Recent fluctuations in precipitation and runoff over the Nile sub-basins and their impact on main Nile discharge

Substantial fluctuations in precipitation and runoff have occurred over the Nile Basin in recent decades. Ten-year mean flows of the Blue Nile (Khartoum gauge) during the 20th century have ranged from 42.2 to 56.7 km³ and for the White Nile (Malakal gauge) from 25.5 to 36.9 km³. These fluctuations have been responsible for changes in decade-mean Main Nile discharge of up to ± 20% which have had important consequences for water resource management in both Egypt and Sudan.This paper provides a review of the Nile Basin hydrology incorporating, for the first time, analyses of the relationships between precipitation and runoff fluctuations in the instrumental period for each of the eight major sub-basins within the Nile Basin. These sub-basins possess very different physical, climatic and hydrological characteristics. Over 90% of the Main Nile discharge originates from only four of the sub-basins; Lake Victoria, Blue Nile, Atbara, and the Sobat. Interbasin correlations of 40 y (1945-84) precipitation and runoff annual time series identify two broadly homogeneous regions; the Ethiopian highlands (Blue Nile and Atbara) and the Lake Victoria and Equatorial Lakes (White Nile). These regions posses contrasting precipitation regimes whose interannual variations are uncorrelated in time and which are therefore associated with different atmospheric circulation anomalies. The observed relationships between catchment precipitation and runoff, however, are not straightforward and the sensitivity of runoff to precipitation fluctuations varies from basin to basin. Some of the water resource management implications of these fluctuations for Egypt are discussed. With water demand in Egypt alone set to increase 17% by the year 2000, it is critical that the role of future climate change in Nile water management is thoroughly assessed based on a correct modelling of the diverse hydrological characteristics of the various Nile sub-basins.     

Virtual water ‘flows’ of the Nile Basin, 1998–2004: A first approximation and implications for water security

This paper interprets an initial approximation of the ‘trade’ in virtual water of Nile Basin states in terms of national water security. The virtual water content (on the basis of weight) of select recorded crop and livestock trade between 1998 and 2004 is provided, and analysed for each state separately, for the Southern Nile and Eastern Nile states as groups, and for the basin states as a whole. To the extent that the datasets allow, the distinction between rainfed and irrigated production is maintained. During the period under study, Nile Basin states ‘exported’ about 14,000 Mm³ of primarily rainfed-derived virtual water outside of the basin annually and ‘imported’ roughly 41,000 Mm³/y. The ‘imports’ are considered to have played a key role in filling the freshwater deficits of Egypt and Sudan, and represent a third of the flow of the Nile River itself. Analysis of food trade within the basin shows that the equivalent of small rivers of water used to raise coffee and tea ‘flow’ from the highlands around Lake Victoria to Egypt and Sudan. Because the bulk of these ‘flows’ derive from rainfed agriculture, the virtual water ‘traded’ annually between the Nile Basin states is not considered to represent a significant demand on the water resources of the basin, nor to significantly remedy the freshwater deficits of the arid basin states. The importance of soil water and rainfed farming is in improving water security is highlighted. The limitations and merits of the inter-state basin-wide approach are also discussed. By highlighting the magnitude of water leaving and entering states in its virtual form, the approach obliges policy-makers to think beyond the basin and reconsider the concept of water security within broader political, environmental, social and economic forces.     

气候变化对澜湄流域上下游水资源合作潜力的影响

澜沧江-湄公河（澜湄）流域南北跨越了25个纬度,流域上下游气候差异明显。同时遭遇干旱或湿润通常不利于上下游水资源合作,而水文气象条件正常或上下游间的干湿条件不一样时有利于缓解流域内的竞争性用水状况。为探究气候变化对澜湄流域上下游水资源合作潜力的影响,基于普林斯顿降水数据集与全球气候模型预估数据,利用标准化降水指数（SPI）和Copula函数计算了历史时期（1985—2016年）与未来时期（2021—2090年）澜湄流域上下游同时面临干旱、湿润以及干湿存在差异的发生概率。基于典型浓度路径RCP4.5和RCP8.5情景的预估结果显示与历史时期相比,未来时期澜湄流域在RCP4.5与RCP8.5情景下具有相似的变化趋势,即：遭遇同期湿润的概率在逐渐增大（最大达到199.5%),遭遇同期干旱的概率则在逐渐减少(最小达到-35.9%),而遭遇干湿差异时期的概率在所有时段均大幅减少（-53.1%~-42.5%)。未来澜湄流域上下游同期湿润概率的增加和遭遇干湿差异概率的减少预计将加大上下游面临水资源竞争的可能性,从而对澜湄流域各国家之间的水资源合作产生不利影响。这一研究可以为澜湄流域水资源合作策略的制定提供科学参考和依据。     

