Differences between CMIP6 and CMIP5 Models in Simulating Climate over China and the East Asian Monsoon

We compare the ability of coupled global climate models from the phases 5 and 6 of the Coupled Model Intercomparison Project(CMIP5 and CMIP6, respectively) in simulating the temperature and precipitation climatology and interannual variability over China for the period 1961–2005 and the climatological East Asian monsoon for the period1979–2005. All 92 models are able to simulate the geographical distribution of the above variables reasonably well.Compared with earlier CMIP5 models, current CMIP6 models have nationally weaker cold biases, a similar nationwide overestimation of precipitation and a weaker underestimation of the southeast–northwest precipitation gradient, a comparable overestimation of the spatial variability of the interannual variability, and a similar underestimation of the strength of winter monsoon over northern Asia. Pairwise comparison indicates that models have improved from CMIP5 to CMIP6 for climatological temperature and precipitation and winter monsoon but display little improvement for the interannual temperature and precipitation variability and summer monsoon. The ability of models relates to their horizontal resolutions in certain aspects. Both the multi-model arithmetic mean and median display similar skills and outperform most of the individual models in all considered aspects.     

2018: The Hottest Summer in China and Possible Causes

In 2018, China experienced the hottest summer since 1961. The maximum, mean, and minimum temperatures all reached the highest. Air temperatures in most regions were much higher than normal; in northern China especially,the temperature anomalies were above double of the standard deviations. Consistent variations of temperature anomalies appeared in the national mean and in northern China on different timescales from intraseasonal to annual, indicating that the above normal temperature in northern China contributed significantly to the record-breaking hot summer of entire China. Relationships among the high temperature in summer 2018, the tropospheric circulation, and the global sea surface temperatures(SSTs) are further analyzed. It is found that the intensified and more northward western Pacific subtropical high(WPSH), weakened Northeast China cold vortex(NECV), and positive geopotential height anomaly from northern China to the Sea of Japan resulted in the abnormally high temperature in summer 2018. From late July to mid August, the WPSH was stronger than normal, with its ridge line jumping to north of 40°N; meanwhile, the NECV was much weaker and more northward than normal; both of the two systems led to the persistent high temperature in northern China during this period. In addition, the SSTs in Kuroshio and its extension area(K–KE) in summer 2018 were also the highest since 1961 and the greatest positive SST anomaly in K–KE was favorable for the above normal geopotential height over North China–Northeast China–Japan at 500 hPa, giving rise to the exceptionally high temperature in northern China.     

Model Projections of East Asian Summer Climate under the ‘Free Arctic’ Scenario

This paper addresses the ‘ice-free Arctic’ issue under the future global warming scenario. Four coupled climate models used in the third phase of the Coupled Model Intercomparison Project (CMIP3) were selected to project summer climate conditions over East Asia once the Arctic becomes ice-free. The models project that an ice-free Arctic summer will begin in the 2060s under the SRESA1B (according to IPCC Special Reports on Emissions Scenarios) simulations. Our results show that the East Asian summer monsoons will tend to be stronger and that the water vapor transport to central northern China will be strengthened, leading to increased summer precipitation in central northern China. The models also project an intensified Antarctic Oscillation, a condition which favors increased precipitation in South China’s Yangtze River Valley. The overall precipitation in Northwest China is projected to increase under ice-free Arctic summer conditions.     

Arctic Climate Changes Based on Historical Simulations(1900-2013) with the CAMS-CSM

