Strong Winter Monsoon Research Articles

Near-surface ocean temperature and air-sea heat flux observed by a buoy array during summer to autumn in year 2014 in the northern South China Sea

The observational data of the air-sea interface and upper ocean elements from a buoy array deployed in the northern South China Sea from June to November 2014 is analyzed. The COARE 3.0 algorithm was adopted to examine the variability of air-sea heat fluxes and their contributions to near-surface ocean temperature changes. During the observation period, air-sea heat exchange in the northern South China Sea is primarily determined by solar shortwave radiation and latent heat flux, with net heat flux mostly negative, indicating that the ocean predominantly loses heat. In autumn, net heat flux loss significantly increased by an average of 34.4 W/m2, primarily due to enhanced latent heat loss driven by stronger winter monsoon. During typhoon period, net air-sea heat flux was significantly suppressed, mainly due to reduced solar shortwave radiation, with minimal contribution from sensible heat flux. Near-surface ocean temperature exhibited a seasonal cooling trend, averaging 29.7°C in summer and 28.4°C in autumn. The maximum cooling during typhoon reached 3.1°C, with minimal contribution from air-sea heat flux. During the monsoon transition period, weaker winds led to a slight increase in near-surface ocean temperature, with air-sea heat flux contributing 70.2% and 56.3% to this process. During winter monsoon, more uniform water column mixing resulted in a gradual decrease in near-surface ocean temperature, with air-sea heat flux contributing over 60% to this process.

Low-latitude forcing on 4.2 ka event indicated by records in the Asian monsoon region

The 4.2 ka event is an important climatic event of the Holocene that is expected to considerably influence the development of human civilizations. However, the forcing mechanism of the 4.2 ka event remains unclear, with current evidence being divided between high-latitude and low-latitude forcings. The Asian climate system contains strong summer and winter monsoon circulations, and thus the region is ideal for studying high- and low-latitude climate forcings on 4.2 ka event. In this study, we present a high-resolution pollen record of the past 5000 years from Wudalianchi Crater Lake in northeastern China in the East Asian monsoon region. An evident decrease in broadleaf trees occurred during the 4.2 ka event, suggesting a decrease in summer monsoon rainfall. Well-dated high-resolution pollen, stalagmite δ18O, and winter monsoon records were collected to study the possible transportation of high- and low-latitude signals. Most of the pollen records showed significant changes during the 4.2 ka event, while no apparent change was found in winter monsoon records. Meanwhile, more obvious changes in stalagmite δ18O records were observed in lower- rather than the higher-latitude regions around 4.2 ka. Our results suggest that the signal of the 4.2 ka event might be transported by low-latitude processes, thus corresponding to a low-latitude forcing.

Large shift in the upper water column characteristics of the west-central Bay of Bengal during the last glacial-interglacial transition

The upper water column structure in the Bay of Bengal is strongly influenced by the monsoon and associated processes. However, the records of variation in water column stratification during different monsoon phases, are scarce from this region. Here, we have reconstructed the variation in monsoon induced upwelling, mixed layer and thermocline during the last glacial-interglacial transition, by using the relative abundance of mixed layer and thermocline dwelling planktic foraminifera, relative abundance of Globigerina bulloides and the stable oxygen isotopic ratio (δ18O) of surface dwelling Globigerinoides ruber. We report a weaker upwelling during the last glacial interval and late Greenlandian-Northgrippian Age, and strong upwelling during the deglaciation and Meghalayan Age. The intense upwelling shoaled the thermocline during the early glacial interval, last glacial maximum, late deglaciation and the Meghalayan Age. The mixed layer depth increased during the early deglaciation and Late Greenlandian-Northgrippian due to the reduced upwelling and stronger winter monsoon. The shift in the position of the core site with respect to the low salinity coastal currents, due to the rising sea level, also influenced the upper water column structure in the west-central Bay of Bengal. Therefore, the reorganization of the upper water column structure in the west-central Bay of Bengal, on the glacial-interglacial time scale, was mainly influenced by a combination of the upwelling, fresh water influx, sea level and winter monsoon.

