Dry conditions over Argentina and the related monthly circulation patterns

Summary This paper presents an analysis of the frequency and spatial distribution of droughts in Argentina during the 20th century. Special attention is given to the mean monthly atmospheric circulation associated with dry conditions in the Pampas during the second half of the century. A reduction in the number of dry cases is found throughout the century, especially during the warm season (October to March). Droughts occurred more often in only one or two regions simultaneously and only a few covered a large territory. Many of the dry months in the Pampas occurred under, or were preceded by, neutral ENSO conditions. During the warm season the ENSO cold phase preceded some of the dry months. Circulation patterns at 1000 hPa and 500 hPa and their temporal evolution are determined using Principal Component Analysis. A coupled 1000-500 hPa level analysis is performed to study the vertical coherence associated with dry conditions. Different patterns related to the warm season are clearly identified using this methodology. The principal feature are positive anomalies over the continent (principally at low levels), high circulation index and an enhancement of the westerlies. Patterns related to the cold season reflect high pressure at the surface, but some differences pre- and post- 1970 have been observed. In this season, some dry cases were found to have high correlations with opposites modes of PC scores. In these cases, a water deficit acted as the principal causes of the dry conditions.     

Abrupt changes in rainfall in the Eastern area of La Pampa Province, Argentina

The eastern area of La Pampa Province, Argentina, lies in a transition zone between the humid temperate climate stretching east and the steppe climate stretching west. The area is thus very sensitive to abrupt changes in rainfall. In order to determine the long-term occurrence of such phenomena, long-term annual precipitation series (1921?C2004) from 17 stations in the study area were analyzed using the Buishand and Pettitt tests. Results showed a sharp increase in annual rainfall at the southern stations in the 1960s and at the northern and central stations in the 1970s. Increased rainfall can be considered one of the reasons for the subsequent expansion in land planted to crops in the region. While a rapid increase in rainfall can be seen as positive, some researchers believe that if an abrupt decrease in rainfall occurred in future and continued for long, the carrying capacity of the environment could be exceeded, leading to decreased production and environmental degradation.     

Large-scale Circulation Anomalies Associated with Interannual Variation in Monthly Rainfall over South China from May to August简

Interannual variation in summer rainfall over South China （SC） was investigated on the monthly timescale.It was found that monthly rainfall from May to August exhibits different features of variation,and the amounts are basically independent of each other.There is a significant negative correlation,however,between May and July SC rainfall,which is partially related to the developing phases of ENSO events.It was also found that stronger （weaker） lower-tropospheric winds over SC and the upstream parts are responsible for more （less） SC rainfall in every month from May to August.Despite this monthly consistent enhancement of horizontal winds,the wind anomalies exhibit distinct differences between May-June and July-August,due to the remarkable change in climatological winds between these two periods.More SC rainfall is associated with a lower-tropospheric anticyclonic anomaly over the SCS and the Philippine Sea in May and June,but with a cyclonic anomaly centered over SC in July and August.     

