土壤湿度年际异常对中国春、夏季气候模拟的影响

基于NCAR大气模式CAM3.1模式,设计了有、无土壤湿度年际异常两组试验对中国区域近40a(1961-2000年)气候进行了模拟。从气候态和年际变率的角度,通过分析两组试验的差值场来探讨土壤湿度年际异常对气候模拟的影响,并初步探讨了影响的可能机制。结果表明:模式模拟的温度和降水对土壤湿度的年际异常非常敏感,土壤湿度的年际变化对中国春夏季气候及其年际变率均有显著影响。当不考虑土壤湿度年际异常时,模式模拟的春夏季平均温度、最高温度、最低温度在我国大范围内降低,春夏季降水在东部大部分地区明显减少,西部增加。而模式模拟的春夏季温度、降水年际变率在中国大部分地区减弱。但当考虑土壤湿度的年际变化,则能在一定程度上提高模式对气候年际变率的模拟能力。在进一步分析表明土壤湿度年际异常时,主要通过改变地表能量通量和环流场,对温度、降水产生影响。当不考虑土壤湿度年际异常时,地表净辐射通量减少,地表温度降低,感热通量减少。感热通量差值场的空间变化和温度差值场的空间变化一致,感热通量对温度有一定影响。而潜热通量差值场的空间变化和降水的差值场的空间变化一致,可见降水受地表潜热通量的影响。土壤湿度年际异常引起的环流场的变化也是导致气候变化的原因之一,地表能量和环流场年际变率的改变对春夏季气候年际变率存在一定影响。     

土壤湿度影响中国夏季气候的数值试验

利用"全球土壤湿度计划第2阶段"提供的土壤湿度资料强迫区域气候模式RegCM3,通过数值试验讨论了土壤湿度对东亚夏季气候模拟效果的影响。结果表明,合理考虑土壤湿度的作用,能够提高区域气候模式对中国夏季降水和2 m气温的空间分布型及逐日变化的模拟效果;模拟结果与观测的相关分析显示,降水和2 m气温的年际变化都得到了有效改进,这种改进在气温上尤为明显。不过上述改进具有区域依赖性。数值试验结果表明,气温对土壤湿度的敏感性强于降水,这也从一个侧面说明提高降水模拟效果的难度。总体而言,合理的土壤湿度能够提高区域气候模式对中国夏季气候的模拟能力。因此,合理描述土壤湿度的变化,是提高中国夏季气候预报技巧的潜在途径之一。     

Relevance of soil moisture for seasonal climate predictions: a preliminary study

In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987–1988) soil moisture climatology at a 1^°×1^° horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture.     

WRF4.0模式陆气耦合对不同物理参数化方案的敏感性初探

模式的不同参数化方案会对模拟的天气和气候产生影响,但如何影响陆气耦合还不是很清楚。利用WRF 4.0模式设计了一组16种不同参数化方案组合的集合试验,对华北地区2013年夏季气候进行模拟。结果表明,不同积云对流参数化方案对模拟的降水平均态影响较大,其次是短波辐射方案,而微物理方案影响较小。进一步利用潜热通量(LH,Latent Heat flux)和土壤湿度(SM,Soil Moisture)、抬升凝结高度(LCL,Lifting Condensation Level)的相关系数R_(SM,LH)和R_(LH,LCL)表征陆气耦合过程,评估了陆气耦合对不同参数化方案的敏感性。模式模拟的R_(SM,LH)在不同区域随着平均降水、相对湿度的增加以及平均短波辐射的减少而减小,而R_(LH,LCL)的变化正好相反。它们内在的物理机制类似,与气候态的干湿变化密切相关,即随着土壤湿度趋于饱和,蒸散发过程逐渐受太阳辐射的限制,陆面对大气的影响减弱。另外,陆气耦合强度受积云对流方案、微物理方案和短波辐射方案的影响而产生差别,这与不同参数化方案模拟的气候态的不同密切相关,因此本研究对于WRF 4.0模式中参数化方案特别是积云对流方案的选择同样有着重要的指导意义。     

