East Asian Monsoon Region Research Articles

Numerical simulation of tropical cyclone generated waves in South China Sea during winter monsoon surge

The South China Sea (SCS) is a highly semi-enclosed marginal sea located in the East Asian monsoon region. This paper proposes interesting aspects of the unique feature of the SCS waves in response to tropical cyclone's passage when large-scale winter monsoon winds prevail. We use the wave model WaveWatch III to study the wave characteristics of typhoon Durian (2006) passing over the middle of SCS in early December 2006, and state the new understanding acquired in the aspects of the tropical cyclone generated waves in the SCS during winter monsoon surge. In light of this, the role of the large-scale NE monsoon winds on winter typhoon wave field characteristics in the SCS are highlighted by conducting sensitivity experiments with and without the NE monsoon winds. The NE monsoon winds weakly affect the SWH field near the typhoon track and strongly away from the track, especially in the deep water area of the northern SCS where the NE monsoon winds produce high waves. Comparisons between the two experiments show the effect of the NE monsoon winds on the directional wave spectra in the SCS, suggesting that the monsoon-generated swells do not decay and remain throughout the typhoon period.

Regional Characteristics of Cloud Radiative Effects before and after the South China Sea Summer Monsoon Onset

The South China Sea summer monsoon (SCSSM) onset is characterized by rapid thermodynamical changes in the atmosphere that are critical to regional weather and climate processes. So far, few studies have focused on the changes in the associated cloud and radiative features. This study investigates spatiotemporal characteristics of top-of-atmosphere (TOA) cloud radiative effects (CREs) before and after the SCSSM onset over the South China Sea (SCS) and South China (SC), based on the 2001–2016 Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) satellite data and ERA-Interim reanalysis data. Before the SCSSM onset, strong net CRE (NCRE) dominated by its cooling shortwave component occurs over SC, while descending motion and weak NCRE prevail over the SCS. In the SCSSM onset pentad, convection, high clouds, and longwave and shortwave CREs (LWCRE and SWCRE) abruptly increase over the southern and central SCS, and their high-value centers subsequently move northeastward and are strongly affected by the western Pacific subtropical high. The strong offset between LWCRE and SWCRE enables the NCRE intensity (TOA radiation budget) to be quite small (large) between the SCS and the western North Pacific after the SCSSM onset. In contrast, low-middle-level clouds and strong cooling SWCRE remain over SC after the SCSSM onset, but the increasing high clouds and LWCRE weaken (intensify) the regional NCRE (TOA radiation budget) intensity. These marked latitudinal differences in CREs between the SCS and SC primarily arise from their respective dominant cloud types and circulation conditions, which manifest the differences between the tropical SCSSM and subtropical East Asian monsoon processes. The results indicate that regional cloud fractions and CREs before and after the SCSSM onset are strongly modulated by quickly changed large-scale circulation over the East Asian monsoon regions, and the spatiotemporal variation of CREs is a response to the monsoonal circulation adjustment to a large extent.

Low Pressure Systems and Extreme Precipitation in Southeast and East Asian Monsoon Regions

AbstractUnderstanding the physical mechanisms behind the secular trends of summer rainfall extremes over the heavily populated Southeast and East Asian monsoon regions is not only of scientific importance but also of considerable socioeconomic implications. In this study, the relevance of the excessive-rain-producing low pressure systems (LPSs) to extreme rainfall is quantified. Using an objective feature-tracking algorithm, the synoptic-scale LPSs are identified and tracked in the 40-yr ECMWF interim reanalysis. The region experiences approximately 16 terrestrial and 18 marine LPSs each summer. The terrestrial LPSs form near the downwind side of the Tibetan Plateau and travel northeastward toward jet latitudes. The marine LPSs form over the western North Pacific Ocean and migrate along the western periphery of the subtropical high. While both types of LPSs account for a large portion of upper-tail rainfall, the terrestrial LPSs predominantly impact the extreme rainfall over inland areas, and the marine LPSs primarily affect the coastal regions where they frequently make landfall. The historical extreme rainfall trend during 1979–2018 aligns with the changes in LPS tracks. The decreasing number of northeastward-moving terrestrial LPSs leads to an extreme rainfall dipole with negative trends in north-central China and positive trends in southern China, while the increasing number of northward-recurving marine LPSs enhances the extreme rainfall in the eastern China coast but suppresses it over the South China Sea. These trends are driven dynamically by the weakening of the monsoonal southwesterlies and the eastward retreat of the subtropical high, which might be attributable to anthropogenic forcings.

