Strong East Asian Summer Monsoon Research Articles

Physical processes of summer extreme rainfall interannual variability in eastern China: Part I—observational analysis

Extreme precipitation can have catastrophic effects in China by triggering floods, landslides, and other natural disasters. We measure extreme precipitation over eastern China by the maximum of five-day precipitation amount (Rx5day) in June, July, and August (JJA), which contributes more than 20% of the climate mean of JJA regional total precipitation. Based on the empirical orthogonal teleconnection (EOT) method, this work identifies four dominant regions of observed Rx5day interannual variability in eastern China: north-eastern China (EOT1), the southern lower reaches of the Yangtze valley (EOT2), southern China (EOT3) and the northern lower reaches of the Yangtze valley (EOT4). EOT1 extreme precipitation is related to a strong East Asian Summer Monsoon (EASM), a weak monsoon front and a northward displaced upper-tropospheric westerly jet. EOT2 and EOT4 extreme precipitation are related to an enhanced and stable monsoon front and a strong western North Pacific subtropical high (WNPSH). The WNPSH associated with EOT4 is stronger than that associated with EOT2, which pushes the monsoon front further north. EOT3 represents extreme precipitation that is related to anomalous southerlies around the western ridge of the WNPSH. The southerlies transport warm and moist air to southern China and increase precipitation there. The four key regions and the related mechanisms are not sensitive to the EOT technique, as the EOT-based extreme precipitation patterns and circulation anomalies are confirmed using Self-Organising Maps (SOMs).

Late Pleistocene-Holocene aeolian loess-paleosol sections in the Yellow River source area on the northeast Tibetan Plateau: chronostratigraphy, sediment provenance, and implications for paleoclimate reconstruction

Two late Pleistocene-Holocene aeolian loess-paleosol sections were identified in the Yellow River source area on the northeast (NE) Tibetan Plateau. Based on the comprehensive study on multiple proxies, AMS 14C and optically stimulated luminescence (OSL) dates, these two sections have a basal age of 14000–11500 yr BP, suggesting that they accumulated since the last deglaciation. Four stages of paleoclimate evolution can be summarized as follows. (1) A cold-dry period was dominated by the Westerlies in the late Pleistocene (ca. 14000–11500 yr BP). (2) A gradual transition to warm-wet with the weakened Westerlies and strengthened East Asian summer monsoon (EASM) occurredin the early Holocene (ca. 11500–9000 yr BP). (3) A warm-wet episode was under strong EASM in the middle Holocene (ca. 9000–3100 yr BP), however, a cold event was identified at 5000–3500 yr BP, which was caused by the abrupt weakening of the EASM and the strengthening of the Westerlies. (4) A phase of gradually shifting to dry-cold was controlled by the weakened EASM and strengthened Westerlies (after ca. 3100 yr BP). The assemblage of grain-size end-member modelling analysis (EM1-EM5) and heavy minerals indicated that aeolian deposit units have multiple sediment provenances and transport processes. EM1 is clay deposits composed of secondary clay minerals produced during weathering and pedogenesis, whereas EM2 is fine silts derived from distant dust transported through long-existing high-level Westerlies. EM3 and EM4 are coarse silts that originated from the moraines and periglacial deposits in the middle and distant source areas transported by the Westerlies and Tibetan Plateau Monsoon. Lastly, EM5 is sand from fluvial deposits and slope clastics transported by strong valley wind within the local area. This study is helpful to understand the mechanisms of paleoclimate evolution on the NE Tibetan Plateau. It also has important implications for characterizing and subdividing aeolian loess in China.

Cellulose Oxygen Isotopes of Sphagnum and Vascular Plants in a Peat Core Reveal Climate Change in Northern Japan Over the Past 2,000 Years

