岷江上游水文气象因子多尺度周期性分析

采用Dmey小波变换法对岷江上游杂谷脑流域1959年至2006年月径流量、月平均气温和月降水量不同时间尺度下的变化周期进行分析,探讨三者在长时间序列周期性变化中的相互响应,并根据主周期预测未来气温、降水和径流的变化趋势,结果表明:气温、径流和降水存在多尺度周期性变化,在不同的尺度周期中,表现出不同的冷暖、丰枯和干湿的振荡规律,总体表现为由小尺度无明显规律的剧烈振荡向大尺度有明显规律的振荡变化。3个要素同以8—12个月的小尺度为周期剧烈振荡,在较大时间尺度上,具有明显规律振荡变化的周期分别为气温500个月、径流150个月和降水120个月。受森林砍伐的影响,研究区域在1962—1988年期间径流对降水变化的周期性响应迟钝,而在1988—2006年期间基本同步。根据大尺度周期性波动趋势预测,未来十几年研究区域处于偏暖的年代际背景下,未来6—7a为多雨期,但径流量偏少。     

云南省植被NDVI时间变化特征及其对干旱的响应

基于云南省74个气象站点的1997—2012年逐日降水资料和逐旬SPOT-NDVI值,利用标准化降水蒸散指数(SPEI)多尺度分析了云南省干旱时间和强度演变与NDVI时间动态特征及其相关性分析,进而探讨气候变化对植被的影响。结果表明,1999—2013年云南省年平均NDVI值和年最大NDVI值均呈现波浪式的发展趋势,其趋势线斜率分别为0.0017和0.0011;NDVI年内各月变化情况大体上相同;不同季节NDVI的年际变化特征呈现出显著差异。1997—2012年不同时间尺度SPEI均体现出干旱化加剧的趋势,并随SPEI的时间尺度增大而增大;3个月尺度的SPEI值(SPEI3)结果表明,各月的变化呈现先增大后减小的趋势;SPEI3反映出多年季节水平的干旱强度为:冬季秋季春季夏季。总体上,云南省的年均NDVI与SPEI的相关性极弱,年最大NDVI与SPEI呈正相关;多年月均NDVI与不同尺度SPEI的相关性较强且存在滞后性;不同季节NDVI与SPEI的相关性及滞后性有较大差异,其中冬季NDVI、秋季NDVI与其当年当季SPEI的负相关性较强。     

基于SPI的石羊河流域气候干湿变化及干旱事件的时空格局特征研究

利用石羊河流域4个气象站1960—2013年逐月降水量资料,应用标准化降水指数(SPI)、游程理论等方法,分析石羊河流域近54年的气候干湿变化及不同时间尺度干旱事件时空演变特征。结果表明:1不同时间尺度SPI对降水量变化的敏感程度存在较大差异,时间尺度越小,SPI对一次降水的反应越明显。2石羊河流域年代际、年际和季的SPI在波动中均呈增加趋势,其中,冬季湿润化速度最快,对年湿润化过程的贡献最显著。31960—2013年,石羊河流域不同时间尺度干旱事件持续时间、干旱程度和干旱强度均呈减少趋势,且波动渐趋平缓;石羊河流域农业干旱和水文干旱最严重的时期分别为1964—1965年和1962—1964年。4两种时间尺度干旱事件持续时间的减少速度从上游至下游均逐渐变慢,上游乌鞘岭地区是农业干旱持续时间最长区域,永昌和下游民勤地区是水文干旱持续时间较长区域;两种时间尺度干旱事件干旱程度减少速度最快区域均在上游乌鞘岭地区;武威和民勤地区3月尺度干旱强度稍有上升,其它地区不同时间尺度干旱事件干旱强度均呈下降趋势,永昌地区是多尺度干旱事件干旱强度较大区域。     

东北地区植被NDVI对不同时间尺度SPEI的响应

植被在陆地生态系统中起至关重要的作用。近年来干旱频率逐渐增加,对植被的生长发育产生严重的影响。本研究基于1982—2015年的归一化植被指数(normalized difference vegetation index,NDVI)和不同时间尺度的标准化降水蒸散指数(standardized precipitation evapotranspiration index,SPEI)的相关性来评估东北地区植被生长状况对干旱的响应。结果表明:年尺度和季尺度内NDVI和SPEI的最大相关系数值在研究区内西部内蒙古草原地区最大,东部长白山脉地区次之,中东部东北平原地区最小,且NDVI与SPEI的相关系数在12个月以上的时间尺度较大;生长季(4—10月)和夏季(6—8月)的NDVI对SPEI响应最为敏感,春季(4—5月)、秋季(9—10月)响应依次减弱;其中,5—7月的NDVI与SPEI-24的显著相关像元比例较高,而8—10月的NDVI则与SPEI-06的显著相关像元比例较高;不同的植被类型对干旱响应不同,森林NDVI在不同月份与不同尺度的SPEI大体上呈负相关,草原和农作物则主要以正相关为主。     

