Interannual to decadal summer drought variability over Europe and its relationship to global sea surface temperature

Interannual to decadal variability of European summer drought and its relationship with global sea surface temperature (SST) is investigated using the newly developed self calibrated Palmer drought severity index (scPDSI) and global sea surface temperature (SST) field for the period 1901–2002. A European drought severity index defined as the average of scPDSI over entire Europe shows quasiperiodic variations in the 2.5–5 year band as well as at 12–13 years suggesting a possible potential predictability of averaged drought conditions over Europe. A Canonical Correlation Analysis between summer scPDSI anomalies over Europe and global SST anomalies reveals the existence of three modes of coupled summer drought scPDSI patterns and winter global SST anomalies. The first scPDSI-SST coupled mode represents the long-term trends in the data which manifest in SST as warming over all oceans. The associated long-term trend in scPDSI suggests increasing drought conditions over the central part of Europe. The second mode is related to the inter-annual ENSO and decadal PDO influence on the European climate and the third one captures mainly the drought pattern associated to Atlantic Multidecadal Oscillation. The lag relationships between winter SST and summer drought conditions established in this study can provide a valuable skill for the prediction of drought conditions over Europe on interannual to decadal time scales.     

Reconstructed cool- and warm-season precipitation over the tribal lands of northeastern Arizona

For over a decade, the Hopi Tribe and Navajo Nation of northeastern Arizona have suffered the effects of persistent drought conditions. Severe dry spells have critically impacted natural ecosystems, water resources, and regional livelihoods including dryland farming and ranching. Drought planning and resource management efforts in the region are based largely on the instrumental climate record, which contains a limited number of severe, sustained droughts. In this study, a new network of moisture-sensitive tree-ring chronologies provides the basis for evaluating the longer-term temporal variability of precipitation in the Four Corners region. By analyzing the earlywood and latewood components within each annual tree ring, we are able to generate separate, centuries-long reconstructions of both cool- (October-April) and warm-season (July-August) precipitation. These proxy records offer new insights into seasonal drought characteristics and indicate that the instrumental record fails to adequately represent precipitation variability over the past 400 years. Through the use of two different analysis techniques, we identify multiyear and decadal-scale drought events more severe than any in the modern era. Furthermore, the reconstructions suggest that many of the historically significant droughts of the past (e.g., 17th century Puebloan drought) were not merely winter phenomena, but persisted through the summer season as well. By comparing these proxy records with historical documents, we are able to independently validate the reconstructions and better understand the socioeconomic and environmental significance of past climate anomalies on the tribal lands of northeastern Arizona.     

Reconstructed Temperature And Precipitation On A Millennial Timescale From Tree-Rings In The Southern Colorado Plateau,U.S.A.

Two independent calibrated and verified climate reconstructions from ecologically contrasting tree-ring sites in the southern Colorado Plateau, U.S.A. reveal decadal-scale climatic trends during the past two millennia. Combining precisely dated annual mean-maximum temperature and October through July precipitation reconstructions yields an unparalleled record of climatic variability. The approach allows for the identification of thirty extreme wet periods and thirty-five extreme dry periods in the 1,425-year precipitation reconstruction and 30 extreme cool periods and 26 extreme warm periods in 2,262-year temperature reconstruction. In addition, the reconstructions were integrated to identify intervals when conditions were extreme in both climatic variables (cool/dry, cool/wet, warm/dry, warm/wet). Noteworthy in the reconstructions are the post-1976 warm/wet period, unprecedented in the 1,425-year record both in amplitude and duration, anomalous and prolonged late 20th century warmth, that while never exceeded, was nearly equaled in magnitude for brief intervals in the past, and substantial decadal-scale variability within the Medieval Warm Period and Little Ice Age intervals.     

