Winter Monsoon Research Articles

Asian winter monsoon controls marine primary productivity in north Arabian Sea during the Holocene

The Arabian Sea (AS), which is strongly impacted by monsoon winds, is one of the most biologically productive regions. The forcing of Asian monsoons on Holocene productivity remains debated in north AS, an area influenced by summer and winter monsoon. A sediment core from Makran margin was utilized to analyze organic biomarkers and high-resolution bromine intensities, unveiling the forces of biological productivity through the past 8000 years. Alkenone sea surface temperature (SST) records show significant cooling in north AS and augmented SST difference between north and west AS toward the late Holocene, suggesting Asian winter monsoon (AWM) strengthening during the period. The Holocene productivity signals in north AS, indicated by the bromine to rubidium ratio and multiple biomarkers, are responsive to SST changes and thus mainly controlled by the AWM variation, not modulated by Asian summer monsoon (ASM). Millennial-to-centennial-scale variability of marine productivity during the Holocene is closely related to North Atlantic climate oscillations, mainly through atmospheric circulations in winter.

Aerosol impacts on the East Asian winter monsoon: Insights from TaiESM1 and CMIP6 simulations

AbstractThe East Asian winter monsoon (EAWM) is an important climate pattern in Southeast Asia. This study delved into the aerosol effects on the EAWM by analysing simulations conducted with the Taiwan Earth System Model version 1 (TaiESM1) model, part of the Coupled Model Intercomparison Project Phase 6 (CMIP6). Our assessment, validated against observational and reanalysis data, revealed that TaiESM1 effectively replicates the robust circulation of the EAWM and aerosol concentrations, placing its performance above average level among CMIP6 models. Notably, the simulated global mean aerosol optical thickness exhibited an approximately 20% increase from 1850 to 2014. The increased aerosols corresponded to a weakened Aleutian Low and an intensified Siberian High, leading to a weakened EAWM in the lower atmosphere over extratropical areas. In addition, the cooling across the Northern Hemisphere might affect enhanced northerly winds at approximately 10°N, which are situated in the southern part of the EAWM. An analysis of the near‐surface temperature budget indicated that the aerosol‐induced cooling in East China and India was primarily driven by changes in shortwave and longwave radiation fluxes, albeit slightly offset by sensible heat and latent heat fluxes. Furthermore, our study also reveals that while the mechanisms through which anthropogenic aerosols affect the northern portion of the EAWM vary across most CMIP6 models, their impacts on the tropical region remain consistent.

Thermal fronts in coastal waters regulate phytoplankton blooms via acting as barriers: A case study from western Guangdong, China

Oceanic fronts play an important role in regulating the distributions of phytoplankton and are subject to climate change. However, the relationship between climate change, oceanic fronts, and coastal phytoplankton blooms requires clarification. Using long-term satellite data, the seasonal, interannual, and decadal variations of the thermal fronts were examined, along with their effects on the distribution of phytoplankton in western Guangdong. Thermal fronts between the cold western Guangdong coastal current waters and the warm South China Sea (SCS) waters were found along the coast of western Guangdong during winter due to northeast winter monsoon. At decadal timescales, an increase (decrease) in frontal activities occurred during the negative (positive) Pacific Decadal Oscillation (PDO) phase. This pattern suggests that the positive (negative) PDO activity decreases (increases) sea level anomaly (SLA) and sea surface temperature in the SCS, thereby weakens (enhances) frontal activities along the coast. Accordingly, extensions and positive anomalies (> 1.5 mg/m3) in chlorophyll-a (Chl a) were observed during the positive PDO phase, whereas Chl a was concentrated in the nearshore areas during the negative PDO phase. These processes are primarily related to the barrier effects of enhanced thermal fronts and positive SLA on offshore materials transport during the negative PDO period. This study highlights the need to incorporate PDO activity into phytoplankton production in coastal oceans.

