Monsoon Region Research Articles

Impacts of reducing scattering and absorbing aerosols on the temporal extent and intensity of South Asian summer monsoon and East Asian summer monsoon

Abstract. The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian summer monsoon and East Asian summer monsoon (SASM and EASM, respectively) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO2. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT) or absorbing (ABS) aerosols or decreases in both. For SCT, the combination of the early transition of land–sea thermal contrast and sea level pressure gradient during the pre-monsoon season, together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South Asia and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian high (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land–sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850 hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone; this is despite the inherent nonlinearity of the atmospheric systems. The opposing adjustments of Asian rainy season forced by the ABS and SCT aerosol emission reductions suggest that emission controls that target factors like emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions.

Contributions of climate and soil properties to geographic variations of soil organic matter across the East Asian monsoon region

Soil organic matter (SOM) plays a crucial role in soil functions and the global carbon cycle. Considering the disconnection of research emphasis between soil and ecology, little is known about the large-scale edaphic controls and their interaction with climate in SOM dynamics. This study aimed to explore the geographic evolution mechanisms of SOM from the perspective of soil intrinsic properties. A wide range of soil types from quaternary sediments were sampled from three ecosystems (forest, grass, and arable lands) and at different pedogenic horizons along the mid-temperate to tropical climate gradient in the East Asian monsoon region. SOM was measured in terms of organic carbon (SOC, 0.06–4.81 %), total nitrogen (TN, 0.04–0.41 %), and their ratio (C/N ratio, 1.17–12.26), as well as a series of soil properties: porosity, soil texture, clay mineralogy, exchangeable cations and soil aggregation. The indicators of SOM were most influenced by soil horizon (F=233.2–2375.2), followed by soil type (F=84.0–924.1, p < 0.001) and land use (F=4.7–92.2, p < 0.05); SOC, TN and C/N ratio overall decreased with increasing soil depth, and exhibited decrease first and then increase from north to south. Traditional predictors of soil texture and climate variables independently contributed little to SOM (p > 0.05). In contrast, bulk density had the greatest explanatory power (R2 = 0.379, p < 0.01), followed by soil aggregation (p < 0.05), while phyllosilicate minerals and iron oxides explained significant but small variances, all of which were climate-dependent. SOM could be jointly explained by soil properties (0.665) and climate (0.220) with an interaction effect of 0.152, among which bulk density, percentage of aggregate disruption, and mean annual temperature (MAT) were the most powerful predictors with negative effects. Structural equation modeling results indicated that MAT could influence SOM through soil aggregation and exchangeable cations. This study unravels the direct and indirect effects of climate on SOM and highlights the predominant roles of soil structural properties rather than abiotic primary agents at the regional scale, which merits consideration in improving SOM modeling and land management.

Extreme precipitation stable isotopic compositions reveal unexpected summer monsoon incursions in the Qilian Mountains

Isotope composition and moisture sources of precipitation are important for understanding water cycles and reconstructing paleoclimate. Based on 15-years' precipitation stable Isotope composition (δ18O and δ2H) from four stations of the Qilian Mountains, we found unique δ18O and δ2H features associated with the incursion of the summer monsoon over the Qilian Mountains, northwestern China. In 12 of the 15 years, similar seasonal variations of δ18O and δ2H confirmed a dominant source of moisture from Westerly circulation, and higher intercepts of the local meteoric water line (LMWL) indicated strong recycling of continental moisture. However, in August 2016 and 2018, extremely low slopes and intercepts of the LMWL, and more negative δ18O and δ2H revealed substantial contributions of the Asian summer monsoon to precipitation of the Qilian Mountains, with extremely heavy precipitation in August 2016. The column moisture flux, land-sea thermal contrast, correlations of precipitation δ18O with East Asian Summer Monsoon Index and Westerlies Index, HYSPLIT modeling results and precipitation δ18O along backward trajectories confirmed incursions of the summer monsoon in August 2016 and 2018. Our redefining of the boundary of the summer monsoon region confirmed the summer monsoon incursion zone can extend to the west of longitude 96°E and north of latitude 40°N in strong monsoon years, corresponding to boundaries of monsoon incursions in the mid-Holocene. Temperature correlated with precipitation δ18O at monthly and shorter time scales, but not for whole seasons or at yearly scale, revealing that summer monsoon incursions are therefore more likely than changing temperature to explain the multi-year cycles in the Qilian Mountains ice archives. Continent-scale shifts in atmospheric circulation strongly influence water resources in the Qilian mountains, and may change in frequency as climate warms. This study therefore has important implications for understanding water resources in the Qilian mountains in the past and into the future.