近40a铜仁夏季旱涝时空分布及典型旱涝年环流特征分析

本文基于铜仁市10个国家自动观测站1981-2020年近40a降水资料,计算多时间尺度标准化降水指数（SPI）,确定旱涝程度,分析旱涝时空分布特征,筛选典型旱、涝年进行环流分析。结果表明：(1)铜仁市20世纪80年代后期总体偏旱,90世纪后期总体偏涝。铜仁市夏季呈现出旱涝交替变化特征。(2)铜仁市中西部、东北部干旱相对频繁,并向四周逐渐减弱;市中西部、东南部发生雨涝的频率相对较高,西北部最低。夏季的干旱和雨涝均以轻、中程度为主,特重程度情况发生很少。(3)夏季偏涝年整个欧亚中高纬均为位势高度的负异常,偏旱年欧亚中高纬的乌拉尔山和中国东北的位势高度异常中心转为正异常分布。（4）赤道中东太平洋的厄尔尼诺现象与南、北太平洋海温异常分布对铜仁夏季偏涝现象的发生发展有密切关系;赤道中东太平洋拉尼娜现象、印度洋及西太平洋的海温异常分布对铜仁夏季偏旱现象的发生发展有密切关系。     

A millennial long March–July precipitation reconstruction for southern-central England

We present a millennial long dendroclimatic reconstruction of spring/summer precipitation for southern-central England. Previous research identified a significant moisture stress signal in ring-width data measured from oak trees growing in southern England. In this study, we build upon this earlier work, specifically targeting south-central England, to derive a well replicated oak ring-width composite chronology using both living and historical material. The data-set includes 352 living trees (AD 1629–2009) and 1540 individual historical series (AD 663–1925). The period expressed by at least 50 trees in any year is AD 980–2009. Calibration experiments identify the optimal seasonal predictand target as March–July precipitation (1901–2007: r² = 0.33). However, comparison with the long Kew Gardens precipitation record indicates a weakening in tree-growth/climate response from ~1800 to 1920 which we speculate may be related to smoke and sulphur dioxide (SO₂) emissions at that time which may have also contributed to a decrease in tree productivity. The time-series derived using the regional curve standardisation method to capture lower frequency information shows a mediaeval period with alternating multi-decade-long dry and wet periods, with AD 1153–1172 being the wettest reconstructed 20-year period in the whole record. Drier conditions are prevalent from ~1300 to the early sixteenth century followed by a period of increasing precipitation levels. The most recent four centuries of the record appear similar to the mediaeval period with multiple decade-long dry and wet periods. The late twentieth century is the second reconstructed wettest period. These centennial hydroclimatic trends are in broad agreement with independent regional scale hydroclimatic reconstructions from tree-ring (East Anglia), historical, speleothem and peat water level proxy archives in the United Kingdom and appear coupled with reconstructed sea surface temperature changes in the North Atlantic which in turn influence the Atlantic meridional overturning circulation and westerly airflow across the UK.     

2010年秋冬季西南地区严重干旱与南支槽关系分析

利用1951~2009年西南地区24个站点逐月降水资料以及NCEP/NCAR再分析资料,分析了2009/2010年冬季我国西南地区严重干旱的演变特征,并使用SVD、小波分析和合成分析揭示了南支槽与我国西南地区冬季严重干旱的关系。结果表明:2009/2010年西南地区冬季严重干旱从秋季10月份云南省开始出现大范围干旱为征兆,11~2月逐步发展到西南三省,3月减弱。从秋季10月到次年2月的降水持续偏少,加重了此次旱情。2009/2010年冬季西南地区严重干旱的开始、发展和减弱与同期500hPa南支槽活动及整层水汽输送有着密切的关系。我国西南地区11~3月降水和前期11月南支槽指数在10~12年周期变化上存在显著的反相关系,前期11月南支槽区负距平,南支槽指数偏弱,南支槽加深,水汽输送充足,西南地区降水偏多;反之,前期11月南支槽区正距平,南支槽指数偏强,南支槽变浅,水汽条件不足,西南地区降水偏少。SVD分析表明,高度场第一模态同性相关场的关键区在青藏高原南侧孟加拉湾地区,反映了南支槽强弱变化信息,第一模态的这对空间分布型表明18~20°N,84~92°E范围11~3月500hPa高度场异常偏低时,同期我国西南地区降水偏多;高度场第二模态同性相关场的关键区在22°N以南区域,反映了西风强弱变化信息。第二模态的这对空间分布性表明70~110°E之间22°N以南区域11~3月500hPa高度场异常偏低时,同期我国西南地区降水偏少。11~3月关键区500hPa高度场与我国西南地区同期降水在时空场上都有着很好的同步关系,并且前期500hPa高度场是我国西南地区11~3月旱涝情况的一种预测信号。      

