Asian Monsoon Research Articles

Responses of hydrodynamic circulation to the climatic change of wind forcing in a subtropical estuary

Under the background of climate change, long-term trends in atmospheric-marine processes have been observed over the past decades. Being a subtropical estuary influenced by East Asian monsoon wind, the Pearl River Estuary (PRE) is chosen as a prototype to investigate the response of estuarine hydrodynamics to the local climatic regime shift in wind forcing. Analyses from ERA5 wind data and local meteorological observations have revealed that summer wind over PRE (prevailed by the up-estuary wind) was decreasing in magnitude and shifting anticlockwise from southwesterly to southeasterly over the past 42 years (1979–2020). Using a fully calibrated three-dimensional numerical model, the hydrodynamic circulation of PRE were simulated for 42 years with realistic daily wind forcing and repeated annual cycles for other forcing fields. Both the least-squares fitting and the EOF analysis demonstrate that the summertime longitudinal flow was strengthened while the lateral circulation was weakened over the past 42 years. Comparative analyses of the results obtained from the process-oriented numerical experiments revealed that the decreasing wind speed is the dominant factor for the strengthening of the longitudinal circulation, while the shifting wind direction is the dominant factor for the weakening of the lateral circulation. Using calculated long-term trends in wind forcing, wind conditions are predicted for 2100. The model results indicate that by 2100, there will be a further strengthening of the longitudinal circulation and a further weakening of the lateral circulation during the summer. The momentum budget reveals that the baroclinic pressure gradient and the diffusion term are the dominant terms modulating the changes of hydrodynamic circulation. The water mass in the PRE will become saltier, and the exchange between the deep channel and shallow shoals of the estuary will be slowed down in the future. The impact of these changes on brackish water habitats and local ecosystems needs further study.

Moist bias in the Pacific upper troposphere and lower stratosphere (UTLS) in climate models affects regional circulation patterns

Abstract. Water vapour in the upper troposphere and lower stratosphere (UTLS) is a key radiative agent and a crucial factor in the Earth's climate system. Here, we investigate a common regional moist bias in the Pacific UTLS during Northern Hemisphere summer in state-of-the-art climate models. We demonstrate, through a combination of climate model experiments and satellite observations, that the Pacific moist bias amplifies local long-wave cooling, which ultimately impacts regional circulation systems in the UTLS. Related impacts involve a strengthening of isentropic potential vorticity gradients, strengthened westerlies in the Pacific westerly duct region, and a zonally displaced anticyclonic monsoon circulation. Furthermore, we show that the regional Pacific moist bias can be significantly reduced by applying a Lagrangian, less-diffusive transport scheme and that such a model improvement could be important for improving the simulation of regional circulation systems, in particular in the Asian monsoon and Pacific region.

Different Configurations of Cross-Equatorial Flows over the Indian Ocean–Western Maritime Continent and Their Implications for Improving Regional Climate Predictability

Abstract Low-level cross-equatorial flows (CEFs) over the Indian Ocean–western Maritime Continent (IO-wMC) play a crucial role in transporting energy, mass, and water vapor into the Northern Hemisphere during the Asian summer monsoon season (May–September). Utilizing ECMWF reanalysis data (ERA5), we investigate three CEFs over the IO-wMC: the Somali-CEF, Bay of Bengal CEF (BoB-CEF), and South China Sea CEF (SCS-CEF). The statistical independence of the Somali-CEF and BoB-CEF implies distinct formation processes on the interannual time scale. To examine the interannual variability of CEFs, we perform an empirical orthogonal function (EOF) analysis of vertically integrated meridional winds from surface pressure to 850 hPa over the equatorial IO-wMC. The first EOF mode reveals a weakening of the Somali-CEF and strengthening of the BoB-CEF and SCS-CEF, which is associated with the concurrent summer Indian Ocean dipole and quasi-biennial phase transition of El Niño events. The second EOF is related to the Indo-western Pacific Ocean capacitor (IPOC) mode that often emerges in post–El Niño summers. Despite the IPOC’s influence on meridional winds over the tropical southwestern Indian Ocean, three CEFs are uncorrelated with the EOF2. On the other hand, the Somali-CEF and BoB-CEF are significantly weakened in the EOF3. The EOF3 is an ENSO-unrelated internal IPOC mode. We further use a multilinear regression model based on preceding sea surface temperature (SST) over the Indo-Pacific and EOF3 to predict regional climate anomalies and compare the prediction skill with the SST-based models. Our results suggest that adding EOF2- and EOF3-related variability to the prediction model can improve Asian summer monsoon predictability. Significance Statement This study examines the interannual variability of low-level cross-equatorial flows (CEFs) over the Indian Ocean and western Maritime Continent, crucial components of the Asian summer monsoon. We identify three CEFs and investigate their relationships with interannual sea surface temperature modes in the Indo-Pacific region. The leading empirical orthogonal function (EOF) modes of equatorial meridional wind reveal diverse CEF configurations and post–El Niño summer wind variability. Also, the EOFs distinguishes between El Niño and El Niño–unrelated equatorial meridional wind variability. Utilizing a multilinear regression model based on both preceding Indo-Pacific SST modes and configurations of CEFs, we demonstrate the potential to enhance regional climate predictions, contributing to a more comprehensive understanding and forecasting of the Asian summer monsoon and its associated climate impacts.