Sub-Seasonal Variability of Meridional Activity of Western Pacific Subtropical High in Boreal Late Summer

Sub-seasonal variability in meridional activity of Western Pacific subtropical high (WPSH) exerts important influences on anomalous weather and climate in East Asian monsoon region. By using a sub-seasonal meridional index of the WPSH, the characteristics and atmospheric evolutions in association with the sub-seasonal meridional activity of the WPSH during boreal late summer are further investigated. The meridional activity of the WPSH exhibits distinctive low frequency oscillation (LFO), with two dominant periods of 10-30 days and 40-50 days. The 10-30 day LFO is influenced by wave trains in mid-high latitudes and convection propagating from tropic. For the northward advance of the WPSH, in the high latitudes, teleconnection wave train originating from Atlantic Ocean propagates eastward, then the positive and negative potential vortex anomalies enhance the high-level divergence in Northeast Asia. In the mid-latitudes, the 10-30 day low frequency wave train from North America propagates westward and influences the pressure field along the East Asia coast, whose energy is gained from the westerly jet through barotropic process. In the tropic, on time scale of 10-30 days, anomalous convections over the equatorial western Pacific propagate northwestward and trigger cyclone-anticyclone circulation anomalies along the East Asia coast, which in turn affect the meridional position of the WPSH. The 40-50 day LFO of meridional activity of WPSH is closely related to the convection anomalies under the interaction between MJO (Madden-Julian Oscillation) and MISO (monsoon intra-seasonal oscillation). The tropical MJO propagates eastward from the equatorial Indian Ocean to West Pacific and then propagates northward, in accompany with the northeastward propagation of anomalous convection teleconnection covering the tropical Indian Ocean-North Indian Peninsula-Northwest Pacific, resulting in the anomalous anti-cyclonic circulation over Northwest Pacific, which contributes to the northward advance of the WPSH. For the southward retreat of the WPSH, the contrary is the case in 10-30 day atmospheric LFO while relatively weaker 40-50 day atmospheric LFO does less contribution to the southward retreat of the WPSH.

Variations of Stable Isotopic Composition in Atmospheric Water Vapor and their Controlling Factors—A 6‐Year Continuous Sampling Study in Nanjing, Eastern China

AbstractLong‐term continuous monitoring of water vapor isotopes has great potential to improve our understanding of mechanisms that control stable isotope variations in hydrological processes, which is essential for accurate interpretation of isotopic proxy records. Here we presented a 6‐year daily data set of stable isotopes in atmospheric water vapor (δ18Ov, δDv, and d‐excessv) and precipitation (δ18Op, δDp, and d‐excessp) at Nanjing in Eastern China, an area under the influence of the East Asian Monsoon. Based on isotopic variations, we divided a year into three seasons, that is, summer monsoon season (June–September), spring (March–May), and fall‐winter (October–February). During summer monsoon season, the low δ18Ov values were mainly caused by the upstream convective rainout process during the large‐scale water vapor transport, and the d‐excessv values were the lowest due to the weak kinetic isotopic fractionation over its oceanic moisture source. Spring had the highest δ18Ov values, which was related to less distillation of local moisture under relatively high air temperatures. The relatively high d‐excessv values resulted from the kinetic isotopic fractionation due to drier condition. The fall‐winter season has the lowest δ18Ov values because the local distillation was the strongest with low air temperature. The d‐excessv values were highest during this season because of the strongest kinetic isotopic fractionation during the continental moisture recycling under dry climate condition. Using the isotopic composition of both precipitation and water vapor, we found that the annual mean subcloud raindrop evaporation is ~11% of precipitation. These findings improve our understanding of hydrological cycle on the East Asian Monsoon region and will potentially improve our interpretation of numerous isotopic proxy records from this region.