2019年4～6月云南持续性高温天气的大气环流异常成因

2019年4～6月云南省发生了历史罕见的持续性极端高温天气,并引发了严重气象干旱。本文利用1961～2019年逐日温度和大气再分析等资料以及CESM-LE计划（Community Earth System Model Large Ensemble Project）模式模拟结果,分析了历史同期云南极端高温天气发生的环流特征,探讨了2019年云南破纪录持续性高温的成因。历史极端高温日的合成分析表明,云南地区对流层上层显著异常反气旋伴随的强下沉异常和到达地表太阳辐射增加,是引发该区域极端高温天气的主要成因。该异常反气旋的形成主要源自北大西洋经东欧平原、西西伯利亚平原向东亚传播的高纬度罗斯贝波和经北非、黑海、伊朗高原向东亚传播的中纬度罗斯贝波之间的相互作用。2019年极端高温的强度和与之相应异常反气旋出现自1961年以来的最强。外强迫导致的增暖对2019年极端暖异常强度的贡献约为37.51%,同时对类似2019年以及更强极端暖事件发生概率的贡献为56.32%,内部变率对该事件也具有重要贡献。2019年4～6月北极涛动（Arctic Oscillation,AO）和ENSO事件分别处于历史极端负位相和暖位相。一方面,在AO强负位相影响下,极地上空深厚的位势高度正异常向南伸至东欧平原,有利于高纬度波列和云南上空的反气旋异常增强。另一方面,ENSO事件暖位相加强了西北太平洋异常反气旋环流,令西北太平洋副热带高压增强西伸至我国内陆地区,维持了云南上空反气旋异常。两者的共同作用,造成了2019年4～6月云南上空持续的深厚异常反气旋,云南地区继而出现持续性极端高温事件。     

The 1983 drought in the West Sahel: a case study

Some drought years over sub-Saharan west Africa (1972, 1977, 1984) have been previously related to a cross-equatorial Atlantic gradient pattern with anomalously warm sea surface temperatures (SSTs) south of 10°N and anomalously cold SSTs north of 10°N. This SST dipole-like pattern was not characteristic of 1983, the third driest summer of the twentieth century in the Sahel. This study presents evidence that the dry conditions that persisted over the west Sahel in 1983 were mainly forced by high Indian Ocean SSTs that were probably remanent from the strong 1982/1983 El Ni?o event. The synchronous Pacific impact of the 1982/1983 El Ni?o event on west African rainfall was however, quite weak. Prior studies have mainly suggested that the Indian Ocean SSTs impact the decadal-scale rainfall variability over the west Sahel. This study demonstrates that the Indian Ocean also significantly affects inter-annual rainfall variability over the west Sahel and that it was the main forcing for the drought over the west Sahel in 1983.     

