A CMIP6-based assessment of regional climate change in the Chinese Tianshan Mountains

Climate warming profoundly affects hydrological changes,agricultural production,and human society.Arid and semi-arid areas of China are currently displaying a marked trend of warming and wetting.The Chinese Tianshan Mountains(CTM)have a high climate sensitivity,rendering the region particularly vulnerable to the effects of climate warming.In this study,we used monthly average temperature and monthly precipitation data from the CN05.1 gridded dataset(1961-2014)and 24 global climate models(GCMs)of the Coupled Model Intercomparison Project Phase 6(CMIP6)to assess the applicability of the CMIP6 GCMs in the CTM at the regional scale.Based on this,we conducted a systematic review of the interannual trends,dry-wet transitions(based on the standardized precipitation index(SPI)),and spatial distribution patterns of climate change in the CTM during 1961-2014.We further projected future temperature and precipitation changes over three terms(near-term(2021-2040),mid-term(2041-2060),and long-term(2081-2100))relative to the historical period(1961-2014)under four shared socio-economic pathway(SSP)scenarios(i.e.,SSP1-2.6,SSP2-4.5,SSP3-7.0,and SSP5-8.5).It was found that the CTM had experienced significant warming and wetting from 1961 to 2014,and will also experience warming in the future(2021-2100).Substantial warming in 1997 was captured by both the CN05.1 derived from interpolating meteorological station data and the multi-model ensemble(MME)from the CMIP6 GCMs.The MME simulation results indicated an apparent wetting in 2008,which occurred later than the wetting observed from the CN05.1 in 1989.The GCMs generally underestimated spring temperature and overestimated both winter temperature and spring precipitation in the CTM.Warming and wetting are more rapid in the northern part of the CTM.By the end of the 21st century,all the four SSP scenarios project warmer and wetter conditions in the CTM with multiple dry-wet transitions.However,the rise in precipitation fails to counterbalance the drought induced by escalating temperature in the future,so the nature of the drought in the CTM will not change at all.Additionally,the projected summer precipitation shows negative correlation with the radiative forcing.This study holds practical implications for the awareness of climate change and subsequent research in the CTM.

Projection of precipitation extremes over South Asia from CMIP6 GCMs

Extreme precipitation events are one of the most dangerous hydrometeorological disasters,often resulting in significant human and socio-economic losses worldwide.It is therefore important to use current global climate models to project future changes in precipitation extremes.The present study aims to assess the future changes in precipitation extremes over South Asia from the Coupled Model Intercomparison Project Phase 6(CMIP6)Global Climate Models(GCMs).The results were derived using the modified Mann-Kendall test,Sen's slope estimator,student's t-test,and probability density function approach.Eight extreme precipitation indices were assessed,including wet days(RR1mm),heavy precipitation days(RR10mm),very heavy precipitation days(RR20mm),severe precipitation days(RR50mm),consecutive wet days(CWD),consecutive dry days(CDD),maximum 5-day precipitation amount(RX5day),and simple daily intensity index(SDII).The future changes were estimated in two time periods for the 21^(st) century(i.e.,near future(NF;2021-2060)and far future(FF;2061-2100))under two Shared Socioeconomic Pathway(SSP)scenarios(SSP2-4.5 and SSP5-8.5).The results suggest increases in the frequency and intensity of extreme precipitation indices under the SSP5-8.5 scenario towards the end of the 21^(st) century(2061-2100).Moreover,from the results of multimodel ensemble means(MMEMs),extreme precipitation indices of RR1mm,RR10mm,RR20mm,CWD,and SDII demonstrate remarkable increases in the FF period under the SSP5-8.5 scenario.The spatial distribution of extreme precipitation indices shows intensification over the eastern part of South Asia compared to the western part.The probability density function of extreme precipitation indices suggests a frequent(intense)occurrence of precipitation extremes in the FF period under the SSP5-8.5 scenario,with values up to 35.00 d for RR1mm and 25.00-35.00 d for CWD.The potential impacts of heavy precipitation can pose serious challenges to the study area regarding flooding,soil erosion,water resource management,food security,and agriculture development.