Distinct responses of Asian summer monsoon to black carbon aerosols and greenhouse gases

Abstract. Black carbon (BC) aerosols emitted from natural and anthropogenic sources induce positive radiative forcing and global warming, which in turn significantly affect the Asian summer monsoon (ASM). However, many aspects of the BC effect on the ASM remain elusive and largely inconsistent among previous studies, which is strongly dependent on different low-level thermal feedbacks over the Asian continent and the surrounding ocean. This study examines the response of the ASM to BC forcing in comparison with the effect of doubled greenhouse gases (GHGs) by analyzing the Precipitation Driver Response Model Intercomparison Project (PDRMIP) simulations under an extremely high BC level (10 times modern global BC emissions or concentrations, labeled BC×10) from nine global climate models (GCMs). The results show that although BC and GHGs both enhance the ASM precipitation minus evaporation (P−E; a 13.6 % increase for BC forcing and 12.1 % for GHGs from the nine-model ensemble, respectively), there exists a much larger uncertainty in changes in ASM P−E induced by BC than by GHGs. The summer P−E is increased by 7.7 % to 15.3 % due to these two forcings over three subregions, including East Asian, South Asian and western North Pacific monsoon regions. Further analysis of moisture budget reveals distinct mechanisms controlling the increases in ASM P−E induced by BC and GHGs. The change in ASM P−E by BC is dominated by the dynamic effect due to the enhanced large-scale monsoon circulation, whereas the GHG-induced change is dominated by the thermodynamic effect through increasing atmospheric water vapor. Radiative forcing of BC significantly increases the upper-level atmospheric temperature over the Asian region to enhance the upper-level meridional land–sea thermal gradient (MLOTG), resulting in a northward shift of the upper-level subtropical westerly jet and an enhancement of the low-level monsoon circulation, whereas radiative forcing of GHGs significantly increases the tropical upper-level temperature, which reduces the upper-level MLOTG and suppresses the low-level monsoonal circulation. Hence, our results indicate a different mechanism of BC climate effects under the extremely high BC level: that BC forcing significantly enhances the upper-level atmospheric temperature over the Asian region, determining ASM changes, instead of low-level thermal feedbacks as indicated by previous studies.

Fire history of the Chinese Loess Plateau under extreme warming scenarios: A case study of the loess-paleosol sequence L5-S5

In order to study the history of natural fires under warming scenarios and to evaluate the possible incidence of fires in the future, we analyzed the carbon concentration and carbon isotope composition of black carbon within the loess-paleosol sequence of the Chinese Loess Plateau. Notably, the interglacials in this East Asian monsoon region were abnormally warm. The results from the interval of L5-S5 of the studied Xifeng section suggest that the principal factors controlling natural fires in an interglacial period (S5) and a glacial period (L5) were fuel load and fuel moisture level, respectively. The difference between the carbon isotope composition of black carbon and soil organic carbon during S5LL1 (marine oxygen isotope stage (MIS) 14) was due to the fact that the former records the more positive carbon isotope values of C3 plants caused by drought and low temperatures in the fire season, while the latter records the carbon isotope of plants throughout the year. The optimum temperature and humidity conditions during S5SS1 (MIS 13) were the most conducive to fires during the interglacial period of S5. However, when the temperature and humidity decreased to a specific range (mean annual temperature 2-12 °C, mean annual precipitation 200–700 mm) during the glacial period of L5, the fire occurrence reached its highest level.

Asynchronous 500-year summer monsoon rainfall cycles between Northeast and Central China during the Holocene