Spatio-temporal variation of mesozooplankton in the northern coastal waters of Bay of Bengal

Mesozooplankton (MSP) are significant primary and secondary consumers in aquatic ecosystems that influence biogeochemical cycles and are considered important bioindicators of ecosystem functioning. Data on the spatio-temporal variability of the MSP population from the estuarine and coastal waters of northwestern Bay of Bengal (BoB) is scarce. The present study is the first detailed investigation on the spatio-temporal dynamics of MSP along the estuarine and coastal waters of northwestern BoB from two contrasting seasons over a period of three years based on univariate and multivariate statistical analyses. The crucial drivers for spatio-temporal variability in the MSP community were water temperature, salinity, dissolved oxygen (DO) concentration, nutrient (nitrate, phosphate, silicate) concentrations, and Chlorophyll (Chl) a concentration. The highest mean MSP abundance was obtained during summer monsoon (SM). The results of this study state that there is a significant seasonal difference in the hydrographical parameters that change the community dynamics of MSP inhabiting the estuarine and coastal waters of BoB. The MSP species assemblages during winter monsoon (WM) were governed by high salinity and DO levels while those during SM were associated with high water temperature, nutrients, and Chl a concentration. A clear seasonal shift from the gelatinous community in WM to the crustacean community in SM was documented. A total of 172 MSP species were identified during WM, and 200 MSP species during SM, with 37 species being exclusively detected during WM and 65 species being exclusively recorded during SM. The most abundant species during WM were Chrysaora caliparea, Cyanea nozakii, Sagitta bipunctata, Pleurobrachia pileus, and Acromitus flagellatus to name a few and those during SM consisted of Undinula vulgaris, Acartia (Odontacartia) spinicauda, Oithona simplex, Oithona similis, and Oncaea venusta. Copepoda was the most dominant taxa during all sampling seasons, while ichthyoplankton and meroplankton fauna were low during WM and comparatively higher during SM, indicating the top-down control of their population by gelatinous taxa. Biodiversity indices reflected well to excellent diversity patterns of MSP. In the current scenario of global warming and climate change, our research highlights the need for comprehensive long-term monitoring of estuarine and coastal waters of western BoB, which harbour a rich marine faunal diversity, to identify the bioindicators of seasonal variability among major MSP groups.

Southward shift of the westerly jet intensified late Holocene dust storms on the Tibetan Plateau

The climate change on the Tibetan Plateau (TP) exhibits a north-south dipole trend during the Holocene. However, it is unclear whether regional differences exist in the variations and driving mechanisms of dust storm activities across the entire TP, and whether these mechanisms align with those found in monsoonal northern China. Here, we present a high-resolution Holocene record of dust storms from an undisturbed lake in the research gap area of the TP, and compile previously published records from other TP regions. Our findings indicate that the dust storm activity increased from the early Holocene to the late Holocene, peaking after 5 cal ka BP. Importantly, our comparison with other dust storm records reveals a unified intensification of dust storm activity across the entire TP during the late Holocene, a pattern distinctly different from the north-south dipole trend of precipitation variation. Moreover, this unified pattern of dust storm intensification across the TP cannot be explained by the decreasing intensity of the East Asian winter monsoon (EAWM) during the Holocene. The observations suggested a limited role for precipitation and EAWM in modulating TP dust storms on the millennial timescale during this period. By contrast, the increases in dust storms on the TP after 5 cal ka BP correlates well to the southward shift of the westerly jet driven by the North Atlantic Oscillation (NAO). As the NAO transitioned to a negative phase during the late Holocene, a meandering westerly jet shifted southward to the interior of the TP, intensifying wind intensity and leading to dust storm outbreaks.

Relationship between fine particulate matter concentration and winter monsoon in the tropics under the influence of climate change and topographic effects: A case study of Kaoping area, Taiwan, since 1959

Relationship between fine particulate matter concentration and winter monsoon in the tropics under the influence of climate change and topographic effects: A case study of Kaoping area, Taiwan, since 1959

Winter Convective Mixing Mediating Coupling of N‐Gain and ‐Loss in the Arabian Sea