2001-2015年天山北坡植被覆盖对干旱的响应——基于土地利用/土地覆盖分析

天山北坡生态系统脆弱,易受干旱影响,全球变暖和不合理的人类活动加剧了干旱的影响,评估植被覆盖对干旱的响应,为改善生态环境和减轻干旱影响提供科学参考。基于MODIS-NDVI遥感数据以及气象数据,计算了2001-2015年天山北坡多尺度标准化降水蒸散发指数（SPEI）和植被覆盖度,总结出植被覆盖度与多尺度SPEI时空动态变化规律,从土地利用/土地覆盖（LUCC）的角度分析了植被覆盖度对气候干旱的响应。结果表明：（1）天山北坡绝大部分区域呈湿润状态,中部（石河子、呼图壁）、西北部（克拉玛依市）呈轻度干旱。3个月、6个月、12个月时间尺度SPEI均表现出干旱化增强的年际变化趋势;（2）天山北坡植被覆盖度整体上属中低覆盖,总体呈南高北低,天山山区、城市绿洲内部高的分布特点。2001-2015年天山北坡植被覆盖度变化总体呈下降趋势;（3）年尺度天山北坡植被覆盖度与SPEI整体呈正相关关系。不同土地利用/土地覆盖的植被覆盖度与12个月时间尺度SPEI（SPEI 12）的相关性不同,大小依次为：草地 > 未利用地 > 城乡用地 > 林地 > 水域 > 耕地;（4）季节尺度夏季和春季干旱对植被覆盖度的影响最明显,不同季节干旱对不同土地利用/土地覆盖植被覆盖度影响程度不同。     

基于主成分分析的广西省干旱时空格局

广西省地处喀斯特地貌区,土壤保水效率低,且年降水时空分布不均,研究干旱时空分布尤为重要。基于广西省1981—2010年20个气象站实测和2011—2100年HadGEM2-ES模型模拟数据,利用标准化降水指数(SPI, Standardized Precipitation Index)和标准化降水蒸散指数(SPEI,Standardized Precipitation Evapotranspiration Index),分析广西省干旱的时空变化。对不同时间尺度的SPI和SPEI应用主成分分析(PCA,Principal Component Analysis)确定干旱的空间模式,结果揭示了3个空间分布明确的区域:桂东北地区(PC1),桂西北地区(PC2)和桂南地区(PC3)。各区域干旱的时空变化和频率分布差异显著。PC1和PC3的SPEI-12呈负增长趋势,PC2的SPEI-12呈正增长趋势,且PC1、PC2和PC3的SPI-12均大于SPEI-12。年尺度(SPEI-12)上PC1和PC3的干旱频率大于PC2,其干旱频率分别为34.24%和35.83%。SPI和SPEI在空间和时间尺度上相关,且具有高度可比性。SPEI因子载荷的空间模式优于SPI,严重干旱年份(1988、1996和2003年)的SPI值明显小于SPEI,且SPEI检测到的干旱频率较高。研究结果可作为广西省干旱预测、评估及其风险管理和应用决策的重要科学基础,也可为广西省区域社会经济发展提供重要的科学参考。     

淮河流域干旱时空演变特征及成因

基于淮河流域149气象站点1962—2016年标准化降水蒸散发指数、16个气候因子和NCEP/NCAR再分析资料,通过小波分析和旋转经验正交函数等手段,分析淮河流域干旱重心的转移轨迹,研究气象干旱与气候因子的相关关系,并通过大气环流的异常特征揭示气象干旱的主要成因。结果表明:(1)干旱重心分布主要从淮河流域中心向四周扩散,淮河流域大范围区域呈干旱化态势。2013年干旱重心从西北部→中部→西南部→中部变化,随着干旱面积的增大,干旱重心由四周向中心移动。(2)PDO、ONI、Nino4、Nino3.4、MEI、BEST与SPEI均呈显著正相关关系,SOI、TNI与SPEI则呈显著负相关关系。(3)干旱周期主要集中在1970年代、1990年代和2000年代存在2—5年显著周期,气候因子在3.4—4.5年存在显著周期。(4)春季高纬度地区的气流南下,与印度洋、孟加拉湾北上气流导致南湿北干;夏季蒙古气旋偏弱与异常偏北风覆盖导致东干西湿;秋季大陆高压控制,偏北风和南风相互影响造成东干西湿;冬季盛行下沉气流与盛行东南风造成东湿西干的气候特征。     