Although the summer monsoon is the dominant source of precipitation in the East Asian monsoon region (EAMR), there is long-standing uncertainty about the spatial and temporal variability of precipitation on the centennial time scale. In this study we analyzed two well-dated, high-resolution, published paleoclimatic records in order to determine the variability of precipitation in the EAMR during the Holocene. One record is a decadal-resolution pollen record from an annually-laminated Maar lake in Northeast (NE) China, and the other is a U Th dated, multi-decadal-resolution speleothem record from Central China. Advanced time-series analysis of the two records provides unambiguous evidence for asynchronous summer monsoon rainfall variability on centennial to millennial time scales in the EAMR. The pollen and speleothem records indicate long-term and ~ 500-yr quasi-periodic changes in humid/dry climatic shifts against the background of an insolation-forced climatic trend throughout the past 8580 years. Phases of wetter climate (during ~7000–5500 cal yr BP) and drier climate (during ~1400–300 cal yr BP) in NE China are closely related to different climatic phases in Central China. However, the ~500-yr cyclic rainfall changes in Central China lag those in NE China by several decades to almost two centuries. During the ~500-yr periodic changes in the monsoon, a climatic pattern encapsulated as “Northern drought, southern flooding” prevailed during an El Niño-like phase, whereas the opposite pattern “Southern drought, northern flooding” occurred during a La Niña-like phase. The asynchronous pattern of ~500-yr climatic cyclicity between the northern and southern EAMR may have been caused by the El Niño-Southern Oscillation, linked to low-latitude oceanic and atmospheric interactions. • Holocene rainfall changes between Northeast and Central China are asynchronous. • 500-year cyclic rainfall changes in Central China lag those in Northeast China. • The lag of the 500-year rainfall cycles is several decades to almost two centuries. • Rainfall changes in Northeast and Central China are anti-phased on the millennial scale.

Statistical Study of the Midlatitude Mesospheric Vertical Winds Observed by the Wuhan and Beijing MST Radars in China

AbstractBased on the long‐term observations of the Wuhan and Beijing Mesosphere‐Stratosphere‐Troposphere (MST) radars, the statistic characteristics of the mesospheric vertical winds at midlatitudes are investigated for the first time. The vertical wind data come from the Wuhan MST radar (29.51°N, 114.13°E) during 2012–2017 and from the Beijing MST radar (39.75°N, 116.96°E) during 2012–2014. The subdaily, seasonal, and annual variations of the mesospheric vertical winds at Wuhan and Beijing are presented. No obvious subdaily and seasonal variations are found and the annual variations of the mean vertical winds are downward in most of time. The mean vertical winds at Wuhan become smaller in summer and turn to upward at some heights, while those of Beijing only become smaller in summer. The difference of summer vertical winds over Wuhan and Beijing is closely related to the gravity wave activity. The Wuhan MST radar is located to the east side of the Qinghai‐Tibet plateau and also in the East Asian monsoon region, thus the mesospheric winds over Wuhan may experience more influence of gravity waves compared with those over Beijing. The possible bias sources in the vertical wind measurement of the two MST radars are carefully analyzed and the presented wind velocities are proved to be credible. The measured velocity magnitude is similar to other Very High Frequency radar observations, but much larger than that of the WACCM simulation. The Stokes drift, multicellular circulation structures, the wave influence on the radar echoes are raised as the potential sources for this discrepancy.

Local and Regional Drivers of Environmental Changes in Two Subtropical Montane Ponds (Central China) Over the Last Two Centuries

Central China, one of the Earth’s distinctive ecoregions due to its endemic subtropical biota, has been subjected to enhanced nitrogen deposition and climate warming during recent decades. However, the extent and timescale of ecological changes are largely unexplored. Multiproxy analyses (diatoms, photosynthetic pigments and geochemistry) of 210Pb-dated sediment cores from two shallow ponds within an alpine basin (central China) were used to investigate the response of primary producer communities to external stressors during the last two centuries. The study sites include one drainage pond and one seepage pond. Both ponds exhibited unambiguous changes in production and composition of photoautotrophs since the early twentieth century, which are linked to climate warming, nitrogen deposition and local factors (for example, lake morphometry, desiccation and macrophyte). Although primary producers responded to regional warming and nitrogen deposition, the ecological responses differed among ponds due to local factors. In the deeper seepage pond, light attenuation due to terrestrial organic matter input caused recent decreases in carotenoids and small fragilarioid taxa. In contrast, the co-occurrence of euterrestrial and tychoplanktonic diatoms in the shallower drainage pond was indicative of its hydrological instability. Our results indicate that subtropical montane ponds in the East Asian monsoon region appear to be strongly influenced by a combination of local (for example, catchment-lake connectivity) and regional driving forces (for example, warming and nitrogen deposition).

