气候与气候变化

气候与气候变化研究进展

Progress in Research on Climate and Climate Change

1 次季节至季节变化特征与机理

1 Features and mechanism of the subseasonal to seasonal variability

1.1 Extremely late onset of the 2018 South China Sea summer monsoon following a La Niña event:Effects of triple SST anomaly mode in the North Atlantic and a weaker Mongolian cyclone

The onset of South China Sea (SCS) summer monsoon (SCSSM) in June 2018 was extremely late following a La Niña event,indicating a breakdown in the traditional positive relationship between ENSO and SCSSM onset.Although the weaker western Pacific subtropical high in early May facilitated the SCSSM establishment,the persistent weaker Mongolian cyclone featured by an anomalous barotropic anticyclone over Northeast Asia during mid-late May dramatically postponed the SCSSM onset in 2018.This not only reduced upper-tropospheric divergence over the SCS,which suppressed monsoon convection by attenuating the South Asian High,but also cooled the mid-upper troposphere to decrease the in-situ vertical easterly shear.The convection over the Maritime Continent was therefore trapped in the tropics,which further delayed the SCSSM onset via an enhanced local Hadley cell.It suggests a potential effect of triple SSTA mode in North Atlantic on the SCSSM onset by changing the mid-latitudinal circulation.(Liu Boqi,Zhu Congwen)

1.2 Weak linkage of winter surface air temperature over Northeast Asia with East Asian winter monsoon during 1993–2003

A colder winter over Northeast Asia (NEA) has traditionally been attributed to the stronger East Asian winter monsoon (EAWM) on interannual timescale.However,the present work proposes this robust linkage becomes weak during 1993–2003,along with the changes of the underlying condition in the last three decades.During 1981–1992 and 2004–2016,the loss of autumn Arctic sea ice condensation (SIC) leads to the stratospheric warming and the polar vortex weakening,which results in the enhancement of the Siberian High (SH) and the colder surface air temperature (SAT) over NEA.In particular,the persisting anomalies of snow cover near the Mongolian Plateau from autumn to winter could amplify the influences of Arctic SIC on the EAWM during 1981–1992.However,during 1993–2003 both the EAWM and the SAT over NEA are modulated by the tropical SSTAs in the western Pacific warm pool,corresponding to a weak association between the EAWM and the SAT over NEA.These tropical warm SSTAs could excite a meridional teleconnection over East Asia to generate a barotropic anticyclonic anomaly in situ.It not only weakens the SH and EAWM but also decreases the SAT over NEA.Therefore,the predictor of winter SAT over NEA is altered from the cryosphere to the tropical SSTAs in this episode.The changes of interdecadal background may contribute to the distinct interannual relationship between EAWM and winter SAT over NEA.This will increase the uncertainty in seasonal forecast on the winter climate over East Asia.(Liu Boqi,Zhu Congwen)

1.3 Record-breaking northward shift of the western North Pacific subtropical high in July 2018

The northward shift of the western North Pacific subtropical high (WNPSH) in July 2018 broke the historical record since 1958 and resulted in extreme heat waves and casualties across Northeast Asia (NEA).In the present work,we associated this extreme WNPSH anomaly with the anomalies of barotropic anticyclone above NEA originating from the strongest positive tripole pattern of sea surface temperature anomaly (SSTA) in North Atlantic in July.Both data analysis and numerical experiments indicated that the positive tripole SSTA pattern could produce an upper-tropospheric wave source over Europe,which stimulated an eastward propagating wave train along the subpolar westerly jet over the Eurasian Continent.When its anticyclonic node reached NEA,the WNPSH started to shift northward.After the cyclonic node in the circulation anomaly encountered the Tibetan Plateau (TP),atmospheric diabatic heating was enhanced over the eastern TP,initiating another subtropical wave train,which furthered the northward shift of the WNPSH.Therefore,the wave source over Europe was critical for the northward shift of the WNPSH in July,connecting the tripole SSTA pattern in the North Atlantic with the WNPSH anomaly and maintaining the downstream effects of thermal forcing over the eastern TP on the East Asian summer monsoon.(Liu Boqi,Zhu Congwen,Su Jingzhi)

1.4 Subseasonal mode of cold and wet climate in South China during the cold season:A climatological view

The authors investigate the dominant mode of climatological intraseasonal oscillation (CISO) of surface air temperature (SAT) and rainfall in China,and discuss the linkage of cold and wet climate in South China (SC) with the Arctic circulation regime during the cold season (from November to March).Results show that a positive CISO displays a cold-dry climate in North China,whereas a cold-wet pattern prevails in SC with a quasi-30-day oscillation during the peak winter season.In SC,the intraseasonal variability of SAT plays a leading role,altering the cold-wet climate by the southward shift of a cold front.Evidence shows that the circulation regime related to the cold and wet climate in SC is mainly regulated by a pair of propagating ISO modes at the 500-hPa geopotential height in the negative phase of Arctic Oscillation.It is demonstrated that the local cyclonic wave activity enhances the southward movement of the Siberian high,favoring an unstable atmosphere and resulting in the cold-wet climate over SC.Therefore,the cold-air activity acts as a precursor for subseasonal rainfall forecasting in SC.(Yu Minjie,Zhu Congwen,Jiang Ning)

1.5 Asymmetry in the dominant co-variation mode of boreal summer monsoon rainfall regulated by the ENSO evolution

Interannual variability of boreal summer monsoon (BSM) rainfall exhibits a prominent co-variation mode,which affects the regions with the highest population density in the world.The mode intensity depends on the rainfall variation over the western North Pacific (WNP) and the North America (NAM) monsoon regions.This study suggests that relative importance of the WNP and NAM rainfall anomalies is asymmetric between strong positive (SP) and strong negative (SN) phases of this mode,which can be attributed to the distinct seasonal evolution of ENSO events.In the SP-phase,the suppressed summer rainfall over the WNP determines the mode following a decaying El Niño event.When the tropical Indian Ocean (TIO) warms and the eastern Equatorial Pacific (EEP) cools,the emanating Kelvin wave- and zonal SST difference-induced low-level WNP anticyclone thus gets enhanced to damp the local rainfall,which further affects the other members of the BSM.However,the SN-phase of this mode,showing closer association with anomalous NAM rainfall,appears with a developing El Niño event.The tropical North Atlantic (TNA) becomes colder from spring to summer,when the EEP warms due to the zonal circulation anomalies.A stronger zonal SST difference between warm EEP and cold TNA suppresses the NAM rainfall by strengthening the low-level anticyclone in situ.Such asymmetry in different phases of the BSM co-variation mode is validated by the pre-industrial control runs in a state-of-theart CGCM,indicating the roles of ENSO evolution in regulating the interaction between the BSM members.(Hao Yuqian,Liu Boqi,Zhu Congwen,He Bian)

1.6 Possible impact of North Atlantic warming on the decadal change in the dominant modes of winter Eurasian snow water equivalent

A west-east dipole mode of winter Eurasian snow water equivalent (SWE) is found during the period of 1979−2015.It accounts for about 23.4% of the total variance,and displays a significant decadal change in the early-2000s.The basin warming footprint of the North Atlantic possibly exerts an influence on this decadal change,and the observed evidence is reproduced by numerical experiments with the Community Atmosphere Model (CAM3.1).A basin-wide warming of North Atlantic sea surface temperature induces atmospheric anomalies by exciting a stationary Rossby wave train,which prorogates from the subtropical North Atlantic to the mid-to-high latitudes of the Eurasian continent.Along with the Rossby wave train,a blocking ridge occurs over the Ural Mountain,and an upper-level trough appears over the eastern Siberian Plateau,which promotes heavy snowfall over the eastern Siberian Plateau and light snowfall to its west.Thus,it is plausible that the North Atlantic warming plays a role in exciting the Rossby wave train to modulate the decadal change in the west-east dipole SWE mode of the extratropical Eurasian continent.The possible moisture transport paths associated with the decadal change in the dipole SWE mode are also discussed.(Sun Chenghu,Zhang Ruonan,Li Weijing,Zhu Jieshun,Yang Song)

