Modeling East Asian climate and impacts of atmospheric CO2 concentration during the Late Cretaceous (66 Ma)

Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 190–201

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Modeling East Asian climate and impacts of atmospheric CO2 concentration during the Late Cretaceous (66 Ma) Junming Chen a, Ping Zhao b,⁎, Chengshan Wang c, Yongjian Huang c, Ke Cao d a

Chinese Academy of Meteorological Sciences, Beijing 100081, China National Meteorological Information Center, China Meteorological Administration, Beijing 100081, China Research Center f or Tibetan Plateau Geology, China University of Geosciences (Beijing), Beijing 100083, China d The Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Qingdao Institute of Marine Geology, Qingdao 266071, China b c

a r t i c l e

i n f o

Article history: Received 21 October 2011 Received in revised form 2 July 2012 Accepted 20 July 2012 Available online 27 July 2012 Keywords: Late Cretaceous Greenhouse effects East Asian climate modeling

a b s t r a c t Utilizing the Community Climate System Model version 2 from the National Center for Atmospheric Research (NCAR) and the reconstructed paleogeographic data, we simulate East Asian climate in the Late Cretaceous (66 Ma) and investigate the impacts of atmospheric CO2 concentration on climate. The simulations show that the large-scale pressure systems and prevailing wind directions showed a remarkable seasonal variation over East Asia at 66 Ma, which indicates a monsoon feature over East Asia. The East Asian winter and summer monsoons showed a synchronous variation, that is, a strong (weak) winter monsoon accompanied a strong summer (weak) monsoon. At 66 Ma, there was more precipitation over the eastern coasts of Asia and less precipitation in the mid-latitudes of the inland areas, but there was no meiyu rainy belt in the subtropics of the East Asian land like the present climate. Moreover, the simulated Cretaceous climate over East Asia was warmer relative to the present day. Annual mean surface air temperature was higher over Asia at that time and close to the estimation from the geological evidence. In the Late Cretaceous, when atmospheric CO2 concentration is reduced, the East Asia climate has a significant change, with weaker winter and summer monsoons over East Asia. Annual mean surface air temperature and annual total precipitation reduce in most of land and ocean. Negative difference of surface water budget appeared mainly in the eastern part of East Asia, indicating a drier soil surface, while positive differences appeared in the mid‐latitudes of centralwestern Asia, indicating a wetter soil surface. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The Cretaceous greenhouse climate is one of hot topics in the Earth science during the recent 20 years, because it is one of the best periods for understanding a warming effect of greenhouse gases. A large number of the isotopes, paleontology, and pale-oceanography evidence have shown that the Cretaceous climate was different from the present icehouse pattern (e.g. Berner, 1991, 1994; Yapp and Poths, 1996; Huber et al., 2001; Bice and Norris, 2002). At that time, terrestrial biota in eastern China also showed a large difference from the present day and Songliao Basin, a continental-scale oil/gas basin, just began to take shape. Therefore, reconstructing the Cretaceous climate in East Asia is very helpful to understand evolutions of the past climate and relationships of geological events with greenhouse climate. Based on the geological deposits and the development and uplift of the Cretaceous mountains, the Cretaceous climate over East Asia has been reconstructed extensively. Because the Tibetan Plateau was lower than 1000 m during the Cretaceous, its mechanic block ⁎ Corresponding author. E-mail address: [email protected] (P. Zhao). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2012.07.017

to atmospheric circulation was weak (Zhang, 1995). The planetary winds prevailed over East Asia at that time. Liu et al. (1995) further discovered that the prevalence of the planetary winds in the Cretaceous was associated with a long-term stable Earth's crust over East Asia, the flattened land, a weak topographic function, and the modulation of the Paleo-Tethys Ocean. Using the pollen and plant fossil proxies, Huang et al. (1999) documented a cyclic feature of the Cretaceous climate in Songliao Basin, indicating local humid and subhumid subtropical climate. Jiang and Li (1996), Jiang et al. (2001) studied the climatic implication of the desert spatial pattern and temporal evolution, revealing a subtropical high-pressure zone and a southeast or southwest monsoon over East Asia in the Late Cretaceous. Moreover, Chen (1997) pointed out that during the early Cretaceous to the Tertiary Period, central China became a tropical or subtropical arid and hot semi-desert and saliniferous lake district because the westward moisture transport from the Pacific was blocked by the coastal mountains of East Asia. Global and regional climate models have been extensively applied to study the continental paleoclimate during the Cretaceous. For example, Barron and Washington (1982) simulated atmospheric circulation in the middle Cretaceous with an atmospheric general

