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The East Asian winter monsoon (EAWM) is one of the most important climate systems during boreal winter. It has a large meridional extent and provides a vital bridge between the mid‐to‐high latitudes and the tropics. This paper reviews the latest research on the climate variability of the EAWM, with particular emphasis on the associated processes of extratropical–tropical interaction. Some new results have been achieved in understanding the variation of the EAWM on interdecadal, interannual, and intraseasonal time scales, such as the interdecadal strengthening in the early 21st century, the unstable impact of El Niño–South Oscillation (ENSO) on the EAWM, and the intraseasonal strong cold events in East Asia. In addition, understanding of the extratropical–tropical interactions related to the EAWM has improved in several aspects, including the impacts of the EAWM on ENSO‐related climate effects, and the variation of regional Hadley circulation over the western Pacific and its climate impacts in the Asia–Australia region. Moreover, recent work has systematically evaluated the capacity of climate models to simulate the EAWM, including its climatology, interannual to interdecadal variations, and its relationship with ENSO. Finally, some scientific issues regarding our understanding of the EAWM are proposed for future investigation. [ABSTRACT FROM AUTHOR]

Monsoon onset over the South China Sea occurs in April–May, marking the start of the wet season over East Asia. Skillful prediction of onset timing remains an open challenge. Recently, theoretical studies using idealized models have revealed feedbacks at work during the seasonal transitions of the Hadley cells and have shown that these are relevant to monsoon onset over Asia. Here, I hypothesize that monsoon onset occurs earlier in years when the atmosphere over the South China Sea is already in a state where these feedbacks are more easily triggered. I find that local anomalies in lower‐level moist static energy in the preceding January–March are well correlated with South China Sea Monsoon onset timing. This relationship remains relatively consistent on decadal timescales, while correlations with other teleconnections vary, and is used to develop a simple forecast model for onset timing that shows skill competitive with that of more complex models. Plain Language Summary: Arable land in China is estimated to feed 20% of the world's population on 7% of the world's farmlands, with much of this region watered by rainfall from the East Asian Summer Monsoon. Forecasting the arrival of the monsoon, which occurs first over the South China Sea in April–May, is therefore helpful for agricultural planning. However, producing a reliable forecast remains an open challenge. Recently, simple climate models, for example, models including seasons but with no land, have been used along with observations to investigate the most basic processes controlling climatological monsoon onset over Asia. In this paper, I suggest that year‐to‐year variations in monsoon onset timing are controlled by the same processes and, based on these, I suggest climate variables that may help in predicting when the monsoon will begin. By using this approach, I find that the "Moist Static Energy" (a quantity that combines temperature and humidity) over the South China Sea in January–March is strongly correlated with monsoon onset timing. This insight can be used to predict future onset timing competitively compared with more complex forecast models. Key Points: Recent theoretical studies have revealed feedbacks at work during monsoon onsetIf the spring atmosphere is in a state where these feedbacks are easily triggered, onset is earlierThis insight is used to produce a simple forecast of South China Sea Monsoon onset timing [ABSTRACT FROM AUTHOR]

Obvious biases in simulating tropical cyclone (TC) genesis of the current climate models hamper our understanding of TC changes. In this study, we found a delay of the seasonal cycle of TC genesis frequency over the western North Pacific (WNP) in most Coupled Model Intercomparison Project Phase 6 models. During the active TC season, the simulated south‐warming and north‐cooling surface temperature bias amplifies the meridional gradient and excites thermal winds. This weakens the western North Pacific Subtropical High and easterly monsoon trough, which further reduces TC genesis frequency over the western WNP in summer. But in autumn, positive TC genesis biases were only observed in coupled models over the eastern WNP. Both seasons contribute to the delayed seasonal cycle of TC frequency in models. Our findings highlight the importance of accurate simulation of surface temperature by climate models to TC simulations and aid in future model improvements. Plain Language Summary: Tropical cyclone (TC) is a devastating weather system generated over the tropical ocean, and the climate model serves as an important tool to study the long‐term variability of TC activity. Hence, it is imperative to explore the biases of climate models in simulating TC genesis. In this investigation, we find a substantial influence of surface temperature bias in Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models on the simulation of the seasonal cycle of TC genesis frequency over the western North Pacific (WNP). The delay in the seasonal cycle of TC genesis frequency is widespread among most CMIP6 models. During boreal summer and autumn, the simulated bias of south‐warming and north‐cooling surface temperature increases its meridional gradient and triggers thermal winds. In summer, the weak western North Pacific Subtropical High and easterly monsoon trough, two crucial large‐scale circulation systems, cause a large negative TC genesis bias in models over the western WNP, substantially contributing to the delay of the seasonal cycle. Meanwhile, the relatively small positive bias of coupled models over the eastern WNP in autumn makes a secondary contribution. The combined contributions of these biases cause delays in seasonal simulation of TC genesis frequency. Key Points: Coupled Model Intercomparison Project Phase 6 models produce delayed seasonal cycles of tropical cyclone genesis frequency over the western North PacificBiases in East Asia summer monsoon and western North Pacific subtropical high lead to underestimation of tropical cyclone genesis during summerThe biased large‐scale circulation systems are further related to a warm southern and cold northern surface temperature bias [ABSTRACT FROM AUTHOR]

Based on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current. Plain Language Summary: As an important component of the Asian monsoon system, the Southeast Asian summer monsoon (SEASM) is crucial for the livelihoods of billions of people in East Asia, and is closely connected to a climate phenomenon called El Niño‐Southern Oscillation (ENSO). Understanding how the relationship between SEASM and ENSO will change in the future is important for enhancing our knowledge of climate change in East Asia. Outputs of 28 climate models from the Phase 6 of the Coupled Model Intercomparison Project show that ENSO will exert enhanced impacts on the SEASM in a warmer climate. The enhanced influences of ENSO on the monsoon will exacerbate the reduction of rainfall over the western North Pacific during the post‐El Niño summer. We find that such projected changes are mainly caused by weakened warm sea surface temperature (SST) anomalies (SSTAs) in the western equatorial Pacific (WEP). Further analyses indicate that the change in WEP SSTAs can be linked to the El Niño‐like change in climatological SSTs in the tropical Pacific. This study depicts detailed physical processes responsible for the projected changes in ENSO's impacts on the SEASM. Key Points: The effects of the El Niño‐Southern Oscillation on the Southeast Asian summer monsoon will strengthen under global warmingThe enhanced El Niño's impacts result from the weakened warm sea‐surface temperature (SST) anomalies in the western equatorial Pacific (WEP)The weakened WEP SST anomalies are related to the eastward shift of anomalous Walker circulation and the slackened mean zonal ocean currents [ABSTRACT FROM AUTHOR]

The variability of the summer North Pacific Subtropical High (NPSH) has substantial socioeconomic impacts. However, state‐of‐the‐art climate models significantly disagree on the response of the NPSH to anthropogenic warming. Inter‐model spread in NPSH projections originates from models' inconsistency in simulating tropical precipitation changes. This inconsistency in precipitation changes is partly due to inter‐model spread in tropical sea surface temperature (SST) changes, but it can also occur independently of uncertainty in SST changes. Here, we show that both types of precipitation uncertainty influence the NPSH via the Matsuno‐Gill wave response, but their relative impact varies by region. Through the modulation of low cloud fraction, inter‐model spread of the NPSH can have a further impact on extra‐tropical land surface temperature. The teleconnection between tropical precipitation and the NPSH is examined through a series of numerical experiments. Plain Language Summary: The North Pacific Subtropical High (NPSH) is a semi‐permanent high‐pressure system located in the subtropical North Pacific. The variability in the summer NPSH has a significant impact on the monsoon and typhoons over East Asia and the hydroclimate of California. However, future projections of the NPSH using state‐of‐the‐art climate models remain highly uncertain. By evaluating how much individual models deviate from the multi‐model mean at different locations, we find four hot spots of high uncertainty in NPSH projections. Our analysis further reveals that the primary source of model variance in changes in the NPSH is tropical precipitation, which can be attributed to both inter‐model SST‐driven and non‐inter‐model SST‐driven factors. Through numerical experiments, we demonstrate that the teleconnection between tropical precipitation and the NPSH is achieved through wave propagation. Key Points: Model spread in the response of the summer North Pacific Subtropical High (NPSH) to CO2 stems from model spread in simulating tropical processesModel spread in tropical sea surface temperature (SST) changes modulates the NPSH by influencing tropical precipitationModel spread in tropical precipitation changes independent of model spread in SST changes also adds to the uncertainty of the NPSH response [ABSTRACT FROM AUTHOR]

