- Email: [email protected]
Northward extension of the East Asian summer monsoon during the mid-Holocene
Global and Planetary Change 184 (2020) 103046
Contents lists available at ScienceDirect
Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha
Review Article
Northward extension of the East Asian summer monsoon during the midHolocene
T
Jinling Piaoa, Wen Chena,b, , Lin Wangc, Francesco S.R. Pausatad, Qiong Zhange ⁎
a
Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China c Key Laboratory of Regional Climate-Environment for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China d Centres ESCER (Étude et la Simulation du Climat à l'Échelle RÉgionale) and GEOTOP (Research Center on the dynamics of the Earth System), Department of Earth and Atmospheric Sciences, University of Quebec in Montreal, Quebec, Canada e Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden b
ARTICLE INFO
ABSTRACT
Keywords: Mid-Holocene East Asian summer monsoon Green Sahara
Previous studies based on multiple paleoclimate archives suggested that during the mid-Holocene (MH, ~ 6000 years before present day), the East Asian summer monsoon (EASM) had a prominent intensification and northward extension. However, current climate model simulations with orbital forcing alone present an underestimation of the magnitude of changes in the EASM. In the current work, we show that considering a vegetated and dust-reduced Sahara in the MH can significantly strengthen the EASM intensity and expand its northernmost boundary northward compared to the results with orbital forcing alone. The vegetation change over the Sahara is the dominant factor for the variation in the EASM, while the dust reduction plays a smaller role. The vegetated Sahara causes a westward shift of the Walker circulation, accompanied with enhancement of the western Pacific subtropical high (WPSH), which then results in a strengthened EASM. On one hand, the change in the Walker circulation induces decreased rainfall over the western equatorial Pacific, intensifying the WPSH through the Gill-Matsuno response. On the other hand, the shift in the Walker circulation is associated with a stronger local Hadley circulation, reinforcing the WPSH. Finally, our results show that the westward expansion of the WPSH is mainly caused by the local strengthening of the Hadley circulation.
1. Introduction As a major component of the Asian climate system, the East Asian summer monsoon (EASM) can affect rainfall intensity not only at regional scale (Huang and Sun, 1992; Kwon et al., 2007; Zhou et al., 2005; Ding et al., 2008; Chen et al., 2013, 2017) but also far afield through atmospheric and oceanic teleconnections (Lau, 1992; Liu et al., 2004; Wang et al., 2005). Due to its importance on both the regional and global scale, much effort has been devoted to investigating future climate change associated with the EASM (Min et al., 2004; Sun and Ding, 2010; Seo et al., 2013; Lee and Wang, 2014; Qu et al., 2014). For the projection of the EASM-related precipitation under a global warming scenario, positive anomalies are identified over central and northern China, the Korean Peninsula and Japan (Min et al., 2006; Kripalani et al., 2007; Kusunoki and Arakawa, 2012; Qu et al., 2014), with negative signals over the East China Sea (Seo et al., 2013). The possible influencing factors might be the warming of the sea surface temperature over the tropical Pacific and Indian Ocean (Hu et al., 2000; ⁎
Ueda et al., 2006), and the strenghtening of the North Pacific subtropical high under future conditions (Kusunoki and Arakawa, 2012; Chen and Sun, 2013; Seo et al., 2013; Lee and Wang, 2014). Previous studies demonstrated that the EASM response to past climate change plays a crucial role in constraining potential future changes (Maher and Hu, 2006; Maher, 2008; Jiang et al., 2013). The mid-Holocene (MH, approximately 6000 years before present) is one of the mostinvestigated periods. During the MH, changes in Earth's orbital parameters increased the amount of incoming boreal summer solar radiation at the top of atmosphere in comparison with present day (Berger, 1988; COHMAP, 1988). The EASM was significantly strengthened during the MH with increased (decreased) rainfall over northern (southern) China according to various paleoclimate archives (Shi et al., 1993; Zhou et al., 2004; Sun et al., 2010; Wang et al., 2014; Goldsmith et al., 2017). Liu et al. (2015) employed diverse proxy records from Northern China and indicated that the EASM reached its maximum intensity during the MH on geological and orbital time scales. Goldsmith et al. (2017) also inferred that the EASM shows a striking intensification and northward
Corresponding author at: Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (W. Chen).
https://doi.org/10.1016/j.gloplacha.2019.103046 Received 23 April 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 Available online 17 October 2019 0921-8181/ © 2019 Elsevier B.V. All rights reserved.
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
extension during the early and middle Holocene, using a closed-basin lake area record over northeastern China. Generally, the variation of orbital forcing is believed to dominate the EASM change during the Holocene, with summer solar radiation modulating the land-sea thermal contrast (Feng et al., 2004; Xiao et al., 2006; Selvaraj et al., 2007; Wen et al., 2010; Jin et al., 2014). Consistent with the paleoclimate archives, the simulated EASM during the MH shows remarkable enhancement (Wei and Wang, 2004; Wang et al., 2010; Jiang et al., 2013; Zheng et al., 2013). Jiang et al. (2013) demonstrated that 27 of 28 models within the Paleoclimate Modeling Intercomparison Project (PMIP) simulated stronger EASM during the MH than during the reference pre-industrial period, with the intensity increased by 32% on average. Using ten coupled models participating in PMIP3, Zheng et al. (2013) also identified stronger EASM features with intensified southerly winds over eastern China during the MH, which then led to the increase in summer rainfall over northern China via intensified northward water vapor transportation. However, MH simulations generally fail to fully reproduce the extent of the EASM variation (Wang et al., 2010; Bartlein et al., 2011; Zhao and Harrison, 2012; Zheng et al., 2013), and other global-scale changes that occurred during the MH, in particular the West African Monsoon strenghtening (Harrison et al., 2014) as well as the likely decrease in El Niño–Southern Oscillation variability (Rodbell et al., 1999; Conroy et al., 2008; Koutavas and Joanides, 2012; White et al., 2018). For instance, Goldsmith et al. (2017) used the precipitataion increase in northern China as representation of the East Asian monsoon extent during the MH, and suggested that PMIP3 models underestimate the annual precipitation amounts by up to 50% compared to paleoclimate records. They pointed out the covariation of the intensity and the northward extent of the East Asian summer monsoon on oribital and millennial timescales. Earlier studies also showed that previous PMIP2 simulations present an underestimation of Mid-Holocene summer monsoon precipitation in Asian monsoon regions, including India and East Asia (Braconnot et al., 2012; Zhao and Harrison, 2012). The source of the discrepancy must lie in that most paleoclimate simulations follow the Paleoclimate Modeling Intercomparison Project Phase/Coupled Model Intercomparison Project (PMIP/CMIP) Protocol (Taylor, 2009), which uses the prescribed preindustrial vegetation cover and dust emissions and ignores the large differences of the two fields between the MH and the preindustrial (McGee et al., 2013; Harrison et al., 2014; Hely et al., 2014). Prentice et al. (2000) shows that steppe, savanna, and xerophytic woods and scrubs once covered the present-day Sahara during the MH, with an expansion of North African lakes and wetlands. deMenocal et al. (2000) and McGee et al. (2013) present a decrese of over 60% in terrigenous fluxes from the Sahara relative to present day. Recent studies have shown that changes in both land cover and dust emission may help filling the gap between data and models (Pausata et al., 2016; Gaetani et al., 2017; Pausata et al., 2017; Egerer et al., 2018; Messori et al., 2019). For example, through a set of sensitivity studies perfomed using a Earth system model, Pausata et al. (2017) shows that the ENSO activity is significantly decreased by 25% when vegetated Sahara and dust-reduced concentrations are accounted for compared to the 10% decrease simulated when only orbital forcing are considered. As the EASM has a close connection with the ENSO activity (Ju and Slingo, 1995; Wang et al., 2000; Lau and Wu, 2001; Wang et al., 2001; Ding and Chan, 2005; Li and Zhou, 2012), we decided to investigate how the vegetated and dust-reduced Sahara may influence the simulation performance of the EASM variation during the MH. The present study mainly analyzes the relative impacts of increased vegetation cover over the Sahara and reduced dust emissions on the EASM during the MH and the potential mechanisms underlying the simulated changes. Section 2 contains a brief introduction to the data and methods. Section 3 presents the influence of the vegetation cover change over the Sahara on the northward shift of the EASM system and the mechanism behind their relationship. The discussion and conclusions are included in Section 4.
2. Data and methods 2.1. Model description The numerical simulations are performed using a fully coupled global climate model, version 3 of the EC-Earth (Hazeleger et al., 2010). The atmospheric model is based on the Integrated Forecast System (IFS cycle 36r4) together with the H-TESSEL land model, which is developed by the European Centre for Medium-range Weather Forecasts. The simulations are carried out at T159 horizontal spectral resolution (approximately 1.125° × 1.125°) with 62 vertical levels. In addition, we used version 3.3.1 of the Ocean General Circulation Model-NEMO (Madec, 2008) as the ocean component, which has a horizontal resolution of approximately 1° × 1° and 46 vertical levels. The surface part is coupled with the Louvain-la-Neuve Ice Model-LIM3 (Vancoppenolle et al., 2009) for every model hour. The coupler OASIS3 (Valcke, 2006) is employed for the coupling between the atmospheric (IFS) and oceanic models (NEMO-LIM). EC-Earth has been widely applied in climate simulations for the past and future time periods (Bosmans et al., 2012; Wouters et al., 2012), contributing to a better understanding of the mechanisms of climatic variations and projection of future changes. Previous studies have proven the good capability of EC-Earth in reproducing monsoonal precipitation (Bosmans et al., 2012; Pausata et al., 2016), the Walker circulation and tropical situations over the Pacific (Bayr et al., 2014; Pausata et al., 2017). 2.2. Experimental design We adopt the model simulation perfomed in Pausata et al. (2016), where a set of four MH experiments were carried out together with a reference pre-industrial climate (PI) using EC-Earth. The standard MH simulation (MHOB) was performed following the PMIP3/CMIP5 protocol (Taylor, 2009), with boundary conditions set at preindustrial values except for the orbital forcing and greenhouse gases. The orbital forcing is set at values for 6000 years BP and the methane concentration is changed from 760 ppmv for PI to 650 ppmv. The second simulation (MHGS) was carried out with shrubland covering the Sahara domain (11°–33°N, 15°W–35°E). Such vegetation change is related to a reduction in surface albedo from 0.3 to 0.15 and an increase in the leaf area index from 0.2 to 2.6. In the third experiment (MHRD), the PI dust concentration were reduced by up to 80% over the Sahara desert and its surrounding regions, in accordance with the decrease in Saharan dust flux during the MH estimated in recent studies (deMenocal et al., 2000; McGee et al., 2013), but vegetation cover was prescribed as in the PI and MHOB eperiment. Then, in the last experiment (MHGS+RD) both changes in vegetation over the Sahara and reduction in dust concentration were considered. A more detailed description of the experimental design can be found in Pausata et al. (2016). The model simulations are carried out for nearly 300 years, with initial conditions derived from a 700-year preindustrial spin-up run. The climate quasiequilibrium was reached after 100–200 years and only the last 100 years of each sensitivity experiment are analyzed in this study. 3. Results 3.1. EASM changes in the MH experiments The experiment in which only orbital forcing is considered (MHOB), shows a northward extension and an intensification of the EASM relative to PI simulation, with anomalous easterlies over the equatorial western Pacific, southerlies over the eastern coast of Asia and southeasterlies over Northeast Asia (Fig. 1a). The results agree well with those of previous studies, in terms of both the amplitude and the spatial pattern of the wind anomalies (e.g., Jiang et al., 2013). The vegetation change over the Sahara and dust reduction (MHGS+RD) further 2
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 1. Summer (June to August) mean anomalies in horizontal wind at 850 hPa (m/s, vectors) between the (a) MHOB, (b) MHGS+RD, (c) MHGS, (d) MHRD and the reference PI experiment. The shadings denote the differences significant at the 95% confidence level by the Student t-test. Wind differences smaller than 0.3 m/s are not shown.