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Fig. 1. (a) Sea-land distribution and bathymetry for 66 Ma (unit: m). (b) Vegetation types for 66 Ma, in which the number 1 is for the tropical rainforest, 2 for the tropical semi-deciduous forest, 3 for the subtropical broadleaved evergreen forest and woodland, 4 for the desert and semi-desert, 5 for the temperate evergreen broadleaved and coniferous forest, 6 for the tropical savanna, 7 for the polar deciduous forest, and 8 for the bare soil.

circulation model (AGCM). Using the Global and Environmental and Ecological Simulation of Interactive Systems (GENESIS) model, Otto-Bliesner and Upchurch (1997) examined the impacts of vegetation on the Late Cretaceous climate. Upchurch et al. (1998) investigated the sensitivity of the Late Cretaceous climate to vegetation changes. Their results show that the existence of forest at high latitudes contributed to a warm climate at that time. Otto-Bliesner et al. (2002) also examined the Late Cretaceous ocean features with a coupled climate system model. Haywood et al. (2004) simulated the early Cretaceous climate over Southeast England with a regional climate model, showing the importance of moisture balance to long-term climate changes. Palaeoclimate simulations over East Asia have also been carried out with a variety of global and regional climate models (e.g., Wang, 1999; Chen et al., 2002; Liu et al., 2002; Zhao et al., 2003; Zhou et al., 2005; Ju et al., 2007; Jiang, 2008). Some researches focused on simulating the impacts of the Tibetan Plateau uplift on climate. For example, the simulation of Chen et al. (1999) showed that the Tibetan Plateau uplift during 40–50 Ma ago might lead to a cool East Asian climate. Liu et al. (2002) pointed out that the East Asia

monsoon is more sensitive to the Tibetan Plateau uplift compared to the South Asia monsoon and that this uplift has a greater effect on the East Asia winter monsoon compared to the East Asian summer monsoon. Using an AGCM, Zhang et al. (2007) analyzed the impacts of the Paratethys retreat on the East Asian monsoon and the causes of the palaeoenvironment transition during the Cenozoic, showing the important roles of both the Paratethys retreat and the Tibetan Plateau uplift in the formation of the monsoon environmental pattern. The Paratethys retreat might strengthen the East Asian monsoon and greatly increase humidity and aridity respectively in the East Asian monsoon region and northwestern China. Jiang et al. (2008) explored the impacts of the uplift and expansion of the Tibetan Plateau on East Asian climate during the mid-Pliocene about 3 Ma ago. Jin et al. (2009) modeled impacts of snow and glacier over the Tibetan Plateau on Afro-Asian monsoon climate during the Holocene. Their result indicates that the development of Tibetan snow and ice cover represented an additional climate feedback, which amplified the orbital forcing and produced a significant synergy with the positive vegetation feedback. Moreover, using a coupled climate system model,

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Fig. 2. The temporal curves of the simulated global and annual mean surface air temperature (unit: °C) in the control (red) and Cretaceous (blue) experiments.

Chen et al. (2009a, 2009b) investigated the characteristics of East Asian climate and the effects of atmospheric CO2 concentration during the Late Cretaceous (80 Ma). It is evident that the previous studies paid little attention to simulations of East Asian climate and associated mechanisms in the Cretaceous. The general features of the Cretaceous East Asian climate are still unclear. Therefore, it is necessary to simulate climatic characteristics over East Asia in the Cretaceous and to further explore the associated mechanisms. In the present study, East Asian climate is simulated by using a climate system model and paleogeographic data during the Last Maastrichtian (66 Ma) that corresponds to the Mingshui Formation in Songliao Basin (Wan et al., 2012–this volume). The rest of this paper is organized as follows. The features of the model and data used in this study are described in Section 2. In Section 3, we depict the characteristics of the simulated atmospheric circulation and climate over East Asia. The impacts of atmospheric CO2 concentration on East Asian climate are analyzed in Section 4. Summary and discussion are presented in Section 5.