AbstractMarine dinitrogen (N2) fixation fuels primary production and thereby influences the Earth's climate. Yet, its geographical distribution and controlling environmental parameters remain debatable. We measured N2 fixation rates from the two spatially and physicochemically contrasting regions of the Arabian Sea during the winter monsoon: (a) the colder and nutrient‐rich waters in the northern region owing to winter convection and (b) the warmer and nutrient‐poor waters in the southern region unaffected by winter convection. We found higher N2 fixation rates at the surface of northern region due to convective mixing driven supply of phosphate (intuitively iron also) from the underlying suboxic waters. N2 fixation was favored by high nutrient concentrations in the euphotic waters, whereas remained unaffected by nutrient availability in the aphotic waters. We conclude that diazotrophs dwelling in the euphotic zone chose phosphate over fixed nitrogen‐poor waters. However, we found that among oligotrophic waters, anticyclonic eddy extremes the barrier of fixed nitrogen supply, and thereby, elevates N2 fixation. While the Arabian Sea loses about 20%–40% of the global ocean fixed nitrogen, we estimate that N2 fixation in the Arabian Sea offsets only up to 42% of its fixed nitrogen‐loss by denitrification, but this offset could be higher if diazotrophic activity is further examined up to the deeper depths of the Arabian Sea.

Winter monsoon brings substantial terrestrial aerosols to Bay of Bengal and adjacent Indian Ocean: Insights from carbon and nitrogen isotopes

It has been currently recognized that air pollutants (e.g., carbon and nitrogen) from South and Southeast Asia have a significant impact on the coastal waters of the northern Indian Ocean. However, the understanding of dynamics and fates of these pollutants in aerosols to the remote ocean through long-distance transportation still needs to be improved. To address this issue, stable isotopic compositions (δ13C, δ15N) of total carbon (TC) and nitrogen (TN) and other geochemical indexes in aerosols are investigated over the Bay of Bengal (BOB) and the adjacent Indian Ocean during the winter monsoon period. The concentrations of TN, TC and aerosol nutrients exhibited a decreasing trend from the northern BOB (N-BOB) to the southern BOB (S-BOB) and Indian Ocean, reflecting the weakening trend of polluted continental influence from the N-BOB to the ocean. The high concentrations of TC (4.28–18.21 μg m−3) in the aerosols of the N-BOB were mainly influenced by the emissions of coal combustion (68 ± 21%), while the high concentrations of TN (0.69–7.31 μg m−3) in the N-BOB were mainly from biofuel/biomass burning (52 ± 31%). In the S-BOB, the lower concentrations of TC (1.42–2.98 μg m−3) and TN (0.38–2.03 μg m−3) mainly originated from the formation of secondary aerosols and the changes in aerosol chemical composition during long-distance transportation from the continent. The primary marine source was responsible for the low TC and TN concentrations over the Indian Ocean. This study quantifies the nitrogen and carbon source shifts from continental anthropogenic activities in the N-BOB to dominant marine and photochemical transformation sources in the remote ocean, quantitatively deepening the understanding of the impact of terrestrial aerosols on the ocean.

Characterization of Mixing at the Edge of a Kuroshio Intrusion into the South China Sea: Analysis of Thermal Variance Diffusivity Measurements

Abstract The Kuroshio occasionally carries warm and salty North Pacific Water into fresher waters of the South China Sea, forming a front with a complex temperature–salinity (T–S) structure to the west of the Luzon Strait. In this study, we examine the T–S interleavings formed by alternating layers of North Pacific Water with South China Sea Water in a front formed during the winter monsoon season of 2014. Using observations from a glider array following a free-floating wave-powered vertical profiling float to calculate the fine-scale parameters Turner angle, Tu, and Richardson number, Ri, we identified areas favorable to double-diffusion convection and shear instability observed in a T–S interleaving. We evaluated the contribution of double-diffusion convection and shear instabilities to the thermal variance diffusivity, χ, using microstructure data and compared it with previous parameterization schemes based on fine-scale properties. We discover that turbulent mixing is not accurately parameterized when both Tu and Ri are within critical ranges (Tu > 60; Ri < ¼). In particular, χ associated with salt finger processes was an order of magnitude higher (6.7 × 10−7 K2 s−1) than in regions where only velocity shear was likely to drive mixing (8.7 × 10−8 K2 s−1).

