1999). Although the nature of this inverse relationship is still not fully understood, it is almost always the case both on annual/interannual timescales and on decadal/interdecadal timescales (e.g. Kumar et al., 1999; Krishnamurthy and Goswami, 2000; etc.). Taken together, a strong ENSO synchronizes with an intensified EASM but a weaker ISM; while a weak ENSO synchronizes with a weaker EASM but an intensified ISM on decadal/interdecadal scales, resulting in an inverse relation between the EASM and the ISM. The decadal/interdecadal variability of ENSO has been revealed by numerous studies (e.g. Torrence and Webster, 1999; Krishnamurthy and Goswami, 2000; Cane, 2005). We therefore suggest that the variability of ENSO on decadal/interdecadal scales may be one of the main factors that sustain the inverse relationship between the intensity of the EASM and that of the ISM on corresponding scales. This is possibly why precipitation at Lake Qinghai is inversely correlated with that in southern Oman on decadal/interdecadal scales, and why the D/F indices of the EASMdominated regions are inversely correlated with those of the ISM-dominated regions in China. 4. Summary The inverse relationship between the trend of precipitation at Lake Qinghai and that in southern Oman, together with the correlations between the D/F index of Xining, southwestern China, and northern China, suggests that the source of moisture to Lake Qinghai during the past 500 years is mainly determined by the EASM on decadal/interdecadal scales. It is also suggested that the intensity of the EASM is inversely correlated to that of the ISM on decadal/interdecadal scales. Increasing evidence supports such an inverse relationship between the EASM and the ISM during the Holocene on centennial to millennial scales. ENSO may play an important role in sustaining the inverse relationship between the EASM and the ISM on decadal/interdecadal scales. During the warm phase of ENSO, the increased SSTNiño3 correlates to a strengthened west Pacific subtropical high, which subsequently leads to an intensified EASM and an increase in monsoon rainfall in areas where EASM dominates, such as Lake Qinghai. On the other hand, higher SSTNiño3 correlates to a decreased ISM, and therefore a
548
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
decrease in monsoon rainfall in the areas where ISM dominates, such as areas in southern Oman and areas in southwestern China. Acknowledgements We thank Prof. D. Fleitmann for providing the stalagmite data and Prof. X.M. Shao for providing tree ring data. This work is supported by the National Basic Research Program of China (No.: 2004CB720207), the projects (No.: 40599422 and No.: 40673071) funded by the Natural Science Foundation of China. References Agnihotri, R., Dutta, K., Bhushan, R., Somayajulu, B.L.K., 2002. Evidence for solar forcing on the Indian monsoon during the last millennium. Earth and Planetary Science Letters 198, 521–527. Burns, S.J., Fleitmann, D., Mudelsee, M., Neff, U., Matter, A., Mangini, A., 2002. A 780-year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman. Journal of Geophysical Research 107, 4434. CAMS (Chinese Academy of Meteorological Sciences), 1981. Yearly Charts of Dryness/Wetness in China for the Last 500-year Period. SinoMaps Press, Beijing, pp. 321–332 (in Chinese). Cane, M.A., 2005. The evolution of El Niño, past and future. Earth and Planetary Science Letters 230, 227–240. Chao, W.C., Chen, B., 2001. The origin of monsoons. Journal of the Atmospheric Sciences 58, 3497–3507. Chen, F.H., Wu, W., Holmes, J.A., Madsen, D.B., Zhu, Y., Jin, M., Oviatt, C.G., 2003. A mid-Holocene drought interval as evidenced by lake desiccation in the Alashan Plateau, Inner Mongolia, China. Chinese Science Bulletin 48, 1401–1410. Enfield, D.B., 1989. El Niño, past and present. Reviews of Geophysics 27, 159–187. Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A., Matter, A., 2003. Holocene forcing of the Indian Monsoon recorded in a stalagmite from southern Oman. Science 300, 1737–1739. Fleitmann, D., Burns, S.J., Neff, U., Mudelsee, M., Mangini, A., Matter, A., 2004. Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from southern Oman. Quaternary Science Reviews 23, 935–945. Gao, Y.X., Xu, S.Y., Guo, Q.Y., et al., 1962. Monsoon region and regional climate in China. In: Gao, Y.X., Xu, S.Y. (Eds.), Some Problems of East Asian Monsoon. Science Press, Beijing, pp. 49–63 (in Chinese). Graham, N.E., 1995. Simulation of recent global temperature trends. Science 267, 666–671. Grove, R.H., 1998. Global impact of the 1789–93 El Niño. Nature 393, 318–319. Gupta, A.K, Anderson, D.M., Overpeck, J.T., 2003. Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean. Nature 421, 354–357. Henderson, A.C.G., Holmes, J.A., Zhang, J.W., Leng, M.J., Carvalho, L.R., 2003. A carbon- and oxygen-isotope record of recent environmental change from Lake Qinghai, NE Tibetan Plateau. Chinese Science Bulletin 48, 1463–1468.