Review: Biogeographic and environmental history of Fagus and beech‐dominant forest in Japan

AbstractThere is a rich beech fossil record in Japan dating from the Paleogene to the Holocene, which allows for the continuous tracing of the evolutionary history of the genus and allows us to understand the environmental background that facilitated the genus to obtain a dominant state in present‐day forests. In addition to the temperate climate, the oceanic climate was an important factor in controlling beech distribution and dominance in the forests in Japan. With the cooling climate, beech species began expanding their distribution in the Northern Hemisphere in the middle Eocene and appeared in Japan in the late middle Eocene. Beech species became dominant in forests in Japan during the Miocene along with the development of the oceanic climate by the opening of the Sea of Japan and with the strengthening of the monsoon. Fagus crenata appeared until the Late Pliocene and became dominant in forests in central Japan in the late Early Pleistocene coinciding with the onsetting inflow of the Tsushima Warm Current to the Sea of Japan in interglacial stages. During the full‐glacial stages since the late Middle Pleistocene, the cold and dry climate caused by the strong winter monsoon limited the distribution of beeches into refugia at humid places, including the inland. During the deglaciation since ca 19 ka, the beech population began to grow and its distribution expanded, controlled by the development of the Tsushima Warm Current, as well as latitudinal and altitudinal temperature clines.

Chemical characteristics of wintertime aerosol and cloud over the Sea of Japan based on aircraft and ground measurements

Chemical characteristics of aerosols and clouds from the boundary layer to the middle troposphere (0–5.2 km) were investigated over the Sea of Japan during winter seasons and were shown to be influenced by the Asian winter monsoon blowing from Siberia. The Na+/Cl− ratio of aerosols and clouds indicates that most Na+ and Cl− originated from sea salts. Positive correlations were found between sea spray aerosol production flux and (Na+ + Cl−) concentrations of boundary layer aerosols (R2 = 0.53–0.71) or clouds (R2 = 0.45–0.69), suggesting that strong winter monsoon winds intensified the production of primary sea salts and the associated clouds. The degree of Cl loss for aerosols near ground level was higher than that at high altitudes, which may be attributed to the distance from emission sources, whereas clouds were scarcely accompanied by Cl loss. The higher (NO3− + non-sea salt (nss)-SO42–)/(NH4+ + nss-Ca2+) ratio of aerosols in the boundary layer compared with the free troposphere may be attributed to the effect of relative humidity on acid gases (SO2, H2SO4, and HNO3) uptake to aerosols. The (NO3− + nss-SO42–)/(NH4+ + nss-Ca2+) ratio of clouds (0.67 on average) implies insufficient amounts of base ions to neutralize acid ions. The Se(VI)/Se(IV) ratios of aerosols in the boundary layer were mostly higher than those of the free troposphere, which may be attributed to the concentrations of oxidants for selenium in aerosol particles. The Se(IV)/Se(VI) ratios of clouds (1.34 ± 0.62) were found to differ significantly from those of aerosols (0.50 ± 0.32 for the boundary layer, 27.92 ± 52.42 for the free troposphere), which strongly suggests that the oxidation state of Se in cloud indicates redox equilibrium.

Distinctive Sedimentary Evolution at the East Edge of the Central Yellow Sea Mud Over the Past 45 kyr: Evidence From Detrital Mineralogy