East Asian monsoon variations in the loess–desert transitional zone (northern China) during the past 14 ka and their comparison with TraCE21K simulation results

Abstract The Holocene is a critical period for understanding the East Asian monsoon system (EAM) over long timescales, but high-precision dating and high-resolution records from the Holocene epoch at monsoonal margins of East Asia are lacking. Here, on the basis of closely spaced radiocarbon and optically stimulated luminescence dating results obtained from a typical loess–paleosol sequence on the northern Chinese Loess Plateau (CLP), we provide an independent age-based, high-resolution depositional record of East Asian summer (EASM) and winter monsoons (EAWM) variations over the past ~14 ka. We find that both the EASM and EAWM simultaneously strengthened sometime during the Holocene optimum (~7–5 ka BP), with greater seasonality, and weakened during the Late Holocene. These findings are counterintuitive to our understanding of the EAM variations based on loess records at suborbital scales during interglacial periods, providing an alternative scenario of the monsoon system evolution. We postulate that high-latitude forcing and surface feedbacks, such as vegetation change, have modulated the EAM variations during the Holocene warmth.

Climatological Madden-Julian Oscillation during boreal spring leads to abrupt Australian monsoon retreat and Asian monsoon onsets

Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.

Impact of Solar Activity and ENSO on the Early Summer Asian Monsoon During the Last Millennium

AbstractThe Asian Monsoonal rainfall accounts for the majority of annual regional precipitation in East and South Asia and could be remotely regulated by El Niño‐Southern Oscillation (ENSO). Besides, several paleoclimate records and simulations have indicated solar signals in the Asian Monsoon, implying the impact of solar activity on the regional monsoon precipitation. By conducting multi‐linear regression analysis to the solar irradiance forced single‐forcing experiment in the last millennium, this study presents the comparison of solar and ENSO effects on monsoonal precipitation in South and East Asia during early summer (May–June). Increased total solar irradiance during high solar activity years tends to trigger a favorable environment for developing monsoon onset, leading to more precipitation against ENSO‐related patterns over Southeast and South Asia before peak‐summer (July–August). The result supports reconstructed terrestrial records and underlines considerable influences of the solar cycle on the variation of the Asian Summer Monsoon.

How Can We Improve the Seamless Representation of Climatological Statistics and Weather Toward Reliable Global K‐Scale Climate Simulations?