Global air-sea CO_(2) exchange fl ux since 1980s: results from CMIP6 Earth System Models

The ocean could profoundly modulate the ever-increasing atmospheric CO_(2) by air-sea CO_(2) exchange process,which is also able to cause signifi cant changes of physical and biogeochemical properties in return.In this study,we assessed the long-term average and spatial-temporal variability of global air-sea CO_(2) exchange fl ux(F CO_(2))since 1980s basing on the results of 18 Coupled Model Intercomparison Project Phase 6(CMIP6)Earth System Models(ESMs).Our fi ndings indicate that the CMIP6 ESMs simulated global CO_(2) sink in recent three decades ranges from 1.80 to 2.24 Pg C/a,which is coincidence with the results of cotemporaneous observations.What’s more,the CMIP6 ESMs consistently show that the global oceanic CO_(2) sink has gradually intensifi ed since 1980s as well as the observations.This study confi rms the simulated F CO_(2) could reach agreements with the observations in the aspect of primary climatological characteristics,however,the simulation skills of CIMP6 ESMs in diverse open-sea biomes are unevenness.None of the 18 CMIP6 ESMs could reproduce the observed F CO_(2) increasement in the central-eastern tropical Pacifi c and the midlatitude Southern Ocean.Defi ciencies of some CMIP6 ESMs in reproducing the atmospheric pressure systems of the Southern Hemisphere and the El Niño-Southern Oscillation(ENSO)mode of the tropical Pacifi c are probably the major causes.

CMIP6模式对中国东部地区水循环的模拟能力评估

本文基于观测和再分析资料,采用Brubaker二元模型评估了第六次国际耦合模式比较计划(CMIP6)中19个模式对中国东部季风区气候态水循环过程的模拟能力,并分析了模拟误差来源。结果表明,CMIP6模式集合平均(MME)能够合理再现观测降水和蒸发的年平均气候态空间分布及年循环特征,与观测值的空间相关系数分别为0.92和0.87。较之观测,MME高估了华北地区降水(0.55 mm d^(−1)),低估了华南沿海地区降水(−0.3 mm d^(−1))。所有CMIP6模式均高估蒸发强度(偏差0.03~0.98 mm d^(−1)),使得模拟的降水与蒸发之差偏少。模式整体能够模拟出我国东部季风区降水再循环率及不同边界水汽来源的贡献率,但低估了由南边界进入季风区的水汽贡献,导致东亚季风区偏干。通过分析模式对影响水汽通量的两个气象要素(风速和大气比湿)的模拟能力,发现研究区南边界的风速大小决定了模式间水汽输送差异。南边界风速越大的模式,由南边界进入的水汽通量越大,模式模拟的降水越多。西北太平洋辐合带的东西位置是影响南边界南风速的重要系统之一,辐合带位置偏东的模式模拟的南风强度较弱,使得水汽输送偏弱、降水偏少;反之,南边界水汽输送偏强、降水偏多。本文通过评估最新一代CMIP6模式在东亚水循环方面的模拟性能,指出了当前气候模式在模拟西太平洋辐合带位置方面存在的偏差及其对东亚水循环的影响。

Hydrologic Response to Future Climate Change in the Dulong-Irra-waddy River Basin Based on Coupled Model Intercomparison Project 6