Boreal winter rainfall anomaly over the tropical indo-pacific and its effect on northern hemisphere atmospheric circulation in CMIP5 models

Experimental outputs of 11 Atmospheric Model Intercomparison Project （AMIP） models from phase 5 of the Coupled Model Intercomparison Project （CMIP5） are analyzed to assess the atmospheric circulation anomaly over Northern Hemisphere induced by the anomalous rainfall over tropical Pacific and Indian Ocean during boreal winter.The analysis shows that the main features of the interannual variation of tropical rainfall anomalies,especially over the Central Pacific （CP） （5°S-5°N,175°E-135°W） and Indo-western Pacific （IWP） （20°S-20°N,110°-150°E） are well captured in all the CMIP5/AMIP models.For the IWP and western Indian Ocean （WIO） （10°S-10°N,45°-75°E）,the anomalous rainfall is weaker in the 11 CMIP5/AMIP models than in the observation.During El Ni（n）o/La Ni（n）a mature phases in boreal winter,consistent with observations,there are geopotential height anomalies known as the Pacific North American （PNA） pattern and Indo-western Pacific and East Asia （IWPEA） pattern in the upper troposphere,and the northwestern Pacific anticyclone （cyclone） （NWPA） in the lower troposphere in the models.Comparison between the models and observations shows that the ability to simulate the PNA and NWPA pattern depends on the ability to simulate the anomalous rainfall over the CP,while the ability to simulate the IWPEA pattern is related to the ability to simulate the rainfall anomaly in the IWP and WIO,as the SST anomaly is same in AMIP experiments.It is found that the tropical rainfall anomaly is important in modeling the impact of the tropical Indo-Pacific Ocean on the extratropical atmospheric circulation anomaly.     

Warm and cold dry months and associated circulation in the humid and semi-humid Argentine region

This paper studies the climatic conditions of warm and cold dry months in the humid and semi-humid Argentine region and some aspects of the regional circulation related to these cases. The climatic analysis of warm (temperatures above percentile 80) and cold (temperatures below percentile 20) dry months is based on precipitation and temperature data registered at reference stations over a period of at least 70 years, while the associated circulation is derived from daily data of geopotential height at 500 hPa from NCEP-DOE Reanalysis 2 database. The reference station for the center of the country registered a greater number of warm dry months during both the warm season (October–March) and the cold season (April–September), whereas the reference stations in the north-east and center-east showed differences depending on the time of the year, with more cold dry months during the April–September season and more warm dry months in the October–March season. A classification of daily fields of geopotential height anomalies at 500 hPa was used to analyze the atmospheric circulation related to warm and cold dry months. The circulation patterns were obtained by applying principal component analysis and cluster analysis. Findings show that some mid-level circulation patterns occur with a significant different frequency during the warm dry months or the cold dry months studied. Finally, cases of spatially extended precipitation-deficit conditions (hereinafter generalized droughts) were studied, noting dominant patterns that are coherent with the previous results.     

2009/2010年冬季云南严重干旱的原因分析

2009/2010年冬季我国云南省出现严重干旱,这次大范围严重干旱是较长时期降水稀少所造成的。首先讨论云南省冬季降水偏多和偏少时大气环流和海温的统计特征,基于它们的统计关系,再对2009/2010年冬季我国云南省的严重干旱进行个例对比分析。研究表明西风带环流系统异常是造成这次干旱灾害的主要成因。贝加尔湖为高度负距平,东亚沿海为高度正距平,从贝加尔湖以西到东亚中高纬度西风带较平直,冬季冷空气偏弱,很难影响西南地区。尤其是副热带中东急流减弱,从欧洲东部到里海为高压脊控制,西风带的扰动系统不易东移到东亚上空;青藏高原上空为稳定的高压脊,孟加拉湾南支槽减弱,云南省受异常西北气流控制。对太平洋和印度洋海温的分析表明,虽然海温异常可以影响冬季的云南降水,但海温异常并不是2009/2010年冬季云南省降水偏少的最重要原因。     