Measurement of oxygen and hydrogen isotopic ratios of speleothem fluid inclusion water using Picarro

A speleothem fluid inclusion extraction and isotope analysis system was established based on a vacuum extraction device and a Picarro L2140-i wavelength scanned cavity ring-down spectroscopy (WS-CRDS) water isotope analyzer. WS-CRDS allows to measure the stable oxygen and hydrogen isotope composition ( δ 18O and δ D) of small water samples simultaneously, which is faster, simpler, and requires less maintenance compared to currently used continuous-flow isotope ratio mass spectrometry (CF-IRMS). The precision of water isotopic analysis utilizing the WS-CRDS is often superior to that of CF-IRMS. Therefore, the use of Picarro for isotope analysis has rapidly gained popularity. The oxygen isotopic composition of speleothem calcite ( δ 18Oc) is one of the main proxies of speleothem-based paleoclimate reconstructions, but the interpretation of δ 18Oc remains complex as it can be influenced by many factors. Speleothem fluid inclusions contain the characteristics of cave drip water. The δ 18O and δ D values of fluid inclusion water provide direct information about paleoprecipitation, and can be further used to reconstruct paleotemperature when combined with δ 18Oc. The present paper contains a detailed description of the system as we have designed it, and provides an introduction to the system’s performance. The vacuum extraction device, connected to the Picarro analyzer, is designed to release the water trapped in speleothem fluid inclusions by pneumatically-operated crushing of discrete speleothem calcite chips. The whole device is kept at a temperature of 120°C to ensure the total and instantaneous vaporization of the water without isotopic fractionation. The vaporized water is carried by a flow of high-purity nitrogen to a 40 mL volume for homogenization, and is then introduced into the Picarro analyzer for δ 18O and δ D analysis. A syringe injection unit was added to the vacuum extraction device to be able to measure standard water, with δ 18O and δ D values similar to the speleothem samples, before and during measurements. To verify the precision and accuracy of the system, the same standard water samples were measured with an automated Picarro L2140-i set-up that includes an autosampler and a Picarro evaporation unit. The δ 18O and δ D values of standard waters as measured by both systems are comparable. Using the crushing device, speleothem samples from the East Asian Monsoon region yield paired δ 18O/ δ D values within ±0.5‰ of the Global Meteoric Water Line (GMWL), indicating no isotopic fractionation during the measurements. The precision is a function of the water amount released and of the isotopic composition of the sample. The reproducibility of crushed speleothem samples, whose water vapor concentration is between 2500 and 50000 ppm, is 0.5‰ for δ 18O and 2‰ for δ D (1 SD), similar to results obtained from other published measuring systems.

Relations between the quasi‐biweekly oscillation over the East Asian monsoon region and the East Asian tropical monsoon depressions

AbstractAn empirical orthogonal function analysis was applied to 34 years of anomalous daily 10–20 day filtered outgoing longwave radiation data to determine the north‐westward propagating mode of quasi‐biweekly oscillations (QBWOs) over tropical East Asia during boreal summer. The centres of active QBWO convection coincided with positive potential vorticity (PV) anomalies at 500·hPa and cyclonic gyres at 850·hPa, whereas the centres of suppressed convection were superimposed upon negative PV anomalies and anticyclonic gyres. These circulations formed an inclined northwest–southeast wave train. The vertical dynamic and thermodynamic structures of the QBWO mode evince two centres of PV at 500 and 750·hPa, respectively, and an almost upright structure in the troposphere up to 300·hPa. The QBWO's convective cell has a warm‐over‐cold structure in the troposphere. These features have several similarities to East Asian tropical monsoon depressions (EAMDs), implying a close relationship between the north‐westward propagating mode of QBWOs and EAMDs. Further investigation indicates that EAMDs are prone to appear in the region of deep convection and move north‐westward with the QBWO circulation. The westernmost position of the western Pacific subtropical high (WPSH), which is linked to the QBWO phase, strongly influences the route of EAMDs. An EAMD generated in the easterly winds to the south of WPSH moves westward under the influence of the horizontal adiabatic advection of PV. More eastward‐moving and turning EAMDs appear when WPSH weakens and retreats eastward due to enhancement of QBWO convection over the South China Sea (SCS) and western North Pacific. Most EAMDs reach peak intensity when moving over the SCS, and show a strong relation to the vigorous convection. However, there is no significant relation between the QBWO convection and the points of disappearance of the EAMDs.