AbstractThe cellulose δ18O of plant tissues in peat is a potential climate proxy. However, understanding what is driving the shifts in δ18O of cellulose is required for its application. Here, we analyzed the δ18O values of Sphagnum and vascular plant cellulose, as well as the δ18O and δD values of pore water, in a 4‐m‐long peat core from the Bekanbeushi ombrotrophic bog to understand paleoclimatic changes in northern Japan over the past 2,000 years. The cellulose δ18O values of Sphagnum were lower than those of vascular plant tissues, although both draw water from the same layer. Whereas the cellulose δ18O values of Sphagnum more directly reflect those of precipitation, those of vascular plants become enriched through transpiration. Thus, the difference between vascular plants and Sphagnum (Δδ18Ovp–sp) is a potential proxy for relative humidity. Cellulose δ18O of Sphagnum revealed centennial variations with maxima around 800, 1300, and 1500 CE and minima around 500, 1000, 1700, and 1900 CE. The Δδ18Ovp–sp was inversely correlated with Sphagnum cellulose δ18O, indicating that the axis of summer westerlies was more frequently located to the north and the climate was moist in the former periods, whereas the axis of summer westerlies was generally located to the south and the climate was dry in the latter periods. These results suggest a warm and moist climate during the former periods due to frequent rainy summers driven by strong East Asian summer monsoon activity, and the opposite conditions in the latter periods.

Quantitative estimations of iron oxide minerals in the Late Pleistocene paleosol of the Yangtze River Delta: Implications for the chemical weathering, sedimentary environment, and burial conditions

Iron oxide minerals are widely present in soil and known to be closely associated with soil-forming environments. In this study, a detailed quantitative analysis of the iron oxide minerals in the Late Pleistocene hard clay of the Yangtze River Delta (YRD) was conducted using new integrated methods of magnetism, selective chemical-dissolution, and diffuse reflectance spectroscopy. A variety of established chemical weathering indexes and optically stimulated luminescence (OSL) chronology were also applied in the investigations of the hard clay in the study area. These methods allowed for detailed examinations of the chemical weathering, sedimentary environmental conditions, and the post-depositional processes of the hard clay deposits in the YRD. This study’s A-CN-K diagram revealed that the hard clay had entered an intermediate weathering stage. In addition, various chemical weathering indexes demonstrated that the hard clay had undergone three stages of chemical weathering from bottom to top as follows: 1) A relatively high stage of chemical weathering; 2) A low chemical weathering stage with a slightly increasing trend; and 3) A relatively high chemical weathering stage with a weakly decreasing trend. It was found that when χ, χfd, Fed/Fet, Gt, and Hm/Gt were not reliable indicators of chemical weathering for the hard clay, Fed could be a useful proxy. In the current study, based on the OSL ages and several paleoclimatic proxies, it was suggested that the climate conditions in the YRD had transitioned from a relatively weak to a strong East Asian summer monsoon phase during the periods between the marine oxygen isotope stages (MIS) 4 and MIS 3. Furthermore, the low values of χ and χfd in the hard clay indicated an intense dissolution of fine-grained strong magnetic minerals. Meanwhile, the relatively high Feo/Fed ratios in the superficial horizon displayed the predominance of poorly crystalline forms of iron oxides. In summary, the analysis results implied that the hard clay had undergone post-burial processes of ferrolysis, as well as dissolved organic matter translocation from the overlying Holocene sediment. The aforementioned post-burial processes for the hard clay had occurred under reduction–oxidation cycles, which were mainly driven by the rise of the groundwater levels with the Holocene transgression.

Hydrological and palynological evidence of wetland evolution on the Sanjiang Plain (NE China) in response to the Holocene East Asia summer monsoon

The responding mechanism of wetland ecosystem to climate change is currently unclear due to a lack of long-term records incorporating multiple ecological factors. This paper presents a well-dated and multi-proxy cored record from a natural freshwater wetland developed in a paleo-pingo-related depression on the Sanjiang Plain. The study aims to reconstruct the wetland water-level and vegetational regimes during the past ~6.0 ka BP and discuss the wetland evolution response to the East Asian summer monsoon (EASM) circulation. The results show that a shallow-water lake with hygrophilous herbs developed in the depression with a strong EASM during ~6.0–4.5 ka BP. The paleolake was succeeded by a wetland with a lowered mean water level and an increase in tree cover controlled by the weakened EASM during the past 4.5 ka BP. Such an ecological lake-to-wetland transition corresponds well to the sharp decline of the EASM around 4.5 ka BP which has been well documented in various geological records across the EASM domain. During the wetland developing stage over the past 4.5 ka BP, the water-level and vegetational changes in the studied depression were also closely related to the EASM variations. The strengthened EASM usually generated higher water levels with more hygrophilous herbs around 4.0 ka BP, 3.5 ka BP, 3.0 ka BP, 2.0 ka BP, and 1.3 ka BP. Considering the prevalent EASM climate serving as the predominant water supply for the wetlands on the modern Sanjiang Plain, we suggest the EASM circulation has played a critical role in driving the wetland ecological evolution during the mid-to-late Holocene Epoch. The past 0.4 ka BP is an exception as the local Trees/Herbs ratio visibly increased with the declined water level, while the monsoonal precipitation shows no equivalent shift. The wetland evolution at this stage was attributed to human interference.