AbstractToward the achievement of reliable global kilometer‐scale (k‐scale) climate simulations, we improve the Nonhydrostatic ICosahedral Atmospheric Model (NICAM) by focusing on moist physical processes. A goal of the model improvement is to establish a configuration that can simulate realistic fields seamlessly from the daily‐scale variability to the climatological statistics. Referring to the two representative configurations of the present NICAM, each of which has been used for climate‐scale and sub‐seasonal‐scale experiments, we try to find the appropriate partitioning of fast/local and slow/global‐scale circulations. In a series of sensitivity experiments at 14‐km horizontal resolution, we test (a) the tuning of terminal velocities of rain, snow, and cloud ice, (b) the implementation of turbulent diffusion by the Leonard term, and (c) enhanced vertical resolution. These tests yield reasonable convection triggering and convection‐induced tropospheric moistening, and result in better performance than in previous NICAM climate simulations. In the mean state, double Intertropical Convergence Zone bias disappears, and the zonal contrast of equatorial precipitation, top‐of‐atmosphere radiation balance, vertical temperature profile, and position/strength of subtropical jet are reproduced dramatically better. Variability such as equatorial waves and the Madden–Julian oscillation (MJO) is spontaneously realized with appropriate spectral power balance, and the Asian summer monsoon, boreal‐summer MJO, and tropical cyclone (TC) activities are more realistically simulated especially around the western Pacific. Meanwhile, biases still exist in the representation of low‐cloud fraction, TC intensity, and precipitation diurnal cycle, suggesting that both higher spatial resolutions and further model development are warranted.

Water conservation pattern of Fangcheng River Basin in Beibu Gulf and its response to precipitation.

Assessing the spatiotemporal patterns of watershed water conservation under the influence of the South Asian monsoon climate and its response to precipitation is essential for revealing the evolving patterns of water conservation under different temporal scales. Following the principles of water balance and using the Soil and Water Assessment Tool (SWAT) model, we investigated the spatiotemporal patterns of water conservation and its response to precipitation in the Fangcheng River Basin of Beibu Gulf. The results showed that water conservation in Fangcheng River Basin calculated by SWAT model were 1637.4 mm·a-1, accounting for 50.7% of the mean annual precipitation. The variation of water conservation in different sub-basins was obviously different. Sub-basins with high forest coverage and steep slopes exhibited higher water conservation, while sub-basins with other land use types (such as cropland and grassland), gentle slopes, and intense human activities showed lower water conservation. At the monthly scale, both water conservation and its variation showed similar response characteristics to precipitation in the basin. The response of water conservation variation to sub-precipitation events could be classified into two types. For the short-term rainfall events (duration≤2 days), water conservation variation showed a linear relationship. For the medium to long-term rainfall events (2 days<duration≤10 days), water conservation variation showed a nonlinear curve, being influenced mainly by processes such as evapotranspiration. High-frequency short-duration precipitation events were more conducive to increasing ecosystem water conservation compared to the long-duration precipitation events.

Attribution of Moisture Sources for Summer Precipitation in the Upstream Catchment of the Three Gorges Dam

Abstract Currently, there is a lack of investigating moisture sources for precipitation over the upstream catchment of the Three Gorges Dam (UCTGD), the world’s largest dam. Using the dynamical recycling model (DRM), trajectory frequency method (TFM), and the Climate Forecast System Reanalysis (CFSR), this study quantifies moisture sources and transport paths for UCTGD summer precipitation from 1980 to 2009 based on two categories of sources: region-specific and source-direction. Overall, the land and oceanic sources contribute roughly 63% and 37%, respectively, of the moisture to UCTGD summer precipitation. UCTGD and the Indian Ocean are the most important land and oceanic sources, respectively, in which the southern Indian Ocean with over 10% of moisture contribution was overlooked previously. Under the influence of the Asian monsoon and prevailing westerlies, the land contribution decreases to 57.3% in June, then gradually increases to 68.8%. It is found that for drought years with enhanced southwest monsoon, there is a weakening of the moisture contribution from the C-shaped belt along the Arabian Sea, South Asia, and UCTGD, and vice versa. TFM results show three main moisture transport paths and highlight the importance of moisture from the southwest. Comparison analysis indicates that, generally, sink regions are more affected by land evaporation with their locations more interior to the center of the mainland. Furthermore, correlations between moisture contributions and indices of general circulation and sea surface temperature are investigated, suggesting that these indices affect precipitation by influencing moisture contributions of the subregions. All of these are useful for comprehending the causes of summer UCTGD precipitation. Significance Statement Quantitative research on the moisture sources of summer precipitation has been implemented for the upstream catchment of the Three Gorges Dam (UCTGD), which is of particular hydrological significance but has not been investigated previously. The dynamical recycling model (DRM)–trajectory frequency method (TFM) approach is used to quantify and interpret the results of the moisture sources both in different specific subregions and directions, which produce more meaningful results than a single method for the areal division of moisture sources. Furthermore, antecedent indices that significantly influence the following moisture contributions of the subregions and then summer UCTGD precipitation are studied in terms of large-scale general circulation indices, which would help our understanding of precipitation forecast for UCTGD.