1.7 Role of Eurasian snow cover in linking winter-spring Eurasian coldness to the autumn Arctic sea ice retreat

An anomalous“north-south”dipole mode of snow water equivalent (SWE) persisting from winter to spring is detected over the Eurasian mid-to-high latitudes in this study.We show that this mode plays a key role in modulating the prolonged winter-spring coldness in Eurasian mid-latitudes,and is linked closely to the declined November Arctic sea ice concentration (SIC) using observational datasets and numerical experiments of the Community Atmospheric Model 5.0.The declined SICs over the Barents-Laptev seas can induce a teleconnection pattern over the mid-to-high latitudes in the following winter,accompanied by an anomalous ridge over the Ural Mountains and an anomalous trough over Europe and East Asia,respectively.Such changes in the large-scale circulation further lead to increased cold surges and heavy snowfall in mid-latitudes and light snowfall in high-latitudes,forming an anomalous“north-south”dipole mode of SWE.Due to the seasonal memory,this SWE pattern can persist into the following spring and lead to occurrence of mid-latitude coldness in spring via both thermodynamic and dynamic processes.For the thermodynamic process,the anomalous SWE condition can lead to anomalous wet soil,reduced incoming surface solar radiation,and cooling air in mid-latitudes.This in turn induces an enhanced Siberian high and a deepened East Asian trough via the snow-Siberian high feedback mechanism,which favors a cold spring in the northern East Asia.Further analysis suggests that an empirical seasonal prediction model based on the SWE factor has a reasonably good prediction skill for East Asian spring temperature.(Zhang Ruonan,Sun Chenghu,Zhang Renhe,Li Weijing,Zuo Jinqing)

1.8 Interannual relationship between intensity of rainfall intraseasonal oscillation and summer-mean rainfall over Yangtze River Basin in the eastern China

The analysis of observational rainfall shows that the intensity of rainfall intraseasonal oscillation (ISO) and the summer-mean rainfall over the middle-lower reaches of the Yangtze River Basin (YRB) exhibit a significant positive correlation during 1979–2007.A stronger (weaker) ISO variability is often associated with wet (dry) summer in the YRB.The composite ISOs in both the wet and dry summers are further analyzed.In the wet summers,the rainfall ISO in YRB is primarily associated with the northward propagation of a lowlevel cyclone-anticyclone pair from the tropics.Cyclonic vorticity and associated boundary layer convergence strengthen the rainfall in situ.In contrast,the rainfall ISO in YRB in the dry summers is primarily associated with the westward propagation of an anomalous anticyclone.Southerly flow to the west of the anomalous anticyclone enhances rainfall in YRB through anomalous moisture advection.In addition to the difference in ISO propagation,the background mean state also shows a marked difference.The diagnosis of water vapor flux budget shows that the convergence and advection of seasonal mean moisture play a critical role in maintenance of the intraseasonal rainfall in the YRB.A greater mean ascending motion and associated higher mean moisture in YRB in the wet summers favor greater intraseasonal rainfall variability in situ.The mean state difference is responsible for distinctive vertical structures of boundary layer vertical velocity.A possible feedback of the ISO to the summer-mean rainfall over the YRB is also discussed.(Qi Yanjun,Li Tim,Zhang Renhe)

1.9 Influence of intraseasonal oscillation on the asymmetric decays of El Niño and La Niña

Warm and cold phases of El Niño-Southern Oscillation (ENSO) exhibit a significant asymmetry in their decay speed.To explore the physical mechanism responsible for this asymmetric decay speed,the asymmetric features of anomalous sea surface temperature (SST) and atmospheric circulation over the tropical Western Pacific (WP) in El Niño and La Niña mature-to-decay phases are analyzed.It is found that the interannual standard deviations of outgoing longwave radiation and 850 hPa zonal wind anomalies over the equatorial WP during El Niño (La Niña) mature-to-decay phases are much stronger (weaker) than the intraseasonal standard deviations.It seems that the weakened (enhanced) intraseasonal oscillation during El Niño (La Niña) tends to favor a stronger (weaker) interannual variation of the atmospheric wind,resulting in asymmetric equatorial WP zonal wind anomalies in El Niño and La Niña decay phases.Numerical experiments demonstrate that such asymmetric zonal wind stress anomalies during El Niño and La Niña decay phases can lead to an asymmetric decay speed of SST anomalies in the central-eastern equatorial Pacific through stimulating different equatorial Kelvin waves.The largest negative anomaly over the Niño3 region caused by the zonal wind stress anomalies during El Niño can be threefold greater than the positive Niño3 SSTA anomalies during La Niña,indicating that the stronger zonal wind stress anomalies over the equatorial WP play an important role in the faster decay speed during El Niño.(Song Xiaomeng,Zhang Renhe,Rong Xinyao)

1.10 Why do we have El Niño:Quantifying a diabatic and nonlinear perspective using observations

El Niño,due to its global impact on weather patterns,ecosystems,agriculture and public health,has become as commonly known to the public as the recent global warming.But why we have El Niño is not yet as well answered as it may have been assumed.Linear theories have been successful in explaining the transition from the warm phase to the cold phase of the eastern tropical Pacific that results from the rise and fall of El Niño,but failed to explain the asymmetry between the two phases.A nonlinear theory for El Niño has suggested that there exist two equilibrium states for the tropical Pacific—one is zonally symmetric (or nearly so) with the warm-pool extending all the way to the eastern Pacific,and the other is strongly zonally asymmetric with the warm-pool confined to the western half of the tropical Pacific.Under this hypothesis,ENSO results from the fact that under the current radiative heating,both states are unstable,resulting in the apparent“wandering”behavior in between these two states as seen in the observations.To test this hypothesis,the authors have obtained the best approximations for the two equilibrium states empirically using updated ocean assimilation data,and quantified the stability of these two empirically obtained equilibrium states using two stability analysis methods.The results suggest that the two states are unstable,offering support for the nonlinear view of why we have El Niño.(Hua Lijuan,Sun Dezheng,Yu Yongqiang)

1.11 Possible causes of the flooding over South China during the 2015/2016 winter

Sequential heavy rainfall clusters and resultant severe winter flooding were observed over South China during the 2015/2016 super El Niño event.In the present,the intra-seasonal oscillations (ISOs) related with heavy rainfall over East Asia from late autumn to early winter in 2015/2016 were investigated using the daily in situ rainfall records and the NCEP-DOE reanalysis data set.Results suggested that the successive heavy winter rainfall events in 2015/2016 were contributed by the ISOs with significant periods of 10−25 and 30−45-days.The wet phase of 10−25-day ISO was jointly induced by the anomalous convergence of the low-level water vapor associated with the tropical-subtropical interaction and the upper-level divergence attributed to the eastwards propagating wave train.However in the wet phase of the 30−45-day ISO,the anomalous upperlevel divergence was possibly caused by the weaker East Asian trough and the stronger barotropic westerly in subtropics.The 30−45-day ISO can facilitate the eastwards propagation of the 10−25-day ISO by providing a benefit upper-level waveguide.Afterwards,the low-level southwesterly surrounding the Tibetan Plateau gets enhanced to bring more moisture into South China where the winter flooding was serious in 2015/2016.In contrast,the more-than-normal winter rainfall was weaker in the super El Niño of 1997/1998 due to the lack of the 30−45-day ISO.Therefore,the strongest winter rainfall anomaly over SC was ascribed to the superposition between the wet phases of 10-25 and 30−45-day ISOs in 2015/2016.(Guo Li,Zhu Congwen,Liu Boqi)

1.12 Interdecadal changes in the asymmetric impacts of ENSO on wintertime rainfall over China and atmospheric circulations over the western North Pacific

Based on data diagnoses,in this study the interdecadal changes in asymmetric impacts of ENSO on wintertime East Asian climate are investigated.It is found that the Pacific Decadal Oscillation (PDO) can significantly modulate the asymmetry in the responses of the East Asian climate to El Niño and La Niña events.In positive PDO phase the response is asymmetric with a strong anomalous anticyclone over the western North Pacific (WNP) and significant positive rainfall anomalies over the southern China during El Niño winters,but a weak anomalous cyclone over WNP and insignificant negative rainfall anomalies over the southern China during La Niña winters.However,such asymmetric responses do not appear in negative PDO phase,with comparable amplitudes of anomalous circulations over WNP and rainfall over the southern China in El Niño and La Niña winters.Further analyses reveal that the warm background of the tropical Pacific in positive PDO phase causes significant difference in the amplitudes of convection anomalies over the tropical western Pacific,resulting in asymmetric responses of the WNP circulation and rainfall over the southern China to El Niño and La Niña events.Nevertheless,the cold background in negative PDO phase reduces the amplitude difference between El Niño and La Niña winters.A comparable convection anomalies appear in the tropical western Pacific in El Niño and La Niña winters and the asymmetric responses do not occur.(Gao Ronglu,Zhang Renhe,Wen Min,Li Tianran)