The central Yellow Sea Mud (CYSM) is a vital part of the coastal sand and distal mud depositional system in the South Yellow Sea (SYS). Previous studies concerning the sedimentary evolution of this area have almost exclusively concentrated on its interior during the Holocene instead of its periphery. In this study, we used a sediment core (H10), with a significantly slow sedimentary rate, to reconstruct the sedimentary evolution at the east edge of the CYSM since MIS3a (∼45 kyr). This mainly involved using detrital minerals, the chemical compositions of garnet, and grain size. The provenance of coarser sediments has remarkable Yellow River-derived characteristics, especially during MIS2 and MIS1. The sedimentary evolution was primarily controlled by hydrodynamic regimes accompanied by changes in relative sea levels (RSLs) and climates. During MIS3a, frequent RSL fluctuations and powerful tidal current erosion were responsible for two facie shifts from the neritic sea to the shore and the lower sedimentary rate in the study area. A paleo-cold water mass and muddy deposition occurred during the high RSL stages with authigenic pyrite enrichment. During MIS2, the paleo-Yellow River was distributed on the SYS and flowed through the study area. Fluvial deposition on the shelf may be eroded by the strong winter monsoon, with an extremely dry and cold climate. Since ∼9.6 kyr, intense hydrodynamic regimes, which were induced by tidal current and upwelling, were responsible for the very much thin deposition, and coarser sediments remained in the study area. Notably, combined with previously studied cores, a much more detailed and intuitional cognition for CYSM formation can be obtained via our special perspective: mud periphery. This study elucidates the sedimentary system evolution and mud area formation of continental shelf seas.

Quaternary subsidence history of Xisha Islands (northern South China Sea): Evidences from the reef-bank system

Subsidence is a potentially important factor influencing the evolution of carbonate platforms. Yet, few studies have been carried out on the quantitative reconstruction of subsidence history of carbonate platforms in the South China Sea. Using paleo-water depth constraints from core XK-1, drilled in the Xisha Islands, northern South China Sea, the tectonic and total subsidence history of Xisha Islands during the Quaternary is quantitatively reconstructed in this study. Slow tectonic subsidence between 1.945 and 0.988 Ma was related with magmatic intrusion beneath the Xisha Islands, while the relatively high tectonic subsidence rate between 0.988 and 0.781 Ma and the subsequent gradual decline in tectonic subsidence rate between 0.781 and 0.07 Ma were linked to the thermal cooling process. A rapid tectonic uplifting process after 0.07 Ma occurred as a result of compression and volcano eruption. Total subsidence rate showed an increase from the stage of 1.945–0.988 Ma to the stage of 0.988–0.781 Ma, and a gradual decline during the stage of 0.781–0.07 Ma. After 0.07 Ma, there was a rapid uplifting in response to the tectonic movement. The high eustatic sea level and intermediate total subsidence rate from 1.945 to 1.0 Ma caused the domination of bank deposits on the Xisha carbonate platform. From 0.988 to 0.65 Ma, the long-term eustatic sea-level lowering and the relatively high total subsidence rate favored the production of thick reef units. By contrast, the low eustatic sea level and the slow total subsidence between 0.65 and 0.42 Ma forced the accommodation space to be limited, resulting in formation of thin reef-bank deposits and occurrence of exposure horizons. During the stage of slow subsidence, the high eustatic sea level around 0.4 Ma allowed the last presence of thick reef unit in the Quaternary section, after which the lower eustatic sea level reduced the accommodation space again and only thin bioclastic bank deposits and iron-stained exposure horizon occurred. Around 0.07 Ma, the Xisha Islands were continuously exposed due to the uplifting and low eustatic sea level. The strong winter monsoon caused a heterozoan factory nearby the Xisha Islands, and blew the carbonate sediments onshore forming aeolianites. Our results proves that the total subsidence has indeed greatly influenced the evolution of Quaternary carbonate system in the Xisha area. The long-term (at scale of Myr) sea-level lowering could facilitate the flourishment of corals, and a moderate total subsidence in the context of secular sea-level lowering would provide more effective accommodation space for the continuous reef accretion, favoring the formation of thick reef units in the tropical areas.

Impacts of the Interannual Variability of the Kuroshio Extension on the East Asian Trough in Winter