The Chinese Academy of Meteorological Sciences Climate System Model(CAMS-CSM) is a newly developed global climate model that will participate in the Coupled Model Intercomparison Project phase 6. Based on historical simulations(1900-2013), we evaluate the model performance in simulating the observed characteristics of the Arctic climate system, which includes air temperature, precipitation, the Arctic Oscillation(AO), ocean temperature/salinity,the Atlantic meridional overturning circulation(AMOC), snow cover, and sea ice. The model-data comparisons indicate that the CAMS-CSM reproduces spatial patterns of climatological mean air temperature over the Arctic(60°-90°N) and a rapid warming trend from 1979 to 2013. However, the warming trend is overestimated south of the Arctic Circle, implying a subdued Arctic amplification. The distribution of climatological precipitation in the Arctic is broadly captured in the model, whereas it shows limited skills in depicting the overall increasing trend. The AO can be reproduced by the CAMS-CSM in terms of reasonable patterns and variability. Regarding the ocean simulation, the model underestimates the AMOC and zonally averaged ocean temperatures and salinity above a depth of 500 m, and it fails to reproduce the observed increasing trend in the upper ocean heat content in the Arctic. The largescale distribution of the snow cover extent(SCE) in the Northern Hemisphere and the overall decreasing trend in the spring SCE are captured by the CAMS-CSM, while the biased magnitudes exist. Due to the underestimation of the AMOC and the poor quantification of air–sea interaction, the CAMS-CSM overestimates regional sea ice and underestimates the observed decreasing trend in Arctic sea–ice area in September. Overall, the CAMS-CSM reproduces a climatological distribution of the Arctic climate system and general trends from 1979 to 2013 compared with the observations, but it shows limited skills in modeling local trends and interannual variability.     

The 2009 Summer Low Temperature in Northeast China and Its Association with Prophase Changes of the Air-Sea System

Under the background of global warming, summer (JJA) low temperature events in Northeast China had not occurred for about 15 yr since 1994, but one such event took place in 2009. By using the NCEP/NCAR reanalysis data, the 100-yr station temperature data at Harbin and Changchun, and the Hadley Center sea surface temperature (SST) data, this paper intends to reveal the cause, circulation background, and influencing factors of this event. Analysis of both horizontal and vertical circulations of a low-value system over Northeast China in summer 2009 during the low temperature event shows that anomalous activities of the Northeast China cold vortex (NECV) played the most direct role. A decadal cooling trend of-0.8 C (10 yr)-1 over 1999-2008 at Changchun and Harbin was found, which is obviously out-of-phase with the linear warming trend (0.2 C (10 yr)-1 ) over 1961-2000 for Northeast China in response to the global warming. The previous winter North Pacific polar vortex (NPPV) area index, significantly positively related to the observed summer temperatures of Harbin and Changchun, was also in a significantly declining tendency. These provide favorable decadal backgrounds for the 2009 low temperature event. Different from the average anomaly field of 500-hPa height for summer 1994-2008 in Northeast China, in the summer of 2009, the Arctic Oscillation (AO) showed a strong negative phase distribution, and significant negative height anomalies dominated Northeast Asia, Aleutian Islands, and North Atlantic. Furthermore, the negative phase of North Pacific Oscillation (NPO) in the winter of 2008 was obviously strong, and it maintained in the spring of 2009. Meanwhile, the SSTA in the equatorial eastern-central Pacific Ocean in the winter of 2008 showed a La Nina phase, but the strength of the La Nina weakened obviously in the spring of 2009. The abnormally strong activities of NECV in June and July of 2009 were related to the disturbances of stationary waves that replaced the original ultra-long waves over the North Pacific region in April and May 2009. The singular value decomposition (SVD) and harmonic analysis results suggest that the anomalous phase of NPO is an important precursor for summer temperature variations over Northeast China, and also a stable planetary-scale component that can be extracted from the atmospheric circulation in addition to the chaotic components on the synoptic scale.     

The Role of Changes in the Annual Cycle in Earlier Onset of Climatic Spring in Northern China

Climatic changes in the onset of spring in northern China associated with changes in the annual cycle and with a recent warming trend were quantified using a recently developed adaptive data analysis tool, the Ensemble Empirical Mode Decomposition. The study was based on a homogenized daily surface air temperature (SAT) dataset for the period 1955–2003. The annual cycle here is referred to as a refined modulated annual cycle (MAC). The results show that spring at Beijing has arrived significantly earlier by about 2.98 d (10 yr)-1, of which about 1.85 d (10 yr)-1 is due to changes in the annual cycle and 1.13 d (10 yr)-1 due to the long-term warming trend. Variations in the MAC component explain about 92.5% of the total variance in the Beijing daily SAT series and could cause as much as a 20-day shift in the onset of spring from one year to another. The onset of spring has been advancing all over northern China, but more significant in the east than in the west part of the region. These differences are somehow unexplainable by the zonal pattern of the warming trend over the whole region, but can be explained by opposite changes in the spring phase of the MAC, i.e. advancing in the east while delaying in the west. In the east of northern China, the change in the spring phase of MAC explains 40%–60% of the spring onset trend and is attributable to a weakening Asian winter monsoon. The average sea level pressure in Siberia (55°–80°N, 50°–110°E), an index of the strength of the winter monsoon, could serve as a potential short-term predictor for the onset of spring in the east of northern China.     

