GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L18609, doi: /2008gl034792, 2008

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L18609, doi: /2008gl034792, 2008 Sea surface temperature differences between the western equatorial Pacific and northern South China Sea since the Pliocene and their paleoclimatic implications Guodong Jia, 1,2 Fajin Chen, 2 and Ping an Peng 2 Received 26 May 2008; revised 28 July 2008; accepted 8 August 2008; published 25 September [1] Alkenone sea surface temperatures (SSTs) in the northern South China Sea (SCS) were reconstructed for the last 3.8 Ma. The SST difference between the western equatorial Pacific and northern SCS was then estimated, showing a general increase since 2.8 Ma. Three features of the SST-gradient evolution were prominent: 1) low values (<2 C) in the late Pliocene; 2) increased values during the late Pliocene/early Pleistocene (from 1 to 4 C); and 3) high values (4 C) by the end of the early Pleistocene. These features were also shown in zonal SST gradient across equatorial Pacific, implying an intimate relationship of the meridional Hadley and East Asian monsoon circulations with the zonal Walker circulation. Moreover, the SST gradient records displayed some characters unique to the equatorial Pacific and not found in benthic d 18 O record, suggesting that zonal tropical climate change could spread into higher latitude by interactions with meridional circulations. Citation: Jia, G., F. Chen, and P. Peng (2008), Sea surface temperature differences between the western equatorial Pacific and northern South China Sea since the Pliocene and their paleoclimatic implications, Geophys. Res. Lett., 35, L18609, doi: /2008gl Introduction 1 CAS Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. 2 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. Copyright 2008 by the American Geophysical Union /08/2008GL [2] The general circulation of the troposphere is now well described. In the Pacific, the converging warm tropical air over the western Pacific warm pool (WPWP) is forced to rise, and on reaching the upper troposphere it spreads poleward or eastward until it reaches the subtropics or the eastern Pacific cold tongue, where it descends and flows back to the WPWP in the surface layers. The meridional and zonal circulations are the Hadley and Walker circulations (HC and WC), respectively. It has been found that the circulations could change and reorganize not only on interannual [e.g., Oort and Yienger, 1996], but also on decadal, or centennial time scales [Cane et al., 1997]. However, whether the observed variability on these longer time scales is due to natural processes, or human activities, or some combination of these influences, remains a matter of debate. Reconstruction of variability of the circulations from geological or biological archives is an important means to address the question. Because underlying SST distribution is crucial to drive these thermally direct atmospheric circulations [Rind and Perlwitz, 2004; Otto-Bliesner and Clement, 2005], SST records are pertinent to reconstruct circulation histories. [3] Earth s climate system has developed from a state of relative global warmth during the mid-pliocene (3.2 to 4.5 Ma), with ice sheets restricted to the Antarctic, to a globally cooler state during the late Pliocene, with extensive bipolar ice sheets and increased pole-equator temperature gradients [e.g., Shackleton et al., 1995]. Many efforts have contributed to the understanding of the long term SST evolution since the Pliocene using geochemical proxies, such as the alkenone unsaturation index (U K 37 or U K0 37) and foraminiferal Mg/Ca ratios [Marlow et al., 2000; Liu and Herbert, 2004; Haug et al., 2005; McClymont et al., 2005; Medina-Elizalde and Lea, 2005; McClymont and Rosell-Melé, 2006; Wara et al., 2005; Lawrence et al., 2006]. Some of these works were concentrated on the SST gradient across equatorial Pacific [Medina-Elizalde and Lea, 2005; McClymont and Rosell-Melé, 2006; Wara et al., 2005], which was applied to discuss the WC history. But how the HC-associated SST gradient between the tropics and extra-tropics has evolved since the Pliocene has not been explored. In this study, U K0 37 SST record from the northern South China Sea (SCS) is presented. By focusing on the reconstruction of SST gradient between the WPWP and northern SCS, we aim to discuss the evolution of meridional atmospheric circulations and its paleoclimatic implications. 