Synergy of the westerly winds and monsoons in lake evolution of glo bal closed basins since the Last Glacial Maximum

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 mid-to-low latitudes and having synergy effect to those regions. Relevant researches on global millennial-scale climate change in monsoon and westerlies regions are mostly devoted to multi-proxy analyses of lakes, stalagmites, ice cores, marine and eolian sediments. Different responses 10 from these proxies to long-term environmental change make understanding climate change pattern in monsoonal and westerlies regions difficult. Accordingly, we disaggregated global closed basins into areas governed by monsoon and westerly winds and unified palaeoclimate indicators, as well as combined with the lake models and paleoclimate simulations for tracking millennial-scale evolution characteristics and mechanisms of global monsoon and westerly winds since the Last Glacial Maximum (LGM). Our results concluded that the effective moisture in most closed basins of the mid-latitudes 15 Northern Hemisphere is mainly a trend on the decrease since the LGM, and of the low-latitudes is mainly a trend on the rise. Millennial-scale water balance change exhibits an obvious boundary between global westerlies and monsoon regions in closed basins, particularly in the Northern Hemisphere. In the monsoon dominated closed basins of the Northern Hemisphere, humid climate prevails in the early-mid Holocene and relative dry climate appears in the LGM and late Holocene. While in the westerly winds dominated closed basins of the Northern Hemisphere, climate is characterized by relative humid LGM 20 and mid-Holocene (MH) compared with the dry early Holocene, which is likely to be connected with precipitation brought by the westerly circulation. This study provides insights into long-term evolution and synergy of monsoon and westerly wind systems and basis for projection of future hydrological balance in the low-to-mid latitudes.