1.13 The influence of wave trains in mid-high latitudes on persistent heavy rain during the first rainy season over South China

Based on daily precipitation data from the China Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy,the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated.The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features,with a period of 8–24 days.The influence of wave trains in the mid-high latitudes on the low-frequency persistent heavy rain event (PHR-LF event,the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed.During the first rainy season over South China,there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet.Analysis of wave activity flux indicates that the wave energy disperses toward the eastern China along these two lowfrequency wave trains from north to south and from west to east,and then propagates downward over South China.Accordingly,the disturbance of the relative vorticity of the cyclonic anomalies over the eastern China is strengthened,which enhances the meridional gradient of relative vorticity.Owing to the transport of lowfrequency relative vorticity and geostrophic vorticity by meridional wind,the ascending motion over South China intensifies and lasts for a long time,triggering a PHR-LF event.In addition,the tropical system is also a key factor to PHR-LF event.The QBWO of the convection over the South China Sea provides moisture for PHR-LF events,maintaining persistent rainfall and vertical ascending motion over South China.(Miao Rui,Wen Min,Zhang Renhe,Li Lun)

1.14 Development and eastward movement mechanisms of the Tibetan Plateau vortices moving off the Tibetan Plateau

Based on the final operational global analysis (FNL) data from the Global Forecasting System of the National Centers for Environment Prediction (NCEP) and the radiosonde data,the development and eastward movement mechanisms of fifteen Tibetan Plateau vortices (TPVs) after they move off the plateau are investigated.The results show that the convergence to the east of the TPVs at 500 hPa,the divergence associated with the westerly jet stream at 200 hPa,as well as the corresponding ascending motion,provide favorable conditions for the development and eastward movement of the TPVs.The spatial structures of the atmospheric apparent heat source (Q1) and the apparent moisture sink (Q2) are studied,showing that the heating centers of Q1at 400 hPa mainly sourced from the condensation latent heat are beneficial to the eastward movement of the TPVs,while the horizontal distribution of Q1at 500 hPa goes against that.The development and eastward movement mechanisms of the TPVs after they move off the plateau are further discussed through diagnosing the potential vorticity (PV) tendency equation.It is revealed that the horizontal PV flux convergence to the east of the TPVs related to the convergence at 500 hPa plays the dominant role,exerting positive contribution to the PV tendency.Meanwhile,the heating fields induce feeble PV tendency,indicating more important effect of the dynamic factor.The development and eastward movement mechanisms of the TPVs after they move off the plateau are different from those before the TPVs move off,and the dynamic effect is vital in the former stage while the effect of Q1is revealed as the dominant influencing factor in the latter.(Li Lun,Zhang Renhe,Wen Min)

1.15 Large-scale backgrounds and crucial factors modulating the eastward moving speed of vortices moving off the Tibetan Plateau

Tibetan Plateau vortices (TPVs) are major rain producer over the Tibetan Plateau,which trigger heavy rainfall in the southwestern and eastern China when moving off the plateau.In this work,two groups of TPVs moving off the plateau are selected according to their eastward moving speeds.The features of the atmospheric dynamic and thermodynamic fields associated with the two groups of TPVs are compared,based on the final (FNL) operational global analysis data from the Global Forecasting System of the National Centers for Environment Prediction (NCEP).The results show that the large-scale circulations and heating fields have a close relationship with the moving speed of the TPVs.The TPVs move eastward faster when wider and stronger convergence at 500 hPa,divergence at 200 hPa and the related ascending motion are observed to the east of TPVs.In addition,the stronger and further eastward stretching unstable stratification and water vapor convergence,as well as the more intensive heating field above 500 hPa to the east of TPVs,correspond to larger eastward moving speed of TPVs.Furthermore,the crucial factors modulating the moving speed of TPVs are explored through potential vorticity (PV) budget analyses,in which the physical variables are partitioned into zonal means and disturbances.The convergence of the mean zonal winds and disturbance winds at 500 hPa,as well as the vertical distribution of disturbance heating to the east of TPVs are the crucial factors influencing the eastward moving speed of TPVs,among which the vertical distribution of disturbance heating is the most dominant.(Li Lun,Zhang Renhe,Wen Min)

1.16 Characteristics of the Tibetan Plateau vortices and the related large-scale circulations causing different precipitation intensity

Tibetan Plateau vortices (TPVs) are important synoptic systems generated over the Tibetan Plateau,triggering heavy rainfall over southwestern and eastern China when they move off the plateau.In the present work,the precipitation associated with the moving-off TPVs is explored,and two groups of TPVs are selected according to the precipitation intensity related to the TPVs (PI).The characteristics of these two groups of TPVs and the large-scale circulations during the period after the TPVs move off the plateau are compared.The results show that the impact of TPVs is mainly found in the region to the west of 115°E between 30°−36°N,becoming weaker along with the longitude.Sichuan Basin is the area with the greatest influence from the TPVs.PI is attributed to both the TPVs and the large-scale background.The TPVs related to high (low) PI are stronger (weaker) and travel further eastward (travel shorter distance),and the ascending motion associated with the TPVs is stronger (weaker) and deeper (shallower).The Western Pacific subtropical high (WPSH),southwesterlies to the south of TPVs in middle and lower troposphere,the South Asia High (SAH) and upperlevel jet stream are crucial systems influencing the PI.Stronger and northwestward stretching WPSH,more intensive southwesterlies and the associated water vapor transportation to the south of TPVs,stronger upperlevel jet stream,as well as the more powerful and eastward extending SAH,correspond to higher PI,and vice versa.(Li Lun,Zhang Renhe,Wen Min)

1.17 Effects of the atmospheric dynamic and thermodynamic fields on the eastward propagation of Tibetan Plateau vortices

Tibetan Plateau vortices (TPVs) are major rain-producing systems over the Tibetan Plateau.Some TPVs can move off the plateau under certain conditions,and impact rainfall over the eastern China.Accordingly,the eastward propagation distances of the TPVs moving off the plateau (EPDs) are closely related to the areas of rainfall associated with TPVs.In this study,the moving-off TPVs during May−August of 1998−2015 are classified into two groups according to their EPDs,and the circulations and heating fields at the times when the TPVs move off the plateau (i.e.,moving-off times) are investigated based on reanalysis data.The dynamic and thermodynamic conditions to the east of the Tibetan Plateau are found to significantly impact the EPDs.In the middle and lower troposphere,the zonal ranges of negative geopotential height anomalies to the east of the Tibetan Plateau are in accordance with the EPDs of the TPVs,indicating that anomalous lows play a favorable role in the eastward movement of TPVs.In addition,the anomalous highs to the northeast of the Tibetan Plateau and over Southeastern China also benefit the maintenance of cyclonic circulation to the east of the plateau.Meanwhile,in the upper troposphere,the jet stream over Northeast Asia is beneficial for divergence at 200 hPa.Accordingly,ascending motion associated with the upper-level divergence and lowerlevel convergence is observed,with the zonal extent corresponding well to the EPDs in the two situations.The atmospheric thermodynamic factors also show a remarkable effect on the EPDs.The TPVs move farther away when the unstable stratification and water vapor convergence extend further eastward.The heating ranges above 500 hPa coincide with the EPDs of TPVs,implying a close relationship between the heating fields and the EPDs.These results benefit prediction on EPDs and further on rainfall to the east of the Tibetan Plateau.(Li Lun,Zhang Renhe,Wen Min,Duan Jianping,Qi Yanjun)

1.18 东亚夏季风次季节变化研究进展

东亚夏季风次季节（10～90 d）变化是中国夏季持续性强降水、高温热浪等高影响天气事件的重要环流载体，处于天气预报上限和气候季节预测下限之间的预报过渡区。研究表明：东亚夏季风次季节变化是东亚夏季风的固有物理特征，它和季节进程之间的时间锁相关系是东亚夏季风次季节变化潜在可预报性的重要来源。东亚夏季风次季节变化与Madden-Julian振荡（MJO）存在显著差异，试图通过MJO来预测东亚夏季风次季节变化不确定性较大。东亚夏季风次季节预测的另一重要来源是下垫面外强迫，包括欧亚大陆春季积雪、 中国东部春季土壤湿度和厄尔尼诺-南方涛动（ENSO）事件。此外，去趋势偏交叉相关分析统计方法能够分析东亚夏季风多因子和多时间尺度问题。目前，亟需解决的科学问题包括：东亚夏季风次季节模态的客观定量描述、 造成东亚夏季风次季节模态年际变化的关键物理过程、不同外强迫因子对东亚夏季风次季节模态的共同影响。（祝从文，刘伯奇，左志燕，袁乃明，刘舸）