The responses of the East Asian Trough (EAT) to the Kuroshio Extension (KE) interannual fluctuation and the underlying mechanisms in the boreal winter are investigated through the lag regression approach in this study. When the KE is in the stable state, the sea surface temperature (SST) front is strengthened, with cold (warm) SST anomaly in the western (eastern) region of the KE, releasing less (more) heat into the atmosphere. The opposite patterns hold for the KE unstable periods. The analysis of the observations shows that the stable KE corresponds to a deeper EAT, accompanied with a stronger winter monsoon over Mongolia and northeastern China. The atmospheric Rossby waves, transient eddies, and thermal winds are found to be responsible for this relationship between the KE and EAT. The SST warming in the lower reaches of the KE excites the Rossby wave activity that propagates toward East Asia, leading to 25% of the EAT amplification. Meanwhile, influenced by the KE-induced Rossby waves, the background baroclinicity is intensified over Japan, which enhances the transient eddy activity, contributing to another 42% magnitude of the EAT deepening. In addition, as depicted by the thermal wind theory, the strong SST cooling in the upper branch of the KE forces an anomalous cyclonic circulation through modifying the meridional temperature gradient, facilitating the EAT development. The finding points to the better understandings of the EAT and associated East Asian winter climate variability, which are crucial for their major economic and social impacts on the large populations in the region.

Decrease of winter cyclone passage over northern Japan due to the reduction in the regional cyclogenesis associated with cold air outbreak

AbstractIn East Asia, a strong winter monsoon and high baroclinicity cause enhanced cyclone activity, which has been the subject of intensive study. Despite the high frequency of genesis of mesoscale cyclones in this area, most of the studies on the decadal timescale have focused only on synoptic‐scale cyclones. To clarify the interannual variation of the passage of winter cyclones including synoptic‐scale and mesoscale cyclones over and around Japan, a tracking algorithm was applied to 62 winter seasons (December–March) from 1958/1959 to 2019/2020. During the study period, the passage of cyclones around northern Japan showed a decreasing trend. Most cyclones that pass around northern Japan are generated over the northern part of the Sea of Japan, which is an area where cyclogenesis has decreased during the study period. Weather pattern classification based on synoptic scale atmospheric conditions revealed that passage of upper‐level troughs and occurrence of lower‐level cold air outbreaks are the preferential conditions for cyclogenesis in this region. In addition, the intensity of the cold air outbreak in this region was weakened during the study period. These results suggest that changes in atmospheric conditions due to climate change have reduced mesocyclone genesis over the northern Sea of Japan, resulting in the reduction in the number of cyclones passing over and around northern Japan.

A ~ 30 kyr sub-centennial to millennial Indian summer monsoon variability record from the southern Andaman Sea, northeastern Indian Ocean

This paper reports centennial-scale variations in the Indian summer monsoon (ISM) that induced stratification and resultant productivity changes in the southern Andaman Sea. The study is based on foraminifera assemblages and sediment geochemistry, along with statistical, spectral and wavelet analyses of faunal data of a well-dated sediment core. The Accelerator Mass Spectrometer (AMS) 14C dated record spans the past 30 kyr with an average temporal resolution varying between 38 and 150 years. The spectral and wavelet analyses of planktonic foraminiferal record reveal significant periodicities during the past 30 kyr suggesting varied forcing mechanisms on the ISM. The characteristic suborbital periodicities correspond mainly to solar irradiance variability. The varying productivity in the southern Andaman Sea over the northern region suggests control of surface salinity, which modulated stratification in the upper water column. The large north to south gradient in productivity during the Last Glacial Maximum (LGM; 23–19 ka) to Heinrich Stadial 1 (HS1; 16–15 ka) indicates the prevalence of strong winter monsoon and well-mixed upper water column in the south. A distinctive high productivity period was identified from the southern Andaman Sea during 16.0–11.2 ka with a peak during the Bølling/Allerød (B/A; 14.5–12.6 ka). On the other hand, a gradual decrease in productivity from the early Holocene to mid-Holocene reflects a response to strong stratification induced by intensified ISM. This enhanced ISM during the early to mid-Holocene resulted in low productivity in the entire basin, albeit with much reduced productivity in the northern part. However, during the marine isotope stage (MIS) 2, the southern Andaman Sea was relatively more productive than the northern Andaman Sea. We recognise three distinctive spatial and temporal changes in salinity structure in the Andaman Sea: 1) ‘normal’ salinity pattern (weak ISM) in which relatively less saline water in the northern part of the basin existed during the late Holocene; 2) ‘strong’ salinity pattern (strong ISM) with extensive freshwater tongue in the northern part existed during early to mid- Holocene; 3) ‘reverse’ salinity pattern with decreased north to south salinity gradient during LGM to early deglacial (19–15 ka), probably driven by the increased isolation of the Andaman basin during the sea-level lowstand, which would have restricted the mixing of northern surface waters with the more saline southern Bay of Bengal (BoB).