Introduction
As important components of atmospheric circulation systems, the mid-latitude westerly winds and low-latitude monsoon 25 systems play key roles in global climate change. Whether on the decadal scale or the millennial scale, researches about this aspect always attract widespread attention from scientists. Examination of global monsoon precipitation changes in land suggested an overall weakening over the recent half-century   (Zhou et al., 2008). Individual monsoon indexes reconstructed by Wang et al. (2017) indicated the moisture in the tropical Australian monsoon, the East African monsoon, https://doi.org/10.5194/cp-2020-53 Preprint. Discussion started: 24 April 2020 c Author(s) 2020. CC BY 4.0 License. and the Indian monsoon regions is a gradual decrease since the early Holocene. And it is widely accepted that the East Asian 30 summer monsoon usually follows the variation of low-latitude summer solar radiation 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 global monsoons which have significant seasonality. However, the global westerly winds and their associated storm tracks dominate the mid-latitude dynamics of the global atmosphere and influence the extratropical large-scale temperature and precipitation patterns (Oster et al., 2015;Voigt et al., 2015). Lake records suggested 35 that since the LGM, climate in central and southern regions of north American continent gradually dried out as the ice sheet melted and the westerlies moved north (Qin et al., 1997).
Millennial-scale evolution in global monsoons and westerly winds probably shows different patterns as a result of complex driving mechanisms. Previous arguments about an asynchronous pattern of moisture variations between monsoon and westerly winds evolution underscore the importance for studying their millennial-scale differentiation (Chen et al., , 40 2008(Chen et al., , 2019An and Chen, 2009;Li et al., 2011;An et al., 2012). Covering one-fifth of the terrestrial surface, global closed basins are mostly located in arid and semi-arid areas of global mid-low-latitudes. Furthermore, closed basins with relative independent hydrological cycle system have a plenty of terminal lakes records that provide more evidences for retrospecting climate change, and can be regarded as ideal regions for studying spatiotemporal differences between monsoons and westerly winds (Li et al., 2017). On account of lake water balance system constantly responding to climatic conditions 45 changes, lake water balance model has become one of the common methods to track past climate change, and makes up the deficiency in qualitative method of multi-proxy analysis (Qin and Yu, 1998;Xue and Yu, 2000;Morrill et al., 2001Morrill et al., , 2004Li andMorrill, 2010, 2013;Lowry and Morrill, 2019).
Here we constructed virtual lakes systems and applied lake models and a transient climate evolution model to continuously simulating water balance change since the LGM in global closed basins. Meanwhile, P-E simulations and 37 50 lake status records in the LGM, MH and Pre-Industrial (PI) were supplemented. And based on the prominent spatial characteristics of global monsoons and westerly winds revealed by simulations, we focused on the Northern Hemisphere mid-latitude closed basins where are simultaneously influenced by mid-latitude westerly winds and low-latitude monsoons: first, due to the limited time scale of the climate records, the reconstructed moisture index from 25 paleoclimate records and water balance simulations were used to reveal and validate the climate change of the whole the Northern Hemisphere 55 mid-latitude closed basins; second, the Northern Hemisphere mid-latitude closed basins were disaggregated into the areas dominated by monsoon and westerly winds respectively, and we emphatically explored the temporal evolution of the Northern Hemisphere monsoons and westerly winds since the LGM. last, we comprehensively considered the determinant that controls the direction of climate change in the Northern Hemisphere westerly winds and monsoons since the LGM, according to records of Quaternary ice sheets, low-mid latitudes summer insolation and winter insolation, δ 18 O of Greenland 60 ice core, etc. This study not only reveals millennial-scale climate change from the perspective of water balance, but also provides a new method for studying the synergy of the westerly winds and monsoons.  (He, 2011). We applied this model to continuously simulating effective moisture change represented by virtual water balance variation since the LGM.
Precession, obliquity and eccentricity values are specified according to Berger (1978). CO2, CH4, and N2O values are set on the basis of reconstructions from ice cores (Monnin et al., 2004;Flückiger et al., 1999Flückiger et al., , 2002. A remnant Laurentide ice sheet in the LGM and a modern-day ice sheet configuration in the MH and PI simulations were specified by the ICE-5G reconstruction (Peltier, 2004). And the vegetation is prescribed to modern values. Ice sheet configuration and vegetation  . For all grid cells in closed basins, we assumed that the virtual lake at each grid cell is a 1 meter deep lake with freshwater, and then the virtual lake evaporation is calculated by a lake energy balance model that is modified according to Hostetler and Bartlein's model (Hostetler and Bartlein, 1990). The evaporation of lake surface depends on the heat capacity of water, water density, lake depth, lake surface temperature, shortwave radiation and longwave 85 radiation absorbed by the water surface, longwave radiation emitted by the water surface, latent heat flux, sensible heat flux, etc. If the surface energy balance is negative (positive), the ice forms (melts). Besides, lake depth and lake salinity are https://doi.org/10.5194/cp-2020-53 Preprint. Discussion started: 24 April 2020 c Author(s) 2020. CC BY 4.0 License.
important input parameters influencing lake surface evaporation (Dickinson et al., 1965), however, only small changes appear in lake evaporation when adding lake depth to 5 and 10 m and increasing lake salinity to 10 ppt. More details of lake energy balance model were described in Morrill, 2004 andLi andMorrill, 2010. For better assessing the relative change of water balance since the LGM, the virtual lakes were supposed in hydrological equilibrium with steady state. The lake water balance equation is shown as follows: where is discharge from the lake (m 3 year -1 ), is area of the drainage basin (m 2 ), R is runoff from the drainage basin (m year -1 ), is area of the lake (m 2 ), is precipitation over the lake(m year -1 ) and is lake evaporation (m year -1 ). Given , and these regions maintain water balance by changes in the ratio of to , as described by setting 濠 in Eq. (2): where represents virtual lake level. Accordingly, for grid cells with , the values were calculated and compared to represent relative water balance change, and more details about lake water balance model were described in Li and Morrill, 2010. We combined the values of , and with Eq. (2) and (3) and simulated the continuous water balance change since the LGM using TraCE 21 kyr model.

Records selection and moisture index inference
We collected 37 lake status information in or near global closed basins to compare relative changes among three characteristic periods, and lake status information sorted by latitudes are shown in Table 2. Then, 25 climate records were compiled in or near the mid-latitude closed basins of the Northern Hemisphere with reliable chronologies and successive sedimentary sequences from published literatures, which can reflect the continuous dry and wet change (Table 3). We 110 interpolated climate data at intervals of 10 years and unified the time scale according to the chronology accuracy of the extracted data. Lastly, the data were standardized to indicate a humid climate with a relative high value and a dry climate with a relative low value, and the signals of moisture change were transformed into a range of 0 to 1 index. 115

