East Asian Summer Monsoon Circulation Research Articles

The offshore wind speed changes in China: an insight into CMIP6 model simulation and future projections

Offshore wind speed in China plays a key role in affecting air–sea interactions, coastal tides, and wind energy, but its changes in a warming climate and the associated causes remain unclear. Based on the ERA5 reanalysis and the Coupled Model Intercomparison Project Phase 6 (CMIP6) models, this study evaluates the past and future variations of wind speed at 10 m (WS10) over China’s offshore seas in summer and winter. The results show that the CMIP6 multi-model mean performs well in simulating the climatological patterns (1981–2010) of WS10 for both seasons. The trends and leading variabilities in WS10 are also reasonably reproduced in the South China Sea (SCS). In the northern SCS, WS10 has strengthened during both seasons in the recent decades. In contrast, in the East China Sea (ECS), WS10 has increased (decreased) during summer (winter). Further attribution analysis suggests that the forcing of greenhouse gasses (aerosols) may make WS10 stronger (weaker) in the two seas and for both seasons, while natural variability tends to slow down (speed up) WS10 in the SCS and ECS during summer (winter). In addition, according to the CMIP6 model projections under various warming scenarios, WS10 is likely to increase over both the northern SCS and the ECS in summer, while WS10 will increase over the northern SCS but decrease over the ECS in winter. Differences in the projected WS10 changes in the ECS during summer and winter are attributed to the projected intensification (weakening) of the East Asian summer (winter) monsoon circulation.

Characteristics of warming and humidification in the Yellow River's upper reaches and their impact on surface water resources

AbstractBased on multi‐source data, including observation, analysis, model calculation, satellite remote sensing, and scenario estimation data, this study has systematically analysed the warming and dry–wet change characteristics of the Yellow River's upper reaches in the last 60 years. The causes and features of the current warming and humidification and their impact on surface water resources are also discussed. This study found that since 1961, the average temperature in the Yellow River's upper reaches has increased at a rate of 0.37°C/10a, which is more significant than the increases observed for the world, China, and even northwest China in the same period. At the beginning of this century, the rate of change in precipitation in the Yellow River's upper reaches reversed direction from decreasing to increasing, with a significant increase (35 mm/10a) since 2000. Therefore, climate change has shown a significant warming and humidification trend in the upper reaches of the Yellow River. From the dynamic change perspective, the Yellow River's upper reaches were warming and drying in each climatic period from 1961 to 2010; however, the tendency towards dryness slowed period by period. In contrast, the region showed consistent warming and humidification from 1991 to 2020. From the perspective of multiple time scales, the change in average temperature is mainly dominated by the multi‐decadal scale. In contrast, precipitation is dominated by interannual change. The increase in the multi‐decadal component in average temperature and precipitation explains the current warming and humidification. Precipitation changes in the upper reaches of the Yellow River Basin are mainly affected by the westerly circulation, the East Asian summer monsoon, and the South Asian summer monsoon circulation systems. However, precipitation was affected by different circulation factors on different time scales. On an interannual scale, it was mainly affected by the South Asian summer monsoon circulation. On an interdecadal scale, it was mainly affected by the westerly wind and the East Asian summer monsoon circulation. On a multi‐decadal scale, it was jointly influenced by the westerly wind, the East Asian summer monsoon, and the South Asian summer monsoon circulation. Under the control of multi‐decadal synergistic enhancement of the westerly, the East Asian summer monsoon, and the South Asian summer monsoon circulations, the vertical field shows an abnormal upward movement. There has been a significant abnormal easterly and southerly airflow that transports water vapour, and cold air is transported along the northwest airflow at the back of the Mongolian cyclone, which makes the climate in the upper reaches of the Yellow River more humid. The warming and humidifying of the upper reaches of the Yellow River has significantly increased runoff and lake area and significantly degraded the frozen soil. The surface water budget in the Yellow River's upper reaches has significantly responded to the current warming and humidification. The actual surface evapotranspiration and surface runoff increased significantly, but the net surface water budget still showed a decreasing trend. The increase in precipitation did not change the surface water deficit.

