Dryland expansion in northern China from 1948 to 2008

This study examines the expansion of drylands and regional climate change in northern China by analyzing the variations in aridity index(AI), surface air temperature(SAT), precipitation and potential evapotranspiration(PET) from 1948 to 2008.It is found that the drylands of northern China have expanded remarkably in the last 61 years. The area of drylands of the last 15 years(1994–2008) is 0.65 × 106km2(12%) larger than that in the period 1948–62. The boundary of drylands has extended eastward over Northeast China by about 2° of longitude and by about 1° of latitude to the south along the middleto-lower reaches of the Yellow River. A zonal band of expansion of semi-arid regions has occurred, stretching from western Heilongjiang Province to southern Gansu Province, while shifts to the east of semi-arid regions in dry subhumid regions have also occurred. Results show that the aridity trend of drylands in northern China is highly correlated with the long-term trend of precipitation and PET, and the expansion of semi-arid regions plays a dominant role in the areal extent of drylands, which is nearly 10 times larger than that in arid and subhumid regions.     

Comparison of dryland climate change in observations and CMIP5 simulations

A comparison of observations with 20 climate model simulations from the Coupled Model Intercomparison Project, Phase5(CMIP5) revealed that observed dryland expansion amounted to 2.61 × 106km2 during the 58 years from 1948 to 2005,which was four times higher than that in the simulations(0.55 × 106km2). Dryland expansion was accompanied by a decline in aridity index(AI)(drying trend) as a result of decreased precipitation and increased potential evapotranspiration across all dryland subtype areas in the observations, especially in the semi-arid and dry subhumid regions. However, the CMIP5multi-model ensemble(MME) average performed poorly with regard to the decreasing trends of AI and precipitation. By analyzing the factors controlling AI, we found that the overall bias of AI in the simulations, compared with observations, was largely due to limitations in the simulation of precipitation. The simulated precipitation over global drylands was substantially overestimated compared with observations across all subtype areas, and the spatial distribution of precipitation in the MME was largely inconsistent in the African Sahel, East Asia, and eastern Australia, where the semi-arid and dry subhumid regions were mainly located.     

How Large Precipitation Changes over Global Monsoon Regions by CMIP5 Models?

Future changes in precipitation over global monsoon domains and their adjacent dry regions are investigated using present-day climate simulations(1986–2005)and future climate simulations under the Representative Concentration Pathways(RCP4.5)scenario by the Coupled Model Intercomparison Project Phase 5(CMIP5)models.In the present-day climate simulations,high reproducibility of the extents of global monsoon domains and dry regions is observed from the multi-model ensemble(MME)result;the associated local summer precipitation variation and its interannual variability are also successfully reproduced.In the future,the global monsoon domains are projected to be expanded,while the dry regions are expected to initially increase and then decrease.The summer precipitation and its variability show significant increases over most global monsoon domains and obvious decreases over their adjacent dry regions.These results indicate that currently wet regions will become wetter and dry areas will be dryer under global warming conditions.Further analysis indicates that changes in summer precipitation over global monsoon and dry regions can be interpreted as moisture convergence changes associated with changes in horizontal moisture transport.     

Progress in Semi-arid Climate Change Studies in China

This article reviews recent progress in semi-arid climate change research in China.Results indicate that the areas of semiarid regions have increased rapidly during recent years in China,with an increase of 33%during 1994-2008 compared to 1948-62.Studies have found that the expansion rate of semi-arid areas over China is nearly 10 times higher than that of arid and sub-humid areas,and is mainly transformed from sub-humid/humid regions.Meanwhile,the greatest warming during the past 100 years has been observed over semi-arid regions in China,and mainly induced by radiatively forced processes.The intensity of the regional temperature response over semi-arid regions has been amplified by land-atmosphere interactions and human activities.The decadal climate variation in semi-arid regions is modulated by oceanic oscillations,which induce land-sea and north-south thermal contrasts and affect the intensities of westerlies,planetary waves and blocking frequencies.In addition,the drier climates in semi-arid regions across China are also associated with the weakened East Asian summer monsoon in recent years.Moreover,dust aerosols in semi-arid regions may have altered precipitation by affecting the local energy and hydrological cycles.Finally,semi-arid regions in China are projected to continuously expand in the 21st century,which will increase the risk of desertification in the near future.     