Mid-Century Ensemble Regional Climate Change Scenarios for the Western United States

To study the impacts of climate change on water resources in the western U.S., global climate simulations were produced using the National Center for Atmospheric Research/Department of Energy (NCAR/DOE) Parallel Climate Model (PCM). The Penn State/NCAR Mesoscale Model (MM5) was used to downscale the PCM control (20 years) and three future(2040–2060) climate simulations to yield ensemble regional climate simulations at 40 km spatial resolution for the western U.S. This paper describes the regional simulations and focuses on the hydroclimate conditions in the Columbia River Basin (CRB) and Sacramento-San Joaquin River (SSJ) Basin. Results based on global and regional simulations show that by mid-century, the average regional warming of 1 to 2.5 °C strongly affects snowpack in the western U.S. Along coastal mountains, reduction in annual snowpack was about70% as indicated by the regional simulations. Besides changes in mean temperature, precipitation, and snowpack, cold season extreme daily precipitation increased by 5 to 15 mm/day (15–20%) along theCascades and the Sierra. The warming resulted in increased rainfall at the expense of reduced snowfall, and reduced snow accumulation (or earlier snowmelt) during the cold season. In the CRB, these changes were accompanied by more frequent rain-on-snow events. Overall, they induced higher likelihood of wintertime flooding and reduced runoff and soil moisture in the summer. Changes in surface water and energy budgets in the CRB and SSJ basin were affected mainly by changes in surface temperature, which were statistically significant at the 0.95 confidence level. Changes in precipitation, while spatially incoherent, were not statistically significant except for the drying trend during summer. Because snow and runoff are highly sensitive tospatial distributions of temperature and precipitation, this study shows that (1) downscaling provides more realistic estimates of hydrologic impacts in mountainous regions such as the western U.S., and (2) despite relatively small changes in temperature and precipitation, changes in snowpack and runoff can be much larger on monthly to seasonal time scales because the effects of temperature and precipitation are integrated over time and space through various surface hydrological and land-atmosphere feedback processes. Although the results reported in this study were derived from an ensemble of regional climate simulations driven by a global climate model that displays low climate sensitivity compared with most other models, climate change was found to significantly affect water resources in the western U.S. by the mid twenty-first century.     

空间分辨率对中国夏季降水时空演变特征的影响

本文使用1979—2021年国家气候中心160站(R₁₆₀)和国家级地面气象观测站2 314站(R₂₃₁₄)逐月降水观测资料,利用EOF (Empirical Orthogonal Function)分解和相关分析等方法,研究两类资料夏季降水时空变化特征及其与海气因素之间物理联系的表征水平差异,并分析了差异的可能原因。结果表明,R₁₆₀在我国西北、青藏高原等地区站点极为稀疏,导致其各EOF模态对上述地区降水的时空变化特征描述失真,中东部地区偏低的空间分辨率会使局地强降水的变化特征信号损失,造成降水的年际变率降低。而R₂₃₁₄主模态能够更为真实地反映我国降水的时空演变特征,特别是在极端降水频发的江淮地区以及降水受局地地形影响较大的山区,其EOF主模态的空间分布和时间系数演变与ENSO (El Niño-Southern Oscillation)、西太平洋副热带高压、青藏高原雪盖等气候强迫信号之间的相关性更为显著。     

The potential of vegetation feedback to alleviate climate aridity over the United States associated with a 2×CO2 climate condition

