Northward migration of the East Asian summer monsoon northern boundary during the twenty-first century

The northern fringe area of the East Asian summer monsoon (EASM) between arid and semiarid regions is a fragile eco-environment zone and ecological transition zone, and it is highly sensitive to climate change. Predicting the future migration of the northern boundary of the EASM is important for understanding future East Asian climate change and formulating of decisions on ecological protection and economic development in arid and semiarid regions. The reanalysis dataset and simulations of 23 models from the Coupled Models Intercomparison Project Phase 6 (CMIP6) were used to investigate the response of the boundary of the ESAM to the global warming. The multi-model ensemble showed a northwestward migration of the EASM northern boundary during the near-term (2020–2060) and late-term (2061–2099) of the twenty-first century under various Shared Socioeconomic Pathways (SSPs). The northern boundary migrated northwestward by 23–28 and 74–76 km in the near-term and late-term respectively, under SSP1-2.6, 2-4.5 and 3-7.0 and by ~ 44 km and ~ 107 km respectively during the near-term and late-term under SSP5-8.5. During the twenty-first century, under various SSPs, the surface of the East Asian subcontinent warmed more than the ocean, thereby increasing the contrast of near-surface temperature and sea level pressure in summer between the East Asian subcontinent and the surrounding oceans. In turn, the intensified land–sea thermal contrast reinforced the EASM meridional circulation and thus transported more moisture from the Indian Ocean into northern China. Additionally, a poleward migration and weakening of the East Asian subtropical westerly jet would also favor an increase in precipitation, eventually caused a northwestward migration of the EASM northern boundary. The results suggest that the arid and semiarid ecotone will become wetter, which could dramatically improve the eco-environment in the future.

East Asia is a monsoon climate dominated region, and the low-level prevailing wind in East Asia is seasonally reversed from southerly in summer to northerly in winter. In summer, the lower troposphere southerly winds transport abundant water vapor from the tropical oceans to East Asia, thus inciting the rainy season in East Asia [1][2][3][4] . The East Asian summer monsoon (EASM) is the most active and influential circulation system, which brings more than half of the annual precipitation to East Asia 5,6 , influencing nearly one-fifth of the world's population living in eastern China, Japan, and the Korean peninsula. The precipitation associated with EASM circulation has far-reaching influences on the local ecosystem, agriculture and industry through its regulation of the available water resources 7,8 .
The EASM northern boundary is the northernmost position EASM can reach. The northern boundary is not static and exhibits significant interannual fluctuations following the variation of EASM. The range of the fluctuation thus form the monsoon transition zone 9,10 . The monsoon transition zone is a narrow belt-shaped region with a northeast-southwest direction. It is a transition zone between not only monsoon and non-monsoon regions but also arid and semiarid regions. The annual mean precipitation in the monsoon transition zone is often at a

Materials and methods
Observational and model data. To evaluate the capabilities of the models, the Global Precipitation Climatology Project (GPCP) Version 2.3 Combined Precipitation dataset 31 was obtained, which describes monthly precipitation from 1981 to 2010. The global geopotential height, wind, and temperature from 1981 to 2010 were obtained from the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1 dataset 32 . The horizontal resolution of the above two datasets is 2.5° × 2.5°.
Most (more than two thirds) of the CMIP6 models simulated larger standard deviations (SDs) of the precipitation than the observation, and the CMIP6 multi-model ensemble mean (MME) shows a slightly smaller SD relative to the observations. In terms of spatial correlation, the correlation coefficient between the CMIP6 MME and observations is 0.993. The Taylor diagram indicates that the CMIP6 MME could convincingly capture the present day climatological summer precipitation over East Asia.
The MME method is commonly applied to suppress model drift and internal variability and improve the reliability of climate model prediction 34 , but this method has common background bias in ensemble models, which leads to bias in MME changes and limits the reliability of climate predictions. As shown in Fig. 1b, the MME method smooths the EASM northern boundary, which makes it more reliable than that described by individual model. However, this method leads to more northerly migration of the EASM northern boundary than that in the observation. Therefore, the method of linear correction was used to reduce the uncertainties of future climate prediction 35,36 . All data were remapped to a common regular 1.25° × 1.25° grid using bilinear interpolation remapping, and all simulation data were bias-corrected.  [35][36][37] . After the bias-corrected, even though the MME exhibits almost same position to the observation (Fig. 1b), this method does not completely reduce the uncertainties in the climate simulations.