Trends of Regional Precipitation and Their Control Mechanisms during 1979–2013

Trends in precipitation are critical to water resources. Considerable uncertainty remains concerning the trends of regional precipitation in response to global warming and their controlling mechanisms. Here, we use an interannual difference method to derive trends of regional precipitation from GPCP(Global Precipitation Climatology Project) data and MERRA(ModernEra Retrospective Analysis for Research and Applications) reanalysis in the near-global domain of 60?S–60?N during a major global warming period of 1979–2013. We find that trends of regional annual precipitation are primarily driven by changes in the top 30% heavy precipitation events, which in turn are controlled by changes in precipitable water in response to global warming, i.e., by thermodynamic processes. Significant drying trends are found in most parts of the U.S. and eastern Canada,the Middle East, and eastern South America, while significant increases in precipitation occur in northern Australia, southern Africa, western India and western China. In addition, as the climate warms there are extensive enhancements and expansions of the three major tropical precipitation centers–the Maritime Continent, Central America, and tropical Africa–leading to the observed widening of Hadley cells and a significant strengthening of the global hydrological cycle.     

Changes in Seasonal Cycle and Extremes in China during the Period 1960–2008

Recent trends in seasonal cycles in China are analyzed, based on a homogenized dataset of daily temperatures at 541 stations during the period 1960–2008. Several indices are defined for describing the key features of a seasonal cycle, including local winter/summer (LW/LS) periods and local spring/autumn phase (LSP/LAP). The Ensemble Empirical Mode Decomposition method is applied to determine the indices for each year. The LW period was found to have shortened by 2–6 d (10 yr)-1, mainly due to an earlier end to winter conditions, with the LW mean temperature having increased by 0.2°C–0.4°C (10 yr)?1, over almost all of China. Records of the most severe climate extremes changed less than more typical winter conditions did. The LS period was found to have lengthened by 2–4 d (10 yr)?1, due to progressively earlier onsets and delayed end dates of the locally defined hot period. The LS mean temperature increased by 0.1°C–0.2°C (10 yr)-1 in most of China, except for a region in southern China centered on the mid-lower reaches of the Yangtze River. In contrast to the winter cases, the warming trend in summer was more prominent in the most extreme records than in those of more typical summer conditions. The LSP was found to have advanced significantly by about 2 d (10 yr)-1 in most of China. Changes in the autumn phase were less prominent. Relatively rapid changes happened in the 1980s for most of the regional mean indices dealing with winter and in the 1990s for those dealing with summer.     

Changing Spring Phenology Dates in the Three-Rivers Headwater Region of the Tibetan Plateau during 1960–2013

The variation of the vegetation growing season in the Three-Rivers Headwater Region of the Tibetan Plateau has recently become a controversial topic. One issue is that the estimated local trend in the start of the vegetation growing season(SOS)based on remote sensing data is easily affected by outliers because this data series is short. In this study, we determine that the spring minimum temperature is the most influential factor for SOS. The significant negative linear relationship between the two variables in the region is evaluated using Moderate Resolution Imaging Spectroradiometer–Normalized Difference Vegetation Index data for 2000–13. We then reconstruct the SOS time series based on the temperature data for 1960–2013.The regional mean SOS shows an advancing trend of 1.42 d(10 yr)~(-1) during 1960–2013, with the SOS occurring on the 160th and 151st days in 1960 and 2013, respectively. The advancing trend enhances to 6.04 d(10 yr)~(-1) during the past 14 years. The spatiotemporal variations of the reconstructed SOS data are similar to those deduced from remote sensing data during the past 14 years. The latter exhibit an even larger regional mean trend of SOS [7.98 d(10 yr~(-1) )] during 2000–13. The Arctic Oscillation is found to have significantly influenced the changing SOS, especially for the eastern part of the region,during 2000–13.     

