Variability of modeled runoff over China and its links to climate change

Runoff is a key component of the water cycle over land, with direct impact on regional ecosystems and water resources. This study investigates historical runoff variability and change over China in 1951–2008 using the Community Land Model and in situ observations of atmospheric forcing fields. Model simulations are first evaluated against in situ observations of streamflow for four major rivers, as well as soil moisture and water table depths, before further analysis is conducted. Then, quantile regression is used to analyze runoff variability and its relation to precipitation and temperature. The spatial pattern of monthly climatological runoff over China is characterized by maxima in the humid south and a gradual decrease toward the arid northwest. Runoff increases in the humid south, slightly decreases in the transition zone, and shows nonsignificant trends in the arid northwest. The footprint of decadal variability can be seen from 1951 to 2008. The annual precipitation advances the spatiotemporal variability of runoff despite locally distinct runoff–precipitation responses. The runoff-temperature relationship shows complex spatiotemporal characteristics that depend on the feedback from precipitation.     

Flood Events of Selected European Rivers in the Sixteenth Century

The severity and frequency of sixteenth-century floods of the Rhine, the Main, the middle and upper Elbe with its tributaries, rivers of northern and central Italy, the Garonne and rivers in Catalonia and Andalusia are analyzed using documentary evidence. The basic topographical and hydrological characteristics of the rivers investigated as well as the synoptic causes of their flooding during the instrumental period are presented. Different examples of modifications of the run-off process due to anthropogenic activity are discussed. Prevalence in flood occurrence during the second half of the sixteenth century in comparison to the first half is typical for central European and Andalusian rivers (mainly in the 1560s and 1590s) and agrees with the evolution of precipitation patterns. On the other hand, Italian and Catalonian rivers, in part, had a higher occurrence of floods during the first half of the century. Changes in the flooding seasons in both halves of the century are not unambiguous. Results of an analysis on a broader European scale show floods to be a random natural phenomena with limited areal extent defined by the spatial influence of forcing meteorological factors (continuous heavy rains, sudden melting of thick snow cover, etc.). Despite some limitations of documentary evidence, series of reconstructed historical floods are valuable sources of proxy data which can be utilized for the study of the flooding fluctuations in the pre-instrumental period.     

Exceptional drought events over eastern China during the last five centuries

Climate extremes, particularly the droughts sustaining over a prolonged period and affecting extended area (defined as “exceptional drought events”), can have long-lasting effects on economic and social activities. Here we use the Chinese drought/flood proxy data of the past five hundred years to identify the cases of exceptional drought events over eastern China (east of 105°E), and to study their spatial patterns and temporal evolutions. The associated circulations for the contemporary case are analyzed using available meteorological data. Possible linkage of these cases to climatic forcing and natural climate events is also explored. After considering the intensity, duration, and spatial coverage, we identified three exceptional drought events, which occurred in 1586–1589, 1638–1641, and 1965–1966 in chronological order. They were the most severe droughts of last five centuries in eastern China, with more than 40% of affected area and the drought center encountered a significant summer rainfall reduction (about 50% or more). These three droughts all developed first in North China (34–40°N), and then either expanded southward or moved to the Yangtze River Valley (27–34°N) and the northern part of the southeastern coastal area (22–27°N). For the 1965–1966 case, the significant reduction of summer precipitation was caused by a weakening of summer monsoon and an anomalous westward and northward displacement of the western Pacific subtropical high. Our analyses also suggest that these three exceptional drought events might be triggered by large volcanic eruptions and amplified by both volcanic eruptions and El Niño events.     

Climate change and river flooding: part 1 classifying the sensitivity of British catchments

Effective national and regional policy guidance on climate change adaptation relies on robust scientific evidence. This two-part series of papers develops and implements a novel scenario-neutral framework enabling an assessment of the vulnerability of flood flows in British catchments to climatic change, to underpin the development of guidance for the flood management community. In this first part, the sensitivity of the 20-year return period flood peak (RP20) to changes in precipitation (P), temperature (T) and potential evapotranspiration (PE) is systematically assessed for 154 catchments. A sensitivity domain of 4,200 scenarios is applied combining 525 and 8 sets of P and T/PE mean monthly changes, respectively, with seasonality incorporated using a single-phase harmonic function. Using the change factor method, the percentage change in RP20 associated with each scenario of the sensitivity domain is calculated, giving flood response surfaces for each catchment. Using a clustering procedure on the response surfaces, the 154 catchments are divided into nine groups: flood sensitivity types. These sensitivity types show that some catchments are (very) sensitive to changes in P but others buffer the response, while the location of catchments of the same type does not show any strong geographical pattern. These results reflect the range of hydrological processes found in Britain, and demonstrate the potential importance of catchment properties (physical and climatic) in the propagation of change in climate to change in floods, and so in characterising the sensitivity types (covered in the companion paper).     