Possible driving mechanisms of millennial-scale water balance change
In this section, the possible driving mechanism that affects the millennial-scale water balance change in the global closed basins was explored. The spatial-temporal decomposition was applied to obtaining the PCA1 and PCA2 with contribution rate of 51% and 14% respectively. Spatial distributions of the EOF1 and EOF2 clearly exhibit that a prominent boundary exists the interactional zones between Asian monsoon and westerly winds in Eurasia (Fig. 3). Positive signs of the EOF1 are 160 most monsoon regions of mid-latitudes and low-latitudes, while negative signs of that are mainly located in the Northern and https://doi.org/10.5194/cp-2020-53 Preprint. Discussion started: 24 April 2020 c Author(s) 2020. CC BY 4.0 License.
Southern Hemisphere westerlies, especially in the Northern Hemisphere westerlies. Spatial characteristics of the EOF2 have an opposite trend with the EOF1, except for the Caspian Sea. The PCA1 fluctuation corresponds well with stalagmite records of Dongge Cave which documents east summer Asian monsoon change. Thus, we further speculated that the water balance change in monsoon regions of global closed basins is mainly driven by mid-latitude and low-latitude summer solar radiation.

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The PCA2 corresponding with not obvious positive signs presents a gradual increase trend in most westerlies during the late Holocene.

Evolutionary differentiation of millennial-scale monsoons and westerly winds in the Northern Hemisphere mid-latitude closed basins
According to the spatial characteristics of the EOF analysis, closed basins in the Northern Hemisphere that affected by both low-latitude monsoon and mid-latitude westerly winds are ideal regions for revealing synergy of the westerly winds and monsoons. Between 30°N and 60°N, 25 paleoclimate records indicating dry or wet 175 climate were collected from the Northern Hemisphere mid-latitude closed basins. As described in Sect. 2.2, we reconstructed moisture index from the early Holocene to late Holocene around that regions (Fig. 4). Simulated mean water balance curve corresponds well with mean moisture index in the Northern Hemisphere mid-latitude closed basins, indicating a transition from humid climate of the early-mid Holocene to arid climate of late Holocene. Therefore, continuous simulations, well validated by the paleoclimate indicators, could be better used to track climate change during the LGM.  Triangles indicate locations of paleoclimate records (Table 3).
Water balance simulations show a humid climate not only appears in early and mid-Holocene but also occurs during the

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LGM. And the maintained high moisture in the LGM is possibly influenced by low evaporation and high precipitation (Fig.   5). Using the paleoclimate modelling, Yu et al. (2000) mentioned that the low temperature during the glacial period causes a decrease in evaporation, resulting in the loss of lake water reduces and the high lake level sustains. Afterward, solar radiation, atmosphere radiation, temperature, evaporation and precipitation simulations gradually increase during the Last Glacial. When entering the warm Holocene, precipitation continues to increase and reaches the maximum in the 190 mid-Holocene, while solar radiation, atmosphere radiation and evaporation decrease during the early-mid Holocene and then increase around the late Holocene. Low evaporation and high precipitation are responsible for the mid-Holocene relative humid climate (Fig. 5).

Figure 5.
Time series of longwave radiation, shortwave radiation, temperature, precipitation, evaporation and 500hpa wind 195 speed between 30°N and 60°N closed basins since the LGM.
The regions dominated by monsoons and westerly winds were then selected respectively on the basis of spatial characteristics of two mode extracted from the EOF, to explore millennial-scale evolution features of two climate systems ( Fig. 6). In the westerly winds dominated regions, the LGM and mid-Holocene were characterized by humid climate, and 200 relative dry climate prevailed in the early Holocene. Whereas, the water balance in the monsoon dominated regions was generally affected by Asian summer monsoon which brings more water vapor over the early-mid Holocene, and relative dry climate occurred in the LGM. Different climate changing patterns between arid central Asia and monsoonal Asia were demonstrated by numerous paleoclimate records . Li (1990) first proposed the "monsoon" and "westerly" modes on the millennial scale since the late Pleistocene in northwest China. Millennial-scale Asian summer monsoon change 205 is possibly driven by summer insolation change in low-latitudes Dykoski et al., 2005;Hu et al., 2008;Fleitmann et al., 2003). However, Chen et al. (2008)