A Modeling Study of the Tripole Pattern of East China Precipitation Over the Past 425 ka

AbstractThe traditional paleoclimatological view suggests that monsoonal China was overall wetter when the East Asian summer monsoon (EASM) was strong. However, this view is at odds with the understanding of modern climate, which demonstrates an analogous “tripole precipitation pattern” in monsoonal China. Little is known about the spatial pattern of paleo‐precipitation in monsoonal China, particularly during the past glacial‐interglacial cycles. Here, we provide evidence for a potential tripole precipitation pattern in monsoonal China over the past 425 ka using climate modeling, and compare the results with paleoclimate records available from China. Our simulations illustrate that more (less) precipitation in North China and South China, and less (more) precipitation in the Central‐East China during strong (weak) EASM periods associated with high (low) boreal summer insolation. Our results agree with the present understanding of modern East Asian climate, and furthermore confirm that the boreal summer insolation is the dominant forcing for the intensity of EASM and the response of subtropical high pressures is fundamental in modulating the precipitation pattern in monsoonal China on orbital timescales. This temporal and spatial variability of precipitation, as revealed in our simulations, shows the potential high complexities in hydroclimatic conditions in monsoonal China throughout the past glacial‐interglacial cycles. Nevertheless, whether the tripole precipitation pattern in monsoonal China over the past 425 ka is robust enough still should be tested, preferable by taking advantage of existing and new well‐dated and explicit indicative precipitation archives in Central‐East China.

Seasonal evolution differences of east Asian summer monsoon precipitation between Bølling-Allerød and younger Dryas periods

The rapid warming trend during the last deglaciation triggered significant global climate instabilities due to a complex non-linear response of the climate system to the gradual increase in insolation over the northern hemisphere. Although climate impacts can be detected globally, major regional imprints such as seasonal evolution and changes of the East Asian Summer Monsoon (EASM) during the last deglaciation are still poorly constrained due to a lack of comprehensive proxy data. In this study, we compare how the extreme climate shifts are linked to changes in EASM precipitation over China between the unusually warm Bolling-Allerod (BA) interstadial and the following strong cooling of the Younger Dryas (YD) stadial. Our analysis is based on the transient atmosphere-ocean simulations of TraCE-21ka, in addition to new results from high-resolution simulations of the CESM1 model for the BA and YD time slices. We find that the earlier onset and stronger intensity of the EASM in the BA interstadial lead to more precipitation in early summer (May–June) but drier conditions during mid-summer (July–August) over Southern China compared to a stadial climate during the YD episode. For Northern China, we find the opposite response. The insolation change in spring and the forced response of the atmospheric system are thought to be responsible for these differences. Relative to the YD episode, the hemispheric temperature gradient during the BA period is enhanced due to the asymmetric warming between the two hemispheres, leading to an intensified northward equatorial cross flow. Combined with a stronger sensible heating of the Tibetan Plateau in spring and the related earlier northward shift of the westerly jet, the early onset of the EASM is triggered. The latent heat release, which is accompanied by the onset of the EASM and the sudden increasing precipitation over Southern China in early summer, contributes to the westward shift of the Western Pacific Subtropical High (WPSH) and eastward movement of the South Asia High (SAH) in mid-summer. Under the above conditions, Southern China experiences a hot and dry climate, while Northern China receives more precipitation. Additionally, the La Nina-like pattern of the equatorial Pacific also partly contributes to the strong EASM in the warm period by influencing the WPSH location and Pacific-North American (PNA) teleconnection pattern.