Results
Spatial evolution of the EASM northern boundary. In this study, the EASM northern boundary is measured by a 2 mm/day isohyet in summer (May-September, MJJAS), and this boundary index has a good capability for describing the northern boundary of the EASM and capturing the variations in summer monsoon circulation 9 . The changes of EASM northern boundary are gradually evident from western to eastern China, the most obvious northwestward migration of the EASM northern boundary is between 110° E and 120° E. It migrates ~ 27 km (SSP1-2.6, Fig. 2a Table 2). The EASM northern boundary generally exhibits a northwestward migration during the twenty-first century compared to that in the present day. In the near-term or late-term future, the largest northwestward migration of the EASM northern boundary occurs under the high-end forcing pathway (SSP5-8.5).
Temporal evolution of the EASM northern boundary. According to the spatial distribution of the EASM northern boundary (Fig. 2), it has a smaller migration range in western China and a larger migration range in eastern China. Therefore, we divided the three regions to analyze the migration of the northern bound-     ary of the EASM. Figure 3 shows the meridional migration of the EASM northern boundary in different regions during the twenty-first century compared to the present day.
In general, the meridional migration of the EASM northern boundary generally exhibits a poleward migration in eastern China under the different emission scenarios, and from the low forcing sustainability pathway (SSP1-2.6) to the high-end forcing pathway (SSP5-8.5), the magnitude of the northward migration gradually increases, and the amplitude of migration reaches its maximum under the high-end forcing pathway scenario (SSP5-8.5).

Mechanisms of the migration of the EASM northern boundary and discussion. Previous stud-
ies suggested that the EASM circulation system largely contributes to the changes in precipitation in northern China 6,18,38,39 . Herein, we focus on the impacts of the global warming on the EASM circulation system and northward migration of the EASM northern boundary and the possible corresponding dynamic mechanisms. EASM intensity is regionally measured by the averaged meridional wind at 850 hPa within the region of 20°-40°N and 105°-120°E 2,38,40 . The temporal evolution characteristics of the EASM are shown in Fig. 4, which indicates that EASM intensity has gradually increases with the increasing emissions relative to the present day under the different future emission scenarios. The results are consistent with the results of previous studies 38, [40][41][42][43][44] . With the enhanced monsoon circulation, the East Asian rainband extends northward and increases the precipitation in northern China 6,19,[45][46][47] . Corresponding to the variations in the intensity of the EASM, relative to the present day, the meridional migration of the EASM northern boundary displays consistent change trends with the intensity of the EASM (Figs. 3, 4). We further analyze the correlations between the meridional migration of the EASM northern and the EASM intensity. As shown in Fig. 4, the significant positive correlation between meridional migration of the EASM northern boundary and the EASM intensity under SSP2-4.5, SSP3-7.0, and SSP5-8.5 are indicate that the higher the emissions are, the stronger the EASM, and the stronger northward advancing of the EASM northern boundary.
The seasonal variation in the uneven heat distribution between the sea and the land is the original driving force of the monsoon, which plays an important role in the formation and maintenance of the monsoon 4,48 . The EASM is the result of a combination of thermal contrast and topography between the Asian continent and surrounding oceans 1,48-50 . Therefore, changes in the thermal conditions of either one of the continents or the oceans may change the land-sea thermal contrast, leading to variations in the EASM.
To understand the dynamic mechanisms of the migration of the EASM northern boundary, the near-term and late-term surface air temperature under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 are analyzed (Fig. 5).
In the near-term, as the thermal inertia of the ocean is much greater than that of the land, the East Asian subcontinent exhibits stronger surface warming than its surrounding oceans due to global warming (Fig. 5a-d), thereby increasing the contrast of near-surface temperature and sea level pressure in summer between the East Asian subcontinent and its surrounding oceans (Fig. 6a-d). The enhanced meridional land-sea thermal contrast and sea-level pressure gradient enhanced the monsoon circulation and the southwesterly wind, and this enhanced southwesterly wind was beneficial for transporting the warm and moist air to northern China 20,38,40,49,51 . Additionally, the moisture transported from the north Pacific resulted from the increases in sea-level pressure gradient (Fig. 6). On the other hand, as shown in Fig. 6, anomalous high and anticyclone patterns over the Indian Ocean can facilitate the transport of moisture from the Indian Ocean to northern China 52 . The interaction between the warm and humid air transported by the southwesterly wind and the cold air transported by the mid-latitude westerly wind has caused an increase in precipitation in most parts of northern China (Fig. 7a-d).
The distribution of land-sea temperature (Fig. 5e-h), sea-level pressure (Fig. 6e-h) and precipitation (Fig. 7e-h) in the late-term follow the same change patterns as in the near-term, and the amplitudes of the enhancements are much greater in the late-term.
Under global warming, the changes in temperature and circulation facilitate moisture transport to the East Asian continent and a northwestward advance of the EASM northern boundary. Therefore, under future emission www.nature.com/scientificreports/ scenarios, changes in land-sea temperature and circulation will induce water vapor transport to the East Asian subcontinent and northwestward migration of the northern boundary of the EASM (Fig. 2). Previous studies have investigated future variations in the EASM and associated precipitation. Recently, some studies evaluated the variations in the EASM and associated precipitation through the twenty-first century based on the multimodel results of Representative Concentration Pathway scenarios (RCPs) experiments from CMIP5 and Shared Socioeconomic Pathway scenarios experiments from CMIP6. The EASM and associated precipitation are projected to intensify over East Asia in the future under various scenarios [40][41][42]44,46 . In addition, the precipitation changes in East Asia were most obvious in the high emission scenarios. Figure 7 shows that the MME forecasts greater precipitation intensity over almost all of East Asia through the twenty-first century under the Shared Socioeconomic Pathway scenarios, which confirms that global warming increases EASM precipitation [40][41][42]44,46 .
In addition to the land-sea thermal contrast, the East Asia subtropical westerly jet (EASWJ) plays a key role in the seasonal migration of the East Asian rain belt [53][54][55] . Previous studies confirmed that poleward EASWJ displacement causes precipitation to increase over northern China during summer 10,19,53 . Similar to the jet transition hypothesis proposed by Chiang et al. 56 , the change in the meridional position of the westerly belt modulates the precipitation in different regions of the monsoon domain. Despite a difference in amplitude between the near-term and the late-term during the twenty-first century, the spatial patterns of responses are similar in the near-term and the late-term, for conciseness, the following analyzes are based mainly on the late-term. Figure 8 indicates the anomalous patterns of EASWJ. As shown in Fig. 8, the weakening of the EASWJ under the future global warming scenarios causes the EASWJ to contract and migrate poleward. Due to the anomalous easterly wind in most of China and the anomalous westerly winds in Northeast Asia, an anomalous anticyclone appears over Northeast Asia. The poleward migration of the EASWJ and the anticyclone suggests stronger divergence in  www.nature.com/scientificreports/ the upper troposphere, which is favorable for convection and precipitation in northern China, which is confirmed by the anomalous upward motions at 500 hPa (Fig. 7). Therefore, poleward migration of the EASWJ brings heavier precipitation over northern China, contributing to the poleward migration of the EASM northern boundary.
Notably, although the mid-Pliocene (~ 3.3-3.0 Ma) climate is regarded as an analogue for a near-future climate scenario, larger northward migration of the EASM northern boundary was evident during the mid-Pliocene relative to the present day 17,57 . In the contrast, a smaller northward migration of the EASM northern boundary was visual in the future warming scenarios. The possible reasons for this may be as follows. First, based on the framework of the Pliocene Model Intercomparison Project (PlioMIP) 58 , this period is characterized by a similar atmospheric CO 2 level (405 ppmv) to that of the present day, reduced land ice in Greenland and Antarctica 59 , and much more complete vegetation cover 60 . Thus, the reduced land ice in Greenland and Antarctica 61 , and the enhanced meridional land-sea thermal contrast with an anomalous anticyclone in the northwestern Pacific Ocean, are responsible for strengthening southerly winds 17 . However, in the future, anomalous cyclones in the northwestern Pacific Ocean (Fig. 6) may prevent the strengthening of southwesterly winds. Previous studies have attributed the northward migration of the EASM northern boundary to the enhanced WNPSH in the mid-Pliocene warm period 17 , but we noted a weak change or slight weakening of the WNPSH under future global warming scenarios 51,62 (Fig. S1). During the mid-Pliocene, the lower temperature and less precipitation in the subtropical North Pacific reduced the latent heat in the western Pacific and intensified the WNPSH, enhanced the EASM circulation. However, these changes in WNPSH are not observed in future projections, which may explain the much less migration of the EASM northern boundary under the global warming scenarios than in the mid-Pliocene. Second, an important feature of the SSPs is that they cover a much wider range of aerosol and air pollutant emissions. Previous studies have investigated whether aerosols play an important role in driving the weakened EASM 5,50 . Therefore, aerosols may counteract the intensification of the EASM in the future. Finally, the nonlinear hydrological responses to warming in different scenarios 62,63 can be a reason for the stronger EASM in the mid-Pliocene than in the future. This can also explain the similar fluctuations in the northern boundary of the EASM under the SSP1-2.6, SSP2-4.5 and SSP3-7.0 scenarios.