2. Model experimental design and statistical method The Community Climate System Model version 2 (CCSM2), developed by the National Center for Atmospheric Research (NCAR) (Kiehl and Peter, 2004), is used in the present study. This model includes atmosphere, ocean, sea-ice, and land surface components. The atmosphere component is the Community Atmosphere Model version 2.0 with an approximate horizontal resolution of 3.75° in longitude and latitude and 26 layers in the vertical direction (Kiehl et al., 1996). The land component is the Community Land Model version 2, including a 10-layer soil model, a multilayer snow model, and a hydrographic scheme (Bonan et al., 2002). The ocean component is the Parallel Ocean Program model with an approximate horizontal resolution of 1.5° × 3.6° in longitude and latitude and 25 layers in the vertical direction (Smith and Gent, 2002). The sea-ice component is the Sea Ice Model and has the same horizontal resolution as that of the ocean component (Bitz and Lipscomb, 1999). Some studies (e.g., Sperber, 2004;

Peng et al., 2010) have shown that the CCSM2 model can capture the major features of East Asian climate. To compare the Late Cretaceous (66 Ma) East Asian climate with that of the present day and analyze impacts of atmospheric CO2 concentration on East Asian climate, we design three experiments. The first experiment is for the present climate, called the control experiment. In this experiment, the orbital parameters, the sea-land distribution, the topography, and the vegetation types are same as those of the present day. Moreover, some trace gas concentrations as well as influences of chlorofluorocarbons are considered. The second experiment is for the Late Cretaceous climate, called the Cretaceous experiment. In this experiment, the continental configuration and the paleotopography with a horizontal resolution of 2°× 2° in longitude and latitude come from the reconstructions of Ziegler (1998). At 66 Ma, the terrain was relatively flat over East Asia and there was no uplift of the Tibetan Plateau (Fig. 1a). The vegetation data come from the reconstruction of lithologic and paleontological indicators by Upchurch et al. (1998) (Fig. 1b). The solar isolation is set to the present-day solar orbital configuration and the solar constant decreases by 0.664% relative to the present value. Because the reconstructed Cretaceous atmospheric CO2 concentration has a large variation of 2–9 times as large as the pre-industrial one (Berner, 1991, 1994; Yapp and Poths, 1996), following Upchurch et al. (1998), atmospheric CO2 concentration in the Cretaceous experiment is set to a middle value of about 6 times (1680 ppmv). The sea-land distribution, the topographic height, the ocean depth, and the vegetation in the Late Cretaceous are interpolated to the grids of the CCSM2 model by using a bilinear interpolation method. The third experiment is for testing impacts of atmospheric CO2 concentration at 66 Ma, called the CO2 experiment. This experiment is same as the Cretaceous experiment but with an atmospheric CO2 concentration of 560 ppmv. The other inputs in the CCSM2 model for each experiment come from the original model. Because there was no long-term polar ice cover during the Cretaceous, the CCSM2 model is initialized without sea ice in the two Cretaceous experiments. For each experiment, the CCSM2 model is run for 250 years. Fig. 2 shows the temporal curves of the annually and globally averaged surface air temperature (SAT) simulated in the present-day and Cretaceous experiments during the 250 model years, in which the reference height (2 m) temperature of the model is used to indicate SAT. In the figure, the global average temperature in the Cretaceous experiment varies in magnitude by about 0.3 °C after 180 years, which indicates that the model reaches quasi-equilibrium under the Cretaceous boundary forcing. For each experiment, the mean values over the last 50 years are used in analyzing its climatic features. The Student t-test at the 95% confidence level is applied to assess the significance of composite differences unless otherwise stated. Moreover, Asia is defined in the areas of Eurasian continent to the east of 60°E and East Asia is defined in the area of the Asian continent to the east of 90°E. Winter is for December, January, and February (DJF) and summer is for June, July, and August (JJA). 3. Simulated East Asian climate at 66 Ma 3.1. Atmospheric circulation Fig. 3a shows the simulated winter mean 500-hPa geopotential height and wind fields in the Cretaceous experiment. It is seen that the westerly or northwesterly winds prevailed to the north of 20°N over East Asia, with a deep trough appearing near the coasts of East Asia and a ridge covering the central part of the Eurasian continent. Although the simulated Cretaceous ridge and trough locations are