2. Study Site, Materials, and Methods [4] The SCS is a climatically sensitive area with its southern part located in the WPWP, and its northern part close to the subtropical mainland China (Figure 1). This geographical setting makes the SCS sensitive to both meridional East Asian monsoon climate and zonal ENSO oscillations. There is an obvious northwest-southeast annual SST gradient from northern SCS (26 C) to the center of the WPWP (29 C) (Figure 1). The annual distribution of velocity potential at 925-hPa atmospheric pressure level in the western Pacific is a suitable variable reflecting largescale features of tropical atmospheric circulations and is roughly parallel to the SST distributions (Figure 1). Because the gradient of velocity potential is the divergent wind velocity, the parallel suggests that SST gradient is the main factor for the low level divergent winds from the northern SCS to the WPWP. This suggestion is supported by some modeling studies demonstrating that underlying SST distribution is crucial to drive thermally direct atmospheric circulations [Rind and Perlwitz, 2004; Otto-Bliesner and Clement, 2005]. L of5

2 Figure 1. Annual distributions of sea surface (0 30 m water depth) temperature and velocity potential at 925-hPa atmospheric pressure level (in isograms) in the tropicalsubtropical western Pacific. Sea surface temperatures were derived from and velocity potentials were from iridl.ldeo.columbia.edu/sources/.noaa/.ncep-ncar/.cdas-1/.monthly/.intrinsic/.pressurelevel/.vpot/. The unit for velocity potential is 10 6 m 2 s 1. [5] ODP Sites 1147 ( N, E, 3246 m water depth) and 1148 ( N, E, 3294 m water depth) were collected on the lowermost continental slope of the northern SCS. They were combined to form a meters composite depth (mcd) scale from 0 to 852 mcd [Wang et al., 2000]. In this study, results from the upper part of mcd, where alkenone concentration is high enough for U K0 37 determination, are reported. The age model of the site was constructed by oxygen isotope stratigraphy relying on the chronologic framework established by biostratigraphy and magnetostratigraphy. The reliability of the isotope stratigraphy was further validated by spectral analysis showing a dominant 100-ka periodicity after the Marine Isotope Stage 22/23 boundary (0.9 Ma), whereas a more important 41-ka periodicity before the boundary [Jian et al., 2003]. According to the age model, the upper mcd of the sites covers a history of 3.8 Ma. A total of 91 samples were collected at average intervals of 1.3 m from 0 to 50 mcd and 2.3 m from 50 to mcd, with the average time resolutions of 20 ky and 60 ky, respectively. [6] Alkenone extraction was adapted from Villanueva et al. [1997]. Fractions containing alkenones were analyzed using a HP 6890 gas chromatography, and the concentrations of C 37:2 and C 37:3 alkenones were used to determine the U K0 37 index [= C 37:2 /(C 37:2 +C 37:3 )]. In the SCS, U K0 37 values in surface sediments show a good linear relationship for modern annually averaged 0 30 m SSTs (from 26.7 to 28.2 C), giving an equation U K0 37 = 0.031SST [Pelejero and Grimalt, 1997]. [7] We calculated differences between the SST records from ODP Site 806 in the WPWP and our results to obtain a meridional SST gradient record (Figure 2c). The SSTs of Site 806 were reconstructed from Mg/Ca ratios of surfacedwelling Globoritalia sacculifer with the average time resolution of 10 ky [Wara et al., 2005]. Recently, this Mg/Ca-SST record was adjusted by Medina-Elizalde et al. [2008] based on the past variations of seawater Mg/Ca. We use the adjusted Mg/Ca-SST record in this work (Figure 2b). Before the calculation, the two records were respectively smoothed to obtain 0.2-Ma running mean data. The smoothed data of Site 806 were then interpolated to get data points with ages corresponding to those from Site 1147/1148. Finally, the time series of the SST differences between the two locations were calculated. Smoothing lowers resolution of SST record, but this is necessary to minimize the errors from interpolations, age uncertainties and proxy discrepancies between the two locations. We made a comparison of the gradients calculated using adjusted and unadjusted Mg/Ca data, respectively, and found that their secular trends are similar since the late Pliocene. The mid Pliocene (>3 Ma) SST differences from the adjusted data are apparently larger than from the unadjusted data, because the adjusted mid Pliocene Mg/Ca SSTs are C greater than the unadjusted SSTs. However, the mid-pliocene SST gradient here contains large uncertainties as suggested below, and hence will not be our focus for this study. 