Sensitivity Studies of the Impacts of Climate Change on White Nile Flows

Several exploratory studies are presented on the sensitivity of the water balance of the White Nile to climate change, using both observed and stochastic time series to drive the models. Example results are presented using various assumed climate change scenarios and results from a General Circulation Model (GCM). The relative merits and shortcomings of each modelling approach are also discussed. A simple analytical model for Lake Victoria is also used to illustrate some of the overall features of the lake's likely response. Particular difficulties with the White Nile system are that, due to the huge area of open water in the basin, transient responses to short-lived events can occur over timescales comparable with those for which long term climate change impacts are being studied, and predicted changes in flows are extremely sensitive to estimates for the rainfall and evaporation at lake and swamp surfaces. Of the modelling approaches considered, the network simulation approach with stochastic inputs is recommended as a way of smoothing out these transient effects, and assessing the uncertainty in the results due to inaccuracies in the data, the model parameters and the climate change predictions. The paper concludes with a brief discussion of some other areas of uncertainty in the hydrological modelling of White Nile flows and possible alternative external forcing mechanisms for flows in the next few decades.     

Ethiopian decadal climate variability

Ethiopian decadal climate variability is characterized by application of singular value decomposition to gridded rainfall data over the period 1901–2007. Two distinct modes are revealed with different annual cycles and opposing responses to regional and global forcing. The northern zone that impacts the Nile River and underlies the tropical easterly jet has a unimodal rainy season that is enhanced by Atlantic Multidecadal Oscillation warm phase. This rainfall mode is linked with the Atlantic zonal overturning circulation and exhibits 10–12-year cycles through much of the twentieth century. The southern zone has a bimodal rainy season that is enhanced by Pacific Decadal Oscillation cool phase and the southern meridional overturning circulation. Multiyear wet and dry spells are characterized by sympathetic responses in the near-equatorial trough extending from Central America across the African Sahel to Southeast Asia. The interaction of Walker and Hadley cells over Africa appears to be a key feature that modulates Ethiopian climate at decadal frequency through anomalous north–south displacement of the near-equatorial trough.     

Natural variability of summer rainfall over China in HadCM3

Summer rainfall over China has shown decadal variability in the past half century, which has resulted in major north–south shifts in rainfall with important implications for flooding and water resource management. This study has demonstrated how multi-century climate model simulations can be used to explore interdecadal natural variability in the climate system in order to address important questions around recent changes in Chinese summer rainfall, and whether or not anthropogenic climate change is playing a role. Using a 1,000-year simulation of HadCM3 with constant pre-industrial external forcing, the dominant modes of total and interdecadal natural variability in Chinese summer rainfall have been analysed. It has been shown that these modes are comparable in magnitude and in temporal and spatial characteristics to those observed in the latter part of the twentieth century. However, despite 1,000 years of model simulation it has not been possible to demonstrate that these modes are related to similar variations in the global circulation and surface temperature forcing occurring during the latter half of the twentieth century. This may be in part due to model biases. Consequently, recent changes in the spatial distribution of Chinese summer rainfall cannot be attributed solely to natural variability, nor has it been possible to eliminate the likelihood that anthropogenic climate change has been the driving factor. It is more likely that both play a role.     