Hong, Y.T., Hong, B., Lin, Q.H., Zhu, Y.X., Shibata, Y., Hirota, M., Uchida, M., Leng, X.T., Jiang, H.B., Xu, H., Wang, H., Yi, L., 2003. Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene. Earth and Planetary Science Letters 211, 371–380. Hong, Y.T., Hong, B., Lin, Q.H., Shibata, Y., Hirota, M., Zhu, Y.X., Leng, X.T., Wang, Y., Wang, H., Yi, L., 2005. Inverse phase oscillations between the East Asian and Indian Ocean summer monsoons during the last 12000 years and paleo-El Niño. Earth and Planetary Science Letters 231, 337–346. Hong, B., Lin, Q.H., Hong, Y.T., 2006. Interconnections between the Asian monsoon, ENSO, and high northern latitude climate during the Holocene. Chinese Science Bulletin 51 (18), 2169–2177. Huang, R.H., Chen, W., 2002. Recent progresses in the research on the interaction between Asian monsoon and ENSO cycle. Climatic and Environmental Research 7, 146–159 (in Chinese with English abstract). Ji, J.F., Shen, J., Balsam, W., Chen, J., Liu, L.W., Liu, X.Q, 2005. Asian monsoon oscillations in the northeastern Qinghai–Tibet Plateau since the late glacial as interpreted from visible reflectance of Qinghai Lake sediments. Earth and Planetary Science Letters 233, 61–70. Krishnamurthy, V., Goswami, B.N., 2000. Indian monsoon–ENSO relationship on interdecadal timescale. Journal of Climate 13, 579–595. Kumar, K.K., Rajagopalan, B., Cane, M.A., 1999. On the weakening relationship between the Indian monsoon and ENSO. Science 284, 2156–2159. Lister, G.S., Kelts, K., Chen, K.Z., Yu, J.Q., Niessen, F., 1991. Lake Qinghai, China: closed-basin lake levels and the oxygen isotope record for Ostracoda since the latest Pleistocene. Palaeogeography, Palaeoclimatology, Palaeoecology 84, 141–162. Maher, B.A., Hu, M., 2006. A high-resolution record of Holocene rainfall variations from the western Chinese Loess Plateau: antiphase behaviour of the African/Indian and East Asian summer monsoons. The Holocene 16, 309–319. Mann, M.E., Gille, E., Bradley, R.S., Hughes, M.K., Overpeck, J., Keimig, F.T., Gross, W., 2000. Global temperature patterns in past centuries: an interactive presentation. Earth Interactions 4, 1–29. Pant, G.B., Kumar, R.K., 1997. Climates of South Asia. John Wiley & Sons, Chichester, UK, p. 320. Shao, X.M., Huang, L., Liu, H.B., Liang, E.Y., Fang, X.Q., Wang, L. L., 2005. Reconstruction of precipitation variation from tree rings in recent 1000 years in Delingha, Qinghai. Science in China (D) 48, 939–949. Shen, J., Liu, X.Q., Wang, S.M., Matsumoto, R., 2005. Palaeoclimatic changes in the Qinghai Lake area during the last 18,000 years. Quaternary International 136, 131–140. Sun, S.Q, Ying, M., 1999. Subtropical high anomalies over the western Pacific and its relations to the Asian monsoon and SST anomaly. Advances in Atmospheric Sciences 16, 559–568. Tao, S., Chen, L., 1987. A review of recent research on the East Asian summer monsoon in China. In: Chang, C.P, Krishnamurti, T.N (Eds.), Monsoon Meteorology. Oxford University Press, Oxford, pp. 60–92. Torrence, C., Webster, P.J., 1999. Interdecadal changes in the ENSO– monsoon system. Journal of Climate 12, 2679–2690. Wang, Y., Cheng, H., Edwards, R.L., He, Y., Kong, X., An, Z., Wu, J., Kelly, M.J., Dykoski, C.A., Li, X., 2005. The Holocene Asian monsoon: links to solar changes and North Atlantic climate. Science 308, 854–857. Wang, C.L., Zhou, L., 2004. West Pacific subtropical high's interannual variability and relativity to ENSO. Journal of Tropical Meteorology 20, 137–144 (in Chinese with English abstract).
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549 Wang, B., Clemens, S.C., Liu, P., 2003. Contrasting the Indian and East Asian monsoons: implications on geologic timescales. Marine Geology 201, 5 21. Webster, P.J., Magana, V.O., Palmer, T.N., Shukla, J., Tomas, R.A., Yanai, M., Yasunari, T., 1998. Monsoons: processes, predictability, and the prospects for prediction. Journal of Geophysical Research 103, 14451–14510. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006a. Geochronology of a surface core in the northern basin of Lake Qinghai: evidence from 210Pb and 137Cs radionuclides. Chinese Journal of Geochemistry 25 (4), 301–306. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006b. Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology, doi:10.1016/j. chemgeo.2006.07.005.
549
Yu, J.Q., Kelts, K.R., 2002. Abrupt changes in climatic conditions across the Late-glacial/Holocene transition on the N. E. Tibet– Qinghai Plateau: evidence from Lake Qinghai, China. Journal of Paleolimnology 28, 195–206. Zhang, R.H., 2001. Relations of water vapor transport from Indian monsoon with that over East Asia and the summer rainfall in China. Advances in Atmospheric Sciences 18, 1005–1017. Zhang, D.E., Li, X.Q., Liang, Y.Y., 2003. Complement of the Chinese Drought/Flood index during 1993–2000. Journal of Applied Meteorological Science 14, 379–383 (in Chinese). Zhang, E.L., Shen, J., Wang, S.M., Yin, Y., Zhu, Y.X., Xia, W.L., 2004. Quantitative reconstruction of the paleosalinity at Qinghai Lake in the past 900 years. Chinese Science Bulletin 49, 730–734.