THE FLUCTUATIONS OF DRY AND WET CLIMATE BOUNDARY AND ITS CAUSAL ANALYSES IN CHINA*

Based on yearly precipitation and Φ20 evaporation pan data during 1951 to 1999 of 295 stations,the aridity index is calculated in this paper.According to the aridity index,the climatic regions in China are divided into three types:the arid zone,the semi-arid zone and the humid zone. Isoline 0.20 is the boundary between arid and semi-arid zones.Isoline 0.50 is the boundary between semi-arid and humid zones.The fluctuations of dry and wet climate boundaries are very substantial,have greatly regional difference,and have the features of the whole shifting along the same direction and of the opposite moving along the contrary direction over the past 50 years.The semi-arid zone is a transitional zone between humid and arid zones,a border belt of monsoon,and a susceptible zone of environmental evolution in China. In the period of the late 1960s to the early 1970s,remarkable change had occurred for dry and wet climate in China.It manifests significantly that climate is from wetter into drought in most regions of northern China.Moreover,drought has an increasing trend.The fluctuations of climatic boundaries and the dry and wet variations in climate have substantial inter-decadal features. The main factors affecting the dry and wet climate boundary fluctuations and the dry and wet variations of climate in China are East Asian summer monsoon,Indian Monsoon,plateau monsoon in the Tibetan Plateau,westerly circulation,and West Pacific subtropical high.The different types of circulations and the strong and weak combinations of these circulations result in the regional differences of dry and wet climate changes in China.Inter-decadal variations of the dry and wet climate boundary fluctuations and of the arid and humid climate result from the inter-decadal changes of East Asian summer monsoon,Indian Monsoon,plateau monsoon,westerly circulation, and West Pacific subtropical high.The anomalous general atmospheric circulation in the Northern Hemisphere during the late 1960s to the early 1970s is the causes of arid and humid climate remarkable change in 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.     

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia, The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However,the opposite interdecadal variation was found in the rainfall anomaly in North China and South China.The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.     

NUMERICAL SIMULATIONS OF EFFECTS OF LAND SURFACE PROCESSES ON CLIMATE——IMPLEMENTING OF SSiB IN IAP/LASG AGCM AND ITS PERFORMANCE

This is an investigation of exchanges of energy and water between the atmosphere and thevegetated continents,and the impact of and mechanisms for land surface-atmosphere interactionson hydrological cycle and general circulation by implementing the Simplified Simple Biosphere(SSiB)model in a modified version of IAP/LASG global spectral general model(L9R15 AGCM).This study reveals that the SSiB model produces a better partitioning of the land surface heat andmoisture fluxes and its diurnal variations,and also gives the transport of energy and water amongatmosphere,vegetation and soil explicitly and realistically.Thus the coupled SSiB-AGCM runslead to the more conspicuous improvement in the simulated circulation,precipitation,mean watervapor content and its transport.particularly in the Asian monsoon region in the real world thanCTL-AGCM runs.It is also pointed out that both the implementation of land surfaceparameterizations and the variations in land surface into the GOALS model have greatly improvedhydrological balance over continents and have a significant impact on the simulated climate.particularly over the massive continents.Improved precipitation recycling model was employed to verify the mechanisms for landsurface hydrology parameterizations on hydrological cycle and precipitation climatology in AGCM.It can be argued that the recycling precipitation rate is significantly reduced,particularly in the aridand semi-arid region of the boreal summer hemisphere,coincident with remarkable reduction inevapotranspiration over the continental area.Therefore the coupled SSiB-AGCM runs reduce thebias of too much precipitation over land surface in most AGCMs,thereby bringing the simulatedprecipitation closer to observations in many continental regions of the world than CTL-AGCMruns.     