Dynamical Downscaling of the Twentieth Century Reanalysis for China:Climatic Means during 1981–2010

This study presents a dynamically downscaled climatology over East Asia using the non-hydrostatic Weather Research and Forecasting(WRF) model, forced by the Twentieth Century Reanalysis(20CR-v2). The whole experiment is a 111-year(1900–2010) continuous run at 50 km horizontal resolution. Comparisons of climatic means and seasonal cycles among observations, 20CR-v2, and WRF results during the last 30 years(1981–2010) in China are presented, with a focus on surface air temperature and precipitation in both summer and winter. The WRF results reproduce the main features of surface air temperature in the two seasons in China, and outperform 20CR-v2 in regional details due to topographic forcing. Summer surface air temperature biases are reduced by as much as 1°C–2°C. For precipitation, the simulation results reproduce the decreasing pattern from Southeast to Northwest China in winter. For summer rainfall, the WRF simulation results reproduce the correct magnitude and position of heavy rainfall around the southeastern coastal area, and are better than 20CR-v2. One of the significant improvements is that an unrealistic center of summer precipitation in Southeast China present in 20CR-v2 is eliminated. However, the simulated results underestimate winter surface air temperature in northern China and winter rainfall in some regions in southeastern China. The mean seasonal cycles of surface air temperature and precipitation are captured well over most of sub-regions by the WRF model.     

A 38-Year Climatology of Explosive Cyclones over the Northern Hemisphere

Explosive cyclones(ECs)over two basins in the Northern Hemisphere(20°-90°N)from January 1979 to December2016 are investigated using ERA-Interim and Optimum Interpolation Sea Surface Temperature(OISST)data.The classical definition of an EC is modified considering not only the rapid drop of the central sea level pressure of the cyclone,but also the strong wind speed at the height of 10 m in which maximum wind speeds greater than 17.2 m s^-1are included.According to the locations of the northern Atlantic and northern Pacific,the whole Northern Hemisphere is divided into the"A region"(20°-90°N,90°W-90°E)and"P region"(20°-90°N,90°E-90°W).Over both the A and P regions,the climatological features of ECs,such as their spatial distribution,intensity,seasonal variation,interannual variation,and moving tracks,are documented.     

Towards More Snow Days in Summer since 2001 at the Great Wall Station,Antarctic Peninsula:The Role of the Amundsen Sea Low

The variation in the precipitation phase in polar regions represents an important indicator of climate change and variability.We studied the precipitation phase at the Great Wall Station and Antarctic Peninsula(AP)region,based on daily precipitation,synoptic records and ERA-Interim data during the austral summers of 1985?2014.Overall,there was no trend in the total precipitation amount or days,but the phase of summer precipitation(rainfall days versus snowfall days)showed opposite trends before and after 2001 at the AP.The total summer rain days/snow days increased/decreased during 1985?2001 and significantly decreased at a rate of?14.13 d(10 yr)?1/increased at a rate of 14.31 d(10 yr)?1 during 2001?2014,agreeing well with corresponding variations in the surface air temperature.Further,we found that the longitudinal location of the Amundsen Sea low(ASL)should account for the change in the precipitation phase since 2001,as it has shown a westward drift after 2001[?41.1°(10 yr)?1],leading to stronger cold southerly winds,colder water vapor flux,and more snow over the AP region during summertime.This study points out a supplementary factor for the climate variation on the AP.     

Impact of Urbanization on Low-Temperature Precipitation in Beijing during 1960–2008简

