Weak East Asian Summer Monsoon Research Articles

East Asian summer monsoon evolution during Heinrich Stadial 5

The bi-millennial Heinrich Stadial 5 (HS 5) event, occurring approximately 48 thousand years ago (kyr BP, before 1950 AD), represents a notable cooling episode during the last glacial period in the Northern Hemisphere. However, our comprehension of its impact on Asian summer monsoon variability remains incomplete, largely because of the scarcity of precisely dated and high-resolution records. Here, we present stalagmite δ18O-inferred absolutely dated decadal-resolved evolution of the East Asian summer monsoon (EASM) during HS 5 from Xianyun Cave, southeastern China. Stalagmite chronology constrains the occurrence of HS 5 from 48.67 ± 0.13 to 46.42 ± 0.10 kyr BP. The Xianyun stalagmite record distinctly reveals the three-phase evolution of the EASM during HS 5. It includes a gradual onset lasting for 0.79 kyr (phase 1) from 48.67 to 47.88 kyr BP, a weak EASM oscillation phase (phase 2) with three centennial-scale intensified monsoon events during 47.88–46.94 kyr BP, followed by a gradual 520-yr recovery phase (phase 3). In phases 1 and 2, the synchronous climatic shifts and resemblance to three notable centennial-scale events observed in both the Xianyun and Greenland ice core records suggest a rapid atmospheric teleconnection between the North Atlantic and EASM regions. During phase 3, the 5-century monsoon recovery registered in the Xianyun record contrasts sharply with the rapid 60-yr rebound in the Greenland ice core record. This disagreement suggests a plausible decoupling between low- and high-latitude climates, probably related to tropical sea surface temperatures variabilities and the distinct responses of the regional Intertropical Convergence Zone to El Niño Southern Oscillation variabilities. Our study highlights the essential role of tropical ocean-atmosphere coupling systems in the termination stages of millennial-scale HS events.

How the enhanced East Asian summer monsoon regulates total gross primary production in eastern China

Recognizing the relationship between gross primary production (GPP) and precipitation in eastern China, the East Asian Summer Monsoon (EASM) plays a crucial role in shaping GPP. Despite confirmation of the strong link between EASM and GPP, there remains a notable research gap in understanding the specific impact of the EASM on GPP in different regions of eastern China. Here we used simulations from Trends in Net Land–Atmosphere Carbon Exchanges (TRENDY) models from 1951 to 2010 and divided eastern China into five subregions for the study. We also used the New East Asian Summer Monsoon Index (NEWI) as a quantitative metric to distinguish between periods of strong and weak EASM. Building on this, this study aims to investigate the response of GPP in different subregions of eastern China. Regionally, under strengthened EASM years (1954, 1957, 1965, 1969, 1977, 1980, 1983, 1987, 1993 and 1998), East China experienced the most pronounced increase in GPP at 12 ± 21 (mean ± 1 sigma) gC m−2 mon−1 compared to the weak EASM years (1958, 1961, 1972, 1973,1978, 1981, 1985, 1994, 1997 and 2004). In contrast, Southwest China showed a decline in GPP at −4 ± 10 gC m−2 mon−1. Moreover, GPP also increased in Northeast and North China when EASM strengthened, while South China showed a decline in GPP. This indicated that GPP changed with monsoon intensity. According to the mechanism analysis, during strong EASM, there was intense moisture convergence through alterations in the atmospheric circulation field over East China and abundant precipitation, which further contributed to the increase in GPP. Downward solar radiation in Southwest China decreased with EASM enhancement, which suppressed GPP and hindered vegetation growth. Overall, the results highlight the importance of accurately predicting the impact of different EASM intensities of regional carbon fluxes.

Why does a decrease in cloud amount increase terrestrial evapotranspiration in a monsoon transition zone?