1.19 全球变化背景下北半球冬季MJO 传播的年代际变化

利用1979—2013年实时多要素MJO（Madden-Julian Oscillation）监测（RMM）指数、美国NOAA逐日长波辐射资料和NCEP/NCAR再分析资料等，分析了全球变化背景下北半球冬季MJO传播的年代际变化特征。从全球平均气温快速增暖期（1985—1997年）到变暖趋缓期（2000—2012年），MJO 2～4位相频次减少，5～7位相频次增多，即MJO对流活跃区在热带印度洋地区停留时间缩短、传播速度加快，而在热带西太平洋停留时间加长、传播明显减缓。进一步分析发现，以上MJO的年代际变化特征与全球变化年代际波动有关。当太平洋年代际涛动（PDO）处于负位相时，全球变暖趋缓，热带东印度洋—西太平洋海温异常偏暖，使其上空对流加强，垂直上升运动加强，对流层低层辐合，大气中的水汽含量增多，该区域的湿静力能（MSE）为正异常。当MJO对流活跃区位于热带印度洋地区时，MJO异常环流对季节平均MSE的输送在强对流中心东侧为正、西侧为负，有利于东侧MSE扰动增加，使得MJO对流扰动东移加快；而当MJO对流活跃区在热带西太平洋地区，MJO异常环流对平均MSE 的输送形成东负西正的形势，东侧MSE扰动减小，不利于MJO快速东传。因此，全球变化背景下PDO引起的大气中水汽含量及MSE的变化可能是MJO传播年代际变化的重要原因。（修军艺，温敏，王遵娅，张人禾）

1.20 欧亚大陆上环球遥相关波列与丝绸之路波列的关系

分析了北半球夏季欧亚大陆上空CGT和SRP之间的联系与区别，结果表明这两种遥相关波列的空间分布形态高度相似，在时间变化上也显著关联。采用偏相关分析方法讨论了这两种遥相关波列的相对独立性，研究发现去除SRP的影响后，CGT在欧亚大陆上的波列状结构消失；而去除CGT的影响后，SRP在欧亚大陆上依然能维持自身的结构。这说明在欧亚大陆上SRP相对独立于CGT，而CGT依赖与SRP而存在，是SRP的一部分。（周馥荔，张人禾，韩晋平）

1.21 2018/2019年冬季江南超长连阴雨天气特征及其成因

2018/2019年冬季，我国长江以南（简称江南）地区发生了超长连阴雨天气。区域平均有效降水日数长达51天，打破了1981年以来的最高纪录，造成了极大的社会影响。本研究利用JRA55再分析资料，通过诊断分析，指出2018/2019年冬季中部型El Niño导致的江南地区异常低空水汽辐合和东亚冬季风次季节变化异常活跃，是造成此次江南地区超长连阴雨天气的主要原因。一方面，受此次中部型El Niño影响，西太平洋副热带高压异常偏强西伸，为江南地区的持续性降水异常提供了稳定的异常暖湿背景。另一方面，东亚冬季风准双周活动的异常偏强是江南地区超长连阴雨天气的维持条件。当冬季风偏强时，中、高纬度冷空气向南输送至江南地区，触发了局地强降水过程; 而当冬季风偏弱时，副热带地区的大气东传低频波列活跃，当该波列到达青藏高原后，在高原大地形的机械阻挡作用下，加强了江南地区上空的动力抬升，并引发弱降水过程。与历史事件相比，2018/2019年冬季江南地区的低空水汽辐合最强，东亚冬季风次季节显著周期最长，两者均达自1981年以来的极值。因此，江南地区次季节强、弱降水过程交替出现，最终造成了此次当地超长连阴雨天气。（郭莉，刘伯奇，祝从文）

1.22 东亚夏季环流多齿轮耦合特征及其对中国夏季降水异常的影响分析

东亚夏季风成员的相互作用，构成了东亚夏季风高、低层环流的“多齿轮耦合”形态。本文利用多变量主成分分析（MV-EOF）等方法诊断分析了东亚夏季风多齿轮耦合的变化特征、耦合机制、时间稳定性、空间稳定特征及其对中国夏季降水的影响机制，并在此基础上构建了典型多齿轮耦合形态影响夏季降水的概念模型。结果表明，多齿轮耦合受到垂直温、压场的强迫和青藏高原大地形的影响，主要表现在年际变化上（周期为2～6年）。其前两个模态稳定地反映了东亚夏季风成员典型联动作用。在第一模态中，北方气旋、南亚高压和西太平洋副热带高压为主要耦合系统。其中北方气旋为正压结构，在高层通过南侧偏西气流与南亚高压耦合，南亚高压则通过中纬东部地区下沉辐散气流与西太平洋副热带高压联动。当该耦合模态增强时，有利于中国夏季降水呈自北向南“＋－＋－”分布。第二模态主要反映中高纬气旋、东亚副热带西风气流、南亚高压、西北太平洋反气旋系统和西太平洋副热带高压耦合特征。其中，中高纬气旋和西北太平洋反气旋为正压系统，两者通过其间的东南气流联动。气旋系统在高层通过南侧西风与东亚副热带西风急流和南亚高压联动。反气旋在中低层通过南侧的偏东气流影响副热带高压强度和面积。当该耦合模态增强时，中国黄河以北及河套地区降水偏多，黄河以南降水偏少。（庞轶舒，祝从文，马振峰，秦宁生）

1.23 动力和统计预测信息融合预测方法及对我国夏季降水预测的检验

短期气候预测中如何将气候模式和统计方法的预测结果科学、客观的集成起来，一直是非常重要的问题。本文针对动力模式和统计方法预测结果相结合的问题，引入资料同化中信息融合的思想，采用最优内插同化方法，实现了动力模式和统计季节降水预测结果的融合。检验表明，对1982—2015年我国夏季降水百分率的回报，融合预测结果与观测的平均空间相关系数可达0.44，也分别较统计预测和CFSv2模式统计降尺度订正的技巧提高了0.1左右，而均方根误差较两者可以降低5%～20%。可见，该方法可以进一步提升对我国夏季降水的预测技巧，具有显著的业务应用价值。（孙丞虎，崔童，李维京，左金清）

1.24 次季节至季节气候预测业务支撑与决策服务

在季节预测方面，气候所次季节至季节研究团队（S2S团队）积极参加汛期气候预测，为决策部门提供预测意见。我们先后参加了5次不同部委组织的国家级气候会商，包括：（1）2019年3月，国家海洋局海洋环境预报中心的ENSO季节预测会商会；（2）2019年3月，国家气候中心汛期气候预测全国会商会；（3）2019年5月，水利部黄河水利委员会的汛期黄海河流域气候预测会商会；（4）2019年5月，国家气候中心汛期气候预测滚动订正会商（全国电视电话会议）；（5）2019年6月，国家气候中心盛夏气候预测会商（全国电视电话会议）。在春季成功地预测出2018/19年的厄尔尼诺事件将持续到今年夏季，基于“气科院东亚季风统计预测自动化系统”成功地预测出今年夏季我国东北地区降水异常偏多的情况，为决策部门提供了重要的预报信息。

在次季节预测方面，S2S团队积极参与2022年北京冬奥会气候预测保障，围绕冬奥会赛区温度的次季节至季节温度预测开展基础研究。针对泛冬奥会地区（东北亚）冬季气温的年际变化开展了专项研究，揭示了冬季西伯利亚高压影响东北亚冬季气温的多样性特征，并将气候变化背景引入冬奥会气温预测研究中，指出不同气候背景下影响东北亚地区冬季气温的前兆信号存在明显差异。此外，我们还揭示了影响赛区关键区的最高和最低极端温度变化特征和中纬度波列结构，为进一步开展冬奥会的次季节尺度温度波动预测工作奠定了科学依据。同时，团队成员代表气科院先后参加了数次由北京市气象局组织的2020年冬奥会测试赛专题会商，建立了针对赛区气温异常的次季节统计预测模型。

