Interactive comment on “ Modeling the climatic implications of the Guliya δ 18 O record during the past 130 ka ” by D

Modeling the climatic implications of the Guliya δ18O record during the past 130 ka D. Xiao, P. Zhao, Y. Wang, and X. Zhou Chinese Academy of Meteorological Sciences, Beijing, 100081, China National Meteorological Information Center, Beijing, 100081, China State Key Laboratory of Marine Geology, Tongji University, Shanghai, 200092, China State Key Laboratory of Severe Weather, Beijing, 100081, China


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Full in the Antarctica (Lorius et al., 1985), the Greenland GRIP (Greenland Ice-Core Project Members, 1993) and GRIP2 ice cores (Grootes et al., 1993;Taylor et al., 1993) in the Arctic, the Devon Island ice cores in Canada (Paterson et al., 1977), and the Guliya ice cores at the North Tibetan Plateau (Thompson et al., 1997;Yao et al., 1997).These ice core records have all been used to reconstruct the record of environmental changes dating to the last interglacial stage.The major climatic events over the past 130 ka include the last interglacial stage, the last glacial stage, and a new interglacial stage, viz., the Holocene (Shi et al., 2000).The environmental changes recorded by the above ice cores greatly extend our temporal range since the last interglacial stage.Recently, the climatic implications of the ice core records have been increasingly investigated.
Among the long-term ice core records, the Guliya ice core is located in a northern subtropical region and is thus helpful for understanding the effects of climate change in the middle and low latitudes.The Guliya ice cap is the largest (with a total area of 376.1 km 2 ), the highest (with an elevation of 6700 m), and the thickest (with an average thickness of approximately 200 m and a maximum thickness of approximately 350 m) ice body found in the middle and low latitudes of the Northern Hemisphere (NH) (Yao et al., 1994(Yao et al., , 2000)).In 1992, Chinese and American scientists drilled into the Guliya glacier in the North Tibetan Plateau at 81.5 • E, 35.2 • N and retrieved three ice cores with lengths of 34.5, 93.2, and 308.6 m.The stable oxygen isotope records in these ice cores constitute reliable indicators of environmental change (Li et al., 2000).The environmental changes over the past 130 ka are recorded as fluctuations in the concentration of oxygen-18 (δ 18 O) in precipitation found in the Guliya ice core (hereinafter referred to as the Guliya δ 18 O).The observed monthly δ 18 O in Guliya precipitation is well correlated with local the monthly surface air temperature (SAT) during the past decades ) (Yao et al., 1996a).
Therefore, the Guliya δ 18 O record is considered to be representative of the temperature of the Tibetan Plateau (Yao et al., 1995(Yao et al., , 1996a)), which witnessed significant warm and cold periods, either in the interglacial stage or in the glacial stage, over the past 130 ka.The alternation of these warm and cold periods indicated obvious 23-ka cycles (Yao Introduction

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Full  , 1997).The Guliya δ 18 O has been used to investigate the features of the Younger Drays and Heinrich events and to compare with the Arctic and Antarctic records (Yang et al., 1997), and has also been used as a temperature index to investigate temporal structures over many periods including the orbital and sub-orbital scales, the millennial scale, the century scale, the decadal scale, and others (Yao et al., 1996b(Yao et al., , 1997(Yao et al., , 2001;;Shi et al., 1999).Moreover, due to the influence of precessional motion on the seasonal and latitudinal asymmetries of solar insolation (Milankovitch, 1969;Liu and Shi, 2009), the Guliya δ 18 O may contain climate signals for a certain season and at particular latitudes.For example, the Guliya δ 18 O record is closely related to June solar insolation at 60 • N.
However, June solar insolation at this latitude leads the Guliya δ 18 O trace by approximately one-quarter phase of the precession cycle (5 ka) (Yao et al., 1997), which suggests that the Guliya δ 18 O does not represent the June SAT.
Although the climatic implications of the Guliya δ 18 O record have been extensively investigated, the temporal and spatial climate ranges represented by the Guliya δ 18 O and the climatic relationship on longer timescales between the Guliya δ 18 O and global climate change remain unclear (Zhang et al., 1995;Yao et al., 1996a).Therefore, it is important to further investigate the temporal and spatial climate ranges represented by the Guliya δ 18 O and the indicative capacity of the Guliya δ 18 O for the SAT and sea surface temperature (SST) during the warm and cold phases of the 23-ka precession cycle.
In the context of the Milankovitch astronomic climate theory (Milankovitch, 1969), the latitudinal and seasonal cycles of solar insolation modulated by the earth orbital parameters (precession, obliquity, and eccentricity) are the ultimate driving forces for the millennial-and orbital-scale climate changes and glacial cycles.Employing climate models to investigate the responses of the earth climate system to this orbital forcing is useful for exploring the influences of these orbital parameters.Equilibrium simulations are often used to provide "snapshots" of the equilibrium climate for a prescribed insolation or orbital parameter (Kutzbach, 1981;Hewitt and Mitchell, 1998;Montoya et al., Introduction Conclusions References Tables Figures