Holocene climatic controls on flooding regime along the Ussuri River in Northeast Asia

Knowledge of the long-term flooding response to climatic changes is critical for probing the flooding future in an oncoming warmer world. In this paper, three well-dated wetland sedimentary cores with high-resolution grain-size records were employed to reconstruct the historical flooding regime along the Ussuri River during the past 7000 years. The results show that five flooding-prone intervals marked by increased mean rates of sand-fraction accumulation occurred at 6.4–5.9 ka BP, 5.5–5.1 ka BP, 4.6–3.1 ka BP, 2.3–1.8 ka BP, and 0.5–0 ka BP, respectively. These intervals are generally consistent with the higher mean annual precipitation controlled by the strengthened East Asian summer monsoon which has been widely documented in geological records across the monsoonal regions of East Asia. Considering the prevalent monsoonal climate along the modern Ussuri River, we suggest that the regional flooding evolution during the Holocene Epoch should be generally controlled by the East Asian summer monsoon circulation which was initially linked to the ENSO activities in the tropical Pacific Ocean. While for the last interval spanning 0.5–0 ka BP, human influence, compared with the long-serving climatic controls, has played a more critical role in driving the regional flooding regime.

Northwestward advance of the northern boundary of the East Asian summer monsoon over the 21st century in CMIP6 projections

AbstractThe East Asian summer monsoon (EASM) projection has attracted much attention, whereas there are few investigations on the future changes of the EASM northern boundary. The boundary migration would influence the distribution of precipitation and the related vegetation, and its projection is important for policy development of climate change adaptation. In this study, based on the median of 22 selected Coupled Model Intercomparison Project Phase 6 (CMIP6) models, the linear trends of meridional movement of the EASM northern boundary are found to be 0.45−1.39° of latitude during 2015–2099 based on the three precipitation‐based metrics under three shared socioeconomic pathway scenarios. Spatially, the multimetric climatological EASM northern boundary displays a 70–170 km northwestward advance during 2080–2099 compared to 1981–2010. Such an advance also holds true for most individual models, but the migration magnitudes vary with metrics, scenarios and models. The strengthened EASM in association with the intensified land–sea thermal contrast and the enhanced atmospheric water vapour content in response to global warming account for the northwestward advance of the EASM northern boundary, which is related to the increased possibility of the negative phase of the Pacific decadal oscillation in the future. Additionally, the thermodynamic component due to the increased moisture content contributes more than the dynamic term arising from the reinforced EASM circulations to the intensified precipitation and northwestward migration of the EASM northern boundary. The future advance of the EASM northern boundary favours a “northern flood and southern drought” precipitation pattern over eastern China, which would partly mitigate drought conditions in northern arid regions.

Oceanic and atmospheric anomalies associated with extreme precipitation events in China 1983-2020.

Observed synoptic anomalies in connection with China's extreme precipitation events/floods in the summers of 1982/83, 1997/98, 2010, 2014, 2015/16, and 2020 are studied. These events mainly occur within the middle and lower Yangtze basins. The dominant moisture source is the Northern Indian Ocean and the Southwestern Pacific Ocean of the Indo-Pacific warm pool (IPWP). Both of these bodies of water have warmed since 1979. In East Asia, the strong land-sea thermal contrast driven by global warming drives the increased East Asian summer monsoon (EASM) circulation, which develops deep convective precipitation. The total precipitable water in the Indo-Pacific region has also been increasing since 1979. The intense southwest Indian monsoon transports moist air to the Yangtze basin in mid-June and forms the Meiyu (plum rain) front. Strengthened Okhotsk/Ural blocking highs in East and West Asia, as well as the Western Pacific subtropical high (WPSH) and the South Asian high (SAH) over south Eurasia, remain stationary for long periods and interact to exacerbate the precipitation. The western edge of the WPSH expands westward towards East Asia to transport moisture. To the north, the WPSH combines with the two blocking highs to trigger more rain. The intensified SAH expands eastward and merges with the extended WPSH to add rain. On the other hand, rainfall is modulated by the El Niño-Southern Oscillation (ENSO), notably in relation to the super El Niño events in 1982-1983, 1997-1998, 2015-2016, and 2020. The research described in this paper highlights changes in the weather systems with warming and, in particular, the enormous and dominating impact of the warming and expanding IPWP on rainfall extremes. Improved seasonal forecasts and planning ahead will protect lives and livelihoods.