Variability of droughts in the Czech Republic, 1881–2006

We analyze droughts in the Czech Republic from 1881–2006 based on the Palmer drought severity index (PDSI) and the Z-index using averaged national temperature and precipitation series for the calculations. The standardized precipitation index (SPI), PDSI and Z-index series show an increasing tendency towards longer and more intensive dry episodes in which, for example, droughts that occurred in the mid-1930s, late 1940s–early 1950s, late 1980s–early 1990s and early 2000s were the most severe. Cycles at periods of 3.4–3.5, 4.2–4.3, 5.0–5.1 and 15.4 years exceeded 95% confidence levels in application of maximum entropy spectral analysis. These are expressed at different intensities throughout the period studied. The occurrence of extremely dry and severely dry months is associated with a higher frequency of anticyclonic situations according to the classification employed by the Czech Hydrometeorological Institute. Principal component analysis documents the importance of the ridge from the Siberian High over Central Europe when extreme and severe droughts in months of the winter half-year are considered in terms of sea-level pressure. In the summer half-year, the ridge of the Azores High over Central Europe is the most important. Drought episodes have a profound effect on national and regional agricultural production, with yields being consistently lower than in normal years, as is documented through the example of spring barley, winter wheat, forage crops on arable land, and hay from meadows. Seasons with pronounced drought during the April–June period (e.g., 1947 and 2000) show the most significant yield decreases. Forests appear to be very vulnerable to long-term drought episodes, as it was the case during the dry years of 1992–1994. This study clearly confirms the statistically significant tendency to more intensive dry episodes in the region, driven by temperature increase and precipitation decrease, which has already been suggested in other studies.     

广东前汛期异常旱涝的大气环流特征

用1957～1999年广东省19个代表站4～6月降水资料和500hPa高度场资料，着重分析和讨论了广东省前汛期异常旱、涝年的同期和前期500hPa合成平均场的主要差异,以及前期关键月、关键区的特点，为广东省前汛期异常旱、涝的预测提供判别依据。     

东亚夏季风异常活动的多模态特征:不同再分析资料的比较分析

利用1979～2002年ERA-40、ERA interim、JRA-25和NCEP-DOE AMIP-Ⅱ(简称为NCEP-2)再分析资料,采用扩展经验正交分解(EEOF)、相关分析等方法,对比分析了不同资料所揭示的东亚夏季风异常活动的多模态特征,在此基础上探讨了东亚夏季风异常活动各模态对应的大气环流异常分布型及其与中国夏季降水的可能联系。结果表明:(1)四套再分析资料所揭示的东亚夏季风异常活动均存在三种差异显著的空间模态,且各套资料对东亚夏季风异常活动空间多模态特征具有很好的一致性,仅NCEP-2的结果与其他资料略有差异。(2)第一模态体现了夏季风年际异常在中国南方和北方的反相变化,并具有显著的3～6年和准8年周期;与正(负)时间系数相对应,850 hPa风场、500 hPa高度场、SLP均显示东亚沿岸存在从西北太平洋经过日本以南到达鄂霍次克海的“-+-”(“+-+”)经向三极型结构;相应的降水变化在长江中下游为显著的负(正)异常,而在我国东北东部、东南沿海及云南西部则为正(负)异常。(3)第二模态反映了夏季风活动主导模态的一致性变化且在1993年左右发生年代际转折,并呈现准12年周期的强弱交替分布。当对应的时间系数为正(负)时,850 hPa风场在环贝加尔湖地区受强大的异常反气旋(气旋)控制;500 hPa高度场上,中高纬地区表现为异常的纬向波列结构,具体表现为起源于欧洲大陆西部经西西伯利亚平原向东南方向延伸至东北亚地区的“+-+”(“-+-”)的波列;SLP在我国大陆主要为正(负)异常,东亚夏季风整体减弱(加强);对应的夏季降水异常场呈现“南涝北旱”(“南旱北涝”)的分布形势。(4)第三模态表明了夏季风异常活动的东西反相变化,且有12～16年的准周期变化。对应正(负)的时间系数,115°E 以东地区盛行异常偏南(北)风,而115°E 以西地区主要盛行异常偏北(南)风;500 hPa高度场、SLP均显示出东亚沿岸地区、鄂霍次克海至日本以南洋面的“-+”(“+-”)波列以及欧亚大陆北部的准纬向遥相关波列;夏季降水在我国大部分地区偏多(偏少),显著变化主要位于黄淮及附近地区。     