Verification of a global weather forecasting system for decision-making in farming over Africa

In the framework of AfriCultuReS project, operational deterministic weather forecasts provide valuable information on the expected weather conditions over the African continent as a part of federation of services within the project. In this study, we investigate the performance of the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) over pilot regions in Africa by utilizing available surface observations, satellite and reanalysis data. The verification period covers two consecutive years (June 2018–May 2020). In addition, we assess the ability of the model to provide skillful forecasts through three high-impact precipitation events that occurred during this period. The results show that the model presents both positive and negative biases with respect to its predicted near surface air temperature, underestimates the near surface relative humidity and the mean sea-level pressure, while overestimates the wind speed at 10 m. The neighborhood-based statistical verification of the 24-h accumulated precipitation reveals that the model forecasts the precipitation events more accurately as the verification area is increasing but at higher precipitation thresholds its performance deteriorates. Different variability, errors and correlation between simulated and observed precipitation exist in each forecast lead day and region. A range of model behavior and forecast skill is found with respect to the examined three precipitation events. Skillful forecasts up to four days ahead were provided in the cases of the Tropical Cyclone IDAI and the flash flooding events in northern Tunisia, while the lowest performance was found in the region of the West African Monsoon.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

The difference in time delay of δ18O, δ13CDIC and Mg/Ca to the hydroclimate change monitored in a subtropical cave, South China

Speleothem δ18O, δ13CDIC, and Mg/Ca are widely used to reconstruct past climatic and environmental changes. However, due to the rainfall regime and hydrological structure of caves, their application in reconstructing past hydroclimate in the East Asian monsoon region is poorly understood. In this study, we monitored two types of drip water in a subtropical cave to investigate the different responses of δ18O, δ13CDIC, and Mg/Ca to hydroclimate changes at monthly to yearly scales. Our results showed that drip rate at all drip sites responded quickly to precipitation, whereas δ18O, δ13CDIC, and Mg/Ca responded with some lag. The lag time for the parameters were in the order of δ13CDIC (lag time ranging from 1 to 2 months) < Mg/Ca ratio (lag time of 2 months) < δ18O (lag time up to 5 months), and for fissure-fed drip sites (M-5 and M-6) < seepage flow drips (M-1 and M-2), as well as single-reservoir supply (M-5) < multi-reservoirs supply (M-6). A comprehension of both fast and slow flows is crucial in order to comprehend karst reservoirs and their paths, which in turn enables the exploration of the hydrological properties of unsaturated zones under the context of Asian monsoon. We also found that the hydrological structure was crucial in generating the different response times of δ18O, δ13CDIC, and Mg/Ca in drip water. Additionally, the relationship between δ18O and δ13CDIC may be due to the different residence times in different drip sites on seasonal- to annual timescales.