Five regional climate models (RCMs)—CCLM, RegCM, HadGEM3‐RA, SNURCM and WRF—participating in the Coordinated Regional Climate Downscaling Experiment‐East Asia (CORDEX‐EA) Phase 2 project are evaluated for their ability to simulate spatiotemporal variability in Asian summer precipitation. For this purpose, two dynamical downscaling sets, experiments forced by ERA‐Interim reanalysis data (reproduction experiment) and historical data from three Coupled Model Intercomparison Project 5 (CMIP5) general circulation models (GCMs) (historical experiment) are analysed. The horizontal resolution of the downscaled results is 25 km, and the analysis period is from 1981 to 2005 (25 years). The RCMs show reasonable performance in simulating the spatial and temporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain. To assess spatiotemporal patterns in Asian precipitation, cyclostationary empirical orthogonal function (CSEOF) analysis is used. In the first mode representing seasonal variations, the model ensemble results of both the reproduction experiment (Rep_ENS) and the historical experiment (His_ENS) simulate the peak times, location of precipitation and progression of the monsoons in Asia reasonably. Their features are greatly influenced by the moisture flux, indicating that the relation between precipitation and 850 hPa moisture flux is depicted well by RCM ensembles. In the second mode, which represents intraseasonal variations, the Rep_ENS depicts the western North Pacific summer monsoon break (WNPSM break) phenomenon similar to the observation. Although the His_ENS simulates the WNPSM break later than the observation, it does present the intraseasonal variation in the East Asian summer monsoon region. Based on these results, the five RCM ensemble results show the ability to capture spatiotemporal characteristics of summer precipitation in CORDEX‐EA Phase 2 domain, as well as added value from dynamical downscaling. [ABSTRACT FROM AUTHOR]

Strong East Asian winter monsoon (EAWM) can enhance the Maritime Continent rainfall (MCR) via frequent southward penetration of cold air along the coast of East Asia. However, such impact diminished after the late‐1990s, primarily due to the weakened covariation between the Siberian high and Aleutian low (AL). During 1979–1998, the co‐variability between the Siberian high and AL facilitates a strong east‐west pressure gradient over East Asia. The resultant northerlies invade southward and facilitate the enhanced MCR. In contrast, such covariation is weak during 2000–2018, and it impedes the southward invasion of northerly winds and diminishes EAWM's impact. Further inspections indicate that the weakened covariation is primarily caused by the reduced variability of the AL. The underlying mechanism is confirmed by the historical simulation of version 6 Model for Interdisciplinary Research on Climate (MIROC6) that participates in phase 6 of the Coupled Model Intercomparison Project and numerical experiments with European Center‐Hamburg atmospheric model version 6.3 (ECHAM6). Plain Language Summary: East Asian winter monsoon (EAWM) is an important extratropical system to drive the variations of the Maritime Continent rainfall (MCR) via frequent southward penetration of cold air along the coast of East Asia. However, the EAWM's impact becomes diminished after the late‐1990s, and it is primarily rooted in the weakened covariation between the Siberian high (SH) and Aleutian low (AL). During 1979–1998, the co‐variability between the SH and AL causes a strong east‐west pressure gradient over East Asia. The resultant northerly winds invade southward along the coast of East Asia and facilitate the enhanced MCR. In contrast, during 2000–2018, the covariation is weak and diminishes EAWM's impact. Further analysis indicates that it is the weakened AL variability that determines the weakening of the covariation between the SH and AL. The phenomenon and the underlying mechanism are confirmed by the climate models. This study highlights the interdecadal variations of the EAWM in linking the Asian winter climate. Key Points: The impact of the East Asian winter monsoon on the Maritime Continent rainfall diminished after the late‐1990sIt arises from the weakened Siberian high–Aleutian low covariation that weakens northerly winds reaching the tropicsThe phenomenon and mechanisms are reproduced and validated by numerical experiments with two climate models [ABSTRACT FROM AUTHOR]

The extended‐range predictability of three simulated extreme cold events in East Asia in the Community Atmosphere Model (version 4) control experiment, whose atmospheric circulation backgrounds are similar to two recent observational cases, is investigated. The results show that they have a predictability of four pentads. Then, we evaluate the extent of the forecast uncertainty in the 4th pentad caused by the initial atmospheric uncertainties in the Arctic, which are large due to sparse instrumental observations. The initial uncertainty leading to the largest forecast uncertainty is obtained by the conditional nonlinear optimal perturbation method, and referred to as the CNOP‐type initial uncertainty. The forecast of the cold surge in the 4th pentad fails after adding the CNOP‐type initial uncertainty at day 0. In comparison, the forecast uncertainties are much weaker when the initial conditions are perturbed by noises, and the weaker influence may be due to the noises' lack of spatial structure. In terms of how the CNOP‐type initial uncertainty develops, a baroclinic structure is seen in the uncertainties on the first 2 days, followed by a propagating Rossby wave feature from the 2nd pentad to the 4th pentad. Meanwhile, synoptic transient eddy feedback also plays an essential role. The results suggest that the CNOP‐type initial uncertainty has potential to identify sensitive areas for targeted observations; plus, it could serve as a member of ensemble initial perturbations, since it indicates the largest uncertainty growth. Plain Language Summary: Although the notable global warming trend continues, extreme cold events in East Asia remain a common feature during the winter season and cause huge socio‐economic impacts. The atmospheric initial conditions are crucial for the prediction over East Asia. However, they have large uncertainties in the Arctic due to the sparse coverage of conventional data. Utilizing the conditional nonlinear optimal perturbation method, what extent the initial uncertainty in the Arctic can influence the prediction of cold surges is investigated. The results demonstrate that initial Arctic uncertainty can lead to the prediction of cold events in fourth pentad failure. Such initial uncertainty may provide potential application in the identification of sensitive areas for targeted observations. Key Points: The predictability of three simulated extreme cold events in East Asia by CAM4 reaches four pentadsThe conditional nonlinear optimal perturbation‐type initial uncertainty in the Arctic can lead to an extreme cold event appearing as a normal or warm event in the 4th pentadThe forecast uncertainty caused by the noise in the Arctic is weak in the 4th pentad [ABSTRACT FROM AUTHOR]

In this study, we developed a regional coupled model for the western North Pacific (WNP) and East Asian region. The atmospheric/oceanic component of the coupled model was the Weather Research and Forecasting and the Regional Ocean Modeling System, coupled through the National Center for Atmospheric Research Coupler version 7 (CPL7). An evaluation is provided through the simulation of the climatology and interannual variability of the summer monsoon climate from 1982 to 2015. The coupled model is also compared with a solely atmospheric run, which is driven by the observed sea surface temperature (SST) as the surface boundary. The results show that the coupled model has good capability in modeling the climate in the WNP and East Asian domain, including the climatology of summer precipitation and circulation, the subseasonal (month‐to‐month) variation in the western Pacific subtropical high and associated precipitation, and the seasonal cycle of the East Asian rain belt. One merit of this model is its ability to simulate the northward and eastward shift from early to late summer. With respect to the interannual variability, the model captures the spatial pattern of the leading mode of summer precipitation, with a north–south dipole mode in the studied region. The model also reproduces the larger SST variability in the northern part of this region. The temporal correlation coefficient between the coupled simulation and observation was 0.70 for the precipitation and 0.53 for the SST leading mode. The coupled model also captures the precipitation–SST relationship, which results from reasonably considering the air–sea coupling processes in the coupled run. Plain Language Summary: Due to the resolution of global models is relatively coarse and cannot describe the regional features, especially in the monsoon regions, regional climate model is an essential solution for regional climate simulation and projection. Compared with atmospheric alone model which is driven by prescribed sea surface temperature, regional air‐sea coupled model can describe reasonably the air‐sea coupling processes and has better performance. In this study, an regional coupled model is developed based on the Community Earth System Model coupler version 7 framework. Based on our analysis, the regional coupled model reasonably reproduced most key characteristics in the East Asian monsoon region. In addition to being used in regional climate simulation and projection, it can also be conveniently coupled with global models through the coupler 7. Thus with this useful tool in future we can develop global‐regional two‐way coupling simulation system for climate predictions and projections. Key Points: A regional coupled model is developed for East Asia and western North Pacific region based on Community Earth System Model frameworkClimatology and interannual variability of the summer monsoon climate have been evaluatedThe model can well capture the climatological northward and eastward movements of the western Pacific subtropical high from June to August [ABSTRACT FROM AUTHOR]

The impact of autumn Arctic sea ice reduction on the winter climate in Eurasia has been widely studied, but the role of the atmospheric circulation of the preceding summer has been mostly neglected. This study investigates a linkage between the summer Arctic circulation anomaly and the ensuing East Asian Winter Monsoon (EAWM) variability. The Arctic circulation anomaly, characterized by an anomalous anti‐cyclone occupying the Arctic Ocean and its marginal seas, can strongly melt Arctic sea ice in the Kara Sea‐Laptev Sea‐East Siberian Sea‐Chukchi Sea‐Beaufort Sea through dynamic and thermodynamic processes. The combination of the summer anti‐cyclone (cyclone) anomaly and light (heavy) autumn sea ice leads to an enhanced (weakened) EAWM. By introducing the coupling relationship between summer circulation and autumn sea ice into numerical experiments, the Community Atmospheric Model version 4 model has a good capability for capturing EAWM variability. Nevertheless, the dominant features of EAWM variability cannot be reproduced when the autumn sea ice mismatches the preceding summer circulation in the model. Therefore, the preceding summer circulation may modulate the impact of sea ice on Eurasia to some extent, which may increase uncertainties in the impact of Arctic sea ice loss on winter atmospheric circulation variability. Plain Language Summary: The enhanced East Asian Winter Monsoon (EAWM) leads to cold winter and has a significant impact on the economy, agriculture, and the lives of more than one billion people in East Asia, and the reduced autumn Arctic sea ice is generally believed to influence EAWM variability with great uncertainties due to strong atmospheric internal variability and impacts of other factors. In this study, through observational analysis and large ensemble model simulation experiments, a coupling relationship between the summer Arctic anti‐cyclone anomaly and the Arctic sea ice and their synergistic effect on EAWM variability are investigated. The summer anti‐cyclone anomaly over the Arctic corresponds to light sea ice and enhanced EAWM. Model experiments show that the summer initial conditions may modulate the lag impact of sea ice boundary conditions on winter circulation. Therefore, it is important to consider the synergistic effect of initial conditions and boundary conditions for predicting EAWM and winter surface temperature. Key Points: The Arctic anti‐cyclone (cyclone) anomaly in summer is a great predictor of the East Asian Winter Monsoon (EAWM)The combination of summer anti‐cyclone (cyclone) anomaly and light (heavy) autumn sea ice leads to an enhanced (weakened) EAWMFailure to consider the synergistic effect of initial conditions and sea ice may be one reason for the uncertainty of sea ice effect [ABSTRACT FROM AUTHOR]