在决策气象服务方面，S2S团队针对2018年冬季以来我国长江以南地区的连阴雨天气成因、春季云南地区的持续性干旱成因和未来发展趋势、在全球变暖背景下近5年东北地区的6月持续低温现象及其对农业的潜在影响，我们基于研究，分别撰写了3份决策服务材料上报给中国气象局。在决策服务材料的基础上，我们进一步加强科学研究，撰写研究论文。其中对我国江南地区冬季连阴雨天气的分析已在今年的《科学通报》上发表，针对春季云南干旱的分析投稿《中国科学：地球科学》，关于东北地区冷夏的决策服务材料被局应急办选送上报“两办”（《 全球变暖给东北地区带来更频繁持久异常冷夏专家建议关注冷夏对区域粮食产量的不利影响》）。（刘伯奇，祝从文，马双梅，蒋宁，苏京志）

2 气候特征与气候变化

2 Climate characteristics and climate change

2.1 Half-a-degree matters for reducing and delaying global land exposure to combined daytimenighttime hot extremes

The Paris Agreement has motivated rapid analysis differentiating changes in frequency/intensity of weather and climate extremes in 1.5 versus 2 warmer worlds.However,implications of these global warming levels on locations,spatial scales and emergence timings of hotspots to extremes are more relevant to policy-making,but remain strikingly under-addressed.Based on a bivariate definitional framework,we show that compared to 2 ,the 1.5 target could avoid a transition of prevailing type of summertime hot extremes from daytime-/nighttime-only events to combined daytime-nighttime hot extremes in approximately 18% of global continents,and protect 14%−26% of land areas from seeing over three-to-tenfold increases in occurrence of combined hot extremes.This half-a-degree reduction also matters for around 21% of global lands,mostly within the tropics,in constraining historically-unprecedented combined hot extremes from becoming the new norm within just 1−3 decades ahead.By contrast,previous analyses based on univariatedefined hot days substantially underestimate the magnitude,areal extent,and emergence rate of 0.5 -caused aggravation of summertime hot extremes.These projected changes of bivariate-classified hot extremes,therefore,underline not only the imperative but also the urgency of striving for the lower Paris target.(Chen Yang,Zhou Baiquan,Zhai Panmao,Wilfran Moufouma-Okia)

2.2 Anthropogenic warming has substantially increased the likelihood of July 2017-like heat waves over the central eastern China

Heat waves in the central eastern China like the record-breaking July 2017 event were rare in natural worlds but have now become approximately 1-in-5-year events due to anthropogenic forcings.(Chen Yang,Chen Wei,Su Qin,Luo Feifei,Sarah Sparrow,Tian Fangxing,Dong Buwen,Simon F B Tett,Fraser C Lott,David Wallom)

2.3 Coincidence of increasingly volatile winters in China with Arctic sea-ice loss during 1980—2018

Despite intense discussions on the recent boom of mid-latitude wintertime cold extremes,co-variations of warm and cold extremes,i.e.winter temperature volatility,garnered substantially less attention.Apart from using temperature extremes’ frequency and intensity,we also define“temperature whiplash”,which depicts rapid switches between warm and cold extremes,to measure winter temperature volatility in China.Results show that Northeast-,Northwest-,Southwest-,Southeast-China and the Yangtze River Valley have experienced increasingly volatile winters after 1980,co-occurring with precipitous decline in Arctic sea-ice.This enhanced volatility has a strong expression in significant increases in temperature whiplash events,with some hotspots also seeing both warm and cold extremes more frequent and/or intense.An observation-based detection analysis highlights the dominance of intrinsic atmospheric variability over both anthropogenic warming and sea-ice decline during 1980−2018 in driving winters in China more volatile over this period.(Chen Yang,Liao Zhen,Zhai Panmao)

2.4 Extreme cold wave over East Asia in January 2016:A possible response to the larger internal atmospheric variability induced by Arctic warming

It is argued that anthropogenic global warming may decrease the global occurrence of cold waves.However,a historical record-extreme cold wave,popularly called the“boss-level”cold wave,attacked East Asia in January 2016,which gives rise to the discussion of why this“boss-level”cold wave occurred during the winter with the warmest recorded global mean surface air temperature (SAT).To explore the impacts of human-induced global warming and natural internal atmosphere variability,we investigated the cold waverelated circulation regime (i.e.,the large-scale atmospheric circulation pattern) and compared the observation with the large ensemble simulations of the MIROC5 model.Our results showed that this East Asian extreme cold wave-related atmospheric circulation regime mainly exhibited an extremely strong anomaly of the Ural blocking high (UBH) and a record-breaking anomaly of the surface Siberian high (SH),and it largely originated from the natural internal atmosphere variability.However,due to the dynamic effect of Arctic amplification,anthropogenic global warming may increase the likelihood of extreme cold waves through shifting the responsible natural atmospheric circulation regime towards a stronger amplitude.The probability of occurrence of extreme anomalies of UBH,SH,and the East Asia area-mean SAT have been increased by 58%,57%,and 32%,respectively,as a consequence of anthropogenic global warming.Therefore,extreme cold waves in East Asia,such as the one in January 2016,may be an enhanced response to the larger internal atmospheric variability modulated by human-induced global warming.(Ma Shuangmei,Zhu Congwen)

2.5 Persistent precipitation extremes in the Yangtze River Valley prolonged by opportune configuration among atmospheric teleconnections

This study investigates whether and how three synoptic-scale teleconnections,i.e.,East Asia/Pacific teleconnection (EAP),Silk-road teleconnection (SR) and Eurasia teleconnection (EU),induce persistent precipitation extremes (PPEs) in the Yangtze River Valley.Results show that only the EAP teleconnection has the potential of independently incurring PPEs; while the other two teleconnections’ influences on PPEs need to be exerted via their liaison with the EAP pattern.Cases are accordingly grouped into two subsets,i.e.,single EAP-PPEs and three teleconnection-PPEs.In both groups,the EAP teleconnection evolves following a similar pathway that poleward energy dispersion dynamically links a westward-extended subtropical high,a deepened mid-latitude trough and the Okhotsk blocking.EAP-induced circulation anomalies enhance low-level convergences and upper-level divergences,convey exceptionally abundant moisture,and hence give birth to a quasi-stationary front leading to PPEs in the Yangtze River Valley.Despite similarities,PPEs last noticeably longer in the three-teleconnection context.The EU teleconnection-related downstream energy dispersion at higher latitudes favors both earlier establishment and latter decay of the Okhotsk blocking.Constructed meridional flows at mid-high latitudes continuously steer migratory disturbances southeastward into the EAP trough.The SR plays a bigger role during the latter half of PPE lifespan (3 days after onset) via stimulating new cyclonic disturbances that largely overlap with the EAP trough spatially.Moreover,EU- and SR-excited disturbances could effectively extract baroclinic and barotropic energies from local mean flows to replenish components of the EAP tripole pattern,which therefore survives longer and prolongs PPEs by several days.(Chen Yang,Zhai Panmao,Liao Zhen,Li Lei)

2.6 Diurnal variations of summer precipitation over the Qilian Mountains in Northwest China

Based on the high-density hourly rain-gauge data from 265 stations over the Qilian Mountains in Northwest China,climatic mean diurnal variations of summer rainfall over different topographies of this area are investigated.Influences of the gauge elevations on the diurnal variation of rainfall are also revealed.Distinct regional features of diurnal variations in rainfall are observed over the Qilian Mountains.Rainfall over the Qinghai Lake area shows a single nocturnal peak.A dominant,late-afternoon peak of rainfall occurs over the mountain tops.Over the northeastern and southeastern slopes,a dominant diurnal peak appears in the late afternoon,and an evident second peak is found in the early morning,respectively.The strengths of the earlymorning peaks in the rainfall frequency are closely related to the rainfall events with different durations over the two slopes.The early-morning peak is dominant across plains with low elevations.From the mountain tops to the plains,the diurnal peaks of rainfall gradually vary from the dominant late-afternoon peak to the dominant early-morning peak with the enhanced early-morning peak in concurrent with the decreasing gauge elevation over the northeastern and southeastern slopes.Further examination indicates that the rainfall at higher elevations over the northeastern and southeastern slopes occurs more readily in the afternoon,compared to the lower elevations.This phenomenon corresponds to the result that the proportion of the rainfall frequency occurring during the early-morning period decreases with increasing elevations over the two slopes.(Li Liangliang,Li Jian,Chen Haoming,Yu Rucong)