Within the context of the Belt and Road Initiative(BRI)and the China-Myanmar Economic Corridor(CMEC),the Dulong-Ir-rawaddy(Ayeyarwady)River,an international river among China,India and Myanmar,plays a significant role as both a valuable hydro-power resource and an essential ecological passageway.However,the water resources and security exhibit a high degree of vulnerabil-ity to climate change impacts.This research evaluates climate impacts on the hydrology of the Dulong-Irrawaddy River Basin(DIRB)by using a physical-based hydrologic model.We crafted future climate scenarios using the three latest global climate models(GCMs)from Coupled Model Intercomparison Project 6(CMIP6)under two shared socioeconomic pathways(SSP2-4.5 and SSP5-8.5)for the near(2025-2049),mid(2050-2074),and far future(2075-2099).The regional model using MIKE SHE based on historical hydrologic processes was developed to further project future streamflow,demonstrating reliable performance in streamflow simulations with a val-idation Nash-Sutcliffe Efficiency(NSE)of 0.72.Results showed that climate change projections showed increases in the annual precip-itation and potential evapotranspiration(PET),with precipitation increasing by 11.3%and 26.1%,and PET increasing by 3.2%and 4.9%,respectively,by the end of the century under SSP2-4.5 and SSP5-8.5.These changes are projected to result in increased annual streamflow at all stations,notably at the basin’s outlet(Pyay station)compared to the baseline period(with an increase of 16.1%and 37.0%at the end of the 21st century under SSP2-4.5 and SSP5-8.5,respectively).Seasonal analysis for Pyay station forecasts an in-crease in dry-season streamflow by 31.3%-48.9%and 22.5%-76.3%under SSP2-4.5 and SSP5-8.5,respectively,and an increase in wet-season streamflow by 5.8%-12.6%and 2.8%-33.3%,respectively.Moreover,the magnitude and frequency of flood events are pre-dicted to escalate,potentially impacting hydropower production and food security significantly.This research outlines the hydrological response to future climate change during the 21st century and offers a scientific basis for the water resource management strategies by decision-makers.

CMIP 6 models simulation of the connection between North/South Pacific Meridional Mode and ENSO

The subtropical North and South Pacific Meridional Modes(NPMM and SPMM)are well known precursors of El Niño-Southern Oscillation(ENSO).However,relationship between them is not constant.In the early 1980,the relationship experienced an interdecadal transition.Changes in this connection can be attributed mainly to the phase change of the Pacific decadal oscillation(PDO).During the positive phase of PDO,a shallower thermocline in the central Pacific is responsible for the stronger trade wind charging(TWC)mechanism,which leads to a stronger equatorial subsurface temperature evolution.This dynamic process strengthens the connection between NPMM and ENSO.Associated with the negative phase of PDO,a shallower thermocline over southeastern Pacific allows an enhanced wind-evaporation-SST(WES)feedback,strengthening the connection between SPMM and ENSO.Using 35 Coupled Model Intercomparison Project Phase 6(CMIP6)models,we examined the NPMM/SPMM performance and its connection with ENSO in the historical runs.The great majority of CMIP6 models can reproduce the pattern of NPMM and SPMM well,but they reveal discrepant ENSO and NPMM/SPMM relationship.The intermodal uncertainty for the connection of NPMM-ENSO is due to different TWC mechanism.A stronger TWC mechanism will enhance NPMM forcing.For SPMM,few models can simulate a good relationship with ENSO.The intermodel spread in the relationship of SPMM and ENSO owing to SST bias in the southeastern Pacific,as WES feedback is stronger when the southeastern Pacific is warmer.