We evaluate the performance of climate models from the Coupled Model Intercomparison Project phases 5 (CMIP5) and 6 (CMIP6) in simulating the weather regimes over East Asia during the period from 1961 to 2005. The circulation patterns, frequencies and transition probabilities of the weather regimes are obtained by categorizing the daily 500‐hPa geopotential height (Z500) anomalies using the k‐means clustering algorithm. We identify four winter (WWR1‐4) and three summer weather regimes (SWR1‐3) in both reanalysis datasets and CMIP5/6 models. Significant increasing (declining) frequency trends in WWR3 (WWR4) and SWR2 (SWR1) are found in the ERA5 and NCEP/NCAR reanalysis datasets. On the whole, both generations of models demonstrate reasonable abilities to reproduce the circulation patterns of weather regimes based on the Z500 anomaly fields but overall underestimate the intensities of weather regime patterns, underestimate the occurrence frequencies of WWR3, SWR2, and SWR3, and overestimate the frequencies of WWR1, WWR2, WWR4, and SWR1. In terms of both the ensemble‐mean and pairwise comparisons, the CMIP6 models generally perform better than their CMIP5 counterparts in reproducing the circulation pattern intensities, occurrence frequencies during the whole period and the linear frequency trends and transition probabilities of the weather regimes both in winter and summer. Further analysis shows a highly positive correlation between the total frequency and transition performances but a lower correlation between the biases in the total frequency and intensity of weather regime patterns. The improved skill of CMIP6 over the CMIP5 models in reproducing the weather regimes over East Asia may be partly related to the increased vertical atmospheric resolution in the model. [ABSTRACT FROM AUTHOR]

This study focused on the interdecadal variability in water vapour content (WVC) over East Asia during winter and its associated mechanisms. The interdecadal variation in the WVC over East Asia during winter was investigated using the empirical orthogonal function (EOF) method. The leading mode (EOF1, explained variance is 52.7%) of interdecadal variability in the WVC, which explains most of the interdecadal variation, exhibits a monopole pattern characterized by consistent WVC anomalies over East Asia. The mechanisms associated with the interdecadal variability in winter WVC over East Asia were further investigated, and it was found that the interdecadal variability in winter WVC over East Asia is modulated by three water vapour transport (WVT) branches from the eastern, western, and southern boundaries of East Asia that are associated with Eurasian winter circulation. Further analyses revealed that increased sea ice concentration over the Barents‐Kara Sea (SIC‐BKS) in October–November (ON) serves as the source of wave activity flux and stimulates a stationary wave train along the Eurasian continent to change the downstream atmospheric circulation. As a response, an abnormal cyclone, an abnormal anticyclone, and an abnormal cyclone are triggered over western Siberia, the northern area of East Asia, and the southern area of East Asia, which are together favourable for WVC sufficiency over East Asia. Sensitivity experiments by the Community Atmosphere Model, version 4 (CAM4) using sea ice forcing in the Barents Sea yield similar results to the observed data, further confirming our conclusion. Therefore, the SIC‐BKS in ON may be an important potential predictor for the interdecadal variability in winter WVC over East Asia. [ABSTRACT FROM AUTHOR]

Within the frameworks of the Coordinated Regional Downscaling Experiment‐East Asia first and second phase (CORDEX‐EA‐I and CORDEX‐EA‐II), the impact of spatial resolution, physical parameterization schemes and spectral nudging (SN) on simulating inland and offshore near‐surface wind speed over China is evaluated. By using WRF and RegCM4 models, the CORDEX‐EA simulations driven by the ERA‐Interim reanalysis data are conducted from 1989 to 2008. The regional climate models (RCMs) have the capacity to simulate the spatial patterns of surface wind, although significant overestimations exist over inland China. The WRF CORDEX‐EA‐II experiments with increasing horizontal resolution perform better in reproducing both inland and offshore wind speed than the CORDEX‐EA‐I experiments. Moreover, the experiments with SN can significantly reduce the positive biases of surface wind speed. For the temporal variation of wind speed, almost all the simulations show larger biases in winter than in summer. Particularly, the simulated wind speed in summer in the RegCM4 experiment with CLM3 land surface model (RCC‐II) is close to the observations. The extreme wind speed can be well reproduced by WRF simulations, and the SN experiments perform better than the experiments without SN (NOSN). In addition, the overestimations of surface wind speed can be partly explained by the overestimations of the wind field at 850 hPa level inherited from ERA‐Interim reanalysis and the increase of temperature gradient between the land and ocean in winter. [ABSTRACT FROM AUTHOR]

The model uncertainty, one of the major sources of projection uncertainty, is still a challenge in the climate change projection. In this study, the model uncertainty in simulating the East Asian summer monsoon rainfall and its physical link to the upper‐level jet streams and the western North Pacific subtropical high (WNPSH) are investigated from the Coupled Model Inter‐comparison Project Phase 6 (CMIP6) models. The results show that extracted by the inter‐model Empirical Orthogonal Function method, the leading inter‐model mode of the rainfall is characterized as a meridional pattern. The positive mode (a continent‐wet‐ocean‐dry pattern) is closely associated with a northward shift of the oceanic branch of the East Asian subtropical jet and a northward movement of the WNPSH, while the negative mode (a continent‐dry‐ocean‐wet pattern) is highly correlated with an enhanced East Asian polar‐front jet combined with a weakened land branch of the East Asian subtropical jet as well as a weakened and eastward recess of the WNPSH. This systematic link among the atmospheric circulations and the leading inter‐model mode is maintained by the negative phase of the Victoria Mode in the North Pacific and warming over the Atlantic Ocean in the positive mode, and the cooling over the Indian Ocean, Atlantic Ocean and the warming over the central Pacific in the negative mode. Our findings suggest that the physical linkage between the atmospheric circulations and monsoon rainfall is useful to understand the model uncertainty, which would be helpful in providing potential indicators of the East Asia summer monsoon change. Plain Language Summary: Monsoon rainfall has profound economic and social impacts, however large uncertainties lie in the climate model simulations. In this study, the inter‐model spread is used to extract the model uncertainty for the East Asian summer monsoon rainfall. The leading inter‐model mode is a meridional dipole pattern, which is closely associated with the concurrent variations of jet streams and the intensity and meridional movement of the western North Pacific subtropical high (WNPSH), but with the asymmetric response. The associated jet stream and the WNPSH variations are sustained by the oceanic conditions over the Atlantic Ocean, Pacific Ocean, and Indian Ocean, and therefore results in the inter‐model spread of the monsoon rainfall. Our findings suggest a useful metric to understand the traces of model uncertainties, focused on the physical linkage between the atmospheric circulations and the monsoon rainfall. Key Points: The leading inter‐model spread in East Asian summer monsoon rainfall is a meridional pattern from the Coupled Model Inter‐comparison Project Phase 6 modelsThe associated jet streams and western North Pacific subtropical high (WNPSH) variations exhibit asymmetric responses to this inter‐model spreadThe uncertainties in the north Pacific, Atlantic, and Indian Ocean have physically linked the jet stream and WNPSH variations to this inter‐model spread [ABSTRACT FROM AUTHOR]

Under the Coordinated Regional Downscaling Experiment‐East Asia second phase (CORDEX‐EA‐II) framework, the performances of three regional climate models (RCMs), including Weather Research and Forecasting (WRF) model, RegCM4 and Consortium for Small‐Scale Modelling model in Climate Mode (CCLM), in simulating the climatic distribution, diurnal variation and extremes of precipitation during 1989–2009 are evaluated. The RCMs are driven by the ERA‐Interim reanalysis and the evaluation is based on both the gridded gauge‐based and the satellite‐derived precipitation data. In general, the RCMs can reproduce the spatial pattern of the mean daily precipitation, with the spatial correlations (SCORs) larger than 0.66, and CCLM has the lowest root‐mean‐square error over East Asia. For the diurnal cycle of precipitation, WRF outperforms the other two RCMs despite the peak time about 3 hr ahead in some areas. RegCM4 overestimates the amplitude of the diurnal curve, and CCLM has limited ability to simulate the diurnal variation over most areas. All the three RCMs can reproduce the spatial patterns of the extreme indices, with the SCORs larger than 0.57. WRF and CCLM perform the best for the frequency of wet days (R1mm) and moderate rain (R20mm), respectively. The intensity indices are overestimated over most land areas, with relatively higher SCORs in WRF and relatively lower biases in RegCM4. The consecutive wet days (CWD) can be well simulated by WRF and CCLM; however, RegCM4 overestimates it over most regions in East Asia. The overestimation of the extreme intensity and frequency indices may be related to the higher water vapour content and the weaker water vapour divergence in the lower layer, as well as the higher mid‐layer geopotential height, especially in CCLM. [ABSTRACT FROM AUTHOR]