3. Results [8] Mercer and Zhao [2004] presented a low-resolution (28 data points for the last 3 Ma) U K0 37 SST record for Site 1147/1148, showing a general post-pliocene cooling trend, and high amplitude of SST oscillations for the last 1 Ma. Our higher-resolution record agrees with theirs and reveals more details of SST evolution (Figure 2a). Three stages could be identified from our record: 1) hot (>28 C) mid- Pliocene (pre 2.7 Ma), 2) warm (26.8 C average) and moderately varying (from 28.1 to 24.9 C) period during the late Pliocene to early Pleistocene (from 2.7 to 1.1 Ma), and 3) cool (24.7 C average) and highly varying (from 27.2 to 22.4 C) mid- to late Pleistocene (after 1.1 Ma). The mid- Pliocene SSTs are probably distorted and might even be underestimated due to the fact that U K0 37 values pre 2.7 Ma are all above the upper end (0.97) of the U K0 37 values used for the temperature calibration in the SCS [Pelejero and Grimalt, 1997], and that several studies have proposed deviations from linearity resulting in the rise of SST upper limit for U K0 37 calibration [Pelejero and Calvo, 2003]. However, the long-term cooling trend since the mid-pliocene is not an artifact, because it is also evidenced by foraminifer studies in northern SCS [Jian et al., 2003; Huang et al., 2003, 2005; Zheng et al., 2005]. The large cooling from 1.1 to 0.9 Ma is the most distinct event. This event started before the mid-pleistocene climate transition (MPT) [Mudelsee and Schulz, 1997], and has also been indicated at nearby ODP Site 1144 [Zheng et al., 2005], and discovered in the Eastern Equatorial Pacific and off West Africa [Marlow et al., 2000; McClymont and Rosell-Melé, 2006; Medina-Elizalde and Lea, 2005]. After this event, SSTs varied with high amplitude from 22.4 to 27.2 C, 2of5

3 Figure 2. (a) U K0 37-SST record from ODP Site 1147/1148 in the northern South China Sea (SCS). (b) Adjusted Mg/Ca-SST record from ODP Site 806 in the western Pacific warm pool (WPWP) [Medina-Elizalde et al., 2008; Wara et al., 2005]. Heavy smooth curves in Figures 2a and 2b represent 0.2-Ma running mean. (c) Changes in SST difference between the WPWP and northern SCS. (d) and (e) Changes in SST difference (0.2-Ma running mean) between the western and eastern equatorial Pacific. Figure 2d was calculated using adjusted Mg/Ca-SSTs from the western Site 806 and the eastern Site 847 [Medina-Elizalde et al., 2008], and Figure 2e using adjusted Mg/Ca-SST from the western Site 806 and U K0 37-SST from the eastern Site 846 [Lawrence et al., 2006]. (f) Benthic d 18 O record from Site 1147/1148 [Jian et al., 2003]. similar to SST difference from the last glacial to the Holocene estimated for the same area [e.g., Pelejero et al., 1999], although our time resolution is not high enough to resolve every glacial-interglacial cycles. [9] SST difference between the WPWP and northern SCS shows a general increase since 2.8 Ma (Figure 2c). Since southern SCS is a part of the WPWP, this SST gradient evolution is comparable to the recent finding that SCS south-north thermocline gradient increased after 3.0 Ma [Jian et al., 2006]. Interestingly, our SST-difference pattern is quite similar to those across the equatorial Pacific (Figures 2d and 2e), although there are some discrepancies in fine structures between them. In general, they are common in the following features: 1) onset of the gradient increase in the late Pliocene (at 2.3 or 2.8 Ma); 2) low values in the Pliocene (>1.7 Ma); 3) pivotal increase in the gradient from the late Pliocene to the end of early Pleistocene, and 4) high values during mid- to late Pleistocene (<1.0 Ma). The higher SST-difference values in the mid- Pliocene shown in Figures 2c and 2e might be a distortion by either the underestimation of alkenone SST as stated above or overestimation of changes in past seawater Mg/Ca [Medina-Elizalde et al., 2008]. 