一种偏差校正方法在青藏高原夏季CMIP6降水数据订正中的应用评估

利用第六次国际耦合模式比较计划(CMIP6)中的18个模式,基于欧洲中期天气预报中心第五代再分析资料(ERA5)再分析数据对青藏高原夏季降水数据进行了偏差校正,并从平均降水和极端降水两方面评估了校正前后的CMIP6数据以及单个模式在1979-2014年的表现。研究结果表明,该校正方法高度依赖于用于偏差校正的ERA5再分析数据在研究区域的质量,尽管偏差校正后的青藏高原夏季平均降水的误差和误差率上有所改善,但在年际时间变化特征方面却不如偏差校正前的数据。大多数CMIP6模式能够较好地模拟1979-2014年青藏高原上由西北至东南逐渐递增的平均降水空间变化特征。偏差校正前的降水数据在高原上会出现显著的高估,误差率为60.4%,经过偏差校正后的数据相对观测数据误差降低,误差率为-13.9%,并且偏差校正后的数据与ERA5的平均误差仅为0.003 mm·d^(-1),与ERA5的空间相关性高达0.999。空间趋势方面,观测数据表明青藏高原大部分地区夏季降水在1979-2014年呈现轻微增加的趋势,只有东缘出现明显降低的趋势。偏差校正前后的数据都能够大致刻画出这一空间分布特征,然而,未经偏差校正的大多数单个CMIP6模式与ERA5的空间相关系数未超过0.5。与由独立观测降水数据的年际变化特征相比,偏差校正前的数据高估了高原上的降水量,而偏差校正后的数据相比观测结果则偏低。通过确定95%分位阈值选取了极端降水个例,其集合平均极端降水空间分布与年平均降水类似,也呈西北向东南递增的趋势。部分CMIP6模式较好地模拟了这一特征,如MRI-ESM2-0(The Meteorological Research Institute Earth System Model version 2.0)和ACCESSCM2(Australian Community Climate and Earth System Simulator Climate Model Version 2),与观测结果的空间相关系数分别为0.851和0.821。但偏差校正后的数据在空间相关性方面下降,由偏差校正前的0.861降为0.730,未能准确刻画高原极端降水阶梯式递增的特点。偏差校正后的极端降水数据误差分布与偏差校正前相似,偏低区域主要集中在高原南部腹地和东部。进一步的极端降水贡献率分析结果表明,观测结果与CMIP6降水数据均显示1979-2014年期间极端降水贡献率变化趋势不明显。单个CMIP6模式中,EC-Earth3-Veg(European Community Earth-Vegetation model version 3)和EC-Earth3(European Community Earth Model version 3)及CanESM5(The Canadian Earth System Model version 5)在多个统计评估指标上排名靠前,展示出较好的模拟能力;IPSL-CM6A-LR(Institut Pierre-Simon Laplace Climate Model 6A Low Resolution)在平均降水误差和极端降水的误差指标上表现出色。

未来北极夏季陆面气温变化区域特征及其与北大西洋海面温度的关系

未来变暖背景下北极气候变化特征研究具有重要意义,基于国际耦合模式比较计划第六阶段(Coupled Model Intercomparison Project Phase 6,CMIP6)中对北极气候变化模拟能力较好的模式模拟结果,研究SSP2-4.5情景下21世纪北极2 m气温的时空变化特征及其影响因素。结果表明:(1)极地陆地的欧亚大陆(Eurasia, EA)和北美-格陵兰(Greenland, GL)对全球变暖具有不同的响应。EA在21世纪中叶前变暖趋势显著,之后主要表现为年代际尺度的冷暖振荡;GL则始终保持增暖趋势。EA、GL分区气温均存在年际、年代际(10~20 a)尺度上的波动,GL分区还存在20~40 a的准周期变化。(2)前冬北大西洋涛动正位相会引起次年夏季北大西洋呈南北向“-、+、-”三极型海面温度异常,并通过影响大气环流导致EA分区气温正异常,这种影响主要体现在年代际尺度上。(3)北大西洋多年代际振荡为正异常时,北美至格陵兰位势高度偏高,GL分区增暖,并且这种影响在21世纪70年代后更重要;北太平洋北部的海面温度正异常对GL分区增温也有贡献。

Comparative Assessment of Impacts of Future Climate Change on Runoff in Upper Daqinghe Basin,China

Assessing runoff changes is of great importance especially its responses to the projected future climate change on local scale basins because such analyses are generally done on global and regional scales which may lead to generalized conclusions rather than specific ones.Climate change affected the runoff variation in the past in the upper Daqinghe Basin,however,the climate was mainly considered uncertain and still needs further studies,especially its future impacts on runoff for better water resources management and planning.Integrated with a set of climate simulations,a daily conceptual hydrological model(MIKE11-NAM)was applied to assess the impact of climate change on runoff conditions in the Daomaguan,Fuping and Zijingguan basins in the upper Daqinghe Basin.Historical hydrological data(2008–2017)were used to evaluate the applicability of the MIKE11-NAM model.After bias correction,future projected climate change and its impacts on runoff(2025–2054)were analysed and compared to the baseline period(1985–2014)under three shared social economic pathways(SSP1-2.6,SSP2-4.5,and SSP5-8.5)scenarios from Coupled Model Intercomparison Project Phase 6(CMIP6)simulations.The MIKE-11 NAM model was applicable in all three Basins,with both R^(2)and Nash-Sutcliffe Efficiency coefficients greater than 0.6 at daily scale for both calibration(2009–2011)and validation(2012–2017)periods,respectively.Although uncertainties remain,temperature and precipitation are projected to increase compared to the baseline where higher increases in precipitation and temperature are projected to occur under SSP2-4.5 and SSP5-8.5 scenarios,respectively in all the basins.Precipitation changes will range between 12%–19%whereas temperature change will be 2.0℃–2.5℃ under the SSP2-4.5 and SSP5-8.5 scenarios,respectively.In addition,higher warming is projected to occur in colder months than in warmer months.Overall,the runoff of these three basins is projected to respond to projected climate changes differently because runoff is projected to only increase in the Fuping basin under SSP2-4.5 whereas decreases in both Daomaguan and Zijingguan Basins under all scenarios.This study’s findings could be important when setting mitigation strategies for climate change and water resources management.