干旱和林分因子对树木死亡的影响——以美国德克萨斯州东部国家森林为例

为探讨不同时间尺度、气候因子及林分因子对森林中树木死亡的影响,本研究以美国德克萨斯州东部的4个国家森林中264个重复调查的森林样地为对象,使用近20年来美国森林清查4个周期的数据,估算其在清查周期和年度水平上的树木死亡率变化,并使用广义线性混合效应模型来分析气候因子(干旱强度、干旱持续时间、年均温和年降水量)、树木大小(胸径)和林分因子(树木胸高断面积、林分密度和林分年龄)对树木存活的影响。结果表明: 在重度干旱当年和重度干旱的清查周期中,森林的树木死亡率分别增加了151%和123%,天气干扰(干旱和飓风)和植物之间的竞争是其主要的影响因素;干旱强度(标准化降水蒸散发指数,SPEI)和干旱持续时间对树木的存活具有显著的负效应,年降水量对树木的存活具有显著的正效应;树木胸高面积对树木存活具有显著的负效应,树木大小、林分年龄和林分密度对树木存活均具有显著的正效应,但是大树比小树更容易受到天气影响而死亡;在重度干旱的清查周期中,松树种组的树木死亡率(2.1%)比阔叶树木种组(3.9%)低,天然林的树木死亡率(3.0%)高于人工林(1.9%)。在分析树木死亡率时,需同时考虑个体树木大小、林分因子与气候因子的相对重要性。     

SPEI指数在中国东北地区干旱研究中的适用性分析

干旱指数的区域适用性是准确表征区域干旱的重要前提,本文以中国东北地区为典型研究区,探讨标准化降水蒸散指数(SPEI)在该地区应用的有效性。基于研究区90个气象台站的逐日气象资料,计算1961—2014年多时间尺度的SPEI指数。从Kolmogorov-Smirnov(K-S)拟合优度检验、SPEI与典型干旱事件核准、SPEI与农作物受旱灾面积及与土壤湿度相关性分析等方面,验证SPEI指数在东北地区的适用性。分析结果表明:1)东北地区多时间尺度的累积水分亏缺量符合Log-logistic分布,SPEI指数在东北地区的应用具备数学统计理论基础;2)生长季平均SPEI值与黑龙江省、吉林省和辽宁省农作物受旱灾面积比例均呈极显著负相关(P0.01);3)在1、3、6和12个月尺度下,SPEI与土壤湿度呈显著正相关(P0.05)的站点比例分别为90.2%、92.16%、90.2%和88.24%。综上所述,SPEI指数不仅满足数学理论统计的要求,而且与干旱灾情数据和土壤水分监测值均具有极度的关联性,说明其在东北地区干旱预测和定量化研究中具有较好的适用性。     

基于SPEI-PM指数的黄淮海平原干旱特征分析

利用黄淮海平原45个气象站点1961—2014年月值气象数据,基于Penman-Monteith蒸散模型计算了标准化降水蒸散指数(SPEI),对黄淮海平原近54年干旱变化趋势、发生频率和持续性特征进行了分析,并探讨了SPEI指数与河南、河北和山东省农业干旱面积的关系,结果表明:(1)改用Penman-Monteith蒸散公式后,SPEI干旱指数在黄淮海平原呈整体上升趋势,即趋于湿润;(2)近54年干旱演变具有明显的年代际差异,20世纪60年代干旱频率最高,而21世纪初(2000—2014)干旱频率整体偏低;(3)黄淮海平原干旱发生具有持续性的特点,20世纪60年代遭受的持续性干旱最为严重,平均干旱持续时长约2.6个月,21世纪初下降到1.5个月;(4)河南、河北和山东省的农业干旱面积年际变化表明,干旱面积呈减少趋势,2000年以后年均受灾面积、成灾面积和绝收面积比2000年之前分别下降了58.0%、44.4%和49.1%;(5)农业干旱面积与SPEI具有中等以上的相关强度,其中对山东省受灾、成灾和绝收面积相关系数r达到-0.7以上,表明基于Penman-Monteith蒸散模型的SPEI指数在黄淮海平原具有良好的适用性。     

