The monsoon system and westerly circulation, to which climate change responds differently, are two important components of global atmospheric circulation interacting with each other in the middle to low latitudes. Relevant research on global millennial-scale climate change in monsoon and westerly regions is mostly devoted to multi-proxy analyses of lakes, stalagmites, ice cores, and marine and eolian sediments. Different responses from these proxies to long-term environmental change make understanding climate change patterns in monsoon and westerly regions difficult. Accordingly, we disaggregated global closed basins into areas governed by monsoon and westerly winds, unified paleoclimate indicators, and added lake models and paleoclimate simulations to emphatically track millennial-scale evolution characteristics and mechanisms of East Asian summer monsoon and westerly winds since the Last Glacial Maximum (LGM). Our results reveal that millennial-scale water balance change exhibits an obvious boundary between global monsoon and westerly regions in closed basins, particularly in the Northern Hemisphere. The effective moisture in most closed basins of the midlatitude Northern Hemisphere mainly exhibits a decreasing trend since the LGM, while that of the low latitudes shows an increasing trend. In the monsoon-dominated closed basins of Asia, a humid climate prevails in the early to mid-Holocene, and a relatively dry climate appears in the LGM and late Holocene. In the westerly-wind-dominated closed basins of Asia, the climate is characterized by a humid LGM and mid-Holocene (MH) compared with the dry early and late Holocene, which is likely to be connected to precipitation brought by the westerly circulation. This study provides insight into the long-term evolution and synergy of westerly winds and monsoon systems as well as a basis for the projection of future hydrological balance.
As important components of atmospheric circulation systems, the midlatitude westerly winds and low-latitude monsoon systems play key roles in global climate change. Whether on the decadal or the millennial scale, research on this aspect always attracts widespread attention from scientists. Examination of global monsoon precipitation changes on land suggests an overall weakening over the recent half-century (1950–2000) (Zhou et al., 2008). Individual monsoon indexes reconstructed by Wang et al. (2017) indicate that moisture in the tropical Australian, East African, and Indian monsoon regions has exhibited a gradual decrease since the early Holocene. It is widely accepted that the East Asian summer monsoon usually follows the variation of low-latitude summer solar radiation (Yuan et al., 2004; Chen et al., 2006; An et al., 2015). Charney (1969) and Wang (2009) also proposed that the seasonal migration of the intertropical convergence zone (ITCZ) profoundly influences the seasonality of the global monsoons. However, the global westerly winds and their associated storm tracks dominate the midlatitude dynamics of the global atmosphere and affect extratropical large-scale temperature and precipitation patterns (Oster et al., 2015; Voigt et al., 2015). Since the Last Glacial Maximum (LGM), the climate in central and southern regions of the North American continent has gradually dried out as the ice sheet melted and the westerlies moved north (Qin et al., 1997). As mentioned in previous studies, millennial-scale evolution in global monsoons and westerly winds probably shows different patterns as a result of complex driving mechanisms. Arguments about an asynchronous pattern of moisture variations between monsoon and westerly wind evolution underscore the importance of studying their millennial-scale differentiation (Chen et al., 2006, 2008, 2019; An and Chen, 2009; Li et al., 2011; An et al., 2012).
A way to examine past climate variability is traditional methods of studying various archives that truly document the evolution of regional climate, including lake sediments (Madsen et al., 2008), stalagmites (Dykoski et al., 2005; Wang et al., 2008), and tree rings (Linderholm and Braeuning, 2006). However, due to the limited timescale of paleoclimate records, most research on the evolution of monsoons and westerly winds is concentrated in the Holocene and lack an exploration during the LGM. With the development of paleoclimatology in recent decades, numerical simulations of paleoclimate continue to emerge and develop to a relatively mature system, which provides a useful tool for reviewing paleoclimate change over long timescales. On account of the water balance system constantly responding to climatic condition changes, a combination of numerical simulations and lake water balance models can be used to effectively track past climate change and make up the deficiency in qualitative methods of multi-proxy analysis (Qin and Yu, 1998; Xue and Yu, 2000; Morrill et al., 2001, 2004; Li and Morrill, 2010, 2013; Lowry and Morrill, 2019; Li et al., 2020). Covering one-fifth of the terrestrial surface, global closed basins are distributed in both low-latitude monsoon regions and midlatitude westerlies. Furthermore, closed basins with a relatively independent hydrological cycle system have plenty of terminal lake records that provide more evidence for retrospecting climate change (Li et al., 2017), and these can be regarded as ideal regions to study the spatiotemporal difference between monsoons and westerly winds.