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The global low-latitudes and mid-latitudes are mainly controlled by the global monsoon and westerly winds systems. As a result of different driving mechanisms, millennial-scale climate change in global low-latitudes and mid-latitudes probably exhibits diverse variations. Qin (1997) made a large-scale spatial analysis and presented that lake levels in south-central North America range from high to low since the LGM and reach the lowest in early-mid Holocene, while the wettest period in the African and South Asian monsoon regions is the early and mid-Holocene. The LGM proxies indicated the 220 southwestern America experiences a climate that was wetter than present, and the Pacific Northwest through the Rockies experiences a climate that was drier than present, as well as a transition from wetter to drier conditions happened along a northwest-southeast trending band across the northern Great Basin (Oster et al., 2015). For the African and Asian tropics in the Northern Hemisphere, the increase summer solar radiation from 12000 to 6000 yr induced the enhancement of thermal contrast between land and sea, and further caused the strengthening of summer monsoons, so that more water vapor was 225 brought (COHMAP Members, 1988).
Collected records in the Northern Hemisphere indicate evolution of westerly winds and monsoons system (Fig. 7).
Speleothem records from central and southern China confirmed that the periods of weak East Asian summer monsoons are https://doi.org/10.5194/cp-2020-53 Preprint. Discussion started: 24 April 2020 c Author(s) 2020. CC BY 4.0 License.
coincided with the cold periods of the North Atlantic , Dykoski et al., 2005Wang et al., 2008). Major trend of moisture conditions revealed by the Australian monsoon, the east African monsoon and the Indian monsoon regions is a 230 gradual decrease since the early Holocene, and reaches the wettest status between 8 and 6 kyr in the East Asian monsoon region . According to the longest and highest-resolution drill core from Lake Qinghai, An et al. (2012) presented that the Lake Qinghai summer monsoon record generally resembles the changing trends of Asian summer monsoon records derived from Dongge and Hulu speleothems over the last 20 kyr, and the mid-latitude Westerlies climate dominates the Lake Qinghai area in glacial times. Low-latitude summer insolation is broadly recognized as a major control 235 on low-latitudes monsoon systems, as a result, the tropical monsoons are weak during the LGM and strong monsoons prevail in the early-mid Holocene (Fig. 7). consequent existence of persisting strong glacial anticyclone leads to the southward displacement of the westerly winds . Many researches suggested the Northern Hemisphere westerlies in the LGM moves south to the southwest of 250 the United States and the eastern Mediterranean region (Lachniet et al., 2014;Rambeau, 2010). Therefore, the narrowed temperature difference between sea and land causes the East Asian summer monsoon weaken, and may further induces the strong westerly winds throughout the year and the precipitation increases . The moisture transport in the arid central Asia mainly comes from the Northern Hemisphere westerlies of which the moisture source derives from the Black Sea, the Mediterranean Sea, the Arctic Ocean and the Atlantic Ocean. In these regions, winter precipitation in this region 255 accounts for a large proportion of annual precipitation (Li et al., 2008).
The above views emphasize the complexity of climate change in the interactional zones between mid-latitude westerlies and Asian summer monsoon. Our results separated the climate systems of the monsoon and westerly dominated regions, revealing humid climate characterized the LGM and mid-Holocene in the westerly winds dominated regions, and drier climate prevailed during the LGM in the monsoon dominated regions. Besides, lots of evidences about Holocene different 260 moisture evolution features between Asian monsoon regions and westerlies dominated arid central Asia were provided by scholars An and Chen, 2009;Li et al., 2011;Chen et al., 2019). However, the intensity of monsoon system and westerly winds varies in different periods so that the main control system in the interactional regions depends largely on which system was much stronger during that period.

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On the basis of 37 lake status records near global closed basins and 25 paleoclimatic records near mid-latitude closed basins of the Northern Hemisphere, we applied a lake energy balance model, a lake water balance model and paleoclimate simulations to exploring the millennial-scale differentiation between global monsoons and westerly winds. Water balance simulation showed that the effective moisture in most closed basins of the mid-latitudes Northern Hemisphere gradually decreases since the LGM, which matches well with reconstructed moisture index. Effective moisture change in most closed 270 basins of the low-latitudes (monsoon regions) presents an opposite changing trend with that in the mid-latitudes. In the Northern Hemisphere mid-latitude closed basins, climate change in regions dominated by westerly winds exhibits a relative humid climate in the LGM and MH, and a relative dry climate in early Holocene, whereas, Asian summer monsoon generally influences the climate change in regions dominated by monsoons, which brings more water vapor over the early-mid Holocene but less water vapor in the LGM and late Holocene.