Simulated Influence of the Atlantic Multidecadal Oscillation on Summer Eurasian Nonuniform Warming since the Mid-1990s

Based on ensemble experiments with three atmospheric general circulation models(AGCMs), this study investigates the role of the Atlantic Multidecadal Oscillation(AMO) in shaping the summer nonuniform warming over the Eurasian continent since the mid-1990 s. The results validate that the positive-phase AMO can indeed cause nonuniform warming,with predominant amplified warming over Europe–West Asia and Northeast Asia, but with much weaker warming over Central Asia. The underlying mechanism is then diagnosed from the perspective that the boundary forcing modulates the intrinsic atmospheric variability. The results highlight the role of the Silk Road Pattern(SRP), an intrinsic teleconnection pattern across the subtropical Eurasian continent propagating along the Asian jet. The SRP can not only be identified from the AGCM control experiments with the climatological sea surface temperature(SST), but can also be simulated by the AMO-related SST anomaly(SSTA) forcing. Furthermore, diagnostic linear baroclinic model experiments are conducted, and the results suggest that the SRP can be triggered by the AMO-related tropical diabatic heating. The AMO-triggered SRP-like responses feature anticyclonic circulations over Europe–West Asia and Northeast Asia, but cyclonic circulation over Central Asia. These responses cause increased warm advection towards Europe–West Asia and Northeast Asia, reduced precipitation and cloud cover, and then increased downward shortwave radiation. This increased warm advection and increased downward shortwave radiation together cause amplified warming in Europe–West Asia and Northeast Asia. The situation is opposite for Central Asia.     

Conceptual model for millennial climate variability: a possible combined solar-thermohaline circulation origin for the ~1,500-year cycle

Dansgaard-Oeschger and Heinrich events are the most pronounced climatic changes over the last 120,000 years. Although many of their properties were derived from climate reconstructions, the associated physical mechanisms are not yet fully understood. These events are paced by a ~1,500-year periodicity whose origin remains unclear. In a conceptual model approach, we show that this millennial variability can originate from rectification of an external (solar) forcing, and suggest that the thermohaline circulation, through a threshold response, could be the rectifier. We argue that internal threshold response of the thermohaline circulation (THC) to solar forcing is more likely to produce the observed DO cycles than amplification of weak direct ~1,500-year forcing of unknown origin, by THC. One consequence of our concept is that the millennial variability is viewed as a derived mode without physical processes on its characteristic time scale. Rather, the mode results from the linear representation in the Fourier space of nonlinearly transformed fundamental modes.     

Hydroclimatic change disparity of Peruvian Pacific drainage catchments

Peruvian Pacific drainage catchments only benefit from 2% of the total national available freshwater while they concentrate almost 50% of the population of the country. This situation is likely to lead a severe water scarcity and also constitutes an obstacle to economic development. Catchment runoff fluctuations in response to climate variability and/or human activities can be reflected in extreme events, representing a serious concern (like floods, erosion, droughts) in the study area. To document this crucial issue for Peru, we present here an insightful analysis of the water quantity resource variability of this region, exploring the links between this variability and climate and/or anthropogenic pressure. We first present a detailed analysis of the hydroclimatologic variability at annual timescale and at basin scale over the 1970–2008 period. In addition to corroborating the influence of extreme El Niño events over precipitation and runoff in northern catchments, a mean warming of 0.2 °C per decade over all catchments was found. Also, higher values of temperature and potential and actual evapotranspiration were found over northern latitudes. We chose to apply the Budyko-Zhang framework that characterizes the water cycle as a function of climate only, allowing the identification of catchments with significant climatic and anthropogenic influence on water balance. The Budyko-Zhang methodology revealed that 11 out of 26 initial catchments are characterized by low water balance disparity related to minor climatic and anthropogenic influence. These 11 catchments were suitable for identifying catchments with contrasting change in their hydroclimatic behavior using the Budyko trajectories. Our analysis further reveals that six hydrological catchment responses can be characterized by high sensitivity to climate variability and land use changes.     