北京地区短时强降水过程的多尺度环流特征

为了探讨不同天气尺度背景下,北京地区短时强降水过程的基本特征,利用2007-2014年6-8月北京地区自动气象站观测数据和ECMWF ERA-Interim（0.5°×0.5°）全球再分析数据,在对北京地区短时强降水日的大尺度环流特征进行分型的基础上,基于分型合成场和距平场分析了北京地区短时强降水天气过程的基本环流背景及相应的中尺度环流特征。结果表明：（1）造成北京地区出现短时强降水过程的天气系统,依据其出现的频次,大体可分为副热带高压（副高）与西来槽相互作用型、西风小槽型、东北冷涡型和黄淮低涡倒槽型等4类;从低层水汽来看,除东北冷涡型主要来自于渤海、黄海外,其他3型短时强降水过程的水汽主要来自中国南海或东海。（2）不同天气系统主导下的短时强降水时空分布存在较大差异：在空间分布上,黄淮低涡倒槽型短时强降水带分布从北京东南平原穿过城区至西北山前成东南-西北走向,其余3型大体上沿北京地形成西南-东北走向,其中,西南山前、城区和东北山前地区是3个短时强降水事件的多发中心;在时间分布上,东北低涡型造成的短时强降水过程主要发生在午后,副高与西来槽相互作用型主要集中在傍晚至前半夜,而西风小槽型和黄淮低涡倒槽型短时强降水表现出较强的夜雨特征。（3）从中尺度环流特征上看,副高与西来槽相互作用型短时强降水过程主要是低层冷空气从北京西部、北部进入,首先触发山区对流,与之对应的雷暴高压逐渐组织化,外侧辐散气流（冷池出流）和山前的偏南风暖湿气流辐合造成对流过程加强;西风小槽型主要是边界层内较强东南风在北京西北部山前受地形阻挡,向两边绕流,西南支气流在西部形成气旋性环流,造成城区西部的对流性天气,东北支气流在东北部山前形成地形辐合线,夜间随着东南气流中偏南分量显著加强,东北部山前地区的辐合上升运动加强,造成东北部山前对流性天气,因此在短时强降水落区上表现为两个分离的多发中心且具有夜发性;东北冷涡型主要是系统性的冷空气从北京北部或西部南下,在山前与低空偏东风形成辐合切变线,触发午后对流性天气;黄淮低涡倒槽型主要是黄淮低涡顶部的低层偏东气流在北京西部山前辐合抬升,触发对流,并逐步演变为中尺度气旋性环流,形成相对组织化的短时强降水。     

江淮夏季降水异常与西印度洋地区大气环流异常的关系

运用NCEP/NCAR再分析资料和中国气候中心整编的160站月平均降水资料应用经验正交函数、线性相关分析等,分析了江淮地区夏季降水异常特征及其与西印度洋区域大气环流年际异常关系的变化及其可能的机理。结果表明:当500 hPa中纬度低槽活动偏多(少),西太平洋副热带高压偏强(弱),东亚夏季风偏强(弱)时,江淮地区降水偏多(少)。进一步分析还发现西印度洋上空的垂直环流与江淮夏季降水存在较好的关系,但这种年际异常之间的联系受到背景场的影响明显:1979—1993年西印度洋垂直上升运动与江淮夏季降水的变化趋势基本相反,两者线性相关系数为-0.43;1994—2010年两者的变化趋势基本一致,相关系数达0.71。即当西印度洋地区存在环流异常下沉(上升)时,西太平洋副热带高压通常异常减弱东退(增强西伸),副热带季风减弱(增强),有利于雨带偏南(北)。因此,在西太平洋副热带高压和副热带季风年代际偏强(弱)阶段,西印度洋环流与江淮夏季降水呈负(正)相关。     

大气环流系统组合性异常与极端天气气候事件发生

根据2008年1月我国南方发生的持续严重雨雪冰冻灾害、1998年长江流域的特大洪涝灾害和2009/2010年冬季云南的极端干旱灾害的分析结果,再次强调指出,对于一些小概率的极端天气气候事件的发生,大气环流系统的组合性异常起着极其重要的作用。对于2008年的严重雨雪冰冻灾害的发生,多个大气环流系统的组合性异常包括:乌拉尔山阻塞高压和贝加尔湖-巴尔喀什湖的横槽,这为不断有冷空气从西路向南爆发提供了条件;东亚和日本地区的高度正异常使得北方冷空气的势力不是很强,适于锋面在我国南岭及其以北地区较长时间停留,为持续降水确立了背景;西太平洋副高偏强和偏西也对冷空气的向南推进起了阻挡作用;印-缅槽的持续偏强和西太平洋副高的偏强共同使暖湿空气源源不断地输送到华南地区,有利持续降水的发生,为冰冻造成了条件。对于1998年夏季长江流域的特大洪涝的发生,多个环流系统的异常包括:夏季西南季风涌的活动,西太平洋副热带高压活动,北方冷空气活动和青藏高原对流系统东传的共同作用。对于2009/2010年冬季云南的极端干旱灾害的发生,多个环流系统的异常包括:对流层高层中东地区副热带西风急流减弱,影响Rossby波的活动,不利于青藏高原-孟加拉湾槽的建立;西太平洋副热带高压偏强、位置略为偏南,对低层水汽输入云南起到抑制作用;NAO的负异常所导致的遥相关波列异常,使得东亚北方冷空气活动偏东,不易到达云南地区,还使得南支槽偏弱,暖湿气流也不易到达云南地区。ENSO虽然对中国天气气候变化有相当重要的影响,但并非每次异常天气气候事件的发生都是它的直接影响。对于ENSO影响必须具体分析,才能决定它在异常事件、特别是极端天气气候事件中的确切作用。     