The Seesaw Pattern of PM2.5 Interannual Anomalies Between Beijing‐Tianjin‐Hebei and Yangtze River Delta Across Eastern China in Winter

AbstractSince 2013, the winter mean fine particulate matter (PM2.5) had been decreased significantly due to stringent emission controls in most of China. Nevertheless, we found a seesaw pattern of PM2.5 interannual anomalies between Beijing‐Tianjin‐Hebei (BTH) and Yangtze River Delta (YRD). Using the multiple linear regression method, meteorology‐driven PM2.5 interannual anomalies show that the low (high) PM2.5 relative difference between BTH and YRD (RDB&Y) was associated with the strong (weak) East Asian winter monsoon (EAWM). The strong EAWM transported more air pollutants from BTH to YRD. During the COVID‐19 lockdown period, due to the weak EAWM, air pollution still occurred in northern BTH, while the PM2.5 was relatively low in YRD, causing high RDB&Y values. Our results imply that the activity of EAWM and characteristics of regional transport have obvious interannual variations, which is indispensable in evaluating the achievements of PM2.5 quality management between up and downstream regions.

The Ba/Ca record of coral from Weizhou Island: Contributions from oil-drilling muds and the winter monsoon

Oil exploration and drilling activities are known to have catastrophic effects on marine environments and ecosystems, yet full understanding of these effects is hindered by sparse observations in many areas of the world. We present annual and bimonthly records of coral Ba/Ca from Weizhou Island in Beibu Gulf in the northern South China Sea (SCS) – the largest oilfield in the SCS. On interannual timescales, a significant increase in Ba/Ca ratios is associated with high barite consumption used for oil exploration and drilling activities. On seasonal timescales, a strong winter monsoon signal is seen in the skeletal Ba/Ca records. Our data provide a long-term and continuous on-site record for oil exploration and drilling activities that can be used for offshore oil management. Furthermore, our approach offers a means to study the influence of oil-related activities on the marine environment when no oil exploration and drilling records are available.

Possible reasons for an extreme ecological disaster event in Zhoushan Archipelago during the period 5900–5600 yr BP

A 3.3-m-thick, 74.5-m-long ancient wood layer was discovered in Zhoushan Archipelago, East China Sea. The ancient woods were well preserved except for weak silicification of wood bark, and abundant amount of leaves, fruits and seeds were found therein. The distribution characteristics of the ancient woods and well-preserved ancient wood stumps indicated the native origin of these ancient woods. The ancient plants as well as their leaves, fruits and seeds were all preserved well, pointing to a possible sudden deposition event. As inferred from the radiocarbon dating results, the abrupt deposition of ancient wood layer was during the period 5900–5600 yr BP, when the palaeoclimatic records reveal intensified summer monsoon winds, strong winter monsoon winds, and very low ENSO activity. We suggest that the formation of the ancient wood layer was very likely caused by the coaction of a strong storm surge and rapid sea level rise, which became more frequent and intensive during this period. The findings of this study imply the potential risk of strong storm surges in the coastal area of the East China Sea, which should be considered in future planning and construction of buildings.

Influence of East Asian winter monsoon on particulate matter pollution in typical regions of China