Fig. 3. (a) The simulated winter mean geopotential height (contour; unit: dagpm) and wind (vector; unit: m/s) at 500 hPa in the Cretaceous experiment. (b) As in (a) but for the composite difference of geopotential height between the Cretaceous and control experiments (Cretaceous minus control, unit: gpm). (c) As in (a) but for 850-hPa wind. (d) As in (b) but for the composite difference of 850-hPa wind (unit: m/s). In (b) and (d), values significant at the 95% confidence level are shaded.

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Fig. 4. (a) The simulated winter mean SLP (contour; unit: hPa) and surface air temperature (shaded; unit: °C) in the Cretaceous experiment. (b) As in (a) but for the control experiment.

similar the present day, some remarkable differences are also observed between the Cretaceous and control experiments. Fig. 3b shows the composite difference of the winter mean 500-hPa geopotential height between the Cretaceous and control experiments. In the figure, positive anomalies of the 500-hPa geopotential height appeared over the mid and high latitudes of Asia, with the central values exceeding 100 gpm, while negative anomalies appear over the Northwest Pacific Ocean, with the central values below − 160 gpm. Compared to the climatology of the winter 500-hPa geopotential height at 66 Ma (Fig. 3a), the positive (negative) anomalies over Asia (the Northwest Pacific) shown in Fig. 3b reflect a stronger and more northward ridge (a stronger trough) at 66 Ma relative to the present day. Fig. 3c shows the simulated winter mean 850-hPa wind field in the Cretaceous experiment. The westerly wind generally prevailed in the mid and high latitudes of the Northern Hemisphere. The northwesterly winds appeared over East Asia between 30°N and 60°N and the northeasterly winds prevailed over Northeast Asia and the lower latitudes of East Asia. Compared to the present day, the northerly wind anomalies generally prevail at the lower latitudes of East Asia (Fig. 3d), indicating the local stronger winter northerly wind at 66 Ma. Fig. 4a shows the simulated winter mean sea level pressure (SLP) in the Cretaceous experiment. In this figure, a large-scale high-pressure system appeared to the north of 30°N over Asia, with its central value of 1024 hPa in central Asia near 40°N. A low-pressure system appeared in the high latitudes of the North Pacific, with a high-pressure system

appearing in the ocean near 30°N. These features in SLP over East Asia are similar to those in the present day (Fig. 4b). Compared to the present day, the Cretaceous high-pressure system over Asia is weaker while the Cretaceous low-pressure system in the high latitudes of the North Pacific is stronger. Fig. 5a exhibits the simulated summer mean 500-hPa geopotential height and wind fields in the Cretaceous experiment. In the figure, a ridge appears in the high latitudes of central Asia, with a low-pressure system in the high latitudes of the North Pacific. Meanwhile, a trough appears in the mid-low latitudes of East Asia. It is evident that the ridge and trough in the high latitudes are weaker in summer than in winter (shown in Fig. 3a). Meanwhile, a large-scale high-pressure zone covers the globe oceans between 20°S and 30°N during summer. Fig. 5b shows the composite difference of the summer mean 500-hPa geopotential height between the Cretaceous and control experiments. Positive anomalies of the 500-hPa geopotential height cover the entire Northern Hemisphere. They indicate a stronger ridge in central Asia, a weaker trough in the mid-low latitudes of East Asia, and a stronger subtropical high over the western North Pacific at 66 Ma compared to the present day. At 850 hPa (Fig. 5c), the southwesterly or southerly winds in the Late Cretaceous prevailed over East Asia and extended to 60°N. The southwesterly or southerly winds originated mainly from two seas. One came from the PaleoTethys Ocean and another came from the tropical western North Pacific. Meanwhile, the northerly winds prevailed over western

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Fig. 5. As in Fig. 3 but for summer.