Relationship between fire,climate oscillations,and drought in British Columbia,Canada, 1920–2000

Climate oscillations such as El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are known to affect temperature and precipitation regimes and fire in different regions of the world. Understanding the relationships between climate oscillations, drought, and area burned in the past is required for anticipating potential impacts of regional climate change and for effective wildfire‐hazard management. These relationships have been investigated for British Columbia (BC), Canada, either as part of national studies with coarse spatial resolution or for single ecosystems. Because of BC's complex terrain and strong climatic gradients, an investigation with higher spatial resolution may allow for a spatially complete but differentiated picture. In this study, we analyzed the annual proportion burned–climate oscillation–drought relationships for the province's 16 Biogeoclimatic Ecosystem Classification (BEC) zones. Analyses are based on a digital, spatially explicit fire database, climate oscillation indices, and monthly precipitation and temperature data with a spatial resolution of 400 m for the period 1920–2000. Results show that (1) fire variability is better related to summer drought than to climate oscillations, and that (2) fire variability is most strongly related to both, climate oscillations and summer drought in southeastern BC. The relationship of area burned and summer drought is strong for lower elevations in western BC as well. The influence of climate oscillations on drought is strongest and most extensive in winter and spring, with higher indices being related to drier conditions. Winter and spring PDO and additive winter and spring PDO+ENSO indices show BC's most extensive significant relationship to fire variability. Western BC is too wet to show a moisture deficit in summer that would increase annual area burned due to teleconnections.     

Changes in spring arrival of Nearctic-Neotropical migrants attributed to multiscalar climate

Climate-related changes associated with the California marine ecosystem have been documented; however, there are no studies assessing changes in terrestrial vertebrate phenology on the Pacific coast of western North America. We analyze the spring phenology of 21 Nearctic-Neotropical migratory songbird species in central and northern CA. Using observational and banding data at multiple sites, we evaluate evidence for a change in arrival timing being linked to either nonclimatic or multiscalar climatic explanations. Using correlation analysis, of the 13 species with a significant ( P <0.10) change in arrival, the arrival timing of 10 species (77%) is associated with both temperature and a large-scale climate oscillation index (El Niño Southern Oscillation, ENSO; North Atlantic Oscillation, NAO; and/or Pacific Decadal Oscillation, PDO) at least at one location. Eight of the 13 species (62%) are advancing their migratory timing. All species for which spring arrival is associated with climate at multiple locations are exhibiting changes ( n =5) and all species lacking evidence for association between migration phenology and climate ( n =3) exhibit no change. Migrants tend to arrive earlier in association with warmer temperatures, positive NAO indices, and stronger ENSO indices. Twelve species negatively correlate ( P ≤0.05) with local or regional temperature at least at one location; five species negatively correlate with ENSO. Eleven species' arrival is correlated ( P ≤0.05) with NAO; 10 are negatively associated. After an exhaustive literature search, this is apparently the first documentation of an association between NAO and migratory phenology in western North America.     

近57年来黄土高原干旱特征及其与大气环流的关系

通过黄土高原地区52个气象站点1961-2017年的气象资料,利用不同尺度的标准化降水指数（Standardized Precipitation Index,SPI）和标准化降水蒸散指数（Standardized Precipitation Evapotranspiration Index,SPEI）对该区57年来干旱的时空变化特征进行了分析,并利用交叉小波变换探讨了干旱指标与大气环流的遥相关分析,得到了以下结论：（1）时间变化上,黄土高原57年来不同干旱指标均呈下降趋势,整体逐渐变干旱。但SPEI6指标较SPI6指标相比,干旱年份更多,干旱特征更明显,说明SPEI的计算由于考虑了蒸散发输入因此结果偏重.不同指标均显示,1999年以来,黄土高原地区干旱时有发生,但整体有降低的趋势。（2） SPEI6和SPI6的站次比和干旱强度最高点都出现在1999年,但SPEI6的站次比和干旱强度的变化幅度更剧烈,且出现全域性干旱的年份（5年）也多于SPI6（3年）;SPEI12相较于SPI12,站次比和干旱强度较为相似,都在1966年达到顶峰,虽然出现全域性干旱的年份SPEI12（9年）多于SPI12（3年）,但SPI12的干旱强度更高。（3）平原区的汾渭平原是轻旱多发区,河套平原、宁夏平原易发生中旱,同时宁夏平原还是重旱多发区。丘陵区西部的中宁、同心两地易发生重旱,乌审旗出现特旱。山地区干旱频率普遍较高,尤其是西部山地区的乌鞘岭重旱、特旱频发。（4） SPEI指数对环流指数的变化更敏感。AMO对区域各干旱指标的影响较小,ENSO、WPI对SPI6、SPEI6有显著的响应;而PNA对6个月尺度的干旱指标（SPI6、SPEI6）影响较小,对12个月尺度的干旱指标（SPI12、SPEI12）影响较大。区域干旱是一个复杂的自然现象,为了进一步探索不同干旱指标在不同区域的运用,必要时可采用多种指标,从不同角度比较多种干旱指标的相似性,从而避免单一指标对结果的局限性。     