Late-Holocene fire regimes under climate and vegetation change in the Sanjiang plain, northeastern China

Understanding historical fire regimes and their driving mechanisms provides valuable insights for future hazard prediction. Here, we reconstructed the fire and vegetation histories of the Sanjiang Plain over the last 5000 years based on microscopic (<125) charcoal content and morphology of sedimentary coring in the coastal marshes of the middle reaches of the Ussuri River, respectively. The results of the study indicate that four high fire frequency phases were identified in the last 5000 years, at 4.3-4.0 ka BP, 2.8-2. ka BP, 1.4-1.2 ka BP, and 0.5 ka BP to date. These intervals coincide with the weakening phase of the East Asian summer monsoon (EASM) during the late Holocene and are generally consistent with the decreasing trend of precipitation in the region. Thus, we suggest that the EASM circulation, and especially its accompanying precipitation, played a key role in the evolution of fire regimes in the Sanjiang Plain during the Late Holocene. In addition to climate change, regional vegetation conditions are also significant factors driving fire occurrence. In contrast, the gradual increase in fire frequency over the last 500 years has been caused mainly by human activities rather than climate and vegetation.

Characteristics of the extreme precipitation and its impacts on ecosystem services in the Wuhan Urban Agglomeration

Summer extreme precipitation is one of the most frequent, intense, and influential extreme weather events that occurs frequently in the Wuhan Urban Agglomeration (WUA). Preventing meteorological disasters and coping with climate change necessitate understanding the characteristics and causes of extreme precipitation and its impact on ecosystems. In this study, the spatiotemporal characteristics of summer extreme precipitation in the WUA are analysed from 1961 to 2020. Then, NCEP reanalysis data and the relevant circulation index are used to explore the causes of extreme precipitation. Finally, how extreme precipitation influences key ecosystem services, such as water yield, water regulation, and soil conservation, is investigated. The results reveal that (1) extreme precipitation in the WUA has shown an obvious upwards trend over the past 60 years. Huanggang, Xianning, Huangshi, Wuhan, and E'zhou city demonstrate the highest values. The extreme precipitation increased significantly after 1980s, especially the R97.5P and PRCPTOT with change rate of 12.1 mm/10a and 18.82 mm/10a respectively. (2) Atmospheric circulation variation is a dominant factor affecting extreme precipitation in the WUA and causes the meridional distribution of the “+ − +” wave train in eastern China. The intensity and location of the Western Pacific subtropical high are closely related to extreme precipitation. Furthermore, the weakening of the East Asian summer monsoon circulation is also conducive to the occurrence of extreme precipitation. (3) The spatial distribution of water yield and runoff retention in abnormal extreme precipitation years are similar to the variation patterns of the total amount of extreme precipitation. Water yield and runoff retention in high-value extreme precipitation years are higher than that in low-value extreme precipitation years, while soil conservation shows no difference. In addition, ecosystem services have a synergistic relationship in high-value areas and a trade-off relationship in low-value areas. This study can contribute to the understanding of extreme precipitation in the WUA and its interaction with ecosystem services.