Fast responses of Northern Hemisphere winter daily circulation to anthropogenic aerosols

Anthropogenic aerosols (AA) have significantly caused anomalous winter mean atmospheric circulation over the Northern Hemisphere, but the main daily patterns of winter large-scale circulation change are not well understood. Here a self-organizing map analysis is applied to identify the leading patterns in AA-induced winter daily geopotential height (Z) anomaly fields simulated by three atmospheric general circulation models, with a focus on fast adjustments. Two winter daily circulation response patterns with a synoptic time scale are found: one pattern shows concurring Z anomalies over North America and North Asia with the same sign and the Bering Sea seeing the opposite, resembling the Asia–Bering–North American teleconnection; while the other is the Arctic Oscillation-like pattern with similar Z anomalies over North Pacific and North Atlantic and the opposite over the Arctic region. The AA-induced anomalous precipitation over the tropics and anomalous synoptic eddy activities over the extratropical oceans concur to support and maintain these circulation anomaly patterns. The winter-mean climate responses to AA can be understood as a result of these daily anomaly patterns, especially over the higher latitudes. Specifically, the associated changes in surface air temperature (SAT) over the mid-high latitudes are caused by the AA-driven meridional movements of polar (cold and dry) airmass and midlatitude (warm and moist) airmass in the regions, mainly through the relevant surface downward longwave radiation. This study highlights the role of AA in altering daily weather patterns, which is not sufficiently captured by seasonal mean responses.     

Circulation dynamics and its influence on European and Mediterranean January–April climate over the past half millennium: results and insights from instrumental data, documentary evidence and coupled climate models

We use long instrumental temperature series together with available field reconstructions of sea-level pressure (SLP) and three-dimensional climate model simulations to analyze relations between temperature anomalies and atmospheric circulation patterns over much of Europe and the Mediterranean for the late winter/early spring (January–April, JFMA) season. A Canonical Correlation Analysis (CCA) investigates interannual to interdecadal covariability between a new gridded SLP field reconstruction and seven long instrumental temperature series covering the past 250 years. We then present and discuss prominent atmospheric circulation patterns related to anomalous warm and cold JFMA conditions within different European areas spanning the period 1760–2007. Next, using a data assimilation technique, we link gridded SLP data with a climate model (EC-Bilt-Clio) for a better dynamical understanding of the relationship between large scale circulation and European climate. We thus present an alternative approach to reconstruct climate for the pre-instrumental period based on the assimilated model simulations. Furthermore, we present an independent method to extend the dynamic circulation analysis for anomalously cold European JFMA conditions back to the sixteenth century. To this end, we use documentary records that are spatially representative for the long instrumental records and derive, through modern analogs, large-scale SLP, surface temperature and precipitation fields. The skill of the analog method is tested in the virtual world of two three-dimensional climate simulations (ECHO-G and HadCM3). This endeavor offers new possibilities to both constrain climate model into a reconstruction mode (through the assimilation approach) and to better asses documentary data in a quantitative way.     