中国南方冬季持续性温湿异常事件的分类和特征分析

利用1981-2010年中国测站逐日气温和降水异常序列,将中国南方冬季持续性异常事件分为冷湿、冷干、暖湿和暖干4类持续性异常事件,并用NCEP-DOE逐日再分析资料对各类持续性异常事件的环流特征进行了分析。结果表明,欧亚大陆中高纬度上空"北高南低"的异常环流形势使得温带急流减弱、副热带急流增强,有利于冷空气向南爆发;而中纬度地区"东高西低"的异常环流则对应西太平洋副热带高压增强北移和南支西风槽的活跃,有利于偏南风水汽输送达到中国南方地区,中国南方降水偏多。受南北异常环流的共同影响,中国南方冬季经常出现持续性异常天气,冷湿（低温雨雪冰冻）事件正是在上述两种异常环流型影响下发生的。因此,考虑与冷湿事件相联系的关键环流系统可能有助于提高中国南方冬季低温雨雪冰冻事件的预报能力。      

近57a南疆帕米尔地区春季降水季节内差异及环流异常分析

基于1961—2017年帕米尔地区3站的日降水资料及NECP/NCAR再分析资料,利用气候诊断分析及多元统计学方法,研究了近57 a来帕米尔地区春季降水特征及季节内差异,并讨论了降水偏多年大气环流异常特征。研究表明:(1)近57 a来,帕米尔地区季节尺度上和月尺度上降水增加明显,旬尺度上有不显著的增加趋势。21世纪初帕米尔地区进入新的多雨期,降水正距平年份强度和频率均有显著增加。(2)500 hPa高度场上3月东欧沿岸脊发展、东欧—西西伯利亚槽加深,4—5月欧洲沿岸槽加深、乌拉尔山脊发展、巴尔喀什湖槽加深是帕米尔地区降水异常偏多的关键系统及指标。(3)降水异常年的高空急流较常年强度更强、位置更偏东,在旬尺度上是一个逐渐减弱西退的过程;其低层风场距平分为西南风距平辐合型、西南风和东北风距平辐合型和气旋式环流距平辐合型,均有利于低层辐合。(4)降水异常偏多年水汽输送以偏西路径为主,同时还有偏东、西南和偏南路径,水汽辐合强度较常年更强,更有利于降水的产生。     

Intraseasonal variations of the Yangtze rainfall and its related atmospheric circulation features during the 1991 summer

The intraseasonal variations of the Yangtze rainfall over eastern China and its related atmospheric circulation characteristics during the 1991 summer are examined based on the gauge-observed rainfall and the NCEP/NCAR reanalysis data. Wavelet analysis shows that during the 1991 summer, the active and break sequences of rainfall over the middle and lower Yangtze Basin are mainly regulated by an oscillatory mode with a period of 15–35 days. An investigation of the circulation features suggests that the 15–35-day oscillation is associated with an anomalous low-level cyclone (anticyclone) appearing alternatively over the northern South China Sea (SCS) and the Philippine Sea, and related to a northeastward (southwestward) shift of the western Pacific subtropical anticyclone over the SCS, leading to a lower tropospheric divergence (convergence) over the Yangtze Basin. In the upper troposphere, the 15–35-day oscillation exhibits a dipole anomaly characterized by an anomalous cyclone (anticyclone) over eastern China and an anomalous anticyclone (cyclone) over the northern Tibetan Plateau, resulting in a southwestward shrinking (northeastward extending) of the South Asian anticyclone, and forming a convergence (divergence) over eastern China. Such a coupled anomalous flow pattern between the lower and upper troposphere favors large-scale descending (ascending) motion, and hence reduced (enhanced) rainfall over the Yangtze Basin. Dynamically, the intraseasonal variations in the Yangtze rainfall are mainly determined by the coupling between the low-level relative vorticity and the upper-level divergence. In the middle troposphere, the 15–35-day oscillation of the subtropical high is originated over the central North Pacific north of Hawaii, then propagates westward to the SCS-Philippine Sea, and finally modulates the intraseasonal variations of the Yangtze rainfall.     