2.7 Regional differences in hourly precipitation characteristics along the western coast of South China

The regional differences in hourly precipitation characteristics along the western coast of South China are investigated.The coastal area can be divided into three regions:two regions with large precipitation amounts to the west and east of the Leizhou Peninsula and another region with less precipitation over the Leizhou Peninsula.The precipitation center west of the Leizhou Peninsula (Dongxing station) is dominated by heavy precipitation,which frequently peaks in the early morning and has a long duration.The precipitation center east of the Leizhou Peninsula (Yangjiang station) has a high frequency of weak precipitation.There are two kinds of precipitation that occur at Yangjiang:short-duration precipitation in the afternoon and longlasting precipitation in the morning.Over the Leizhou Peninsula (Zhanjiang station),precipitation mainly occurs over a short duration in the afternoon.The possible reasons for the different precipitation features are discussed.The morning precipitation at Dongxing is usually caused by deep convection.A large proportion of the morning precipitation events are locally generated events,which are closely related to the southerly winds and topographic barriers.Among the many possible factors affecting the morning precipitation at Yangjiang,the convergence between the land breeze and prevailing onshore airflow is an important one.The afternoon precipitation events affecting Yangjiang mostly originate over the mountains northwest of Yangjiang.Influenced by the curvature of the coastline and the relatively flat terrain over the Leizhou Peninsula,there is no convergence and less convection in the morning.However,the Leizhou Peninsula is prone to being influenced by daytime thermal forcings,which trigger afternoon precipitation.(Li Jian,Li Nina,Yu Rucong)

2.8 Differences in the rainfall characteristics between Mount Tai and its surrounding areas

As a typical small-scale,isolated topography,Mount Tai exhibits great differences in precipitation characteristics from the surrounding areas.It is found that the amount of rainfall occurring over Mount Tai is significantly larger than what observed in the surrounding areas.Based on hourly rain gauge records for the warm season (May to September) of 1996–2015,differences between Mount Tai and its surrounding areas were further revealed in terms of rainfall diurnal variation,spatial scale,and evolution process.The diurnal variation of the enhancement on rainfall amount exhibit“dual peaks”occurring in the early morning and afternoon,and the dual peaks are mainly attributable to rainfall frequency.The diurnal phase of the rainfall amount in the surrounding areas lags 1 h behind that over Mount Tai.Regarding differences in rainfall spatial scale,compared to those of surrounding areas,precipitation over Mount Tai is characterized by a smaller coverage,especially in the early morning.Mount Tai also tends to have a kind of unique,small-scale rainfall in the afternoon and at night.Based on statistical analysis of precipitation events,differences in rainfall evolution process were identified as well.Rainfall over Mount Tai often starts earlier in the afternoon and ends later at night than it does in the surrounding areas.Furthermore,nocturnal rainfall events over Mount Tai are prone to peaking over a shorter period and enduring for a longer period after reaching their maximum intensity,compared with nocturnal rainfall events occurring in the surrounding areas.Rainfall events over Mount Tai always last longer,especially those occurring in the early morning.In general,Mount Tai has a large enhancement effect on rainfall.(Gan Yuting,Li Nina,Li Jian)

2.9 The relationship between heavy precipitation in the eastern region of China and atmospheric heating anomalies over the Tibetan Plateau and its surrounding areas

The relationships among heavy precipitation in the eastern region of China,the atmospheric heat source over the Tibetan Plateau and its surrounding areas,and atmospheric circulation in East Asia were investigated using the multi-variate empirical orthogonal function (MV-EOF) method and synthetic analysis on daily meteorological data from May through August 2010,which were compared with data from 2013.The MVEOF decomposition results revealed that the atmospheric heating over the eastern region of the Tibetan Plateau and the Bay of Bengal exhibited opposite trends when heavy precipitation events occurred in South China,West China,and the middle and lower reaches of the Yangtze River.These results indicated that the landsea thermal contrast between the eastern region of the Tibetan Plateau and the Bay of Bengal was likely one of the key factors leading to the occurrence of heavy precipitation events in the eastern region of China.The results of the synthetic analysis revealed a possible physical mechanism:When the atmospheric heating was weak over the Tibetan Plateau and strong over the Bay of Bengal,there was a strong ascending motion over the Bay of Bengal and its surrounding areas,which was conducive to maintaining the South Asian high and the western Pacific subtropical high (WPSH) in southerly positions.This also resulted in weak water vapor transport in the southwest,thus forming continuous heavy precipitation in South China.After the increase in atmospheric heating over the Tibetan Plateau,the convergence and ascending motion of the lower atmosphere were strengthened,and the South Asian high moved northward to the plateau,with a strengthened eastward extension.The WPSH then lifted northward,and the airflow around it conveyed more water vapor to West China and the middle and lower reaches of the Yangtze River,resulting in heavy precipitation in these regions.(Shi Xiaohui,Chen Jinqiu,Wen Min)

2.10 Climatic characteristics of East Asian tropical monsoon depressions

The climatic characteristics of 260 East Asian tropical monsoon depressions (EAMDs) are investigated using the ERA-Interim reanalysis dataset and a tracking dataset of global monsoon low-pressure systems.Most EAMDs form over the South China Sea (SCS) and the western tropical Pacific Ocean in July–October and have an average lifetime of 10 days.The vertical structures of EAMDs are usually upright or tilt slightly westward with height.The warm-over-cold thermal structure is a distinctive characteristic of EAMDs and two potential vorticity (PV) centers are related to the warm core in the upper level and the specific humidity center in the lower level,respectively.We divided the EAMDs into four groups:eastward-moving,westward-moving,turning,and northwestward-moving EAMDs.Most of the eastward-moving EAMDs form over the SCS in May and June,whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October.The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes.The structures of the eastward-moving and turning EAMDs show common characteristics in each stage.Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage,especially for the specific humidity.By contrast,the structures of westward- and northwestward-moving EAMDs show little change during their lifetime.They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.(Hu Yi,Wen Min,Li Lun,Zhang Renhe)

2.11 气候变化和城市可持续发展

越来越多的研究表明，城市受到全球气候变化以及城市化本身引起的局地气候变化的多重影响，使得高温热浪、强降水和严重污染天气更加频繁和严重。加之城市在密集的空间内容纳了全世界55%的人口，集聚了人类大部分的资产和经济活动，很容易受到气候变化所带来的负面影响。同时，城市又是化石能源利用的中心，二氧化碳和污染物排放占比很大。未来全世界城市将进一步扩展，城市人口也将进一步增加，这意味着未来城市所面临的气候变化风险形势可能会更为严峻。由此，从气候变化应对角度来看，城市具有举足轻重的地位。显然，气候变化与未来城市可持续发展将是目前人类面临的重大研究课题之一。政府间气候变化委员会（IPCC）从第6次评估报告（AR6）开始，将更加关注气候变化的区域问题; 并会结合巴黎协定、联合国2030年可持续发展目标（SDGs）等，来更多地关注气候变化和城市可持续发展问题。IPCC计划将在2023年第7次气候变化评估报告期内开展“气候变化与城市”特别报告的编写。2018年4月，IPCC与其他相关国际组织在加拿大Edmond市召开了城市与气候变化大会，会议评估了全球城市与气候变化有关科学、政策和应用等方面的现状，发现了有关研究方面的不足。本文在简要回顾国际上城市气候变化及其应对有关研究进展和活动的最新动态基础上，分析中国城市气候变化有关风险的严峻形势，并阐述城市应对气候变化与可持续发展的密切关系，围绕对我国城市气候变化问题的科学认识和应对行动，梳理出中国针对城市应对气候变化问题上值得关注的几个关键科学问题。（翟盘茂，袁宇锋，余荣）

