Asynchronous responses of East Asian and Indian summer monsoons to mountain uplift shown by regional climate modelling experiments
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It has been demonstrated in climate models that both the Indian and East Asian summer monsoons (ISM and EASM) are strengthened by the uplift of the entire Asian orography or Tibetan Plateau (TP) (i.e. bulk mountain uplift). Such an effect is widely perceived as the major mechanism contributing to the evolution of Asian summer monsoons in the Neogene. However, geological evidence suggests more diachronous growth of the Asian orography (i.e. regional mountain uplift) than bulk mountain uplift. This demands a re-evaluation of the relation between mountain uplift and the Asian monsoon in the geological periods. In this study, sensitivity experiments considering the diachronous growth of different parts of the Asian orography are performed using the regional climate model COSMO-CLM to investigate their effects on the Asian summer monsoons. The results show that, different from the bulk mountain uplift, the regional mountain uplift can lead to an asynchronous development of the ISM and EASM. While the ISM is primarily intensified by the thermal insulation (mechanical blocking) effect of the southern TP (Zagros Mountains), the EASM is mainly enhanced by the surface sensible heating of the central, northern and eastern TP. Such elevated surface heating can induce a low-level cyclonic anomaly around the TP that reduces the ISM by suppressing the lower tropospheric monsoon vorticity, but promotes the EASM by strengthening the warm advection from the south of the TP that sustains the monsoon convection. Our findings provide new insights to the evolution of the Asian summer monsoons and their interaction with the tectonic changes in the Neogene.
KeywordsMountain uplift East Asian summer monsoon (EASM) Indian summer monsoon (ISM) Regional climate model Asynchronous evolution Thermal effect
The Tibetan Plateau (TP) and the surrounding orography, which peak above 8,000 m, cover more than 70 % of the land surface in Asia. Most of the Asian orography was built in the Cenozoic as a result of the continental collisions between India (or Arabia) and Eurasia (Yin 2010). The growth of these mountains, especially the TP, has had crucial impact on the formation and development of the Asian monsoon climate on geological time scale (An et al. 2001; Kutzbach et al. 1989; Liu and Yin 2002; Prell and Kutzbach 1992; Ramstein et al. 1997; Zhang et al. 2007).
A prominent feature of the Asian monsoon climate is the warm, wet summer monsoon. It can be divided into the Indian and East Asian summer monsoon (EASM) systems, which share common features but also exhibit unique attributes. In general, the Indian summer monsoon (ISM) is embedded in tropical climate systems and the meridional Hadley circulation, while the EASM is more subject to the mid-latitude processes, such as westerly jet stream and frontal systems (Molnar et al. 2010). In spite of these differences, it has been demonstrated that both the ISM and EASM are strengthened by the increasing height of the entire TP or Asian orography (referred to as “bulk mountain uplift” hereafter) (e.g. An et al. 2001; Kutzbach et al. 1993; Song et al. 2010). In India, this is represented by the emergence and deepening of the monsoon trough over northern India, the enhancement of the low-level southwesterly flow over the Arabian Sea and India, and the northward migration of the Intertropical Convergence Zone (ITCZ) (Abe et al. 2003; An et al. 2001; Fluteau et al. 1999; Hahn and Manabe 1975). In East Asia, it is characterized by the intensification and northward penetration of the low-level southerly flow, the inland-ward expansion of the Western North Pacific (WNP) Subtropical High and the rain front along its northern periphery (e.g. An et al. 2001; Jiang et al. 2008; Kitoh 2004; Liu and Yin 2002). Such effects on the ISM and EASM have been largely attributed to the diabatic heating of the uplifted plateau which maintains an upper tropospheric thermal high pressure over the TP. This leads to a divergent flow in the upper troposphere and a low-level convergent flow around the plateau that brings precipitation to both Indian (Kutzbach et al. 1993; Li and Yanai 1996) and East Asian (Duan and Wu 2005; Duan et al. 2008; Wu et al. 2012) continent. The monsoonal precipitation can further enhance the upper-level thermal high pressure over the TP by latent heat release, therefore amplify the thermal pumping effect of the TP (Yanai and Wu 2006).