Precipitation at Lake Qinghai, NE Qinghai–Tibet Plateau, and its relation to Asian summer monsoons on decadal/interdecadal scales during the past 500 years H. Xu ⁎, Z.H. Hou, L. Ai, L.C. Tan Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, Shaanxi Province, 710075, China Received 31 October 2006; received in revised form 7 July 2007; accepted 12 July 2007
Abstract Knowledge of the variability of precipitation at Lake Qinghai and its relation to the Asian summer monsoons is helpful in constraining global climatic dynamics. Based on the high-resolution precipitation indicators of δ13C of the organic matter (δ13Corg), C/N atomic ratio, and the detrended total organic carbon content (TOCdetrended), we found that the trend of precipitation at Lake Qinghai is inversely correlated to that of the Indian Summer Monsoon (ISM) inferred from layer thickness of a stalagmite (S3) in southern Oman on decadal/interdecadal scales. The Chinese Drought/Flood (D/F) indices, which can indicate the dryness/wetness over large geographic areas, are also used to indicate the intensity of the monsoon rainfall. The D/F index of Xining near Lake Qinghai is synchronous with those of the regions in northern China where the East Asian Summer Monsoon (EASM) dominates; while it is anti-phase with those of southwestern China where ISM prevails. These materials suggest that, during the past 500 years, the source of moisture to Lake Qinghai on decadal/interdecadal scales is controlled mainly by the EASM, but not by the ISM. It is also suggested that the intensity of EASM is inversely related to that of the ISM on decadal/interdecadal scales. The decadal/ interdecadal variability of ENSO may be responsible for the inverse relationship between the intensity of EASM and that of ISM. © 2007 Elsevier B.V. All rights reserved. Keywords: Lake Qinghai; Precipitation; EASM; ISM; ENSO
1. Introduction Lake Qinghai, the largest lake in China, is located in the arid/semi-arid area on the northeast Qinghai–Tibet Plateau. It is one of the most sensitive regions for studying the global climate, with four planetary-scale atmospheric circulations present there. These include the East Asian summer monsoon (EASM), Indian summer monsoon (ISM), East Asian winter monsoon, and the westerly jet stream. Knowledge of the variability in local ⁎ Corresponding author. E-mail address: [email protected] (H. Xu). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.07.007
precipitation is also crucial to the local economy, to landuse management, and to the evaluation and prediction of the sustainability of ecology and the environment. Great efforts have been devoted to exploring the dynamics of precipitation at Lake Qinghai (e.g. Lister et al., 1991; Yu and Kelts, 2002; Shen et al., 2005; Ji et al., 2005). However, because of the variable atmospheric circulations, the complex topography, etc., the nature of precipitation at Lake Qinghai is far from fully understood. The relatively short instrumental climate records and the scarcity of reliable high-resolution proxy records further limit our understanding. For example, it remains unclear what is the major source
542
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
of moisture to Lake Qinghai, and how it is related to the EASM and the ISM? On the other hand, in recent years, increasing evidence suggests that the intensity of the EASM is inversely correlated to that of the ISM (refer to Hong et al. (2006) and references therein). This is especially important for understanding the activities and the linkages of some key components in the monsoon system, such as the west Pacific subtropical high, the Intertropical Convergence Zone (ITCZ), the Mascarene high, and El Niño-Southern Oscillation (ENSO), etc. However, much more evidence is necessary to support such an important relationship, especially on different timescales. Moreover, if it is true that the EASM is inversely correlated to ISM, what mechanism is responsible and how does it modulate the variability of precipitation at Lake Qinghai? In this study, we examined the precipitation inferred from proxy indices of the recent sediments from Lake Qinghai. We discussed the mechanism of precipitation at Lake Qinghai and suggested an inverse relationship between the EASM and the ISM on decadal/interdecadal scales for the past 500 years. 2. Background and materials Mean annual precipitation at Lake Qinghai is about 300–400 mm (Fig. 1A). Mean annual temperature at Lake Qinghai is about − 0.7 °C (Fig. 1B). More than
80% of the total annual precipitation occurs from May to September (Fig. 1C). Two cores (QH0407-C-1, QH0407-C-2) collected at the same site in the north basin of Lake Qinghai were studied (Xu et al., 2006a,b). Core QH0407-C-2 was dated by 210Pb and 137Cs dating methods (Xu et al., 2006a). The sedimentation rate, derived from 210Pb radioactivity, correlates quite well with that inferred from 137Cs radioactivity (Xu et al., 2006a). We calculated the dates of core QH0407-C-2 in terms of mass accumulation rate and verified the dates by comparison to the AMS 14C dating model of a surface core in the lake (refer to Xu et al., 2006a for details). Mass accumulation rate is about 0.018 g cm− 2 a− 1. The average depth sedimentation rate is about 0.1 cm/a for the upper 5 cm and about 0.05 cm/a for the lower parts respectively. Such an obvious difference in depth sedimentation rate has been ascribed to the compaction of the surface sediments during early diagenesis. As a result, the 210Pb dates and/or 137Cs dates of some surface cores in Lake Qinghai examined by some other workers (e.g. Henderson et al., 2003; Zhang et al., 2004), which were calculated in terms of depth sedimentation rate (e.g. cm/a), may have suffered from the compaction of the surface sediments and therefore are considerably different from our dates from core QH0407-C-2. Dates of the sediments of core QH0407-C-1 were based on the dating model of core QH0407-C-2. The resolution of the samples of core QH0407-C-1 is about
Fig. 1. Climates at Lake Qinghai and Xining (see locations in Fig. 3). A. Mean annual precipitation; B. Mean annual temperature; C. Monthly precipitation; D. Mean annual precipitation at Lake Qinghai and Drought/Flood (D/F) index of Xining during 1960 to 2000 (Zhang et al., 2003) (r = − 0.45, α b 0.01). Meteorological records for Lake Qinghai are collected from the nearest Gangcha station, which is about 10 km northwards to Lake Qinghai. Xining is about 150 km eastwards to Lake Qinghai.