strengthen the EASM intensity and expand the EASM domain northward, causing anticyclonic anomalies over the East China Sea (Fig. 1b). In the MHGS simulation, the result is quite similar to that of MHGS+RD in regard to both spatial distributions and amplitude (Fig. 1c). For the situation of MHRD, the EASM variations do not show notable changes compared with those in MHOB (Fig. 1d). The summer precipitation changes over the EASM region show a good correspondence to the wind anomalies (Fig. 2). Positive anomalies exist over northeastern and southern Asia, and negative ones over northwestern Mongolia, the Bay of Bengal and the western part of the Pacific in the MHOB relative to PI (Fig. 2a). The simulated anomalies in MHGS+RD resemble those of MHOB; however they are amplified in particular over southern (more negative) and northern (more positive) China (Fig. 2b), which is in accordance with the stronger southerlies over the EASM domain (Fig. 1b). The precipitation anomalies between MHGS and MHGS+RD are very similar in both magnitude and spatial aspects in agreement with the identical wind patterns (Fig. 2c). In addition, weak precipitation anomalies can be found in MHRD (Fig. 2d), which are even weaker than those in MHOB (Fig. 2a). Given the similarity between MHGS and MHGS+RD anomalies, the vegetation cover change is the dominant factor affecting the strengthening and northward extension of the EASM during the MH. Since much work has already been done on the strengthening of the EASM during the MH (Wang et al., 2010; Zhao and Harrison, 2012; Jiang et al., 2013), we focus on the impacts of the vegetated Sahara on the northward shift of the EASM and the possible mechanisms. To better depict the northward extension of the EASM system, a climatological northern boundary index (CNBI) is adopted, which is defined as the summer (May–September) 2 mm day-1 isoline (Chen et al., 2018). The good capability of this index to describe the EASM northern boundary and capture the major shifts of the summer monsoon circulation has been demonstrated (Chen et al., 2018). Here, we mainly focus on the variations within 100°–120°E. The climatological mean position of the EASM northern boundary presents a northward extension in both MHOB and MHGS compared with PI, with a more northward position identified in the MHGS experiment (Fig. 3). Considering a more quantitative view, we further calculated the climatological mean position of the CNBI within different sections of the PI,
MHOB and MHGS. The results show that the differences within 100°–105°E and 115°–120°E are much smaller than those within 105°–110°E and 110°–115°E. Within 105°–110°E, the northern boundary has a northward extension of approximately 68 (193) km in the MHOB (MHGS) compared to that in the PI. For the 110°–115°E section, the northward extension reaches 155 (362) km for the MHOB (MHGS). The change in vegetation cover over the Sahara induces the further northward shift of the CNBI within 105°–110°E (110°–115°E) by approximately 125 (207) km compared to that of the orbital forcing alone. These results indicate the prominent role of the vegetated Sahara on the northward expansion of the EASM system during the MH. The western Pacific subtropical high (WPSH) is one of the major components of the EASM system, and the intensity and spatial pattern of the WPSH are closely related to the onset, withdrawal and the circulation anomalies of the EASM (Hwang and Yue, 1962; Tao and Chen, 1987; Huang and Wu, 1989; Ninomiya and Kobayashi, 1999). The values of geopotential height within the western Pacific in MHGS are larger than those in MHOB and PI, which indicates the strengthening of the WPSH in the MH and in particular in the MHGS experiment (Fig. 4). In addition, the 5820-gpm isoline is taken as a reference to measure the position change of the WPSH, the western boundary of which reaches 135°E for the PI (Fig. 4a) and 120°E in the MHOB (Fig. 4b) while stretching into eastern China in the MHGS at approximately 110°E (Fig. 4c). The WPSH experiences a remarkable intensification and westward extension under the impact of the forcing of the vegetated Sahara (Fig. 4). This favors the strengthening of the prevailing southerly wind over eastern China, contributing to the northward shift of the EASM. 3.2. Saharan vegetation cover change and the strength of the WPSH During the MH, summer rainfall presents positive anomalies over North Africa (deMenocal et al., 2000; Wu et al., 2007; Pausata et al., 2016). Such increase in precipitation is associated with a divergent flow over North Africa in the upper atmosphere for both the MHOB and MHGS (Fig. 5a, b). A negative center is instead found over the equatorial eastern Pacific, which is stronger in the MHGS (Fig. 5b) relative to the MHOB (Fig. 5a). To emphasize the role of the change in vegetation 3
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 2. Summer (June to August) mean anomalies in precipitation (shadings, mm/day) between the (a) MHOB, (b) MHGS+RD, (c) MHGS, (d) MHRD and the reference PI experiment. The dotted areas denote the differences significant at the 95% confidence level by the Student t-test.
Fig. 3. The climatological mean position of the EASM northern boundary in the PI (black solid line), the MHOB (purple dotted line), and the MHGS (green dotted line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cover, the differences are further calculated between the MHGS and MHOB (Fig. 5c). The positive and negative centers can still be identified over North Africa and the equatorial eastern Pacific, indicating the strengthening of the Walker circulation when the desert areas are turned into shrubland over the Sahara. Zhou et al. (2009) investigated the westward extension of the WPSH since the late 1970s and demonstrated that the Indian Ocean-western Pacific warming intensifies the Walker circulation, which then modulates the WPSH through the ENSO/Gill response. Hence, the Walker circulation might be a factor contributing to the relationship between the Saharan land surface change and the changes in WPSH intensity. To better understand the Walker circulation changes, we also calculate the the differences in the mean vertical velocity profile between
Fig. 4. The climatological mean of geopotential height at 500 hPa in the (a) PI, (b) MHOB and (c) MHGS (contours: 10 m interval from 5010 to 5040 m).
4
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 5. Summer (June to August) mean anomalies in the velocity potential (contours, 105 m2 s−1) and divergent wind (vectors, m/s) at 200 hPa between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The shadings denote the differences in the velocity potential significant at the 95% confidence level by the Student t-test.
20°S and 20°N (Fig. 6). By comparison, the Walker circulation is intensified and shifted to the west in both the MHOB and MHGS compared to that in the PI (Fig. 6a, b). In addition, the differences between MHGS and MHOB show anomalous ascending motions over the equatorial Indian Ocean and the equatorial eastern Pacific, and anomalous descending motions over the equatorial western Pacific and the equatorial Atlantic (Fig. 6c). This indicates a westward shift of the Walker circulation under the forcing of the Saharan vegetation cover change. A vegetated and less dusty Sahara during the MH lead to the strengthening of the West African Monsoon (WAM). The intensification of the WAM and the northward shift of the Inter-Tropical Convergence Zone result in a decreased upwelling (warming, i.e. Atlantic Niño) and SST variability in the western equatorial Atlantic (Pausata et al., 2017). The Atlantic Niño response and the reduction of the equatorial Atlantic SST variability then induce the westward shift of the Walker circulation, as also mentioned in many studies on present day climate (e.g., RodríguezFonseca et al., 2009; Li et al., 2016). Correspondingly, rainfall anomalies exhibit negative values over the equatorial western Pacific and the equatorial Atlantic with positive values over the subtropical Pacific and North Africa in the MHOB and MHGS (Fig. 7a, b). Over the Indian Ocean, significant discrepancies in rainfall anomalies can be identified between the MHOB and MHGS. In the MHOB, positive values are simulated over the equatorial western Indian Ocean with negative values over the equatorial eastern Indian Ocean relative to PI (Fig. 7a). In the MHGS experiment, a dipole rainfall pattern is shown with positive anomalies in the northern Indian Ocean and negative anomalies in the southern Indian Ocean relative to PI (Fig. 7b). The rainfall differences between the MHGS and MHOB present similar spatial patterns (Fig. 7c), but with positive rainfall anomalies over the equatorial Indian Ocean, consistent with the local anomalous ascending motions (Fig. 6c) caused by the Walker circulation change. As the WPSH appears to be the maximum in the low-level stream function field during the boreal summer (Lu and Dong, 2001; Zhou et al., 2009), we further display the differences of the nondivergent wind and stream function at 850 hPa between sensitivity experiments and the PI (Fig. 8). Cyclonic anomalies are identified over the North
Fig. 6. Summer (June to August) mean anomalies in the longitude-height cross section of the vertical velocity (shadings, Pa/s) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The differences are shown as 20°S–20°N average. The contours denote the climatological mean of the vertical velocity in the PI.
Africa in the MHOB and MHGS in response to the postive rainfall anomalies, with stronger intensity for the latter one (Fig. 8a, b). This implies that the latent heat released by the positive rainfall anomalies might act as heating sources and force the anomalous atmospheric circulation following the study of Gill (1980). A subtropical anticyclonic circulation center is located over the western Pacific (Fig. 8a, b), which can still be recognized with the Saharan vegetation forcing alone (Fig. 8c). The anomalous anticyclonic circulation seems to contribute to the westward extension and intensification of the WPSH during the MH, the formation of which might be related to the negative heating source implied by the decreased rainfall over the equatorial western Pacific (Fig. 7c) through the Gill-Matsuno response. In addition, other studies provided evidence that the WPSH variation can result from the anomalous convection induced by changes in the Hadley and Walker circulations (Wang, 1995; Li, 1997; Chang et al., 2000). In consideration of the close relationship between the Hadley and Walker circulations revealed by previous studies (Nigam and Shen, 1993; Oort and Yienger, 1996), the differences of the vertical velocity averaged between 110°–140°E are displayed to represent the change of the Hadley circulation (Fig. 9). In the MHOB, anomalous upward motions are identified along the equator, with downward motions approximately around 10°–20°N (Fig. 9a). In the MHGS simulation, the anomalies mainly lie in the apparent subtropical descending motions along 30°N (Fig. 9b), which is a sign of the intensified Hadley 5
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 7. Summer (June to August) mean anomalies in precipitation (mm/day) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The dotted areas denote the differences significant at the 95% confidence level by the Student t-test.
Fig. 9. Summer (June to August) mean anomalies in the latitude-height cross section of the wind (m/s) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The vertical velocity is multiplied by a factor of 100. The differences are shown as the average between 110°–140°E.
to the westward shift of the Walker circulation, which then induces the westward extension and intensification of the WPSH through the GillMatsuno response and an intensified local Hadley circulation, resulting in the northward extension of the EASM system. Then, what is the relative importance of the Hadley circulation and the negative heating source over the equatorial western Pacific? The differences in the stream function at 200 hPa and 850 hPa between the MHGS and MHOB, show positive centers over the subtropical northwestern Pacific (Fig. 10), indicating a barotropic vertical structure. As the Rossby response to the negative heating anomalies is baroclinic, this suggests that the strengthened local Hadley circulation plays a more important role in the westward extension of the WPSH. Furthermore, the center of the downward motions is quite in phase with the anticyclonic circulation center at 850 hPa over the subtropical northwestern Pacific (Fig. 10b). Based on the study of Chung et al. (2011), this phase consistency inferred that the intensified WPSH might be a direct response to the anomalous local Hadley circulation, confirming the larger influence of the local Hadley circulation.