Terrestrial evapotranspiration plays a critical role in drought monitoring and water resource management. Changes in evapotranspiration are significantly influenced by cloud-related precipitation and radiation effects. However, the impact of cloud amount (CA) on evapotranspiration through its influence on precipitation remains uncertain, especially in the transition zone affected by the East Asian summer monsoon (EASM), which limits the understanding of the water cycle. Therefore, this study deeply explores the impact of CA on evapotranspiration and its potential physical mechanisms in Northwest China. The results show that the correlation between 31-year average evapotranspiration and CA is negative only in the semi-arid region and is positive in other climatic regions of Northwest China. This unique negative correlation is related to the change of precipitation pattern in the semi-arid region caused by the weak EASM. Smaller CA in weak monsoons results in more short-wave radiation reaching the surface, larger sensible heat, and weaker convective inhibition. Consequently, the proportion of convective clouds (CCs) increases and precipitation from these CCs enhances evapotranspiration. Less CA increases evapotranspiration and potentially exacerbates aridity in the semi-arid region of Northwest China. These results emphasize the role of cloud type in evapotranspiration. It is well known that global warming can change cloud type with more CCs. Therefore, this study sheds new light on evapotranspiration change under global warming.

Contrasted changes of sea-surface salinity in the southern and northern Okinawa Trough since the mid-Holocene

A reconstruction of sea-surface salinity (SSS) in the southern Okinawa Trough (OT) since 6800 cal. yr BP has been presented in this study. We found that the SSS record increased gradually from the middle to late Holocene, indicating reduced summer precipitation in Taiwan. Our results also suggest regional patterns of SSS with SSS increasing in the southern OT and decreasing in the northern OT since the mid-Holocene. Observational data indicate that the SSS in the southern and northern OT are strongly affected by fluvial discharge from Taiwan and the Changjiang River, linked to summer precipitation in South and Central Eastern China, respectively. The contrasting changes in SSS from the OT since the mid-Holocene suggest different spatial patterns of precipitation, with decreasing summer precipitation in South China and increasing summer precipitation in central-eastern China due to the weakened East Asian Summer Monsoon (EASM) and solar insolation. The delayed seasonal transitions of the EASM, with a longer Meiyu stage and a shorter midsummer stage from the mid-Holocene to late Holocene, are consistent with the delayed northward migration of the westerlies in the late Holocene.

Impacts of a Recent Interdecadal Shift in the Summer Arctic Dipole on the Variability in Atmospheric Circulation over Eurasia

This study investigated the relationship between the summer Arctic Dipole (AD) anomaly and the climatic variability in Eurasia during the period 1979–2021. It was found that the summer AD anomaly experienced a phase shift from frequent negative phases before 2006 to positive phases after 2007, as manifested by the shift of the center of the positive (negative) AD anomaly to Greenland (in the Laptev Sea and East Siberian Seas) in the more recent period (2007–2021) from the vicinity of the Kara Sea and Laptev Sea (the Canadian archipelago) in the earlier period (1979–2006). Before the mid-2000s, a wave train was shown in the middle troposphere of Eurasia, and this teleconnection pattern of atmospheric circulation could have resulted in local warm and wet (cool and dry) anomalies over northern Russia and East Asia (Western Europe and the Far east). Since the mid-2000s, the wave train has experienced a notable adjustment that was conducive to East Asian and Arctic cooling, displaying anticyclonic anomalies around northern Eurasia and two cyclonic anomalies centered near the Arctic and East Asia. The presence of a cold Arctic anomaly was found to enhance westerly winds at high latitudes by modulating the meridional temperature gradient (MTG) and impeding the southward propagation of cold Arctic air. Additionally, the warmth of northern Eurasia may have also resulted in a reduction in the MTG between northern Eurasia and the mid-lower latitudes, favoring a weakening of zonal winds over the central region of Eurasia. The increased upper-level westerly winds over southern East Asia implied a weakened East Asian Summer Monsoon, which inhibited precipitation in northeast China.

Holocene provenance variations and palaeofloods response to ENSO-driven monsoon precipitation in the subalpine peatland in southern China