This study examines the potential impact of vegetation feedback on changes in summer climate aridity over the contiguous United States (US) due to the doubling of atmospheric CO₂ concentration using a set of 100-year-long climate simulations made by a global climate model interactively coupled with a dynamic vegetation model. The Thornthwaite moisture index (I _m ), which quantifies climate aridity on the basis of atmospheric water supply (i.e., precipitation) and atmospheric water demand (i.e., potential evapotranspiration, PET), is used to measure climate aridity. Warmer atmosphere and drier surface resulting from increased CO₂ concentration increase climate aridity over most of the contiguous US. This phenomenon is due to larger increments in PET than in precipitation, regardless of the presence or absence of vegetation feedback. Compared to simulations without active dynamic vegetation feedback, the presence of vegetation feedback significantly alleviates the increase in aridity. This vegetation-feedback effect is most noticeable in the subhumid regions such as southern, midwestern and northwestern US, primarily by the increasing vegetation greenness. In these regions, the greening in response to warmer temperatures enhances moisture transfer from soil to atmosphere by evapotranspiration (ET). The increased ET and subsequent moistening over land areas result in weaker surface warming (1–2?K) and PET (3–10?mm?month^?1), and greater precipitation (4–10?mm?month^?1). Collectively, they result in moderate increases in I _m . Our results suggest that moistening by enhanced vegetation feedback may prevent aridification under climatic warming especially in areas vulnerable to climate change, with consequent implications for mitigation strategies.     

土壤湿度年际变化对中国区域极端气候事件模拟的影响研究Ⅱ.敏感性试验分析

利用NCARCAM3.1大气环流模式,设计了有、无土壤湿度年际异常的两组数值试验,探讨了土壤湿度年际异常对极端气候事件模拟的可能影响。结果表明,模式模拟的极端气候事件对土壤湿度异常十分敏感,土壤湿度异常对极端气候指标的多年平均空间分布、年际变率以及年际变化均具有重要影响。当不考虑土壤湿度的年际异常时:(1)模拟的暖夜日数、暖昼日数和热浪持续指数的发生频次在全国范围内均明显减少,而霜冻日数则明显增加。极端降水指标的响应表现出明显的空间差异,极端降水频次在江淮流域明显减小,而极端降水强度则表现为东北减弱、长江流域增强;中雨日数和持续湿期在我国大部分地区减少。(2)极端气温指标的年际变率在我国大部分地区呈减小趋势;而极端降水事件的变化则较为复杂,极端降水频次和极端降水强度的年际变率在长江以南有所增强,而北方地区则有所减弱。中雨日数和持续湿期的年际变率在我国呈现出较为一致的减少趋势。(3)模式对暖夜日数、霜冻日数的年际变化的模拟能力明显下降,并对4个极端降水指标的年际变化的模拟能力在全国多数区域均有不同程度的下降。     

Tree mortality in response to climate change induced drought across Beijing,China

Tree mortality in response to climate change induced drought has emerged as a global concern. Small changes of tree mortality rates can profoundly affect forest structure, composition, dynamics and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from natural stands (82 plots) in Beijing showed that tree mortality rates have increased significantly over the two decades from 1986 to 2006. In contrast, recruitment rates decreased significantly over this period. The increase in overall mortality rates resulted from an increase in tree deaths dominantly attributed to changes in temperature and precipitation resulting in drier conditions across latitudes, elevations, tree species, and tree sizes. In addition, the results showed that mortality rates of Chinese pine (Pinus tabuliformis) (β ₁ ?=?0.0874) as a result of climate change induce drought were much smaller than oak (Quercus) (β ₁ ?=?0.1583).     

雅鲁藏布江流域夏季降水的年际变化及其原因

本文通过多套观测与再分析降水资料的比较,分析了雅鲁藏布江流域夏季降水的特征,从水汽含量与水汽输送的角度检验了雅鲁藏布江水汽通道的特点,研究了流域夏季降水的年际变化及其原因。分析表明:（1）该流域夏季降水大值位于雅鲁藏布江出海口至大峡谷一带,观测中流域平均降水可达5.8 mm d-1。不同资料表现的降水空间分布一致,但再分析降水普遍强于观测,平均为观测的2倍左右。（2）该流域夏季的水汽主要来自印度洋和孟加拉湾的偏南暖湿水汽输送,自孟加拉湾出海口沿布拉马普特拉河上溯至大峡谷,即雅鲁藏布江水汽通道。水汽收支诊断表明,夏季流域南部（即水汽通道所在处）是水汽辐合中心,流域平均的辐合约9.5 mm d-1,主要来自风场辐合与地形坡度的贡献。（3）不同再分析资料表现的流域降水和水汽分布特征总体一致,但量值差异较大。NCEP（美国国家环境预报中心）气候预报系统再分析资料CFSR、日本气象厅再分析资料JRA-25较欧洲中期天气预报中心再分析ERA-Interim资料更适于研究该流域（青藏高原东南部）的水汽特征,因为后者给出的流域降水和水汽偏强。（4）近30年该流域夏季降水无显著趋势,以年际变率为主。年际异常的水汽辐合（约为气候态的35.4%）源自异常西南风导致的局地水汽辐合（纬向、经向辐合分别贡献了16.5%、83.5%）,地形作用很小。流域夏季降水的年际变化是由印度夏季风活动导致的异常水汽输送造成的,其关键系统是印度季风区北部的异常气旋（反气旋）式水汽输送。     