Fig. 8. Summer (June to August) mean anomalies in the stream function (contours, 105 m2 s−1) and non-divergent wind (vectors, m/s) at 850 hPa between the (a) MHOB, (b) MHGS and PI experiment and between (c) the MHGS and MHOB. The shadings denote the differences in the stream function significant at the 95% confidence level by the Student t-test.
4. Summary and discussions
circulation. The differences between the MHGS and MHOB further confirm the prominent impacts of the Saharan vegetation cover change on the strengthening of the Hadley circulation with the upward and downward motions found over the equator and the subtropics, respectively (Fig. 9c). The downward motions within the ridge area favor the variation in WPSH during the MH. Hence, a vegetated Sahara leads
Previous studies indicate that the models fail to fully reproduce the extent of the EASM variations during the MH, which might stem from the fact that the large differences in the vegetation cover and dust concentrations between the MH and the preindustrial are not taken into account. Here, with a vegetated Sahara and reduce dust concentrations, the EASM presents an intensification and northward extension 6
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
during the MH is clearly improved by considering the vegetation cover change over the Sahara desert. Hence, it is important for model simulations to include the Saharan vegetation cover change to obtain a better reproduction of climatic variations during the MH. This study, therefore, suggests that having a good simulation of the vegetation cover may also be relevant for a better projections of future climate change. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We thank the three anonymous reviewers for their valuable comments and suggestions, which led to significant improvement in the manuscript. This work was supported jointly by the Swedish Research Council (VR) project “Simulating Green Sahara With Earth System Model” (2017–04232), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT: CH2016-6711), the National Natural Science Foundation of China (Grants 41721004 and 41875115), and the Jiangsu Collaborative Innovation Center for Climate Change. F.S.R.P. acknowledges funding from the Swedish Research Council (FORAMS) as part of the Joint Programming Initiative on Climate and the Belmont Forum for the project “Palaeo-constraints on Monsoon Evolution and Dynamics (PACMEDY) and the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC Grant RGPIN-2018-04981). The EC-Earth model simulations were performed on a supercomputer provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and Cray XC30 HPC systems at ECMWF.
Fig. 10. Summer (June to August) mean anomalies in the stream function (contours, 105 m2 s−1) between the MHGS and MHOB at (a) 200 hPa and (b) 850 hPa. The shadings denote the differences in the vertical velocity at 500 hPa (shadings, Pa/s) between the MHGS and MHOB.
compared with the simulation where only orbital forcing are considered. A climatological northern boundary index is further employed to describe the variations in the northern boundary of the EASM system. The results show that with a green Sahara state, the distance of the mean EASM northern boundary between the MHGS and MHOB can reach approximately 125 (207) km between 105°–110°E (110°–115°E), indicating that the vegetated Sahara can exert a great impact on the northward extension of the EASM system during the MH. The intensification and westward extension of the WPSH is regarded as the bridge between the Saharan vegetation cover change and the northward extension of the EASM during the MH. The possible mechanism behind the impact of the vegetated Sahara on the WPSH can be expressed as follows:
References Bartlein, P.J., Harrison, S.P., Brewer, S., Connor, S., Davis, B.A.S., Gajewski, K., Guiot, J., Harrison-Prentice, T.I., Henderson, A., Peyron, O., Prentice, I.C., Scholze, M., Seppa, H., Shuman, B., Sugita, S., Thompson, R.S., Viau, A.E., Williams, J., Wu, H., 2011. Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis. Clim. Dyn. 37 (3–4), 775–802. Bayr, T., Dommenget, D., Martin, T., Power, S.B., 2014. The eastward shift of the Walker Circulation in response to global warming and its relationship to ENSO variability. Clim. Dyn. 43 (9–10), 2747–2763. Berger, A., 1988. Milankovitch theory and climate. Rev. Geophys. 26 (4), 624–657. Bosmans, J.H.C., Drijfhout, S.S., Tuenter, E., Lourens, L.J., Hilgen, F.J., Weber, S.L., 2012. Monsoonal response to mid-holocene orbital forcing in a high resolution GCM. Clim. Past 8 (2), 723–740. Braconnot, P., Harrison, S.P., Kageyama, M., Bartlein, P.J., Masson-Delmotte, V., AbeOuchi, A., Otto-Bliesner, B., Zhao, Y., 2012. Evaluation of climate models using palaeoclimatic data. Nat. Clim. Chang. 2 (6), 417. Chang, C.P., Zhang, Y.S., Li, T., 2000. Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of the subtropical ridge. J. Clim. 13 (24), 4310–4325. Chen, H., Sun, J., 2013. Projected change in East Asian summer monsoon precipitation under RCP scenario. Meteorog. Atmos. Phys. 121 (1–2), 55–77. Chen, W., Feng, J., Wu, R., 2013. Roles of ENSO and PDO in the link of the East Asian Winter Monsoon to the following Summer Monsoon. J. Clim. 26 (2), 622–635. Chen, J., Wen, Z., Wu, R., Wang, X., He, C., Chen, Z., 2017. An interdecadal change in the intensity of interannual variability in summer rainfall over southern China around early 1990s. Clim. Dyn. 48, 191–207. Chen, J., Huang, W., Jin, L., Chen, J., Chen, S., Chen, F., 2018. A climatological northern boundary index for the East Asian summer monsoon and its interannual variability. Sci. China Earth Sci. 61 (1), 13–22. Chung, P.-H., Sui, C.-H., Li, T., 2011. Interannual relationships between the tropical sea surface temperature and summertime subtropical anticyclone over the western North Pacific. J. Geophys. Res. 116. COHMAP, 1988. Climatic changes of the last 18,000 years: observations and model simulations. Science 1043–1052. Conroy, J.L., Overpeck, J.T., Cole, J.E., Shanahan, T.M., Steinitz-Kannan, M., 2008. Holocene changes in eastern tropical Pacific climate inferred from a Galápagos lake sediment record. Quat. Sci. Rev. 27 (11−12), 1166–1180. deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quat. Sci. Rev. 19 (1–5), 347–361.
1. Previous studies suggested that the vegetated Sahara could induce the westward shift of the Walker circulation through an Atlantic Niño response and reduced equatorial Atlantic SST variability. The westward shift of the Walker circulation has corresponding rainfall anomalies, with positive values over the equatorial eastern Indian Ocean and the North Africa and negative values over the equatorial western Pacific. The negative rainfall anomalies over the equatorial western Pacific could act as a negative heating source. This then forces anticyclonic circulation over the western Pacific based on the Gill-Matsuno response and favors the intensification of the WPSH. 2. Corresponding to the westward shift of the Walker circulation, the local Hadley circulation is strengthened through the anomalous heating source over the equatorial eastern Indian Ocean, with ascending motions located over the equator and descending motions over the subtropics. The descending branch can further contribute to the intensification of the WPSH. In addition, further analysis shows that the strengthened local Hadley circulation exerts more important influence on the WPSH variation during the MH, compared to the role of the Rossby response to the negative heating anomalies over the equatorial western Pacific. In conclusion, our study suggests that the simulation of the EASM system 7
Global and Planetary Change 184 (2020) 103046
J. Piao, et al. Ding, Y.H., Chan, J.C.L., 2005. The East Asian summer monsoon: an overview. Meteorog. Atmos. Phys. 89 (1–4), 117–142. Ding, Y., Wang, Z., Sun, Y., 2008. Inter-decadal variation of the summer precipitation in East China and its association with decreasing Asian summer monsoon. Part I: Observed evidences. Int. J. Climatol. 28 (9), 1139–1161. Egerer, S., Claussen, M., Reick, C., 2018. Rapid increase in simulated North Atlantic dust deposition due to fast change of northwest African landscape during the Holocene. Clim. Past 14 (7), 1051–1066. Feng, Z.D., An, C.B., Tang, L.Y., Jull, A.J.T., 2004. Stratigraphic evidence of a Megahumid climate between 10,000 and 4000 years BP in the western part of the Chinese Loess Plateau. Glob. Planet. Chang. 43 (3–4), 145–155. Gaetani, M., Messori, G., Zhang, Q., Flamant, C., Pausata, F.S.R., 2017. Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene. J. Clim. 30 (19), 7621–7642. Gill, A.E., 1980. Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc. 106 (449), 447–462. Goldsmith, Y., Broecker, W.S., Xu, H., Polissar, P.J., Demenocal, P.B., Porat, N., Lan, J., Cheng, P., Zhou, W., An, Z., 2017. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proc. Natl. Acad. Sci. U. S. A. 114 (8), 1817–1821. Harrison, S.P., Bartlein, P.J., Brewer, S., Prentice, I.C., Boyd, M., Hessler, I., Holmgren, K., Izumi, K., Willis, K., 2014. Climate model benchmarking with glacial and midHolocene climates. Clim. Dyn. 43 (3–4), 671–688. Hazeleger, W., Severijns, C., Semmler, T., Stefanescu, S., Yang, S., Wang, X., Wyser, K., Dutra, E., Baldasano, J.M., Bintanja, R., Bougeault, P., Caballero, R., Ekman, A.M.L., Christensen, J.H., van den Hurk, B., Jimenez, P., Jones, C., Kallberg, P., Koenigk, T., McGrath, R., Miranda, P., Van Noije, T., Palmer, T., Parodi, J.A., Schmith, T., Selten, F., Storelvmo, T., Sterl, A., Tapamo, H., Vancoppenolle, M., Viterbo, P., Willen, U., 2010. EC-Earth a Seamless Earth-System Prediction Approach in Action. Bull. Amer. Meteor. Soc. 91 (10), 1357–1363. Hely, C., Lezine, A.M., Contributors, A.P.D., 2014. Holocene changes in African vegetation: tradeoff between climate and water availability. Clim. Past 10 (2), 681–686. Hu, Z.Z., Latif, M., Roeckner, E., Bengtsson, L., 2000. Intensified Asian summer monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations. Geophys. Res. Lett. 27 (17), 2681–2684. Huang, R.H., Sun, F.Y., 1992. Impacts of the tropical western Pacific on the East Asian summer monsoon. J. Meteor. Soc. Japan 70 (1B), 243–256. Huang, R.H., Wu, Y., 1989. The influence of ENSO on the summer climate change in china and its mechanism. Adv. Atmos. Sci. 6 (1), 21–32. Hwang, S., Yue, T., 1962. On the structure of the sub-tropical highs and some associated aspects of the general circulation of atmosphere. Acta Meteor. Sin. 31 (4), 339–359. Jiang, D., Lang, X., Tian, Z., Ju, L., 2013. Mid-Holocene East Asian summer monsoon strengthening: Insights from Paleoclimate Modeling Intercomparison Project (PMIP) simulations. Palaeogeogr. Palaeoclimatol. Palaeoecol. 