There have been few investigation of the provenance of sediments and hydrological response to past climatic change in the subalpine peatland in southern China. The source of sediments and regional climatic variations during the Holocene have been deduced from Rare Earth Elements (REEs), SrNd isotopic data and grain size end-member modeling analysis (EMMA) derived from peat sediments from the southwestern mountainous region of Hunan Province, China. From the distinctive SrNd isotopes patterns, LREEs enrichment, and the Eu anomaly observed in these sediments, it can be concluded that local weathering residues have been the primary sedimentary source throughout the Holocene. Based on the method of EMMA, four distinct end members were identified. EM2, signifying weathering products of moderate particle size, is sensitive to climate variations. This sensitivity makes it an effective marker to track the historical evolution of the East Asian summer monsoon (EASM) intensity. The results suggest that the prevalence of the relatively weak EASM during the interval 11,600–9000 cal yr BP, followed by an overall weakening trend during the past 9000 years. EM3 + EM4, the coarsest component in the peat sediments, indicate strong runoff typically caused by the extreme weather events. Notably, the occurrence of relatively high EM3 + EM4 component during the interval 5400–4500 cal yr BP and 3500–2600 cal yr BP is consistent with the previous reported palaeoflood events in the Yangtze River basin. We suggest that the regional flood patterns are predominantly controlled by the East Asian monsoon system, which in turn, is driven by ENSO activities. For a comprehensive understanding of regional flood patterns, it is imperative to extend research into high-altitude areas.

Different responses of precipitation patterns to the East Asian summer monsoon weakening: The 7.2 and 8.2 ka events

The 8.2 ka abrupt cooling event has been extensively studied, whereas the 7.2 ka event and their comparations rarely mentioned. Here, we present a high resolution (∼4 years) and precisely dated (±25 years) speleothem multi-proxy record from Zhangjia Cave (northern margin of Sichuan basin, China) spanning the 7.2 and 8.2 ka events within the transition of early to middle Holocene. The speleothem δ18O mainly reflects the dynamic facets of the East Asian summer monsoon (EASM) system, while its δ13C and trace elements ratios serve as an indicator of local hydrological conditions. Our results reveal intense dry conditions in southwest China during the 7.2 ka event, despite the 7.2 event was characterized by a relatively stronger monsoon compared to the 8.2 ka event. Furthermore, the hydrological conditions during the 7.2 ka event and the middle Holocene appear to exhibit “north flood and south drought” pattern when compared to different paleoclimate records in EASM domains, while it can be summarized as “spatially consistent drought” in the 8.2 ka event. These different hydrological conditions are a response to the weakening of the EASM, a phenomenon also observed in modern climatic observations and empirical simulations, which underpins our understanding that the speleothem δ18O variations are mainly dynamical manifestations of the EASM system, likely different with its thermodynamical appearances.

Middle Miocene evolution of East Asian summer monsoon precipitation in the northeast part of the Tibetan Plateau based on a quantitative analysis of palynological records

Characterized by elevated pCO2 levels and global warmth, the mid-Miocene climate is a valuable analogue for investigating how the East Asian Summer Monsoon (EASM) may evolve at different time scales. In this paper, we present a quantitative EASM precipitation record with a temporal resolution of ∼14 kyr during the mid-Miocene (∼15.97 to 13.64 Ma) by applying the probabilistic CREST (Climate Reconstruction Software) method to palynological records from the northeast part of the Tibetan Plateau. Reconstructed mid-Miocene EASM precipitation (∼860 mm) was almost twice that of today (∼450 mm), indicating much stronger EASM intensity. The reconstruction shows a gradual long-term decline on which was superimposed a stronger EASM period (∼15.97–14.54 Ma) followed by a relatively stable period (∼14.54–13.84 Ma) and a short period of reduced precipitation (∼13.84–13.64 Ma). The correspondence of EASM precipitation changes with the mid-Miocene climate optimum and west-east thermal gradients in equatorial Pacific suggests these two factors were the main driving forces for EASM evolution from ∼15.97 to 14.54 Ma, whereas the combined impact of global cooling and the northward shift of the Intertropical Convergence Zone is probably responsible for the slight decline of the EASM from ∼14.54 to 13.84 Ma. The weaker EASM between ∼13.84 and 13.64 Ma was most likely a response to the global significant cooling event Mi-3. On orbital time scales, the precipitation records exhibit a dominant ∼400 kyr periodicity, indicating EASM changes were mainly paced by eccentricity via the modulation of precessional amplitude, and the East Antarctic Ice Sheet variations were probably another important driving force.