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Full 2000; Liu et al., 2006).However, such equilibrium simulations are unable to simulate the forcing effects of the varying orbital parameters on the climate system.Thus, transient simulations are employed to study the influence of varying orbital parameters on climate (Lorenz and Lohmann, 2004;Kutzbach et al., 2008). In

Model and methods
This study employs version 2.9 of the UVic Earth System Climate Model (UVic Model) with a resolution of 3.6 • in longitude and 1.8 • in latitude.The UVic Model is an intermediate-complexity coupled atmosphere-ocean model (Weaver et al., 2001) in which the atmospheric model comprises a single-layer energy-moisture balance model and the ocean component utilizes version 2.2 of the GFDL Modular Ocean Model with 19 vertical levels.The UVic Model can capture several major features of global surface temperature and precipitation and has been widely used in paleoclimate research (Weaver et al., 2001;Matthews et al., 2004;Stouffer et al., 2006;Weber et al., 2007;Fyke et al., 2011).Transient simulations are used to display the evolution of climate changes over time.However, it is difficult to simulate a period of one hundred thousand years using the complicated coupled models to explore climate responses to orbital forcing.Thus, integrated acceleration schemes were applied in transient paleoclimate Introduction

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Full simulations (Jackson and Broccoli, 2003;Timm and Timmermann, 2007;Timmermann et al., 2007;Kutzbach et al., 2008).Lorenz and Lohmann (2004) found that acceleration factors of 10 and 100 yield similar results.The used time-dependent orbital forcing (due to changes in longitude of perihelion, axial tilt and eccentricity) was calculated according to Berger (1978).The UVic Model had been run for 200 model years under the prescribed orbital forcing.Then, we accelerate the orbital forcing by a factor of 100, i.e., the orbital parameters are advanced by 100 yr at the end of each year in the simulation.The accelerated simulation runs from 130.8 ka BP to the present.The 1308 time samples (corresponding to the period from 130.8 ka BP to the present) of the model output are analyzed in this study.
The temperature over the Guliya region in the model is the averaged SAT over the region (80-85 • E, 33-38 • N).Because there are 1308 model years in this simulation, the Guliya δ 18 O data obtained from Yao et al. (1997) are linearly interpolated over the 1308 model years.The smoothness of the time series of an approximately 51-yr moving mean of the Guliya δ 18 O is similar to that of the simulated Guliya temperature.
For the 51-yr moving mean time series over 1308 yr, a Monte Carlo Simulation gives the critical value of the correlation coefficient at the 95 % confidence level as 0.26.In this study, the late summer is defined as the period from August to September.
A correlation analysis was applied to examine the relation of the Guliya δ 18 O to the simulated SAT and SST.The lagging and leading correlations were also employed to examine the relationship between the two variables.A composite analysis was used to evaluate differences in the SAT and SST during the high and low phases of the 23-ka precession cycles in the Guliya SAT.A power spectrum analysis was performed to display the periods of the Guliya δ 18 O record using the REDFIT software in MAT-LAB (Schulz and Mudelsee, 2002).A squared wavelet coherence analysis (Grinsted et al., 2004) was used to determine the relative phase relationship between the Guliya temperature and the Guliya δ 18 O.Introduction