近50 a华南盛夏降水的季节内差异及大气环流异常特征分析

基于CRU、CMAP、PREC/L、CN05.1、NCEP/NCAR以及全国160个台站的月降水资料,采用经验正交函数(EOF)分解、依赖于季节的经验正交函数(SEOF)分解、滑动平均、空间相关、回归以及合成分析等多元统计方法研究了近50 a华南盛夏降水异常的基本特征及其季节内差异,并讨论了其大气环流异常。结果表明:(1)盛夏7、8月华南降水异常的空间分布都表现为区域一致性,即整个华南地区都为正(负)异常。(2)华南盛夏降水异常在月季变化的时间尺度上存在着同位相和反位相演变,1963—1993年,华南7、8月降水大致为反相演变,即7月华南全区一致偏涝(旱)而8月一致偏旱(涝);1994—2015年,二者总体表现为同相演变,即7月华南全区降水一致偏涝(旱)时8月亦一致偏涝(旱)。(3)大气遥相关型的变化是同相和反相两种演变模态产生的主要原因,同相期间对流层中层7月表现为欧亚遥相关(EU)和东亚太平洋遥相关(EAP)相互配置,8月表现为类似EU和太平洋北美遥相关(PNA)型;反相期间对流层中层7月表现为类似北美东西遥相关(NAEW)型,8月表现为类似EAP型。(4)西太平洋副热带高压的变化与华南盛夏降水季节内差异密切相关。反相期间7月与8月西太平洋副热带高压的差异主要体现在东西位置变化较大,而同相期间变化不大。     

Seasonal extreme rainfall over Indian monsoon region: a moisture budget analysis to distinguish the role of ENSO and non-ENSO forcing

Theoretical and Applied Climatology - Indian summer monsoon rainfall (ISMR) variability of ± 10% of its long-term mean leads to flood and drought, affecting the life and economic...     

Precipitation climate of Central Asia and the large-scale atmospheric circulation

The precipitation climate in the larger Tian Shan region of Central Asia is described in terms of the climatological seasonal moisture fluxes and background circulation based on the ERA-40 reanalysis data and a precipitation reanalysis. The study area is partitioned into (1) the Tarim river basin, (2) bordering regions of China, Kyrgyzstan and Kazakhstan, and (3) Northwestern China. Moisture supply to these areas is primarily due to the midlatitude westerlies with contributions from higher latitudes. In addition, moisture from the Indian Ocean is notably imported into the Tarim drainage area. Monthly interannual precipitation variability relates to the variability of hemispheric circulation patterns. Extreme precipitation above and below normal in Western China and Central Asia is analyzed using the standardized precipitation index. Related circulation composites show that, despite regional and seasonal differences, episodes of extreme and severe dryness are dominated by various upstream standing wave patterns from the North Atlantic to Central Asia. These features extend further downstream to the North Pacific. Non-symmetry between wet and dry composites is noted upstream and in regional moisture flux composites.     

Decadal variation of summer rainfall in the yangtzehuaihe river valley and its relationship to atmospheric circulation anomalies over east asia and western north pacific

Decadal variations of summer rainfall during 1951 through 1990 are analyzed by using summer rainfall data of 160 stations in China. Four major patterns of decadal variations are identified. The decadal variations of summer rainfall showed northward shift in the eastern China from South China through the Yangtze-Huaihe River to North China. Summer rainfall in the Yangtze-Huaihe River valley underwent two obvious decadal transitions during the 40 years: one from rainy period to drought period in the end of the 1950’s, the other from drought period to rainy period in the late 1970’s. Correspondingly, the atmospheric circulation over East Asia through the western North Pacific showed two similar obvious transitions. The East Asian/ Pacific (EAP) pattern switched from high index to low index in the end of the 1950’s and from low index to high index in the late 1970’s, respectively. Hence, summer rainfall in the Yangtze-Huaihe River valley is closely associated with the EAP pattern not only in the interannual variation but also in the decadal variation.     