ENSO-like evolution of the tropical Pacific climate mean state and its potential causes since 300ka

The tropical Pacific Ocean plays a significant role in climate change, and the El Niño-Southern Oscillation (ENSO) is considered to be closely related to extreme climate phenomenon worldwide. However, the evolution of the ENSO-like patterns in the tropical Pacific during the Pleistocene glacial cycles remains controversial. In this study, we present geochemical indices and a transient model simulation to explore the ENSO-like evolution during the glacial-interglacial cycles and its driving mechanisms. Our results indicate that during the interglacial periods, the Western Pacific Warm Pool (WPWP) is characterized by the modern El Niño-like conditions with relatively decreased precipitation and weakened East Asian Summer Monsoon (EASM). By contrast, the mean state of WPWP during glacial periods is more similar to the modern La Niña-like conditions, with increased precipitation and the intensified of EASM. Spectral analyses showed that the WPWP hydrodynamics were controlled by the earth's orbital parameters through their impacts on ocean circulations. Combined with published data, we find that expansion of water masses formed in the Southern Ocean would change the deep-water redox conditions, subsurface water upwelling and surface productivity in the WPWP, and ultimately influence the ENSO-like conditions through CO2-related Walker Circulation on glacial-interglacial cycles.

Fire History in the Qinling Mountains of East‐Central China Since the Last Glacial Maximum

AbstractThe study of fire history and driving mechanisms at long time scales can provide a theoretical background for future fire management in forested regions. The alpine lake sediments from Daye Lake in the Qinling Mountains, east‐central China, were used to explore the influence of climate, vegetation and human activity on fires since the LGM, based on charcoals and black carbon. During the last glacial period, fire activity was mostly controlled by regional aridity under a weak East Asian summer monsoon, while human‐induced fires were commonly dominated in the late Holocene. Fire activity was found to be linked to biomass through temperature variability. The biofuel dominated by conifers induced high intensity fires in the last glacial, and herbs contributed more to the high fire frequency over the mid‐late Holocene. With predicted future rising temperatures, increased vegetation cover and extreme climate events may increase the fire risk in the region.

Mechanisms of hydrological responses to volcanic eruptions in the Asian monsoon and westerlies-dominated subregions

Abstract. Explosive volcanic eruptions affect surface climate, especially in monsoon regions, but responses vary in different regions and to volcanic aerosol injection (VAI) in different hemispheres. Here, we use six ensemble members from the last-millennium experiment of the Coupled Model Intercomparison Project Phase 5 to investigate the mechanisms of regional hydrological responses to different hemispheric VAIs in the Asian monsoon region (AMR). Northern hemispheric VAI (NHVAI) leads to an intensified aridity over the AMR after northern hemispheric VAI (NHVAI); spatially, a distinct inverse response pattern to the climatological conditions emerges, with an intensified aridity in the relatively wettest area (RWA) but a weakened aridity in the relatively driest area (RDA) of the AMR. Southern hemispheric VAI (SHVAI) shows a weakened aridity over the AMR, but the spatial response pattern is not that clear due to small aerosol magnitude. The mechanism of the hydrological impact relates to the indirect change of atmospheric circulation due to the direct radiative effect of volcanic aerosols. The decreased thermal contrast between the land and the ocean after NHVAI results in a weakened East Asian summer monsoon and South Asian summer monsoon. This changes the moisture transport and cloud formation in the monsoon and westerlies-dominated subregions. The subsequent radiative effect and physical feedbacks of local clouds lead to different hydrological effects in different areas. Results here indicate that future volcanic eruptions may temporarily alleviate the uneven distribution of precipitation in the AMR, which should be considered in the near-term climate predictions and future strategies of local adaptation to global warming. The local hydrological responses and mechanisms found here can also provide a reference for stratospheric aerosol engineering.

A Case Study on the Impact of East Asian Summer Monsoon on Surface O3 in China

The East Asian summer monsoon (EASM) was extremely strong in 2018, which substantially affected surface ozone (O3) in China. Taking 2018 and the average synthesis of 2003 and 2010 to represent the strong and weak EASM cases, respectively, GEOS-Chem with constant anthropogenic emission was employed to investigate the impact of the EASM on surface O3 in the east of China. Simulations show that surface O3 decreased in the northeast and the eastern coast of China and increased in most of the remaining regions during strong EASM. The difference in surface O3 between strong and weak EASM was around −15~7 ppbv. After analyzing relevant meteorological fields, it is found that the decrease in northeast China was mainly attributed to the large increase in vertical upward transport. The considerable decrease in the Huang-Huai-Hai region depended on the dilution and diffusion of eastward anomalous horizontal circulation. The increase in Hunan-Hubei-Guangdong Province was largely due to input from the north. In addition, the vast areas between the Yangtze River and the Yellow River were supported by higher temperatures and stronger shortwave solar radiation that promoted photochemical reactions. The reasons for changes in Shanxi-Sichuan-Yunnan Province were relatively more complex and thus require more in-depth exploration.