Environmental memory of microbes regulates the response of soil enzyme kinetics to extreme water events: Drought-rewetting-flooding

How environmental history mediates the response of microbial activities to dry-wet cycle (D/W) in monsoon regions is poorly investigated. In this study, soil from Red River Delta of Vietnam was incubated for 52 days, which included 14 days at 20% water holding capacity (WHC), followed by 14 days rewetted to 60% WHC (rewetting) or 100% WHC (flooding). The kinetics of β-glucosidase, chitinase and acid phosphomonoesterase enzymes, microbial respiration, microbial biomass C and P were measured.The activities of all soil enzymes (Vmax) decreased by 20–42% due to drought but partially or totally recovered after two weeks of rewetting or flooding. The substrate affinity (Km) of all measured enzymes were fluctuating at initial drought and then dropped down by 33–55% after 14 days prolonged drought. Chitinase was the only enzyme showing the decoupling of Vmax and Km at the onset of drought, indicating different factors regulating enzyme synthesis and substrate affinity for the N demands of microorganisms. Pulse rewetting significantly reduced catalytic efficiency (Ka) but boosted turnover time (Tt) of chitinase and β-glucosidase. Rewetting prolonged Tt as compared to flooding by the end of the experiment. Moreover, both soil basal respiration and substrate induced respiration were significantly inhibited by drought and recovered to the control level after rewetting. In conclusion, soil with history of D/W triggers the recovery of microbial activities after D/W cycles and rewetting has a great impact on enzyme systems, while flooding induces more effects on enzyme synthesis.

Composition, Distribution, and Attribution of Hydrochemistry in Drainage Systems in the North of Tianshan Mountains, China

The characteristics and sources of the hydrochemical composition of natural water are mainly influenced and limited at the basin scale by what factors, this has become a focal issue of environmental change in the middle latitude areas and even globally. In this study, three large drainage systems (Junggar, Yili, and Erlqis) in the north of the Tianshan Mountains were selected to study the hydrochemistry of different river basins and understand the relevant causes and attributions of different water bodies in arid environments in the Central Asian Orogenic Belt (CAOB). Natural water samples from the three drainage systems and their hydrochemical data, combined with literature data of the north and south Tianshan Mountains and surrounding areas, were systematically collected and comprehensively compared with other higher, middle, and low latitude watersheds with different climate conditions. The results show that the total dissolved solid concentrations in the CAOB drainage systems are generally higher than those of rivers in Chinese monsoon and humid regions, and also higher than the world average level. The relative concentrations of different ions are similar to those of rivers in the monsoon region of eastern China and most rivers in the world dominated by carbonate weathering. The ionic compositions of surface water bodies from different sub-basins in the study area are distributed near the Ca apex in the piper diagram, while those of phreatic and confined groundwater samples tend to the Na apex. The compositional differences in the anions are not like those in the cations, rarely distinguishable for these different water types. In a sub-drainage basin scale from I to VIII sub-basins in the study area, major ion concentrations and distributions in these basins are evidently heterogeneous. Almost all the cation and anion concentrations span &gt; 1 order of magnitude, especially sodium and chlorine; however, the calcium and alkalinity concentrations and distributions are relatively homogeneous for some basins in the Junggar and Yili drainage systems. The relative homogeneity between anions and the relative heterogeneity between cations can be recognized in the ion chemistry of natural water samples between different types, indicating that the composition and distribution of cations in water is more conducive than those of anions to identifying the differences and commonalities between different regions. Atmospheric precipitation plays a key role on the hydrogeochemical process in the north of the Tianshan Mountains, and the elevation differentiation of chemical weathering and ion concentrations are closely related to it. The positive correlation between total solute flux and runoff cannot be explained by the dilution effect, indicating that hydroclimatic factors such as runoff and aridity have a strong effect in limiting the hydrochemistry of natural water bodies in the study area.

The Tibetan Plateau bridge: Influence of remote teleconnections from extratropical and tropical forcings on climate anomalies