2.12 2018年7月北半球极端天气气候事件及环流特征分析

2018年7月北半球天气气候显著异常，极端事件高发。欧洲、北非、东亚以及北美的大部分地区均遭受严重的高温热浪侵袭;印度、东南亚、中国西南部以及日本西部等地出现极端降水;西太平洋台风活动异常活跃，移动路径偏北。初步诊断表明，北半球中高纬度，由低层到高层稳定维持的异常高压系统是导致北半球中高纬度大部分地区高温热浪持续发生的直接原因。其中异常偏强、偏北的副热带高压，以及增强、东伸的南亚高压与东亚地区持续高温和极端降水事件直接相关;低层菲律宾周围异常活跃的对流活动和强盛的西南水汽输送共同导致南亚、东南亚地区极端降水发生。热带太平洋大部分地区偏暖的海温条件和菲律宾附近异常气旋性环流则与异常活跃的台风活动有关。更需要关注的是，北半球尤其是东亚地区大气环流的异常主要受海洋表面热力状况以及其他区域大气环流遥相关的影响。（王倩，翟盘茂，余荣）

3 气候模式研发

3 Climate model development

3.1 Recent progress in numerical atmospheric modeling in China

This review summarizes the scientific and technical progress in atmospheric modeling in China since 2011,including the dynamical core,model physics,data assimilation,ensemble forecasting,and model evaluation strategies.In terms of the dynamical core,important efforts have been made in the improvement of the existing model formulations and in exploring new modeling approaches that can better adapt to massive parallel computers and global multiscale modeling.With regard to model physics,various achievements in physical representations have been made,especially a trend toward scale-aware parameterization for accommodating the increase of model resolution.In the field of data assimilation,a 4D-Var system has been developed and is operationally used by the National Meteorological Center of China,and its performance is promising.Furthermore,ensemble forecasting has played a more important role in operational forecast systems and progressed in many fundamental techniques.Model evaluation strategies,including key performance metrics and standardized experimental protocols,have been proposed and widely applied to better understand the strengths and weaknesses of the systems,offering key routes for model improvement.The paper concludes with a concise summary of the status quo and a brief outlook in terms of future development.(Yu Rucong,Zhang Yi,Wang Jianjie,Li Jian,Chen Haoming,Gong Jiandong,Chen Jing)

3.2 A layer-averaged nonhydrostatic dynamical framework on an unstructured mesh for global and regional atmospheric modeling:Model description,baseline evaluation and sensitivity exploration

This paper describes the development and evaluation of a new nonhydrostatic dynamical framework for global and regional atmospheric modeling,with an emphasis on the numerical performance of dry dynamics.The model is formulated in a layer-averaged manner using a generalized hybrid sigma-mass vertical coordinate and an unstructured mesh.The mass-based equations allow a flexible and effective switch between the hydrostatic and nonhydrostatic solvers.The unstructured mesh treats the conventional icosahedral grid and the more general Voronoi polygon in a consistent manner,allowing a flexible switch between quasi-uniform and variable-resolution modeling.The horizontal and vertical discretization is formulated in an explicit Eulerian approach,while those terms describing the vertically propagating fast waves are solved implicitly.The model is equipped with physically based Smagorinsky diffusion as a tuning tool.A suite of multiscale test cases from hydrostatic to nonhydrostatic regimes is used to assess the model performance.The general strategies for evaluation focus on two aspects:(i) the nonhydrostatic solver should behave similarly to its hydrostatic counterpart under the hydrostatic regime; and (ii) the nonhydrostatic solver should produce unique nonhydrostatic responses under the nonhydrostatic regime.In the context of model evaluation,model sensitivity to numerical configurations is further explored to understand the impact of isolated components,helping to identify appropriate configurations for realistic modeling applications.The present framework is a prototype towards a Global-Regional Integrated forecast SysTem (GRIST).(Zhang Yi,Li Jian,Yu Rucong,Zhang Shixun,Liu Zhuang,Huang Jiahao,Zhou Yihui)

3.3 Understanding the performance of an unstructured-mesh global shallow water model on kinetic energy spectra and nonlinear vorticity dynamics

A strategy for evaluating global shallow water model based on aspects of kinetic energy spectra and nonlinear vorticity dynamics is proposed in this study.The kinetic energy spectra and nonlinear vorticity dynamics of a recently developed global shallow water model on an unstructured mesh are evaluated,and are compared against the benchmark solutions from a global high-resolution spectral model.The kinetic energy spectra,the rotational and divergent components,the stationary and transient components,and the nonlinear spectral fluxes of the shallow water model are studied.Results show that the developed model agrees well with those results generated by the reference model.The influence of different flux operators for transporting the potential vorticity is assessed in detail.It is shown that the second-order flux operator leads to a spurious increase in the kinetic energy at the tail of the spectrum,whereas the upwind third-order flux operator does not support this behavior owing to implicit numerical diffusion.The nonlinear vorticity dynamics is studied using colliding modons.The grid-point model maintains the symmetrical pattern of vortices,and generates similar kinetic energy spectra and nonlinear spectral fluxes to the reference model.The evaluation provides a reference for evaluating the shallow water model in terms of nonlinear dynamics,and the developed global shallow water model presents a good example.(Wang Lei,Zhang Yi,Li Jian,Liu Zhuang,Zhou Yihui)

3.4 Boreal summer intraseasonal oscillation in the Asian-Pacific monsoon region simulated in CAMS-CSM

The simulation of boreal summer intraseasonal oscillation (BSISO) by the climate system model (CSM) developed at Chinese Academy of Meteorological Sciences (CAMS) is used in this study.The results indicate that this new model is able to simulate reasonable annual cycle and seasonal mean of the precipitation,as well as the vertical shear of large-scale zonal wind in tropics.The model reproduces the eastward- and northward-propagating signals similar to those found in observations.The simulation of BSISO is generally in agreement with the observations in terms of variance center,periodicity and propagation,with the exception that the magnitude of BSISO anomalous convections are underestimated in both eastward propagation along the equator and northward propagation over the Asian-Pacific summer monsoon domain.Our preliminary evaluation of the simulated BSISO by CAMS-CSM suggests that this new model has the capability,to a certain extent,in representing BSISO features,including its zonal propagation along the equator and meridional propagation over the Asian-Pacific summer monsoon domain.(Qi Yanjun,Zhang Renhe,Rong Xinyao,Li Jian,Li lun)

3.5 ENSO asymmetry in the CAMS-CSM

This study presents an overview of El Niño-Southern Oscillation (ENSO) asymmetry using the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM).We discover that the coupled run of the CAMS-CSM has an obvious bias of ENSO opposite-sign asymmetry compared to observation,mainly in the eastern Pacific.Further analysis shows that the spatial distributions of sea surface temperature anomalies (SSTA) during both phases of the ENSO present individual biases,consisting of a warmer field during the warm phase and a colder field during the cold phase,in comparison with observation.The bias of ENSO asymmetry during both phases is partly due to the unrealistic simulation of shortwave (SW) radiation flux and the corresponding total cloud cover (TCC).The Atmospheric Model Intercomparison Project (AMIP) run demonstrates that biases of the SW radiation flux and the associated TCC originate in the atmospheric component of the model,which could be attributed to its unrealistic cloud microphysical scheme.Through air-sea interaction,these biases are amplified significantly during both ENSO phases of the coupled run.Moreover,another cause for the bias of ENSO asymmetry during the warm phase is the relatively slow decay of the ENSO in the simulation,with the thermocline anomalies propagating eastward more slowly.The bias of ENSO asymmetry in the cold phase is attributed to oceanic internal dynamic advection,mainly associated with zonal and meridional terms.Further analysis also highlights the essential role of reasonably representing the climatological mean state in ENSO model simulation.(Hua Lijuan,Chen Lin)

3.6 An assessment of ENSO stability in CAMS climate system model simulations

We present an overview of the El Niño-Southern Oscillation (ENSO) stability simulation using the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM).The ENSO stability was quantified based on the Bjerknes (BJ) stability index.Generally speaking,CAMS-CSM has the capacity of reasonably representing the BJ index and ENSO-related air-sea feedback processes.The major simulation biases exist in the underestimated thermodynamic damping and thermocline feedbacks.Further diagnostic analysis reveals that the underestimated thermodynamic feedback is due to the underestimation of the shortwave radiation feedback,which arises from the cold bias in mean sea surface temperature (SST) over central-eastern equatorial Pacific (CEEP).The underestimated thermocline feedback is attributed to the weakened mean upwelling and weakened wind-SST feedback (μa) in the model simulation compared to observation.We found that the weakenedμais also due to the cold mean SST over the CEEP.The study highlights the essential role of reasonably representing the climatological mean state in ENSO simulations.(Hua Lijuan,Chen Lin,Rong Xinyao,Li Jian,Zhang Guo,Wang Lu)