Interaction between different mixing aerosol direct effects and East Asian summer monsoon

Aerosol mixtures, which are still unclear in current knowledge, may cause large uncertainties in aerosol climate effect assessments. To better understand this research gap, a well-developed online coupled regional climate-chemistry model is employed here to investigate the influences of different aerosol mixing states on the direct interactions between aerosols and the East Asian summer monsoon (EASM). The results show that anthropogenic aerosols have high-level loadings with heterogeneous spatial distributions in East Asia. Black carbon aerosol loading accounts for more than 13% of the totals in this region in summer. Thus, different aerosol mixing states cause very different aerosol single scattering albedos, with a variation of 0.27 in East Asia in summer. Consequently, the sign of the aerosol instantaneous direct radiative forcing at the top of the atmosphere is changed, varying from − 0.95 to + 1.50 W/m2 with increasing internal mixing aerosols. The influence of aerosol mixtures on regional climate responses seems to be weaker. The EASM circulation can be enhanced due to the warming effect of anthropogenic aerosols in the lower atmosphere, which further induces considerable aerosol accumulation associated with dynamic field anomaly, decrease in rainfall and so on, despite aerosol mixtures. However, this interaction between aerosols and the EASM will become more obvious if the aerosols are more mixed internally. Additionally, the differences in aerosol-induced EASM anomalies during the strongest and weakest monsoon index years are highly determined by the aerosol mixing states. The results here may further help us better address the environmental and climate change issues in East Asia.

Evaluation and projections of the East Asian summer monsoon in a perturbed parameter ensemble

The East Asian summer monsoon (EASM) is a dominant driver of East Asian climate, with variations in its strength potentially impacting the livelihoods of millions of people. Understanding, predicting, and assessing uncertainties in these variations are therefore important area of research. Here, we present a study of the projected twenty-first century changes in the EASM using a ‘perturbed parameter ensemble’ (PPE) of HadGEM3-GC3.05 coupled climate models, which samples uncertainties arising from differences in model parameter values. We show that the performance of PPE members for leading order EASM metrics is comparable to CMIP5 and CMIP6 models in many respects. But the PPE also exposes model biases which exist for almost all parameter combinations. These ‘structural’ biases are found mainly to affect metrics for the low-level circulation. We also show that future changes in regional circulation and precipitation are projected consistently across the PPE members. A more detailed moisture budget analysis of the precipitation changes in a region covering the Yangtze River valley shows that the spread of these changes is mainly due to spread in dynamic responses. We also perform parameter sensitivity analyses and find that a parameter controlling the amplitude of deep-level entrainment is the main driver of spread in the PPE’s representation of the EASM circulation. Finally, we discuss how the information provided by the PPE may be used in practice, considering the plausibility of the models, and giving examples of ways to sub-select ensemble members to capture the diversity in the moisture budget changes.

Interdecadal to Multidecadal Variability of East Asian Summer Monsoon Over the Past Half Millennium

AbstractThe East Asian summer monsoon (EASM) plays a crucial role for ecosystems and societies in East Asia past, present, and future. However, substantial uncertainties remain regarding EASM variability on interdecadal to multidecadal timescales because of the short length of instrumental data in East Asia. This study extended the EASM circulation index in the modern meteorological studies to the paleoclimate over the past half‐millennium (1470–1998 CE) to reconcile the understanding of the EASM variability in paleoclimate and modern meteorological studies. The EASM index is reconstructed based on the common signal from the three main types of the proxy records (the tree rings, speleothems, and historical documentary data) related to EASM. The reconstructed EASM index captures the simultaneous changes of the “Meiyu precipitation” and the southwesterly anomalies in South China on interdecadal to multidecadal timescales, which is a dynamic pattern visible and well‐documented in the modern meteorology. Analysis of the reconstructed EASM index suggests that the interdecadal to multidecadal EASM variability is closely associated with the Pacific‐Japan teleconnection pattern, which acts as a bridge between the negative phase of the Pacific Decadal Oscillation and the anomalous anticyclonic circulation over the western North Pacific. It also indicates that the EASM variability over the recent 30 years (1992–2021 CE) falls within the range of natural variability over the past half‐millennium.