Inconsistencies in the Eocene climates of East Asia have been revealed in both geological studies and simulations. Several earlier reconstructions showed an arid zonal band in mid‐latitude China, but others showed a humid climate in the same region. Moreover, previous Eocene modeling studies have demonstrated that climate models can simulate both scenarios in China. Therefore, it is essential to investigate the cause of this model spread. We conducted a series of experiments using Norwegian Earth System Model 1‐F and examined the impact of mountains in Southern China on the simulated Eocene climate. These mountains, including the Gangdese and Southeast Mountains, are located along the main path of water vapor transport to East Asia. Our results reveal that the Southeast Mountains play the dominant role in controlling the simulated precipitation in Eastern China during the Eocene. When the heights of the Southeast Mountains exceed ∼2,000 m, an arid zonal band appears in mid‐latitude China, whereas humid climates appear in Eastern China when the elevation of the Southeast Mountains is relatively low. Key Points: Southeast Mountains control simulated Eocene precipitation in Eastern ChinaWhen the Southeast Mountains are high, an arid zonal band appears in mid‐latitude ChinaThe early Eocene climate in East Asia is not monsoonal climate [ABSTRACT FROM AUTHOR]

The evolution of hydroclimate in East Asia during the Pliocene—a potential analog of the future greenhouse climate—remains highly uncertain. Here we reconstruct changes in the hydroclimate of Northern China during this epoch. Our results reveal previously undocumented severe drought conditions that persisted through the early Pliocene with a significantly greater magnitude than suggested by other records. Using a broad hierarchy of climate simulations, we find that reduction in the Pacific meridional sea surface temperature (SST) gradient has particularly strong impacts on precipitation over Northern China, which suggests its key role in maintaining drought conditions during the early Pliocene. The weaker Pacific meridional SST gradient causes the weakening of the East Asian summer monsoon precipitation over this region via a reduction of atmospheric northward moisture transport from the ocean, which enhances aridity inland. Our results highlight the fundamental control of the tropical ocean on the extra‐tropical hydrological cycle. Plain Language Summary: Understanding East Asian rainfall changes with global warming is an important issue of climate sciences. The Pliocene epoch provides a potential analog to the future greenhouse climate. Here, for the first time we reconstruct high‐resolution aridity changes over the Chinese Loess Plateau (CLP) in northern East Asia during the Pliocene. Our results indicate persistent drought conditions over CLP during the early Pliocene. Analyzing a broad range of climate model simulations, we find that the reduced meridional sea surface temperature gradient is a critical factor in the weakening of the East Asian summer monsoon over Northern China and in maintaining drought conditions during the early Pliocene relative to the late Pliocene. Our results expand the Asian inland paleoclimate records during the Pliocene, providing new information on past Asian monsoon precipitation variations during the warm Pliocene with potential implications for future hydroclimate changes under greenhouse warming in East Asia. Key Points: Severe drought conditions persisted throughout the early Pliocene epoch over Northern ChinaWe link these conditions to the reduced meridional temperature gradient in the Pacific OceanThe reduced Pacific meridional temperature gradient acts to weaken the summer East Asian monsoon precipitation over Northern China [ABSTRACT FROM AUTHOR]

It has been challenging to simulate the East Asian summer monsoon (EASM) using general circulation models. By evaluating the cloud layers unified by binormals (CLUBB) model and its revised version in the version‐5 Community Atmosphere Model, we find that EASM simulations benefit from improving the reproduction of low clouds over the Tibetan Plateau. When a cloud‐top radiative cooling scheme (RAD) is coupled with CLUBB, it significantly improves the resulting EASM rain belt and western Pacific subtropical high (WPSH) simulations compared to the default simulations without RAD; in these default simulations, the low‐level southwesterlies, WPSH ridge, and EASM rain belt are displaced northward. The moisture budget analyses indicate that the improvements in EASM simulations are mainly contributed to by the improved presentation of low‐level stationary eddy meridional flow convergence over East Asia; this convergence shifts northward during the default model runs. Because the RAD scheme enables the model to better represent the subgrid radiation–turbulence interaction, the model produces stronger turbulent fluxes and lower clouds but reduces incoming solar radiation over the Tibetan Plateau. It thus shifts the Tibetan High southward, ultimately resulting in an improved simulation of the low‐level southwesterlies. These improvements in CLUBB_RAD highlight the importance of improving the representation of low clouds when simulating EASM rainfall. [ABSTRACT FROM AUTHOR]

Meteorological to hydrological drought propagation has been widely studied to reflect the relationship between these drought categories and better understand drought mechanisms. However, global warming may alter the drought propagation features, which are not fully understood. This study aims to investigate changes in meteorological and hydrological drought conditions, especially their propagation features in 1.5–3.0°C warmer climates for 8,655 watersheds globally. First, the three‐month scale standardized precipitation index and the standardized runoff index are calculated based on the precipitation simulated by the 15 global climate models and the runoff simulated by the four hydrological models, respectively. Drought events are then identified using the run theory, followed by the calculation of drought propagation features (i.e., pooling, lag, and lengthening) for matched meteorological and hydrological droughts. As a result, both meteorological and hydrological drought conditions (i.e., duration and severity) would relieve in warmer climates due to increased precipitation for regions excluding Western North America, South America, the Mediterranean, Southern Africa, East Asia, and Australia. However, the drought conditions would be more severe during drought propagation from meteorological to hydrological droughts over most regions. During drought propagation, the worsening drought conditions over half of the regions would be more serious first and then relieved with the rising temperature. These results indicate that efforts to slow down global warming can suppress the deterioration of drought conditions in the propagation. Plain Language Summary: Droughts can be categorized into meteorological, hydrological, agricultural, and socioeconomic types from different impacted aspects. Investigating meteorological to hydrological drought propagation can reflect the relationship between these two drought categories to understand the drought mechanism better. However, it is not fully understood whether global warming would alter the drought propagation features or not. Therefore, we evaluated the change of meteorological and hydrological drought conditions, especially their propagation features in 1.5 to 3.0°C warmer climates for 8,655 watersheds around the world. Here we show that both meteorological and hydrological drought conditions would relieve in warmer climates due to the increase of precipitation for regions excluding Western North America, South America, the Mediterranean, Southern Africa, East Asia, and Australia. However, the drought conditions would be more severe during drought propagation over most regions in warmer climates. Our findings promote a better understanding of drought propagation changes under global warming. Key Points: Both meteorological and hydrological drought conditions are projected to relieve in warmer climates for around half of global regionsThe duration and severity developing from meteorological to hydrological droughts would be more severe over most regionsThe worsening duration and severity during the propagation over half of the regions would intensify first and then ease in warmer climates [ABSTRACT FROM AUTHOR]

Accurate subseasonal prediction of East Asian summer precipitation (EASP) requires a deep understanding of the origins of its intraseasonal variability (ISV). However, the relative contributions to the EASP ISV from local processes, tropical intraseasonal oscillation (ISO), and extratropical influence remain unknown. We conducted a set of numerical experiments with a grid nudging method to confirm the roles of external forcing from mid‐to‐high latitude wave trains and tropical ISO and quantify their relative contributions. The Community Earth System Model can reproduce realistic EASP ISV and associated preceding signals. The tropical and mid‐to‐high latitude forcings account for, respectively, 53% and 40% of the total variance for the two leading ISV modes of EASP. The tropical forcing contributes more significantly to the two leading modes (31% and 40%, respectively) compared to the extratropical forcing (24% and 15%). These external forcings mainly affect EASP regional averages rather than the values of local (grid) scale. Plain Language Summary: East Asian summer monsoon precipitation exhibits significant fluctuations on a 2–5‐week time scale beyond the synoptic scale. These fluctuations are called intraseasonal variability (ISV). Since East Asia is located in between the largest continent (Eurasia) and the largest ocean (the Pacific), its ISV is affected by the southeastward propagating high‐latitude disturbances (wave trains) and northwestward propagating tropical ISV. However, the relative roles of these outside influences have never been quantified. We applied a climate model that could reproduce realistic ISV and designed a suite of numerical experiments to quantitatively determine the contributions of these "external forcings" to the East Asian ISV. About one half of the total intraseasonal variance (a measure of ISV intensity) arises from the mid‐to‐high latitude and tropical forcings. Thus, accurate simulations of tropical ISV and mid‐to‐high latitude wave trains are critical for improving subseasonal prediction over East Asia. Key Points: Community Earth System Model performs well in simulating the leading intraseasonal variability (ISV) modes of East Asian summer precipitation (EASP) and their preceding tropical and mid‐to‐high latitude signalsTropical and mid‐to‐high latitude forcings together contribute half of the total variance of the two leading ISV modes of EASPExternal tropical and mid‐to‐high latitude forcings affect EASP ISV more apparently on a regional, rather than a local, spatial scale [ABSTRACT FROM AUTHOR]