Roles of ENSO in the Link of the East Asian Summer Monsoon to the Ensuing Winter Monsoon

AbstractThe link between the East Asian summer monsoon (EASM) and the ensuing East Asian winter monsoon (EAWM) is investigated. The results found that strong (weak) EASM years are often followed by the strong (weak) EAWM years, demonstrating that the in‐phase EASM to EAWM transition dominate the EASM‐EAWM link. The strong EASM to the strong EAWM transition is established by an anomalous anticyclone over the western North Pacific (WNPAC) in summer and an anomalous cyclone (WNPC) in winter. The weak EASM to the weak EAWM transition is connected by the same way but with the WNPC in summer and the WNPAC in winter. Further studies suggest that ENSO plays a crucial role in the in‐phase EASM to EAWM transition, which forces the transition between the WNPC and WNPAC from summer to winter via inducing Rossby wave responses to the ENSO‐induced convection anomalies. Numerical simulation analysis further confirms the dominant role of ENSO in the in‐phase EASM to EAWM transitions. The asymmetric analysis suggests that the in‐phase EASM to EAWM transition is stronger under El Niño than under La Niña.

Response of summer extreme precipitation over East Asia during the mid-Holocene versus future global warming

Changes in extreme precipitation have significant impacts on society and the environment. In recent years, the response of extreme precipitation over East Asia to global warming has received considerable attention; however, few studies have focused on the extreme precipitation in geological warm periods, especially the mid-Holocene—the most recent warm period on the orbital timescale. In this study, the outputs of mid-Holocene, pre-industrial and CO2-induced warming experiments using a coupled climate model (MPI-ESM-P) were employed to examine the changes in extreme precipitation over East Asia in the mid-Holocene and under future warming scenario, respectively. The results show that, during the mid-Holocene, summer extreme precipitation increases over the east of China but decreases over Japan and the surrounding oceans. However, under CO2-induced warming, summer extreme precipitation increases over the southeast of China, Japan, and the surrounding oceans, but decreases over the north of East Asia. Moisture budget analysis shows that these distinct responses come from different changes in moisture and circulation during the two periods. During the mid-Holocene, higher insolation over the Northern Hemisphere leads to a larger ocean–land thermal contrast and a stronger East Asian summer monsoon (EASM). The strengthened EASM brings more moisture to continental East Asia, resulting in an increase in both mean and extreme precipitation. In contrast, the CO2-induced warming facilitates an anomalous heating center in the upper troposphere over the tropical part of Northwestern Pacific ocean, which weakens the EASM. However, enhanced atmospheric moisture counterbalances the effect of the weakened EASM, resulting in more extreme precipitation over the south of East Asia.

Penetration of monsoonal water vapour into arid central Asia during the Holocene: An isotopic perspective

Monsoonal water vapour transport is an active component of the hydrological cycle, which has a profound influence on regional climate changes. This study explored how far can the East Asian summer monsoon (EASM) transport water vapour to the west during the Holocene using published speleothem oxygen isotope (δ18O) records inferred from modern analogues of precipitation δ18O in the summer monsoon season (May to September) at mid-latitudes. Modern climate analyses suggest that there are two water vapour transport pathways to arid central Asia in the summer monsoon season: westerly transport and monsoonal easterly transport. Westerly transport from the North Atlantic and Arctic Oceans is associated with 18O-depleted water vapour, while monsoonal easterly transport from the tropical Indian Ocean, South China Sea, and East Asia is associated with 18O-enriched water vapour. The mixture of these water vapour dominates precipitation δ18O in arid central Asia from May to September. The difference of δ18O between precipitation in arid central Asia and the southern Urals reflects the penetration of monsoonal water vapour into arid central Asia. Speleothem δ18O records from Kesang Cave in the central Tianshan Mountains and Kinderlinskaya Cave in the southern Urals were therefore selected to indicate monsoonal water vapour transport into arid central Asia during the Holocene. Their differences suggest prominent monsoonal water vapour were transported to arid central Asia during the early Holocene, 8.2 ka BP event and late Holocene when the EASM was very weak. The results indicate that monsoonal water vapour could be transported at least as far west as the central Tianshan Mountains (∼81.75° E) in arid central Asia during the Holocene under weak EASM conditions rather than as previously supposed under strong EASM conditions. Although the amount of monsoonal water vapour is relatively less compared with that of westerly water vapour, it cannot be neglected because this source plays an important role in changes of precipitation and its δ18O in arid central Asia during the summer monsoon season.