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Fig. 6. As in Fig. 4 but for summer.

Asia. For the present summer, the southwesterly winds prevail mainly over the eastern part of East Asia and only arrive to about 40°N (figures not shown). On the composite difference map of the summer mean 850-hPa wind between the Cretaceous and control experiments (Fig. 5d), the southerly wind anomalies prevail over the mid and lower latitudes of East Asia. On the Cretaceous summer SLP map (Fig. 6a), a low-pressure system appeared in the mid and high latitudes of Asia, with its central value of 996 hPa, and a high-pressure system appeared in the mid‐latitudes of the North Pacific, with its central value of 1020 hPa. Compared to the present climate (Fig. 6b), the low-pressure (high-pressure) system over Asia (the subtropical North Pacific) was stronger (weaker) at 66 Ma. The East Asia monsoon may be defined as a reversal of the large-scale prevailing wind in the direction with the seasonal variation. Fig. 7a shows the month-latitude cross‐section of the simulated 850-hPa meridional wind in the Cretaceous experiment along 90°E-120°E. In this figure, the southerly wind mainly prevailed between 0°N and 60°N during April to September, with the strongest southerly wind of 5 m/s in July, while the northerly wind prevailed in other months, with the strongest northerly wind of −6 m/s in November to December. This seasonal change in the wind direction also corresponded to a seasonal reversal of surface pressure systems over Asia at 66 Ma (shown in Figs. 4a and 6a), showing a pronounced monsoon feature over East Asia, which is consistent with the geological evidence of Jiang and Li (1996), Jiang et al. (2001). In their studies, there possibly existed a

subtropical high zone and a southeast or southwest monsoon over East Asia in the Late Cretaceous summer. Our simulation further demonstrates the existence of the Cretaceous monsoon circulation over East Asia. Because of the existence of the Tibetan Plateau in the present day, we show the cross‐section of the 850-hPa meridional wind along 110–120°E in the control experiment (Fig. 7b). It is seen that the 850-hPa southerly wind generally prevails to the south of 40°N in summer, with the strongest southerly wind in July, which is similar to the present observation (Zhao et al., 2007). These results show that compared to the present day, the monsoon feature in the Late Cretaceous was more remarkable and extended to a more northern area. Meanwhile, both winter and summer monsoons were also stronger in these latitudes at that time. 3.2. Surface air temperature Fig. 4a also shows the simulated winter mean SAT in the Cretaceous experiment. There was a large-scale cold area in the mid-high latitudes of Asia and the Arctic, with the coldest center below − 20 °C appearing to the northeast of the high-pressure center in Asia, similar to the simulated result of Upchurch et al. (1998). Their study showed the lowest temperature of − 34 °C over the Asian interior. This feature is also similar to that of the present day (Fig. 4b). Combining with SLP, a large-scale cold high-pressure system generally appeared at the mid and high latitudes of Asia. In summer (Fig. 6a),

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results show the consistence between the simulated temperature and the geological evidence over East Asia, also supporting the reliability of the model results.

3.3. Precipitation and surface water budget

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The simulated annual total precipitation over East Asia in the Cretaceous experiment (Fig. 9a) was similar to that of the present day (figure not shown) in pattern and amount. Precipitation of 3000–4000 mm mainly appeared in the south part of East Asia and its adjacent oceans between 10°S and 10°N and rapidly decreased toward the inland area. Little precipitation below 400 mm appeared in the inland areas of Asia between 20°N and 40°N. There was precipitation above 1200 mm in the western Pacific and the eastern coasts of East Asia. However, some remarkable differences are also observed between the Cretaceous and control experiments. For example, the Cretaceous precipitation exceeded 800 mm at the mid and high latitudes of East Asia, larger than that of the present day (about 600 mm). In the present day, there is a rainy belt over East Asia and its adjacent seas near 30°N, which indicates the summer meiyu rainfall (Zhao et al., 2007, 2009). At 66 Ma, the meiyu rainy belt mainly appeared over the western Pacific rather than the subtropics of the East Asian land. Fig. 9b shows the month-latitude cross‐section of the precipitation along 90–120°E in the Cretaceous experiment. Precipitation showed a remarkable seasonal variation and there were generally more precipitation in summer and less precipitation in other seasons. The major rainfall exceeding 2 mm/day appeared over the subtropics in May and arrived to the north of 60°N. In autumn the major rainy belt

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Fig. 7. (a) The month-latitude cross‐section of the 850-hPa meridional wind (unit: m/s) along 90–120°E in the Cretaceous experiment. (b) As in (a) but along 110–120°E in the control experiment.