4. Discussion and Conclusions [10] During normal conditions in the modern tropical Pacific, an east-west SST difference (5 C) across the equator maintains the WC [Ravelo et al., 2006]. But a weakening of the circulation and a reduction in SST gradient temporarily occur during El Niño events. Reconstructions of east-west SST difference have revealed that the Pliocene east-west SST asymmetry was greatly reduced, suggesting a permanent El Niño like state, and the modern east-west SST pattern was established after 1.7 Ma [Wara et al., 2005; Ravelo et al., 2006] or by MPT [McClymont and Rosell-Melé, 2005]. These results thus suggest a large Pliocene warm pool across the equatorial Pacific. From our results, the small WPWP-northern SCS SST gradient (<2 C) during the late Pliocene suggests that the warm pool might also expand towards northwest, which is consistent with the conclusion of northward expansion of the WPWP during the Pliocene proposed by Wang [1994]. This small meridional SST gradient was thus coincident with the small zonal gradient in the low-latitude Pacific before 1.7 Ma (Figures 2c, 2d, and 2e), which consequently suggests a super-sized late Pliocene Pacific warm pool. From the late Pliocene (2.3 Ma) to the end of early Pleistocene (1.0 Ma) both the meridional and zonal SST differences kept increasing, indicating the area of the warm pool was in shrinkage. After 1.0 Ma, the mean state of the WPWP was relatively stable on the 0.2-Ma time scale. [11] We further use the reconstructed SST difference between the WPWP and northern SCS to infer the evolution of atmospheric velocity potential in the western Pacific due to their intimate relationship (Figure 1). Because the tropical velocity potential actually reflects the mixture of the interacted WC, HC, and monsoon circulation [e.g., Tanaka et al., 3of5

4 2004], we believe that variations in the SST differences contain the information of tropospheric circulation patterns. Because HC intensity is associated with the gradient in latent heat release from the tropics to the subtropics, driven by the underlying SST gradient [Rind and Perlwitz, 2004], our meridional SST gradient could consequently suggest that the HC started strengthening at 2.8 Ma, but was relatively weak before 1.7 Ma (SST gradient < 2 C), then continued enhancing from 1.7 to 1.0 Ma, and finally persisted in a relatively stable state after 1.0 Ma. This evolution pattern of the HC is generally similar to that of the WC suggested by the equatorial zonal SST-difference records (Figures 2c, 2d, and 2e), suggesting the coupling between the circulations. The coupling between the HC and WC is partly supported by some simulations and observations [Chen et al., 2002; Wang, 2005; Lu et al., 2007]. In terms of the coupled ocean-atmosphere system, the HC can be viewed as the fundamental driver of a tightly coupled Pacific cold tongue, warm pool, and WC system [Liu and Huang, 1997]. However, from the present work it is difficult to determine how the coupling developed before 1.7 Ma. From the Mg/Ca data of Wara et al. [2005] the onset of WC enhancement was at 2.3 Ma (Figure 2d), while from the alkenone data of Lawrence et al. [2006] it was at 2.7 Ma (Figure 2e). Our record suggests the onset of HC intensification was 2.8 Ma (Figure 2c), similar to the results from the data of Lawrence et al. [2006]. Nevertheless, it is important to note that our inference of past changes in the HC is based only on two-site SST differences, which is insufficient to get a complete view of how the HC changes as suggested by Otto-Bliesner and Clement [2005]. Moreover, our inference is a simple application of the present is the key to the past, which might be unrealistic as the mean position of the Inter-Tropical Convergence Zone (ITCZ), and hence of the ascending branch of the atmospheric circulations, has been suggested to vary in the past [Barreiro et al., 2006]. However, we hope this work is a prospective step forward for better understanding of the HC in the past. [12] East Asian monsoon circulation, another meridional circulation system with seasonally changing surface wind directions as a result of land-sea heating gradient, exists in the low- to high-latitude East Asia [Webster et al., 1998]. With its hot waters, the WPWP is the cradle of the East Asian summer monsoon (EASM) [Webster et al., 1998]. Different from the EASM, the East Asian winter monsoon (EAWM) originates from Siberian High and is closely associated with the Northern Hemisphere Glaciation (NHG) [e.g., Xiong et al., 2003]. Due to the opposite surface flow direction of the HC (equatorward) to that of EASM and the same direction as that of EAWM, strengthening of the HC could help enhance EAWM, and weaken EASM. Therefore, our suggested enhancing of the HC since the late Pliocene could