The influence of different parts of the Asian orography (referred to as “regional mountain uplift”) on the summer monsoons has been tentatively investigated in some modeling studies. Chakraborty et al. (2002) find that the presence of the western TP is more instrumental to the formation of the ISM than its eastern part. Boos and Kuang (2010) further show that the presence of the narrow orography of the Himalayas and the adjacent mountain ranges is sufficient to sustain an ISM of present-day strength. They inferred that it is the orographic insulation of low entropy extratropical air mass by the narrow mountain chains (i.e. “thermal insulation”) rather than the diabatic heating of the entire elevated plateau which maintains the ISM. They also noticed that the present-day EASM does not emerge with only the presence of such narrow mountain chains. This leads the authors to the contention that the formation of the EASM must be tied to the meridional extent of the TP and its mechanical impact on the westerly jet stream (Boos and Kuang 2010; Molnar et al. 2010). This idea, however, has recently been challenged by Wu et al. (2012), who emphasize the importance of the elevated heating of different regional mountains in driving either the ISM or the EASM.
So far, the more specific effects of the regional mountain uplift on both the ISM and EASM in different geological periods remain elusive. Most studies rely on global climate models, which, hampered by their coarse spatial resolution, cannot sufficiently capture the influence of a regional-scale mountain uplift. Compared to global models, regional climate models allow higher spatial resolutions and more detailed description of small-scale dynamical and physical processes, therefore, provide valuable tools in identifying the influence of meso-scale orographic features on the Asian summer monsoon climate (Chow et al. 2006; Gao et al. 2006; Park and Hong 2004; Shi et al. 2008; Song et al. 2010; Xie et al. 2006). In this study, a regional climate model—COSMO-CLM driven by the output of the fully-coupled atmosphere–ocean general circulation model (AOGCM) COSMOS—is employed to further explore the impact of regional mountain growth on the Asian summer monsoons. The model configuration and experimental design are described in Sect. 2. In Sect. 3, we show that distinct from the effect of bulk mountain uplift, the regional mountain uplift triggers asynchronous changes between ISM and EASM. The mechanisms for such asynchronous summer monsoon response are examined in Sect. 4. Finally, the implication of our results to the evolution of the Asian monsoon in the Neogene is discussed in Sect. 5.
2.1 The COSMO-CLM model and model setup
The COSMO-CLM is the climate mode of a non-hydrostatic regional weather prediction model COSMO (Consortium for Small-scale Modelling) (available at http://www.clm-community.eu). It can simulate the modern Asian monsoon circulation and precipitation well compared to other regional climate models (Dobler and Ahrens 2010; Lucas-Picher et al. 2011; Rockel and Geyer 2008) and has been applied to projecting the Asian summer monsoon changes into the future (Dobler and Ahrens 2011). The model uses a rotated geographical coordinate system. Its vertical domain is represented by a terrain-following hybrid coordinate system (η coordinate). Consistent with our previous regional model study on the Asian climate in the Late Miocene (11–7 Ma) (Tang et al. 2011), we adopted the model version 2.4.11 and chose the leapfrog numerics, the Tiedtke convection scheme (Tiedtke 1989), the radiation transfer scheme based on Ritter and Geleyn (1992), the prognostic turbulent kinetic energy closure (TKE) (Raschendorfer 2001) and the TERRA-ML multi-layer soil–vegetation–atmosphere-transfer model (Schrodin and Heise 2002) for all the experiments in this study. The model domain covers the Asian monsoon area (0–60°N and 50–140°E) with a spatial resolution of 1° × 1° on the rotated model grid (the north pole is at 60°N, 80°W) and 20 vertical levels.