Impacts of atmospheric transport and biomass burning on the inter-annual variation in black carbon aerosols over the Tibetan Plateau

Abstract. Atmospheric black carbon (BC) in the Tibetan Plateau (TP) can largely impact regional and global climate. Still, studies on the inter-annual variation in atmospheric BC over the TP and associated variation in BC sources and controlling factors are rather limited. In this study, we characterize the variations in atmospheric BC over the TP surface layer through analysis of 20-year (1995–2014) simulations from a global chemical transport model, GEOS-Chem. The results show that surface BC concentrations over the TP vary largely in space and by season, reflecting complicated interplays of BC sources from different origins. Of all areas in the TP, surface BC concentrations are highest over the eastern and southern TP, where surface BC is susceptible to BC transport from East Asia and South Asia, respectively. Applying a backward-trajectory method that combines BC concentrations from GEOS-Chem and trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we assess the contributions of worldwide source regions to surface BC in the TP. We estimate that on the 20-year average, 77 % of surface BC in the TP comes from South Asia (43 %) and East Asia (35 %). Regarding seasonal variation in non-local influences, South Asia and East Asia are dominant source regions in winter and summer, respectively, in terms of the amount of BC imported. However, in terms of affected areas in the TP, South Asia is the dominant contributor throughout the year. Inter-annually, surface BC over the TP is largely modulated by atmospheric transport of BC from non-local regions year-round and by biomass burning in South Asia, mostly in spring. We find that the extremely strong biomass burning in South Asia in the spring of 1999 greatly enhanced surface BC concentrations in the TP (31 % relative to the climatology). We find that the strength of the Asian monsoon correlates significantly with the inter-annual variation in the amount of BC transported to the TP from non-local regions. In summer, a stronger East Asian summer monsoon and a stronger South Asian summer monsoon tend to, respectively, lead to more BC transport from central China and north-eastern South Asia to the TP. In winter, BC transport from central China is enhanced in years with a strong East Asian winter monsoon or a strong Siberian High. A stronger Siberian High can also bring more BC from northern South Asia to the TP. This study underscores the impacts of atmospheric transport and biomass burning on the inter-annual variation in surface BC over the TP. It reveals a close connection between the Asian monsoon and atmospheric transport of BC from non-local regions to the TP.

Variations in northeast Asian summer precipitation driven by the Atlantic multidecadal oscillation

AbstractBy analysing a 113–year (1900–2012) observational dataset, it is shown that the interdecadal fluctuation of the summer precipitation over Northeast Asia differs from that in central East Asia during the 20th century, and has experienced three interdecadal shifts in the 1920s, mid‐1960s and late 1990s. That fluctuation coincides well with the multidecadal fluctuation of the sea surface temperature in the North Atlantic, known as the Atlantic Multidecadal Oscillation (AMO). The AMO affects Northeast Asia via a circumglobal teleconnection pattern extending from the North Atlantic to North America. Corresponding with the positive phase of this teleconnection pattern, a tripolar pattern emerges in the Asian–North Pacific sector, with anomalous low pressure over Northeast Asia and high pressure over Lake Baikal and the mid‐latitude western North Pacific. These results suggest that the positive phase of the AMO favours the occurrence of cold vortex over Northeast Asia and anomalous highs over Lake Baikal and the mid‐latitude western North Pacific in summer, which enhances the East Asian summer monsoon and southward intrusion of high‐latitude cold air, and eventually increases the summer precipitation in Northeast Asia. Furthermore, initialized decadal prediction simulations using the CCSM4 model reproduce well the observed variations of the AMO and its associated atmospheric teleconnection but with slightly shifted geographic locations. The simulated anomalous Northeast Asia cold vortex and strong East Asian summer monsoon, lead to above‐normal summer precipitation over Northeast Asia. The modelling results confirm that the multidecadal variability in the North Atlantic can cause the observed interdecadal variations of Northeast Asia summer precipitation. Our results suggest that to understand and predict interdecadal climate change over Northeast Asia, it is important to consider the key role of the AMO.