Recent advances in the bridging roles played by the Tibetan Plateau (TP) are reviewed in terms of the remote influence of circulation anomalies over the North Atlantic Ocean on Asian monsoon and El Niño-Southern Oscillation (ENSO) events, and in a clear link between the tropical oceans and Asian climate anomalies. The authors firstly introduce how the winter and spring anomalies in the North Atlantic Ocean affect the seasonal transition over the South Asian monsoon region and subsequent ENSO events on the interannual timescale. A distinct negative sensible heating-baroclinic structure in May over the TP is found to provide an intermediate bridging effect in this Atlantic-Asian-Pacific connection. In summer, the North Atlantic Oscillation is significantly correlated with the variations of East China summer rainfall, and it is the TP's latent heating that plays the bridging role within. On the other hand, such a TP bridging effect also exists in the connection from the tropical oceans to extreme precipitation events over eastern China in summer, and from the midlatitude wave train to the biweekly oscillation of South China rainfall in spring.摘要本文回顾了青藏高原桥梁作用方面的最新研究进展, 涉及北大西洋气候异常对春, 夏亚洲季风和厄尔尼诺-南方涛动 (ENSO) 事件的遥相关影响, 热带海洋异常和中国东部极端气候异常之间的联系以及华南春雨的季节内变化等. 介绍了年际时间尺度上, 冬-春季北大西洋海表温度强迫如何影响南亚季风的季节性转变以及随后ENSO事件的触发. 5月份青藏高原上空显著的负感热斜压结构, 为北大西洋影响亚洲季风和ENSO提供了桥梁效应. 夏季北大西洋涛动与华东夏季降水变化显著相关, 高原潜热在这一关系中起着桥梁作用. 另一方面, 这种高原桥梁效应也存在于从热带海洋异常到东亚夏季极端降水事件的连接中, 以及从中纬度波列到华南春雨准双周振荡的联系中.

Hysteretic Behavior of Global to Regional Monsoon Area Under CO2 Ramp‐Up and Ramp‐Down

AbstractWhen projecting future monsoon changes by carbon dioxide (CO2) pathway, most studies have analyzed precipitation responses without considering monsoon area (total area of monsoon domain, MA) variations. However, how MA responds to CO2 removal remains uncertain. This study evaluates MA variations and impacts in idealized CO2 ramp‐up (toward CO2 quadrupling), ramp‐down, and stabilized simulations using the Community Earth System Model version 1. Global MA negatively overshoots (i.e., recovery with decreasing tendency beyond the original MA) during the ramp‐down period due to reduced or rapidly recovered MA in several regional monsoons, including Northern and Southern Africa, South and East Asia, and South America, showing hysteresis when comparing CO2 ramp‐up and ramp‐down periods despite similar global warming levels. These non‐linear regional MA variations come from distinct regional summer and winter precipitation variations, which are found to be mostly associated with Intertropical Convergence Zone movements and El Niño‐like response. Further, regional monsoon precipitation characteristics also vary through CO2 ramp‐up and ramp‐down periods consistently with overall hysteresis. Changes in total monsoon precipitation resemble the distinct responses of MA. Our results suggest that regions characterized by a monsoonal climate may experience reduced seasonal rainfall variations under net‐negative CO2 emissions.

Modeling Extreme Rainfall Events in the Tropical Monsoon Region: A Generalized Pareto Distribution Approach

Modeling Extreme Rainfall Events in the Tropical Monsoon Region: A Generalized Pareto Distribution Approach

Moisture sources of summer heavy precipitation in two spatial patterns over Northeast China during 1979–2021

AbstractThis study classifies the spatial distribution of heavy precipitation in summer (June–August) from 1979 to 2021 in the three provinces of Northeast China (TPNC) into two patterns by using the self‐organizing maps (SOM) neural network, and then quantitatively analyzes their moisture transport channels and sources using the Lagrangian model. The results show that the summer heavy precipitation in TPNC can be divided into the northern and southern patterns according to the distribution of the heavy precipitation. Both patterns of heavy precipitation are affected by the low‐level vortex west of TPNC, but the strength and shape of the low vortex are different. The northern pattern is mainly influenced by the westerly flow in the vortex in the mid‐high latitudes, which transports moisture from the upstream westerly region into TPNC. The southern pattern is mainly affected by the southerly jet stream southeast of TPNC, which conveys a large amount of moisture from the East Asian summer monsoon region into TPNC. In terms of the summer climatological mean, the northern pattern has a higher precipitation recycling rate, while the southern pattern has a lower recycling rate.