SAT was high over Asia, with the central value above 30 °C, which accompanied a low-pressure system over Asia. Compared to the present climate (Fig. 6b), Asian land is warmer at 66 Ma. Fig. 8a shows the simulated annual mean SAT at 66 Ma. Over East Asia, annual mean SAT was generally above 20 °C between 0° and 30°N, with the value of 28 °C near the equator, and it is in 0–20 °C between 30°N and 60°N and below 0 °C to the north of 60°N, with the lowest temperature of − 4 °C. On the composite difference map of annual mean SAT between the Cretaceous and control experiments (Fig. 8b), over Asia, there are positive SAT anomalies of 0–4 °C between 0° and 20°N, 4–12 °C between 20°N and 60°N, and 4 °C-8 °C in more northern areas. Thus, annual mean SAT was higher at 66 Ma relative to the present day, with the largest increase in the mid‐latitudes. To examine the reliability of the simulated SAT in the Cretaceous experiment, we compare the simulated SAT with the geological evidence. For example, Yang et al. (1993) found out that the annual mean SAT in Nanxiong Basin near 16°N (corresponding to Guangdong Province of southern China) during the Cretaceous was 31.6 °C, much close to the simulated temperature of 29 °C. Some reconstructions (Shen et al., 2008) showed that the annual mean SAT in Songliao Basin between 38°N and 47°N (corresponding to Heilongjiang Province of Northeast China) (Gao and Cai, 1997) was 14–17 °C during the Mingshui Formation of the Cretaceous. Our simulated regional mean temperature over 119–128°E/38–47°N was about 12 °C. These

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Fig. 8. (a) The simulated annual mean surface air temperature (unit: °C) at 66 Ma. (b) As in (a) but for the composite difference between the Cretaceous and control experiments, in which values significant at the 95% confidence level are shaded.

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Fig. 10. The simulated annual mean surface water budget (unit: mm/day) in the Cretaceous experiment.

difference between precipitation and surface evaporation. At 66 Ma, negative values of annual mean SWB mainly appeared in Asia between 10°N and 40°N (Fig. 10), indicating that the local surface soil lost water and became dry, which corresponded to a drier climate. There were positive SWB values of 1–4 mm/day over the Eurasian continent to the north of 40°N, which shows that the local surface soil obtained water and became wet, corresponding to a relatively wetter climate. Some geological evidence suggests that the central part of China was a tropic (subtropical) arid or hot semi-desert and saliniferous Lake District from the early Cretaceous to the Tertiary Period (Chen, 1997) and that the paleoclimate of Songliao Basin in the Mingshui Formation was warm and moist (Huang et al., 1999). These results from the geological evidence are close to those of the simulations.

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b Fig. 9. (a) The simulated annual total precipitation at 66 Ma (unit: mm). (b) As in Fig. 7a but for precipitation (unit: mm/day). (c) As in Fig. 7b but for precipitation (unit: mm/day).

gradually retreated southwards and in winter finally returned to the tropics. This advance of the major rainy belt was associated with a northward shift of the southerly wind over East Asia (shown in Fig. 7a). In the present day, the major rainfall exceeding 2 mm/day appeared to the south of 60°N (Fig. 9c), which is similar to the present observation (Zhao et al., 2007, 2009). Thus the major rainy belt at 66 Ma arrived to a more northern position compared to the present day, which also indicates a stronger East Asian summer monsoon. Following Zhao et al. (2003), we further analyze surface water budget (hereafter SWB) at 66 Ma, in which SWB is defined as the

Fig. 11. (a) The composite difference of winter mean 850-hPa wind (unit: m/s) between the CO2 and Cretaceous experiments. (b) As in (a) but for summer. Values significant at the 95% confidence level are shaded.