Dominant Coupling Mode of SST in Maritime Continental Region and East Asian Summer Monsoon Circulation

AbstractUsing a series of data from the National Oceanic and Atmospheric Administration (NOAA) and singular value decomposition (SVD) method, the dominant coupled mode between sea surface temperature (SST) anomaly in Maritime Continent (MC) and circulation anomaly in East Asia in summertime and its underlying mechanism are revealed. Results show that the first SVD mode is characterized by the in‐phase variations of SST anomaly in the entire MC and sea level pressure (SLP) anomaly in East Asia, and their co‐variation is measured by an SVD joint index, . Based on composite difference fields, the ocean mixed layer heat budget reveals that the maintenance of the MC regional SST anomaly is jointly determined by ocean dynamics and heat flux. The anomalous SST warming near Java Island is dominated by the meridional advection of ocean current, while the SST warming in the Bay of Bengal, the South China Sea and the Northwest Pacific are mainly contributed by the increased downward latent heat flux and shortwave radiation flux. The SLP anomaly pattern in East Asia coupled with SST anomaly in the MC region is represented by the anomalous anticyclone in the Northwest Pacific, which can be maintained by the ‐related atmospheric vertical cell. The ‐related circulation anomalies tend to favor the summer precipitation in the middle and lower reaches of the Yangtze River and the south of Yunnan‐Guizhou Plateau, but lead to negative precipitation anomaly in central and Northeast China.

Response of East Asian summer monsoon climate to North Atlantic meltwater during the Younger Dryas

The Younger Dryas (YD) event, recognized as one of the most typical abrupt climate changes on the millennial time scale, results in striking cooling in most regions of the North Atlantic. The most acceptable hypothesis believes that this event is related to a large volume of meltwater fluxes injected into the North Atlantic. In remote Asia, various paleoclimate reconstructions have revealed that the East Asian summer monsoon (EASM) is significantly depressed during the cold YD episode. However, the effect of North Atlantic meltwater-induced cooling on the whole downstream Eurasian regions and its potential dynamics remains been not fully explored till now. In this study, the responses of Asian climate characteristics during the YD episode, especially the EASM, are evaluated based on modeling data from the Simulation of the Transient Climate of the Last 21,000 years (TraCE 21ka). The results show that the cooling signal during the YD, which is mainly caused by meltwater flux, spreads from the North Atlantic to the whole Eurasia. In agreement with the paleoclimatic proxies, the simulated EASM is obviously weakened. The summer precipitation is also suppressed over East, South, and Central Asia. Dynamically, the North Atlantic cooling produces an eastward propagated wave train across the mid-latitude Eurasia, which facilitates weaker EASM circulation. The weakened land-sea thermal contrast over East Asia also contributes to the monsoon decrease during YD cooling.

Mid-Late Holocene vegetation and hydrological variations in Songnen grasslands and their responses to the East Asian Summer Monsoon (EASM)