中国地球气候系统模式的发展及其模拟和预估

地球气候系统模式是开展多学科、多圈层集成研究的重要平台,其发展是国际地学领域特别是全球变化领域竞争的前沿。中国的地球气候系统模式研发工作始于20世纪80年代,最近10年得到快速发展。研发格局上已经形成中国科学院、有关部委和高校三足鼎立的局面。文中在简要回顾中国地球气候系统模式早期发展历史的基础上,总结了中国参加第6次耦合模式比较计划的9个地球气候系统模式的技术特点,初步评估了中国4个模式对全球和东亚气候模拟的基本性能,分析了其在4种共享社会经济路径情景下对全球降水与温度的预估变化及其与平衡态气候敏感度的联系。最后,结合国际态势,从发展的角度提出未来中国气候模式研发工作需要加强的8个方向。

基于CMIP6的中国主要地区极端气温/降水模拟能力评估及未来情景预估

基于耦合模式比较计划第6阶段(the coupled model intercomparison program in phase 6,CMIP6)的全球气候模式模拟数据,评估分析了全球气候模式对中国主要地区1979―2014年极端气温与极端降水特征的模拟能力。通过泰勒图及各种统计参数的对比表明,全球气候模式对中国各个地区的极端降水/气温的模拟存在显著差异,对中国地区极端气温与降水模拟最好的模式是EC-Earth3。对EC-Earth3在4种未来情景下的气温/降水数据进行偏差校正,并对中国地区未来时期极端降水/气温进行了预估。结果表明:2021—2100年,中国地区升温趋势和极端降水的增加趋势显著,4种情景下的极端气温指标和极端降水指标在21世纪50年代之前是相似的,但之后变化趋势差异增加;SSP1-2.6情景下,2021—2100年升温速率和极端降水速率增加趋势较为平缓;SSP2-4.5、SSP3-7.0、SSP5-8.53种情景下,升温速率和极端降水增加速率随着模式和时间增加逐渐增大。

乌江中上游地区极端降水特征及未来预估

选取乌江中上游地区1961-2019年的逐日降水数据,采用趋势分析、EOF、小波分析等方法,研究极端降水在时空上的变化特征,同时为了使研究具有整体性,利用CMIP6中5个GCMs下的3种情景数据(SSP126、SSP245、SSP585),在降尺度处理后预估未来(2020-2100年)极端降水的变化。结果表明:1961-2019年整个流域地区极端降水事件虽然有增有减,但无显著性变化;在变化周期上,信号强烈的周期主要在23~30 a的时间尺度上,且贯穿整个时序;5个极端降水指数的第1模态表明其在空间变化上具有一致性,第2模态则有差异;未来极端降水事件整体上随SSPs情景的升高而愈发显著,且多以正趋势为主。研究结果可为乌江流域地区水安全管控、规划建设、防灾减灾等提供参考。