Variation of the stable isotopes of water in the soil-plant-atmosphere continuum of a Cinnamomum camphora woodland in the East Asian monsoon region

The hydrogen and oxygen stable isotopic (2H and 18O) tracer method has become an important means of studying water transfer between the atmosphere, rivers and lakes, soils, plants, and aquifers. We conducted a study on the variations of the stable isotopes of water, and the seasonal variations in water use of Cinnamomum camphora, in the soil-plant-atmosphere continuum in a C. camphora woodland in Changsha, central-southern China, which lies within the monsoon region of East Asia. Stable isotopes were monitored in precipitation, soil water (0–130-cm depth), groundwater, twig xylem water of C. camphora, and at the same time relevant environmental variables (soil water content, and meteorological variables) were measured, from March 2017 to August 2019. The monitoring data were collated and analyzed to determine the variations of stable isotopes in different water pools and mutual exchanges. The results indicate that daily precipitation (P) greater than 3.7 mm (regarded as effective precipitation) contributed to soil water replenishment. Isotopic composition of soil water and twig xylem water fell below the LMWLP>3.7 (the local meteoric water line eliminated rainfall events with P ≤ 3.7 mm). Isotopic composition of shallow soil water (0–60 cm) and twig xylem water was positively correlated with that of cumulative antecedent precipitation within two months. These patterns of precipitation isotopic signal propagation delineate a clear pathway of water transfer in this soil and plant continuum. The slope of the soil water line in the study area was higher than that of the LMWLP>3.7 because of the situation that precipitation in warm season with a larger evaporative demand is more depleted with heavy isotopes (i.e., more negative delta values) than that in cold season when the evaporative demand is low. Therefore, it was not feasible to determine the strength of isotopic fractionation based on the isotopic water line method. According to the MixSIR model, C. camphora water uptake mainly sourced from shallow soil water during the observation period. However, in July-September when soil was dry, deep soil water (60–130 cm) contributed to C. camphora water uptake for ~50%. These findings improve our understanding of the water cycle of the soil-plant-atmosphere continuum in the East Asian monsoon region.

Assessment of the Forecast Skill of Multiphysics and Multistochastic Methods within the GRAPES Regional Ensemble Prediction System in the East Asian Monsoon Region

Abstract To more comprehensively and accurately address model uncertainties in the East Asia monsoon region, a single-physics suite, where each ensemble member uses the same set of physics parameterizations as the control member in combination with multiple stochastic schemes, is developed to investigate if the multistochastic schemes that combine different stochastic schemes together can be an alternative to a multiphysics suite, where each ensemble member uses a different set of physics parameterizations (e.g., cumulus convection, boundary layer, surface layer, microphysics, and shortwave and longwave radiation). For this purpose, two experiments are performed for a summer monsoon month over China: one with a multiphysics suite and the other with a single-physics suite combined with multistochastic schemes. Three stochastic schemes are applied: the stochastically perturbed parameterizations (SPP) scheme, consisting of temporally and spatially varying perturbations of 18 parameters in the microphysics, convection, boundary layer, and surface layer parameterization schemes; the stochastically perturbed parameterization tendencies (SPPT) scheme; and the stochastic kinetic energy backscatter (SKEB) scheme. The combination of the three stochastic schemes is compared with the multiphysics suite in the Global and Regional Assimilation and Prediction Enhanced System–Regional Ensemble Prediction System with a horizontal grid spacing of 15 km. Verification results show that, overall, a single-physics suite that combines SPP, SPPT, and SKEB outperforms the multiphysics suite in precipitation verification and verification for upper-air weather variables, 10-m zonal wind, and 2-m temperature in the East Asian monsoon region. The indication is that a single-physics suite combining SPP, SPPT, and SKEB may be an appropriate alternative to a multiphysics suite. This finding lays a foundation for the development and design of future regional and global ensembles.