Variation in Summer Rainfall in North China during the Period 1956--2007 and Links with Atmospheric Circulation

Using gauge precipitation data and NCEP-NCAR reanalysis data, the interdecadal changes in summer precipitation during the period 1956-2007 in North China and the link with atmospheric circulation change over Eurasia are studied. Results show that precipitation amounts decreased by 16.2 mm per decade, which was attributable to a significant reduction in precipitation frequency. Contrary wave trains were found in the subtropical westerly jet (wave guide) over Eurasia for the wet and dry years of North China. When the wave trains had a ridge (trough) around the Korean Peninsula, conditions favored (disfavored) the westward and northward extension of the West Pacific subtropical high. The westward and northward extension of the West Pacific subtropical high is, and was, beneficial to rainfall in North China. The downstream propagation of Rossby waves was found to favor the maintenance of these wave trains. Sensible heating in the south of Lake Baikal and latent heating from the Korean Peninsula to the south of Japan increased during the period 1980-2007, as compared to that during 1957-1979. the wet period. These changes had positive influences on the maintenance of Anticyclonic-Cyclonic anomaly centers in the wave trains. Furthermore, northerly winds were prevalent in the lower troposphere during the dry period (1980-2007). which prohibited the transportation of water vapor to North China from the seas and thereby led to a decrease in rainfall in North China. The weakening of the Indian Monsoon during the dry period might be one of reasons for the reduction in water vapor transportation.     

Geographical Patterns and Temporal Variations of Regional Dry and Wet Heatwave Events in China during 1960–2008

Daily maximum/minimum temperatures and relative humidity records from 510 stations in China for the period 1960–2008 were used to investigate geographical patterns and temporal variations of heatwave (HW) events. Dry and wet HW events were compared by different definitions. Regionally, both dry and wet HW events are commonly located in southeastern China in the monsoon area, with neither type occurring in the northeast part of Northeast China and Southwest China, while the north-northwest region of the country experiences dry HW events and a few wet HW events. In the southeast of the country, site dry HW events occurred from April to September and mostly in June, while site wet HW events occurred from April to October and mostly in September. In total, 163 regional wet HW events were identified. The ten longest regional wet HW events lasted for more than 20 days, while the mean duration for 163 events was about 11 days. For the top ten events, six occurred after the 1990s, compared with four before this time. Global surface warming was clear since 1979, but the frequency and severity of regional wet HW events were relatively low in the 1980s, increasing remarkably since the 1990s. Possible reasons for this might be the strong interdecadal and interannual variations in regional atmospheric circulations, as well as water transport related directly to temperature contrasts in different regions, rather than global-mean temperature changes.     

末次间冰期亚洲中部干旱区干湿变化的模拟研究

利用国际古气候模拟对比计划第四阶段的多模式结果,分析了末次间冰期亚洲中部干旱区的干湿变化及机制。多模式集合平均结果表明,末次间冰期亚洲中部干旱区年降水减少0.7%,其中中亚地区的年降水减少2.8%,新疆地区年降水增加1.8%。水汽收支方程表明,末次间冰期中亚地区在雨季(冬春季)的降水变化主要与垂直动力项有关,新疆地区在雨季(夏季)的降水变化主要与垂直动力与热力项有关。此外,基于Penman-Montieth方法计算的亚洲中部干旱区的干旱指数在末次间冰期减小约10.2%,表明末次间冰期亚洲中部干旱区气候明显变干且存在旱区扩张的现象,这主要受到潜在蒸散变化的调控。潜在蒸散的增加进一步受到有效能量增加与地面风速增大的调控。本研究从模拟的角度揭示了末次间冰期亚洲中部干旱区干湿变化的可能特征及机制,在一定程度上有助于理解旱区气候在增暖情景下对轨道参数的响应特征。     