Daily precipitation and temperature records at 13 stations for the period 1960-2008 were analyzed to identify climatic change and possible effects of urbanization on low-temperature precipitation [LTP, precipitation of ≥ 0.1 mm d^-1 occurring under a daily minimum temperature （Tmin） of ≤ 0℃] in the greater Beijing region （B JR）, where a rapid process of urbaniza tion has taken place over the last few decades. The paper provides a climatological overview of LTP in B JR. LTP contributes 61.7% to the total amount of precipitation in B JR in the cold season （November-March）. There is a slight increasing trend [1.22 mm （10 yr）^-1] in the amount of total precipitation for the cold season during 1960-2008. In contrast, the amount of LTP decreases by 0.6 mm （10 yr）^-1. The warming rate of Train in B JR is 0.66℃ （10 yr）^-1. Correspondingly, the frequency of LTP decreases with increasing Tmin by -0.67 times per ℃. The seasonal frequency and amount of LTP in southeast B JR （mostly urban sites） are 17%-20% less than those in the northwestern （rural and montane sites）. The intensity of LTP for the urban sites and northeastern B JR exhibited significant enhancing trends [0.18 and 0.15 mm d^- 1 （10 yr）^- 1, respectively]. The frequency of slight LTP （〈0.2 mm d^-1） significantly decreased throughout B JR [by about -15.74% （10 yr）^-1 in the urban area and northeast B JR], while the contribution of the two heaviest LTP events to total LTP amount significantly increased by 3.2% （10 yr） ^-1.     

Extreme Temperature Change of the Last 110 Years in Changchun,Northeast China

In China and East Asia,the long-term continuous observational data at daily resolution are insufficient,and thus there is a lack of good understanding of the extreme climate variation over the last 100 years plus.In this study,the extreme temperature indices defined by ETCCDI(Expert Team on Climate Change Detection and Indices)and local meteorological administrations were analyzed for Changchun City,Northeast China,by using the daily maximum temperature(Tmax)and daily minimum temperature(Tmin)over 1909?2018.The results showed that extreme cold events,such as cold days,cold nights,frost days,icing days,and low temperature days,decreased significantly at rates of?0.41 d(10 yr)^?1,?1.45 d(10 yr)^?1,?2.28 d(10 yr)^?1,?1.16 d(10 yr)?1 and?1.90 d(10 yr)^?1,respectively.Warm nights increased significantly at a rate of 1.71 d(10 yr)^?1,but warm days decreased slightly and the number of high temperature days decreased at a rate of?0.20 d(10 yr)?1.The frequency of cold surge events increased significantly at a rate of 0.25 d(10 yr)^?1,occurring mainly from the mid-1950s to late-1980s.The average Tmax,average Tmin and extreme Tmin increased at rates of 0.09℃(10 yr)^?1,0.36℃(10 yr)^?1 and 0.54℃(10 yr)^?1,respectively;and extreme Tmax decreased significantly at a rate of?0.17℃(10 yr)^?1.In 1909?2018,1951?2018 and 1979?2018,the indices related to cold events decreased,while the trends of the indices related to warm events were different for different periods.     

青藏高原西侧地区冬季降水的年际变率及其影响因子

基于全球降水气候中心（GPCC）和全球降水气候计划（GPCP）的降水数据及ERA-interim再分析资料,分析了1979~2012年冬季青藏高原（简称高原）西侧地区降水的基本特征及影响其年际变率的潜在因子。结果表明高原冬季降水主要发生在其西侧地区且为全区变化一致型,降水所需的水汽主要来自上游地区,从该区域的西边界输入。然而,高原西侧地区冬季降水的年际变率主要由水汽输送的动力过程所决定,表现为高原西侧的西南风异常。此外,高原西侧冬季降水的年际变率与其上游典型的大气内部变率北大西洋涛动和北极涛动相关性不强,而与赤道西印度洋和热带中东太平洋的海温显著相关。热带中东太平洋海温异常通过影响大气环流变化,在印度洋北部激发一个反气旋式的环流异常,使得高原西侧地区出现异常西南风,从而加强了水汽通量输送的动力作用。同时在赤道异常东风的作用下,暖水也向印度洋西部输送堆积。赤道中东太平洋海温的异常可进一步导致西风急流发生南北移动,从而也在一定程度上影响了高原西侧冬季水汽输送以及降水的年际变率。     