Interdecadal shift of the North Pacific Oscillation and its nonstationary relationship with East Asian climate

A pronounced shift of the North Pacific Oscillation (NPO), including its teleconnection with the East Asian winter monsoon, has been observed in recent decades, but the underlying mechanism remains unclear. In this study, we found that interannual variation of the NPO and its relationship with the dominant modes of winter mean and extreme temperatures over East Asia (EA) experienced a significant interdecadal shift in the mid-1980s. Before the shift (during 1950–1985), the NPO emerged and was maintained as a strong regional north-south dipole, manifesting a remarkable tropical-extratropical teleconnection. It significantly modulated the second mode of air temperature over EA (depicted by a north-south dipole) and extreme temperature events over southern EA, mainly through horizontal advection and adiabatic heating induced by the anomalous circulation in the southern lobe of the NPO. After the shift (during 1986–2021), the NPO was characterized by an intensified northern lobe and a weakened and northwestward shifted southern lobe, and by a close teleconnection with the Rossby wave trains at the mid-higher latitudes. In the meantime, tropical-extratropical teleconnections became apparently weak. Correspondingly, the NPO was mainly linked to the interannual variation of air temperature and extreme events over northern EA via horizontal advection. Importantly, the decadal shift of the relationships of the NPO with the Arctic Oscillation and El Niño–Southern Oscillation played an apparent role in modulating EA climate.

Distribution Dynamics of Diplopanax stachyanthus Hand.-Mazz. (Mastixiaceae) and Its Implications in Relict Mastixioid Flora Conservation

Climate is a key driver shaping the distribution pattern of organisms. Cenozoic climate change has led to extensive biota turnover. Untangling the distribution dynamics of a representative lineage of flora can provide deep insights into biodiversity conservation. Diplopanax is a notable relict lineage of the Tertiary mastixioid flora with abundant fossils in the Northern Hemisphere. Diplopanax stachyanthus Hand.-Mazz. is a representative relic lineage of the mastixioid flora, which was once widespread in the Northern Hemisphere of the early Tertiary period, but with only endemic distribution in the (sub)tropical humid forests of East Asia. It offers a unique chance to understand how climatic drivers shape the Boreotropical flora. In this research, we investigated the distribution dynamics of D. stachyanthus at the last glacial maximum (LGM), mid-Holocene (MH), current, and three periods of the future (2041–2060, 2061–2080, and 2081–2100) at four shared socio-economic emissions scenarios pathways. Our results indicated that the Precipitation of the Wettest Quarter (32.6%), the Precipitation of the Driest Quarter (21.2%), and the Precipitation of the Coldest Quarter (17.3%) are the key factors affecting its distribution. The current high suitable distribution areas are primarily in southern China and northern Indo-China. The enforced winter monsoon seasons in East Asia since the late Pliocene period are the key climatic drivers reducing its once widespread distribution in the Northern Hemisphere. Under future scenarios, centroid transfer analysis suggests that its distribution center will shift southwestward, but the potentially suitable habitats in the coastal regions of southern China and northern Indo-China will be lost. These coastal populations should be prioritized for ex situ conservation. Expanding the nature reserve within its long-term stable distribution range in southwest China is an effective strategy for the in situ conservation of the ancient mastixioid flora.

Spatiotemporal characterization of PM2.5, O3, and trace gases associated with East Asian continental outflows via drone sounding

East Asian continental outflows with PM2.5, O3, and other species may determine the baseline conditions and affect the air quality in downwind areas via long-range transport (LRT). To gain insight into the impact and spatiotemporal characteristics of airborne pollutants in East Asian continental outflows, a versatile multicopter drone sounding platform was used to simultaneously observe PM2.5, O3, CO2, and meteorological variables (temperature, specific humidity, pressure, and wind vector) above the northern tip of Taiwan, Cape Fuiguei, which often encounters continental outflows during winter monsoon periods. By coordinating hourly high-spatial-resolution profiles provided by drone soundings, WRF/CMAQ model air quality predictions, HYSPLIT-simulated backward trajectories, and MERRA-2 reanalysis data, we analyzed two prominent phenomena of airborne pollutants in continental outflows to better understand their physical/chemical characteristics. First, we found that pollutants were well mixed within a sounding height of 500 m when continental outflows passed through and completely enveloped Cape Fuiguei. Eddies induced by significant fluctuations in wind speeds coupled with minimal temperature inversion and LRT facilitated vertical mixing, possibly resulting in high homogeneity of pollutants within the outflow layer. Second, the drone soundings indicated exceptionally high O3 concentrations (70–100 ppbv) but relatively low concentrations of PM2.5 (10–20 μg/m3), CO2 (420–425 ppmv), and VOCs in some air masses. The low levels of PM2.5, CO2, and VOCs ruled out photochemistry as the cause of the formation of high-level O3. Further coordination of spatiotemporal data with air mass trajectories and O3 cross sections provided by MERRA-2 suggested that the high O3 concentrations could be attributed to stratospheric intrusion and advection via continental outflows. High-level O3 concentrations persisted in the lower troposphere, even reaching the surface, suggesting that stratospheric intrusion O3 may be involved in the rising trend in O3 concentrations in parts of East Asia in recent years in addition to surface photochemical factors.