1949—2019 年影响山东的热带气旋时空分布及极端降水和大气环流异常

使用中国气象局热带气旋资料中心的热带气旋最佳路径数据集和NCEP/NCAR再分析资料提供的月平均数据,对北上影响山东的热带气旋（tropical cyclone,TC）及其造成的极端降水进行统计分析,并揭示了有利于 TC北移影响山东的大气环流特征。结果表明：影响山东的 TC主要出现 于 6—9 月,其中盛夏时节（7、8 月）TC对山东影响最大;TC影响山东时,强度主要为台风及以下等 级,或已发生变性;TC会引发山东极端降水事件,TC极端降水多出现在夏秋季（7—9 月）,其中8月的占比最大,9月次之,TC降水在极端降水事件中的占比约为 10%,但年际变化大,有些年份占比达60%以上,特别是1990 年以来 TC对极端降水的贡献显著增强;影响山东的 TC主要生成于西 北太平洋,多为转向型路径;当500 hPa位势高度异常场呈太平洋一日本遥相关型的正位相时,TC更易北上影响山东,此时西北太平洋副热带高压位置偏北,其外围气流会引导TC北上转向,对华东地区造成影响;850 hPa上,南海至西北太平洋存在异常气旋式环流,对流活跃,夏季风环流和季风槽加强,有利于TC的生成和发展,同时,华东、华南上空有异常上升运动,涡度增大,垂直风切变减小,水汽充沛,TC登陆后强度能得到较好的维持。     

利用CAM5.1模拟中国东部大规模城市化对东亚地区夏季大气环流及降水分布的影响

利用CAM5.1大气环流模式研究中国东部大规模城市化对东亚夏季大气环流及降水分布的影响。通过在模式中修改中国东部地区(22～42°N,110～125°E)城市比重的方法,探讨东亚地区夏季大气环流与降水等气象要素在一般城市化及极端城市化两种情景下的响应。结果表明:(1)CAM5.1模式能够很好地模拟出东亚地区夏季大气环流形势及降水分布。(2)城市比重增大后,晴空时地面吸收的净辐射增多,近地层气温升高,低层增温中心上空的大气由于受热产生上升运动,35°N以南的气流向增温区辐合,东亚夏季风出现增强的趋势,大量暖湿水汽往北输送,导致降水在中国北方地区增多而南方减少。(3)城市化的发展程度越高,它所产生的气候效应对各气象要素的影响就表现得越明显。     

广东6月持续性暴雨期间的大气环流异常

利用1979—2011年共33年广东86个观测站日降水和全球大气多要素日平均资料,分析广东前汛期降水异常(包括暴雨和无雨)的环流特征。结果表明,广东6月持续性暴雨和持续性无雨期间大气存在显著的经向遥相关波列,其中,持续性暴雨过程波列更完整,非持续性暴雨(或非持续性无雨)则波列不显著,而4、5月的持续性暴雨或持续性无雨过程都没有波列出现。在经向波列存在的情况下,对流层中高层大气西风带环流经向度增大、槽脊发展增强,中高纬度这种持续稳定的环流形势,有利于冷空气和高空槽影响华南;在高层200 hPa,华南处于偏西风和西南风异常之间的气流辐散区域,有利于高层辐散;对流层中低层西太平洋副热带高压偏强、西脊点偏西,华南上升、南海下沉的垂直异常经圈环流建立;同时对流层低层来源于印度和孟加拉湾北部以及热带太平洋的水汽输送明显加强,从而为持续性暴雨过程提供有利的环流背景以及暴雨区所需的动力和水汽条件,可见经向波列通过对流层高、中、低层大气环流异常影响持续性暴雨。在没有经向波列的情况下,当500 hPa华南地区有西风槽活动、850 hPa南海北部西风偏强,广东局地动力上升条件和水汽输送条件达到一定程度,则只能出现非持续性暴雨。因此,经向波列可为区分持续暴雨与非持续暴雨预报提供参考。与广东降水持续异常相关的经向波列受中高纬度罗斯贝波、热带对流以及中低纬度太平洋地区大气异常等多方面的共同影响。     