Decadal/interdecadal variations of the ocean temperature and its impacts on climate

Decadal/interdecadal climate variability is an important research focus of the CLIVAR Program and has been paid more attention. Over recent years, a lot of studies in relation to interdecadal climate variations have been also completed by Chinese scientists. This paper presents an overview of some advances in the study of decadal/interdecadal variations of the ocean temperature and its climate impacts, which includes interdecadal climate variability in China, the interdecadal modes of sea surface temperature (SST) anomalies in the North Pacific, and in particular, the impacts of interdecadal SST variations on the Asian monsoon rainfall. As summarized in this paper, some results have been achieved by using climate diagnostic studies of historical climatic datasets. Two fundamental interdecadal SST variability modes (7– 10-years mode and 25–35-years mode) have been identified over the North Pacific associated with different anomalous patterns of atmospheric circulation. The southern Indian Ocean dipole (SIOD) shows a major feature of interdecadal variation, with a positive (negative) phase favoring a weakened (enhanced) Asian summer monsoon in the following summer. It is also found that the China monsoon rainfall exhibits interdecadal variations with more wet (dry) monsoon years in the Yangtze River (South China and North China) before 1976, but vice versa after 1976. The weakened relationship between the Indian summer rainfall and ENSO is a feature of interdecadal variations, suggesting an important role of the interdecadal variation of the SIOD in the climate over the south Asia and southeast Asia. In addition, evidence indicates that the climate shift in the 1960s may be related to the anomalies of the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO). Overall, the present research has improved our understanding of the decadal/interdecadal variations of SST and their impacts on the Asian monsoon rainfall. However, the research also highlights a number of problems for future research, in particular the mechanisms responsible for the monsoon long-term predictability, which is a great challenge in climate research.     

年代际气候变率问题的研究

对年代际气候变率的研究工作和主要结果作了综合评述，并对该领域的研究前景作了一定的阐述。     

Hydrologic impacts of climate change on the Nile River Basin: implications of the 2007 IPCC scenarios

We assess the potential impacts of climate change on the hydrology and water resources of the Nile River basin using a macroscale hydrology model. Model inputs are bias corrected and spatially downscaled 21st Century simulations from 11 General Circulation Models (GCMs) and two global emissions scenarios (A2 and B1) archived from the 2007 IPCC Fourth Assessment Report (AR4). While all GCMs agree with respect to the direction of 21st Century temperature changes, there is considerable variability in the magnitude, direction, and seasonality of projected precipitation changes. Our simulations show that, averaged over all 11 GCMs, the Nile River is expected to experience increase in streamflow early in the study period (2010–2039), due to generally increased precipitation. Streamflow is expected to decline during mid- (2040–2069) and late (2070–2099) century as a result of both precipitation declines and increased evaporative demand. The predicted multimodel average streamflow at High Aswan Dam (HAD) as a percentage of historical (1950–1999) annual average are 111 (114), 92 (93) and 84 (87) for A2 (B1) global emissions scenarios. Implications of these streamflow changes on the water resources of the Nile River basin were analyzed by quantifying the annual hydropower production and irrigation water release at HAD. The long-term HAD release for irrigation increases early in the century to 106 (109)% of historical, and then decreases to 87 (89) and 86 (84)% of historical in Periods II and III, respectively, for the A2 (B1) global emissions scenarios. Egypt’s hydropower production from HAD will be above the mean annual average historical value of about 10,000 GWH for the early part of 21st century, and thereafter will generally follow the streamflow trend, however with large variability among GCMs. Agricultural water supplies will be negatively impacted, especially in the second half of the century.     

Role of the Indian Ocean sea surface temperature in shaping the natural variability in the flow of Nile River

A significant fraction of the inter-annual variability in the Nile River flow is shaped by El Niño Southern Oscillation (ENSO). Here, we investigate a similar role for the Indian Ocean (IO) sea surface temperature (SST) in shaping the inter-annual variability of the Nile River flow. Using observations of global SST distribution and river flow in addition to atmospheric general circulation model sensitivity experiments, we show that North and Middle IO SSTs play a significant intermediate role in the teleconnection between ENSO and the Nile flow. Applying partial coherency analyses, we demonstrate that the connection between North and Middle IO SSTs and Nile flow is strongly coupled to ENSO. During El Niño events, SST in the North and Middle IO increases in response to the warming in the Tropical Eastern Pacific Ocean and forces a Gill-type circulation with enhanced westerly low-level flow over East Africa and the Western IO. This anomalous low-level flow enhances the low-level flux of air and moisture away from the Upper Blue Nile (UBN) basin resulting in reduction of rainfall and river flow. SSTs in the South IO also play a significant role in shaping the variability of the Nile flow that is independent from ENSO. A warming over the South IO, generates a cyclonic flow in the boundary layer, which reduces the cross-equatorial meridional transport of air and moisture towards the UBN basin, favoring a reduction in rainfall and river flow. This independence between the roles of ENSO and South IO SSTs allows for development of new combined indices of SSTs to explain the inter-annual variability of the Nile flow. The proposed teleconnections have important implications regarding mechanisms that shape the regional impacts of climate change over the Nile basin.     