369, 422–429. Jin, L., Schneider, B., Park, W., Latif, M., Khon, V., Zhang, X., 2014. The spatial–temporal patterns of Asian summer monsoon precipitation in response to Holocene insolation change: a model-data synthesis. Quat. Sci. Rev. 85, 47–62. Ju, J., Slingo, J., 1995. The Asian summer monsoon and ENSO. Quart. J. Roy. Meteor. Soc. 121 (525), 1133–1168. Koutavas, A., Joanides, S., 2012. El Niño–Southern oscillation Extrema in the holocene and last glacial maximum. Paleoceanography 27 (4). Kripalani, R., Oh, J., Chaudhari, H., 2007. Response of the East Asian summer monsoon to doubled atmospheric CO 2: coupled climate model simulations and projections under IPCC AR4. Theor. Appl. Climatol. 87 (1–4), 1–28. Kusunoki, S., Arakawa, O., 2012. Change in the precipitation intensity of the East Asian summer monsoon projected by CMIP3 models. Clim. Dyn. 38 (9–10), 2055–2072. Kwon, M., Jhun, J.G., Ha, K.J., 2007. Decadal change in east Asian summer monsoon circulation in the mid-1990s. Geophys. Res. Lett. 34 (21). Lau, K.-M., 1992. East Asian summer monsoon rainfall variability and climate teleconnection. J. Meteor. Soc. Japan 70 (1B), 211–242. Lau, K., Wu, H., 2001. Principal modes of rainfall–SST variability of the Asian summer monsoon: a reassessment of the monsoon–ENSO relationship. J. Clim. 14 (13), 2880–2895. Lee, J.-Y., Wang, B., 2014. Future change of global monsoon in the CMIP5. Clim. Dyn. 42 (1–2), 101–119. Li, T., 1997. Phase transition of the El Niño–Southern Oscillation: a stationary SST mode. J. Atmos. Sci. 54 (24), 2872–2887. Li, X., Zhou, W., 2012. Quasi-4-year coupling between El Niño-Southern Oscillation and water vapor transport over East Asia-WNP. J. Clim. 25, 5879–5891. Li, X., Xie, S.-P., Gille, S.T., Yoo, C., 2016. Atlantic-induced pan-tropical climate change over the past three decades. Nat. Clim. Chang. 6 (3), 275. Liu, Z., Harrison, S., Kutzbach, J., Otto-Bliesner, B., 2004. Global monsoons in the midHolocene and oceanic feedback. Clim. Dyn. 22 (2–3), 157–182. Liu, J., Chen, J., Zhang, X., Li, Y., Rao, Z., Chen, F., 2015. Holocene East Asian summer monsoon records in northern China and their inconsistency with Chinese stalagmite δ18O records. Earth-Sci. Rev. 148, 194–208. Lu, R., Dong, B., 2001. Westward extension of North Pacific subtropical high in summer. J. Meteor. Soc. Japan 79 (6), 1229–1241. Madec, G., 2008. NEMO ocean engine, Note du Pole de modélisation, Institut PierreSimon Laplace (IPSL), France, No 27. (ISSN-1288-1619). Maher, B.A., 2008. Holocene variability of the East Asian summer monsoon from Chinese cave records: a re-assessment. Holocene 18 (6), 861–866. Maher, B.A., Hu, M., 2006. A high-resolution record of Holocene rainfall variations from the western Chinese Loess Plateau: antiphase behaviour of the African/Indian and East Asian summer monsoons. Holocene 16 (3), 309–319. McGee, D., Winckler, G., Stuut, J., Bradtmiller, L., 2013. The magnitude, timing and
abruptness of changes in North African dust deposition over the last 20,000 yr. Earth Planet. Sci. Lett. 371, 163–176. Messori, G., Gaetani, M., Zhang, Q., Zhang, Q., Pausata, F.S.R., 2019. The water cycle of the mid-Holocene West African Monsoon: the role of vegetation and dust emission changes. Int. J. Climatol. 39 (4), 1927–1939. Min, S.-K., Park, E.-H., Kwon, W.-T., 2004. Future projections of East Asian climate change from multi-AOGCM ensembles of IPCC SRES scenario simulations. J. Meteor. Soc. Japan 82 (4), 1187–1211. Min, S.-K., Legutke, S., Hense, A., Cubasch, U., Kwon, W.-T., Oh, J.-H., Schlese, U., 2006. East Asian climate change in the 21st century as simulated by the coupled climate model ECHO-G under IPCC SRES scenarios. J. Meteor. Soc. Japan 84 (1), 1–26. Nigam, S., Shen, H.-S., 1993. Structure of oceanic and atmospheric low-frequency variability over the tropical Pacific and Indian Oceans. Part I: COADS observations. J. Clim. 6 (4), 657–676. Ninomiya, K., Kobayashi, C., 1999. Precipitation and moisture balance of the Asian summer monsoon in 1991. J. Meteor. Soc. Japan 77 (1), 77–99. Oort, A.H., Yienger, J.J., 1996. Observed interannual variability in the Hadley circulation and its connection to ENSO. J. Clim. 9 (11), 2751–2767. Pausata, F.S.R., Messori, G., Zhang, Q., 2016. Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period. Earth Planet. Sci. Lett. 434, 298–307. Pausata, F.S.R., Zhang, Q., Muschitiello, F., Lu, Z., Chafik, L., Niedermeyer, E.M., Stager, J.C., Cobb, K.M., Liu, Z., 2017. Greening of the Sahara suppressed ENSO activity during the mid-Holocene. Nat. Commun. 8, 16020. Prentice, I.C., Jolly, D., Participants, B., 2000. Mid-Holocene and glacial-maximum vegetation geography of the northern continents and Africa. J. Biogeogr. 27 (3), 507–519. Qu, X., Huang, G., Zhou, W., 2014. Consistent responses of East Asian summer mean rainfall to global warming in CMIP5 simulations. Theor. Appl. Climatol. 117, 123–131. Rodbell, D.T., Seltzer, G.O., Anderson, D.M., Abbott, M.B., Enfield, D.B., Newman, J.H., 1999. An~ 15,000-year record of El Niño-driven alluviation in southwestern Ecuador. Science 283 (5401), 516–520. Rodríguez-Fonseca, B., Polo, I., García-Serrano, J., Losada, T., Mohino, E., Mechoso, C.R., Kucharski, F., 2009. Are Atlantic Niños enhancing Pacific ENSO events in recent decades? Geophys. Res. Lett. 36 (20). Selvaraj, K., Chen, C.T.A., Lou, J.Y., 2007. Holocene East Asian monsoon variability: links to solar and tropical Pacific forcing. Geophys. Res. Lett. 34 (1). Seo, K.-H., Ok, J., Son, J.-H., Cha, D.-H., 2013. Assessing future changes in the East Asian summer monsoon using CMIP5 coupled models. J. Clim. 26 (19), 7662–7675. Shi, Y.F., Kong, Z.C., Wang, S.M., Tang, L.Y., Wang, F.B., Yao, T.D., Zhao, X.T., Zhang, P.Y., Shi, S.H., 1993. Mid-Holocene climates and environments in China. Glob. Planet. Chang. 7 (1–3), 219–233. Sun, Y., Ding, Y., 2010. A projection of future changes in summer precipitation and monsoon in East Asia. Sci. China Earth Sci. 53 (2), 284–300. Sun, Q., Wang, S., Zhou, J., Chen, Z., Shen, J., Xie, X., Wu, F., Chen, P., 2010. Sediment geochemistry of Lake Daihai, north-central China: implications for catchment weathering and climate change during the Holocene. J. Paleolimnol. 43 (1), 75–87. Tao, S., Chen, L., 1987. A review of recent research on the East Asian summer monsoon in China. Monsoon Meteorology 60–92. Taylor, K.E., 2009. A summary of the CMIP5 experiment design. http://cmip-pcmdi.llnl. gov/cmip5/docs/Taylor_CMIP5_design.pdf. Ueda, H., Iwai, A., Kuwako, K., Hori, M.E., 2006. Impact of anthropogenic forcing on the Asian summer monsoon as simulated by eight GCMs. Geophys. Res. Lett. 33 (6). Valcke, S., 2006. OASIS3 user guide (prism_2-5). In: PRISM Support Initiative Report. 3. pp. 64. Vancoppenolle, M., Fichefet, T., Goosse, H., Bouillon, S., Madec, G., Maqueda, M.A.M., 2009. Simulating the mass balance and salinity of Arctic and Antarctic Sea ice. 1. Model description and validation. Ocean Model 27 (1–2), 33–53. Wang, B., 1995. Interdecadal changes in El Nino onset in the last four decades. J. Clim. 8 (2), 267–285. Wang, B., Wu, R., Fu, X., 2000. Pacific–East Asian teleconnection: how does ENSO affect East Asian climate? J. Clim. 13 (9), 1517–1536. Wang, B., Wu, R., Lau, K., 2001. Interannual variability of the Asian summer monsoon: contrasts between the Indian and the western North Pacific–East Asian monsoons. J. Clim. 14 (20), 4073–4090. Wang, P., Clemens, S., Beaufort, L., Braconnot, P., Ganssen, G., Jian, Z., Kershaw, P., Sarnthein, M., 2005. Evolution and variability of the Asian monsoon system: state of the art and outstanding issues. Quat. Sci. Rev. 24 (5–6), 595–629. Wang, T., Wang, H., Jiang, D., 2010. Mid-Holocene East Asian summer climate as simulated by the PMIP2 models. Palaeogeogr. Palaeoclimatol. Palaeoecol. 288 (1–4), 93–102. Wang, H., Chen, J., Zhang, X., Chen, F., 2014. Palaeosol development in the Chinese Loess Plateau as an indicator of the strength of the East Asian summer monsoon: evidence for a mid-Holocene maximum. Quat. Int. 334, 155–164. Wei, J., Wang, H., 2004. A possible role of solar radiation and ocean in the mid-Holocene East Asian monsoon climate. Adv. Atmos. Sci. 21 (1), 1–12. Wen, R., Xiao, J., Chang, Z., Zhai, D., Xu, Q., Li, Y., Itoh, S., 2010. Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas 39 (2), 262–272. White, S.M., Ravelo, A.C., Polissar, P.J., 2018. Dampened El Niño in the early and MidHolocene due to Insolation-Forced Warming/Deepening of the Thermocline. Geophys. Res. Lett. 45 (1), 316–326. Wouters, B., Drijfhout, S., Hazeleger, W., 2012. Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH. Clim. Dyn. 39 (11), 2695–2712. Wu, H., Guiot, J., Brewer, S., Guo, Z., 2007. Climatic changes in Eurasia and Africa at the
8
Global and Planetary Change 184 (2020) 103046
J. Piao, et al. last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling. Clim. Dyn. 29 (2–3), 211–229. Xiao, J., Wu, J., Si, B., Liang, W., Nakamura, T., Liu, B., Inouchi, Y., 2006. Holocene climate changes in the monsoon/arid transition reflected by carbon concentration in Daihai Lake of Inner Mongolia. Holocene 16 (4), 551–560. Zhao, Y., Harrison, S.P., 2012. Mid-Holocene monsoons: a multi-model analysis of the inter-hemispheric differences in the responses to orbital forcing and ocean feedbacks. Clim. Dyn. 39 (6), 1457–1487. Zheng, W., Wu, B., He, J., Yu, Y., 2013. The East Asian summer monsoon at midHolocene: results from PMIP3 simulations. Clim. Past 9 (1), 453–466.
Zhou, W., Yu, X., Jull, A.T., Burr, G., Xiao, J., Lu, X., Xian, F., 2004. High-resolution evidence from southern China of an early Holocene optimum and a mid-Holocene dry event during the past 18,000 years. Quat. Res. 62 (1), 39–48. Zhou, W., Chan, J.C.L., Li, C.Y., 2005. South China Sea summer monsoon onset in relation to the off-equatorial ITCZ. Adv. Atmos. Sci. 22, 665–676. Zhou, T., Yu, R., Zhang, J., Drange, H., Cassou, C., Deser, C., Hodson, D.L.R., SanchezGomez, E., Li, J., Keenlyside, N., Xin, X., Okumura, Y., 2009. Why the western Pacific subtropical high has extended westward since the late 1970s. J. Clim. 22 (8), 2199–2215.