Exploring the Mid-Late Holocene interactions between ecological and climate variations in semi-arid areas such as Songnen grasslands (northeastern China) provide insights into how future vegetation changes and hydrological variations may have an impact on semi-arid ecosystems, in general. In this study, we present a high-resolution palaeoecological dataset covering the past 7300 years. For this purpose, a peat section has been sampled from the shore of Dabusu lake situated in the southwestern part of the Songnen grasslands. We use pollen analyses to reconstruct the regional and local vegetation dynamics, and measure TOC (total organic carbon) and the degree of peat humification to study changes in regional and local biomass. Diatom analysis, grain size analysis, and CaCO3 contents are used to assess the water-table fluctuations of the Dabusu peatlands. The results show that from 7300 to 6000 cal BP, Dabusu peatlands were characterized by grasslands, dominated by Poaceae plants and high-water level. From 6000 to 910 cal BP, forest-steppes and dry-steppes dominated in the region alternatively. The water level of Dabusu peatlands experienced relatively low levels following an earlier rapid decline around 6000–5100 cal BP. Then after a slight increase between 5100 and 4000 cal BP, it remained at a relatively low level until 910 cal BP. After 910 cal BP, steppe vegetation dominated in the region and the water level of Dabusu peatlands slightly rose. By comparing these features with regional climate change, we show that EASM circulations might be the main driving forces controlling the regional paleovegetation dynamics and hydrological variations. This outcome is critical for the understanding of the long-term interactions between vegetation, hydrology, and climate of mid-latitude semi-arid grasslands.

Strong coupling of the East Asian summer monsoon and hydroclimate footprints during 53–47 ka

A multi-proxy (δ18O, δ13C and trace elements) analysis of an absolutely-dated stalagmite in northern China is conducted to reconstruct the East Asian summer monsoon (EASM) intensity and local hydroclimate conditions during the period of 53.8–47.4 ka (BP, before 1950 CE). Our δ18O and δ13C records, with 24-yr resolution, clearly register the Dansgaard-Oeschger (DO) events 14 and 13. A clear double-peak structure for DO13, as previously detected in the NGRIP ice core, is identified in our δ18O and δ13C records, for the first time. This strong correspondence at the finer time-scale indicates a sensitive and rapid response of the EASM system to the North Atlantic climate changes through air-sea coupling. Our δ18O record shows that the EASM intensity deceased abruptly during latter stage of DO14 (51.1–50.3 ka). This abrupt change is different from other stalagmite δ18O records from central and southern China. On the other hand, both the δ13C and trace element records, representing local hydroclimatic changes, match well with the δ18O record, reflecting an integrated EASM system, on the timescale of the DO cycles. The covariance suggests that the hydrological variation in northern China could be modulated by the movement of the large-scale EASM circulation. Further comparison displays an anti-correlated relationship between the millennial precipitation changes in northern and southeastern China during the period of 51.5–47.4 ka. This anti-correlation is analogous to the “North-South” dipole mode in observed present-day precipitation. Overall, this study advances our understanding of the spatial and temporal evolution of the EASM precipitation on the millennial timescale.

Differences in East Asian summer monsoon responses to Asian aerosol forcing under different emission inventories

Opposite anthropogenic aerosol emission trends in Asia can lead to different responses of the climate. Here, we examined the responses of the East Asian summer monsoon (EASM) to changes in Asian anthropogenic aerosol emissions during 2006–2014 using a global aerosol/atmospheric chemistry–climate coupled model (BCC_AGCM2.0_CUACE/Aero) with two sets of emission inventories: the Community Emissions Data System (CEDS) inventory adopted by the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the inventory developed at Peking University (PKU). The changes in Asian anthropogenic aerosol emissions during 2006–2014 between the two inventories were remarkably different, particularly in eastern China where completely opposite trends were observed (i.e., increase in the CEDS inventory, but significant reduction in the PKU inventory). The perturbation simulations with the Asian anthropogenic aerosol forcing from the two inventories showed opposite changes in aerosol optical depth, aerosol effective radiative forcing, cloud liquid water path, and total cloud cover in eastern China. The simulated ‘dipole-type’ changes (i.e., increase in India but decrease in China) in Asian aerosols and the resulting changes in local radiation budget under the PKU inventory were consistent with the corresponding observations. The summer surface temperatures over eastern China decreased by 0–0.4 K because of the Asian anthropogenic aerosol forcing under the CEDS inventory, while they increased by 0.1–0.8 K under the PKU inventory. The weakening of the EASM index caused by the Asian aerosol forcing under the PKU inventory was twofold greater than that under the CEDS inventory (−0.4 vs. −0.2). The Asian ‘dipole-type’ aerosol forcing contributed to the observed summer ‘southern drought and northern flood’ phenomenon in eastern China during 2006–2014. The slow ocean-mediated response to the regional ‘dipole-type’ aerosol forcing dominated the weakening of the EASM circulation and the precipitation changes in eastern China in the total response. This study further confirms that the biases in anthropogenic aerosol emissions over Asia can affect the CMIP6-based regional climate attribution.