A timescale decomposed threshold regression downscaling approach to forecasting South China early summer rainfall

A timescale decomposed threshold regression(TSDTR) downscaling approach to forecasting South China early summer rainfall(SCESR) is described by using long-term observed station rainfall data and NOAA ERSST data. It makes use of two distinct regression downscaling models corresponding to the interannual and interdecadal rainfall variability of SCESR.The two models are developed based on the partial least squares(PLS) regression technique, linking SCESR to SST modes in preceding months on both interannual and interdecadal timescales. Specifically, using the datasets in the calibration period 1915–84, the variability of SCESR and SST are decomposed into interannual and interdecadal components. On the interannual timescale, a threshold PLS regression model is fitted to interannual components of SCESR and March SST patterns by taking account of the modulation of negative and positive phases of the Pacific Decadal Oscillation(PDO). On the interdecadal timescale, a standard PLS regression model is fitted to the relationship between SCESR and preceding November SST patterns. The total rainfall prediction is obtained by the sum of the outputs from both the interannual and interdecadal models. Results show that the TSDTR downscaling approach achieves reasonable skill in predicting the observed rainfall in the validation period 1985–2006, compared to other simpler approaches. This study suggests that the TSDTR approach,considering different interannual SCESR-SST relationships under the modulation of PDO phases, as well as the interdecadal variability of SCESR associated with SST patterns, may provide a new perspective to improve climate predictions.     

Pacific decadal oscillation and variability of the Indian summer monsoon rainfall

Recent studies have furnished evidence for interdecadal variability in the tropical Pacific Ocean. The importance of this phenomenon in causing persistent anomalies over different regions of the globe has drawn considerable attention in view of its relevance in climate assessment. Here, we examine multi-source climate records in order to identify possible signatures of this longer time scale variability on the Indian summer monsoon. The findings indicate a coherent inverse relationship between the inter-decadal fluctuations of Pacific Ocean sea surface temperature (SST) and the Indian monsoon rainfall during the last century. A warm (cold) phase of the Pacific interdecadal variability is characterized by a decrease (increase) in the monsoon rainfall and a corresponding increase (decrease) in the surface air temperature over the Indian subcontinent. This interdecadal relationship can also be confirmed from the teleconnection patterns evident from long-period sea level pressure (SLP) dataset. The SLP anomalies over South and Southeast Asia and the equatorial west Pacific are dynamically consistent in showing an out-of-phase pattern with the SLP anomalies over the tropical central-eastern Pacific. The remote influence of the Pacific interdecadal variability on the monsoon is shown to be associated with prominent signals in the tropical and southern Indian Ocean indicative of coherent inter-basin variability on decadal time scales. If indeed, the atmosphere–ocean coupling associated with the Pacific interdecadal variability is independent from that of the interannual El Niño-Southern Oscillation (ENSO), then the climate response should depend on the evolutionary characteristics of both the time scales. It is seen from our analysis that the Indian monsoon is more vulnerable to drought situations, when El Niño events occur during warm phases of the Pacific interdecadal variability. Conversely, wet monsoons are more likely to prevail, when La Niña events coincide during cold phases of the Pacific interdecadal variability.     