Characterization of precipitation δ 18O variation in Nagqu, central Tibetan Plateau and its climatic controls

The stable isotopic composition of precipitation in different regions reflects climatic factors such as temperature, precipitation, moisture sources, and transport process. However, the isotopic variation in the region is usually much complicated due to the combined influences of these factors. A good understanding of climatic controls on the isotopic composition of precipitation can contribute to the study on isotopic tracer for climate and hydrology. To investigate the isotopic variation of precipitation and its climatic controls in the middle of the Tibetan Plateau, a monitoring station for stable isotope in precipitation has been established in Nagqu region, central Tibetan Plateau. We obtained 79 daily samples at Nagqu Meteorological Station in 2000. The observed δ ¹⁸O in precipitation showed a distinctly seasonal pattern with higher values in spring and winter and lower values in summer, despite of individually low values in winter due to extremely low temperature. To further understand this pattern, we evaluated the influence of temperature, precipitation, moisture sources, and moisture transport process on precipitation δ ¹⁸O. A multiple linear regression model represents quantitatively the dependence of precipitation δ ¹⁸O on precipitation and temperature: δ ¹⁸O_ppt?=??0.30P???0.11T???14.8 (R ²?=?0.13, n?=?79, P?=?0.005), which indicates δ ¹⁸O values in precipitation are more dependent on precipitation amount than on temperature. In contrast, when the temperature is low enough (<2°C), δ ¹⁸O values in precipitation are mainly dependent on temperature: δ ¹⁸O_ppt?=?0.53T???10.2 (R ²?=?0.44, n?=?19, P?=?0.002). The variation of δ ¹⁸O in precipitation is also closely related to moisture origins and transport trajectories. A model is set up to trace the trajectories for air masses arriving in the observed region, and the results demonstrated that humid marine air masses from the Indian Ocean generally have significantly lower δ ¹⁸O values than dry continental air masses from the north or local re-evaporation. During monsoon precipitation, the distance and depth of moisture transport as well as convective precipitation all lead to the large variability of δ ¹⁸O in precipitation.     

Impact of soil moisture�Catmosphere coupling on European climate extremes and trends in a regional climate model

Processes acting at the interface between the land surface and the atmosphere have a strong impact on the European summer climate, particularly during extreme years. These processes are to a large extent associated with soil moisture (SM). This study investigates the role of soil moisture?Catmosphere coupling for the European summer climate over the period 1959?C2006 using simulations with a regional climate model. The focus of this study is set on temperature and precipitation extremes and trends. The analysis is based on simulations performed with the regional climate model CLM, driven with ECMWF reanalysis and operational analysis data. The set of experiments consists of a control simulation (CTL) with interactive SM, and sensitivity experiments with prescribed SM: a dry and a wet run to determine the impact of extreme values of SM, as well as experiments with lowpass-filtered SM from CTL to quantify the impact of the temporal variability of SM on different time scales. Soil moisture?Cclimate interactions are found to have significant effects on temperature extremes in the experiments, and impacts on precipitation extremes are also identified. Case studies of selected major summer heat waves reveal that the intraseasonal and interannual variability of SM account for 5?C30% and 10?C40% of the simulated heat wave anomaly, respectively. For extreme precipitation events on the other hand, only the wet-day frequency is impacted in the experiments with prescribed soil moisture. Simulated trends for the past decades, which appear consistent with projected changes for the 21st century, are identified to be at least partly linked to SM-atmosphere feedbacks.     