基于CMIP 6多模式的长江流域气温、降水与径流预估

【目的】探究未来气候变化对长江流域径流的影响,为长江流域及其他地区的早期洪水预警和防御措施提供依据。【方法】采用国际耦合模式比较计划第6阶段(CMIP 6)多模式集合平均(MME),结合SWAT水文模型,对长江流域1961—2014年气温、降水量和径流等进行评估,并预估了长江流域2020—2099年SSP1-1.9、SSP1-2.6、SSP2-4.5、SSP3-7.0、SSP5-8.5排放情景下的气温、降水量和径流。【结果】(1)相比单一模式,MME历史时期模拟气温和降水效果更好,与观测值的相关系数均大于0.90,MME可以很好地模拟出气温和降水量的空间分布规律。(2)由MME分析可知,2020—2099年长江流域在所有情景下的气温增幅低于50%,降水增量小于20%,在SSP5-8.5情景下模拟的温度值比SSP1-1.9时的温度值高1.23℃,比SSP1-2.6时的温度值高0.99℃。(3)总体上,长江流域未来的年均径流量增加显著,到21世纪末,SSP5-5.8情景下年均径流量将达到40380 m^(3)/s。【结论】本研究揭示了长江流域径流变化趋势与气温与降水之间的相互关系,以及模拟未来气候变化的准确度,同时认为未来长江流域气温、降水量与径流量均呈上升趋势,低碳排放情景下的洪涝灾害相对较少,可为以后长江流域洪涝灾害的预估和区域性气候变化提供依据。

Development of Climate and Earth System Models in China: Past Achievements and New CMIP6 Results

The Earth–Climate System Model(ECSM)is an important platform for multi-disciplinary and multi-sphere integration research,and its development is at the frontier of international geosciences,especially in the field of global change.The research and development(R&D)of ECSM in China began in the 1980 s and have achieved great progress.In China,ECSMs are now mainly developed at the Chinese Academy of Sciences,ministries,and universities.Following a brief review of the development history of Chinese ECSMs,this paper summarized the technical characteristics of nine Chinese ECSMs participating in the Coupled Model Intercomparison Project Phase 6 and preliminarily assessed the basic performances of four Chinese models in simulating the global climate and the climate in East Asia.The projected changes of global precipitation and surface air temperature and the associated relationship with the equilibrium climate sensitivity under four shared socioeconomic path scenarios were also discussed.Finally,combined with the international situation,from the perspective of further improvement,eight directions were proposed for the future development of Chinese ECSMs.

共享社会经济路径(SSPs)下未来30年长江流域夏季降水预估

本文利用国际耦合模式比较计划第六阶段(CMIP6)中22个全球气候模式试验数据,通过MR评分(Comprehensive Rating Metrics)方法进行评估,择优选取了GFDL-CM4、EC-Earth3、MIROC6等10个模式,使用这些模式在SSP245和SSP585两种共享社会经济路径(SSPs)下的预估试验数据,对未来30年(2021~2050年)长江流域夏季降水,特别是极端降水事件的时空演变特征,进行了预估。结果显示,两种不同排放情景下,相对于1980~2010年的平均,流域总降水量(PRCPTOT)、降水强度(SDII)均呈现显著增加趋势,特别是在上游高原和中下游平原地区;降水频次(R1mm)上游减少,中下游增加,二者相抵导致了流域降水频次整体变化不明显;强降水量(R95p)增加约9.6%、16.5%(SSP245、SSP585),极端降水量(R99p)增加约10.2%、15.5%,最大5日降水量(RX5day)也为增长趋势;连续无降水日数(CDD)在整个流域呈现增多,特别是上游地区。两种排放情景相比较,高情景(SSP585)下的变化强度要比中低情景(SSP245)大。这些结果意味着,未来30年长江流域降水气候可能变得更为极端,不仅总降水量增加,暴雨、大暴雨易于出现,而且连续无雨日长度也增加,干旱变得更为频繁。特别地,中下游地区要警惕发生极端暴雨、极端洪涝的风险,而上游地区要警惕发生干旱、极端干旱的风险。

Climate Sensitivity and Feedbacks of BCC-CSM to Idealized CO2 Forcing from CMIP5 to CMIP6