Damaging Rainfall and Flooding: The Other Sahel Hazards

Damaging rainfall and rain-induced flooding occur from time to time in the drought-prone Sahel savanna zone of Niger in West Africa but official records of these events and their socioeconomic impacts do not exist. This paper utilized newspaper accounts between 1970 and 2000 to survey and illustrate the range of these flood hazards in the Sahel. During the study interval, 53 newspaper articles reported 79 damaging rainfall and flood events in 47 different communities in the Sahel of Niger. Collectively, these events destroyed 5,580 houses and rendered 27,289 people homeless. Cash losses and damage to infrastructure in only three events exceeded $4 million. Sahel residents attribute these floods to five major causes including both natural and anthropogenic, but they view the flood problem as driven primarily by land use patterns. Despite such awareness, traditional coping strategies appear inadequate for dealing with the problems in part because of significant climatic variability. Analysis of several rainfall measures indicates that the cumulative rainfall in the days prior to a heavy rain event is an important factor influencing whether or not heavy rainfall results in flooding. Thus, despite some limitations, newspaper accounts of historical flooding are largely consistent with measured climatic variables. The study demonstrates that concerted effort is needed to improve the status of knowledge concerning flood impacts and indeed other natural and human hazards in the Sahel.     

基于隐马尔可夫模型的中小河流致灾雨量阈值研究

基于数字高程模型(digital elevation model,DEM)高程数据和D8算法实现了湖南中小河流流域子流域划分。利用洪水预报网络传播模型,构建中小河流各子流域径流拓扑结构并提出了空间子流域等效面雨量的概念。基于等效面雨量序列和径流观测序列,构建了基于时空概念的隐马尔可夫降水-径流模型,并利用该模型计算了中小河流各子流域不同时间尺度条件下暴雨致灾临界面雨量阈值,最后利用2010—2015年实况资料对阈值进行检验,结果证明相比传统统计法,新方法的计算结果与传统方法结果一致且具有很好的准确性和稳定性。基于2015年6月的一次暴雨灾害预报,证明了该方法适应于业务化运行。     

The influence of volcanic, solar and CO2 forcing on the temperatures in the Dalton Minimum (1790–1830): a model study

The Dalton Minimum (1790–1830) was a period with reduced solar irradiance and strong volcanic eruptions. Additionally, the atmospheric CO₂ concentrations started to rise from the background level of previous centuries. In this period most empirical climate reconstructions indicate a minimum in global or hemispheric temperatures. Here, we analyse several simulations starting in 1755 with the coupled atmosphere-ocean model ECHO-G driven by different forcing combinations to investigate which external forcing could have contributed most strongly to the reduced temperatures during the Dalton Minimum. Results indicate that on global and hemispheric scales, the volcanic forcing is largely responsible for the temperature drop in this period, especially during its second half, whereas changes in solar forcing and the increasing atmospheric CO₂ concentrations were of minor importance. At regional scales, especially the extratropical, the impact of volcanic forcing is much less discernible due to the large regional variability, a finding that agrees with empirical temperature reconstructions.     

Archival explorations of climate variability and social vulnerability in colonial Mexico

In this paper, unpublished archival documentary sources are used to explore the vulnerability to–and implications of–climatic variability and extreme weather events in colonial Mexico. Attention focuses on three regions covering a variety of environmental, social, economic, and political contexts and histories and located at key points along a north-south rainfall gradient: Chihuahua in the arid north, Oaxaca in the wetter south and Guanajuato located in the central Mexican highlands. A number of themes are considered. First, the significance of successive, prolonged, or combined climate events as triggers of agrarian crisis. Second, a case study demonstrating the national and regional impacts of a particularly devastating climate induced famine, culminating with the so-called ‘Year of Hunger’ between 1785 and 1786, is presented. The way in which social networks and community engagement were rallied as a means of fortifying social resilience to this and other crises will be highlighted. Third, the impacts of selected historical flood events are explored in order to highlight how the degree of impact of a flood was a function of public expectation, preparedness and also the particular socio-economic and environmental context in which the event took place. An overview of the spatial and temporal variations in vulnerability and resilience to climatic variability and extreme weather events in colonial Mexico is then provided, considering those recorded events that could potentially relate to broader scale, possibly global, climate changes.     

Combining climatological and participatory approaches for assessing changes in extreme climatic indices at regional scale

This paper combines the climatological and societal perspectives for assessing future climatic extremes over Kangasabati River basin in India using an ensemble of four high resolution (25 km) regional climate model (RCM) simulations from 1970 to 2050. The relevant extreme indices and their thresholds are defined in consultation with stakeholders and are then compared using RCM simulations. To evaluate the performance of RCM in realistically representing atmospheric processes in the basin, model simulations driven with ERAInterim global re-analysis data from 1989 to 2008 are compared with observations. The models perform well in simulating seasonality, interannual variability and climatic extremes. Future climatic extremes are evaluated based on RCM simulations driven by GCMs, for present (1970–1999) and for the SRES A1B scenario for future (2021–2050) period. The analysis shows an intensification of majority of extremes as projected by future ensemble mean. The study suggests that there is a marked consistency in stakeholder observed changes in climate extremes and future predicted trends.     