Asian summer monsoon responses to the change of land‒sea thermodynamic contrast in a warming climate: CMIP6 projections

It is of practical significance to use the updated Coupled Model Intercomparison Project Phase 6 (CMIP6) models to study the impact of changes in land‒sea thermodynamic contrast (TC) on the Asian summer monsoon under different scenarios and to compare the similarities and differences of the impact mechanisms between different monsoon regions. In this study, we investigated future changes of the Asian summer monsoon under four Shared Socioeconomic Pathway (SSP) scenarios using 19 CMIP6 models. The intensity of the South Asian summer monsoon (SASM) is projected to decrease by 2.6%, 6.3%, 10.1%, and 11.1%, while the East Asian summer monsoon (EASM) intensity is projected to increase by 4.6%, 7.9%, 7.4%, and 9.8% until the end of the 21st century for SSP126, SSP245, SSP370, and SSP585 scenarios, respectively. Moreover, summer precipitation in Asia is projected to increase remarkably in 2015–2099 under all four scenarios. The inconsistent warming trends over the Tibetan Plateau (TP), Northwest Pacific, and tropical Indian Ocean would greatly impact the monsoon circulations. The upper-troposphere warming trend over the surrounding oceans is remarkably greater than that over the TP, while the near-surface warming trend over the surrounding oceans is smaller than that over the TP. The decrease of upper-troposphere TC between the TP and tropical Indian Ocean results in a weakening of the SASM circulation. The enhancement of the lower-troposphere TC between the TP and Northwest Pacific would strengthen the EASM circulation. Moisture budget analysis shows that the water-vapor would increase in the future, which would thermodynamically enhance summer precipitation through the anomalous vertical moisture transport associated with mean flow. The strengthening of the meridional circulation of the EASM would increase monsoon precipitation, while the weakening of zonal circulation of the SASM would dynamically reduce South Asian summer precipitation.

The sensitivity of simulated aerosol climatic impact to domain size using regional model (WRF-Chem v3.6)

Abstract. Domain size can have significant impact on regional modeling results, but few studies examined the sensitivities of simulated aerosol impact to regional domain size. This study investigates the regional modeling sensitivities of aerosol impact on the East Asian summer monsoon (EASM) to domain size. The simulations with two different domain sizes demonstrate consistently that aerosols induce the cooling of the lower troposphere that leads to the anticyclone circulation anomalies and thus the weakening of EASM moisture transport. The aerosol-induced adjustment of monsoonal circulation results in an alternate increase and decrease pattern of precipitation over China. Domain size has a great influence on the simulated meteorological fields. For example, the simulation with larger domain size produces weaker EASM circulation, which also affects aerosol distributions significantly. This leads to the difference of simulated strength and area extent of aerosol-induced changes of lower-tropospheric temperature and pressure, which further results in different distributions of circulation and precipitation anomalies over China. For example, over southeastern China, aerosols induce the increase (decrease) of precipitation from the smaller-domain (larger-domain) simulation. Different domain sizes consistently simulate an aerosol-induced increase in precipitation around 30∘ N over eastern China. This study highlights the important influence of domain size on regional modeling results of aerosol impact on circulation and precipitation, which may not be limited to East Asia. More generally, this study also implies that proper modeling of meteorological fields with appropriate domain size is one of the keys to simulating robust aerosol climatic impact.