By constructing virtual lake systems, here we applied lake models and a transient climate evolution model to continuously simulate water balance
change since the LGM in global closed basins. Meanwhile, precipitation minus evaporation (
Boundary conditions in CMIP5/PMIP3 simulations at PI, MH, and LGM.
The transient climate evolution experiment (TraCE 21
Before calculating, we linearly interpolated different-resolution grid cells of the TraCE model and multi-model ensemble into a uniform resolution of
0.5
To better assess the relative change in water balance since the LGM, the virtual lakes are assumed to be in hydrological equilibrium at steady
state. The lake water balance equation is shown as follows:
Status information for 37 lakes in or near global closed basins was collected to compare relative changes among the three characteristic periods. Lake status information sorted by latitude is shown in Table 2. Then, 27 climate records were compiled in or near the midlatitude closed basins of the Northern Hemisphere with reliable chronologies and successive sedimentary sequences from published literature, which can reflect the continuous dry and wet change (Table 3). We interpolated climate data at intervals of 10 years and unified the timescale according to the chronological accuracy of the extracted data. Finally, the data were standardized to indicate a humid climate with a relatively high value and a dry climate with a relatively low value, and the signals of moisture change were transformed into an index range of 0 to 1. Due to the different timescales of the collected continuous paleoclimate records, we can only reconstruct the regional moisture change from the early to late Holocene after unifying the timescales, but the purpose of this part is only to check the simulation results.
Summary of lake-level change in or near global closed basins.
Paleoclimatic records indicating dry or wet status.
Linear tendency estimation is a common trend analysis method, which was chosen to measure the variation degree of simulated water balance in this paper. We also used the empirical orthogonal function (EOF), a method of analyzing the structural features in matrix data and extracting the feature vector of main data to examine the spatial and temporal variability of simulated water balance. The spatial distribution of the EOF first (second) mode is denoted by EOF1 (EOF2), and the time series of the first (second) mode is denoted by PCA1 (PCA2).
As Fig. 1 shows, we intercepted LGM (18 000–22 000
Annual mean precipitation, evaporation, and runoff from TraCE 21
In continuous simulations, we partitioned the trend map of water balance into positive and negative components to highlight the spatial patterns of water balance change (Fig. 2). In the global midlatitude westerlies, simulations indicate that widespread effective moisture has declined since the LGM except in the northern Caspian Sea, whereas effective moisture has increased since the LGM over the global tropics. Meanwhile, the trend map exactly exhibits the spatial differentiation of the millennial-scale water balance change between the global low-latitude monsoon-dominated regions and the midlatitude westerly-wind-dominated regions. This differentiation provides the basis to explore the continuous evolution of monsoons and westerly winds in closed basins since the LGM.
In this section, the possible driving mechanism that affects the millennial-scale water balance change in global closed basins is
explored. Positive signs of EOF1 represent most monsoon regions in the midlatitudes and low latitudes, while negative signs are mainly
located in the Northern and Southern Hemisphere westerlies. Spatial characteristics of EOF2 have an opposite trend to EOF1, except for the
Caspian Sea. The contribution rate of PCA1 and PCA2 is 51 % and 14 %, respectively; therefore, the following discussion mainly focuses on PCA1
with a high contribution rate (Fig. 3). PCA1 extracted from the water balance simulation tends to represent the effective moisture fluctuation of
closed basins in low-latitude monsoon regions, indicating a relatively humid climate during the early to mid-Holocene. By comprehensively analyzing a
variety of paleoclimate proxies, Wang et al. (2017) suggested that moisture change revealed by the Australian monsoon, the East African monsoon, and
the Indian monsoon regions reaches the wettest status in the early Holocene, while the wettest condition in the East Asian summer monsoon regions
occurs between 8 and 6
The climatic significance of
On the basis of the spatial characteristics of the EOF analysis, closed basins in the Northern Hemisphere, affected both by low-latitude monsoons and
midlatitude westerly winds, are ideal regions for revealing the synergy of the westerly winds and monsoons. Between 30
Comparison between simulated water balance change and reconstructed moisture index in the midlatitude closed basins of the Northern Hemisphere during the Holocene. Triangles indicate locations of paleoclimate records (Table 3).