Sedimentary rhythm of Mn-carbonate laminae induced by East Asian summer monsoon variability and human activity in Lake Ohnuma, southwest Hokkaido, northern Japan

Past and ongoing precipitations of Mn-carbonate (rhodochrosite) have occurred in the freshwater Lake Ohnuma, which is situated about 8 km south of the Hokkaido-Komagatake volcano, southwest Hokkaido (northern Japan). Lake sediments from a surface and a longer ∼4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro-XRF fluorescence analyses. The three tephra ash layers from the Hokkaido-Komagatake volcano, characterized by peaks of Al, Si, K, Ca, and Ti, correspond to regionally recognized tephras (1640, 1929 and 1996 CE) based on ashfall isopachs and 137Cs, 210Pb and 14C ages determined from the two cores. The sedimentation rate showed a drastic change around 1929 CE; ∼0.3 cm/yr for 1640–1929 CE and ∼2.7 cm/yr for 1929–2012 CE. The Mn content record of Lake Ohnuma sediments, composed of rhodochrosite laminae, showed a periodic variation with a decadal–centennial scale over the interval 1640–2012 CE. Sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated, followed by the crystal growth of rhodochrosite within the anoxic sediments. On the basis of these results, high Mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff. The Ti/Si ratios are representative of the relative abundance of detrital inputs from the watershed. The combined signals of elevated Mn content and Ti/Si ratio for 1935–2012 CE indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed, associated with the post-1970s intensive land use such as livestock farming. For 1640–1935 CE, the Lake Ohnuma record shows an antiphase relationship between the Mn content and Ti/Si ratio, which implies that the detrital inputs reflected the strength of the East Asian summer monsoon (EASM), and the reduced rainfall runoff resulted in weaker mixing, followed by depleted O2 in the water column. The antiphase variation in Mn content and Ti/Si ratio suggest weak EASM during the Little Ice Age (∼1850 CE), followed by strong EASM between 1860 and 1929 CE.

Holocene vegetation-hydrology-climate interactions of wetlands on the Heixiazi Island, China

An integrated view of wetland's evolution is currently poorly understood due to a lack of knowledge on long-term interactions of multiple ecological factors. Here, we present a cored palynological record covering the Holocene Epoch from a depressional wetland on Heixiazi Island (China). With the aid of principal component analysis and cluster analysis of the palynological data, the historical vegetation regime has been well reconstructed for wetlands on the island. With further assistance from the published data on local hydrology and regional East Asian summer monsoon (EASM) variations, the interactions of vegetation, hydrology, and climate in the island's wetlands have been thoroughly analyzed with correlation analysis of the three factors. The results indicate that a strong EASM generally led to a high water level in the wetlands from increased monsoonal precipitation, causing an increase in arboreal vegetation and a decrease in herbs. Alternatively, a weak EASM generally led to low water levels due to decreased monsoonal precipitation, causing a decrease in arboreal vegetation and an increase in herbs. The local vegetation regime during the early Holocene was marked by an increase in tree/herb ratio due to rising water levels under the influence of an increasingly strengthened EASM. Subsequently, a general decline of the tree/herb ratio occurred from a gradual decrease in water level during the mid and late Holocene when the EASM gradually retreated. The wettest stage marked by the highest water level and tree/herb ratio occurred 8.0–4.6 ka BP with the strongest EASM. The driest stage occurred during the last 0.4 ka BP, which was attributed to both the weakest EASM influence and anthropogenic disturbance. Our study provides an integrated view of the wetlands' ecological dynamics incorporating multi-factor interactions, which further sheds light on the EASM driving mechanisms on wetlands evolution during the Holocene Epoch.

Effects of Subgrid Terrain Radiative Forcing on the Ability of RegCM4.1 in the Simulation of Summer Precipitation Over China

AbstractWe implemented a subgrid terrain radiative forcing (STRF) scheme into the International Centre for Theoretical Physics, Italy, Regional Climate Model Version 4.1 (RegCM4.1) and evaluated its impacts on the summer precipitation simulation over China by comparing the model results from the experiments with and without the STRF scheme. The RegCM4.1 without the STRF scheme simulates strong East Asian summer monsoon and overestimates the summer precipitation over China. Adopting the STRF scheme in the RegCM4.1 improves the representation of surface radiation process over complex terrains, by reducing the net gain of the surface radiation energy over the Tibetan Plateau (TP). As a result, the heat released from the surface to the overlying air column decreases, thus suppressing the development of local convection over the TP. In addition, the weakened TP thermal forcing due to the STRF effect reduces the land‐sea thermal contrast, resulting in a weakened East Asian summer monsoon that produces less transport of water vapor from tropical oceans and hence weakens summer precipitation over China. Our analysis indicates that such improvement in the precipitation simulation induced by the STRF effect is mainly due to the improved simulation of precipitation intensity, particularly that associated with the moderate and heavy precipitation events, and convective processes.