Trends in flood seasonality of the River Ouse (Northern England) from archive and instrumental sources since AD 1600

The last decade has witnessed an increase in the application of historical records (historical and documentary) in developing a more complete understanding of high-magnitude flood frequency; but little consideration has been given to the additional information that documentary accounts contain, particularly relating to flood seasonality. This paper examines the methods and approaches available in long-term flood seasonality analysis and applies them to the River Ouse (Yorkshire) in Northern England since AD 1600. A detailed historical flood record is available for the City of York consisting of annual maxima flood levels since AD 1877, with documentary accounts prior to this. A detailed analysis of long-term flood seasonality requires confidence in the accuracy and completeness of flood records; as a result the augmented flood series are analysed using three strategies: firstly, considering all recorded floods since AD 1600; secondly, through detailed analysis of the more complete record since AD 1800; and finally, applying a threshold to focus on high-magnitude flood events since AD 1800. The results identify later winter flooding, particularly in the second half of the twentieth century, with a notable reduction in summer flood events at York during the twentieth century compared to previous centuries. Flood generating mechanisms vary little between the periods considered, with a general pattern of stability in the ratio of floods incorporating a snowmelt component.     

How the aggressiveness of rainfalls in the Mediterranean lands is enhanced by climate change

Rainfall and overland flow are fundamental processes for Earth’s ecosystems but can also be land disturbing forces, particularly when triggered by extreme hydro-meteorological events. Examples of these extremes are rainstorms and related phenomena due to rainfall aggressiveness. They produce high-impact land processes such as soil erosion and nutrient losses. Economic and social consequences of these processes can be quite severe. However, hydrological extremes and their environmental implications are still poorly understood, particularly if analyzed in the context of climate change. Here, we analyze a 300 year long times series of historical rainfall patterns across the Mediterranean in the last three centuries and we investigate changes in the erosive forcing as related to climate changes. Our results show that the erosive forcing increased towards the end of the Little Ice Age (~1850) over western and central Mediterranean and that has been increasing in the recent warming period at low Mediterranean latitudes, due to a higher frequency of intensive storms. Such increased concentrated precipitation may lead to an intensification of land degradation processes triggered by soil erosion and transport across a range of scales from hillslopes to small catchments.     

天山山区树轮气候研究若干进展

由于分布广泛、分辨率高、定年准确和气候敏感性好等原因,树木年轮在重建过去区域、半球甚至全球气候环境变化中扮演着重要角色。天山地处中亚干旱区,气候变化波动大,对全球变化响应敏感,植物生长的干旱胁迫作用强烈,天山山区分布有大量雪岭云杉和西伯利亚落叶松等长龄且对气候敏感的针叶树种,因此天山山区是树轮气候研究的理想区域。天山山区树轮气候研究始于20世纪70年代,尤其是近10 a有了长足的进步,有关天山山区树轮气候研究已经在国际上有一定影响。本文通过综述国内外对天山山区树轮气候研究的现状和进展,总结了近200 a基于树轮资料的天山山区较为一致的气候变化规律,并为进一步开展天山山区树轮气候研究提出建议。天山山区未来树木年轮气候学研究应在开展大量不同区域树木年轮气候学重建基础上,尝试理解树木径向生长对气候的响应机理研究,同时选用不同数理方法和多树木年轮指标进行长时间尺度和大空间范围重建工作,并讨论中亚干旱区过去千年气候变化的影响机制。     

Does global warming favour the occurrence of extreme floods in European Alps? First evidences from a NW Alps proglacial lake sediment record

Flood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19^th century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170?m?a.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (¹³⁷Cs and ²⁴¹Am), historic lead contamination and the correlation of major flood- and earthquake-triggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270?years (AD 1740–AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20^th century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments.     

A method to study the impact of climate change on variability of river flow: an example from the Guadalupe River in Texas

This work introduced a method to study river flow variability in response to climate change by using remote sensing precipitation data, downscaled climate model outputs with bias corrections, and a land surface model. A meteorological forcing dataset representing future climate was constructed via the delta change method in which the modeled change was added to the present-day conditions. The delta change was conducted at a fine spatial and temporal scale to contain the signals of weather events, which exhibit substantial responses to climate change. An empirical transformation technique was further applied to the constructed forcing to ensure a realistic range. The meteorological forcing was then used to drive the land surface model to simulate the future river flow. The results show that preserving fine-scale processes in response to climate change is a necessity to assess climatic impacts on the variability of river flow events.     