Conclusion
By analyzing the model outputs of the CMIP6 SSPs and Historical simulations, we explored the dynamic mechanisms for the migration of the EASM northern boundary under global warming. It is projected that both the circulation and precipitation of the EASM will be strengthened through the twenty-first century under the future SSP scenarios, and the EASM northern boundary will migrate northwestward. From the low forcing sustainability pathway (SSP1-2.6) to the high-end forcing pathway (SSP5-8.5), the amplitude of migration along the EASM northern boundary gradually increases, and the amplitude of migration reaches its maximum under the high-end forcing pathway scenario (SSP5-8.5).
The enhanced land-sea thermal contrast and the poleward migration and weakened EASWJ in summer explain the substantial enhancement of the EASM circulation and northwestward migration of the EASM   www.nature.com/scientificreports/ northern boundary under future global warming. Due to the thermal inertia of the ocean, the summertime warming of the East Asian subcontinent is much stronger than that of the surrounding oceans under future global warming. The enhanced land-sea thermal contrast enhances the EASM circulation, which helps transport the warm and moist air to northern China. In addition, the poleward migration of the EASWJ triggered upward motions in the lower troposphere, which also provided favorable conditions for the increase in precipitation in northern China, eventually caused the northwestward migration of the EASM northern boundary. These results shed light on the relationship between the migration of the EASM northern boundary and global warming, and provide useful information for the formulation of decisions on ecological protection and economic development in arid and semiarid regions. The EASM domain is near the Pacific Ocean, which is directly and indirectly associated with the Hadley circulation and the El Niño-Southern Oscillation (ENSO) phenomenon. The possible roles of these factors in the responses of the EASM to future warming deserve further investigation.