Quantifying the Influences of PM2.5 and Relative Humidity on Change of Atmospheric Visibility over Recent Winters in an Urban Area of East China

Fine particulate matters (PM2.5) and relative humidity (RH) in the ambient atmosphere are the leading anthropogenic and natural factors changing atmospheric horizontal visibility. Based on the analysis of environmental and meteorological data observed over 2013–2019 in Nanjing, an urban area in East China, this study investigated the influences of PM2.5 and RH on atmospheric visibility changes over recent years. The visibility had significantly negative correlations with the PM2.5 concentrations and RH changes. The nonlinear relationships existed between PM2.5 concentrations and visibility, as well as between RH and visibility, with the inflection points in the atmospheric visibility changes. The PM2.5 inflection concentrations were 81.0 μg m−3, 76.0 μg m−3, 49.0 μg m−3, and 33.0 μg m−3, respectively, for the RH ranges of RH < 60%, 60% ≤ RH < 80%, 80% ≤ RH < 90%, and RH ≥ 90%, indicating that the improvement of visibility with reducing PM2.5 concentrations could be more difficult under the humid meteorological condition. The visibility changes were most sensitive to PM2.5 concentrations in the RH range of 60–80% in this urban area of East China. The relative contributions of natural factor RH and anthropogenic factor PM2.5 to variations of wintertime atmospheric visibility were quantified with 54.3% and 45.7%, respectively, revealing an important role of natural factor RH in the change of atmospheric visibility in the urban area of East Asian monsoon region.

Impacts of moisture sources on the temporal and spatial heterogeneity of monsoon precipitation isotopic altitude effects

Moisture sources affect the temporal variations of the elevation effects on the stable isotopes in monsoon precipitation. However, whether it will lead to spatial variations and form a specific spatial-temporal heterogeneity pattern remains to be explored. Based on the monthly precipitation samples collected in Ailao Mountains of East Asian monsoon region, we computed and identified five moisture source regions to facilitate its comparison to the monthly oxygen isotope composition (δ18O) of precipitation with HYSPLIT model. We analysed the influence mechanisms of the moisture sources on the temporal and spatial heterogeneity of the elevation effect on the hydrogen and oxygen stable isotopes during dry and rainy seasons. We found that the temporal heterogeneity of the elevation effects on the precipitation 18O depends on the proportion of the moisture sources in the dominant direction, and as the proportion increases, the elevation effect becomes more significant. The spatial heterogeneity depends on the combination complexity of the moisture sources, the simpler the combinations, the more significant the elevation effects. The results demonstrate that for precipitation δ18O in Ailao Mountains, the dominant of monsoon moisture sources (the ratio higher than 50%) can result in significant elevation effects, while the mixtures of multiple moisture sources and the dominant of local moisture sources will lead to no elevation effects. Future studies should pay more attention to the influence mechanism of the spatial and temporal heterogeneity of the elevation effects on a relatively larger or global scale.

Impacts of extremely asymmetrical polar ice sheets on the East Asian summer monsoon during the MIS-13 interglacial

Marine Isotope Stage (MIS) 13 (∼500 ka ago) was a relatively weak interglacial according to the benthic δ18O and Antarctic ice core records. However, proxy records from the Northern Hemisphere indicate that MIS-13 was at least as warm as or even warmer than more recent interglacials, with an extremely strong summer monsoon and possible melting of the Greenland ice sheet. In this study, assuming an asymmetric hemispheric climate, we investigate the response of the East Asian summer monsoon to various Greenland and Antarctic ice sheet sizes using a set of sensitivity experiments with the HadCM3 model through factor separation analysis. Results show that with no Greenland ice sheet and a large Antarctic ice sheet there is an enhancement of summer monsoon precipitation in the East Asian monsoon region of up to ∼7%, in agreement with the direction of change from proxy reconstructions for China. This enhancement is associated with a stronger land–sea thermal contrast, more water vapor transport to East Asia and a northward shift of the Intertropical Convergence Zone. The response to the Greenland ice sheet removal is related to thermodynamic and topographical effects that influence the East Asian monsoon through an orographically induced wave train. The response to the larger Antarctic ice sheet involves stronger upwelling of circumpolar deep water in the Southern Ocean melts the sea ice, leading to a warmer SST. This warming propagates to the North, affecting the East Asian summer monsoon.