Weakening of the Summer Monsoon Over the Past 150 Years Shown by a Tree‐Ring Record From Shandong, Eastern China, and the Potential Role of North Atlantic Climate

AbstractThe causes of the decreased intensity of the East Asian summer monsoon (EASM) over the past 150 years are still not fully understood, although several studies have linked the monsoon weakening to the warming of tropical oceans. Here, we use pine tree‐rings to reconstruct the precipitation total for April–August from 1810 to 2018, in south‐central Shandong Province, in the EASM region. The reconstruction accounts for 41.8% of the instrumental precipitation variance during 1965–2018. The EASM precipitation reconstruction shows extreme pluvial conditions in 1832, 1833, 1886, and 1998, and extreme droughts in 1878, 1901, and 1910, which correspond precisely to extreme climatic events recorded in historical documents. The reconstructed precipitation reveals a drying trend since the 1870s, which matches well with the decreasing trend of the EASM inferred from stalagmite oxygen isotope (δ18O) records and climate simulations. The trend of decreasing precipitation since the 1870s, indicated by our reconstruction, is significantly correlated with the spring sea surface temperature (SST) of the North Atlantic Ocean, which suggests that the EASM weakening was linked to North Atlantic SST variations during the past 150 years. This potential role of North Atlantic SST variability is supported by climate sensitivity simulations of the Community Earth System Model. North Atlantic SST variability induces two teleconnections of Rossby‐like wave propagation from the North Atlantic into East Asia, resulting in anomalous precipitation in this region.

Composite Effects of ENSO and EASM on Summer Ozone Pollution in Two Regions of China

AbstractCombining the 3‐D reanalysis with the surface observational O3 data, this study investigated the composite effects of the El Niño‐Southern Oscillation (ENSO) and East Asian summer monsoon (EASM) on summer O3 pollution in two regions of China, that is, the Beijing‐Tianjin‐Hebei (BTH) and the Yangtze River Delta (YRD). The results show that the El Niño has significantly enhanced the near surface O3 pollution in the BTH, but has little effect on surface O3 in the YRD. On the contrary, the La Niña has mainly reduced the surface O3 pollution in the YRD with little effect in the BTH. When the El Niño and a strong EASM occurred at the same time, the effect on O3 was more significant than that by the El Niño event alone due to the extra intensification of the vertical transport of O3 with about 10–30 μg/m3 from stratosphere and the worsening of the horizontal diffusion conditions (easterly wind anomalies) in the BTH. The La Niña with a weak EASM played a dominant role in the reduction of O3 pollution in the YRD by improving the meteorological conditions (especially in total cloud cover) and horizontal diffusion conditions (westerly wind anomalies). These composite effects were reflected in the trends of summer surface O3 in the BTH and the YRD, and became the crucial contributors to the abnormal changes of summer surface O3 in some years when the ENSO/EASM happened.

Multi-decadal to centennial scales variability in the East Asian Summer Monsoon around the 5.5 kyr B.P. climate event

Understanding the variability of the mid-Holocene climate on multi-decadal to centennial scales, including the well-known 5.5 kyr B.P. cold event, is important to better evaluate possible changes of the East Asian Summer Monsoon (EASM) as global warming progresses. Over the past few decades, stalagmite oxygen isotope records have been widely used as a proxy for the EASM; however, most high-resolution stalagmite records are from southern China. In this study, a 4-yr-resolution δ18O record obtained from stalagmite TW704 in northeastern China was used to reconstruct regional precipitation and the EASM between 5.82 and 4.77 kyr B.P. (where B.P. indicates “before 1950 A.D.”). In general, the inferred EASM variations agree with previously published stalagmite records from southwestern and central China, suggesting synchronous variations of the EASM on decadal to centennial scales across monsoonal China. Our new record also features relatively large decadal–centennial EASM oscillations from northern China, with two centennial-scale weak EASM events centred at 5.6 and 5.0 kyr B.P. The former corresponds to the well-known 5.5 kyr B.P. cold event that occurred in the high-latitude North Atlantic Ocean. These depressed EASM events were probably caused by prolonged periods of El Niño, resulting in decreased precipitation over northeastern China. Our study provides robust evidence to support the hypothesis that ENSO played an important role in modulating the EASM during the mid-Holocene.