An unstable tree-growth response to climate in two 500 year chronologies,North Eastern Qinghai-Tibetan Plateau

Two new Juniper tree-ring-width (TRW) chronologies spanning more than 500 years were developed in the Yellow River source area, North Eastern Qinghai-Tibetan Plateau (NE-QTP). For the two studied sites, located approximately 50 km apart, split correlation and coherence analysis reveal unstable tree-growth responses to local moisture availability. While significant correlations are obtained with April–June local precipitation, Palmer Drought Severity Index (PDSI) and river flow from 1948/1954 to 1998 and from 1948/1954 to 1970s, these correlations vanish for the time period 1970s-1998. The local instrumental climate data (precipitation, PDSI and river flow) exhibit opposite correlations with large scale modes of variability (El Niño Southern Oscillation, ENSO, and Pacific Decadal Oscillation, PDO) before and after the 1977 PDO shift. One tree-ring chronology is coherent and anti-phased with instrumental ENSO/PDO indices at 5.2-year frequency. On the longer time span, this TRW chronology is compared with PDO reconstructed from historical Chinese data. This comparison also exhibits unstable multi-decadal relationships, notably in the mid 19th century. Altogether, the comparison between our two chronologies, local instrumental climate records, and ENSO/PDO indices suggest a cautious use of local TRW records for paleoclimate reconstructions. Further studies are needed to explore both the spatial coherency of tree-ring records and the temporal stability of their response to local and large scale climate variability.     

Relationships of subalpine forest fires in the Colorado Front Range with interannual and multidecadal‐scale climatic variation

Aim An understanding of past relationships between fire occurrence and climate variability will help to elucidate the implications of climate‐change scenarios for future patterns of wildfire. In the present study we investigate the relationships between subalpine‐zone fire occurrence and climate variability and broad‐scale climate patterns in the Pacific and Atlantic Oceans at both interannual and multidecadal time‐scales. Location The study area is the subalpine zone of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern sector of the Rocky Mountain National Park, which straddles the continental divide of the northern Colorado Front Range. Methods We compared years of widespread fire from AD 1650 to 1978 for the subalpine zone of southern Rocky Mountain National Park, with climate variables such as measures of drought, and indices such as the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). Results Years of extensive subalpine‐zone fires are significantly related to climate variability, phases of ENSO, the PDO, and the AMO, as well as to phase combinations of ENSO, the PDO, and the AMO at both interannual and centennial time‐scales. Main conclusions Years of extensive fires are related to extreme drought conditions and are significantly related to the La Niña phase of ENSO, the negative (cool) phase of the PDO, and the positive (warm) phase of the AMO. The co‐occurrence of the phase combination of La Niña‐negative PDO‐positive AMO is more important to fire occurrence than the individual influences of the climate patterns. Low‐frequency trends in the occurrence of this combination of climate‐pattern phases, resulting from trends in the AMO, are the primary climate pattern associated with periods of high fire occurrence (1700–89 and 1851–1919) and a fire‐free period (1790–1850). The apparent controlling influence of the AMO on drought and years of large fires in the subalpine forests of the Colorado Front Range probably applies to an extensive area of western North America.     