9
Contents lists available at ScienceDirect
Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha
Review Article
Northward extension of the East Asian summer monsoon during the midHolocene
T
Jinling Piaoa, Wen Chena,b, , Lin Wangc, Francesco S.R. Pausatad, Qiong Zhange ⁎
a
Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China c Key Laboratory of Regional Climate-Environment for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China d Centres ESCER (Étude et la Simulation du Climat à l'Échelle RÉgionale) and GEOTOP (Research Center on the dynamics of the Earth System), Department of Earth and Atmospheric Sciences, University of Quebec in Montreal, Quebec, Canada e Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden b
ARTICLE INFO
ABSTRACT
Keywords: Mid-Holocene East Asian summer monsoon Green Sahara
Previous studies based on multiple paleoclimate archives suggested that during the mid-Holocene (MH, ~ 6000 years before present day), the East Asian summer monsoon (EASM) had a prominent intensification and northward extension. However, current climate model simulations with orbital forcing alone present an underestimation of the magnitude of changes in the EASM. In the current work, we show that considering a vegetated and dust-reduced Sahara in the MH can significantly strengthen the EASM intensity and expand its northernmost boundary northward compared to the results with orbital forcing alone. The vegetation change over the Sahara is the dominant factor for the variation in the EASM, while the dust reduction plays a smaller role. The vegetated Sahara causes a westward shift of the Walker circulation, accompanied with enhancement of the western Pacific subtropical high (WPSH), which then results in a strengthened EASM. On one hand, the change in the Walker circulation induces decreased rainfall over the western equatorial Pacific, intensifying the WPSH through the Gill-Matsuno response. On the other hand, the shift in the Walker circulation is associated with a stronger local Hadley circulation, reinforcing the WPSH. Finally, our results show that the westward expansion of the WPSH is mainly caused by the local strengthening of the Hadley circulation.
1. Introduction As a major component of the Asian climate system, the East Asian summer monsoon (EASM) can affect rainfall intensity not only at regional scale (Huang and Sun, 1992; Kwon et al., 2007; Zhou et al., 2005; Ding et al., 2008; Chen et al., 2013, 2017) but also far afield through atmospheric and oceanic teleconnections (Lau, 1992; Liu et al., 2004; Wang et al., 2005). Due to its importance on both the regional and global scale, much effort has been devoted to investigating future climate change associated with the EASM (Min et al., 2004; Sun and Ding, 2010; Seo et al., 2013; Lee and Wang, 2014; Qu et al., 2014). For the projection of the EASM-related precipitation under a global warming scenario, positive anomalies are identified over central and northern China, the Korean Peninsula and Japan (Min et al., 2006; Kripalani et al., 2007; Kusunoki and Arakawa, 2012; Qu et al., 2014), with negative signals over the East China Sea (Seo et al., 2013). The possible influencing factors might be the warming of the sea surface temperature over the tropical Pacific and Indian Ocean (Hu et al., 2000; ⁎
Ueda et al., 2006), and the strenghtening of the North Pacific subtropical high under future conditions (Kusunoki and Arakawa, 2012; Chen and Sun, 2013; Seo et al., 2013; Lee and Wang, 2014). Previous studies demonstrated that the EASM response to past climate change plays a crucial role in constraining potential future changes (Maher and Hu, 2006; Maher, 2008; Jiang et al., 2013). The mid-Holocene (MH, approximately 6000 years before present) is one of the mostinvestigated periods. During the MH, changes in Earth's orbital parameters increased the amount of incoming boreal summer solar radiation at the top of atmosphere in comparison with present day (Berger, 1988; COHMAP, 1988). The EASM was significantly strengthened during the MH with increased (decreased) rainfall over northern (southern) China according to various paleoclimate archives (Shi et al., 1993; Zhou et al., 2004; Sun et al., 2010; Wang et al., 2014; Goldsmith et al., 2017). Liu et al. (2015) employed diverse proxy records from Northern China and indicated that the EASM reached its maximum intensity during the MH on geological and orbital time scales. Goldsmith et al. (2017) also inferred that the EASM shows a striking intensification and northward
Corresponding author at: Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail address: [email protected] (W. Chen).
https://doi.org/10.1016/j.gloplacha.2019.103046 Received 23 April 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 Available online 17 October 2019 0921-8181/ © 2019 Elsevier B.V. All rights reserved.
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
extension during the early and middle Holocene, using a closed-basin lake area record over northeastern China. Generally, the variation of orbital forcing is believed to dominate the EASM change during the Holocene, with summer solar radiation modulating the land-sea thermal contrast (Feng et al., 2004; Xiao et al., 2006; Selvaraj et al., 2007; Wen et al., 2010; Jin et al., 2014). Consistent with the paleoclimate archives, the simulated EASM during the MH shows remarkable enhancement (Wei and Wang, 2004; Wang et al., 2010; Jiang et al., 2013; Zheng et al., 2013). Jiang et al. (2013) demonstrated that 27 of 28 models within the Paleoclimate Modeling Intercomparison Project (PMIP) simulated stronger EASM during the MH than during the reference pre-industrial period, with the intensity increased by 32% on average. Using ten coupled models participating in PMIP3, Zheng et al. (2013) also identified stronger EASM features with intensified southerly winds over eastern China during the MH, which then led to the increase in summer rainfall over northern China via intensified northward water vapor transportation. However, MH simulations generally fail to fully reproduce the extent of the EASM variation (Wang et al., 2010; Bartlein et al., 2011; Zhao and Harrison, 2012; Zheng et al., 2013), and other global-scale changes that occurred during the MH, in particular the West African Monsoon strenghtening (Harrison et al., 2014) as well as the likely decrease in El Niño–Southern Oscillation variability (Rodbell et al., 1999; Conroy et al., 2008; Koutavas and Joanides, 2012; White et al., 2018). For instance, Goldsmith et al. (2017) used the precipitataion increase in northern China as representation of the East Asian monsoon extent during the MH, and suggested that PMIP3 models underestimate the annual precipitation amounts by up to 50% compared to paleoclimate records. They pointed out the covariation of the intensity and the northward extent of the East Asian summer monsoon on oribital and millennial timescales. Earlier studies also showed that previous PMIP2 simulations present an underestimation of Mid-Holocene summer monsoon precipitation in Asian monsoon regions, including India and East Asia (Braconnot et al., 2012; Zhao and Harrison, 2012). The source of the discrepancy must lie in that most paleoclimate simulations follow the Paleoclimate Modeling Intercomparison Project Phase/Coupled Model Intercomparison Project (PMIP/CMIP) Protocol (Taylor, 2009), which uses the prescribed preindustrial vegetation cover and dust emissions and ignores the large differences of the two fields between the MH and the preindustrial (McGee et al., 2013; Harrison et al., 2014; Hely et al., 2014). Prentice et al. (2000) shows that steppe, savanna, and xerophytic woods and scrubs once covered the present-day Sahara during the MH, with an expansion of North African lakes and wetlands. deMenocal et al. (2000) and McGee et al. (2013) present a decrese of over 60% in terrigenous fluxes from the Sahara relative to present day. Recent studies have shown that changes in both land cover and dust emission may help filling the gap between data and models (Pausata et al., 2016; Gaetani et al., 2017; Pausata et al., 2017; Egerer et al., 2018; Messori et al., 2019). For example, through a set of sensitivity studies perfomed using a Earth system model, Pausata et al. (2017) shows that the ENSO activity is significantly decreased by 25% when vegetated Sahara and dust-reduced concentrations are accounted for compared to the 10% decrease simulated when only orbital forcing are considered. As the EASM has a close connection with the ENSO activity (Ju and Slingo, 1995; Wang et al., 2000; Lau and Wu, 2001; Wang et al., 2001; Ding and Chan, 2005; Li and Zhou, 2012), we decided to investigate how the vegetated and dust-reduced Sahara may influence the simulation performance of the EASM variation during the MH. The present study mainly analyzes the relative impacts of increased vegetation cover over the Sahara and reduced dust emissions on the EASM during the MH and the potential mechanisms underlying the simulated changes. Section 2 contains a brief introduction to the data and methods. Section 3 presents the influence of the vegetation cover change over the Sahara on the northward shift of the EASM system and the mechanism behind their relationship. The discussion and conclusions are included in Section 4.
2. Data and methods 2.1. Model description The numerical simulations are performed using a fully coupled global climate model, version 3 of the EC-Earth (Hazeleger et al., 2010). The atmospheric model is based on the Integrated Forecast System (IFS cycle 36r4) together with the H-TESSEL land model, which is developed by the European Centre for Medium-range Weather Forecasts. The simulations are carried out at T159 horizontal spectral resolution (approximately 1.125° × 1.125°) with 62 vertical levels. In addition, we used version 3.3.1 of the Ocean General Circulation Model-NEMO (Madec, 2008) as the ocean component, which has a horizontal resolution of approximately 1° × 1° and 46 vertical levels. The surface part is coupled with the Louvain-la-Neuve Ice Model-LIM3 (Vancoppenolle et al., 2009) for every model hour. The coupler OASIS3 (Valcke, 2006) is employed for the coupling between the atmospheric (IFS) and oceanic models (NEMO-LIM). EC-Earth has been widely applied in climate simulations for the past and future time periods (Bosmans et al., 2012; Wouters et al., 2012), contributing to a better understanding of the mechanisms of climatic variations and projection of future changes. Previous studies have proven the good capability of EC-Earth in reproducing monsoonal precipitation (Bosmans et al., 2012; Pausata et al., 2016), the Walker circulation and tropical situations over the Pacific (Bayr et al., 2014; Pausata et al., 2017). 2.2. Experimental design We adopt the model simulation perfomed in Pausata et al. (2016), where a set of four MH experiments were carried out together with a reference pre-industrial climate (PI) using EC-Earth. The standard MH simulation (MHOB) was performed following the PMIP3/CMIP5 protocol (Taylor, 2009), with boundary conditions set at preindustrial values except for the orbital forcing and greenhouse gases. The orbital forcing is set at values for 6000 years BP and the methane concentration is changed from 760 ppmv for PI to 650 ppmv. The second simulation (MHGS) was carried out with shrubland covering the Sahara domain (11°–33°N, 15°W–35°E). Such vegetation change is related to a reduction in surface albedo from 0.3 to 0.15 and an increase in the leaf area index from 0.2 to 2.6. In the third experiment (MHRD), the PI dust concentration were reduced by up to 80% over the Sahara desert and its surrounding regions, in accordance with the decrease in Saharan dust flux during the MH estimated in recent studies (deMenocal et al., 2000; McGee et al., 2013), but vegetation cover was prescribed as in the PI and MHOB eperiment. Then, in the last experiment (MHGS+RD) both changes in vegetation over the Sahara and reduction in dust concentration were considered. A more detailed description of the experimental design can be found in Pausata et al. (2016). The model simulations are carried out for nearly 300 years, with initial conditions derived from a 700-year preindustrial spin-up run. The climate quasiequilibrium was reached after 100–200 years and only the last 100 years of each sensitivity experiment are analyzed in this study. 3. Results 3.1. EASM changes in the MH experiments The experiment in which only orbital forcing is considered (MHOB), shows a northward extension and an intensification of the EASM relative to PI simulation, with anomalous easterlies over the equatorial western Pacific, southerlies over the eastern coast of Asia and southeasterlies over Northeast Asia (Fig. 1a). The results agree well with those of previous studies, in terms of both the amplitude and the spatial pattern of the wind anomalies (e.g., Jiang et al., 2013). The vegetation change over the Sahara and dust reduction (MHGS+RD) further 2
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 1. Summer (June to August) mean anomalies in horizontal wind at 850 hPa (m/s, vectors) between the (a) MHOB, (b) MHGS+RD, (c) MHGS, (d) MHRD and the reference PI experiment. The shadings denote the differences significant at the 95% confidence level by the Student t-test. Wind differences smaller than 0.3 m/s are not shown.