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Full It is known that the Guliya δ 18 O experienced several approximately 23-ka cycles in the past 130 ka that are directly related to varying earth orbital parameters (Yao et al., 1997).However, the solar insolation differs over the twelve months of the year due to precessional motion.Thus, we first examine the climate implications of the Guliya Figure 1 presents the annual cycles of the simulated SAT anomalies over the Guliya region over the past 130 ka.In this figure, the annual cycle of SAT anomalies is composed of approximately eleven vertical belts toward the right, with lifetimes similar to the 23-ka precession cycle, and the variability of the SAT anomalies is dominant in April to August, with the largest value generally observed in June.The June SAT anomalies vary in magnitude by approximately 6 • C.These results indicate that the distributions of solar insolation and SAT anomalies differ over the twelve months of a year.
Because June insolation at 60 • N is approximately one-quarter phase ahead of the Guliya δ 18 O (Yao et al., 1997), the phases of the precession cycle in the July-December temperature may be ahead of or synchronous with the variation in the Guliya δ 18 O.Here, we examine the relationship between the Guliya δ 18 O and the simulated Guliya SAT in June-December. Figure 2 presents the correlation coefficients between the Guliya δ 18 O and the simulated Guliya SAT.We observed positive correlation coefficients of 0.43 in August and 0.4 in September that are both significant at the 99 % confidence level.Figure 3  two significant periods at 43.6 ka and 20.762 ka and a peak of red noise at 10.634 ka (Fig. 5).It is evident that the two significant periods correspond to the periods of the precession and the obliquity, respectively, which indicates that the Guliya δ 18 O was mainly modulated by precessional motion and obliquity variation over the past 130 ka.The simulated Guliya SAT displays two main periods at 43.599 ka and 22.947 ka and a peak of red noise at 11.179 ka (Figure not shown), in which the precession period at 22.947 ka is more significant than the obliquity period.It can be seen that our simulation is able to successfully capture the main periods of the Guliya δ 18 O. Figure 6 displays the squared wavelet coherence between the Guliya δ 18 O and the simulated late-summer Guliya SAT.The precession and obliquity cycles in the Guliya δ 18 O are manifested in the Guliya SAT as wavelengths of approximately 23 ka and 43 ka because the arrows at these periods generally point toward to the right, which indicates in-phase relationships between the simulated Guliya SAT and the Guliya δ 18 O in both the 23-ka precession cycle and the 43-ka obliquity cycle.In the 23-ka period, the arrows deflect downward (indicating a phase angle less than 90 • ) during the 45-20 ka BP (Fig. 6) that correspond to the fifth peak of the Guliya δ 18 O in Fig. 4, which indicates that the simulated Guliya SAT and the Guliya δ 18 O are not in phase during this period.
These results suggest that the Guliya δ 18 O record generally represents the precession and obliquity cycles and their in-phase relationships over the past 130 ka, and represents the late-summer temperature not only over the Tibetan Plateau and its adjacent areas but also over the NH.