2.2 Experiment design
Summary of the model experiments
No mountains (≤250 m above sea level)
The southern TP is added compared to M00
The central TP is added compared to MsTibet
The northern and southeastern TP are added compared to McTibet
The Zagros and Hindu Kush Mountains are added compared to MnTibet
The Tianshan and Gobi Altai Mountains are added compared to MZagros
50 % of the present-day height of all the orography
The present-day height of all the orography
Our regional model experiments are driven by 6-hourly output from both a present-day control run and a Late Miocene run performed in the AOGCM COSMOS (Eronen et al. 2009; Micheels et al. 2011). Compared to the present-day global control run, the Late Miocene run has lower orography globally. It represents a global climate generally warmer and wetter than at present and exhibits stronger mid-latitude westerlies throughout a year (Micheels et al. 2011). Particularly in Asia, the whole TP is reduced to about 70 % of its present-day height. Both the Indian and East Asian summer monsoon circulation is weaker than in the present-day run (Tang et al. 2011). However, summer precipitation in India is stronger in the Late Miocene run because of the high sea surface temperature over the Indian Ocean (Micheels et al. 2011). We expect that such difference in global forcings may affect the climatic response to mountain uplift in our regional model. However, such influence turns out to be small. Therefore, only the outcome driven by the Late Miocene global run is presented in this paper. The results under the present-day global forcing are briefly analyzed in Online Resource 3.
All the regional model experiments are integrated for 10 years. Because the initial adaptation of the upper-level soil moisture takes only a few months (Tang et al. 2011), the first year integration is left for the model to spin up, and the last 9-year results are used for analysis.
3 Effects of mountain uplift on summer monsoon
3.1 Bulk mountain uplift
3.2 Regional mountain uplift
3.2.1 Precipitation and 850 hPa wind
3.2.2 Monsoon indices
Correlation coefficients of the ISM and EASM indices (same as in Fig. 7) with precipitation in India, N-China and S-China in all the mountain uplift experiments
4 Mechanisms for asynchronous summer monsoon response
4.1 Thermal effect: elevated heating versus thermal insulation
The summer mean surface albedo and energy flux (W m−2) over the uplifted regions
Sensible heat flux
Latent heat flux
It shows that the enhanced heating over the southern TP can induce a cyclonic wind anomaly around the southern TP (Fig. 9a) and a great increase of precipitation over the southern TP. There is also a slight increase of precipitation over East Asia, but little change (or even slight suppression) of precipitation over India are observed. The insensitivity of the ISM to the elevated heating of the southern TP indicates that the “diabatic heating” may not be the major forcing for the enhancement of the ISM due to the presence of the southern TP (Fig. 4a). Other mechanisms, e.g. the “thermal insulation” effect (Boos and Kuang 2010), might be more important. As shown in Fig. 3a, without any orography, there is a northeasterly wind across the TP to northern India. This is a part of the eastern side of the subtropical anticyclone over western and central Asia. The presence of the southern TP blocks this northeasterly wind (Fig. 6b). According to Boos and Kuang (2010), this protects the high near-surface entropy air over India from the extratropical low entropy air, therefore maintains an upper-level thermal high over northern India that supports a stronger ISM.
The increased heating over the central and northern TP (Fig. 9b, c), also gives rise to a cyclonic wind anomaly around the TP, which greatly enhances the precipitation over East China, but reduces the precipitation over India (particularly the northern Bay of Bengal). The heating of the Tianshan-Altai Mountains also have similar effect (data not shown). This largely explains the changes of the ISM and EASM due to the presence of the central TP, northern TP and the Tianshan-Altai Mountains (cf. Figs. 4b, c, 9b, c). It is shown that the elevated heating of the central and northern TP is crucial for the strengthening of the EASM (Fig. 9b, c). This coincides with the studies by Duan et al. (2008) and Wu et al. (2012), but disagrees with the recent studies which imply that the heating of the plateau may have negligible influence on destabilizing the atmosphere over East Asia (Boos and Kuang 2010; Park et al. 2012).