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
543
Fig. 2. Precipitation at Lake Qinghai inferred from TOCdetrended, C/N, and δ13Corg (normalized) (refer to Xu et al., 2006b for details), precipitation at Delingha reconstructed from tree ring widths (thick green curve) (normalized; 31-year running mean; Shao et al., 2005), and layer thickness of a stalagmite (S3) in Southern Oman which indicates the intensity of ISM (thick blue curve) (31-year running mean; Burns et al., 2002). Delingha is about 260 km westwards to Lake Qinghai. Note that the δ13Corg was reversely plotted because it is negatively correlated with the precipitation.
3.7-year for the upper 5 cm and about 7.4-year for the lower part (see Xu et al., 2006b for details). Multi-proxy indices of core QH0407-C-1 were developed and their climatic significances were evaluated (Xu et al., 2006b). Precipitation is the growth-limiting factor of the terrestrial plants around Lake Qinghai because of the relatively dry climatic conditions. Higher precipitation
leads to higher biomass of the terrestrial plants and higher influx of organic matter into the lake, and consequently to higher TOC in the lake sediments (Xu et al., 2006b). Higher precipitation will also lead to an increase of the C/N atomic ratio because terrestrial plants generally have higher C/N ratios than the algae/ plankton in the lake (refer to Xu et al., 2006b for
Fig. 3. Correlation coefficients between D/F index of Xining and those of other sites in China (data from CAMS (1981)). Positive coefficients (triangles) are centralized at northern China (grey shaded area) where the EASM prevails; while negative coefficients (circles) are centralized at southwestern China where ISM prevails (blackish shaded area). Also shown are the locations of Lake Qinghai and Xining, and the general directions of EASM, ISM, East Asian winter monsoon, and the Westerly Jet Stream.
544
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
details). Carbon isotopic fractionation of terrestrial C3 plants during the physiological processes has been well studied. Numerous studies have shown that the relative humidity, as well as the amount of precipitation, is generally negatively related to the δ13C of terrestrial plants. Therefore, higher amount of precipitation will lead to a decrease of δ13C of the organic matter in the lake sediment because of both a physiological decrease of δ13Cinflowing and an increase of the influx of terrestrial organic matter into the lake. As a result, variations in δ13C of the organic matter (δ13Corg), C/N atomic ratios, as well as the total organic carbon content (after removal of the geographic trend), can be mainly ascribed to local precipitation. These inferences have been tested by comparison with instrumental climatic records and by comparison with the reconstructed climates based on tree rings at nearby sites (Xu et al., 2006b). In this study, we use these proxy indices to study the dynamics of precipitation at Lake Qinghai. Several data sets from the literature were used to perform comparisons in a broad context. (1) We collected data of precipitation reconstructed from tree ring widths at Delingha which is about 260 km west of Lake Qinghai (Shao et al., 2005). (2) The layer thickness, δ13C, and δ18O of a stalagmite (S3) in southern Oman show similar trends and have been used to assess precipitation (Burns et al., 2002). Since the time series of the layer thickness is annually scaled,
while those of the δ13C and the δ18O are unevenly scaled, we selected the layer thickness in this study and used it as an indicator of the intensity of ISM. (3) The sea surface temperature anomaly of the Niño3 region (Mann et al., 2000) was used as an indicator of the intensity of ENSO. (4) The instrumental climatic records are less than 50 years for most stations in China, which are too short to study decadal/interdecadal climatic variations, especially in areas with complex topography and variable atmospheric circulations. Instead, we use a Drought/Flood (D/F) index constructed successfully by Chinese scientists to reflect the dry/wet climatic conditions from abundant historical records (CAMS, 1981). The descriptions of climatic changes in the literature were categorized into five grades varying from 1 to 5 (CAMS, 1981). The lowest index indicates the wettest conditions while the highest index indicates the driest conditions. This index reflects changes of precipitation both on annual scales and over large geographic areas (CAMS, 1981). As shown in Fig. 1A, B, temperature and precipitation levels at Lake Qinghai are similar in trends with the corresponding parameter in Xining, suggesting similar climatic patterns between these two sites. As a result, precipitation at Lake Qinghai can be generally reflected by the D/F index of Xining (Fig. 1D). We use the D/F index of Xining during the past 500 years as a surrogate of precipitation at Lake Qinghai in this study (see below).
Fig. 4. Relation between D/F index of Xining and that of northern China (A) (r = 0.39, α b 0.01). Relation between D/F index of Xining and that of southwestern China (B) (r = − 0.42, α b 0.01). All the D/F indices are after 10-year running means.