中国西北干旱、半干旱区春季地气温差的年代际变化特征及其对华北夏季降水年代际变化的影响

利用我国1951~2000年夏季降水观测资料分析了我国夏季降水的年代际变化特征,表明了我国夏季降水在1976年前后发生了一次明显的气候跃变,在1976年之后华北地区和黄河流域夏季降水明显减少,出现了严重持续干旱,而西北地区从20世纪70年代后期开始,降水增多,且西北地区降水振荡位相要超前于华北地区降水振荡位相5~8年。并且,本文从1960~2000年我国西北干旱、半干旱区的地温、气温观测资料分析了我国西北干旱、半干旱地区的地气温差(Ts-Ta)的变化特征,其结果表明了在20世纪70年代后期以前,我国西北干旱、半干旱地区的地气温差大部分年份偏低,低于平均值,而从70年代后期之后到2000年,我国西北干旱、半干旱地区的地气温差偏高。此外,本文还分析了西北干旱区地气温差变化的最大地区新疆西部春季地气温差与我国夏季降水的相关关系,其正相关区分别位于西北地区、东北地区和长江流域,而负相关区分别位于华北地区东部和西南地区,这表明我国西北干旱、半干旱区春季地气温差可能是华北地区夏季降水年代际变化的原因之一。     

我国江淮地区5-7月降水异常的区域特征

采用旋转经验正交展开(REOF)方法，对我国江淮地区50a5—7月降水标准化距平场进行客观分区，并分析了南北两区5—7月降水异常的长期变化趋势及其周期的变化。结果表明，江淮地区5—7月降水方差场可以分为2个区域，各区降水量异常有明显的季节变化，降水异常峰值出现在6月和7月，基本呈单峰型分布；南部区变化趋势比北部区明显，均具有显著的年际和年代际变化特征：南北两区都存在3a的主周期，南部和北部分别有21a和15a的年代际周期；江淮南、北区的降水在降水偏多、偏少年都与全国大部分地区呈同位相分布，但江淮北区的降水始终都与华南地区呈反位相分布；副热带季风系统的强弱及副高南北位置的变化直接影响江淮5—7月降水。     

夏季我国干旱、半干旱区陆面过程能量平衡及其局地大气环流

利用NCEP资料分析得出,夏季我国干旱,半干旱区在整个欧亚大陆上是陆面感热通量最强的地方,与此对应的陆面潜热通量则最弱.陆面所接收的太阳短波辐射主要以感热和长波辐射的能量形式释放.该区降水量很少,降水量的年际变率也很弱;因此,该区的陆面热量通量都显出很弱的年际变率;然而,这些通量的年代际变率信号则比较显著.我国干旱、半干旱区大气环流的热力过程与其陆面过程特征密切相关.该区对流层大气的辐射冷却很强,达-3 K d-1.由于缺乏水汽和上升运动,大尺度凝结加热率、深对流加热率、浅对流加热率都非常弱.因此,600hPa以上的大气以绝热下沉加热来平衡辐射冷却;600hPa以下,陆面感热引起的垂直扩散加热率非常强,多达8 K d-1,它除了平衡辐射冷却以外还制造对流层低层的对流运动,以绝热上升冷却来平衡多余的垂直扩散加热.总之,我国干旱、半干旱区的陆面过程特征决定了该区大气运动的特殊垂直结构,即对流层低层对流上升运动及其上层的下沉运动.我国干旱、半干旱区陆面能量平衡及其局地大气环流的年代际变率,是全球气候系统年代际变率的必然结果.     