To better understand the characteristics of simulated raindrop size distributions (DSDs) and ice microphysical processes for convective systems in the East Asia monsoon region, a typical Meiyu event is simulated with the Weather Research and Forecasting (WRF) model using three two‐moment bulk microphysics schemes in this study. The simulated microphysical characteristics are then evaluated using polarimetric radar observations and retrievals. Although the observed linear storm structures are well simulated in terms of their location, there are significant deviations regarding simulated polarimetric radar variables and DSD parameters when compared to the observations. Compared to radar retrievals, all the simulated low‐level raindrops are found to have lower number concentration and larger mean sizes. To investigate the sources of the simulated DSD biases, vertical distributions of radar reflectivity and specific ice hydrometers (snow, graupel), as well as profiles of liquid/ice water contents from radar retrievals and simulations are further compared. In addition, variations about the occurrence frequency and transfer rate for four categories of ice processes (deposition, aggregation, riming, and melting) among the three schemes are analyzed. Results indicate that the overprediction of snow and graupel from riming processes is likely to be responsible for the production of extremely large raindrops at low levels. Key Points: Simulated microphysical structures for a Meiyu convective system were evaluated using polarimetric radar observations for the first timeRemarkable raindrop size distribution biases exist in the microphysics schemes when simulating this Meiyu caseThe extremely large raindrops in simulations are attributed to the overprediction of specific ice particles at higher levels [ABSTRACT FROM AUTHOR]

The East Asian coastal mountains have been in place during the Late Cretaceous times, thus substantially influencing the Asian climate. So far, their altitude is uncertain. Here we investigate the influence of such mountains on Asian climate using an atmosphere‐ocean general circulation model, Community Earth System Model version 1.2.2. Simulation results show that extensive deserts would develop over the eastern part of the East Asia if the altitude of the coastal mountains was greater than 2 km. This is due to the pumping effect of the coastal mountains which deprives the moisture from the East Asian interior during summer and autumn, leading to less precipitation and greater potential evapotranspiration. The existence of extensive desert areas would be more consistent with the presented Asian paleoenvironmental reconstructions. Therefore, our results independently indicate that the altitude of the coastal mountains had attained 2 km or more by the early Late Cretaceous. Key Points: The East Asian coastal mountains had risen to 2 km or higher during the early Late CretaceousThe East Asian coastal mountains had a drying effect on the Asian climate during the Late CretaceousThe East Asian coastal mountains decreased the Asian precipitation by means of their pumping effect [ABSTRACT FROM AUTHOR]

This study evaluates the precipitation over East Asia simulated by the variable resolution CAM‐MPAS featuring the Model for Prediction Across Scales (MPAS) atmospheric dynamical core coupled with the physics parameterizations of the Community Atmosphere Model (CAM) version 5.4. Two CAM‐MPAS experiments, one with a global quasi‐uniform resolution mesh of 120 km (MPAS‐UR) and the other with a variable resolution mesh of 30–120 km refined over East Asia (MPAS‐VR), are conducted from 1989 to 2005 following the Atmospheric Model Intercomparison Project protocol. Two regional climate model simulations at ∼25 km resolution from the Coordinated Regional Downscaling Experiment East Asia second plan are also analyzed for comparison. Results show that CAM‐MPAS performs better than the selected regional models in simulating the precipitation climatology over East Asia. Compared with MPAS‐UR, MPAS‐VR with refinement over East Asia better simulates the precipitation over and around the Tibetan Plateau (TP), the upper‐level circulation, the precipitation frequency and intensity in various regions of China, and the interannual precipitation pattern associated with the East Asian summer monsoon. Both decreased grid spacing and the use of finer‐scale terrain information contribute to the improvements in MPAS‐VR relative to MPAS‐UR. However, decreased grid spacing is more essential for simulating the precipitation magnitude over TP and the location of peak precipitation south of TP, likely due to the better resolved physical and dynamical processes associated with orographic precipitation. The 30–120 km CAM‐MPAS model is shown to be a promising tool for precipitation simulation over East Asia. Plain Language Summary: Precipitation and associated cloud and radiation processes can affect the energy budgets and water supply over land and thus the livelihoods of billions of people over East Asia. However, traditional low‐resolution global climate models still have difficulty in realistically simulating the mean‐state and variability of precipitation over East Asia, partly due to their insufficient resolution to resolve the complex terrain and associated synoptic‐scale systems. In this work, we perform model experiments using the Community Atmosphere Model coupled with the atmospheric dynamical core from the Model for Prediction Across Scales with a variable resolution mesh to assess the impact of regional refinement on the precipitation simulation over East Asia. Experiment with higher resolution (30 vs. 120 km) over East Asia better simulates the total precipitation amount over and around the Tibetan Plateau (TP), as well as precipitation frequency, intensity, and interannual variability associated with the East Asian summer monsoon. The finer grid spacing is found to contribute more to the improvements in areas adjacent to TP than the use of finer terrain information, likely due to better resolved physical and dynamical processes associated with orographic precipitation. Key Points: The precipitation climatology over East Asia can be reasonably reproduced by a global climate model CAM‐MPASRegional refinement over East Asia improves the precipitation simulation over East Asia in various aspectsFiner grid spacing is more essential for precipitation simulation around large‐scale terrain than using finer‐scale terrain information [ABSTRACT FROM AUTHOR]

A global model formulation for unified weather‐climate forecast is evaluated, with emphasis on the climate simulations at typical hydrostatic resolutions. The internal sensitivity is explored by considering different dynamical configurations (resolution, solver type, transport scheme). After a basic assessment of the global mean climate, a detailed analysis of precipitation characteristics is extended to East Asia. The model shows a reasonable mean state, seasonal variation, frequency–intensity structure, and diurnal phase time. The artificial rainfall around the steep slopes of the Tibetan Plateau can be improved through choices in the dynamical configuration. The regional features characterized by "afternoon versus nocturnal‐to‐early‐morning peaks" are properly distinguished. The hourly climatic features are comparable to super‐parameterized CAM5. Different dynamical configurations demonstrate unique sensitivities related to underling physical mechanisms, which are studied from the perspective of the diurnal cycle for three representative regions. Over South China, the higher‐resolution models decrease the weak‐precipitation while increase intense rainfall, thus reducing the dry biases. This is contributed by enhanced grid and sub‐grid scale motions associated with daytime convection progression. Over central western China, the variable‐resolution model better simulates the eastward propagating episodes characterized by a transition from convective to stratiform rainfall along the eastern slope of the Plateau. This reduces the positive biases at the high topography of the Plateau and alleviates the negative biases at the lower foot. Over central eastern China, the model replicates the dominant role of large‐scale governing factors in regulating the early morning rainfall peaks, and produces stratiform heating patterns. Plain Language Summary: We examine the statistical simulation of a model developed within the Global‐to‐Regional Integrated forecast SysTem, using Atmospheric Model Intercomparison Project style integration. This is among an effort in exploring a unified system for weather and climate modeling. Because of the evolutionary nature of model development, the absolute model performance can change. Thus, special attention is paid to the model sensitivity related to dynamical configurations, which helps to gain understanding that can support continuous research and development. Precipitation is one of the most important variables in a general circulation model as it encapsulates the synergistic effects of resolvable and non‐resolvable processes, the hydrological cycle, and radiative balance. Simulating precipitation faces the challenges in representing the moist physics and its interaction with dynamics. We use precipitation as performance metrics for a better understanding of the model behavior, focusing on a unique region–East Asia. The diurnal cycle is given particular emphasis because it reveals the direct physical mechanism of precipitation, and can help to disentangle the interactions between model dynamics and physics. We show that the models produce hourly rainfall features that are comparable to a global multiscale modeling framework that has two interactive dynamical solvers, and support some beneficial sensitivity. Key Points: To evaluate a model formulation for unified weather‐climate forecast using the Atmospheric Model Intercomparison Project‐style experimentTo understand model precipitation characteristics over East Asia by analyzing hourly scale apparent sources/sinks and grid‐scale tendenciesTo understand model sensitivity related to dynamical configurations, including grid resolution, moisture transport scheme and solver type [ABSTRACT FROM AUTHOR]