Origins of formaldehyde in a mountainous background atmosphere of southern China

Formaldehyde (HCHO) is one of the key indicators of severe photochemical pollution and strong atmospheric oxidation capacity in southern China. However, current information on the origins of regional HCHO and the impacts of polluted air masses remains scarce and unclear. In this study, an intensive observation of HCHO was conducted at a mountainous background site in southern China during typical photochemical pollution episodes. The concentrations of HCHO reached up to 6.14 ppbv and averaged at 2.68 ± 1.11 ppbv. Source appointment using a photochemical age-based parameterization method revealed significant contributions of secondary formation (50 %) and biomass burning (42 %). Meanwhile, under the influence of the East Asian Winter Monsoon, polluted air masses from central and western China can significantly increase the regional HCHO levels. The simulation results adopting the Rapid Adaptive Optimization Model for Atmospheric Chemistry model further demonstrated that the intrusion of active anthropogenic pollutants (e.g., small-molecule alkenes) can accelerate the net production rate of HCHO, particularly through BVOC-oxidation pathways. This study suggests a potential enhanced mechanism of HCHO production resulting from anthropogenic-biogenic interactions. It highlights that polluted air masses carrying abundant HCHO from upwind areas may facilitate severe photochemical pollution in the Greater Bay Area.

Orbital- and millennial-scale Asian winter monsoon variability across the Pliocene–Pleistocene glacial intensification

Intensification of northern hemisphere glaciation (iNHG), ~2.7 million years ago (Ma), led to establishment of the Pleistocene to present-day bipolar icehouse state. Here we document evolution of orbital- and millennial-scale Asian winter monsoon (AWM) variability across the iNHG using a palaeomagnetically dated centennial-resolution grain size record between 3.6 and 1.9 Ma from a previously undescribed loess-palaeosol/red clay section on the central Chinese Loess Plateau. We find that the late Pliocene–early Pleistocene AWM was characterized by combined 41-kyr and ~100-kyr cycles, in response to ice volume and atmospheric CO2 forcing. Northern hemisphere ice sheet expansion, which was accompanied by an atmospheric CO2 concentration decline, substantially increased glacial AWM intensity and its orbitally oscillating amplitudes across the iNHG. Superposed on orbital variability, we find that millennial AWM intensity fluctuations persisted during both the warmer (higher-CO2) late Pliocene and colder (lower-CO2) early Pleistocene, in response to both external astronomical forcing and internal climate dynamics.

Variability and trends of near-surface wind speed over the Tibetan Plateau: The role played by the westerly and Asian monsoon

Near-surface wind speed exerts profound impacts on many environmental issues, while the long-term (≥60 years) trend and multidecadal variability in the wind speed and its underlying causes in global high-elevation and mountainous areas (e.g., Tibetan Plateau) remain largely unknown. Here, by examining homogenized wind speed data from 104 meteorological stations over the Tibetan Plateau for 1961–2020 and ERA5 reanalysis datasets, we investigated the variability and long-term trend in the near-surface wind speed and revealed the role played by the westerly and Asian monsoon. The results show that the homogenized annual wind speed displays a decreasing trend (−0.091 m s−1 per decade, p < 0.05), with the strongest in spring (−0.131 m s−1 per decade, p < 0.05), and the weakest in autumn (−0.071 m s−1 per decade, p < 0.05). There is a distinct multidecadal variability of wind speed, which manifested in an prominent increase in 1961–1970, a sustained decrease in 1970–2002, and a consistent increase in 2002–2020. The observed decadal variations are likely linked to large-scale atmospheric circulation, and the correlation analysis unveiled a more important role of westerly and East Asian winter monsoon in modulating near-surface wind changes over the Tibetan Plateau. The potential physical processes associated with westerly and Asian monsoon changes are in concordance with wind speed change, in terms of overall weakened horizontal air flow (i.e., geostrophic wind speed), declined vertical thermal and dynamic momentum transfer (i.e., atmospheric stratification thermal instability and vertical wind shear), and varied Tibetan Plateau vortices. This indicates that to varying degrees these processes may have contributed to the changes in near-surface wind speed over the Tibetan Plateau. This study has implications for wind power production and soil wind erosion prevention in the Tibetan Plateau.