Water balance simulations since the LGM show that a humid climate not only appears in the early to mid-Holocene but also occurs during the LGM, while the climate is relatively dry in the late Holocene. The maintained high moisture in the LGM is possibly influenced by low evaporation and high precipitation (Fig. 5). Using paleoclimate modeling, Yu et al. (2000) mentioned that the low temperature during the glacial period causes a decrease in evaporation and a reduction of lake water loss, resulting in the appearance of a high lake level. Afterward, solar radiation, atmosphere radiation, temperature, evaporation, and precipitation simulations gradually increase (Fig. 5). When entering the warm Holocene, precipitation continues to increase and reaches a maximum in the MH, while solar radiation, atmospheric radiation, and evaporation decrease during the early to mid-Holocene and then increase around the late Holocene. Low (high) evaporation and high (low) precipitation are responsible for the MH (late Holocene) relatively humid (dry) climate (Fig. 5).
Time series of
Spatial distributions of EOF1 and EOF2 clearly exhibit a prominent boundary in the interactional zones between the East Asian summer monsoon and westerly winds in Asia. Since the boundary of the monsoon will be adjusted accordingly with the change in the East Asian summer monsoon strength, the evolution of Asian lakes on the millennial scale probably does not follow a single climate changing pattern (Wu and Guo, 2000; Editorial Committee of China's Physical Geography, 1984; An et al., 2012). The regions dominated by the East Asian summer monsoon and westerly winds were then selected based on the spatial characteristics of the two modes extracted from the EOF to explore millennial-scale evolution features of the two climate systems (Fig. 6). In westerly-wind-dominated regions, the LGM and MH are characterized by a humid climate, and a relatively dry climate prevails in the early and late Holocene, whereas the water balance in monsoon-dominated regions is generally affected by the East Asian summer monsoon, which brought a lot of water vapor over the early to mid-Holocene and led to a relatively dry climate in the LGM and late Holocene. Li (1990) first proposed the “monsoon” and “westerly” modes on the millennial scale since the late Pleistocene in northwest China; since then different climate changing patterns between arid central Asia and monsoonal Asia have been demonstrated by numerous paleoclimate records (Chen et al., 2006, 2008; An and Chen, 2009; Li et al., 2011; Chen et al., 2019). The viewpoint that millennial-scale East Asian summer monsoon change is possibly driven by summer insolation change in low latitudes is the most widely accepted (Yuan et al., 2004; Dykoski et al., 2005; Hu et al., 2008; Fleitmann et al., 2003); the sea surface temperatures (SSTs) of the North Atlantic and air temperatures at high latitudes are responsible for the Holocene effective moisture evolution in arid central Asia, which is dominated by the westerly winds (Chen et al., 2008). Moisture transport in arid central Asia mainly comes from the Northern Hemisphere westerlies, the moisture source of which derives from the Black Sea, the Mediterranean Sea, the Arctic Ocean, and the Atlantic Ocean. Winter precipitation accounts for a large proportion of annual precipitation in these regions (Li et al., 2008).