This study uses the NCEP/NCAR monthly average reanalysis and number of haze day data during 1958–2017, and the average daily PM2.5 mass concentration data during 2013–2017, to calculate the East Asian winter monsoon index (EAWMI) and statistically analyze the correlation between the winter monsoon index and air quality in China, particularly for the five typical regions (Beijing-Tianjin-Hebei, Fen-Wei Plain, Sichuan-Chongqing Delta, Yangtze River Delta, and Pearl River Delta). Thereafter, the strong and weak winter monsoon years were classified based on the EAWMI, and the atmospheric circulation and temperature fields over China and the five regions in different winter monsoon years were spatially compared. Finally, the study also investigated the various distribution features of the climatic circulation background responsible for the strong and weak winter monsoons and their impact mechanisms on air quality the five typical regions in China. The results show that the effect of the winter monsoon on the air quality of China may be represented by a north–south boundary line located at approximately 30° N. During strong winter monsoon years, pollution was lower in the area north of the boundary but higher to its south. By contrast, the opposite phenomenon was observed during the weak monsoon years. During the strong winter monsoon years, the Beijing-Tianjin-Hebei region and Fen-Wei Plain to the north of the boundary line were less polluted, while the Yangtze River, Chengdu-Chongqing, and Pearl River delta regions to the south of the boundary were more polluted. Diagnostic analysis of the circulation field indicated that during the strong winter monsoon years, an abnormal downward airflow occurred to the south of the boundary, limiting convective diffusion and thereby causing the increased pollution. However, during the weak winter monsoon years, ascending airflows occurred, which favored pollutant diffusion. Furthermore, during the strong winter monsoon years, an abnormal southeast airflow with weak horizontal wind speed occurred in the lower atmosphere of the Chengdu-Chongqing region, causing localized pollutant accumulation, thereby aggravating the pollution. In the Pearl River Delta region, a descending abnormal westerly flow inhibited the local uplift and diffusion of air. Moreover, importing pollutants occurred from the north, aggravating the pollution in the region.

Remote Sensing and Argo Float Observations Reveal Physical Processes Initiating a Winter-Spring Phytoplankton Bloom South of the Kuroshio Current Near Shikoku

BIO-Argo float (chlorophyll a (Chl-a), temperature, and salinity profiles) and remote sensing data (Chl-a, photosynthetic available radiation (PAR), and wind) located south of the Kuroshio current near Shikoku from September 2018 to May 2019 were used to study phytoplankton bloom and their mechanisms of development in open oceans. Results show that higher (lower) Chl-a concentrations are correlated with a deeper (shallower) mixed layer (RPearson = 0.77, Rcrit = 0.12 (alpha = 0.05, n = 263)) compared to the average of Chl-a and mixed layer depth (0.13 mg/m3 and 105 m). The average net accumulation rates (r) of phytoplankton were close to 0.08 d−1. An increasing r corresponds to a gradually increasing surface Chl-a (S (Chl-a): 0–20 m average Chl-a) and integrated Chl-a inventory (I (Chl-a): integrated Chl-a from surface to euphotic depth). These phenomena indicate that the mechanism of winter-spring phytoplankton blooms is consistent with the dilution-recoupling hypotheses (DRH). During the bloom formation, winter deep mixing and eddy-wind Ekman pumping are enhanced by a strong winter monsoon. The enhancement may disturb predator–prey interactions and dilute zooplankton in deep mixed layers. Moreover, winter deep mixing and eddy-wind Ekman pumping can cause the nutrients to be transported into the euphotic layer, which can promote the growth of phytoplankton and increase grazing. During the bloom extinction, the stratification strengthens and the intensity of light increases; this increases grazing and nutrient consumption, and decreases the phytoplankton bloom significantly (S (Chl-a) and I (Chl-a) increase by 0.3 mg/m3 and 27 mg/m2, respectively). The output from a biogeochemistry model shows that nutrients are consistent with the temporal distribution of S (Chl-a) and I (Chl-a). Our results suggest that physical processes (deep winter mixing and eddy-wind Ekman pumping) under the DHR framework are critical factors for winter-spring blooms in open oceans with an anticyclone eddy.

Longshore drift produced by climate-modulated monsoons and typhoons in the South China Sea