strengthen the EASM intensity and expand the EASM domain northward, causing anticyclonic anomalies over the East China Sea (Fig. 1b). In the MHGS simulation, the result is quite similar to that of MHGS+RD in regard to both spatial distributions and amplitude (Fig. 1c). For the situation of MHRD, the EASM variations do not show notable changes compared with those in MHOB (Fig. 1d). The summer precipitation changes over the EASM region show a good correspondence to the wind anomalies (Fig. 2). Positive anomalies exist over northeastern and southern Asia, and negative ones over northwestern Mongolia, the Bay of Bengal and the western part of the Pacific in the MHOB relative to PI (Fig. 2a). The simulated anomalies in MHGS+RD resemble those of MHOB; however they are amplified in particular over southern (more negative) and northern (more positive) China (Fig. 2b), which is in accordance with the stronger southerlies over the EASM domain (Fig. 1b). The precipitation anomalies between MHGS and MHGS+RD are very similar in both magnitude and spatial aspects in agreement with the identical wind patterns (Fig. 2c). In addition, weak precipitation anomalies can be found in MHRD (Fig. 2d), which are even weaker than those in MHOB (Fig. 2a). Given the similarity between MHGS and MHGS+RD anomalies, the vegetation cover change is the dominant factor affecting the strengthening and northward extension of the EASM during the MH. Since much work has already been done on the strengthening of the EASM during the MH (Wang et al., 2010; Zhao and Harrison, 2012; Jiang et al., 2013), we focus on the impacts of the vegetated Sahara on the northward shift of the EASM and the possible mechanisms. To better depict the northward extension of the EASM system, a climatological northern boundary index (CNBI) is adopted, which is defined as the summer (May–September) 2 mm day-1 isoline (Chen et al., 2018). The good capability of this index to describe the EASM northern boundary and capture the major shifts of the summer monsoon circulation has been demonstrated (Chen et al., 2018). Here, we mainly focus on the variations within 100°–120°E. The climatological mean position of the EASM northern boundary presents a northward extension in both MHOB and MHGS compared with PI, with a more northward position identified in the MHGS experiment (Fig. 3). Considering a more quantitative view, we further calculated the climatological mean position of the CNBI within different sections of the PI,
MHOB and MHGS. The results show that the differences within 100°–105°E and 115°–120°E are much smaller than those within 105°–110°E and 110°–115°E. Within 105°–110°E, the northern boundary has a northward extension of approximately 68 (193) km in the MHOB (MHGS) compared to that in the PI. For the 110°–115°E section, the northward extension reaches 155 (362) km for the MHOB (MHGS). The change in vegetation cover over the Sahara induces the further northward shift of the CNBI within 105°–110°E (110°–115°E) by approximately 125 (207) km compared to that of the orbital forcing alone. These results indicate the prominent role of the vegetated Sahara on the northward expansion of the EASM system during the MH. The western Pacific subtropical high (WPSH) is one of the major components of the EASM system, and the intensity and spatial pattern of the WPSH are closely related to the onset, withdrawal and the circulation anomalies of the EASM (Hwang and Yue, 1962; Tao and Chen, 1987; Huang and Wu, 1989; Ninomiya and Kobayashi, 1999). The values of geopotential height within the western Pacific in MHGS are larger than those in MHOB and PI, which indicates the strengthening of the WPSH in the MH and in particular in the MHGS experiment (Fig. 4). In addition, the 5820-gpm isoline is taken as a reference to measure the position change of the WPSH, the western boundary of which reaches 135°E for the PI (Fig. 4a) and 120°E in the MHOB (Fig. 4b) while stretching into eastern China in the MHGS at approximately 110°E (Fig. 4c). The WPSH experiences a remarkable intensification and westward extension under the impact of the forcing of the vegetated Sahara (Fig. 4). This favors the strengthening of the prevailing southerly wind over eastern China, contributing to the northward shift of the EASM. 3.2. Saharan vegetation cover change and the strength of the WPSH During the MH, summer rainfall presents positive anomalies over North Africa (deMenocal et al., 2000; Wu et al., 2007; Pausata et al., 2016). Such increase in precipitation is associated with a divergent flow over North Africa in the upper atmosphere for both the MHOB and MHGS (Fig. 5a, b). A negative center is instead found over the equatorial eastern Pacific, which is stronger in the MHGS (Fig. 5b) relative to the MHOB (Fig. 5a). To emphasize the role of the change in vegetation 3
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 2. Summer (June to August) mean anomalies in precipitation (shadings, mm/day) between the (a) MHOB, (b) MHGS+RD, (c) MHGS, (d) MHRD and the reference PI experiment. The dotted areas denote the differences significant at the 95% confidence level by the Student t-test.
Fig. 3. The climatological mean position of the EASM northern boundary in the PI (black solid line), the MHOB (purple dotted line), and the MHGS (green dotted line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cover, the differences are further calculated between the MHGS and MHOB (Fig. 5c). The positive and negative centers can still be identified over North Africa and the equatorial eastern Pacific, indicating the strengthening of the Walker circulation when the desert areas are turned into shrubland over the Sahara. Zhou et al. (2009) investigated the westward extension of the WPSH since the late 1970s and demonstrated that the Indian Ocean-western Pacific warming intensifies the Walker circulation, which then modulates the WPSH through the ENSO/Gill response. Hence, the Walker circulation might be a factor contributing to the relationship between the Saharan land surface change and the changes in WPSH intensity. To better understand the Walker circulation changes, we also calculate the the differences in the mean vertical velocity profile between
Fig. 4. The climatological mean of geopotential height at 500 hPa in the (a) PI, (b) MHOB and (c) MHGS (contours: 10 m interval from 5010 to 5040 m).
4
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 5. Summer (June to August) mean anomalies in the velocity potential (contours, 105 m2 s−1) and divergent wind (vectors, m/s) at 200 hPa between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The shadings denote the differences in the velocity potential significant at the 95% confidence level by the Student t-test.
20°S and 20°N (Fig. 6). By comparison, the Walker circulation is intensified and shifted to the west in both the MHOB and MHGS compared to that in the PI (Fig. 6a, b). In addition, the differences between MHGS and MHOB show anomalous ascending motions over the equatorial Indian Ocean and the equatorial eastern Pacific, and anomalous descending motions over the equatorial western Pacific and the equatorial Atlantic (Fig. 6c). This indicates a westward shift of the Walker circulation under the forcing of the Saharan vegetation cover change. A vegetated and less dusty Sahara during the MH lead to the strengthening of the West African Monsoon (WAM). The intensification of the WAM and the northward shift of the Inter-Tropical Convergence Zone result in a decreased upwelling (warming, i.e. Atlantic Niño) and SST variability in the western equatorial Atlantic (Pausata et al., 2017). The Atlantic Niño response and the reduction of the equatorial Atlantic SST variability then induce the westward shift of the Walker circulation, as also mentioned in many studies on present day climate (e.g., RodríguezFonseca et al., 2009; Li et al., 2016). Correspondingly, rainfall anomalies exhibit negative values over the equatorial western Pacific and the equatorial Atlantic with positive values over the subtropical Pacific and North Africa in the MHOB and MHGS (Fig. 7a, b). Over the Indian Ocean, significant discrepancies in rainfall anomalies can be identified between the MHOB and MHGS. In the MHOB, positive values are simulated over the equatorial western Indian Ocean with negative values over the equatorial eastern Indian Ocean relative to PI (Fig. 7a). In the MHGS experiment, a dipole rainfall pattern is shown with positive anomalies in the northern Indian Ocean and negative anomalies in the southern Indian Ocean relative to PI (Fig. 7b). The rainfall differences between the MHGS and MHOB present similar spatial patterns (Fig. 7c), but with positive rainfall anomalies over the equatorial Indian Ocean, consistent with the local anomalous ascending motions (Fig. 6c) caused by the Walker circulation change. As the WPSH appears to be the maximum in the low-level stream function field during the boreal summer (Lu and Dong, 2001; Zhou et al., 2009), we further display the differences of the nondivergent wind and stream function at 850 hPa between sensitivity experiments and the PI (Fig. 8). Cyclonic anomalies are identified over the North
Fig. 6. Summer (June to August) mean anomalies in the longitude-height cross section of the vertical velocity (shadings, Pa/s) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The differences are shown as 20°S–20°N average. The contours denote the climatological mean of the vertical velocity in the PI.
Africa in the MHOB and MHGS in response to the postive rainfall anomalies, with stronger intensity for the latter one (Fig. 8a, b). This implies that the latent heat released by the positive rainfall anomalies might act as heating sources and force the anomalous atmospheric circulation following the study of Gill (1980). A subtropical anticyclonic circulation center is located over the western Pacific (Fig. 8a, b), which can still be recognized with the Saharan vegetation forcing alone (Fig. 8c). The anomalous anticyclonic circulation seems to contribute to the westward extension and intensification of the WPSH during the MH, the formation of which might be related to the negative heating source implied by the decreased rainfall over the equatorial western Pacific (Fig. 7c) through the Gill-Matsuno response. In addition, other studies provided evidence that the WPSH variation can result from the anomalous convection induced by changes in the Hadley and Walker circulations (Wang, 1995; Li, 1997; Chang et al., 2000). In consideration of the close relationship between the Hadley and Walker circulations revealed by previous studies (Nigam and Shen, 1993; Oort and Yienger, 1996), the differences of the vertical velocity averaged between 110°–140°E are displayed to represent the change of the Hadley circulation (Fig. 9). In the MHOB, anomalous upward motions are identified along the equator, with downward motions approximately around 10°–20°N (Fig. 9a). In the MHGS simulation, the anomalies mainly lie in the apparent subtropical descending motions along 30°N (Fig. 9b), which is a sign of the intensified Hadley 5
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
Fig. 7. Summer (June to August) mean anomalies in precipitation (mm/day) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The dotted areas denote the differences significant at the 95% confidence level by the Student t-test.
Fig. 9. Summer (June to August) mean anomalies in the latitude-height cross section of the wind (m/s) between the (a) MHOB, (b) MHGS and PI experiment, and between (c) the MHGS and MHOB. The vertical velocity is multiplied by a factor of 100. The differences are shown as the average between 110°–140°E.
to the westward shift of the Walker circulation, which then induces the westward extension and intensification of the WPSH through the GillMatsuno response and an intensified local Hadley circulation, resulting in the northward extension of the EASM system. Then, what is the relative importance of the Hadley circulation and the negative heating source over the equatorial western Pacific? The differences in the stream function at 200 hPa and 850 hPa between the MHGS and MHOB, show positive centers over the subtropical northwestern Pacific (Fig. 10), indicating a barotropic vertical structure. As the Rossby response to the negative heating anomalies is baroclinic, this suggests that the strengthened local Hadley circulation plays a more important role in the westward extension of the WPSH. Furthermore, the center of the downward motions is quite in phase with the anticyclonic circulation center at 850 hPa over the subtropical northwestern Pacific (Fig. 10b). Based on the study of Chung et al. (2011), this phase consistency inferred that the intensified WPSH might be a direct response to the anomalous local Hadley circulation, confirming the larger influence of the local Hadley circulation.