Distinctive South and East Asian monsoon circulation responses to global warming

The Asian summer monsoon (ASM) is the most energetic circulation system. Projecting its future change is critical for the mitigation and adaptation of billions of people living in the region. There are two important components within the ASM: South Asian summer monsoon (SASM) and East Asian summer monsoon (EASM). Although current state-of-the-art climate models projected increased precipitation in both SASM and EASM due to the increase of atmospheric moisture, their circulation changes differ markedly—A robust strengthening (weakening) of EASM (SASM) circulation was projected. By separating fast and slow processes in response to increased CO2 radiative forcing, we demonstrate that EASM circulation strengthening is attributed to the fast land warming and associated Tibetan Plateau thermal forcing. In contrast, SASM circulation weakening is primarily attributed to an El Niño-like oceanic warming pattern in the tropical Pacific and associated suppressed precipitation over the Maritime Continent.

Holocene terrestrialization process on the Sanjiang Plain (China) and its significance to the East Asian summer monsoon circulation

A wide-spread terrestrialization process has been reported occurring across the Sanjiang Plain in the Holocene Epoch, while little is known for its detailed process and links to regional climate variation. Here, we present high-resolution palynological and lithological data of a peatland's sedimentary profile in the central Sanjiang Plain. The study aims to reconstruct the local terrestrialization process and discuss its significance to the Holocene East Asian summer monsoon (EASM) evolution. The results indicate that a paleolake surrounded by broadleaved forests developed during ~7.4-4.5 ka BP, corresponding to a wet and warm stage with strengthened EASM. Thereafter, a wetland dominated by Cyperaceae and Poaceae was initiated with the terrestrialization process. This process lasted until 3.9 ka BP when a lithological mud-to-peat transition occurred indicating a peatland began to develop after the extinction of the paleolake. Considering the prevalent EASM climate on the modern Sanjiang Plain, this paleolake-to-peatland transition during 4.5-3.9 ka BP was attributed to the mid-Holocene EASM retreat in addition to the paleolake's autogenic infilling process. During the peatland developing stage, four dry intervals with weakened EASM occurred in 3.9-3.2 ka BP, 2.0-1.3 ka BP, 1.0-0.5 ka BP, and 0.1-0.0 ka BP. They were alternated with three wet stages of the strengthened EASM during 3.2-2.0 ka BP, 1.3-1.0 ka BP, and 0.5-0.1 ka BP. Our findings are supported by a series of paleoclimatic records across the monsoonal regions of China. We suggest that the EASM evolution associated with the tropical ocean-atmosphere interactions was initially modulated by the solar output variation.

Index‐ and model‐dependent projections of East Asian summer monsoon in Coupled Model Intercomparison Project Phase 6 simulations

AbstractIn terms of five categories of East Asian summer monsoon (EASM) indices and one classic meridional wind index, we project the EASM intensity change over the 21st century using 27 global climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the mid‐range combined scenarios of Shared Socioeconomic Pathways and Representative Concentration Pathways (SSP2‐4.5). Results show that future changes in the EASM intensity are index‐ and model‐dependent. Relative to the reference period 1985–2014, the EASM intensity strengthens slightly based on the east–west thermal contrast indices but varies little as a whole based on the southwest monsoon indices over the 21st century, with large discrepancies among the models. However, it tends to weaken over time based on the other three categories of indices, including the north–south thermal contrast indices, the shear vorticity indices, and the South China Sea monsoon indices. In terms of the meridional wind index that directly measures the EASM circulation in the lower troposphere and holds valid under future global warming, the EASM intensity strengthens significantly over the 21st century as a consequence of increases in zonal and meridional land–sea thermal contrasts between the East Asian continent and the adjacent oceans of western North Pacific and South China Sea. There are certain relationships between the trends of the meridional wind index and other EASM indices. The projections are overall consistent between the ensemble means of all available 27 models and the 14 good models as selected according to their abilities to simulate the present EASM climatology, but with different magnitudes.