Influences of the atmospheric patterns on unstable climate-growth associations of western Mediterranean forests

Quantifying climate-growth associations is needed to evaluate how forest productivity will respond to climate change. Year-to-year fluctuations in forest productivity and radial growth are partly explained by local climatic conditions driven by large-scale atmospheric patterns. This is illustrated by Iberian forests in the western Mediterranean Basin, which are subjected to complex climatic and atmospheric influences such as Atlantic and Mediterranean cyclogenesis. The North Atlantic Oscillation (NAO) is one of the major atmospheric circulation patterns affecting Iberian forests since positive winter NAO phases lead to dry and warm conditions. The Western Mediterranean Oscillation (WeMO) may also explain Iberian forest growth in some areas since this index captures Mediterranean cyclogenesis and WeMO negative phases are linked to warm and wet spring to summer conditions. Here, we analyze the associations between atmospheric patterns, climate and tree growth and we determine if they are changing through time. We use dendrochronology to relate radial growth of four tree species (Pyrenean oak, Sweet chestnut, Maritime pine and Scots pine) growing in western Spain to climate conditions and the NAO and WeMO indices. Winter and early spring temperatures increased since the 1950s in the area whereas the negative association between winter precipitation and the NAO strengthened since then. However, mean temperature rise was particularly evident since the 1970s. Growth was reduced by dry conditions during the growing season (spring and summer), but also by cold and dry conditions during the previous autumn and winter. This explains why the NAO January and the WeMo April indices were negative to growth of three species excluding Pyrenean oak. The early 1970s reflected an inflection point in the instability of climate-growth associations in the study area. We conclude that the winter NAO is a relevant driver of forest growth in the western Iberian Peninsula forests but additional atmospheric patterns (WeMO) also affect, albeit to a minor extent, these forests.     

Influences of the El Niño/Southern Oscillation and the North Atlantic Oscillation on avian productivity in forests of the Pacific Northwest of North America

To model the effects of global climate phenomena on avian population dynamics, we must identify and quantify the spatial and temporal relationships between climate, weather and bird populations. Previous studies show that in Europe, the North Atlantic Oscillation (NAO) influences winter and spring weather that in turn affects resident and migratory landbird species. Similarly, in North America, the El Niño/Southern Oscillation (ENSO) of the Pacific Ocean reportedly drives weather patterns that affect prey availability and population dynamics of landbird species which winter in the Caribbean. Here we show that ENSO‐ and NAO‐induced seasonal weather conditions differentially affect neotropical‐ and temperate‐wintering landbird species that breed in Pacific North‐west forests of North America. For neotropical species wintering in western Mexico, El Niño conditions correlate with cooler, wetter conditions prior to spring migration, and with high reproductive success the following summer. For temperate wintering species, springtime NAO indices correlate strongly with levels of forest defoliation by the larvae of two moth species and also with annual reproductive success, especially among species known to prey upon those larvae. Generalized linear models incorporating NAO indices and ENSO precipitation indices explain 50–90% of the annual variation in productivity reported for 10 landbird species. These results represent an important step towards spatially explicit modelling of avian population dynamics at regional scales.     

Effects of precipitation and temperature on crop production variability in northeast Iran

Climate variability adversely impacts crop production and imposes a major constraint on farming planning, mostly under rainfed conditions, across the world. Considering the recent advances in climate science, many studies are trying to provide a reliable basis for climate, and subsequently agricultural production, forecasts. The El Niño-Southern Oscillation phenomenon (ENSO) is one of the principle sources of interannual climatic variability. In Iran, primarily in the northeast, rainfed cereal yield shows a high annual variability. This study investigated the role played by precipitation, temperature and three climate indices [Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and NINO 3.4] in historically observed rainfed crop yields (1983–2005) of both barley and wheat in the northeast of Iran. The results revealed differences in the association between crop yield and climatic factors at different locations. The south of the study area is a very hot location, and the maximum temperature proved to be the limiting and determining factor for crop yields; temperature variability resulted in crop yield variability. For the north of the study area, NINO 3.4 exhibited a clear association trend with crop yields. In central locations, NAO provided a solid basis for the relationship between crop yields and climate factors.     

Sub-saharan desertification and productivity are linked to hemispheric climate variability

Vegetation productivity and desertification in sub‐Saharan Africa may be influenced by global climate variability attributable to the North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO). Combined and individual effects of the NAO and ENSO indices revealed that 75% of the interannual variation in the area of Sahara Desert was accounted for by the combined effects, with most variance attributable to the NAO. Effects were shown in the latitudinal variation on the 200 mm isocline, which was influenced mostly by the NAO. The combined indices explained much of the interannual variability in vegetation productivity in the Sahelian zone and southern Africa, implying that both the NAO and ENSO may be useful for monitoring effects of global climate change in sub‐Saharan Africa.