The dust emission parameterization in the Chinese Academy of Sciences Earth System Model version 2 (CAS‐ESM2) is described with emphasis on the implementation process and simulations of global dust cycle and East Asia dust event statistics. The parameterization is based on the scheme of Shao (2004; http://doi.org/10.1029/2003JD004372) which considers two major dust emission mechanisms, namely, saltation bombardment and aggregation disintegration. Shao (2004; http://doi.org/10.1029/2003JD004372) scheme was well tested against field observations and has been widely used in regional dust modeling. Here this scheme is implemented into a global climate model for the first time and thus provides an independent solution for simulating global dust emissions. With this scheme, CAS‐ESM2 reasonably simulates the main dust emission regions in the Earth and reproduces the observations of dust deposition flux and surface dust concentrations at most stations. Compared to the synoptic records of dust events, the model also captures the general patterns and seasonal variations of dust activities in East Asia. However, the model underestimates the frequency of strong dust events (instantaneous surface dust concentrations >1,000 μg m−3) due to the weaker surface winds simulated by the model. The model tends to simulate much weaker and longer‐lasting dust events in Eastern Sources (35–49°N, 94–126.5°E) of northern China, suggesting the weaker and slower‐moving synoptic weather systems associated with dust events in the model. Overall CAS‐ESM2 performs well in simulating the key aspects of global dust distribution and East Asian dust events, yet some biases remain to be improved. Plain Language Summary: Dust emitted from the bare soil is an important aerosol type, and dust cycle links the various components of Earth System including atmosphere, land, and oceans. Therefore, dust emission is an essential part of Earth system. In this study, we develop a dust emission module for Chinese Academy of Sciences Earth System Model version 2 (CAS‐ESM2) to simulate the global dust emission. The module incorporates advanced mechanisms of dust emission physics. The results show that with this module, CAS‐ESM2 reproduces well the global distribution of dust aerosol. We also evaluate the model's ability in simulating the individual dust events in terms of dust event frequency, intensity, and duration. We find that the model can reproduce the key regions of dust emission and the highest frequency of dust event in spring in East Asia. However, because the model underestimates surface wind speeds, it simulates weaker and longer‐lasting dust events. Overall CAS‐ESM2 provides a new and promising solution to simulate global dust emission, although there are still some biases for further improvements. Key Points: Chinese Academy of Sciences Earth System Model version 2 (CAS‐ESM2) with a physically based dust emission scheme provides a new and promising solution to simulate global dust emissionsCAS‐ESM2 captures the main global dust emission regions and reproduces observed dust depositions and surface dust concentrationsThe weaker dust events are compensated by overestimated dust event frequency and duration to produce reasonable means in East Asia [ABSTRACT FROM AUTHOR]

The Kamchatka Peninsula (Russian Far East) remains among the least studied regions of eastern Asia. Recent studies revealed a high degree of palaeoenvironmental variability between different parts of the peninsula. We investigated semi‐aquatic (chironomids) and terrestrial (leaf wax biomarkers) proxies from a sediment core collected from Lake Sokoch (southern Kamchatka) to provide reconstruction of the mean July air temperature and variations in limnic conditions. The lake formed after 10.0 cal. ka BP as a result of postglacial warming and was fed by glacial meltwaters from neighbouring glaciers. Our data show a later beginning of the Holocene thermal maximum (HTM) relative to more northern sites in Kamchatka, Siberia and Chukotka and support climate model experiments that suggest that the HTM was delayed in southern and central Kamchatka by about 2000 years compared with Alaska and NE Siberia. Warm conditions prevailed between 10.0 and 6.4 cal. ka BP with a short spell of cool and dry climate around 8.2 cal. ka BP that might be related to the 8.2 ka cooling event. The HTM took place between 6.5 and 3.4 cal. ka BP with the warmest phase from 6.0 to 5.0 cal. ka BP. An onset of Neoglacial cooling at 3.4 cal. ka BP is consistent with the strengthening of both the Siberian High and the Aleutian Low. Warming between 1.2 and 0.9 cal. ka BP can be attributed to the Mediaeval Climate Anomaly. The LIA cooling is related to another strengthening of the Siberian High and the Aleutian Low. The modern warming, though weakly traced in our record, is consistent with the recent meteorological observations. The presented palaeoenvironment record confirms the earlier findings of spatial differences within Kamchatka in timing and magnitude of the major Holocene climate fluctuations and contributes towards understanding the expression of Holocene climate change in Kamchatka. [ABSTRACT FROM AUTHOR]

Based on 10 pairs of atmospheric general circulation models from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project, we find that high‐resolution models (HRMs, ∼50 km) perform better than low‐resolution models (LRMs, ∼100 km) in simulating the East Asian summer monsoon (EASM) rain belt. We attribute the increased rainfall to enhanced vertical advection of water vapor in the HRMs which is mainly contributed by enhanced vertical motion. Based on moist static energy analyses, the stronger vertical motion in the HRMSs is mainly balanced by enhanced net energy flux into the atmosphere column in the "Meiyu‐Baiu" (MB) region due to the increase of air‐sea turbulent latent heat fluxes along the Kuroshio and sensible heat fluxes over land. The increased net energy flux in the HRMs drives stronger ascending motion, resulting in more realistic rainfall in the MB region. Higher saturated specific humidity determined by finer sea surface temperature, less near‐surface specific humidity along the Kuroshio, and stronger near‐surface winds on western side of the western North Pacific Subtropical High strengthened by the enhanced pressure gradient together contribute to higher ventilation efficiencies and enhanced ocean forcing along the Kuroshio, leading to more turbulent latent heat fluxes in the HRMs. Additionally, drier and less cloudy atmosphere in the HRMs result in more downward shortwave radiation heating over the land, which partly contributes to the increased sensible heat flux. This understanding of the sensitivity of simulated MB to model resolution has implications for precipitation simulation and prediction over East Asia. Plain Language Summary: Improving the Meiyu‐Baiu simulation is a long‐term challenge for the climate‐modeling community. One way to achieve such improvement is to develop high‐resolution global climate models, since there are many mesoscale systems embedded in the large‐scale monsoon circulation that need small grid spacing to be resolved. By analyzing ten pairs of models, we show that the high‐resolution models (HRMs) generally show better performance in Meiyu‐Baiu simulations than the low‐resolution models (LRMs), with a reduction in bias of 31%. The increased air‐sea latent heat flux in the HRMs is a dominant energy source driving increased Meiyu‐Baiu rainfall. Stronger near‐surface winds and drier air in the HRMs favor increased evaporation, resulting in a more turbulent latent heat flux. Hence, we explain the processes that can help improve Meiyu‐Baiu simulation when improving the horizontal resolution from ∼100 to ∼50 km in General climate models. Key Points: High‐resolution models (∼50 km) perform better than low‐resolution models (∼100 km) in simulating Meiyu‐BaiuThis added value is mainly attributed to the enhanced air‐sea turbulent latent heat fluxes along KuroshioStronger near‐surface wind and drier air in high‐resolution model favor the more turbulent latent heat fluxes [ABSTRACT FROM AUTHOR]

In this work, we projected future drought risks over East Asia from the perspective of evapotranspiration (ET) deficit using the new climate projections of the Coupled Model Intercomparison Project Phase 6 (CMIP6). For the regional drought projections, we employed a state‐of‐the‐art complementary relationship (CR) method to estimate ET and atmospheric evaporative demand (Ep) that quantify the combined impact of water deficiency and vapor pressure deficit stress. The performance of the CR method was validated using the reanalysis climate data and independent water‐balance ET estimates. The same CR method was again applied to hindcasts and projections of seven global climate models (GCM) in the CMIP6 archive. Results showed that the CR ET estimates and the GCM‐driven ET products were both able to approximately close the basin‐scale water balance and tightly correlated with each other. The application with the CMIP6 GCM hindcasts provided the same indication that the CR ET and the GCM‐driven ET are consistent. The drought assessments using the ET deficit (ET − Ep) from the CMIP6 projections implicates that future drought risks over East Asia would be much higher than indicated by precipitation or soil moisture projections. Rising trends of Ep are expected to outpace those of ET even under the medium‐level greenhouse gas emission scenario. Our new drought projections suggest that Ep amplified by soil water deficiency could raise agricultural and wildfire risks substantially by the end of the 21st century. Plain Language Summary: In this work, drought risks over East Asia were assessed using new climate projections under a medium and a strong level of greenhouse gas emission scenarios. The future drought risks were quantified by comparing plants' water consumption against atmospheric water demand from 1980 to the end of the 21st century. The assessments showed that the atmospheric water demand would increasingly surpass the plant's water consumption in most of East Asia, even though the warming atmosphere would increase precipitation and thus the water consumption. The drying signal was indicated by responses of the lower atmosphere to soil moisture conditions with no synthesis of complex land‐surface processes. This study highlights that the rising atmospheric water demand would pose a stress independent of and in addition to soil water deficiency, and drive increasing agricultural and wildfire risks. Key Points: The complementary evaporation principle can produce reliable evapotranspiration (ET) estimates consistent with land surface modelsET deficits are expected to rise steadily through the 21st century across East Asia even under the medium‐level warming scenarioFuture drought risks assessed with ET deficits are far higher than indications from precipitation‐dependent metrics [ABSTRACT FROM AUTHOR]