青藏高原西部春季降水年代际变化趋势

利用ERA-Interim和APHRO_MA资料分析了1979～2007年间青藏高原西部春季（3～5月）降水的年代际变化趋势及可能原因。结果表明,青藏高原西部局部区域春季降水呈显著减少趋势,降水的变化趋势与其西南部辐合上升运动及阿拉伯海北部水汽含量变化存在联系。发现研究区春季降水增加时伴随其西南部显著的辐合上升异常,同时高层（500 hPa）位势高度场负异常中心与环流的气旋式正异常中心一致,而低层（850 hPa）的辐合上升异常相对较弱;研究区春季降水增加同时伴随阿拉伯海北部至研究区西南部的高、低层比湿正异常,其中低层的比湿正异常更为显著,其正异常中心均位于阿拉伯海北部。上述区域的水汽输送能解释研究区春季降水55.3%的变化,同时两者的变化趋势具有很好相关性。研究显示1979～2007年间研究区春季降水呈下降趋势主要是由阿拉伯海北部低层向印度次大陆水汽输送减少,以及研究区南部高层辐合上升运动减弱造成的。青藏高原西部春季降水变化趋势和相关区域水汽变化的一致性,可以为分析高原气候变化提供依据。     

中国冬季最低气温观测与再分析资料的比较

运用EOF分解、线性趋势分析、方差分析、小波分析、相关分析等方法,从均值、长期变化、年际和年代际变化特征等方面对NCEP/NCAR(N1T)、NCEP/DOE(N2T)、ERA-Interim(ET)、JRA-25(JT)4种再分析资料集和194站观测资料集中1979—2012年我国冬季最低气温(WTMIN)进行比较。结果表明:(1)ET在再现观测冬季最低气温多年平均时最优;(2)除新疆北部和西南部以及东北北部有降低趋势外,我国大部分地区WTMIN有升高的趋势,N1T对观测温度长期变化趋势再现能力优于其他3种资料;(3)观测WTMIN在中国北方和高原地区年际和年代际变化大于南方地区,四种再分析WTMIN对我国冬季最低气温的年际和年代际变化的再现在不同区域各有所长,总体上以ET为优。(4)再分析资料在再现观测WTMIN年代际时间尺度上的能力相当,以ET最优,尤其是在我国中东部地区和新疆北部地区。N1T和N2T对东北地区观测冬季最低气温年际变化的再现能力较好,ET再现观测WTMIN在1990s之前,2005年之后的3~4 a变化周期特征时能力略优于其他3种资料。     

Performance of RegCM4 over Major River Basins in China

A long-term simulation for the period 1990–2010 is conducted with the latest version of the International Centre for Theoretical Physics' Regional Climate Model(RegCM4), driven by ERA-Interim boundary conditions at a grid spacing of 25 km. The Community Land Model(CLM) is used to describe land surface processes, with updates in the surface parameters,including the land cover and surface emissivity. The simulation is compared against observations to evaluate the model performance in reproducing the present day climatology and interannual variability over the 10 main river basins in China,with focus on surface air temperature and precipitation. Temperature and precipitation from the ERA-Interim reanalysis are also considered in the model assessment. Results show that the model reproduces the present day climatology over China and its main river basins, with better performances in June–July–August compared to December–January–February(DJF).In DJF, we find a warm bias at high latitudes, underestimated precipitation in the south, and overestimated precipitation in the north. The model in general captures the observed interannual variability, with greater skill for temperature. We also find an underestimation of heavy precipitation events in eastern China, and an underestimation of consecutive dry days in northern China and the Tibetan Plateau. Similar biases for both mean climatology and extremes are found in the ERA-Interim reanalysis, indicating the difficulties for climate models in simulating extreme monsoon climate events over East Asia.     

夏、秋季北极海冰异常与秋、冬季青藏高原极端低温日数的关系及影响机理

利用1979~2012年青藏高原125个基本、基准站观测日最高及最低气温数据、Hadley中心月平均海冰覆盖率资料、ERA-Interim的风场、高度场等再分析资料,根据相关统计分析、合成分析等方法系统地分析了青藏高原地区秋、冬季冷昼和冷夜日数(低温日数)与关键影响海区海冰的关系及影响机理。结果表明,夏、秋季关键海区海冰偏少时,秋、冬季极地和青藏高原地区500 h Pa位势高度减小,中高纬西伯利亚地区位势高度增强,北极至青藏高原有明显由北向南波动通量,高压反气旋系统在西伯利亚地区形成与壮大,青藏高原以北风场呈现明显偏北风,Rossby波在青藏高原及其以北地区呈现由北向南波动形式,青藏高原以北的西风带地区Rossby波东传减缓,导致经向活动加强,北部冷空气易于通过气流向高原侵袭,秋、冬季青藏高原低温日数将偏多。