Severe Historical Droughts Carved on Rock in the Yangtze

AbstractThe White Crane Ridge (WCR) Rock Fish, now submerged under the backwater of the Three Gorges Reservoir in the Yangtze River, are affirmed as one of the earliest hydrologic observations ever made in any large river in the world. The usually in-water monument provides highly valuable historical records of severe droughts in the upper Yangtze over the last 1,200 years. This article updated the historical drought chronology previously developed based on the WCR inscriptions, which can be applied in assessment of extreme climatic and hydrological risks, and also made a preliminary analysis of changes of the severe drought frequency during the last thousand years in the upper Yangtze. The analysis shows that the severe droughts occurred more frequently during the Medieval Climate Anomaly (MCA), relatively less so during the Little Ice Age (LIA), and once again more often under the background of modern global warming. It was suggested that a generally warmer Euro-Asian continent during the MCA was in favor of the stronger East Asian summer monsoon, and the resulting less precipitation and more severe droughts of the Yangtze and the lower water level at the Three Gorges area on the centennial scale, and vice versa for the period of the LIA. The results would help in understanding the causes and mechanisms of the regional climate change and variability, and also in taking measures in the fields of the watershed management to cope with the long-term change in climatic and hydrologic droughts.

Link between East Asian summer monsoon and sedimentation in river-mouth sandbars since the early Holocene preserved in the Yangtze River subaqueous delta front

AbstractThis paper presents geochemical and grain-size records since the early Holocene in core ECS0702 with a fine chronology frame obtained from the Yangtze River subaqueous delta front. Since ~9500 cal yr BP, the proxy records of chemical weathering from the Yangtze River basin generally exhibit a Holocene optimum in the early Holocene, a weak East Asian summer monsoon (EASM) period during the middle Holocene, and a relatively strong EASM period in the late Holocene. The ~8.2 and ~4.4 cal ka BP cooling events are recorded in core ECS0702. The flooding events reconstructed by the grain-size parameters since the early Holocene suggest that the floods mainly occurred during strong EASM periods and the Yangtze River mouth sandbar caused by the floods mainly formed in the early and late Holocene. The Yangtze River-mouth sandbars since the early Holocene shifted from north to south, affected by tidal currents and the Coriolis force, and more importantly, controlled by the EASM. Our results are of great significance for enriching both the record of Holocene climate change in the Yangtze River basin and knowledge about the formation and evolution progress of the deltas located in monsoon regions.

Possible link of an exceptionally strong East Asian summer monsoon to a La Niña-like condition during the interglacial MIS-13

Previous studies from the central and eastern Chinese Loess Plateau (CLP) showed that the S5-1 paleosol, which corresponding to Marine Isotope Stage (MIS) 13, was the strongest developed during the last million years, suggesting extremely strong East Asian summer monsoon (EASM). However, evidence shows that regional diversity exists in the relative intensity of the S5-1 soil formation. In this study, we first verify systematically the S5-1 soil development across the whole CLP by using numerous loess records. We then examine the spatial variation of the magnetic susceptibility of different loess-paleosol sections to identify the spatial change of the EASM intensity during MIS-13. We also compare the loess records with other monsoon records in China. Our results show that, in central CLP, S5-1 is indeed the strongest developed as suggested by previous studies, whereas in the western CLP, it is weakly developed and the S4 paleosol developed during MIS-11 is the strongest. As compared to MIS-11, the northern front of the EASM during MIS-13 didn’t penetrate into western CLP, but was located more northerly and eastward. Based on sea surface temperature records in the tropical Pacific, we suggest that the temporal and spatial abnormal variation of the EASM during MIS-13 could be related to a strong La Niña or La Niña-like climate condition. Under such condition, the Western Pacific Subtropical High (WPSH) becomes weaker and retreats more northeastward, leading to more precipitation in the mid-east region of northern China but less rainfall in the western inland.