Seasonal variations in the surface energy and CO2 flux over a high‐rise, high‐population, residential urban area in the East Asian monsoon region

AbstractUsing the eddy covariance method, this study reports the one‐year turbulent fluxes of momentum, energy, and CO2, and their seasonal variations over a recently redeveloped high‐rise, high‐population, residential area in the metropolitan city of Seoul, Korea. The study area is affected by the Asian monsoon, which is accompanied by long rain spells and a related mid‐season depression of solar radiation in the summer. Our analysis shows that the urban surface energy balance and turbulence characteristics demonstrate typical urban properties. Unstable conditions dominate all day, and the storage heat flux (night‐time and morning) and sensible heat flux (afternoon) significantly affect the diurnal variations in the urban surface energy balance. Owing to the rough urban surface, the turbulence intensities are higher than those reported previously in other cities. The annual CO2 emission rate is approximately 13.1 kg CO2 m−2⋅year−1 with traffic, which is the major source of CO2 (+2.3 μmol⋅m−2⋅s−1 per 100 vehicles). Ecosystem respiration, including that by vegetation, soil, and humans, becomes dominant in the night‐time (00:00–05:00), thus contributing significantly to the annual CO2 budget. Further analysis indicates a unique coupling of urban surface energy partitioning and CO2 emission rates with the seasonal progression of the Asian monsoon: (a) surface albedo has annual minima in late summer when the sun elevation angle, is relatively higher and the urban surface condition is wetter than in other seasons; (b) the Bowen ratio ranges from 1.7 (summer) to 7.0 (winter); and (c) CO2 emission rates show seasonal variations with the progress of the summer monsoon.

Temporal and Spatial Distribution of Multi-scale Drought in North China

The North China is located in the East Asian monsoon region. The uneven spatial and temporal distribution of water resources leads to frequent drought disasters, and drought has increasingly become a limiting factor for regional development. In this paper, the intrinsic evolution characteristics of multi-scale spatial and temporal distribution of meteorological drought in the past 57 years in North China are analyzed by using standardized precipitation evapotranspiration index and ensemble empirical mode decomposition with multi-time scale features. The results show that the North China region was generally humid in the 1960s and 1970s. Drought in the 1980s and 1990s increased relatively. The interannual SPEI of North China changed in 1991, and entered a relatively dry period in the late 1990s and 2000s. From the SPEI index sequence, the scale characteristics of each scale were extracted. It was found that the SPEI sequence of the three-month time scale in North China mainly existed in the three scale ranges of monthly, interannual and intergenerational, At the same time, through the comparison of the temporal and spatial distribution characteristics of the summer drought in june and august 2002 in North China, it was found that the severe drought in august 2002 was caused by the temporal and spatial overlap of short-term scale and long-term drought events.

Squall Line and Its Vertical Motion Under Different Moisture Profiles in Eastern China

The impacts of different moisture profiles on the structure and vertical motion of squall lines were investigated by conducting a set of numerical simulations. The base state was determined by an observational sounding, with high precipitable water representing moist environmental conditions in the East Asian monsoon region. To reveal the impact of moisture at different levels, the moisture content at the middle and low levels were changed in the numerical simulations. The numerical results showed that more convective cells developed and covered a larger area in the high moisture experiments, which was characteristic of the convection during the Meiyu season in China. In addition, high moisture content at low levels favored the development of updrafts and triggered convection of greater intensity. This was demonstrated by the thermodynamic parameters, including Convective Available Potential Energy (CAPE), Lifted Index (LI), Lift Condensation Level (LCL), and Level of Free Convection (LFC). Dry air at middle levels led to strong downdrafts in the environment and updrafts in clouds. This could be because dry air at middle levels favors the release of latent heat, thereby promoting updrafts in clouds and downdrafts in the environment. Therefore, high relative humidity (RH) at low levels and low RH at middle levels favors updrafts in the cloud cores. Additionally, moist air at low levels and dry air at middle levels promotes the development of convective cells and the intensification of cold pool. The squall line can be organized by the outflow boundary induced by cold pool. The balance of cold pool and environmental wind shear is favorable for the maintenance and strengthening of squall lines.