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
3. Results and discussion 3.1. Inverse relation between precipitation around Lake Qinghai/Delingha and that at southern Oman As shown in Fig. 2, precipitation at Lake Qinghai inferred from δ 13 Corg, the C/N atomic ratio, and TOCdetrended correlates quite well with that at Delingha reconstructed from tree ring widths (Shao et al., 2005). The long-term trends of precipitation both at Lake Qinghai and at Delingha are inversely correlated to that recorded in the layer thickness of a stalagmite (S3) in southern Oman (Burns et al., 2002; Fig. 2). The intervals around 1500, 1650–1715, 1800–1830 are relatively dry both in indices of Lake Qinghai and in tree ring widths of Delingha. However, the ISM is relatively strong as inferred from the layer thickness of S3. On the other hand, during the period 1500–1650, the ISM decreased slightly; while precipitation at Lake Qinghai and Delingha increased. A remarkable decline of ISM occurred during the period of 1760–1800, with a most notable rainfall failure between 1790–1800, which led to about 600,000 deaths in India and low discharge of the Nile River (Grove, 1998; Fleitmann et al., 2004). However, during this period of time, precipitation at Lake Qinghai and Delingha was higher. As shown in Fig. 2, another striking decrease of ISM occurred between 1870–1890. The strong “Indian Famine” occurred around 1877 due to a catastrophic failure of ISM (Pant and Kumar, 1997). However, precipitation at Lake Qinghai and Delingha were relatively high during this interval. Over the last few decades, both the multiproxy indices in S3, and the meteorological records in India and Sahel zone in Africa show that monsoon rainfall decreased (Burns et al., 2002; Fleitmann et al., 2004). Again, the precipitation at Lake Qinghai and
545
Delingha increased slightly (Fig. 2). This inverse relationship possibly suggests that the ISM is not the main source of moisture to Lake Qinghai during the past 500 years. 3.2. Inverse relation between precipitation in the EASM-area and that in the ISM-area inferred from the Chinese Drought/Flood index We investigated the correlation coefficients between the D/F index of Xining and those of other sites for the past 500 years (Fig. 3). The results show that the D/F index (10-year running mean) in Xining is positively correlated with those in northern China along the Yellow River, and in northeast China. On the contrary, it is negatively correlated with those in southern China, especially southwestern China. The higher and more significant coefficients are generally centered on two regions: one in northern China and another in southwestern China (see the shaded areas in Fig. 3). D/F indices of the sites in these two regions were averaged to provide a general measure of D/F variations in northern China and southwestern China, respectively. As shown in Fig. 4A, the D/F index of Xining is positively correlated with the averaged D/F index of northern China; correlation coefficient is 0.39 (a b 0.01) after application of a 10-year running mean. Conversely it is negatively correlated with the averaged D/F index of southwestern China; the correlation coefficient is − 0.42 (a b 0.01) after application of a 10-year running mean (Fig. 4B). The positive relationship between the D/F index of Xining and that of northern China may suggest a common driving force of precipitation in the past 500 years. According to the monsoon divisions suggested by Gao et al. (1962) and by Wang et al. (2003), the grey shaded area in northern China should be
Fig. 5. Relationship between D/F index of Xining and the sea surface temperature anomaly in Niño3 region (SSTANiño3; Mann et al., 2000). Data after 10-year running means. Correlation coefficient is − 0.357 (α b 0.01).
546
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
mainly controlled by the EASM. As a result, the precipitation of Xining is most likely controlled by the EASM on the decadal scale over the past 500 years. Conversely, precipitation in southwestern China is generally controlled by the ISM (Gao et al., 1962; Hong et al., 2003; Wang et al., 2005). For instance, the trend of precipitation inferred from δ13C of peat cellulose at Hongyuan, southwestern China (Hong et al., 2003) correlates well with that recorded in sediments of the Arabian Sea (Gupta et al., 2003). Precipitation recorded in a stalagmite of Dongge cave in southwestern China (Wang et al., 2005) is also similar in trends with that registered in a stalagmite (Q5) in southern Oman (Fleitmann et al., 2003). Therefore, the inverse relationship between the D/F index of Xining and that of southwestern China possibly suggests that precipitation at Xining is inversely related to the intensity of the ISM, similar to the inverse relationship between precipitation at Lake Qinghai and that recorded in S3 in southern Oman (Fig. 2). This suggests again that the main factor controlling precipitation at Lake Qinghai is not the ISM, but maybe the EASM. 3.3. Inverse relationship between the EASM and the ISM The inverse relationship between precipitation around Lake Qinghai and that recorded in S3 in southern Oman on decadal/interdecadal scales (Fig. 2), together with the correlations between the D/F index of Xining, that of northern China, and that of southwestern China (Figs. 3 and 4), suggests that the intensity of EASM is inversely related to that of ISM on decadal/interdecadal scales. Studies based on instrumental records also suggest an inverse relationship between intensity of the EASM and that of the ISM. For example, Sun and Ying (1999), using the 500 hPa geopotential heights and the northern Hemisphere sea surface temperature (SST) between 1951 and 1994, as well as outgoing longwave radiation (OLR) data for 1974–1993, found that when the west Pacific subtropical high (WPSH for short) is strong, the ITCZ and the convective activity in the west Pacific is strengthened. Chao and Chen (2001) showed that the summer monsoons share almost all features with the ITCZ, and suggested further that “the summer monsoon is the off-equator ITCZ and its associated circulations” (Chao and Chen, 2001). Although this viewpoint may be debatable, it supports another very important inference that, on relatively longer timescales, such as interannual to interdecadal scales, an intensified ITCZ would correlate to an intensified EASM, while a weakened ITCZ would correlated to a decreased EASM. As a