Foreign influences on tropospheric ozone over East Asia through global atmospheric transport

Abstract. Tropospheric ozone in East Asia is influenced by the transport of ozone from foreign regions around the world. However, the magnitudes and variations in such influences remain unclear. This study was performed to investigate the influences using a global chemical transport model, GEOS-Chem, through the tagged ozone and emission perturbation simulations. The results show that foreign ozone is transported to East Asia (20–60∘ N, 95–150∘ E) mainly through the middle and upper troposphere. In East Asia, the influence of foreign ozone increases rapidly with altitude. In the middle and upper troposphere, the regional mean concentrations of foreign ozone range from 32 to 65 ppbv, being 0.8–4.8 times higher than its native counterpart (11–18 ppbv). Annually, ∼60 % of foreign ozone in the East Asian middle and upper troposphere comes from North America (5–13 ppbv) and Europe (5–7 ppbv), as well as from foreign oceanic regions (9–21 ppbv). Over the East Asian tropospheric columns, foreign ozone appears most in spring when ozone concentrations in the foreign regions are high and the westerlies are strong and least in summer when the South Asian High blocks eastward foreign ozone from reaching East Asia south of 35∘ N. At the East Asian surface, the annual mean of foreign ozone concentrations is ∼22.2 ppbv, which is comparable to its native counterpart of ∼20.4 ppbv. In the meantime, the annual mean of anthropogenic ozone concentrations from foreign regions is ∼4.7 ppbv, half of which comes from North America (1.3 ppbv) and Europe (1.0 ppbv). Seasonally, foreign ozone concentrations at the East Asian surface are highest in winter (27.1 ppbv) and lowest in summer (16.5 ppbv). This strong seasonality is largely modulated by the East Asian monsoon (EAM) via its influence on vertical motion. The large-scale subsidence prevailing during the East Asian winter monsoon (EAWM) favours the downdraft of foreign ozone to the surface, while widespread convection in the East Asian summer monsoon (EASM) blocks such transport. Interannually, the variation in foreign ozone at the East Asian surface is found to be closely related to the intensity of the EAM. Specifically, the stronger the EAWM is in a winter, the more ozone from North America and Europe reaches the East Asian surface because of the stronger subsidence behind the East Asian trough. In summer, ozone from South and South-east Asia is reduced in strong EASM years due to weakened south-westerly monsoon winds. This study suggests substantial foreign influences on ozone at the East Asian surface and in its tropospheric columns. It also underscores the importance of the EAM in the seasonal and interannual variations in foreign influences on surface ozone in East Asia.

Possible mechanisms for the coupling between late spring sea surface temperature anomalies over tropical Atlantic and East Asian summer monsoon

The relationship between the preceding late spring Sea Surface Temperature (SST) over the tropical Atlantic and the East Asian Summer Monsoon (EASM) is investigated based on the observational data and Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations. The results show that warm (cold) tropical Atlantic SST (TASST) during May tends to be followed by a strong (weak) EASM with positive (negative) precipitation anomalies over the subtropical frontal area. Evidence is also provided that the atmospheric teleconnections propagating in both east and west directions are the key mechanisms linking the EASM with the preceding May TASST. That is, the warm TASST anomaly during late spring can persist through the subsequent summer, which, in turn, induces the Gill-type Rossby wave response in the eastern Pacific, exciting the westward relay of the Atlantic signal, as well as the eastward propagation of the Rossby wave along the jet stream. Furthermore, the westward (eastward) propagating teleconnection signal may induce the anomalous anticyclone in the lower troposphere over the Philippine Sea (anomalous tropospheric anticyclone with barotropic structure over the Okhotsk Sea). The anomalous anticyclonic circulation over the Philippine Sea (Okhotsk Sea) brings warm and humid (cold) air to higher latitudes (lower latitudes). These two different types of air mass merge over the Baiu-Meiyu–Changma region, causing the enhanced subtropical frontal rainfall. To support the observational findings, CMIP5 historical simulations are also utilized. Most state-of-the-art CMIP5 models can simulate this relationship between May TASST and the EASM.