How did the climate and human activities modulate the sedimentary evolution of the central Yellow Sea Mud, China

• The sedimentary evolution and provenance variation of Central Yellow Sea Mud was the result of sea level rising at the begin of Holocene and the established shelf circulation after 7.0 ka. • Drastic environmental changes in the Yellow River basin after 4.0 ka induced by weakening of East Asian Summer Monsoon and enhanced human activities increased the sediment accumulation in the Central Yellow Sea Mud. • The sedimentation of Central Yellow Sea Mud was controlled by the sediment supply under the regulation of monsoon climate and human activities after 4.0 ka. The Central Yellow Sea Mud (CYSM) archived important signals of regional environment and climate changes during the Holocene. We performed high-resolution analysis of two sediment cores taken from the CYSM, including measurements of radiocarbon chronology and grain size, as well as the mineralogical and geochemical measurements on the sediment samples. The results indicate that the Yellow River were likely a dominant source of the CYSM before 6.8 ka BP, with minimal contributions from the local small rivers in the Shandong Peninsula and Korean Peninsula, as confirmed by the mineralogical and geochemical evidences. After 6.8 ka BP, the fine-grained sediments accumulated in the CYSM presented a mixture of those from both the Yangtze and Yellow River with an individual contribution of 55 ± 8% and 45 ± 6%, respectively, as the sea level reached its highstand and initiated the establishment of modern shelf circulation. The drastic environmental changes in the Yellow River basin after 4.0 ka BP driven by the weakened East Asian Summer Monsoon and enhanced human activities that significantly increased the sediment yield from the Chinese Loess Plateau should be responsible for the abrupt increase in the fine-grained sediment (<2 μm) accumulation in the CYSM. After 1.4 ka BP the decrease in precipitation in the Yellow River basin resulted in the imbalance between sediment yield and river runoff, which perhaps induced lower channel aggradation and frequent avulsions, and consequently decreased the accumulation rate of the Yellow River-derived sediment in the CYSM. During the Holocene, the rising sea level played a primary role in shaping the sedimentation of the CYSM including the retreated coastline and establishment of shelf circulation system, whereas the monsoonal climate and human activities dominated the terrestrial sediment supply and accumulation in the CYSM after 4.0 ka BP.

Holocene East Asian Summer Monsoon Variation Recorded by Sensitive Grain Size Component from the Pearl River-Derived Mud in the Northern South China Sea

Abstract To better understand the environmental significance of sediment grain size in continental shelf of the South China Sea (SCS), we carried out a detailed grain size study of sediments from the YJ Core, derived from the mud deposits of the northern SCS. Based on the grain size-standard deviation method, two sensitive grain size components were identified, namely, component 1 (8.2 ~ 9.3 μm) and component 2 (106.8 ~ 120.7 μm), respectively. The results indicate that the sensitive component 1 is likely to derive from fine-grained materials of the Pearl River. These fine-grained materials could be transported by the southwestward coastal current during the wet season, with the domination of the East Asian summer monsoon (EASM). Accordingly, the sensitive component 1 could be sensitive to climate change and has a great potential to reconstruct details of EASM variations. During the period of 7500-6800 cal yr BP, the sensitive component 1 may be controlled by both sea level change and EASM intensity. Besides, the curve of the sensitive component 1 in the YJ Core presents a strong EASM during the interval 6800-3500 cal yr BP and a weak EASM during the period of 3500-2000 cal yr BP, which is synchronous with other paleoclimate records in southern China. In the past 2000 years, the sensitive component 1 may reflect the increasing of human activities. It is essential to carry out more studies with higher resolution in mud areas to clarify a detailed historical evolution of EASM intensity over the whole Holocene.