Fig. 8. Summer (June to August) mean anomalies in the stream function (contours, 105 m2 s−1) and non-divergent wind (vectors, m/s) at 850 hPa between the (a) MHOB, (b) MHGS and PI experiment and between (c) the MHGS and MHOB. The shadings denote the differences in the stream function significant at the 95% confidence level by the Student t-test.
4. Summary and discussions
circulation. The differences between the MHGS and MHOB further confirm the prominent impacts of the Saharan vegetation cover change on the strengthening of the Hadley circulation with the upward and downward motions found over the equator and the subtropics, respectively (Fig. 9c). The downward motions within the ridge area favor the variation in WPSH during the MH. Hence, a vegetated Sahara leads
Previous studies indicate that the models fail to fully reproduce the extent of the EASM variations during the MH, which might stem from the fact that the large differences in the vegetation cover and dust concentrations between the MH and the preindustrial are not taken into account. Here, with a vegetated Sahara and reduce dust concentrations, the EASM presents an intensification and northward extension 6
Global and Planetary Change 184 (2020) 103046
J. Piao, et al.
during the MH is clearly improved by considering the vegetation cover change over the Sahara desert. Hence, it is important for model simulations to include the Saharan vegetation cover change to obtain a better reproduction of climatic variations during the MH. This study, therefore, suggests that having a good simulation of the vegetation cover may also be relevant for a better projections of future climate change. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We thank the three anonymous reviewers for their valuable comments and suggestions, which led to significant improvement in the manuscript. This work was supported jointly by the Swedish Research Council (VR) project “Simulating Green Sahara With Earth System Model” (2017–04232), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT: CH2016-6711), the National Natural Science Foundation of China (Grants 41721004 and 41875115), and the Jiangsu Collaborative Innovation Center for Climate Change. F.S.R.P. acknowledges funding from the Swedish Research Council (FORAMS) as part of the Joint Programming Initiative on Climate and the Belmont Forum for the project “Palaeo-constraints on Monsoon Evolution and Dynamics (PACMEDY) and the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC Grant RGPIN-2018-04981). The EC-Earth model simulations were performed on a supercomputer provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and Cray XC30 HPC systems at ECMWF.
Fig. 10. Summer (June to August) mean anomalies in the stream function (contours, 105 m2 s−1) between the MHGS and MHOB at (a) 200 hPa and (b) 850 hPa. The shadings denote the differences in the vertical velocity at 500 hPa (shadings, Pa/s) between the MHGS and MHOB.
compared with the simulation where only orbital forcing are considered. A climatological northern boundary index is further employed to describe the variations in the northern boundary of the EASM system. The results show that with a green Sahara state, the distance of the mean EASM northern boundary between the MHGS and MHOB can reach approximately 125 (207) km between 105°–110°E (110°–115°E), indicating that the vegetated Sahara can exert a great impact on the northward extension of the EASM system during the MH. The intensification and westward extension of the WPSH is regarded as the bridge between the Saharan vegetation cover change and the northward extension of the EASM during the MH. The possible mechanism behind the impact of the vegetated Sahara on the WPSH can be expressed as follows:
References Bartlein, P.J., Harrison, S.P., Brewer, S., Connor, S., Davis, B.A.S., Gajewski, K., Guiot, J., Harrison-Prentice, T.I., Henderson, A., Peyron, O., Prentice, I.C., Scholze, M., Seppa, H., Shuman, B., Sugita, S., Thompson, R.S., Viau, A.E., Williams, J., Wu, H., 2011. Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis. Clim. Dyn. 37 (3–4), 775–802. Bayr, T., Dommenget, D., Martin, T., Power, S.B., 2014. The eastward shift of the Walker Circulation in response to global warming and its relationship to ENSO variability. Clim. Dyn. 43 (9–10), 2747–2763. Berger, A., 1988. Milankovitch theory and climate. Rev. Geophys. 26 (4), 624–657. Bosmans, J.H.C., Drijfhout, S.S., Tuenter, E., Lourens, L.J., Hilgen, F.J., Weber, S.L., 2012. Monsoonal response to mid-holocene orbital forcing in a high resolution GCM. Clim. Past 8 (2), 723–740. Braconnot, P., Harrison, S.P., Kageyama, M., Bartlein, P.J., Masson-Delmotte, V., AbeOuchi, A., Otto-Bliesner, B., Zhao, Y., 2012. Evaluation of climate models using palaeoclimatic data. Nat. Clim. Chang. 2 (6), 417. Chang, C.P., Zhang, Y.S., Li, T., 2000. Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of the subtropical ridge. J. Clim. 13 (24), 4310–4325. Chen, H., Sun, J., 2013. Projected change in East Asian summer monsoon precipitation under RCP scenario. Meteorog. Atmos. Phys. 121 (1–2), 55–77. Chen, W., Feng, J., Wu, R., 2013. Roles of ENSO and PDO in the link of the East Asian Winter Monsoon to the following Summer Monsoon. J. Clim. 26 (2), 622–635. Chen, J., Wen, Z., Wu, R., Wang, X., He, C., Chen, Z., 2017. An interdecadal change in the intensity of interannual variability in summer rainfall over southern China around early 1990s. Clim. Dyn. 48, 191–207. Chen, J., Huang, W., Jin, L., Chen, J., Chen, S., Chen, F., 2018. A climatological northern boundary index for the East Asian summer monsoon and its interannual variability. Sci. China Earth Sci. 61 (1), 13–22. Chung, P.-H., Sui, C.-H., Li, T., 2011. Interannual relationships between the tropical sea surface temperature and summertime subtropical anticyclone over the western North Pacific. J. Geophys. Res. 116. COHMAP, 1988. Climatic changes of the last 18,000 years: observations and model simulations. Science 1043–1052. Conroy, J.L., Overpeck, J.T., Cole, J.E., Shanahan, T.M., Steinitz-Kannan, M., 2008. Holocene changes in eastern tropical Pacific climate inferred from a Galápagos lake sediment record. Quat. Sci. Rev. 27 (11−12), 1166–1180. deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., 2000. Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quat. Sci. Rev. 19 (1–5), 347–361.
1. Previous studies suggested that the vegetated Sahara could induce the westward shift of the Walker circulation through an Atlantic Niño response and reduced equatorial Atlantic SST variability. The westward shift of the Walker circulation has corresponding rainfall anomalies, with positive values over the equatorial eastern Indian Ocean and the North Africa and negative values over the equatorial western Pacific. The negative rainfall anomalies over the equatorial western Pacific could act as a negative heating source. This then forces anticyclonic circulation over the western Pacific based on the Gill-Matsuno response and favors the intensification of the WPSH. 2. Corresponding to the westward shift of the Walker circulation, the local Hadley circulation is strengthened through the anomalous heating source over the equatorial eastern Indian Ocean, with ascending motions located over the equator and descending motions over the subtropics. The descending branch can further contribute to the intensification of the WPSH. In addition, further analysis shows that the strengthened local Hadley circulation exerts more important influence on the WPSH variation during the MH, compared to the role of the Rossby response to the negative heating anomalies over the equatorial western Pacific. In conclusion, our study suggests that the simulation of the EASM system 7
Global and Planetary Change 184 (2020) 103046
J. Piao, et al. Ding, Y.H., Chan, J.C.L., 2005. The East Asian summer monsoon: an overview. Meteorog. Atmos. Phys. 89 (1–4), 117–142. Ding, Y., Wang, Z., Sun, Y., 2008. Inter-decadal variation of the summer precipitation in East China and its association with decreasing Asian summer monsoon. Part I: Observed evidences. Int. J. Climatol. 28 (9), 1139–1161. Egerer, S., Claussen, M., Reick, C., 2018. Rapid increase in simulated North Atlantic dust deposition due to fast change of northwest African landscape during the Holocene. Clim. Past 14 (7), 1051–1066. Feng, Z.D., An, C.B., Tang, L.Y., Jull, A.J.T., 2004. Stratigraphic evidence of a Megahumid climate between 10,000 and 4000 years BP in the western part of the Chinese Loess Plateau. Glob. Planet. Chang. 43 (3–4), 145–155. Gaetani, M., Messori, G., Zhang, Q., Flamant, C., Pausata, F.S.R., 2017. Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene. J. Clim. 30 (19), 7621–7642. Gill, A.E., 1980. Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc. 106 (449), 447–462. Goldsmith, Y., Broecker, W.S., Xu, H., Polissar, P.J., Demenocal, P.B., Porat, N., Lan, J., Cheng, P., Zhou, W., An, Z., 2017. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proc. Natl. Acad. Sci. U. S. A. 114 (8), 1817–1821. Harrison, S.P., Bartlein, P.J., Brewer, S., Prentice, I.C., Boyd, M., Hessler, I., Holmgren, K., Izumi, K., Willis, K., 2014. Climate model benchmarking with glacial and midHolocene climates. Clim. Dyn. 43 (3–4), 671–688. Hazeleger, W., Severijns, C., Semmler, T., Stefanescu, S., Yang, S., Wang, X., Wyser, K., Dutra, E., Baldasano, J.M., Bintanja, R., Bougeault, P., Caballero, R., Ekman, A.M.L., Christensen, J.H., van den Hurk, B., Jimenez, P., Jones, C., Kallberg, P., Koenigk, T., McGrath, R., Miranda, P., Van Noije, T., Palmer, T., Parodi, J.A., Schmith, T., Selten, F., Storelvmo, T., Sterl, A., Tapamo, H., Vancoppenolle, M., Viterbo, P., Willen, U., 2010. EC-Earth a Seamless Earth-System Prediction Approach in Action. Bull. Amer. Meteor. Soc. 91 (10), 1357–1363. Hely, C., Lezine, A.M., Contributors, A.P.D., 2014. Holocene changes in African vegetation: tradeoff between climate and water availability. Clim. Past 10 (2), 681–686. Hu, Z.Z., Latif, M., Roeckner, E., Bengtsson, L., 2000. Intensified Asian summer monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations. Geophys. Res. Lett. 27 (17), 2681–2684. Huang, R.H., Sun, F.Y., 1992. Impacts of the tropical western Pacific on the East Asian summer monsoon. J. Meteor. Soc. Japan 70 (1B), 243–256. Huang, R.H., Wu, Y., 1989. The influence of ENSO on the summer climate change in china and its mechanism. Adv. Atmos. Sci. 6 (1), 21–32. Hwang, S., Yue, T., 1962. On the structure of the sub-tropical highs and some associated aspects of the general circulation of atmosphere. Acta Meteor. Sin. 31 (4), 339–359. Jiang, D., Lang, X., Tian, Z., Ju, L., 2013. Mid-Holocene East Asian summer monsoon strengthening: Insights from Paleoclimate Modeling Intercomparison Project (PMIP) simulations. Palaeogeogr. Palaeoclimatol. Palaeoecol. 