关于年代际气候变化可能机制的研究

年代际气候变化作为年际和月季气候变化的重要背景,往往影响着年际和月季时间尺度的气候及特征。随着科学的发展进步和社会需求的提高,年代际气候变化已成为人们关注的重要问题。作为气候动力学和气候预测研究的重要内容之一,年代际气候变化及其动力学机制的研究在国内外都在蓬勃开展,并取得了不少的成果。本文除简要介绍了中国气候的年代际变化特征,将着重就年代际气候变化的可能机制作一个系统的综合性讨论,内容主要包括全球主要海温变化模态的影响、气候系统相互关系年代际变化的影响、大气行星尺度系统年代际变化的影响,以及太阳活动及火山爆发的影响等。大家知道,年代际气候变化研究十分重要,但也可以看到年代际气候变化的动力学机制却十分复杂,不少问题还没有搞的十分清楚,需要加大力量进行深入研究;我们相信,深入的研究结果必将对年代际气候变化的预测提供可靠的科学依据,进而推动年代际气候变化的业务预测及其能力的提高。     

Dry spell,onset and cessation of the wet season rainfall in the Upper Baro-Akobo Basin,Ethiopia

In this study, maximum dry spell length and number of dry spell periods of rainy seasons in the upper Baro-Akobo River basin which is a part of the Nile basin, Western Ethiopia, were investigated to analyse the drought trend. Daily rainfall records of the period 1972–2000 from eight rain gauge stations were used in the analysis, and Mann-Kendall test was used to test trends for significance. Furthermore, the beginning and end of the trend development in the dry spell were also tested using the sequential version of Mann-Kendall test. Results have shown that there is neither clear monotonic trend found in dry spell for the basin nor significant fluctuation in the onset, cession and duration of rainfall in the Baro-Akobo river basin. This sufficiently explains why rain-fed agriculture has suffered little in the western part of Ethiopia. The predictable nature of dry spell pattern may have allowed farmers to adjust to rainfall variability in the basin. Unlike many parts of Ethiopia, the Baro-Akobo basin climate variability is not a limiting factor for rain-fed agriculture productivity which may contribute significantly to national food security.     

Climate change impacts on water management in the Puget Sound region, Washington State, USA

Climate change is projected to result, on average, in earlier snowmelt and reduced summer flows in the Pacific Northwest, patterns not well represented in historical observations used in water planning. We evaluate the sensitivities of water supply systems in the Puget Sound basin cities of Everett, Seattle, and Tacoma to historical and projected future streamflow variability and water demands. We simulate streamflow for the 2020s, 2040s, and 2080s using the distributed hydrology–soil–vegetation model (DHSVM), driven by downscaled ensembles of climate simulations archived from the 2007 IPCC Fourth Assessment Report. We use these streamflow predictions as inputs to reservoir system models for the three water supply systems. Over the next century, under average conditions all systems are projected to experience declines and eventual disappearance of the springtime snowmelt peak. How these shifts affect management depends on physical characteristics, operating objectives, and the adaptive capacity of each system. Without adaptations, average seasonal drawdown of reservoir storage is projected to increase in all three systems throughout the 21st century. Reliability of all systems in the absence of demand increases is robust through the 2020s however, and remains above 98% for Seattle and Everett in the 2040s and 2080s. With demand increases, however, reliability of the systems in their current configurations and with current operating policies progressively declines through the century.     

未来气候变化对黄河和长江流域极端径流影响的预估研究

使用NASA-NCAR全球环流模式FvGCM结果驱动高分辨率区域气候模式RegCM3 (20 km),进行1961～1990年当代气候模拟(控制试验)和2071～2100年IPCC A2排放情景下未来气候情景模拟(A2情景模拟试验)。将RegCM3同高分辨率大尺度汇流模型LRM(分辨率0.25°×0.25°)连接,分析水文极端事件在A2情景下相对于当代气候的变化,预估未来气候变化对我国黄河和长江流域水文极端事件的影响。结果表明:(1)未来黄河流域径流年变率增大,月变率减小,日变率在头道拐站以上流域减小,以下流域增大。未来兰州以上半湿润地区,流域东南部湿润区出现径流量峰值的可能性增大,而流域西北部干旱半干旱区出现径流量百分位极值的可能性减小。未来黄河流域中游地区发生流域洪水的风险在夏季月份减少,其余月份均增大。(2)未来长江干流径流年际变率增大,上中游地区径流日和月变率减小,下游地区略有增大;未来汉江流域径流量的年、月和日变率均增大。未来长江干流发生流域洪水的风险在夏季明显降低,而汉江流域各月发生流域洪水的可能性均增大。     