The Intra‐Seasonal Oscillation of Precipitation δ18O Over the Asian Equatorial and Monsoon Regions

AbstractWater isotopes‐climate correlations are used to reconstruct paleoclimate from various natural archives. Intra‐seasonal oscillation is one of the major atmospheric patterns that modulate the ratio of precipitation isotope in low latitudes, yet their spatial and temporal distribution patterns are unclear. Here we presented a detailed analysis of how the intra‐seasonal oscillations (MJO, Madden‐Julian Oscillation, and BSISO, Boreal Summer Intra‐seasonal Oscillation) modulate precipitation δ18O and vapor δ18O over the Asian monsoon region and the equatorial region. This analysis found consistent intra‐seasonal variations between the ISO and rain δ18O. We interpret it as the ISO regulating the active and inactive convective systems on the regional scale, leading to the intra‐seasonal oscillations in precipitation δ18O, with amplitudes from 4‰ up to 15‰. The MJO (BSISO) leads to eastward (northeastward) propagation of δ18O intra‐seasonal oscillations in the Asian equatorial (monsoon) region, coinciding with the spatial patterns of OLR oscillations, reflecting the response of precipitation/vapor δ18O to the oscillation of large‐scale convective activity or accumulated depletion of regional precipitation. We proved that the amplitudes of intra‐seasonal variation of precipitation δ18O are comparable to seasonal variations, and both of them are the main patterns for precipitation δ18O in study regions. Our results are conducive to the accurate interpretation of δ18O in climate proxies such as precipitation, ice cores, tree cellulose, cave stalagmites, et al. on the intra‐seasonal scale in this region.

Hydroclimate changes since the last glacial maximum from sedimentary biomarkers in a crater lake in the Great Khingan Mountains, Northeast China

The scarcity of well-dated, highly temporally resolved, paleoclimate records since the last glacial maximum (LGM) in Northeast China limits our understanding of past climatic variations and the prediction of future hydrological changes in the context of anthropogenic global warming. A high-resolution n-alkane record covering the past ∼25 kyr was retrieved from Lake Tuofengling, a hydrologically closed crater lake in the central Great Khingan Mountains. The sediments contained a suite of mid- and long-chain n-alkanes with a strong odd-to even-carbon number predominance. The relative proportion of mid-to long-chain n-alkane homologues (Paq) was proposed to evaluate the input of submerged/floating plants into lake sediments relative to that from emergent/terrestrial plants, which could be a useful indicator of lake level in lacustrine settings. The Paq proxy demonstrated that lake level increased stably from the shallowest level during the LGM to a maximum depth at 7.5–3.0 cal ka BP, with a minor decreasing trend during the late Holocene. The asynchronous changes in effective moisture in the East Asian summer monsoon (EASM) region since the LGM might be directly or indirectly mediated by the shift in the rainfall belt, the position and intensity of the Western Pacific Subtropical High and Okhotsk High at different timescales. Our study provides new insights into climatic evolution in Northeast China, where the main climate pattern may not always be similar to the typical EASM pattern.

High-resolution GEOS-Chem model for Indian monsoon region: Seasonal cycle and budget of tropospheric CO2