result, a strong WPSH would be synchronous with an intensified ITCZ and an intensified EASM; and vice versa. Conversely, when the WPSH is strong, the intensity of ISM is weak inferred from the OLR data from the Indian Ocean to the Indian subcontinent (Sun and Ying, 1999), suggesting an inverse relationship between the EASM and the ISM. Based on the reanalysis data of monthly rainfall from NCEP-NCAR, monthly rainfall data from the China Meteorological Administration, and monthly OLR data from the National Ocean and Atmospheric Administration, Zhang (2001) showed that stronger moisture transport by the ISM correlates with weaker WPSH, which leads to less moisture transport to East Asia, and thus less rainfall in the middle and lower reaches of the Yangtze River (Zhang, 2001), suggesting again that the intensity of ISM is inversely related to that of the EASM. More and more proxy indices seem to support an inverse relationship between the intensity of EASM and that of ISM on centennial to millennial scales during the Holocene (Hong et al., 2005; Hong et al., 2006; Maher and Hu, 2006). For instance, Hong et al. (2005) found that monsoon precipitation at Hongyuan, southwestern China is clearly inversely correlated to that at Hani, northeastern China, providing very important evidence for the inverse relationship between the ISM and the EASM on centennial to millennial scales during the Holocene. By comparison between the intensity of southeast summer monsoon at Duowa and that of ISM at the west African coast, Maher and Hu (2006) showed that at 6 ka BP, the southeast summer monsoon was weakened, while the northwest African/southwest Asian monsoons were strengthened; After that, from ∼ 5 ka BP, the EASM was intensified, while northwest Africa/ southwest Asia became dry. The ISM was strong between 7–5 ka BP, which can be illustrated by the precipitation recorded in a stalagmite (Q5) in southern Oman (Fleitmann et al., 2003), in the sediments of the Arabian Sea (Gupta et al., 2003), and in peat sediments at Hongyuan (Hong et al., 2003), etc. However, it was obviously dry on the Alashan Plateau, northwestern China, during this period of time (Chen et al., 2003). Chen et al. (2003) pointed out that the EASM may have decreased between 7000 and 5000 years BP; dry conditions spread throughout a quite large region of the southern part of the Inner Mongolian Plateau. 3.4. Possible mechanism for the inverse relationship between EASM and ISM However, little previous work has been undertaken on the inverse relation between the EASM and the ISM
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
on decadal/interdecadal scales as in this study. Much more evidence is necessary to demonstrate this relationship. If true, the next concern is: what is the reason behind the inverse relationship and how does it modulate precipitation at Lake Qinghai? The EASM and the ISM are two interlinked but independent monsoon subsystems (Tao and Chen, 1987). Intensity of EASM is influenced by the west Pacific subtropical high, ITCZ, Indian summer monsoon, the Tibet anticyclone, Eurasian snow cover, etc; while intensity of the Indian summer monsoon is affected by the Mascarene high, the Tibet low, Eurasian snow cover, etc. Solar activities (Agnihotri et al., 2002), thermohaline circulation (Hong et al., 2005), as well as the ENSO (Krishnamurthy and Goswami, 2000; Cane, 2005), may also play important roles in modulating the variability of Asian summer monsoons. Hong et al. (2005) suggested “on the centennial to millennial timescales the inverse phase relationships between EAM and ISM could reflect the more frequent and perhaps more severe ENSO activities that occurred during the period”. They further pointed out that “Each of the El Niño-like patterns seems to be accompanied by the clear change of atmospheric circulation over the tropical Pacific, led to abnormally decreasing or increasing of precipitation in the ISM and EAM regions, respectively”. As pointed out by Wang et al. (2003), ENSO has been recognized as a major cause of the variability of Asian Summer monsoons. Therefore, despite the variable elements behind the EASM and the ISM, we simply focus on the role of ENSO in this study and suggest that ENSO may be one of the key elements sustaining the inverse relationship between the EASM and the ISM. Numerous works have demonstrated that ENSO can influence heat transport through ocean currents and atmospheric circulations (Enfield, 1989; Graham, 1995; Cane, 2005, and references therein). It has been suggested that ENSO is closely linked to the west Pacific subtropical high (Huang and Chen, 2002); higher SSTNiño3 correlates to a higher index of the west Pacific subtropical high, and vice versa (Wang and Zhou, 2004). Since higher pressure in the west Pacific would correspond to a stronger EASM on long-term scales (as mentioned above), the intensity of ENSO would be expected to be synchronous with the intensity of EASM. As a result, precipitation at Lake Qinghai should also be synchronous with the intensity of ENSO since it may be mainly controlled by the EASM, as suggested above. The negative relationship between SSTNiño3 and the D/F index of Xining strongly supports such an inference (Fig. 5).