The water balance change in the Asian monsoon regions we extracted largely represents the hydroclimate variation in regions dominated by the East Asian summer monsoon
since the LGM, while the water balance change in the westerly regions in central Asia can represent the hydroclimate variation in
the entire Northern Hemisphere westerlies. Qin et al. (1997) made a large-scale spatial analysis
and presented lake levels in south-central North America changing from high to low since the LGM, reaching the lowest levels in the early to mid-Holocene. The
LGM proxies indicate that southwestern America experienced a climate that was wetter than present, and the Pacific Northwest through the Rockies
experienced a climate that was drier than present, as well as a transition from wetter to drier conditions along a northwest–southeast-trending band across the northern Great Basin (Oster et al., 2015). Our results generally reflect the climate of the westerlies being relatively wet at
the LGM and relatively dry at the MH. For the Asian tropics in the Northern Hemisphere, the increased summer solar radiation from the past 12 000 to
the past 6000 years ago induced the enhancement of a thermal contrast between the land and sea and further caused the strengthening of summer monsoons so that
more water vapor was brought (COHMAP Members, 1988). Collected records in the Northern Hemisphere indicate the evolution of westerly winds and monsoon
systems (Fig. 7). Speleothem records from central and southern China confirm that the periods of weak East Asian summer monsoons coincide with
cold periods in the North Atlantic (Yuan et al., 2004, Dykoski et al., 2005; Wang et al., 2008). The longest and highest-resolution drill core
from Lake Qinghai (An et al., 2012) indicates that the summer monsoon record generally resembles the changing trends of Asian summer monsoon records
derived from Dongge and Hulu speleothems over the last 20
Comparison of records between the westerly and monsoon regions of the Northern Hemisphere.
The Northern Hemisphere westerlies shifting northward or southward has a significant impact on global atmosphere circulation and inevitably affects the monsoon systems. Quaternary ice sheets of the Northern Hemisphere in the LGM developed to their maximum extension, and the consequent existence of a persistent strong glacial anticyclone led to the southward displacement of the westerly winds (Yu et al., 2000). Most research suggests that the Northern Hemisphere westerlies in the LGM moved to the southwest of the United States and the eastern Mediterranean region (Lachniet et al., 2014; Rambeau, 2010). Therefore, the narrowed temperature difference between the sea and land caused the East Asian summer monsoon to weaken and may have further induced strong westerly winds throughout the year and then precipitation increases (Yu et al., 2000). Furthermore, a growing body of evidence shows that the position and orientation of the westerly jet (WJ) probably controlled the Holocene East Asian summer rainfall patterns. A link between the northward seasonal progression of the WJ and the spatial pattern of East Asian summer monsoon precipitation shows that earlier northward progression of the WJ caused abundant precipitation at high latitudes and less precipitation at low latitudes (Nagashima et al., 2013). The northward evolution of the WJ from south of the Tibetan Plateau and seasonal transition exerted especially strong influences on East Asian paleoclimate change (Chiang et al., 2015). Herzschuh et al. (2019) proposed that the position of the summer monsoon rain band changed as the WJ axis shifted gradually southward, leading to the occurrence of a spatiotemporal difference in Holocene China's maximum precipitation. In summary, the above views emphasize that the complex interaction between the monsoon and the westerly systems on the millennial scale should receive more attention.
On the basis of 37 lake status records near global closed basins and 27 paleoclimatic records near midlatitude closed basins of the Northern Hemisphere, we applied a lake energy balance model, a lake water balance model, and paleoclimate simulations to explore the millennial-scale differentiation between global monsoons and westerly winds. A water balance simulation shows that the effective moisture in most closed basins of the Northern Hemisphere midlatitudes has gradually decreased since the LGM, which matches the reconstructed moisture index well. Effective moisture change in most closed basins of the low latitudes (monsoon regions) presents an opposite changing trend to that in the midlatitudes. In Asian midlatitude closed basins, climate change in regions dominated by westerly winds exhibits a relatively humid climate in the LGM and MH and a relatively dry climate in the early and late Holocene, whereas the East Asian summer monsoon generally influences climate change in closed basins dominated by monsoons, which brought more water vapor over the early to mid-Holocene but less water vapor in the LGM and late Holocene.
The TraCE 21
YL and YZ designed this study and carried it out.
The authors declare that they have no conflict of interest.
We acknowledge Julie Loisel and two anonymous referees for constructive comments.
This research has been supported by the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (grant no. 2019QZKK0202), the National Natural Science Foundation of China (grant nos. 42077415 and 41822708), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA20100102).
This paper was edited by Julie Loisel and reviewed by two anonymous referees.