Atmospheric contribution to hydrologic variations in the Arctic

Abstract

High‐latitude rawinsonde data for 18 years (1973–1990) are used to compute the atmospheric moisture flux convergence over two regions: the Arctic Ocean and the Mackenzie River drainage basin. The primary objectives are to assess the interannual variability and to compare the macroscale hydrologie regimes of the two regions. The moisture flux convergence is positive in all months over the Arctic Ocean, but is occasionally negative during summer over the Mackenzie Basin. The climatological seasonal cycle of the moisture convergence contains a late‐summer (August‐September) maximum over the Arctic Ocean but a late‐summer minimum over the Mackenzie Basin. Evaporation, deduced from the moisture inflow and independent data on precipitation, makes a much greater contribution to the atmospheric moisture budget of the Mackenzie domain, especially during summer. The respective equivalent area averages of the 18‐year annual mean moisture flux convergence, precipitation and derived evaporation are 17.3, 19.5 and 2.2 cm a^‐1 for the Arctic Ocean and 24.9, 33.6 and 8.7 cm a^‐1 for the Mackenzie domain. However, the range of interannual variations of the flux convergence is about ±50% of the annual means and more than twice the monthly means. The annual totals of the flux convergence are correlated with station‐derived precipitation over the Mackenzie domain and with yearly variations of the Mackenzie discharge. The moisture flux convergence over the Mackenzie domain suggests that station reports underestimate precipitation during the winter months by amounts equivalent to several centimetres per annum.     

Stream temperature sensitivity to climate warming in California’s Sierra Nevada: impacts to coldwater habitat

Water temperature influences the distribution, abundance, and health of aquatic organisms in stream ecosystems, so understanding the impacts of climate warming on stream temperature will help guide management and restoration. This study assesses climate warming impacts on stream temperatures in California’s west-slope Sierra Nevada watersheds, and explores stream temperature modeling at the mesoscale. We used natural flow hydrology to isolate climate induced changes from those of water operations and land use changes. A 21 year time series of weekly streamflow estimates from WEAP21, a spatially explicit rainfall-runoff model were passed to RTEMP, an equilibrium temperature model, to estimate stream temperatures. Air temperature was uniformly increased by 2°C, 4°C, and 6°C as a sensitivity analysis to bracket the range of likely outcomes for stream temperatures. Other meteorological conditions, including precipitation, were unchanged from historical values. Raising air temperature affects precipitation partitioning into snowpack, runoff, and snowmelt in WEAP21, which change runoff volume and timing as well as stream temperatures. Overall, stream temperatures increased by an average of 1.6°C for each 2°C rise in air temperature, and increased most during spring and at middle elevations. Viable coldwater habitat shifted to higher elevations and will likely be reduced in California. Thermal heterogeneity existed within and between basins, with the high elevations of the southern Sierra Nevada and the Feather River watershed most resilient to climate warming. The regional equilibrium temperature modeling approach used here is well suited for climate change analysis because it incorporates mechanistic heat exchange, is not overly data or computationally intensive, and can highlight which watersheds are less vulnerable to climate warming. Understanding potential changes to stream temperatures from climate warming will affect how fish and wildlife are managed, and should be incorporated into modeling studies, restoration assessments, and licensing operations of hydropower facilities to best estimate future conditions and achieve desired outcomes.     