Abstract Monsoons and typhoons impact the tropical coastal zones through their signature on winds and waves, leading to increased vulnerability – through erosion, marine submersion and flooding – of an ever growing coastal population. This study addresses wave-driven coastal impacts in the South China Sea (SCS), particularly along the Vietnam coast. Using 38 years of offshore wave fields from ERA-Interim reanalysis, we assess the seasonal and interannual variability of wave patterns, storminess, and wave-induced alongshore sediment transport (longshore drift). Results suggest a large seasonal coastal impact due to high-energy, northeasterly waves generated by winter monsoon winds. The net annual sediment transport is southward with significant interannual and decadal variations (20% standard deviation with particular years at 40% of the mean), presenting strong correlations with the El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) regime changes. The regime shift of 1998, from a warm to a cold PDO phase characterized by more La Nina events, is associated with an increase in winter wave energy and thus higher sediment transport (about 10%). The typhoon activity in the SCS is partly associated with that of the Pacific Northwest basin, but as a large semi-enclosed sea, presents local differences. It is positively correlated with La Nina in summer and with El Nino Modoki in winter. The summer correlation to ENSO phases is opposite to that of the whole Northwest Pacific due to competing local and remote mechanisms driving cyclone formation and trajectory. For the same reason, the effects of ENSO and PDO phases are opposite in the SCS: during the last cold PDO phase, typhoon frequency was reduced by 20%, with significant net impact on estimated sediment transport. The typhoon contribution to sediment transport is a 15% reinforcement of southward transport due essentially to winter activity. If winter monsoon and typhoons appear to work together on average, their low-frequency variability are out-of-phase. This is particularly clear at decadal scale, as cold PDO phases seem favorable to strong winter monsoon waves but detrimental to SCS typhoon numbers. These results confirm that regional climate variability (together with human factors affecting river sediment supply, coastal management of beach-dune systems, land subsidence, mangrove deforestation) is an essential part of coastal vulnerability that needs to be better assessed.

Holocene Moisture Variation Recorded by Aeolian Sand‐Palaeosol Sequences of the Gonghe Basin, Northeastern Qinghai‐Tibetan Plateau, China

AbstractThe northeastern Qinghai‐Tibetan Plateau (QTP) of China is located at the triple junction of the Asian winter and summer monsoons and the westerlies, where paleoclimatic evolution has an important scientific significance for recognizing the spatial‐temporal pattern of Asian monsoons in the past and predicting environmental change in the future. Nevertheless, the framework of the Holocene moisture variation and related mechanisms remain controversial, owing to complex hydroclimatic conditions triggered by the landform of the large mountain‐arid basin. Here, we employed geochemical proxies from typical aeolian sand‐palaeosol sequences in the Gonghe Basin, northeastern QTP, together with Optically Stimulated Luminescence (OSL) dating, to reconstruct the pattern of effective moisture variation and associated mechanisms in this region. Our results indicate that the regional effective moisture was at its lowest until 9–8 ka, and approached a maximum during 8–4/3 ka of the middle Holocene. Afterwards, the climate became relatively dry in general, but with a transient humid interval around 2–1 ka. Our geochemical evidence indicates that the dry early Holocene probably can be attributed to a strong winter monsoon forced by remnant ice sheet, combined with the high evaporation caused by solar insolation. Also, shifts of humid‐dry are closely linked to the Asian summer monsoonal strength and therefore the balance of evaporation‐precipitation in the middle and late Holocene. Thus, the pattern of the Holocene effective moisture variation is characterized as the ‘monsoon model’ in a closed intermontane arid and semi‐arid basin near the western Asian monsoonal limit.

Linking Monsoon Activity with River-Derived Sediments Deposition in the Northern South China Sea

The history of the East Asian monsoon (EAM) since Last Glacial Maximum is reconstructed based on records in a sediment core retrieved from the northern South China Sea. The provenance of organic carbon and the EAM’s evolution are investigated using combined organic carbon, grain size, and AMS 14C dating analysis. Responding to the strong winter monsoon, the marine organic matter was dominated in the sediments, and δ13C of organic matter was high during the last glacial period. During the Holocene, the primary productivity decreased and the dilution effect of riverine inorganic clastics strengthened. Accordingly, the total organic carbon contents and δ13C values decreased, and terrestrial organic matter content increased in the core sediments. The 4 μm grain size fraction, as the environmental sensitive grain size component, is determined to reconstruct the paleoclimatic records. During the last glacial period, the contents of 4 μm grain size component were mainly controlled by the sea level changes. Upon the Holocene, the influence of the sea level changes weakened and the roles of current system and provenance strengthened. The correlation between the EAM and solar insolation forcing is also discussed in this paper.