The role of SST on the South American atmospheric circulation during January, February and March 2001

Precipitation deficits were observed over southeastern, northeastern and Central Brazil during the 2001 Austral Summer. They contributed to the worsening of the energy crisis that was occurring in the country. A low-level anomalous anticyclonic circulation observed over eastern Brazil enhanced the deviation of moisture transport that usually occurs from the Amazon Basin to southeastern Brazil and inhibited the occurrence of South Atlantic Convergence Zone events in that period. However, an anomalous low-level northerly moisture flux was observed over the La Plata Basin, and positive precipitation anomalies occurred over Bolivia, Paraguay, northeastern Argentina and southern Brazil. Using the ensemble technique, a numerical study was carried out to investigate the role of different sea surface temperature (SST) forcings observed over this anomalous South American atmospheric circulation. Reynolds SST monthly means were used as boundary conditions to study the influence of South Atlantic, South Indian, South Pacific and Equatorial Pacific oceans. The simulations were run from September 2000 to April 2001 using the Community Climate Model version 3.6 General Circulation Model. Ten integrations using different initial conditions were done to each experiment. Numerical experiments suggested that the combined influence of South Pacific and Equatorial Pacific oceans could be responsible for the drought observed over Central Brazil. These experiments simulated the low-level anticyclonic anomaly observed over eastern Brazil. However, both experiments have poorly reproduced the intensity of the anomalous low-level northerly moisture flux observed over the La Plata Basin. Therefore, the intensity of the simulated precipitation anomalies over the subtropical regions was much weaker than observed.     

西北东部夏季极端干旱事件机理分析

利用国家气候中心提供的1951—2012年逐日降水、温度、综合气象干旱指数、逐月NCEP/NCAR再分析资料等,采用REOF分析、动力诊断、相关分析以及合成分析等方法,从大气环流异常特征、高空急流与季风异常等方面揭示西北东部夏季极端干旱事件的可能机理。研究发现西北东部夏季发生极端干旱时,副热带急流轴"倾斜",且急流与东亚夏季风强度均处于相对偏弱阶段。极端干旱的成因研究表明:急流轴"倾斜"及其强度减弱导致西北东部地区高层大范围的异常辐合;该地区为水汽源区,对流层整层水汽收支显著亏损;此外,该地区低层盛行来自内陆干旱区的异常西南风,东亚夏季风强度偏弱,高低层配置及大尺度环流形势不利于降水产生。     

2011年8月气候异常及成因分析

在总结2011年8月我国气候异常与大气环流特征的基础上,针对西南干旱和热带气旋活动偏少两大气候异常事件的成因进行了分析。结果表明：高度场偏高、西太平洋副热带高压偏强、夏季风偏弱和水汽条件较差等大气环流异常是导致高温干旱的主要原因;中部型拉尼娜事件的滞后影响和印度洋偏暖的影响是西南干旱的重要外强迫条件。南海对流活动偏弱,菲律宾以东季风槽位置偏北,热带气旋活动区域垂直风切偏大,西北太平洋副热带高压偏强等因素导致热带气旋活动偏少。     

Impact of cold air surges on rainfall variability in the Sahel and wet African tropics: a multi-scale analysis

Satellite-derived rainfall estimates and the ERA-Interim reanalysis are used to better understand cold air surge/precipitation interactions and to identify the implications for rainfall variability in the Sahel and tropical Africa on synoptic to seasonal timescales. At the synoptic timescale, cold air surges are associated with cold conditions over the eastern Sahara throughout the year due to the eastward passage of surface low pressure systems over the Mediterranean and the subsequent ridging over northern Africa. Rainfall decreases over central and eastern Africa approximately 4–5 days after the cold air first arrives in northeastern Africa. These precipitation anomalies persist for 4 or more days. At the seasonal timescale, a significant relationship between eastern Saharan low-level temperatures and rainfall in the Sahel and tropical Africa is identified, with colder conditions associated with reduced convection on the northern flank of the primary convergence zone, and vice versa. During boreal winter, the anomalous rainfall occurs over tropical Africa (0°N–8°N). During the summer, rainfall anomalies associated with cold air surges occur over the Sahel (10°N–16°N). These relationships are mediated by anomalous anticyclonic flow over northwestern Africa and western Europe. The analysis shows that cold air surges are significantly associated with summertime cooling over the Sahara, but less so during the winter.