547
On the other hand, numerous studies have revealed that the intensity of ENSO is negatively correlated with that of the ISM (Webster et al., 1998; Kumar et al., 1999; Cane, 2005), with the exception after 1980 when this inverse relation was broken, as pointed out by Kumar et al. (1999). Although the nature of this inverse relationship is still not fully understood, it is almost always the case both on annual/interannual timescales and on decadal/interdecadal timescales (e.g. Kumar et al., 1999; Krishnamurthy and Goswami, 2000; etc.). Taken together, a strong ENSO synchronizes with an intensified EASM but a weaker ISM; while a weak ENSO synchronizes with a weaker EASM but an intensified ISM on decadal/interdecadal scales, resulting in an inverse relation between the EASM and the ISM. The decadal/interdecadal variability of ENSO has been revealed by numerous studies (e.g. Torrence and Webster, 1999; Krishnamurthy and Goswami, 2000; Cane, 2005). We therefore suggest that the variability of ENSO on decadal/interdecadal scales may be one of the main factors that sustain the inverse relationship between the intensity of the EASM and that of the ISM on corresponding scales. This is possibly why precipitation at Lake Qinghai is inversely correlated with that in southern Oman on decadal/interdecadal scales, and why the D/F indices of the EASMdominated regions are inversely correlated with those of the ISM-dominated regions in China. 4. Summary The inverse relationship between the trend of precipitation at Lake Qinghai and that in southern Oman, together with the correlations between the D/F index of Xining, southwestern China, and northern China, suggests that the source of moisture to Lake Qinghai during the past 500 years is mainly determined by the EASM on decadal/interdecadal scales. It is also suggested that the intensity of the EASM is inversely correlated to that of the ISM on decadal/interdecadal scales. Increasing evidence supports such an inverse relationship between the EASM and the ISM during the Holocene on centennial to millennial scales. ENSO may play an important role in sustaining the inverse relationship between the EASM and the ISM on decadal/interdecadal scales. During the warm phase of ENSO, the increased SSTNiño3 correlates to a strengthened west Pacific subtropical high, which subsequently leads to an intensified EASM and an increase in monsoon rainfall in areas where EASM dominates, such as Lake Qinghai. On the other hand, higher SSTNiño3 correlates to a decreased ISM, and therefore a
548
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549
decrease in monsoon rainfall in the areas where ISM dominates, such as areas in southern Oman and areas in southwestern China. Acknowledgements We thank Prof. D. Fleitmann for providing the stalagmite data and Prof. X.M. Shao for providing tree ring data. This work is supported by the National Basic Research Program of China (No.: 2004CB720207), the projects (No.: 40599422 and No.: 40673071) funded by the Natural Science Foundation of China. References Agnihotri, R., Dutta, K., Bhushan, R., Somayajulu, B.L.K., 2002. Evidence for solar forcing on the Indian monsoon during the last millennium. Earth and Planetary Science Letters 198, 521–527. Burns, S.J., Fleitmann, D., Mudelsee, M., Neff, U., Matter, A., Mangini, A., 2002. A 780-year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman. Journal of Geophysical Research 107, 4434. CAMS (Chinese Academy of Meteorological Sciences), 1981. Yearly Charts of Dryness/Wetness in China for the Last 500-year Period. SinoMaps Press, Beijing, pp. 321–332 (in Chinese). Cane, M.A., 2005. The evolution of El Niño, past and future. Earth and Planetary Science Letters 230, 227–240. Chao, W.C., Chen, B., 2001. The origin of monsoons. Journal of the Atmospheric Sciences 58, 3497–3507. Chen, F.H., Wu, W., Holmes, J.A., Madsen, D.B., Zhu, Y., Jin, M., Oviatt, C.G., 2003. A mid-Holocene drought interval as evidenced by lake desiccation in the Alashan Plateau, Inner Mongolia, China. Chinese Science Bulletin 48, 1401–1410. Enfield, D.B., 1989. El Niño, past and present. Reviews of Geophysics 27, 159–187. Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A., Matter, A., 2003. Holocene forcing of the Indian Monsoon recorded in a stalagmite from southern Oman. Science 300, 1737–1739. Fleitmann, D., Burns, S.J., Neff, U., Mudelsee, M., Mangini, A., Matter, A., 2004. Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from southern Oman. Quaternary Science Reviews 23, 935–945. Gao, Y.X., Xu, S.Y., Guo, Q.Y., et al., 1962. Monsoon region and regional climate in China. In: Gao, Y.X., Xu, S.Y. (Eds.), Some Problems of East Asian Monsoon. Science Press, Beijing, pp. 49–63 (in Chinese). Graham, N.E., 1995. Simulation of recent global temperature trends. Science 267, 666–671. Grove, R.H., 1998. Global impact of the 1789–93 El Niño. Nature 393, 318–319. Gupta, A.K, Anderson, D.M., Overpeck, J.T., 2003. Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean. Nature 421, 354–357. Henderson, A.C.G., Holmes, J.A., Zhang, J.W., Leng, M.J., Carvalho, L.R., 2003. A carbon- and oxygen-isotope record of recent environmental change from Lake Qinghai, NE Tibetan Plateau. Chinese Science Bulletin 48, 1463–1468.