Monsoon-influenced variation of clay mineral compositions and detrital Nd-Sr isotopes in the western Andaman Sea (IODP Site U1447) since the late Miocene

Nd-Sr isotopes (εNd, 87Sr/86Sr) of detrital particles, clay mineral compositions, and δ13C of sediment organic matter (δ13CSOM) at the International Ocean Discovery Program (IODP) Expedition 353 Site U1447 in the western Andaman Sea were measured to reveal the sediment provenance changes and/or weathering intensity variations in association with the Indian monsoon intensity change. The shipboard age model based on biostratigraphic data and paleomagnetic reversals shows that IODP Site U1447 preserves late Miocene (~10 Ma) sediments. Nd/Sr isotope systematics demonstrates that the sediments originated mainly from the Myanmar region, including the Irrawaddy River, Salween River, Sittang River and Indo-Burman-Arakan Ranges without a significant change of sediment provenance since the late Miocene. Thus, temporal variations of clay mineral compositions, represented as smectite/(illite + chlorite) [S/(I + C)], indicate the long-term variations of physical/chemical weathering intensity attributable to intensity changes of Indian winter/summer monsoon. A gradual decreasing trend of S/(I + C) ratios indicates stronger physical and/or weaker chemical weathering since the late Miocene, as a result of strengthening of Indian winter monsoon (and/or weakening of Indian summer monsoon), which seems be closely related to global cooling since the late Miocene. Distinct decrease of S/(I + C) ratios occurred at ~9.2–8.5 Ma, ~3.6 Ma, ~2.4 Ma, and ~1.2 Ma, which may be attributed to the combined effect of both global cooling and Tibetan Plateau Uplift as a local response. In addition, δ13CSOM values at IODP Site U1447 were higher at ~3.5 Ma and after 1.5 Ma when S/(I + C) ratio was minima, which may imply an increase of C4 plant in Myanmar region as a result the strong Indian winter monsoon (or weak Indian summer monsoon).

Mid-late Holocene maar lake-mire transition in northeast China triggered by hydroclimatic variability

The density of maar lake type peatlands in the Changbai Mountain areas, northeast (NE) China is among the highest in the world. These deposition systems include both peat and limnic deposits, providing valuable sites to study their evolutionary history shaped by their sensitivity to climate-forced state shifts. To investigate the maar lake-mire shift in response to hydroclimatic variability, we examined developmental history of the Jinchuan peatland based on sedimentary facies, grain-size, physical and chemical properites, plant macrofossils and peat basal age. The results indicate that the Jinchuan peatland mainly originated from paludification in a maar lake from approximately 7000 cal. BP. Its water level continuously increased since the Middle Holocene, reaching the peak during the period of 5000–4000 cal. yr BP. The outward expansions of Jinchuan peatland suggest a climate–driven model based on which water level variability superimposes the underlying basin morphology. The hydroclimatic variability and peatland development history can be divided into three periods. From 12 to 7 ka cal. BP, the increasing East Asian summer monsoon (EASM) was the major water vapor source of Jinchuan peatland. The strong evaporation effects induced by the strong Winter Asian winter monsoon (WAWM) and the maximum sea surface temperatures (SSTs) of Japan Sea might have changed the Jinchuan maar lake into dry upland. From 7 to 4 ka cal. BP, the maximum EASM and increasing Westerly jet (WJ) provided water vapor for the peatland. The decreasing EAWM and SSTs of Japan Sea resulted in decreased evaporation effects. The moisture and water level in Jinchuan peatland gradually increased. The peat covered 68.97% of the area during this period. Since 4 ka cal. BP, the decreased EASM and the maximum WJ provided water vapor for the Jinchuan peatland. The weak EAWM and the lower SSTs of Japan Sea induced lower evaporation effects. The moisture and water level in the peatland reached the maximum. Subsequently, peat area further expanded and covered the remaining 31.03% area. This study reveals a remarkable link between the maar lake-mire shift and hydroclimatic variability, which is significant for future prediction of maar lake-mire ecosystem shifts under global climate change.