Seasonal patterns and interannual variations of atmospheric circulation over the Bering Sea in 1995–2022

Mean patterns of atmospheric circulation over the Bering Sea in 4 seasons: winter (January-March), spring (April-June), summer (July-September) and autumn (OctoberDecember) are described using the author’s typification of synoptic situations. Frequency of all 6 types of synoptic situations is calculated and the predominant types are determined, by season. Mean values of the meridional and zonal indices of atmospheric circulation and the number and intensity of cyclones in the Bering Sea area are calculated for each season in the period 1995–2022. Cyclonic activity and direction and intensity of general wind transfer, particularly intensity of winter and summer monsoons, are considered separately for the western and eastern parts of the Bering Sea. Interannual variations of all these parameters are traced. For all seasons, the wind transfer over the entire area is determined by cyclonic activity in the western Bering Sea: the higher activity leads to weaker monsoon in winter (northeasterly) and summer (southwesterly) but strengthening in spring (southeasterly) and autumn (northwesterly). Since the middle 1990s, autumns and winters in the Bering Sea became warmer (with the warmest period in the late 2010s when southeasterlies prevailed in winter), but springs became colder (southeastern wind transfer in early 2000s changed to the northeastern one in late 2010s), with no definite tendency for summer, when the meridional index of atmospheric circulation was rather stable and the zonal index had a negative trend (weakening of westerlies). Trends and cycles of oceanographic conditions in the northwestern Bering Sea generally coincided with the changes in wind transfer in any season.

Influence of tidal regime and shelf circulation on sedimentary provenance changes in the middle Okinawa Trough of the East China Sea since the last deglaciation

Deciphering sedimentary provenance in the Okinawa Trough (OT) is vital for understanding the dispersal system from source to sink and the paleoenvironmental evolution in the East China Sea (ECS). Sources include the Huanghe, Changjiang, and Taiwan rivers, as well as the ECS shelf; however, there is controversy over which source has dominated sediment supply since the last deglaciation, and how the tidal currents, shelf circulation, and Kuroshio Current (KC) has controlled the sediment transport. Here, we present a high-resolution record of grain size and clay minerals over the last 14.3 kyrs in an AMS 14C dated piston core from the middle OT. We use these data to reconstruct changes in the KC, identify the provenance of fine-grained sediment, and unravel the impact of paleoenvironmental factors on these provenance changes. A grain size component of 15.6–31.3 μm was extracted to reconstruct the KC changes. These data show that the KC strengthened sharply until 10.2 ka, then kept strong and stable during the Holocene. Clay mineral assemblages indicate that the sediment was mainly derived from the ECS shelf between 14.3 and 8.6 ka, then from the Taiwan rivers after 8.6 ka. The shelf sediment was eroded and transported by intense tidal currents. Taiwan river sediment became the dominant source due to the development of Taiwan Warm Current rather than the strengthening of KC. The ratio of smectite and kaolinite to illite and chlorite content indicates that the sediment contribution from the Huanghe (old Huanghe delta) and Changjiang rivers increased from 7.6 to 5.0 ka and from 3.2 ka to the present. This increase is attributed to the strengthening of East Asian winter monsoon and the consequent intensification of cross-shelf circulation.

Upper Ocean variations at IODP Hole U1505C in the northern South China Sea and their response to the East Asian Monsoon during the middle Miocene