further suggest weakening of EASM and strengthening of EAWM. This suggestion is generally consistent with the terrestrial monsoon records from Chinese Loess Plateau showing strong EASM and weak EAWM during the Pliocene, and prominent EASM weakening and EAWM strengthening after the Pliocene [Sun et al., 2006]. Although this consistency does not necessarily mean the East Asian monsoon is controlled by tropical climate, it provides another possible mechanism of the connections between tropical and NHG forcings to modulate the history of East Asian monsoon. [13] The long-term cooling history in the northern SCS (Figure 2a), especially the cooling events at 2.7 and 0.9 Ma, was generally consistent with the history of NHG [e.g., Ravelo et al., 2004; Mudelsee and Schulz, 1997], indicating the influences of increasing NHG on the northern SCS through EAWM. However, our SST record has some features unique to the tropical system, and not found in NHG-controlled benthic d 18 O record of the study sites (Figure 2f). For example, the pivotal increase in SST gradient during the late Pliocene/early Pleistocene (1.7 Ma) is not correspondingly demonstrated in the benthic d 18 O signal. This phenomenon has also been noted and nicely discussed by Ravelo et al. [2004]. Another feature is that the attainment to the stable meridional SST gradient was at 1.0 Ma, preceding MPT. The precedence is also the character of the tropical Pacific [McClymont and Rosell-Melé, 2006]. Consequently, our record documents both the NHG-related high-latitude events and several additions linked with tropical forcing, suggesting the tropical roles in influencing extra-tropical climate in addition to the NHG forcing. [14] Acknowledgments. Samples for this project were provided by the Ocean Drilling Program. This work is supported by the National Natural Science Foundation of China (grant ). Three anonymous reviewers are thanked for their great help in improving this paper. References Barreiro, M., G. Philander, R. Pacanowski, and A. Fedorov (2006), Simulations of warm tropical conditions with application to middle Pliocene atmospheres, Clim. Dyn., 26, Cane, M. A., A. C. Clement, A. Kaplan, Y. Kushnir, R. Murtugudde, D. Pozdnyakov, R. Seager, and S. E. Zebiak (1997), 20th century sea surface temperature trends, Science, 275, Chen, J. Y., B. E. Carlson, and A. D. Del Genio (2002), Evidence for strengthening of the tropical general circulation in the 1990s, Science, 295, Haug, G. H., et al. (2005), North Pacific seasonality and the glaciation of North America 2.7 million years ago, Nature, 433, Huang, B. Q., X. R. Cheng, Z. M. Jian, and P. X. Wang (2003), Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea, Paleogeogr. Paleoclimatol. Paleoecol., 196, Huang, B. Q., Z. M. Jian, and P. X. Wang (2005), Paleoceanographic evolution recorded in the northern South China Sea since 4 Ma, Sci. China, Ser. D, 48, Jian, Z. M., Q. H. Zhao, X. R. Cheng, J. Wang, P. X. Wang, and X. Su (2003), Pliocene Pleistocene stable isotope and paleoceanographic changes in the northern South China Sea, Paleogeogr. Paleoclimatol. Paleoecol., 193, Jian, Z. M., Y. Q. Yu, B. H. Li, J. L. Wang, X. H. Zhang, and Z. Y. Zhou (2006), Phased evolution of the south north hydrographic gradient in the South China Sea since the middle Miocene, Paleogeogr. Paleoclimatol. Paleoecol., 230, Lawrence, K. T., Z. H. Liu, and T. D. Herbert (2006), Evolution of the eastern tropical Pacific through Plio-Pleistocene glaciation, Science, 312, Liu, Z. H., and T. D. Herbert (2004), High-latitude influence on the eastern equatorial Pacific climate in the early Pleistocene epoch, Nature, 427, Liu, Z. Y., and B. Y. Huang (1997), A coupled theory of tropical climatology: Warm pool, cold tongue, and Walker circulation, J. Clim., 10, Lu, J., G. A. Vecchi, and T. Reichler (2007), Expansion of the Hadley cell under global warming, Geophys. Res. Lett., 34, L06805, doi: / 2006GL Marlow, J. R., C. B. Lange, G. Wefer, and A. Rosell-Melé (2000), Upwelling intensification as part of the Pliocene-Pleistocene climate transition, Science, 290, of5