Interdecadal modulation of Australian rainfall

Interdecadal variability is investigated in a 300 year run of the Parallel Climate Model, a global coupled atmosphere-land-ocean-sea ice model. The model simulates El Niño variability of realistic magnitude and is found to produce interdecadal characteristics similar to those observed, both in frequency, spatial patterns and amplitude. Modulation of Australian rainfall on interdecadal time scales is similar to observed and is found to have contributions from both the modulation of ENSO, changes in the position of the Walker circulation and variations in western Pacific SSTs. A slackening of the equatorial Pacific thermocline slope is associated with diminished ENSO variability during interdecadal periods of positive tropical Pacific SSTs. These interdecadal changes to ENSO and shifts in the position of the Walker circulation are physical mechanisms that contribute to the weakened correlations between the SOI and Australian climate during interdecadal periods of positive tropical Pacific SSTs. Warm anomalies in the western Pacific also contribute to a decrease in Australian rainfall in the model on interdecadal time scales.     

Analysis of Durban rainfall and Nile river flow 1871–1999

Summary  Climatic fluctuations across Africa are analysed from two century+ records of rainfall at Durban, South Africa and the Nile River flow in southern Egypt. A wavelet transform analysis is applied to the rainfall record to determine the strength of intra-seasonal to decadal rhythms. The annual cycle constitutes 33% of variability, whilst 2.3–4 year cycles account for 10% of the variance. A contingency analysis of flood events reveals a bimodal character with peaks in November and March. The Durban rainfall time series is compared with remote environmental variables. Close relationships are found with the meridional gradient of sea surface temperature in the Atlantic and the southern oscillation index. Comparisons are made between the southern summer rainfall at Durban (November–March) and northern summer Nile River flow (July–October). Cross-wavelet analysis of the two records indicates a matching of frequency in quasi-biennial and El Ni?o frequency bands. This suggests that the uptake of ‘teleconnections’ governing African climate occurs in a widespread manner. Received February 25, 2000     

SST versus Climate Change Signals in West African Rainfall: 20th-Century Variations and Future Projections

Rainfall variability is a crucial factor in food production,water resource planning and ecosystems, especially in regions with scarce freshwaterresources. In West Africa rainfall has been subject to largedecadal and interdecadal variations during the 20th century. The most prominent feature is thereduction in rainfall amount throughout the second half of the century with somerecovery at the end. Among the conceivable mechanisms, which might inducesuch low-frequency variability in West African precipitation, this study isfocussed onsea surface temperature (SST) variations and increasing greenhouse gas (GHG)concentrations. A tool is presented to distinguish between both impacts bymeans of various climate model simulations, which are found to reproduce theobserved rainfall characteristics over West Africa reasonably well.Further, a multi-model approach is usedto evaluate the expected future greenhouse signal in West African rainfall with respect to natural variability and intermodel variations.It is found that observed SST fluctuations, forcing two different atmospheric climate models, are able to reproduce the main features ofobserved decadal rainfall anomalies in the southern part of West Africathroughout the second half of the 20th century. The seasonal response to varying SST isstrongest in summer when the region is undergoing intensive monsoondynamics. Whereas both atmospheric models simulate the observeddrought tendency,following the 1960s, there is some indication that the additional GHG forcing in one model inducessome significantly different rainfall anomalies in recent years, re-initiatingeven positive anomalies relative to the climatological mean which has alsobeen observed since the 1990s. However, thisresult is still subject to model uncertainty.Coupled climate model integrations with different climate change scenariosalsopredict that precipitation, particularly over the Guinea Coast and Sahelregion, will steadily increase into the 21st century. The model-comprehensive signal isstatistically significant with respect to natural variability and modeluncertainty, suggesting that the observed recovery of yearly rainfall overparts of West Africa might actually reflect the beginning impact of risinganthropogenic GHG. The physical mechanism, linking the radiative forcing tothe monsoonal rainfall, probably works via warming of the tropicalAtlantic Ocean.     

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。