Eocene monsoon climate expansion in East Asia: Evidence from orbital‐cycle driven terrestrial successions in the Jianghan Basin, Central China

ABSTRACTEast Asia experienced complex climatic changes during the Eocene. Due to the lack of a high‐precision time framework covering the Eocene, the characteristics of sedimentary and climate evolution in this period remain unclear. The Jianghan Basin (Central China) developed an Eocene continental succession, which is composed of lower red beds and upper salt‐lake deposits exhibiting clear sedimentary rhythms, making these strata an ideal target for astronomical cycle analysis. Based on the established floating astronomical timescale, the lower red beds in the basin formed during the early–middle Eocene and were dominated by eccentricity cycles. The upper salt‐lake deposits formed during the middle–late Eocene and recorded obliquity cycles. The change in facies indicates that the climate transitioned from continuously arid to less arid with periodic relative wetness, which was forced by orbital cycles. Significant obliquity signals recorded by salt rhythmites responded to Eocene global temperature changes and Tibetan region uplift, which can be important markers of a monsoon‐like climate. Comparing the sedimentary and climatic characteristics of other Eocene lake basins in East Asia, the extension of the Eocene East Asian monsoon may have initiated during ca 43.6 to 40.0 Ma. This study provides a new perspective for understanding the early evolution of the East Asian monsoon.

Strontium isotope dating influenced by Rubidium contamination from terrestrial material: A case study from the Cenozoic dolomite in the Xisha Islands, South China Sea

AbstractStrontium isotopes (87Sr/86Sr) are commonly used to assess the age of Cenozoic dolostones found on many isolated oceanic islands. The possible influence of Rubidium (Rb) content on the 87Sr/86Sr ratios, however, has generally been ignored. This critical issue is assessed by examining the 87Sr/86Sr ratios and elements (Rb, Sr, Al, Ti and K) of seventy‐one samples that came from a ca 339 m thick cored interval of Cenozoic dolomite on the Xisha Islands, South China Sea. For the dolomite from the upper Pliocene Yinggehai Formation (180.5 to 308.5 m; average 23% dolomite) there is a positive correlation between the 87Sr/86Sr ratios and Rb concentrations, whereas for the dolomite from the lower late Miocene Huangliu Formation (308.5 to 519 m; average &gt;98% dolomite) such a correlation is not evident. Combined with high‐temperature experiments, correlation between Al, Ti, K, Rb and 87Sr/86Sr indicates that Rb containing old 87Rb from terrestrial materials modified the 87Sr/86Sr ratios of the Yinggehai dolomite, but not those from the Huangliu dolomite. This is mainly attributed to the different sources of terrestrial dust carried by different East Asian monsoon systems between the late Miocene and Pliocene, and probably also influenced by the dolomitization process to some extent. To test for 87Rb contamination, extrapolation of results from the Yinggehai dolomite and Huangliu dolomite indicate that concentrations of Rb &lt; 1 ppm, Al &lt; 1000 ppm, Ti &lt; 15 ppm and K &lt; 250 ppm in the dolomite indicates that terrestrial inputs are minimal and that the measured 87Sr/86Sr ratios can be used for determining the dolomitization age. Potentially, these concentration thresholds may serve as a screening criterion for assessing the reliability of Sr isotope chronostratigraphy of other island dolostones throughout the world.

Revisiting East Asian monsoon change during the Last Glacial Maximum using PMIP4 simulations