In contrast to the thermal effect of the central and northern TP, the surface heating over the Zagros Mountains promotes the ISM (Fig. 9d). This agrees with previous studies (Wu et al. 2012; Zaitchik et al. 2007; Zarrin et al. 2011), which emphasize the importance of diabatic heating of the Zagros Mountains in lowering the sea level pressure and inducing the low-level cyclonic wind surround the mountains that may reinforce the ISM, particularly over northwestern India. However, such heating effect is relatively small compared to the full impact of the Zagros Mountains (cf. Figs. 4d, 9d). This indicates that the thermal insulation or the mechanical effect of the Zagros Mountains may also play a substantial role. As shown in Fig. 6d, without the Zagros Mountains, there is a strong northwesterly wind across the Zagros Mountain region to northern India. The presence of the Zagros Mountains blocks this flow (Fig. 6e). This may also act as a thermal insulator as proposed by Boos and Kuang (2010) that sustains a stronger ISM. However, compared to the effect of the southern TP which thermal insulation effect is dominant, the effect of the Zagros Mountains is different in several aspects. For instance, the upper-level thermal maximum over northern India is not as greatly enhanced by the presence of the Zagros Mountains as that by the southern TP (cf. Fig. 8c, f). There is strong westerly (northwesterly) wind anomaly to the north (east) of the TP due to the presence of the Zagros Mountains (Fig. 4d), which is not produced by the presence of the southern TP (Fig. 4a). Therefore, we argue that the mechanical blocking and deflecting of the low-level westerly flows around the TP by the Zagros Mountains might play a more prominent role in affecting the Asian summer monsoons (see more discussion in Sect. 4.2).
In the case of the uplift of the central and northern TP when the diabatic heating effect is pronounced, only a lower and middle level westerly (easterly) wind anomaly to the south (north) of the TP is observed (Fig. 10c, e). The ISM is suppressed primarily by the weakened low-level monsoon vorticity (as depicted by IMI in Fig. 7). The EASM, however, is greatly promoted (Fig. 9b, c). This has been attributed to the coupled lower (upper) level southerly (northerly) wind over E-China that favors ascending motion according to vorticity balance equation (Duan et al. 2008). But this interpretation does not explain the zonal elongated feature of the precipitation increase due to the presence of the central and northern TP (Fig. 4b, c).
4.2 Dynamical effects
It has been long recognized that the uplifted TP also acts as a physical barrier, which splits and deflects the mid-latitude westerly flow, excites topographic Rossby wave response and transient disturbance and hence, modifies the Asian monsoon climate (Hahn and Manabe 1975; Liu et al. 2007; Molnar et al. 2010; Park et al. 2012; Rodwell and Hoskins 2001; Trenberth and Chen 1988). However, since the mid-latitude westerlies weaken and displace northward in summer, the mechanical effect of the TP on the summer monsoons is thought to be much weaker than that on the winter monsoon (Yanai and Wu 2006). As discussed in previous section, the mechanical effect seems to be trivial compared to the thermal isulation or sensible heating effect in modulating the summer monsoons when the southern, central and northern TP are present. However, the mechanical effect of the Zagros Mountains may exert substantial impact on the ISM and EASM. The blocking of the low-level westerly flow by the Zagros Mountains to the south of the TP greatly facilitates the ISM vorticity (Fig. 10g), while suppresses the westerly flow and warm advection from the south of the TP to East Asia (Figs. 10h, 11e). Such effect is just opposite to that of the diabatic heating of the central and northern TP, leading to a stronger ISM but weaker EASM.
We also notice that although the mechanical effects of the central TP on the seasonal average of the summer monsoons are negligible, they are prominent in spring and early summer (e.g. June) when both the ISM and EASM are most subjective to the position and strength of the westerly jet (Sampe and Xie 2010; Schiemann et al. 2009). The mechanical effects of the central TP are similar to its diabatic heating effect that diminishes the ISM but promotes the EASM (Figure not shown).
4.3 Comparison with synchronous summer monsoon response
Compared to regional mountain uplift which gives rise to an out-of-phase summer monsoon changes in India and East Asia, the bulk mountain uplift generally intensifies both the ISM and EASM (Figs. 3, 7). Likewise, in the combined effect of several mountain regions, such as McTibet minus M00, MZagros minus MsTibet, or MTian minus McTibet, a synchronous strengthening of the ISM and EASM may also be observed (Figs. 5, 7).
We propose that such synchronous summer monsoon strengthening can only be induced when the uplifted mountains have reached to the extent that its elevated surface heating effect is sufficient to enhance the EASM, while the thermal insulation (or the mechanical blocking) effect of the mountains can be adequate to intensify the ISM. This is in contrast to the asynchronous summer monsoon response, which is produced because the extent of the uplifted mountains is relatively small and only their thermal insulation effect (e.g. MsTibet − M00), or the diabatic heating (e.g. MnTibet − McTibet) is at work.