Impact of the Atlantic multidecadal oscillation (AMO) on the interannual relationship between the El Niño‐Southern Oscillation (ENSO) and East Asian early summer monsoon (EAESM) is investigated using observations and community atmosphere model (CAM) outputs. We find that the interannual ENSO–EAESM relationship displays a prominent decadal variation during 1951–2018, which is consistent with the AMO phase change, that is, a high ENSO–EAESM correlation appears during the negative AMO phase, while no significant correlation is found during the positive AMO phase. Further analyses reveal that the AMO‐related sea surface temperature anomalies (SSTAs) in the western North Pacific, rather than in the North Atlantic, modulate the interannual relationship between the El Niño and EAESM. These SSTAs can prolong the existence of the anomalous western North Pacific anticyclone (WNPAC) from the ENSO mature winter to the subsequent early summer, thus affecting the ENSO–EAESM relationship for the El Niño events. In contrast, the AMO modulates the interannual relationship between the La Niña and EAESM through the North Atlantic and North Pacific. The North Atlantic warming‐induced anomalous anticyclone weakens the background cyclone over the western North Pacific during the positive AMO phase, and the mid‐latitude air–sea interaction in the North Pacific also influence the anomalous cyclone. The observed AMO's impact on the interannual relationship between the ENSO and EAESM is confirmed by atmosphere general circulation model simulations using the CAM. [ABSTRACT FROM AUTHOR]

Water management is a crucial issue in East Asia and is significantly influenced by climate change. Because East Asia's precipitation characteristics are varied and complex, it is necessary to implement the analysis of the future changes in precipitation on an objectively classified, specific sub‐regional level to effectively solve these problems. In this study, sub‐regions were objectively classified according to their precipitation characteristics, based on the present observations in East Asia. Additionally, the bias‐correction technique was applied to the regional climate model (RCM) to yield more reliable future precipitation predictions for the sub‐regions. We obtained the following four key results in this study. (1) The classified sub‐regions corresponded well to the observed annual precipitation distribution and adequately reflected the influence of the summer monsoon. The annual precipitation of the classified sub‐regions exhibited a decreasing trend in arid regions and an increasing trend in humid areas. (2) Because the bias‐corrected RCM was able to simulate annual precipitation reasonably, this technique was applied to future scenarios to analyse future changes in precipitation for each classified sub‐region. (3) Future annual precipitation will increase significantly in the region spanning from the western part of Southern China to the area around the Bohai Sea, while the temporal variations of the climatological daily precipitation indicated that sub‐regions under the influence of the East Asian summer monsoon (EASM) will be predicted an increase in second peaks during the summer. Hence, in the future, the EASM rain belt will be strengthened and its average location will move northward from its present location. (4) Because the high‐intensity extreme precipitation indices are expected to have a more considerable increase in all the classified sub‐regions, there is a need to solidify response measures against disasters that could arise from these changes in the East Asian region. [ABSTRACT FROM AUTHOR]

Based on hourly temperature data collected from 2,307 meteorological stations over China and two sets of climate change simulations over East Asia using the regional climate model version 4.4 (RegCM4.4) driven by the global models of HadGEM2‐ES and MPI‐ESM‐MR, future changes of heating and cooling degree‐hours (HDH and CDH) at the end of the 21st century under RCP4.5 scenario are investigated. Compared against the observation, the spatial distribution, magnitude and annual cycle of present‐day HDH and CDH at different times, as well as the diurnal cycle of them can be realistically simulated by the model. Meanwhile, some bias can also be found and it varies with the times, especially for CDH. At the end of the century, significant decrease of HDH can be found over the Tibetan Plateau, the northern part of Northeast China, and Northwest China at each times while increase of CDH are observed at all the times except some parts of the Tibetan Plateau where the CDH remains zero due to the cold climate there. But for different times, some differences are found in HDH decrease and CDH increase. For example, the changes of HDH at the fifth to the seventh times present a weaker decrease in the northwestern basins compared to other times and the CDH increase are more significant at the fifth and the sixth times. The regional mean changes of HDH and CDH over China in the 21st century (2006–2098) also diverge among different times, which indicates the degree‐hours is needed to precisely calculate the hourly heating and cooling energy demand and then the heating and cooling degree days. [ABSTRACT FROM AUTHOR]

The winter temperature in East Asia from 1958 to 2001 has shown an obvious warming trend superposed with a large amplitude of decadal oscillations. To separate the relative contributions of external forcing and internal variability to the decadal change in winter temperature, we propose a method which combines reanalysis data, global atmosphere‐ocean coupled model outputs, and regional climate model. The results show that the weakening of the East Asian winter monsoon accounts for warming trend in the south Mongolia, the southeastern coast of Asia, and the adjacent seas to a large extent. In contrast, radiative forcing causes the warming trend in middle and high latitudes. The increasing concentration of aerosols could have resulted in local cooling in some areas. The winter temperature shows a decadal oscillation of 0.6 °C at a time scale of 8–20 a, which is mainly due to the oscillation of the Siberian‐Mongolia High and volcanic activities. The amplitude of temperature oscillations caused by volcanic activities was one third of that induced by the Siberian‐Mongolia High. Key Points: The winter warming in East Asia during 1958–2001 was due to greenhouse gas emissions and the weakening of the East Asia winter monsoonRadiative‐forced warming had a uniform pattern, while the warming caused by weakening monsoon was located in the eastern coast of EurasiaVolcanic aerosols caused an interdecadal oscillation of 0.2 °C, which was one third of the strength caused by internal variability [ABSTRACT FROM AUTHOR]

Based on 34 coupled models from Phase 5 of the Coupled Model Intercomparison Project, we found that the projected changes in the western North Pacific anomalous anticyclone (WNPAC) during El Niño decaying summers show large spread among the models due to different decaying paces of El Niño. Models with a slower (faster) decaying El Niño are followed by weakened (intensified) tropical central Pacific cooling, which further drives a weaker (stronger) WNPAC through inducing weakened (intensified) descending Rossby waves to its west. The decaying pace of El Niño is dominated by the wintertime WNPAC because the easterly surface wind anomalies to its southern flank are critical to El Niño demise. Both the El Niño‐related sea surface temperature anomaly and the precipitation sensitivity to such kind of sea surface temperature anomaly under global warming could affect the wintertime WNPAC. Plain Language Summary: The western North Pacific anomalous anticyclone (WNPAC) is the key atmospheric circulation system that connects El Niño and East Asian climate variability. Understanding how the WNPAC will change in the future is of vital importance to people living in East Asia‐western Pacific region. There is extensive literature on the topic of changes in the WNPAC in response to global warming but how such kind of connection would change under global warming remains inconclusive. The causes of uncertainty in the WNPAC projection and the dominant mechanisms remain unknown. Based on 34 state‐of‐art climate coupled models, we show evidence that uncertainty in El Niño projection is a source of uncertainty in the WNPAC projection. Our results highlight the importance of a reliable projection of El Niño decaying pace to the projection of climate variability over the western Pacific and East Asia. Key Points: The projected changes in the WNPAC during El Niño decaying summers are uncertain among different climate modelsThe uncertainty of El Niño‐northwestern Pacific climate relationship projection is determined by the El Niño decaying pace under global warmingChanges in El Niño decaying pace depend on the strength of wintertime WNPAC [ABSTRACT FROM AUTHOR]

This study presents a comprehensive and quantitative evaluation of the mean state of summer atmospheric circulation over East Asia. Attention is paid to the South Asian high (SAH), western North Pacific subtropical high (WNPSH) and Indian low (IL) at the upper, middle and lower troposphere, respectively. A total of 31 state‐of‐the‐art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used as examples for the analysis. Most models can basically simulate the three closed‐circulation systems (CCSs), although there is a certain inter‐model spread and an underestimation of their intensity. In terms of geographic location, models demonstrated the best performance for SAH and the poorest performance for WNPSH. The latter shows a generally south‐westward shift compared to the National Centers for Environmental Prediction (NCEP) reanalysis. Five atmospheric fields (zonal and meridional wind at 850 hPa, geopotential height at 500 and 100 hPa and sea level pressure) are inspected and generally well reproduced in models, with Taylor‐S indices all larger than 0.84 for 90% of the models. The best performance is for the geopotential height of 500 hPa with an average Taylor‐S index of 0.98. Models' skill in simulating the sea level pressure is the lowest. However, a significant positive correlation with models' resolution is observed. Almost all models underestimate the 100 hPa geopotential height over East Asia, mainly due to the common cold bias in the troposphere. As a whole, CCSM4, CNRM‐CM5, CESM1‐CAM5 and NorESM1‐M are identified as high‐skill models for simulating the East Asian atmospheric circulation. High‐skill models also show better simulation of precipitations in East China, with a 21.3% decrease of dry biases in Southeast China. The physical explanation for this linkage is also reported to reside in the central position of the WNPSH, which determines the quality of 850 hPa winds and water vapour transport in Southeast China. [ABSTRACT FROM AUTHOR]