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Fig. 12. As in Fig. 11 but for annual mean air temperature (unit: °C).

4. Impacts of CO2 concentration on East Asian climate at 66 Ma Fig. 11a shows the composite difference of winter mean 850-hPa wind between the CO2 and Cretaceous experiments. It is seen from this figure that a large-scale anomalous cyclone appears in the mid and high latitudes of Asia, with its center appearing near 40°N/90°E. Meanwhile, anomalous southwesterly or southerly winds to the south or east of the anomalous cyclonic center prevail over the lower latitudes and the eastern part of Asia, indicating a weaker East Asian winter monsoon. Fig. 11b presents the composite difference of summer mean 850-hPa wind between the CO2 and Cretaceous experiments. In the figure, a large-scale anomalous anticyclone covers Asia and the ocean to the west, with its two circulation centers

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Fig. 13. (a) As in Fig. 11 but for annual total precipitations (unit: mm). (b) As in Fig. 11 but annual mean surface water budget (unit: mm/day).

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appearing at the high latitude near 90°E and at the mid‐latitude near 45°E, respectively. The anomalous northerly wind appears over East Asia between 10°N and 50°N, indicating a weaker East Asian summer monsoon. Thus both the East Asian winter and summer monsoons generally weaken at 66 Ma when the CO2 concentration decreases. Fig. 12 shows the composite difference of annual mean SAT between the Cretaceous and CO2 experiments. Corresponding to the reduction of the CO2 concentration, annual mean SAT remarkably drops in East Asia, with a decrease of 2–3 °C between 0° and 30°N and a decrease of 4–6 °C between 30° and 60°N. The similar decreases of SAT also occur in winter and summer (figures not shown). The similar decrease of SAT is also modeled over the almost globe (figure not shown). Fig. 13a shows the composite difference of annual total precipitations between the CO2 and Cretaceous experiments. The simulated precipitation significantly reduces over East Asia, with the largest negative anomalies exceeding 100 mm appearing in the southeastern and northeastern parts of East Asia. Over the central and west parts of Asia near 30°N, precipitation slightly increases. Our analysis shows that these anomalies in annual total precipitation are attributed to the changes of summer precipitation (figure not shown). Fig. 13b shows the composite difference of annual mean SWB. In this figure, negative SWB anomalies appear in the high and low latitudes of East Asia, with a bigger decrease to the south of 30°N, indicating the loss of the local surface soil water when the CO2 concentration reduces. Over the mid‐latitudes (30–50°N) of Asia, there are weaker positive anomalies of 0–0.5 mm/day, indicating that surface soil obtains water and becomes wetter. 5. Summary and discussion We have used the CCSM2 model to simulate East Asian climate and analyzed the impacts of atmospheric CO2 concentration on climate during the Late Cretaceous (66 Ma). The results show that the large-scale pressure systems and prevailing wind directions showed a remarkable seasonal variation over East Asia at 66 Ma. A cold high-pressure (a low-pressure) system appeared over Asia (the high latitudes of the North Pacific) in winter, while a warm low-pressure (a high-pressure) system appeared over Asia (the subtropical North Pacific) in summer. The northerly (southerly) wind prevailed over East Asia during winter (summer). Corresponding to the seasonal variation of the wind, rainfall showed a seasonal variation between dry and wet seasons. These features suggest the existence of the monsoon phenomenon over East Asia at that time. Compared to the present climate, East Asian winter and summer monsoons were stronger during the Late Cretaceous and the summer monsoon rainfall might arrive to a more northern position (to the north of 60°N). Thus, during the warmer Late Cretaceous, the East Asian winter and summer monsoons showed a synchronous variation, which supports the viewpoint of Zhou and Zhao (2009). Their result shows that under a warmer background of the global climate such as the middle Holocene, East Asian winter and summer monsoons show a consistently strengthening, that is, a strong winter monsoon accompanied a strong summer monsoon. However, Jiang and Wang (2005) show that both East Asian winter and summer monsoons were significantly decreased during the middle Pliocene compared to the present day. In the recent global warming background, East Asian winter and summer monsoons show a significantly declining trend (Wang, 2001; Jiang and Wang, 2005; Zhu, 2008). Thus the relationship between East Asian winter and summer monsoons is complicated. Further research may help us to better understand the relationship between East Asian winter and summer monsoons and the associated mechanisms. Compared to the present day, annual mean SAT over Asia was higher at 66 Ma, with the largest increase of temperature in the mid‐latitudes of Asia, and the major rainy belt might arrive to a more northern