浙江秋季连阴雨气候统计特征及其异常环流背景

利用1971—2016年浙江常规气象站观测资料和多种再分析资料,分析了浙江省秋季连阴雨的气候特征及其异常环流背景。结果表明:(1)浙江每年每站平均出现1.8次秋季连阴雨天气,过程平均可持续5.5 d,平均雨量77 mm;秋季3个月中9月出现连阴雨的频次最高;大多年份会发生1～2次持续5～7 d的连阴雨,78%过程累计降水量在100 mm以下;发生时多为全省一致变化型,多发区集中在杭州—绍兴一带。(2)连阴雨过程次数、累计持续天数及累计降水量有一致变化性,均反映出浙江秋季连阴雨有明显的年际变化特征,有4个强年和7个弱年。(3)浙江秋季连阴雨强(弱)年对应南亚高压和西太平洋副热带高压"相向(背)而行"。强(弱)年时,西太平洋副热带高压异常偏西(东),南亚高压异常偏东(西),华东地区上空垂直上升运动强(盛行下沉气流),低层异常偏南(北)风提供了充足的水汽(阻碍了暖湿空气北上),有(不)利于浙江连阴雨的发生。(4)连阴雨强弱年大气环流异常与热带海表温度强迫有关。强(弱)年时,赤道太平洋海温呈拉尼娜状态(距平不明显),使东印度洋—南海—海洋性大陆上空对流活跃(偏弱),潜热加热(不足以)激发低层Matsuno-Gill型响应,(不能)在亚洲大陆激发气旋性环流异常,而赤道中东太平洋强(弱)下沉气流则(不能)在西北太平洋激发反气旋性环流异常,此低层气旋—反气旋异常环流配置(弱环流异常)使华东地区有低层偏南(北)风异常,水汽增多(减少),有(不)利于浙江连阴雨的发生。     

Characteristics of the atmospheric circulation associated with cold-season heavy rainfall and flooding over a complex terrain region in Greece

Raingauge data from four meteorological stations in Chalkidiki peninsula (Greece) were used to identify the characteristics of the synoptic circulation patterns associated with cold-season heavy rainfall events and corresponding flooding over the area. Precipitation climatology over the complex topography of Chalkidiki is characterized by limited annual rainfall, but in the occurrence of heavy rainfall episodes daily accumulations are exceptionally high with increased precipitation rates, leading often to severe flooding. Fifty-five cases of high daily accumulations for the period of 1997 to 2010, mostly observed during December and October, were classified into eight clusters by applying S-Mode Factor Analysis and Cluster Analysis to the ERA-Interim grid point reanalysis data. The results revealed that, in most cases, intense rainfall and flooding is produced by synoptic scale disturbances that generate and sustain cyclonic activity over south Italy, the Ionian, and less frequently over the Aegean Sea. In particular, the atmospheric circulation patterns associated with heavy rainfall are characterized by the presence of a southeasterly–easterly low-level humid flow over Chalkidiki in conjunction with the potentially unstable lower troposphere influenced by mid-level cyclonic vorticity advection and enhanced low-level convergence over the complex terrain.     

夏季亚洲-太平洋遥相关季节演变与大气环流和降水

利用1981-2007年逐日大气再分析资料和降水资料以及统计分析方法,研究了夏季(5-9月)亚洲与太平洋区域的大气遥相关,并分析了其季节演变与夏季亚洲-太平洋大气环流和亚洲季风区降水的联系.结果表明:在5-9月逐日对流层上层扰动温度场上,亚洲与北太平洋中纬度存在着类似于亚洲太平洋涛动的遥相关现象,即在季节尺度上,当亚洲大陆中纬度对流层上层偏暖时,北太平洋中纬度对流层上层偏冷,反之亦然;亚洲与太平洋对流层上层温度的反位相变化特征也出现在对流层中下层和平流层低层.亚洲-太平洋涛动指数不仅可以指示夏季亚洲与太平洋中纬度纬向热力差异的变化特征,也可以较好地指示亚洲与热带印度洋经向热力差异的变化特征.亚洲-太平洋涛动指数最大值常出现在7月中旬到8月初,并且从1981年到2007年该最大值出现时间有偏早趋势.当夏季亚洲太平洋涛动指数偏高(低)时,亚洲大陆上空的南亚高压和其下方的亚洲大陆低压系统偏强(弱),太平洋副热带高压偏强(弱)并偏北(南),亚洲-非洲上空的热带东风急流和低层的西南风偏强(弱),从印度到中国华南的广大地区、中国华北以及东北亚等地降水偏多(少).