Natural Climatic Variability as an Explanation for Historical Climatic Fluctuations

The question as to whether the climatic anomalies associated with the Medieval Warm Period and the Little Ice Age can be attributed to natural climatic variability is explored in this paper. The output from a 500-year run with a global climatic model is used for this purpose. The model exhibits multi-decadal variability in its climatic outputs, which appears to have many of the characteristics of observed climatic data over the last millennium. Global distributions of surface temperature associated with peak warming and cooling phases of the model run highlight the spatial variability which occurs, and the lack of synchroneity in the response from region to region. Considerable year-to-year variability occurs in temperature anomaly patterns during the warming and cooling phases, indicating the complexity of the responses. The model results suggest that such climatic phases should not be considered as lengthy periods of universal warming or cooling. Comparison of observed time series of land surface temperature for the northern hemisphere for the last 500 years with model output indicates that most of the observed features in this climatic record can be reproduced by processes associated with internal mechanisms of the climatic system as reproduced in the model. While the model results do not exclude the possible contribution of external forcing agents as a contributing factor to these climatic episodes, the perception is that such agents would enhance existing naturally-induced climatic features rather than initiate them, at least for this time frame. Given the omnipresent nature of natural climatic variability, it is assumed that such variability rather than external forcing agents has primacy in generating and maintaining the underlying observed climatic variability. An understanding of the mechanisms and behaviour of such climatic features is becoming of increasing importance, in view of their possible role in modulating future climatic trends given the expected influence of the greenhouse effect.     

European climate of the past 500 years: new challenges for historical climatology

Temperature reconstructions from Europe for the past 500 years based on documentary and instrumental data are analysed. First, the basic documentary data sources, including information about climate and weather-related extremes, are described. Then, the standard palaeoclimatological reconstruction method adopted here is discussed with a particular application to temperature reconstructions from documentary-based proxy data. The focus is on two new reconstructions; January–April mean temperatures for Stockholm (1502–2008), based on a combination of data for the sailing season in the Stockholm harbour and instrumental temperature measurements, and monthly Central European temperature (CEuT) series (1500–2007) based on documentary-derived temperature indices of the Czech Republic, Germany and Switzerland combined with instrumental records from the same countries. The two series, both of which are individually discussed in greater detail in subsequent papers in this special edition, are here compared and analysed using running correlations and wavelet analysis. While the Stockholm series shows a pronounced low-frequency component, the CEuT series indicates much weaker low-frequency variations. Both series are analysed with respect to three different long-period reconstructions of the North Atlantic Oscillation (NAO) and are compared with other European temperature reconstructions based on tree-rings, wine-harvest data and various climate multiproxies. Correlation coefficients between individual proxy-based series show weaker correlations compared to the instrumental data. There are also indications of temporally varying temperature cross-correlations between different areas of Europe. The two temperature reconstructions have also been compared to geographically corresponding temperature output from simulations with global and regional climate models for the past few centuries. The findings are twofold: on the one hand, the analysis reinforces the hypothesis that the index-data based CEuT reconstruction may not appropriately reflect the centennial scale variations. On the other hand, it is possible that climate models may underestimate regional decadal variability. By way of a conclusion, the results are discussed from a broader point of view and attention is drawn to some new challenges for future investigations in the historical climatology in Europe.     

Influence of seasonality on glacier mass balance, and implications for palaeoclimate reconstructions

Climates inferred from former glacier geometries in some areas exhibit discrepancies with regional palaeoclimates predicted by General Circulation Models (GCMs) and modelling of palaeoecological data, possibly as a consequence of their differing treatments of climatic seasonality. Since glacier-based climate reconstructions potentially offer an important tool in the calibration of GCMs, which themselves need validation if used to predict future climate scenarios, we attempt to resolve mismatches between these techniques by (1) investigating the influence of seasonality on glacier mass balance, and (2) refining the methodology used for the derivation of glacier-based palaeoclimates. Focussing on the Younger Dryas stadial glaciation of Scotland, northeast Atlantic, we show that sea-ice amplified seasonality led to a significantly drier climate than has been suggested by glacier-based interpretations. This was characterised by a relatively short ablation season and the survival of a more substantial winter snowpack. We suggest that if palaeoglaciological studies were to account for changes in seasonal temperature and precipitation variability, their results would agree more closely with the cold, arid, northeast Atlantic palaeoenvironment predicted by atmospheric modelling and northwest European pollen studies, and would therefore provide more accurate constraints for GCM calibration.