As manifested in our results (Fig. 3), the increase of Indian summer rainfall is greater in the earlier stage of bulk mountain uplift (i.e. M50 − M00). In contrast, the East Asian summer rainfall appears to be more increased by the later stage of bulk mountain uplift (i.e. M100 − M50). This is in agreement with previous modeling study by An et al. (2001). We suggest that such non-linear responses of the two monsoon systems to the height of the mountains, can be particularly related to the mechanisms governing these two monsoons. The ISM is mainly intensified by the thermal insulation of the Asian orography that essentially offers a protection of the sub-cloud high entropy air over India from the low entropy extratropical air (Boos and Kuang 2010). According to this mechanism, the earlier stage of the bulk mountain uplift could have been sufficient to protect the low-level high entropy air over India therefore greatly enhances the ISM. This speculation is supported by the sensitivity experiments showing that increasing the height of the south TP from 0 to 50 % of the present-day height has much stronger impact on the ISM than that from 50 to 100 % of the present-day height (see Fig. S3 in Online Resource 2). In comparison, the EASM is mainly driven by the elevated surface heating of the TP. Such heating at higher elevation has a larger effect on atmospheric circulation than at lower elevation, because the relatively small mass of the overlying air at higher elevation (compared to that at lower elevation) can have greater temperature increase (decrease) per unit heating (cooling) (Kutzbach et al. 1993). As a result, the EASM can be more intensified during the later stage of the bulk mountain uplift. Nevertheless, we note that the sensitivity of the EASM and ISM to bulk mountain height is very model-dependent. While Kutzbach et al. (1989) document a linear response of the summer monsoons to bulk mountain height, Abe et al. (2003) and Kitoh (2004) find that the enhancement of the ISM (EASM) in the later stages of the bulk mountain uplift is larger (smaller) than in the earlier stages. More studies are needed to better understand such discrepancies.
5 Implication for summer monsoon evolution in the Neogene
6 Summary and conclusions
We note several limitations of our regional model experiments. Firstly, the regional model is unable to cover all the area that is vital for the Asian summer monsoons, such as the subtropical anticyclone over western Asia and Western North Pacific, and the tropical Indian Ocean. The one-way nesting of our regional model in the global model does not allow the interaction between the global circulation and orography. Both may affect the monsoon response to mountain uplift. Moreover, the atmospheric–ocean interaction has been shown to be of relevance in determining the effect of mountain uplift on the Asian monsoon (Kitoh 2004). Such interaction, however, is missing in our model. This may be responsible for the lack of changes over the sea with the mountain uplift in our results (Fig. 4). Finally, the scenarios of the regional mountain uplift in this study are not exhaustive. The order in which each mountain region appears may significantly affect their perceived effect on the Asian summer monsoon. For instance, if the Zagros Mountains were added before the presence of the central and northern TP, its impact on the ISM may not be as strong as that shown in our results (cf. Fig. 4d and Fig. S1 in Online Resource 2). In spite of these limitations, our findings still offer useful insights to the Asian monsoon evolution and its interactions with tectonic changes in the Neogene. In this study, we focused on the influence of the regional mountain uplift on summer monsoon. Its impact on winter monsoon is beyond the scope of the paper, but is worth investigating in the future.
We thank the Ella and Georg Ehrnrooth foundation for project funding. This work was also supported by the Humboldt Foundation (JTE) and the federal state Hessen (Germany) within the LOEWE initiative (AM). This is a contribution to the NECLIME framework. We acknowledge the model support of the CLM community especially from Andreas Will, Burkhardt Rockel and Hans-Jürgen Panitz. Our model experiments were performed at the Center for Scientific Computing (CSC) in Espoo (Finland) with special technical support from Juha Lento and Tommi Bergman. We would also like to thank Carl Fortelius, Liping Zhou, Anu Kaakinen, Juhani Rinne and the two anonymous reviewers for their valuable comments and fruitful discussions.
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