The Last Glacial Maximum (LGM; 21 000 years ago) East Asian monsoon was revisited using all available simulations performed by the latest generation of climate models participating in phase 4 of the Paleoclimate Modelling Intercomparison Project (PMIP4). With respect to the preindustrial period, all five models consistently showed a weakening of the East Asian monsoon by 1%–18% in winter and by 2%–32% in summer during that glacial period. Regionally, the LGM monsoon circulation was dominated by southerly wind anomalies in winter but northerly wind anomalies in summer over East Asia in AWI-ESM-1-1-LR, MIROC-ES2L, and MPI-ESM1-2-LR. In the two versions of CESM2, large regional differences existed in the LGM change in the East Asian monsoon, especially for summer. Such changes in the East Asian monsoon can be explained by the various geographical distributions of surface cooling and sea level pressure difference, mainly due to the large volume of ice sheets and lower atmospheric greenhouse gas concentrations during the LGM. The spread among the models and the uncertainty among the proxy data call for further simulation and reconstruction efforts to comprehend the change in the East Asian monsoon during the glacial period.摘要利用PMIP4多模式试验数据, 本文重新检查了末次冰盛期 (距今约21 000年) 东亚季风变化. 结果表明: 相对于工业革命前期, 所有5个模式一致模拟显示末次冰盛期东亚季风减弱, 冬季和夏季减幅分别为1 %–18 %和2–32 %; 不同模式中东亚季风环流变化的空间分布存在差异, 这主要源于该时期大尺度变冷和海平面气压梯度变化的空间分布不同; 由于模式之间的差异和重建记录之间的不确定性, 未来有待开展更多模拟和重建工作以更好地理解冰期东亚季风变化.

Genetic structure of the commercially important Asian monsoon scallop, Amusium pleuronectes (Linnaeus 1758), across the Indonesian Archipelago

AbstractThe Asian monsoon scallop, Amusium pleuronectes, is a key member of the most commercially harvested shellfish community in Asia. Patterns of genetic diversity and natural population structure in the target species were investigated to gain a better understanding of its evolutionary history. Samples were collected from five sites across the Indonesian Archipelago. We characterized sequence variation in an mtDNA control region fragment in 249 individuals. The genetic diversity (h = ranging from 0.83 to 0.92) and nucleotide diversity (π = ranging from 0.24 to 0.32) were low compared to the estimates reported for many other similar shellfish taxa. Nonetheless, analysis of molecular variance revealed significant genetic differentiation, FST = 0.0203 (p &lt; .005 after Bonferroni corrections). Furthermore, both Pairwise fixation index values showed significance among majority population sites, indicating that dispersal potential and gene flow were low in the past. This pattern likely reflects a low dispersal potential, potentially allowing local adaptation to sites that augment any oceanographic and geographic contribution to genetic structure. The results described herein provide a foundation for developing better conservation strategies for the target species in the future.

Phylogeny and diversification of genus Sanicula L. (Apiaceae): novel insights from plastid phylogenomic analyses

BackgroundThe genus Sanicula L. is a unique perennial herb that holds important medicinal values. Although the previous studies on Sanicula provided us with a good research basis, its taxonomic system and interspecific relationships have not been satisfactorily resolved, especially for those endemic to China. Moreover, the evolutionary history of this genus also remains inadequately understood. The plastid genomes possessing highly conserved structure and limited evolutionary rate have proved to be an effective tool for studying plant phylogeny and evolution.ResultsIn the current study, we newly sequenced and assembled fifteen Sanicula complete plastomes. Combined with two previously reported plastomes, we performed comprehensively plastid phylogenomics analyses to gain novel insights into the evolutionary history of this genus. The comparative results indicated that the seventeen plastomes exhibited a high degree of conservation and similarity in terms of their structure, size, GC content, gene order, IR borders, codon bias patterns and SSRs profiles. Such as all of them displayed a typical quadripartite structure, including a large single copy region (LSC: 85,074–86,197 bp), a small single copy region (SSC: 17,047–17,132 bp) separated by a pair of inverted repeat regions (IRs: 26,176–26,334 bp). And the seventeen plastomes had similar IR boundaries and the adjacent genes were identical. The rps19 gene was located at the junction of the LSC/IRa, the IRa/SSC junction region was located between the trnN gene and ndhF gene, the ycf1 gene appeared in the SSC/IRb junction and the IRb/LSC boundary was located between rpl12 gene and trnH gene. Twelve specific mutation hotspots (atpF, cemA, accD, rpl22, rbcL, matK, ycf1, trnH-psbA, ycf4-cemA, rbcL-accD, trnE-trnT and trnG-trnR) were identified that can serve as potential DNA barcodes for species identification within the genus Sanicula. Furthermore, the plastomes data and Internal Transcribed Spacer (ITS) sequences were performed to reconstruct the phylogeny of Sanicula. Although the tree topologies of them were incongruent, both provided strong evidence supporting the monophyly of Saniculoideae and Apioideae. In addition, the sister groups between Saniculoideae and Apioideae were strongly suggested. The Sanicula species involved in this study were clustered into a clade, and the Eryngium species were also clustered together. However, it was clearly observed that the sections of Sanicula involved in the current study were not respectively recovered as monophyletic group. Molecular dating analysis explored that the origin of this genus was occurred during the late Eocene period, approximately 37.84 Ma (95% HPD: 20.33–52.21 Ma) years ago and the diversification of the genus was occurred in early Miocene 18.38 Ma (95% HPD: 10.68–25.28 Ma).ConclusionThe plastome-based tree and ITS-based tree generated incongruences, which may be attributed to the event of hybridization/introgression, incomplete lineage sorting (ILS) and chloroplast capture. Our study highlighted the power of plastome data to significantly improve the phylogenetic supports and resolutions, and to efficiently explore the evolutionary history of this genus. Molecular dating analysis explored that the diversification of the genus occurred in the early Miocene, which was largely influenced by the prevalence of the East Asian monsoon and the uplift of the Hengduan Mountains (HDM). In summary, our study provides novel insights into the plastome evolution, phylogenetic relationships, taxonomic framework and evolution of genus Sanicula.