A nested GEOS-Chem atmosphere transport model has been set up for the Indian monsoon region (40° E-110° E & 15° S–45° N at 0.250X0.3125°) spatial resolution to study the variability of the tropospheric CO2 over India and surrounding oceans during 2012–2020. The model has been constrained by the influxes of CO2 at its four lateral boundaries derived from the GEOS-Chem global simulations and surface fluxes of CO2 over the land and oceans based on historical databases. It is initialized from a known state and driven by the GEOS reanalysis's meteorological forcing. Model simulations were evaluated at three in situ measurement locations and with respect to the satellite retrievals from OCO-2 and GOSAT. Model parameters were tuned to have reasonable solutions to resolve observed latitudinal gradient and seasonal and inter-annual variability. Three additional model runs were made to evaluate the role of long-range transport, net terrestrial ecosystem exchange, air-sea fluxes, and regional emissions in controlling seasonal oscillations of tropospheric CO2 over the study region. It revealed that the net terrestrial ecosystem exchanges are responsible for 35% control in the surface layer CO2 seasonal tendency for India and the Bay of Bengal domains, and 10% for the Arabian Sea. The air-sea fluxes have 8% control on the CO2 seasonal tendency for the Arabian Sea and Bay of Bengal and 5% for India. The anthropogenic emissions have 4% control for India and <1% for the Arabian Sea, and 9% for the Bay of Bengal. The long-range transport, as the residual, is responsible for 60% variability in the seasonal cycle of CO2 over India and the Bay of Bengal and 75% for the Arabian Sea domain.

The spatio-temporal pattern of Asian summer monsoon during glacial Termination II recorded by Chinese stalagmite δ18O

The difficulty in constraining the large-scale Asian summer monsoon (ASM) variability in the Chinese monsoon region (CMR) during glacial Termination II lies in our limited knowledge of the western part, primarily due to sparse paleoclimate records. To get a better picture of the ASM during Termination II, we examined a precisely dated stalagmite δ18O record between 133.1 and 127.0 kyr B.P. from Wanxiang Cave located at the ASM's northern edge in western China. In combination with published δ18O data from this cave, we have identified the ‘Weak Monsoon Interval’ (WMI) in the Wanxiang δ18O record and confirmed that the Heinrich 11 cold event in the North Atlantic caused the weakened ASM over the CMR via reorganization of the large-scale ocean-atmospheric circulation. However, the post-WMI change in δ18O is gradual, in contrast with the abrupt shift shown in the other cave records from southern and northeastern China. The rapid northward migration of the westerly jet relative to the Qinghai-Tibet Plateau is probably responsible for this discrepancy. This northward-positioned westerly jet prevented the more 18O-depleted moisture from the Indian Ocean from reaching the study site. Simultaneously, it facilitated the earlier northward movement of the East Asian summer monsoon (EASM) rainband that carries positive precipitation δ18O to obscure the abrupt decrease in our δ18O record. After the onset of the last interglacial, no obvious Younger Dryas (YD)-like event was recorded in Wanxiang Cave. This result is consistent with most stalagmite δ18O records in the CMR and further suggests a minimal impact of the YD-like event on ASM variabilities. The relatively large amplitude of δ18O variations observed in Wanxiang Cave between the late penultimate glacial and the last interglacial corresponds to a dominant control of the Indian summer monsoon (ISM), whereas smaller δ18O amplitudes were recorded in cave sites mainly under the influence of both ISM and EASM. Therefore, we posit that the heterogeneity of the hydroclimate in the CMR during Termination II resulted from a combination of multiple processes, that is, the westerly jet, ISM and EASM, rather than a single one.

Atmospheric processes control the stable isotopic variability of precipitation in the middle–lower reaches of the Yangtze River Basin, East Asian monsoon region

Paleoclimatic records in the East Asian monsoon region (EAM) depend on precipitation isotopes as proxies for past hydroclimatic variability. However, the potential mechanisms controlling precipitation isotopic variability in this region remain poorly understood. This study collected daily precipitation isotopes in the middle-lower reaches of the Yangtze River Basin from 2017 to 2020 to explore how convective behaviors and moisture sources determining the variability of precipitation stable isotopes. Daily precipitation δ18O presented a weak inverse relationship with local precipitation amount and air temperature. Results of correlation analysis showed that the maximum correlation zones of precipitation δ18O between different cloud levels and outgoing long wave radiation were identified in the upstream regions such as the Indian Peninsula, BoB, and Indo-China Peninsula during the rainy season. Back-trajectory analysis showed that substantial water vapor from maritine sources in the Indian Ocean and the Bay of Bengal (BoB) largely contributed to the lower δ18O values during the rainy season (−7.5 ‰) because of intense rainout processes in the upstream sourced regions, demonstrated by significant regional correlation between precipitation δ18O and regional precipitation amount. The regional correlations were stronger in the upstream regions than the local influences, indicating that precipitation isotopes in this region are more influenced by regional convective processes in the upstream region during the rainy season. Using the modified Stewart model, the mean subcloud evaporation fraction estimated was greater in rainy season (24.1 % vs 36.1 % for HK and HZ, respectively) than that in dry season (15.1 % vs 19.9 % for HK and HZ, respectively). These findings highlight the importance of upstream processes in controlling precipitation isotopes in the EAM region and provide reliable evidence on the interpretation of monsoonal climate variability.