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Full Figure 7a displays the composite difference of the simulated late-summer SATs between 11-6 ka BP and 23-18 ka BP (11-6 ka BP minus 23-18 ka BP), which corresponds to the difference between the early Holocene and the Last Glacial Maximum.Compared with the cold phase in 23-18 ka BP, the SAT in the warm phase during 11-6 ka BP is significantly warmer over the NH middle and lower latitudes, and large temperature anomalies occur over the Asian and African continents, with a maximum value of approximately 1.4 • C. The temperature anomalies over the Southern Hemisphere (SH) and the ocean are weaker than those over the NH and land.The anomalies generally increase toward the Arctic.A similar anomalous pattern also occurs between 59-54 ka BP and 72-67 ka BP (Fig. 7c), and between 105-100 ka BP and 117-112 ka BP (Fig. 7e); that is, there are higher (lower) temperature anomalies over the NH (SH) when the late-summer Guliya SAT is high.However, another anomalous pattern in the SAT also occurs between the high and low late-summer Guliya SAT cases.Figure 7b presents the composite difference in the SATs between 33-28 ka BP and 45-40 ka BP.In this figure, the SAT anomalies generally show an increase toward the equator, and relative to 45-40 ka BP, the SAT in the 33-28 ka BP period was warmer over the majority of regions except the NH high latitudes, with anomalous warm centers above 1.2 • C in Africa and South America.
A similar anomalous pattern is also observed in Fig. 7d.This result reveals the presence of different anomalous patterns associated with the Guliya temperature during the warm and cool phases of the precession cycle.Why do these two anomalous SAT patterns occur?As the obliquity cycle significantly modulates the earth's climate (Short et al., 1991), especially at high latitudes, and is more evident in the Arctic, we compare the temporal evolutions of the Arctic and the Guliya SATs in an attempt to understand the contributions of the precession and obliquity cycles to the two anomalous patterns between the warm and cold phases.former is more significant than the latter (Figure not shown).Thus, the Arctic temperature may well reflect an obliquity influence.As illustrated in Fig. 8, the warm and cold phases of the precession cycle in the Guliya temperature, associated with the first type of anomalous SAT pattern, are generally superposed on those of the obliquity cycle in the Arctic temperature, while the warm and cold phases of the precession cycle in the Guliya temperature, associated with the second type of anomalous SAT pattern, are not superposed on those of the obliquity cycles in the Arctic temperature.Therefore, the occurrence of two anomalous patterns in the SAT may be associated with the superposition of the precession cycle and the obliquity cycle.Specifically, the first anomalous SAT pattern is influenced both by the precession and obliquity cycles, and the second anomalous SAT pattern is mainly influenced by the precession cycle.Figure 9 presents the annual cycles of the modeled Guliya SAT for five warm and cold phases and their differences.It appears that the annual cycles show similar features for each warm and cold phase, with the highest temperatures in June and July and the lowest temperatures in December and January.Compared with the cold phases, the Guliya temperature during the warm phases is generally higher in the warm season (May-September) and lower in the cold season (October-April) (Fig. 9a, c-e), which indicates a larger seasonal variation in the warm phases than in the cold phases.For example, the annual ranges of July (January) temperatures between the warm and cold phases varied from 0.5 (0) • C (Fig. 9b) to 4 (2) • C (Fig. 9e).These results also exhibit a larger temperature difference between the warm and cold phases in summer than in winter.Moreover, the difference of the annual cycles in the Guliya SAT between the warm and cold phases in the late-summer Guliya SAT shows the similar feature (Fig. 9), which indicates no obvious influence of the obliquity cycle on the annual cycles in the Guliya SAT.Further analysis reveals that the NH temperature displays similarly varying features.Therefore, the warm-season (cold-season) Guliya and NH temperatures are higher (lower) in the warm phases than in the cold phases.Introduction

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SST anomalies associated with the Guliya temperature
Similar to the SAT, the SST anomalies between the warm and cold phases also show two different features.One occurs between 11-6 ka BP and 23-18 ka BP (Fig. 10a), between 59-54 ka BP and 72-67 ka BP (Fig. 10c), and between 105-100 ka BP and 117-112 ka BP (Fig. 10e).This pattern is characterized by a warmer NH ocean and a cooler SH ocean with SSTs increasing toward the Arctic.Another feature occurs between 33-28 ka BP and 45-40 ka BP (Fig. 10b) and between 83-78 ka BP and 94-89 ka BP (Fig. 10d), with a warmer SST at the middle and low latitudes and a cooler SST near the high latitude, which indicate increasing SSTs toward the equator.The alternation of two anomalous SST patterns also displays an approximately 43-ka obliquity cycle.Similar to the cause of the anomalous patterns in the SAT field, the first anomalous SST pattern shows contributions of both the precession and obliquity cycles, while the second anomalous SST pattern is mainly influenced by the precession cycle.
Figure 11 presents the correlation coefficients between the Guliya δ 18 O and the SST in the late summer over the past 130 ka.Here, significant positive correlation coefficients mainly appear in the North Atlantic, the Bering Sea, the Bay of Alaska, and the Arctic region to the north of Europe, with the maximum correlation coefficient exceeding 0.57 in the North Atlantic.These results suggest a close link between the Guliya temperature and the NH (particularly the Atlantic) SST in the past 130 ka.
How do the SST anomalies associated with the late-summer Guliya SAT evolve? Figure 12a presents the leading and lagging correlation coefficients between the Guliya temperature and the North Atlantic SST.The Guliya temperature lags the North Atlantic SST by 2.5 ka (Fig. 12a), which possibly indicates an effect of the North Atlantic SST on the Guliya temperature.What physical processes are possibly responsible for this influence?One explanation is given below.
Because the δ 18 O variable is present in the local precipitation, the Guliya precipitation may be closely related to the local temperature.Figure 13 illustrates the simulated