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position at that time. At 66 Ma, annual total precipitation was large in the tropics of East Asia and its adjacent oceans and rapidly decreased toward the inland area. Moreover, the meiyu rainy belt moved eastwards, appearing over the western Pacific at 66 Ma rather than the subtropics of the East Asian land like the present day, which is possibly associated with the distribution of the topography. Some studies have showed that the uplift of the Tibetan Plateau is important to the formation of the present-day East Asian monsoon climate, particularly to the summer meiyu rainfall (Liu and Yin, 2002; Zhang et al., 2007; Jiang et al., 2008). At 66 Ma, there was no uplift of the Tibetan Plateau and there was a highland in the eastern part of East Asia. Under such topography, there was no strengthened thermal contrast between the Tibetan Plateau and its adjacent areas like the present day and then there was no occurrence of the meiyu rainy belt over the subtropics of the East Asian land. This result also implies a role of the Tibetan Plateau in the formation of the present East Asian meiyu rainfall. Moreover, compared to the present day, the Cretaceous surface soil lost water and became dry over the mid and low altitudes of Asia while obtained water and became wet over the high latitudes of the Eurasian continent. A comparison analysis of SAT and soil water budget shows the consistency between the modeling and the geological evidence, which supports the reliability of the model results. Atmospheric CO2 concentration has a great influence on East Asia climate at 66 Ma. When the CO2 concentration is reduced, both East Asian winter and summer monsoons weaken. Meanwhile, annual mean SAT decreases over Asia and the globe and annual total precipitation also pronouncedly reduces at the high and low latitudes of East Asia. Surface soil generally loses water in the high and low latitudes of East Asia and obtains water in the mid‐latitudes of Asia. The simulated positive correlation between SAT and CO2 in the Cretaceous is consistent with the present-day conclusion (Solomon et al., 2007). Their results showed that in the recent decades, global mean SAT rapidly increases with an increase of atmospheric CO2 concentration. However, we also note that the long-term variability of the East Asian summer monsoon is not synchronous with that of global mean SAT (Zhao et al., 2010; Zhou et al., 2011). Under the recent warming background, the East Asian summer monsoon circulation and precipitation exhibits a weakening feature (Zhao et al., 2010), which indicates a “high SAT–weak monsoon” relationship. This result is different from the simulated “low SAT-weak monsoon” link at 66 Ma. Thus, the impact of the atmospheric CO2 concentration on temperature is similar between the Late Cretaceous and the present climate but its impact on East Asian monsoon is different. This result suggests that the responses of the East Asian monsoon to the change of atmospheric CO2 concentration are complicated. Investigating this impact under different Earth's environments is helpful to understand mechanisms of regional climate changes. This should also be addressed in the future work. Acknowledgments We are grateful to the editor, Professor Dave Bottjer, and the anonymous reviewer for their helpful and careful reviews that greatly improved the manuscript. We thank the NCAR for providing the CCSM2 on its homepage. This work was sponsored by the National Key Basic Research Project of China (2006CB701406). References Barron, E.J., Washington, W.M., 1982. Cretaceous climate: a comparison of atmospheric simulations with the geologic record. Palaeogeography, Palaeoclimatology, Palaeoecology 40, 103–133. Berner, R.A., 1991. A model for atmospheric CO2 over Phanerozoic time. American Journal of Science 291, 339–376. Berner, R.A., 1994. 3GEOCARB II: a revised model of atmospheric CO2 over Phanerozoic time. American Journal of Science 294, 56–91.

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