The soil moisture of China in a high resolution climate-vegetation model

1. Introduction The soil moisture plays an important role in in- fluencing the climate change by altering the surface albedo, soil heat capacity and the heat flux between air and land (Ma et al., 2001). Near-surface soil mois- ture controls the partitioni…     

Possible Impacts of Climate Change on Soil Moisture Availability in the Southeast Anatolia Development Project Region (GAP): An Analysis from an Agricultural Drought Perspective

This paper presents probable effects of climate change on soil moisture availability in the Southeast Anatolia Development Project (GAP) region of Turkey. A series of hypothetical climate change scenarios and GCM-generated IPCC Business-as-Usual scenario estimates of temperature and precipitation changes were used to examine implications of climate change for seasonal changes in actual evapotranspiration, soil moisture deficit, and soil moisture surplus in 13 subregions of the GAP. Of particular importance are predicted patterns of enhancement in summer soil moisture deficit that are consistent across the region in all scenarios. Least effect of the projected warming on the soil moisture deficit enhancement is observed with the IPCC estimates. The projected temperature changes would be responsible for a great portion of the enhancement in summer deficits in the GAP region. The increase in precipitation had less effect on depletion rate of soil moisture when the temperatures increase. Particularly southern and southeastern parts of the region will suffer severe moisture shortages during summer. Winter surplus decreased in scenarios with increased temperature and decreased precipitation in most cases. Even when precipitation was not changed, total annual surplus decreased by 4 percent to 43 percent for a 2°C warming and by 8 percent to 91 percent for a 4°C warming. These hydrologic results may have significant implications for water availability in the GAP as the present project evaluations lack climate change analysis. Adaptation strategies – such as changes in crop varieties, applying more advanced dry farming methods, improved water management, developing more efficient irrigation systems, and changes in planting – will be important in limiting adverse effects and taking advantage of beneficial changes in climate.     

江苏省近50a气候干湿特征研究

根据江苏省1960—2009年54个气象台站常规气象观测资料,利用Penman-Monteith公式计算了全省各地区50 a的逐日潜在蒸散量,结合逐日降水量,推算出了相对湿润度指数值,并采用国家标准《气象干旱等级》(GB/T20481-2006)中的相对湿润度分级指标对全省干湿状况进行了评估,分析其时空变化特征。研究表明:1)就全年而言,江苏省半干旱区与湿润区各占50%左右的面积,其中淮北、江淮北部、苏北沿海的北部为半干旱区,江淮南部、苏北沿海的南部、沿江、苏南地区为湿润区;2)降水量和潜在蒸散量是影响相对湿润度指数的两个关键因子,降水量的变化对相对湿润度的时空分布起着主导作用,潜在蒸散量起着辅助作用。3)江苏省1 a中冬季的南北气候干湿反差最大、夏季最小,湿润区范围夏季最大、秋季最小,半干旱区范围秋季最大、夏季没有,干旱区范围春季最大、夏季和秋季没有。夏季气候最湿润、春季气候最干燥。4)淮北和苏北沿海地区的相对湿润度指数年变化呈"单峰型",江淮、沿江和苏南地区的年变化呈"双峰型",苏北沿海地区相对湿润度年内变化最大,沿江地区最小。     

Snowpack and runoff response to climate change in Owens Valley and Mono Lake watersheds

Precipitation from the Eastern Sierra Nevada watersheds of Owens Lake and Mono Lake is one of the main water sources for Los Angeles’ over 4 million people, and plays a major role in the ecology of Mono Lake and of these watersheds. We use the Variable Infiltration Capacity (VIC) hydrologic model at daily time scale, forced by climate projections from 16 global climate models under greenhouse gas emissions scenarios B1 and A2, to evaluate likely hydrologic responses in these watersheds for 1950–2099. Comparing climate in the latter half of the 20th Century to projections for 2070–2099, we find that all projections indicate continued temperature increases, by 2–5 °C, but differ on precipitation changes, ranging from ?24 % to +56 %. As a result, the fraction of precipitation falling as rain is projected to increase, from a historical 0.19 to a range of 0.26–0.52 (depending on the GCM and emission scenario), leading to earlier timing of the annual hydrograph’s center, by a range of 9–37 days. Snowpack accumulation depends on temperature and even more strongly on precipitation due to the high elevation of these watersheds (reaching 4,000 m), and projected changes for April 1 snow water equivalent range from ?67 % to +9 %. We characterize the watershed’s hydrologic response using variables integrated in space over the entire simulated area and aggregated in time over 30-year periods. We show that from the complex dynamics acting at fine time scales (seasonal and sub-seasonal) simple dynamics emerge at this multi-year time scale. Of particular interest are the dynamic effects of temperature. Warming anticipates hydrograph timing, by raising the fraction of precipitation falling as rain, reducing the volume of snowmelt, and initiating snowmelt earlier. This timing shift results in the depletion of soil moisture in summer, when potential evapotranspiration is highest. Summer evapotranspiration losses are limited by soil moisture availability, and as a result the watershed’s water balance at the annual and longer scales is insensitive to warming. Mean annual runoff changes at base-of-mountain stations are thus strongly determined by precipitation changes.     