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Full late-summer SAT and precipitation curves in the Guliya region.Both precipitation and SAT display significant cycles of approximately 23 ka, and there is an out-of-phase relationship between precipitation and SAT, with a correlation coefficient of −0.95.All the peaks (valleys) in precipitation correspond to valleys (peaks) in SAT.Moreover, the variation of late-summer SAT generally lags by 1 ka relative to that of late-summer precipitation (Fig. 12b), demonstrating that the Guliya late-summer precipitation leads the Guliya temperature.Several prior studies have shown that the summer North Atlantic SST modulates atmospheric circulation over the NH and affects Asian precipitation on the decadal or longer time scales (Sutton and Hodson, 2005;Dong et al., 2006;Wang et al., 2009;Feliks et al., 2011).Therefore, the anomalous signal from the North Atlantic SST may be reflected in the Guliya precipitation record and thus affect the Guliya SAT.Such an effect leads to a close link between the Guliya SAT and the North Atlantic SST, in which the Asian precipitation may act as a "bridge" linking the North Atlantic SST and the Guliya temperature.

Summary and conclusion
We employed the UVic Model with accelerated orbital forcing by a factor of 100 to examine the climate implications and indicative nature of the Guliya δ 18 O record for the past 130 ka.The simulated late-summer (August-September) Guliya temperature effectively captures the variation and the major periods of the Guliya δ 18 O including the 43-ka obliquity and 23-ka precession cycles, with a correlation coefficient of 0.47 between the Guliya δ 18 O and the modeled late-summer Guliya temperature.The Guliya the precession and obliquity cycles, and another pattern shows an increase toward the equator, possibly attributed to the precession cycle.In the annual cycles, the Guliya and NH summer (winter) temperatures are warmer (cooler) in the warm phases of Guliya late-summer SAT than in the cold phases and the influence of the obliquity cycle on the annual cycles in the Guliya temperature is not significant.
Moreover, the Guliya temperature is closely related to the North Atlantic SST, lags the North Atlantic SST by approximately 2.5 ka, and lags the Guliya precipitation by 1 ka.These results suggest an effect of the North Atlantic SST on Asian precipitation and then on the Guliya temperature.Thus, the Guliya precipitation may be a "bridge" connecting the local temperature with the North Atlantic SST.Because the UVic Model does not exhibit atmospheric vertical circulation features, studies using a more complex coupled ocean-land-atmosphere model are required in the future.Figures Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | et al.
Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 Temporal and spatial climatic implications of the Guliya δ 18 O further illustrates the distribution of correlation coefficients between the Guliya δ 18 O and the simulated late-summer SAT.Here, significant positive correlation coefficients cover the majority of the NH and exceed 0.48 at latitudes of 45• -80• N.Figure 4 presents the time series of the Guliya δ 18 O and the simulated late-summer Guliya SAT.Both clearly present an approximately 23-ka precession cycle and show an in-phase relationship during the majority of the studied time period, except for 45-20 ka, with a positive correlation coefficient of 0.47 (exceeding the 99.9 % confidence level).The power spectrum of the Guliya δ 18 O reveals the presence of CPD Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure 8 shows the simulated temporal curves for the Arctic (0-360 • , 80-90 • N) SAT and the Guliya SAT during the past 130 ka.The Arctic air temperature displays both the 43.6-ka obliquity and 21.8-ka precession cycles, and the Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

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18 O is also significantly correlated with the simulated NH late-summer SAT.Thus, the Guliya δ 18 O record represents the temperature not only over the Tibetan Plateau but also over the NH.Epochal differences between the warm and cold phases of the Guliya air temperature indicate two types of anomalous patterns in the SAT and SST fields.One pattern shows an increase in the SAT toward the Arctic, possibly associated with the joint influence of Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .
Fig. 1.Annual cycles of the simulated Guliya SAT anomalies relative to the climatological mean over the past 130 ka.The abscissa is time before the present, and the ordinate is the month of a year.