Climate sensitivity represents the response of climate system to doubled CO2 concentration relative to the preindustrial level, which is one of the sources of uncertainty in climate projections. It is unclear how the climate sensitivity and feedbacks will change as a model system is upgraded from the Coupled Model Intercomparison Project Phase 5(CMIP5) to CMIP6. In this paper, we address this issue by comparing two versions of the Beijing Climate Center Climate System Model(BCC-CSM) participating in CMIP6 and CMIP5, i.e., BCC-CSM2-MR and BCC-CSM1.1 m,which have the same horizontal resolution but different physical parameterizations. The results show that the equilibrium climate sensitivity(ECS) of BCC-CSM slightly increases from CMIP5(2.94 K) to CMIP6(3.04 K). The small changes in the ECS result from compensation between decreased effective radiative forcing(ERF) and the increased net feedback. In contrast, the transient climate response(TCR) evidently decreases from 2.19 to 1.40 K, nearly the lower bound of the CMIP6 multimodel spread. The low TCR in BCC-CSM2-MR is mainly caused by the small ERF overly even though the ocean heat uptake(OHU) efficiency is substantially improved from that in BCC-CSM1.1 m.Cloud shortwave feedback(λSWCL) is found to be the major cause of the increased net feedback in BCC-CSM2-MR,mainly over the Southern Ocean. The strong positive λSWCL in BCC-CSM2-MR is coincidently related to the weakened sea ice-albedo feedback in the same region. This result is caused by reduced sea ice coverage simulated during the preindustrial cold season, which leads to reduced melting per 1-K global warming. As a result, in BCCCSM2-MR, reduced surface heat flux and strengthened static stability of the planetary boundary layer cause a decrease in low-level clouds and an increase in incident shortwave radiation. This study reveals the important compensation between λSWCL and sea ice-albedo feedback in the Southern Ocean.

Uncertainty in Projection of Climate Extremes:A Comparison of CMIP5 and CMIP6

Climate projections by global climate models(GCMs)are subject to considerable and multi-source uncertainties.This study aims to compare the uncertainty in projection of precipitation and temperature extremes between Coupled Model Intercomparison Project(CMIP)phase 5(CMIP5)and phase 6(CMIP6),using 24 GCMs forced by 3 emission scenarios in each phase of CMIP.In this study,the total uncertainty(T)of climate projections is decomposed into the greenhouse gas emission scenario uncertainty(S,mean inter-scenario variance of the signals over all the models),GCM uncertainty(M,mean inter-model variance of signals over all emission scenarios),and internal climate variability uncertainty(V,variance in noises over all models,emission scenarios,and projection lead times);namely,T=S+M+V.The results of analysis demonstrate that the magnitudes of S,M,and T present similarly increasing trends over the 21 st century.The magnitudes of S,M,V,and T in CMIP6 are 0.94-0.96,1.38-2.07,1.04-1.69,and 1.20-1.93 times as high as those in CMIP5.Both CMIP5 and CMIP6 exhibit similar spatial variation patterns of uncertainties and similar ranks of contributions from different sources of uncertainties.The uncertainty for precipitation is lower in midlatitudes and parts of the equatorial region,but higher in low latitudes and the polar region.The uncertainty for temperature is higher over land areas than oceans,and higher in the Northern Hemisphere than the Southern Hemisphere.For precipitation,T is mainly determined by M and V in the early 21 st century,by M and S at the end of the 21 st century;and the turning point will appear in the 2070 s.For temperature,T is dominated by M in the early 21 st century,and by S at the end of the 21 st century,with the turning point occuring in the 2060 s.The relative contributions of S to T in CMIP6(12.5%-14.3%for precipitation and 31.6%-36.2%for temperature)are lower than those in CMIP5(15.1%-17.5%for precipitation and 38.6%-43.8%for temperature).By contrast,the relative contributions of M in CMIP6(50.6%-59.8%for precipitation and 59.4%-60.3%for temperature)are higher than those in CMIP5(47.5%-57.9%for precipitation and 51.7%-53.6%for temperature).The higher magnitude and relative contributions of M in CMIP6 indicate larger difference among projections of various GCMs.Therefore,more GCMs are needed to ensure the robustness of climate projections.