European summer climate variability in a heterogeneous multi-model ensemble

Recent results from an enhanced greenhouse-gas scenario over Europe suggest that climate change might not only imply a general mean warming at the surface, but also a pronounced increase in interannual surface temperature variability during the summer season (Schär et al., Nature 427:332–336, 2004). It has been proposed that the underlying physical mechanism is related to land surface-atmosphere interactions. In this study we expand the previous analysis by including results from a heterogeneous ensemble of 11 high-resolution climate models from the PRUDENCE project. All simulations considered comprise 30-year control and enhanced greenhouse-gas scenario periods. While there is considerable spread in the models’ ability to represent the observed summer variability, all models show some increase in variability for the scenario period, confirming the main result of the previous study. Averaged over a large-scale Central European domain, the models simulate an increase in the standard deviation of summer mean temperatures between 20 and 80%. The amplification occurs predominantly over land points and is particularly pronounced for surface temperature, but also evident for precipitation. It is also found that the simulated changes in Central European summer conditions are characterized by an emergence of dry and warm years, with early and intensified depletion of root-zone soil moisture. There is thus some evidence that the change in variability may be linked to the dynamics of soil-moisture storage and the associated feedbacks on the surface energy balance and precipitation.     

Effects of soil moisture on the sensitivity of a climate model to earth orbital forcing at 9000 yr BP

The sensitivity of climate to orbitally-related changes in solar radiation at 9000 yr BP (before present) is examined using fixed and interactive soil moisture versions of a low resolution general circulation model. In both versions of the model increased solar radiation for June–August at 9000 yr BP (compared to present) produced enhanced northern monsoons and warmer continental interiors in comparison to present whereas decreased solar radiation at 9000 yr BP in December–February produced weaker southern monsoons. The increased rainfall in the northern tropics in summer increased soil moisture and runoff at 9000 yr BP in the interactive model; in the southern hemisphere decreased rainfall in summer led to decreased soil moisture and runoff. Conditions in summer became drier (decreased soil moisture and runoff) at 9000 yr BP in the northern extratropics.The experiments showed that the magnitude (but not the sign) of model sensitivity to 9000 yr BP radiation is altered by the effects of interactive soil moisture. Decreased soil moisture (about 20%) over northern Eurasia in the interactive model led to smaller evaporative increases, greater temperature increases and greater reduction of precipitation than for the model with fixed soil moisture. Over northern tropical lands, slightly smaller temperature increases and greater evaporation and precipitation increases in the interactive model are linked to the simulation of increased soil moisture at 9000 yr BP. The differences in sensitivity between the two versions of the model over northern Eurasia are statistically significant at the 95% level whereas those for the tropics are not.Overall, the results of the simulations are generally supported by the geologic evidence for 9000 yr BP; however, the evidence lacks sufficient precision and the model resolution is too coarse for detailed model/data comparisons and for assessment of the relative accuracy of the two 9000 yr BP experiments.The computed sensitivities of temperature and soil moisture to 9000 yr BP radiation differ from those simulated under equilibrium conditions in the various general circulation model experiments for increased atmospheric concentration of CO₂. In contrast to the effects of the enhanced seasonal cycle of solar radiation at 9000 yr BP, a CO₂ increase causes a broad warming of both the ocean and land with little modification of land/ocean temperature difference. The experiments for 9000 yr BP indicate a clearer signal for summer drying than is obtained in the experiments for increased CO₂. The results suggest that the 9000 yr BP climate may be of limited utility as an analog to future warm climates.