Carbon Stock Differentials and Sequestration Potential under Different Cropping Systems in a Tropical Monsoon Region, South-West India

Carbon Stock Differentials and Sequestration Potential under Different Cropping Systems in a Tropical Monsoon Region, South-West India

A strong short-duration convection near Poyang Lake in daytime of warm season

Poyang Lake (PL), the biggest freshwater lake in China, is situated in the East Asian Monsoon region, and has an important impact on local convection. In general, PL can result in convection in local region when it is a heat source in nighttime of warm season. However, at around noon on 4 May 2020 (PL is a cold source), a convection was triggered about 20 km west of PL, and rapidly enhanced and resulted in lightning when approaching PL, and then quickly weakened and disappeared after entering the main body of PL. In order to explore the convection formation, several observational data and the Weather Research and Forecasting model were applied in this study. Results show that when the convection approaches PL, its rapid enhancement is induced by PL, and after entering the main body of PL, its quick weakening is also resulted from PL. However, the initiation of the convection is mainly induced by the local topography west of PL under a favorable large-scale background. Mechanism analysis indicates that the strong low-level convergence near the west shore of PL associated with lake-land breeze is responsible for the rapid enhancement of the convection, and the low-level divergence over the main body of PL associated with the lake-land breeze and the increase of the low-level stability induced by cooling of PL jointly result in the quick weakening of the convection. The prevailing southerly wind in low level passes through the local topography (Meiling Mountain) west of PL, and is divided into southwesterly wind (flow around Meiling) and southerly wind (flow over Meiling), and they converge in the north of Meiling, triggering the convection. This study is not only important to deepen the understanding of PL affecting regional weather, but also helpful for improving the refined forecasting of convection near PL.

Why a large‐scale monsoon does not exist in North America: Orographic effects

AbstractTo understand why a large‐scale monsoon does not exist in North America, we performed a series of sensitivity simulations to investigate orographic effects of the Rocky Mountains (RMs) using the Community Atmosphere Model version 5.1. Results show that the height of the RMs plays a fundamental role in shaping the monsoon over North America that is confined to a small area from northwestern Mexico to the southwestern United States. When the RMs' height is increased by five times their actual height, a larger part of the non‐monsoon region over North America becomes a monsoon region. The mechanical effects of the RMs uplift dominate in winter, while thermal effects dominate in summer. During winter the mechanical effects induce an equivalent barotropic atmospheric response in the troposphere. With the uplift of the RMs, the ridge and trough located on the western and eastern sides of Canada are strengthened. Most areas of North America are influenced by the northerlies during winter such that enhanced descending motion over the eastern RMs favours a dry winter climate. However, thermal effects dominate during summer through enhanced baroclinic atmospheric responses. The Mexico high and lower‐level cyclonic circulation are strengthened with the RMs uplift, triggering large‐scale ascending motion. Eastern North America is mainly controlled by the enhanced southerly wind along the western flank of the North Atlantic subtropical high. Thus, the enhanced water‐vapour transport and upward motion on the eastern side of the RMs increase summer precipitation. As a result, an obvious seasonal variation with the feature of ‘dry winter and wet summer’ finally develops, indicating that the height of the RMs plays a crucial role in shaping the monsoon over the central United States. Additional experiments show that the base area of the RMs has little effect on the large‐scale monsoon formation over North America.