The Reexamination of the Moisture–Vortex and Baroclinic Instabilities in the South Asian Monsoon

Observational analyses reveal that a dominant mode in the South Asian Monsoon region in boreal summer is a westward-propagating synoptic-scale disturbance with a typical wavelength of 4000 km that is coupled with moistening and precipitation processes. The disturbances exhibit an eastward tilt during their development before reaching their maximum activity center. A 2.5-layer model that extends a classic 2-level quasi-geostrophic model by including a prognostic lower-tropospheric moisture tendency equation and an interactive planetary boundary layer was constructed. The eigenvalue analysis of this model shows that the most unstable mode has a preferred zonal wavelength of 4000 km, a westward phase speed of 6 m s−1, an eastward tilt vertical structure, and a westward shift of maximum moisture/precipitation center relative to the lower-tropospheric vorticity center, all of which agree with the observations. Sensitivity experiments show that the moisture–vortex instability determines, to a large extent, the growth rate, while the baroclinic instability helps set up the preferred zonal scale. Ekman-pumping-induced vertical moisture advection prompts an in-phase component of perturbation moisture relative to the low-level cyclonic center, allowing the generation of available potential energy and perturbation growth, regardless of whether or not a low-level mean westerly is presented. In contrast to a previous study, the growth rate is reversely proportional to the convective adjustment time. The current work sheds light on understanding the moisture–vortex and the baroclinic instability in a monsoonal environment with a pronounced easterly vertical shear.

Modulations of NAO on the interplay between monsoon and westerlies in Asia on interdecadal timescales

Modulations of NAO on the interplay between monsoon and westerlies in Asia on interdecadal timescales

Stepwise astronomical tuning of obliquity-driven evaporite cycles in an Eocene salt lake (Jianghan Basin, Hubei Province, China): Implications for middle Eocene East Asian monsoon-like climate evolution

The rhythmic evaporation cycle is an ideal recorder of astronomical cycle signals, but studies on cyclostratigraphy have yet to be directly conducted on halite-rich strata. The Qianjiang Depression of the Jianghan Basin is a representative Eocene East Asian halite-rich basin, and the salt rhythmites that developed therein are important recorders of climate evolution in East Asia. This study selected five wells for basin-scale cyclostratigraphy analysis, taking the Lower Qian 4 member of the Qianjiang Formation as the research object. This study found that the basinal salt lake facies were dominated by obliquity cycles, whereas the shallow-water deposits mainly recorded short eccentricity cycles. The study also found that s3−s6 obliquity cycles could be detected throughout the salt lake sedimentary record. Therefore, a stepwise astronomical tuning scheme was adopted. First, the target intervals in different sedimentary areas were preliminarily tuned to s3−s6 obliquity cycles, and stratigraphic correlation was performed. Then, the tuned data were further adjusted using obliquity cycles to reveal the impact of obliquity on the development of salt rhythmites and establish a floating astronomical time scale (ATS). Sedimentary noise models and pollen analysis further demonstrated that obliquity amplitude modulation cycles drove periodic changes in hydrology and climate. Based on the spatiotemporal distribution of salt rhythmite−rich strata in Paleogene East Asia, this study proposes that the development of rhythmic evaporites can reveal the existence of a monsoon-like climate. Astronomical influence was an important driving force for developing the middle Eocene East Asian monsoon.