湖北省潜在蒸散估算模型对比

利用湖北省74个气象站1961~2011年逐日气象资料,通过与FAO56-Penman-Monteith(FAO-PM)模型潜在蒸散估算结果对比,从月、季、年际变化及不同干湿状况条件比较了PriestleyTaylor、Hargreaves及Thornthwaite 3种简化的经验模型在湖北省的适用性。结果表明:3种简化模型与FAO-PM模型计算的逐月潜在蒸散均存在一定偏差,Thornthwaite模型偏差最大且冬夏季偏差反位相,Priestley-Taylor模型偏差最小,Hargreaves模型各月间的偏差最为稳定。3种简化模型与FAOPM模型估算结果具有良好的线性关系,且在各区域间相对稳定,但不同季节和干湿状况下各有差异,其中Hargreaves模型各季节间和不同干湿状况下与FAO-PM模型的关系均最为稳定。在年际变化上,Priestley-Taylor和Hargreaves模型与FAO-PM模型计算结果年际波动基本一致,Thornthwaite模型与FAO-PM模型计算结果尽管在量值上较为接近,但年际波动偏小;Priestley-Taylor和FAOPM模型年潜在蒸散趋势变化基本一致,Hargreaves模型年潜在蒸散趋势变化微弱,而Thornthwaite模型年潜在蒸散趋势变化与FAO-PM模型相反。建议在湖北省气象资料匮乏或不便应用的情况下,作物模拟模型及气候变化等研究中采用Priestley-Taylor估算模型,日常干旱监测及水资源规划中采用Hargreaves模型,干湿气候区划等工作中可采用Thornthwaite模型。同时,使用中应基于FAO-PM模型对经验模型进行适当订正,模型订正应建立在季节或月尺度。     

Responses of soil carbon variation to climate variability in China using the LPJ model

In this study, we explored the maximal response of soil carbon in a part of China to climate change, including variations in climatology and climate variability, under the condition of global warming. A conditional nonlinear optimal perturbation (CNOP) approach was employed to discuss the above issue using the Lund–Potsdam–Jena (LPJ) model. The variation in the soil carbon was compared with those caused by a linear temperature or precipitation perturbation. The key difference between the CNOP-type and the linear perturbations depended on whether the perturbations brought the variation in the temperature or the precipitation variability in comparison with the reference temperature or the precipitation variability. The model results demonstrated that the variations in the soil carbon resulted from the CNOP-type and linear temperature perturbations in south of the study region, which was corresponding to part of South China, had different variations. We examined three components of the soil carbon in the LPJ model: fast-decomposing soil carbon, slow-decomposing soil carbon, and litter below the ground. The variations of these components derived by the two types of temperature perturbations were different in the chosen region. The reduction in the litter below the ground may be the main cause of decreased soil carbon in arid and semi-arid regions as a result of the two types of temperature perturbations. The different impacts of the two types of temperature perturbations in the south of the study region may be mainly caused by the variations in the fast-decomposing soil carbon. The variations in the soil carbon caused by the two types of precipitation perturbations were similar. In the arid and semi-arid regions, the soil carbon increased due to the two types of precipitation perturbations. This research implies that the variation in temperature variability plays a crucial role in the variations of the soil carbon and its components in the study region.     

大气热源年代际异常与20世纪70年代末期东亚夏季风年代际减弱

利用1958-2001年ERA-40再分析资料计算大气热源,统计分析了亚洲季风区及其邻近海域大气热源年代际变异的典型模态;利用线性斜压干模式,模拟了夏季大气对大气热源年代际异常的响应,揭示了大气热源年代际异常与1970s末期东亚夏季风年代际减弱的关系。结果表明:近50 a来亚洲及其邻近海域夏季整层大气热源变异主要表现为年代际变化特征,其年代际位相转换发生在1970s中后期,这与东亚夏季风年代际减弱的时间一致;菲律宾附近海域和中国西南地区是与东亚夏季风年代际减弱有直接联系的两个热源异常关键区;东亚夏季风年代际减弱最直接地表现为这两个关键区热源异常的共同作用,而赤道中东太平洋、赤道印度洋大气热源增强则通过大气遥响应机制影响菲律宾附近海域低层大气环流异常对东亚夏季风变异起相反的贡献。     

全球干旱/半干旱区年代尺度干湿变化研究的进展及思考

干旱/半干旱区气候变化研究一直是广泛关注的前沿科学问题,尤其是气候干湿变化规律及未来的发展趋势。过去大量的研究揭示了全球不同干旱/半干旱区的干湿变化事实和机理,取得了一系列重要进展,IPCC第6次评估报告明确指出未来全球干旱化将加剧,但也存在诸多问题没有得到一致的认识。本文将对全球变暖背景下有关干旱/半干旱区年代尺度干湿变化,特别是年代尺度干旱研究进行梳理,系统评述当前相关研究的现状并提出干旱/半干旱区研究所面临的关键科学问题。