369, 422–429. Jin, L., Schneider, B., Park, W., Latif, M., Khon, V., Zhang, X., 2014. The spatial–temporal patterns of Asian summer monsoon precipitation in response to Holocene insolation change: a model-data synthesis. Quat. Sci. Rev. 85, 47–62. Ju, J., Slingo, J., 1995. The Asian summer monsoon and ENSO. Quart. J. Roy. Meteor. Soc. 121 (525), 1133–1168. Koutavas, A., Joanides, S., 2012. El Niño–Southern oscillation Extrema in the holocene and last glacial maximum. Paleoceanography 27 (4). Kripalani, R., Oh, J., Chaudhari, H., 2007. Response of the East Asian summer monsoon to doubled atmospheric CO 2: coupled climate model simulations and projections under IPCC AR4. Theor. Appl. Climatol. 87 (1–4), 1–28. Kusunoki, S., Arakawa, O., 2012. Change in the precipitation intensity of the East Asian summer monsoon projected by CMIP3 models. Clim. Dyn. 38 (9–10), 2055–2072. Kwon, M., Jhun, J.G., Ha, K.J., 2007. Decadal change in east Asian summer monsoon circulation in the mid-1990s. Geophys. Res. Lett. 34 (21). Lau, K.-M., 1992. East Asian summer monsoon rainfall variability and climate teleconnection. J. Meteor. Soc. Japan 70 (1B), 211–242. Lau, K., Wu, H., 2001. Principal modes of rainfall–SST variability of the Asian summer monsoon: a reassessment of the monsoon–ENSO relationship. J. Clim. 14 (13), 2880–2895. Lee, J.-Y., Wang, B., 2014. Future change of global monsoon in the CMIP5. Clim. Dyn. 42 (1–2), 101–119. Li, T., 1997. Phase transition of the El Niño–Southern Oscillation: a stationary SST mode. J. Atmos. Sci. 54 (24), 2872–2887. Li, X., Zhou, W., 2012. Quasi-4-year coupling between El Niño-Southern Oscillation and water vapor transport over East Asia-WNP. J. Clim. 25, 5879–5891. Li, X., Xie, S.-P., Gille, S.T., Yoo, C., 2016. Atlantic-induced pan-tropical climate change over the past three decades. Nat. Clim. Chang. 6 (3), 275. Liu, Z., Harrison, S., Kutzbach, J., Otto-Bliesner, B., 2004. Global monsoons in the midHolocene and oceanic feedback. Clim. Dyn. 22 (2–3), 157–182. Liu, J., Chen, J., Zhang, X., Li, Y., Rao, Z., Chen, F., 2015. Holocene East Asian summer monsoon records in northern China and their inconsistency with Chinese stalagmite δ18O records. Earth-Sci. Rev. 148, 194–208. Lu, R., Dong, B., 2001. Westward extension of North Pacific subtropical high in summer. J. Meteor. Soc. Japan 79 (6), 1229–1241. Madec, G., 2008. NEMO ocean engine, Note du Pole de modélisation, Institut PierreSimon Laplace (IPSL), France, No 27. (ISSN-1288-1619). Maher, B.A., 2008. Holocene variability of the East Asian summer monsoon from Chinese cave records: a re-assessment. Holocene 18 (6), 861–866. Maher, B.A., Hu, M., 2006. A high-resolution record of Holocene rainfall variations from the western Chinese Loess Plateau: antiphase behaviour of the African/Indian and East Asian summer monsoons. Holocene 16 (3), 309–319. McGee, D., Winckler, G., Stuut, J., Bradtmiller, L., 2013. The magnitude, timing and
abruptness of changes in North African dust deposition over the last 20,000 yr. Earth Planet. Sci. Lett. 371, 163–176. Messori, G., Gaetani, M., Zhang, Q., Zhang, Q., Pausata, F.S.R., 2019. The water cycle of the mid-Holocene West African Monsoon: the role of vegetation and dust emission changes. Int. J. Climatol. 39 (4), 1927–1939. Min, S.-K., Park, E.-H., Kwon, W.-T., 2004. Future projections of East Asian climate change from multi-AOGCM ensembles of IPCC SRES scenario simulations. J. Meteor. Soc. Japan 82 (4), 1187–1211. Min, S.-K., Legutke, S., Hense, A., Cubasch, U., Kwon, W.-T., Oh, J.-H., Schlese, U., 2006. East Asian climate change in the 21st century as simulated by the coupled climate model ECHO-G under IPCC SRES scenarios. J. Meteor. Soc. Japan 84 (1), 1–26. Nigam, S., Shen, H.-S., 1993. Structure of oceanic and atmospheric low-frequency variability over the tropical Pacific and Indian Oceans. Part I: COADS observations. J. Clim. 6 (4), 657–676. Ninomiya, K., Kobayashi, C., 1999. Precipitation and moisture balance of the Asian summer monsoon in 1991. J. Meteor. Soc. Japan 77 (1), 77–99. Oort, A.H., Yienger, J.J., 1996. Observed interannual variability in the Hadley circulation and its connection to ENSO. J. Clim. 9 (11), 2751–2767. Pausata, F.S.R., Messori, G., Zhang, Q., 2016. Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period. Earth Planet. Sci. Lett. 434, 298–307. Pausata, F.S.R., Zhang, Q., Muschitiello, F., Lu, Z., Chafik, L., Niedermeyer, E.M., Stager, J.C., Cobb, K.M., Liu, Z., 2017. Greening of the Sahara suppressed ENSO activity during the mid-Holocene. Nat. Commun. 8, 16020. Prentice, I.C., Jolly, D., Participants, B., 2000. Mid-Holocene and glacial-maximum vegetation geography of the northern continents and Africa. J. Biogeogr. 27 (3), 507–519. Qu, X., Huang, G., Zhou, W., 2014. Consistent responses of East Asian summer mean rainfall to global warming in CMIP5 simulations. Theor. Appl. Climatol. 117, 123–131. Rodbell, D.T., Seltzer, G.O., Anderson, D.M., Abbott, M.B., Enfield, D.B., Newman, J.H., 1999. An~ 15,000-year record of El Niño-driven alluviation in southwestern Ecuador. Science 283 (5401), 516–520. Rodríguez-Fonseca, B., Polo, I., García-Serrano, J., Losada, T., Mohino, E., Mechoso, C.R., Kucharski, F., 2009. Are Atlantic Niños enhancing Pacific ENSO events in recent decades? Geophys. Res. Lett. 36 (20). Selvaraj, K., Chen, C.T.A., Lou, J.Y., 2007. Holocene East Asian monsoon variability: links to solar and tropical Pacific forcing. Geophys. Res. Lett. 34 (1). Seo, K.-H., Ok, J., Son, J.-H., Cha, D.-H., 2013. Assessing future changes in the East Asian summer monsoon using CMIP5 coupled models. J. Clim. 26 (19), 7662–7675. Shi, Y.F., Kong, Z.C., Wang, S.M., Tang, L.Y., Wang, F.B., Yao, T.D., Zhao, X.T., Zhang, P.Y., Shi, S.H., 1993. Mid-Holocene climates and environments in China. Glob. Planet. Chang. 7 (1–3), 219–233. Sun, Y., Ding, Y., 2010. A projection of future changes in summer precipitation and monsoon in East Asia. Sci. China Earth Sci. 53 (2), 284–300. Sun, Q., Wang, S., Zhou, J., Chen, Z., Shen, J., Xie, X., Wu, F., Chen, P., 2010. Sediment geochemistry of Lake Daihai, north-central China: implications for catchment weathering and climate change during the Holocene. J. Paleolimnol. 43 (1), 75–87. Tao, S., Chen, L., 1987. A review of recent research on the East Asian summer monsoon in China. Monsoon Meteorology 60–92. Taylor, K.E., 2009. A summary of the CMIP5 experiment design. http://cmip-pcmdi.llnl. gov/cmip5/docs/Taylor_CMIP5_design.pdf. Ueda, H., Iwai, A., Kuwako, K., Hori, M.E., 2006. Impact of anthropogenic forcing on the Asian summer monsoon as simulated by eight GCMs. Geophys. Res. Lett. 33 (6). Valcke, S., 2006. OASIS3 user guide (prism_2-5). In: PRISM Support Initiative Report. 3. pp. 64. Vancoppenolle, M., Fichefet, T., Goosse, H., Bouillon, S., Madec, G., Maqueda, M.A.M., 2009. Simulating the mass balance and salinity of Arctic and Antarctic Sea ice. 1. Model description and validation. Ocean Model 27 (1–2), 33–53. Wang, B., 1995. Interdecadal changes in El Nino onset in the last four decades. J. Clim. 8 (2), 267–285. Wang, B., Wu, R., Fu, X., 2000. Pacific–East Asian teleconnection: how does ENSO affect East Asian climate? J. Clim. 13 (9), 1517–1536. Wang, B., Wu, R., Lau, K., 2001. Interannual variability of the Asian summer monsoon: contrasts between the Indian and the western North Pacific–East Asian monsoons. J. Clim. 14 (20), 4073–4090. Wang, P., Clemens, S., Beaufort, L., Braconnot, P., Ganssen, G., Jian, Z., Kershaw, P., Sarnthein, M., 2005. Evolution and variability of the Asian monsoon system: state of the art and outstanding issues. Quat. Sci. Rev. 24 (5–6), 595–629. Wang, T., Wang, H., Jiang, D., 2010. Mid-Holocene East Asian summer climate as simulated by the PMIP2 models. Palaeogeogr. Palaeoclimatol. Palaeoecol. 288 (1–4), 93–102. Wang, H., Chen, J., Zhang, X., Chen, F., 2014. Palaeosol development in the Chinese Loess Plateau as an indicator of the strength of the East Asian summer monsoon: evidence for a mid-Holocene maximum. Quat. Int. 334, 155–164. Wei, J., Wang, H., 2004. A possible role of solar radiation and ocean in the mid-Holocene East Asian monsoon climate. Adv. Atmos. Sci. 21 (1), 1–12. Wen, R., Xiao, J., Chang, Z., Zhai, D., Xu, Q., Li, Y., Itoh, S., 2010. Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas 39 (2), 262–272. White, S.M., Ravelo, A.C., Polissar, P.J., 2018. Dampened El Niño in the early and MidHolocene due to Insolation-Forced Warming/Deepening of the Thermocline. Geophys. Res. Lett. 45 (1), 316–326. Wouters, B., Drijfhout, S., Hazeleger, W., 2012. Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH. Clim. Dyn. 39 (11), 2695–2712. Wu, H., Guiot, J., Brewer, S., Guo, Z., 2007. Climatic changes in Eurasia and Africa at the
8
Global and Planetary Change 184 (2020) 103046
J. Piao, et al. last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling. Clim. Dyn. 29 (2–3), 211–229. Xiao, J., Wu, J., Si, B., Liang, W., Nakamura, T., Liu, B., Inouchi, Y., 2006. Holocene climate changes in the monsoon/arid transition reflected by carbon concentration in Daihai Lake of Inner Mongolia. Holocene 16 (4), 551–560. Zhao, Y., Harrison, S.P., 2012. Mid-Holocene monsoons: a multi-model analysis of the inter-hemispheric differences in the responses to orbital forcing and ocean feedbacks. Clim. Dyn. 39 (6), 1457–1487. Zheng, W., Wu, B., He, J., Yu, Y., 2013. The East Asian summer monsoon at midHolocene: results from PMIP3 simulations. Clim. Past 9 (1), 453–466.
Zhou, W., Yu, X., Jull, A.T., Burr, G., Xiao, J., Lu, X., Xian, F., 2004. High-resolution evidence from southern China of an early Holocene optimum and a mid-Holocene dry event during the past 18,000 years. Quat. Res. 62 (1), 39–48. Zhou, W., Chan, J.C.L., Li, C.Y., 2005. South China Sea summer monsoon onset in relation to the off-equatorial ITCZ. Adv. Atmos. Sci. 22, 665–676. Zhou, T., Yu, R., Zhang, J., Drange, H., Cassou, C., Deser, C., Hodson, D.L.R., SanchezGomez, E., Li, J., Keenlyside, N., Xin, X., Okumura, Y., 2009. Why the western Pacific subtropical high has extended westward since the late 1970s. J. Clim. 22 (8), 2199–2215.
9