Summer ozone pollution in China affected by the intensity of Asian monsoon systems

Ozone in the troposphere is harmful to human health and ecosystems. It has become the most severe air pollutant in China. Here, based on global atmospheric chemistry model simulations during 1981–2019 and nation-wide surface observations, the impacts of interannual variations in Asian summer monsoon (ASM), including East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM), on surface O3 concentrations during June-July-August (JJA) in China are investigated. EASM intensity has a significant positive correlation with the surface O3 concentration in south-central China (97.5°–117.5°E, 20°–35°N) with a correlation coefficient of 0.6. Relative to the weak EASM years, O3 concentrations in strong EASM years increased by up to 5 ppb (10 % relative to the average) due to the weakened transboundary transport of O3 resulting from the decrease in prevailing southwesterlies. SASM can be divided into two components. The one near East Asia has a similar relation with O3 in southern China (100°–117.5°E, 22°–32°N) as that of EASM. The other component of SASM is negatively correlated with surface O3 concentration in eastern China (110°–117.5°E, 22°–34°N) and the maximum difference in O3 concentrations exceeded 5 ppb (10 %) between the strong and weak monsoon years, which can be explained by the O3 divergence caused by the anomalous southerlies blowing pollutants away from the northern boundary of eastern China. This study shows that the ASM has an important impact on the O3 concentrations in China, primarily through changing transboundary transport related to the variability of large-scale circulations, which has great implications for air pollution prevention and mitigation in China. Future projections of ASM suggests that the sustainable and medium development scenarios are the perfect pathways that can help to mitigate O3 pollution, while high social vulnerability and radiative forcing scenarios could enhance future O3 pollution in China.

Late Miocene Tarim desert wetting linked with eccentricity minimum and East Asian monsoon weakening

Periodic wetting is an inherent feature of many monsoon marginal region deserts. Previous studies consistently demonstrate desert wetting during times of Earth’s high orbital eccentricity and strong summer monsoon. Here we report the first evidence demonstrating desert wetting during Earth’s low orbital eccentricity from the late Miocene strata of the northwestern Tarim Basin of northern China, which is commonly thought to be beyond the range of Asian monsoon precipitation. Using mechanisms for modern Tarim wetting as analogs, we propose that East Asian summer monsoon weakening enhanced westward moisture transport and caused opposite desert wetting pattern to that observed in monsoon marginal region deserts. This inference is supported by our model simulations. This result has far-reaching implications for understanding environmental variations in non-monsoonal deserts in the next few thousands of years under high atmospheric CO2 content and low eccentricity.

Increased extreme drought events in south–central China since the last century：Evidence from oxygen isotope signatures preserved in tree ring cellulose

Tree-ring cellulose oxygen isotope ratios (δ18O) is a well-established proxy for hydroclimatic conditions in monsoon Asia. We reconstructed June–October relative humidity (RHJ–O) variations from 1808 to 2017, based on tree-ring cellulose δ18O data, which explain 46.2% of the actual RH variance in the Nanyue region, south–central China. Extreme wet events occurred frequently prior to the 1900s, but there have been more extreme dry events since the 1900s, apart from the late 1930s and early 1950s. Periodicity analysis revealed that the reconstructed RHJ–O records show obvious 15–30 years cycles from the 1830–1970s. The multi-decadal signals may reflect the effect of the Pacific Decadal Oscillation (PDO) on hydroclimate. In the positive PDO phase, there is drying in south–central China, which is related to a weaker East Asian summer monsoon (EASM) via the Pacific–Japan teleconnection. The decadal signal has weakened since the 1970s. In addition, the reconstructed RHJ–O record shows strong interannual variations, which may be related to the Central Pacific El Niño–Southern Oscillation (CP ENSO). During extreme CP El Niño events, there is a weaker EASM due to the Pacific–Japan teleconnection, and the study site experienced drought. Our reconstructed moisture record is characterized by a decreasing influence from the PDO and increasing influence from the CP ENSO in recent decades. Moreover, the frequency of CP ENSO events is projected to increase under anthropogenic warming. Consequently, more extreme droughts which are related to CP ENSO events may increase in the south–central China in near future.