Speleothem δ18O is widely used to reconstruct past hydroclimate variability, particularly over Asia. However, the interpretation of this proxy is still in debate. While this proxy is originally interpreted as regional rainfall amount of the Asian monsoon, other studies have interpreted it as upstream monsoon rainfall or atmospheric circulation changes. To better understand the signal preserved in speleothems over various time scales, this study employs a state‐of‐the‐art isotope‐enabled climate model to quantify contributions to the oxygen isotope composition of precipitation (δ18OP) over China. Results suggest that orbital‐scale speleothem δ18O variations at Chinese sites mainly record the meridional migration of the Asian monsoon circulation, accompanied by an early northward movement of the East Asian rain belt. At interannual scales, Chinese speleothem δ18O is also tied to the intensity of monsoonal circulation, via a change in moisture source locations: Enhanced moisture delivery from remote source regions leads to more negative δ18OP, particularly in late summer and early autumn. Our results have implications for the hydroclimatic interpretation of speleothem δ18O from Chinese caves and suggest that this interpretation is time scale dependent. Key Points: At orbital scales, Chinese speleothem oxygen isotopes mainly represent the meridional migration of the East Asian monsoon circulationAt orbital scales, low values of Chinese speleothem oxygen isotopes are associated with early northward movement of the East Asian rainbeltAt interannual scales, Chinese speleothem oxygen isotope ratios are mainly controlled by the change of moisture source locations [ABSTRACT FROM AUTHOR]

The prediction skill and source of the predictability of the East Asian summer monsoon (EASM) system are examined in this work based on four state‐of‐the‐art seasonal climate forecast models including BCC_CSM1.1, ECMWF_SYS4, NCEP_CFS2 and TCC_CPS2. The prediction of the climatology and interannual EASM pattern and the impact on the prediction are further investigated. It is noted that the four models have some skill in predicting summer rainfall in the East Asia, however, the skill is low on average and also largely regional dependence. The interannual variation of EASM measured by monsoon circulation index is well reproduced, implying that the broad‐scale feature/pattern of EASM has higher predictability than the detailed spatial variation of EASM rainfall. The possible sources of predictability of the interannual variability of EASM are associated with the El Niño‐Southern Oscillation (ENSO) and the north Indian Ocean (NIO) sea surface temperature (SST) anomalies. The correlation pattern of rainfall with the NIO SST is characterized by a tripole pattern from south to north of East Asia, which is different from the correlation distribution of the southern‐northern dipole with ENSO, suggesting that NIO SST may exert influence on the EASM independently. The major biases in climatology of EASM in the models are the northward shift of the western Pacific subtropical high (WPSH) and weak monsoonal southerly over the coast of East Asia, which leads to the prediction bias of the Meiyu/Baiu/Changma (MBC) rainfall belt. The prediction of the interannual EASM pattern presents two deficiencies: too weak rainfall variability and northward shift of the dipole rainfall pattern (opposite variation between MBC and the northwestern Pacific), that may be caused by the biases of WPSH in the models. [ABSTRACT FROM AUTHOR]

Attributing the surface air temperature (SAT) trends at regional scale to internally generated and externally forced components is a major challenge. Based on the observations and ensemble simulations of climate models, we apply a "dynamical adjustment" methodology to estimate quantitatively the contribution of the internal and forced dynamic and thermodynamic components to the East Asian winter SAT trends during 1979–2018. The observed winter SAT trends are strongly influenced by internal variability, especially in the northern East Asia (NEA). The internal component of observed SAT trends is largely dynamically induced, whereas the forced component is controlled by thermodynamics. The internally generated variability offsets the forced warming by more than 70% in the NEA, leading to a weak observed warming in the NEA. In contrast, internal contributions are small in the southern East Asia. The internally generated SAT changes in the NEA are closely tied to the multidecadal changes of Arctic Oscillation. Key Points: The observed winter SAT trends are strongly influenced by internal variability, especially in the northern East Asia (NEA)The internally generated variability offsets the forced warming by more than 70% in the NEA, leading to a weak observed warming in the NEAThe internally generated SAT changes in the NEA are closely tied to the multidecadal changes of Arctic Oscillation [ABSTRACT FROM AUTHOR]

The response of the East Asian winter monsoon (EAWM) circulation to aerosols is studied using a coupled atmosphere‐slab ocean general circulation model. In the extratropics, the aerosol‐induced cooling in the midlatitudes leads to an intensified subtropical jet stream, a deepened East Asian trough, and thus an enhanced EAWM. In the tropics, the local Hadley circulation shifts southward to compensate for the interhemispheric asymmetry in aerosol radiative cooling. Anomalous subsidence at around 10°N leads to a salient anticyclone to the southwest of the Philippines. The associated southwesterlies advect abundant moisture to South China, resulting in local precipitation increases and suggesting a weaker EAWM. The EAWM response to aerosol forcing is thus driven by a competition between tropical and extratropical mechanisms, which has important implications for the future monsoon evolution, as aerosol changes may follow different regional‐dependent trajectories. Plain Language Summary: In this study, we use a global atmospheric model to examine how aerosols affect the East Asian winter monsoon intensity, which modulates the strength of cold air outbreaks in East Asia. The aerosol‐induced cooling mainly appears in the Northern Hemisphere midlatitude land and their downwind ocean regions. The cooling enhances the East Asian winter monsoon north of 30°N, with stronger northerly winds and colder winters there. However, aerosols weaken the East Asian winter monsoon south of 30°N, with intensified southerly winds bringing warm and moist air to South China, thus resulting in enhanced precipitation. The larger magnitude of the southerly south of 30°N than that of the northerly north of 30°N and increased precipitation over South China suggest a considerable influence of aerosols on East Asian winter monsoon through tropical circulation changes. Key Points: Anthropogenic aerosols enhance the East Asian winter monsoon circulation north of 30°N but weaken it south of 30°NThe precipitation increases over South China are primarily associated with an anomalous anticyclone to the southwest of the PhilippinesThe tropical circulation changes are related to a southward shift of the local Hadley circulation driven by asymmetrical aerosol forcing [ABSTRACT FROM AUTHOR]

Future change of the rainy season over East Asia region is investigated using the high horizontal resolution global atmospheric model called the Meteorological Research Institute-Atmospheric General Circulation Model (MRI-AGCM) version 3.2S (20-km grid) and 3.2H (60 km). Higher reproducibility of precipitation during East Asian rainy season gives reliability of future projections. For the present-day simulations (1983-2003, 21 years), the models are forced with observed historical sea surface temperature (SST). For the future simulations (1979-2099, 21 years), the models are forced with future SST projected by Atmosphere-Ocean General Circulation Models (AOGCMs) participated to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). In order to estimate the uncertainty of future change, we execute ensemble simulations with three kinds of cumulus convection schemes and four kinds of SST distributions using 60-km version to meet the limitation of computer resources. For each grid point, the onset and retreat of rainy season are defined from time evolution of pentad mean precipitation. The onset of rainy season becomes earlier in most regions over East Asia, but onset will delay in some area of western Japan. Retreat delays over China and Korea. Duration increases in most part of East Asia especially over China and Korea. This can be caused by enhanced water vapor transport from the tropics. The delay of onset over western Japan can be attributed to the decrease of water vapor transport associated with the southward shift of the subtropical high. Future change presented in higher horizontal resolution compared with previous studies is useful for impact, adaptation and mitigation studies of global warming. [ABSTRACT FROM AUTHOR]

ABSTRACT: The forecast skill of BCC‐CSM1.1 model in summer East Asian climate was evaluated with the decadal prediction experiments launched annually during 1961–2005. Results of the decadal prediction were compared with those of the historical simulation to estimate contribution of the initialization. Improved prediction skill is found for summer surface air temperature (SAT) in central eastern China measured by both root mean square error (RMSE) and anomaly correlation coefficient (ACC) for the forecast years 2–5. The decadal hindcast of precipitation in central eastern China shows increased skill in RMSE but not ACC. Such improvement in East Asia is related to enhanced prediction skill of the western Pacific (WP) sea surface temperature (SST) originated from the oceanic initialization. In the decadal hindcast, the lower tropospheric atmospheric circulation associated with warm WP exhibits an anticyclone over the South China Sea, which resembles the observation. The southwesterly increases over southeast China associated with warm WP, favouring higher SAT in central eastern China. This reveals that the decadal predictions could more realistically reproduce the relationship between summer East Asian atmospheric circulation and WP SST. The 4‐year average forecast within a decade shows that the initialization increases the ACC skill up to forecast years 4–7 for summer SAT in central eastern China. [ABSTRACT FROM AUTHOR]

ABSTRACT The potential role of surface soil moisture ( SSM) in improving the simulation of interannual East Asian summer precipitation is investigated using a coupled land-atmosphere climate model, the Community Atmosphere Model version 3 ( CAM3). Forced by specified observational sea surface temperature ( SST), two ensemble simulations for 22 boreal-summer seasons (1979-2000) are conducted, one with interannually varying SSM ( SSMinter) forcing and another with climatological SSM ( SSMclim) forcing. Results show that, relative to the SSMclim run, interannual variability of East Asian summer precipitation is better simulated in the SSMinter run over the mid- and high-latitudes of East Asia, especially in northwest China, where the correlation coefficient between precipitation simulated and observed from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) is increased from 0.12 to 0.56 during 1979-2000. Meanwhile, positive relationships between anomalies of local soil evaporation and summer precipitation in northwest China can be better reproduced in the SSMinter run than in the SSMclim run. Possible mechanisms for the improved simulations of the interannual summer precipitation variability in northwest China are analysed; the improvement is resulted from the reasonable reproduction of anomalous atmospheric circulation in Siberia and Iranian plateau. [ABSTRACT FROM AUTHOR]