3.7 Cloud radiative feedbacks during the ENSO cycle simulated by CAMS-CSM

This study evaluated the simulated cloud radiative feedbacks (CRF) during the El Niño-Southern Oscillation (ENSO) cycle in the latest version of the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM).We conducted two experimental model simulations:the Atmospheric Model Intercomparison Project (AMIP),forced by the observed sea surface temperature (SST); and the preindustrial control (PIcontrol),a coupled run without flux correction.Both the experiments generally reproduced the observed features of the shortwave and longwave cloud radiative forcing (SWCRF and LWCRF) feedbacks.The AMIP run exhibited better simulation performance in the magnitude and spatial distribution than the PIcontrol run.Furthermore,the simulation biases in SWCRF and LWCRF feedbacks were linked to the biases in the representation of the corresponding total cloud cover and precipitation feedbacks.It is interesting to further find that the simulation bias originating in the atmospheric component was amplified in the PIcontrol run,indicating that the coupling aggravated the simulation bias.Since the PIcontrol run exhibited an apparent mean SST cold bias over the cold tongue,the precipitation response to the SST anomaly (SSTA) changes during the ENSO cycle occurred towards the relatively warmer western equatorial Pacific.Thus,the corresponding cloud cover and CRF shifted westward and showed a weaker magnitude in the PIcontrol run versus observational data.In contrast,the AMIP run was forced by the observational SST,hence representing a more realistic CRF.Our results demonstrate the challenges of simulating CRF in coupled models.This study also underscores the necessity of realistically representing the climatological mean state when simulating CRF during the ENSO cycle.(Chen Lin,Hua Lijuan,Rong Xinyao,Li Jian,Wang Lu,Zhang Guo,Sun Ming,Ge Zi’an)

3.8 Performance of CAMS-CSM in simulating the shortwave cloud radiative effect over global stratus cloud regions:Baseline evaluation and sensitivity test

The ability of climate models to correctly reproduce clouds and the radiative effects of clouds is vitally important in climate simulations and projections.In this study,simulations of the shortwave cloud radiative effect (SWCRE) using the Chinese Academy of Meteorological Sciences climate system model (CAMSCSM) are evaluated.The relationships between SWCRE and dynamic-thermodynamic regimes are examined to understand whether the model can simulate realistic processes that are responsible for the generation and maintenance of stratus clouds.Over the eastern China,CAMS-CSM well simulates the SWCRE climatological state and stratus cloud distribution.The model captures the strong dependence of SWCRE on the dynamic conditions.Over the marine boundary layer regions,the simulated SWCRE magnitude is weaker than that in the observations due to the lack of low-level stratus clouds in the model.The model fails to simulate the close relationship between SWCRE and local stability over these regions.A sensitivity numerical experiment using a specifically designed parameterization scheme for the stratocumulus cloud cover confirms this assertion.Parameterization schemes that directly depict the relationship between the stratus cloud amount and stability are beneficial for improving the model performance.(Zhou Yihui,Zhang Yi,Rong Xinyao,Li Jian,Yu Rucong)

3.9 Robust nocturnal and early morning summer rainfall peaks over continental East Asia in a global multiscale modeling framework

The diurnal cycle of convection and precipitation is an important atmospheric feature.It also poses a great challenge to global numerical atmospheric modeling.Over continental East Asia,most global models cannot well capture the nocturnal and early morning peaks of summer rainfall.This problem may lead to dry biases and limit the modeling skills.This study investigates this problem using a global multiscale modeling framework (Super-Parameterized CAM5; SPCAM5).The nocturnal and early morning peaks,which are almost absent in CAM5 and a coarser-resolution SPCAM5,can be successfully captured by SPCAM5 with a moderate increase in the horizontal resolution.On the lee side of the Tibetan Plateau,SPCAM5 generates robust eastward propagating rainfall signals,which correspond to the moving convective systems,as revealed by the heating and drying profiles.Over the eastern plain of China,the early morning peaks become more evident,corresponding to a stratiform-type heating structure in the midlevel.A sensitivity experiment with altered gridscale forcing also suggests the important preconditioning role of the vertical moisture advection in regulating the early morning peaks.These results highlight the added value of representing multiscale processes to the successful simulation of the diurnal cycle over continental East Asia.(Zhang Yi,Chen Haoming,Wang Dan)

3.10 Revisiting the impact of stochastic multicloud model on the MJO using low-resolution ECHAM6.3 atmosphere model

Based on the preceding work,the influence of the stochastic multicloud model (SMCM) on the Madden-Julian oscillation (MJO) in the state-of-the-art ECHAM6.3 atmospheric general circulation model (AGCM) is further evaluated.The evaluation presented here is based on six recently proposed dynamics-oriented diagnostic metrics.Lag-longitude correlation maps of surface precipitation in the eastern Indian Ocean and West Pacific Ocean confirm the previously discovered improved representation of the MJO in the modified ECHAM6.3 model compared with the standard configuration.In fact,the modified ECHAM6.3 outperforms the default ECHAM6.3 in five of the six MJO-related diagnostics evaluated here.In detail,the modified ECHAM6.3 (1) successfully models the eastward propagation of boundary layer moisture convergence (BLMC); (2) captures the rearward-tilted structure of equivalent potential temperature (EPT) in the lower troposphere and forward-tilted structure of EPT in the upper troposphere; (3) exhibits the rearward-tilted structure of equatorial diabatic heating in the lower troposphere; (4) adequately simulates the MJO-related horizontal circulation at 850 and 200 hPa and the 300 hPa diabatic heating structure.These evaluations confirm the crucial role of convective-parameterization formulation on GCM-simulated MJO dynamics and support the further application and exploration of the SMCM concept in full-complexity GCMs.(Ma Libin,Karsten Peters,Wang Bin,Li Juan)

3.11 Prediction of heavy precipitation in the eastern China flooding events of 2016:Added value of convection-permitting simulations

During the period from 30th June to 6th July 2016,a heavy rainfall event affected the middle and lower reaches of the Yangtze River Valley in the eastern China.The event was characterized by high-intensity,longduration (lasted more than 6 days) precipitation and huge amounts (over 600.0 mm) of rainfall.The rainfall moved eastward from the Sichuan Basin to the middle Yangtze River valley during the first two days,and then entered a“quasi-stationary”(Mei-yu front) phase.During the second-phase,successive heavy rainfall systems occurred repeatedly over the same areas along the front,leading to widespread and catastrophic flooding.In this study,limited-area convection-permitting models (CPMs) covering all of eastern China,and globalmodel simulations from the Met Office Unified Model are compared to investigate the added values of CPMs on the veracity of short-range forecasts of the heavy rainfall event.The results show that all the models can successfully simulate the accumulated amount and the evolution of this heavy rainfall event.However,the global model produces too much light rainfall (10.0 mm day−1),fails to simulate the small-scale features of both atmospheric circulations and precipitation,and tends to generate steady heavy rainfall over the mountainous region.Afternoon precipitation is also excessively suppressed in the global model.By comparison,the CPMs add some value in reproducing the spatial distribution of precipitation,the smaller-scale disturbances within the rain-bands,and the diurnal cycle of precipitation and also reduce the spurious topographical rainfall,although there is a tendency for heavy rainfall to be too intense in CPMs.(Li Puxi,Guo Zhun,Kalli Furtado,Chen Haoming,Li Jian,Sean Milton,Paul R Field,Zhou Tianjun)

3.12 CAMS-CSM模式及其参与CMIP6的方案

世界气候研究计划(WCRP)组织开展的耦合模式比较计划已实施到第6阶段（CMIP6），中国气象科学研究院发展的气候系统模式CAMS-CSM是注册参加CMIP6的模式之一。除CMIP6要求的气候诊断、评估和描述试验（DECK）以及历史气候模拟试验（Historical）外，CAMS-CSM还计划参加情景模式比较计划（ScenarioMIP）、云反馈模式比较计划（CFMIP）、全球季风模式比较计划（GMMIP）和高分辨率模式比较计划（HighResMIP）这4个模式比较子计划（MIPs）。文中通过介绍CAMS-CSM的基本情况和模拟性能，以及计划参加的CMIP6试验及MIPs计划，为模式试验数据使用者提供参考。（容新尧，李建，陈昊明，辛羽飞，苏京志，华莉娟，张正秋）