Hong, Y.T., Hong, B., Lin, Q.H., Zhu, Y.X., Shibata, Y., Hirota, M., Uchida, M., Leng, X.T., Jiang, H.B., Xu, H., Wang, H., Yi, L., 2003. Correlation between Indian Ocean summer monsoon and North Atlantic climate during the Holocene. Earth and Planetary Science Letters 211, 371–380. Hong, Y.T., Hong, B., Lin, Q.H., Shibata, Y., Hirota, M., Zhu, Y.X., Leng, X.T., Wang, Y., Wang, H., Yi, L., 2005. Inverse phase oscillations between the East Asian and Indian Ocean summer monsoons during the last 12000 years and paleo-El Niño. Earth and Planetary Science Letters 231, 337–346. Hong, B., Lin, Q.H., Hong, Y.T., 2006. Interconnections between the Asian monsoon, ENSO, and high northern latitude climate during the Holocene. Chinese Science Bulletin 51 (18), 2169–2177. Huang, R.H., Chen, W., 2002. Recent progresses in the research on the interaction between Asian monsoon and ENSO cycle. Climatic and Environmental Research 7, 146–159 (in Chinese with English abstract). Ji, J.F., Shen, J., Balsam, W., Chen, J., Liu, L.W., Liu, X.Q, 2005. Asian monsoon oscillations in the northeastern Qinghai–Tibet Plateau since the late glacial as interpreted from visible reflectance of Qinghai Lake sediments. Earth and Planetary Science Letters 233, 61–70. Krishnamurthy, V., Goswami, B.N., 2000. Indian monsoon–ENSO relationship on interdecadal timescale. Journal of Climate 13, 579–595. Kumar, K.K., Rajagopalan, B., Cane, M.A., 1999. On the weakening relationship between the Indian monsoon and ENSO. Science 284, 2156–2159. Lister, G.S., Kelts, K., Chen, K.Z., Yu, J.Q., Niessen, F., 1991. Lake Qinghai, China: closed-basin lake levels and the oxygen isotope record for Ostracoda since the latest Pleistocene. Palaeogeography, Palaeoclimatology, Palaeoecology 84, 141–162. Maher, B.A., Hu, M., 2006. A high-resolution record of Holocene rainfall variations from the western Chinese Loess Plateau: antiphase behaviour of the African/Indian and East Asian summer monsoons. The Holocene 16, 309–319. Mann, M.E., Gille, E., Bradley, R.S., Hughes, M.K., Overpeck, J., Keimig, F.T., Gross, W., 2000. Global temperature patterns in past centuries: an interactive presentation. Earth Interactions 4, 1–29. Pant, G.B., Kumar, R.K., 1997. Climates of South Asia. John Wiley & Sons, Chichester, UK, p. 320. Shao, X.M., Huang, L., Liu, H.B., Liang, E.Y., Fang, X.Q., Wang, L. L., 2005. Reconstruction of precipitation variation from tree rings in recent 1000 years in Delingha, Qinghai. Science in China (D) 48, 939–949. Shen, J., Liu, X.Q., Wang, S.M., Matsumoto, R., 2005. Palaeoclimatic changes in the Qinghai Lake area during the last 18,000 years. Quaternary International 136, 131–140. Sun, S.Q, Ying, M., 1999. Subtropical high anomalies over the western Pacific and its relations to the Asian monsoon and SST anomaly. Advances in Atmospheric Sciences 16, 559–568. Tao, S., Chen, L., 1987. A review of recent research on the East Asian summer monsoon in China. In: Chang, C.P, Krishnamurti, T.N (Eds.), Monsoon Meteorology. Oxford University Press, Oxford, pp. 60–92. Torrence, C., Webster, P.J., 1999. Interdecadal changes in the ENSO– monsoon system. Journal of Climate 12, 2679–2690. Wang, Y., Cheng, H., Edwards, R.L., He, Y., Kong, X., An, Z., Wu, J., Kelly, M.J., Dykoski, C.A., Li, X., 2005. The Holocene Asian monsoon: links to solar changes and North Atlantic climate. Science 308, 854–857. Wang, C.L., Zhou, L., 2004. West Pacific subtropical high's interannual variability and relativity to ENSO. Journal of Tropical Meteorology 20, 137–144 (in Chinese with English abstract).
H. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 254 (2007) 541–549 Wang, B., Clemens, S.C., Liu, P., 2003. Contrasting the Indian and East Asian monsoons: implications on geologic timescales. Marine Geology 201, 5 21. Webster, P.J., Magana, V.O., Palmer, T.N., Shukla, J., Tomas, R.A., Yanai, M., Yasunari, T., 1998. Monsoons: processes, predictability, and the prospects for prediction. Journal of Geophysical Research 103, 14451–14510. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006a. Geochronology of a surface core in the northern basin of Lake Qinghai: evidence from 210Pb and 137Cs radionuclides. Chinese Journal of Geochemistry 25 (4), 301–306. Xu, H., Ai, L., Tan, L.C., An, Z.S., 2006b. Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications. Chemical Geology, doi:10.1016/j. chemgeo.2006.07.005.
549
Yu, J.Q., Kelts, K.R., 2002. Abrupt changes in climatic conditions across the Late-glacial/Holocene transition on the N. E. Tibet– Qinghai Plateau: evidence from Lake Qinghai, China. Journal of Paleolimnology 28, 195–206. Zhang, R.H., 2001. Relations of water vapor transport from Indian monsoon with that over East Asia and the summer rainfall in China. Advances in Atmospheric Sciences 18, 1005–1017. Zhang, D.E., Li, X.Q., Liang, Y.Y., 2003. Complement of the Chinese Drought/Flood index during 1993–2000. Journal of Applied Meteorological Science 14, 379–383 (in Chinese). Zhang, E.L., Shen, J., Wang, S.M., Yin, Y., Zhu, Y.X., Xia, W.L., 2004. Quantitative reconstruction of the paleosalinity at Qinghai Lake in the past 900 years. Chinese Science Bulletin 49, 730–734.