We used the samples from International Ocean Discovery Program (IODP) Hole U1505C in the northern South China Sea (SCS) to reconstruct the upper ocean profiles and discussed the East Asian Monsoon (EAM) variations during 15.0–12.5 Ma. 15 genera and 41 species were identified, and 7 planktonic foraminifer datums were recognized, then a reliable chronostratigraphic framework was updated. A principal component analysis (PCA) was used to identify the suitable assemblages for reconstructing upper water profiles, based on planktonic foraminifera relative abundance and isotope records. The sea surface temperature (SST), paleo-productivity, and the depth of thermocline (DOT) were reconstructed by the content of warm water species, high productivity species, and the ratio of mixed layer to thermocline species, respectively. Three main phases were identified: (1) during 15.0–14.7 Ma, the SST and paleo-productivity were high, while the thermocline was shallow, indicating a warm and rainy climate influenced by East Asian Summer Monsoon (EASM); (2) during 14.7–13.8 Ma, the SST was lower, the paleo-productivity increased significantly, and the thermocline deepened, indicating an enhanced East Asian Winter Monsoon (EAWM); (3) during 13.8–12.5 Ma, the SST increased slightly, the paleo-productivity deceased, while the DOT showed neglectable change, indicating a stable EAWM and a waning EASM. The results of the Morlet wavelet spectrum revealed that EAM and the upper ocean profiles in the northern SCS were predominantly regulated by the eccentricity cycles, emphasizing the significant impact of low-latitude processes on climate variations.

East Asian monsoon and westerly jet driven changes in climate and surface conditions in the NE drylands of China since the Late Pleistocene

The East Asian summer monsoon (EASM) is a major component of the global climate yet, the causes for the past spatiotemporal variability of EASM rainfall, its interactions and impacts remain unresolved. Here we use the Sr–Nd isotopes and rare earth elements composition of dust in a peat record from northeast (NE) China to investigate the relationship between aeolian dust, the East Asian monsoon and Westerly Jet (WJ) over East Asia since the Late Pleistocene (14 cal ka BP). The NE drylands of China dominate the dust fraction (Hunshandake, Horqin; 44–88%) with a contribution from the deserts of NW China (Badain Jaran, Tengger, Taklamakan; 10–44%), suggesting an influence of the WJ. Dust deposition varied during the Holocene, displaying a minimum between 8.0 and 6.0 cal ka BP and two peaks at 5.8–3.8 and 1.7–0.3 cal ka BP. Changes in dust flux are opposite to the East Asian winter monsoon intensity profile, suggesting a limited influence of the winter monsoon. The EASM appears to have played a more significant role in the dust cycle, with increasing dust fluxes corresponding with lower EASM precipitation. Variations in dust flux from NE drylands display shifts reflecting changes in EASM precipitations and dune activity along the EASM margin, where the dust originates from. To account for the influence of the WJ, we propose that the meridional position and intensity of the WJ also affected dust emission in the drylands’ region. A more northward position of the WJ allows the EASM front further north, generating more precipitations over the NE drylands, reducing the extent of arid areas, and resulting in less dust emission from dune activity, while the opposite occurs with a strong, more southerly WJ. Anthropogenic activities are likely to have had an increasing impact on the dust cycle over the late Holocene. Nevertheless, the presence of inconsistencies in records, coupled with a simultaneous decline in climatic conditions (mainly precipitations) during the same timeframe, hinders the precise assessment of the influence of human activities on dust emissions in the region.

The Toba Eruption 74,000 Years ago Strengthened the Indian Winter Monsoon‐Evidence From Coccolithophores

AbstractThe global impact of the Youngest Toba Tuff (YTT) supereruption is still heavily debated, ranging from having little effects on climate to significantly affecting modern human evolution. Climate models and proxy records show that the eruption may have caused cooling of the Asian landmass, thus impacting regional climate such as the Indian monsoon system. However, the immediate effect of the eruption on the Indian monsoon has not been indisputably demonstrated in any proxy record. Here, we present a paleo‐primary productivity (PP) record in core SO130‐289KL from the northeastern Arabian Sea based on the coccolithophore species Florisphaera profunda transfer function. Florisphaera profunda decreased from ∼30% before the YTT eruption to ∼8% right after the YTT eruption, which translates to an increase in PP by ∼65% from a long‐term average of ∼200 gC/m2/yr for about 8–19 years after the eruption. The duration was estimated using a new error‐weighted mean age of the YTT eruption (73.9 ± 0.1 ka, 2σ uncertainties) based on recent age estimates from ice cores, radiometric dating, and speleothem records. The elevated PP is most likely linked to the deepening of the surface ocean mixed layer driven by strengthened northeasterly Indian winter monsoon winds. This hypothesis is supported by stable oxygen isotope records from speleothem and ice cores, which show indications of a strengthening of the Indian winter monsoon during times of increased PP. Our results support previous modeling studies and provide unequivocal evidence from a marine record for an Indian monsoon response to the supereruption.