Impact of spring land-surface conditions over the Tibetan Plateau on the early summer Asian monsoon using an AGCM large ensemble

Influence of land-surface memory effects over the Tibetan Plateau on the Asian summer monsoon has long been studied, but not quantified because of the difficulty of extracting only these effects from observational data. This study examines the impact of spring land-surface conditions, including surface air temperature (SAT) and snow cover, over the Tibetan Plateau and its surrounding regions on the early stage of the Asian summer monsoon using large-ensemble experiments produced by an atmospheric global climate model (AGCM). The results show that the above-normal SAT over and around the Tibetan Plateau in May can enhance June monsoon circulation without the sea surface temperature (SST) forcing. The physical mechanism behind this involves warmed surface air over the plateau enhancing the north–south SAT gradient, leading to strengthening the Asian summer monsoon circulation and precipitation. Inter-ensemble correlation analysis indicated that SAT warming over the plateau is clearly associated with upper-tropospheric anti-cyclonic circulation anomalies in May. Also, although winter land-surface signals do not persist until late spring, SAT anomalies in March tend to persist until May, explaining 10–20% of the total SAT variance in May. Interestingly, the spring SAT impacts on June monsoon circulations vary interannually, indicating that the contribution of the land–atmosphere (L–A) coupling process to interannual monsoon variability differs from year to year. Active L–A coupling years tend to coincide with cooler SSTs with anomalous low-level divergence and upper-level convergence over and around the Maritime continent, implying that SST forcing is associated with the L–A coupling strength. Specifically, in May of the developing stage of monsoon circulation, the weak Walker circulation over the equatorial and northern Indian Ocean and undeveloped monsoon circulation are suitable for the L–A coupling to influence the development of the monsoon circulation effectively. However, the well-known oceanic events, such as El Nin~\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\widetilde{n}}$$\\end{document}o, are found to be less connected with the interannual L–A coupling variations.

Impacts of Mid‐Pliocene Ice Sheets and Vegetation on Afro‐Asian Summer Monsoon Rainfall Revealed by EC‐Earth Simulations

AbstractThe impact of mid‐Pliocene boundary conditions on Afro‐Asian summer monsoon (AfroASM) rainfall is examined using the fully coupled Earth System Model EC‐Earth3‐LR. Our focus lies on the effects of varying CO2 concentration, diminished ice sheets and vegetation dynamics. We find that the enhanced AfroASM rainfall is predominantly caused by the “warmer‐gets‐wetter” mechanism due to elevated CO2 levels. Additionally, the ice sheet, similar in size to that of the mid‐Pliocene era, creates several indirect effects. These include sea ice‐albedo feedback and inter‐hemispheric atmosphere energy transport. Such influences result in the southward shift of Hadley circulation and formation of Pacific‐Japan pattern, leading to reduced rainfall in North African and South Asian monsoon regions but increased rainfall in East Asian monsoon region. Interestingly, while dynamic vegetation feedback has a minimal direct effect on AfroASM rainfall, it significantly influences rainfall in the mid‐high latitudes of the North Hemisphere by enhancing water vapor feedback.

Short communication: Extreme glacier mass loss triggered by high temperature and drought during hydrological year 2022 / 2023 in Qilian Mountains

In the hydrological year 2022/2023, the glaciers in the Qilian Mountains experienced unprecedented mass loss. The glacier-wide mass balance was −1,188 mm w.e., in contrast to −350 mm of average mass balance since 1990 over the Bailanghe Glacier No. 12 in the middle of Qilian Mountains. The temperature during 2022–2023 reached the highest value ever recorded, second only to 2022, while at the same time the precipitation amount was less compared to other year since 2000, which together led to the strongest glacier mass loss during 2022–2023. The atmospheric circulation analysis shows that the high temperature in the Qilian Mountains in 2023 was jointly caused by the Arctic air mass and East Asian monsoon.