Comparison of trends in the Hadley circulation between CMIP6 and CMIP5

There have been extensive studies on poleward expansion of the Hadley cells and the associated poleward shift of subtropical dry zones in the past decade.In the present study,we study the trends in the width and strength of the Hadley cells,using currently available simulation results of the Coupled Model Intercomparison Project Phase-6(CMIP6),and compare the trends with that in CMIP5 simulations.Our results show that the total annual-mean trend in the width of the Hadley cells is 0.13°±0.02°per decade over 1970–2014 in CMIP6 historical All-forcing simulations.It is almost the same as that in CMIP5.The trend in the strength of the Northern-Hemisphere(NH)cell shows much greater weakening in CMIP6 than in CMIP5,while the strength trend in the Southern-Hemisphere(SH)cell shows slight strengthening.Single-forcing simulations demonstrate that increasing greenhouse gases cause widening and weakening of both the NH and SH Hadley cells,while anthropogenic aerosols and stratospheric ozone changes cause weak strengthening trends in the SH cell.CMIP6 projection simulation results show that both the widening and weakening trends increase with radiative forcing.

Present-Day PM_(2.5)over Asia:Simulation and Uncertainty in CMIP6 ESMs

This study assesses the ability of 10 Earth System Models(ESMs)that participated in the Coupled Model Intercomparison Project Phase 6(CMIP6)to reproduce the present-day inhalable particles with diameters less than 2.5 micrometers(PM_(2.5))over Asia and discusses the uncertainty.PM_(2.5)accounts for more than 30%of the surface total aerosol(fine and coarse)concentration over Asia,except for central Asia.The simulated spatial distributions of PM_(2.5)and its components,averaged from 2005 to 2020,are consistent with the Modern-Era Retrospective Analysis for Research and Applications version 2(MERRA-2)reanalysis.They are characterized by the high PM_(2.5)concentrations in eastern China and northern India where anthropogenic components such as sulfate and organic aerosol dominate,and in northwestern China where the mineral dust in PM_(2.5)fine particles(PM_(2.5)DU)dominates.The present-day multimodel mean(MME)PM_(2.5)concentrations slightly underestimate ground-based observations in the same period of 2014–2019,although observations are affected by the limited coverage of observation sites and the urban areas.Those model biases partly come from other aerosols(such as nitrate and ammonium)not involved in our analyses,and also are contributed by large uncertainty in PM_(2.5)simulations on local scale among ESMs.The model uncertainties over East Asia are mainly attributed to sulfate and PM_(2.5)DU;over South Asia,they are attributed to sulfate,organic aerosol,and PM_(2.5)DU;over Southeast Asia,they are attributed to sea salt in PM_(2.5)fine particles(PM_(2.5)SS);and over central Asia,they are attributed to PM_(2.5)DU.They are mainly caused by the different representations of aerosols within individual ESMs including the representation of aerosol size distributions,dynamic transport,and physical and chemistry mechanisms.

Variation of Dust in Northern China and Its Reproduction in BCC-ESM1 since 1980

In this paper,we explore the possible causes and mechanisms for the variation of dust in northern China from 1980to 2014 using the Modern-Era Retrospective analysis for Research and Applications version 2(MERRA-2)data,observational data,and BCC-ESM1(Beijing Climate Center Earth System Model version 1)simulation data.Two important dust centers are identified in China:one in the Taklamakan Desert in southern Xinjiang Region and the other in the Badain Jaran Desert in western Inner Mongolia Plateau.Both centers display distinct seasonal variations,with high dust concentration in spring and summer and low in autumn and winter.BCC-ESM1 is able to generally capture the main spatial and temporal characteristics of dust in northern China.Both the MERRA-2 reanalysis data and BCCESM1 simulation data show a decreasing trend in spring dust,which is evident during 1980–2000 and 2001–2014.The analysis based on daily mean dust loads and wind fields from MERRA-2 and BCC-ESM1 indicates that dusty weather in North China may be mainly caused by transport of the dust,especially that from the central and western Inner Mongolia Plateau during the prior 0–2 days,through the westerly winds from the upstream“dust core”region(38°–45°N,90°–105°E).This is one of the important paths for dust to move into North China.The weakened westerly wind in the lower troposphere in this“dust core”region may be responsible for the reduction of spring dust in North China.