5 McClymont, E. L., and A. Rosell-Melé (2006), Links between the onset of modern Walker circulation and the mid-pleistocene climate transition, Geology, 33, McClymont, E. L., A. Rosell-Melé, J. Giraudeau, C. Pierre, and J. M. Lloyd (2005), Alkenone and coccolith records of the mid-pleistocene in the k southeast Atlantic: Implications for the U 0 37 index and South African climate, Quat. Sci. Rev., 24, Medina-Elizalde, M., and D. W. Lea (2005), The mid-pleistocene transition in the tropical Pacific, Science, 310, Medina-Elizalde, M., D. W. Lea, and M. S. Fantle (2008), Implications of seawater Mg/Ca variability for Plio-Pleistocene tropical climate reconstruction, Earth Planet. Sci. Lett., 269, Mercer, J. L., and M. X. Zhao (2004), Alkenone stratigraphy of the northern South China Sea for the last 35 m.y., Sites 1147 and 1148, ODP Leg 184 [online], Proc. Ocean Drill. Program Sci. Results, 184, 17 pp. (Available at pdf). Mudelsee, M., and M. Schulz (1997), The mid-pleistocene climate transition: Onset of 100 ka cycle lags ice volume build-up by 280 ka, Earth Planet. Sci. Lett., 151, Oort, A. H., and J. J. Yienger (1996), Observed interannual variability in the Hadley circulation and its connection to ENSO, J. Clim., 9, Otto-Bliesner, B. L., and A. Clement (2005), The sensitivity of the Hadley circulation to past and future forcings in two climate models, in The Hadley Circulation: Past, Present and Future, editedbyh.f.diaz, and R. S. Bradley, pp , Springer, New York. Pelejero, C., and E. Calvo (2003), The upper end of the U K0 37 temperature calibration revisited, Geochem. Geophys. Geosyst., 4(2), 1014, doi: / 2002GC Pelejero, C., and J. O. Grimalt (1997), The correlation between the U K 37 index and sea surface temperature in the warm boundary: The South China Sea, Geochim. Cosmochim. Acta, 61, Pelejero, C., J. O. Grimalt, S. Heilig, M. Kienast, and L. Wang (1999), High-resolution U k 37 temperature reconstructions in the South China Sea over the past 220 kyr, Paleoceanography, 14, Ravelo, A. C., D. H. Andreasen, M. Lyle, A. O. Lyle, and M. W. Wara (2004), Regional climate shifts caused by gradual global cooling in the Pliocene epoch, Nature, 429, Ravelo, A. C., P. S. Dekens, and M. McCarthy (2006), Evidence for El Niño-like conditions during the Pliocene, GSA Today, 16, Rind, D., and J. Perlwitz (2004), The response of the Hadley circulation to climate changes, past and future, in The Hadley Circulation: Past, Present and Future, edited by H. F. Diaz, and R. S. Bradley, pp , Springer, New York. Shackleton, N. J., M. A. Hall, and D. Pate (1995), Pliocene stable isotope stratigraphy of Site 846, Proc. Ocean Drill. Program, Sci. Results, 138, , doi: /odp.proc.sr Sun, Y. B., S. C. Clemens, Z. S. An, and Z. W. Yu (2006), Astronomical timescale and palaeoclimatic implication of stacked 3.6-Myr monsoon records from the Chinese Loess Plateau, Quat. Sci. Rev., 25, Tanaka, H. L., N. Ishizaki, and A. Kitoh (2004), Trend and interannual variability of Walker, monsoon and Hadley circulations defined by velocity potential in the upper troposphere, Tellus, Ser. A, 56, Villanueva, J., C. Pelejero, and J. O. Grimalt (1997), Clean-up procedures for the unbiased estimation of C37 alkenone sea surface temperatures and terrigenous n-alkane inputs in paleoceanography, J. Chromatogr. A, 757, Wang, C. (2005), ENSO, Atlantic climate variability, and the Walker and Hadley circulations, in The Hadley Circulation: Past, Present and Future, edited by H. F. Diaz, and R. S. Bradley, pp , Springer, New York. Wang, L. J. (1994), Sea surface temperature history of the low latitude western Pacific during the last 5.3 million years, Paleogeogr. Paleoclimatol. Paleoecol., 108, Wang, P. X.,, W. L. Prell, and P. Blum (Eds.) (2000), Proceedings of the Ocean Drilling Program: Initial Reports, vol. 184, Ocean Drill. Program, College Station, Tex. Wara, M. W., A. C. Ravelo, and M. L. Delaney (2005), Permanent El Niño like conditions during the Pliocene warm period, Science, 309, Webster, P. J., V. O. Magana, T. N. Palmer, J. Shukla, R. A. Tomas, M. Yanai, and T. Yasunari (1998), Monsoons: Processes, predictability, and the prospects for prediction, J. Geophys. Res., 103, 14,451 14,510. Xiong, S. F., Z. L. Ding, W. Y. Jiang, S. L. Yang, and T. S. Liu (2003), Initial intensification of east Asian winter monsoon at about 2.75 Ma as seen in the Chinese eolian loess-red clay deposit, Geophys. Res. Lett., 30(10), 1524, doi: /2003gl Zheng, F., Q. Y. Li, B. H. Li, M. H. Chen, X. Tu, J. Tian, and Z. M. Jian (2005), A millennial scale planktonic foraminifer record of the mid- Pleistocene climate transition from the northern South China Sea, Paleogeogr. Paleoclimatol. Paleoecol., 223, F. Chen, G. Jia, and P. Peng, State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou , China. (jiagd@gig.ac.cn) 5of5