Seasonal stability of snow cover in Poland in relation to the atmospheric circulation

The seasonal stability of snow cover (ISS) was defined as a percentage ratio of the real and the potential snow cover duration in a winter season. Main results of the study are as follows: (1) alternately occurring periods of high and low values of the index of snow cover stability did not appeared simultaneously in mountainous and non-mountainous areas; (2) in the majority of Poland area both zonal and meridional components of the atmospheric circulation influence the ISS; however, in south the meridional air flow reveals the stronger impact, mostly due to the intensification of the southern advection by the foehn effect; and (3) changes of two or three indices describing atmospheric circulation explain up to 50 % of the ISS in Poland. The diminishing stability of snow cover in Poland corresponds with an increasing intensity of the advection from the western sector in winter in the second half of the twentieth century in Europe.     

Changes in atmospheric circulation over Europe detected by objective and subjective methods

Summary Changes in atmospheric circulation over Europe since 1958 were examined using both objective (modes of low-frequency variability and objective classification of circulation types) and subjective (Hess-Brezowsky classification of weather types) methods. The analysis was performed with an emphasis on the differences between the winter (DJF) and summer (JJA) seasons, and between objectively and subjectively based results. Majority of the most important changes in atmospheric circulation are same or similar for the objective and subjective methods: they include the strengthening of the zonal flow in winter since the 1960s to the early 1990s; the increase (decrease) in frequency of anticyclonic (cyclonic) types in winter from the late 1960s to the early 1990s, with a subsequent decline (rise); and the sharp increase in the persistence (measured by the mean residence time) of all groups of circulation types in winter around 1990 and of anticyclonic types in summer during the 1990s. Differences between the findings obtained using the objective and subjective methods may result from the intrinsically different approach to the classification (e.g. the Hess-Brezowsky weather types have a typical duration of at least 3 days while objective types typically last 1–3 days). Generally, changes in atmospheric circulation which have taken place since the 1960s were more pronounced in winter than in summer. The most conspicuous change seems to be the considerable increase in the persistence of circulation types during the 1990s, which may be also reflected in the increase in the occurrence of climatic extremes observed in Europe during recent years.     

Detection of climate change in Europe by analyzing European atmospheric circulation patterns from 1881 to 1989

Summary The time series of classified European atmospheric circulation patterns for the time period of 1881–1989 are analyzed. Frequencies of several circulation types changed considerably during the last years, showing extremes never reached before. Changes can be observed both in the annual frequencies and in the seasonal ones for many circulation types. The time series were analyzed for homogeneity using non-parametric statistical methods. The annual cycle shows an increase of the frequency of zonal circulations in winter (December and January) and a corresponding decrease of cold meridional circulations. The climatological consequences of this are more frequent mild and humid winters in Central Europe, with precipitation mostly falling as rain. Change points in the series occurred with high probability during the seventies. Examples demonstrate the coupling of circulation patterns to temperature and precipitation.With 9 Figures     

Response of Northern Hemisphere storm tracks to Indian-western Pacific Ocean warming in atmospheric general circulation models

With 40 years integration output of two atmospheric general circulation models (GAMIL/IAP and HadAM3/UKMO) forced with identical prescribed seasonally-varying sea surface temperature, this study examines the effect of the observed Indian-western Pacific Ocean (IWP) warming on the Northern Hemisphere storm tracks. Both models indicate that the observed IWP warming tends to cause both the North Pacific storm track (NPST) and the North Atlantic storm track (NAST) to move northward. Such a consistent effect on the two storm tracks is closely associated with the changes in the low-level atmospheric baroclinicity, high-level jet stream and upper-level geopotential height. The IWP warming can excite a wavelike circum-global teleconnection in the geopotential height that gives rise to an anticyclonic anomaly over the midlatitude North Pacific and a positive-phase NAO anomaly over the North Atlantic. These geopotential height anomalies tend to enhance upper-level zonal westerly winds north of the climatological jet axes and increase low-level baroclinicity and eddy growth rates, thus favoring transient eddy more active north of the climatological storm track axes, responsible for the northward shift of the both storm tracks. The IWP warming-induced northward shift of the NAST is quite similar to the observed, suggesting that the IWP warming can be one of the key factors to cause decadal northward shift of the NAST since the 1980s. However, the IWP warming-induced northward shift of the NPST is completely opposite to the observed, implying that the observed southward shift of the NPST since the 1980s would be primarily attributed to other reasons, although the IWP warming can have a cancelling effect against those reasons.     

欧亚土壤湿度异常对北半球大气环流的显著影响

用44 a ERA40再分析资料的土壤湿度和大气环流变量场, 研究持续性的欧亚大陆土壤湿度异常对后期北半球大尺度大气环流的反馈作用。首先,运用经验正交函数分解去除ENSO遥相关及趋势影响后,分析了欧亚大陆中高纬度土壤湿度变率主要模态的季节变化特征,及相对应主分量时间序列显示的土壤湿度异常的衰减时间,结果表明土壤湿度异常的主要模态在全年都表现出很好的连续性。其次,对不同季节的连续3个月的月平均土壤湿度和500 hPa高度场进行滞后最大协方差分析,研究欧亚地区中高纬度土壤湿度异常与北半球大气环流异常之间的线性耦合。第一最大协方差模态的结果表明:全年的主导信号是大气强迫土壤湿度的变化,但在冬季和夏季,大气中类似于负位相北极涛动的环流型与之前月份(最长达4个月)土壤湿度的持续变化显著相关。最后,基于土壤湿度变率中心的回归分析也证实了秋季和春季欧亚土壤湿度,特别是北非副热带,欧亚内陆和西伯利亚地区的土壤湿度异常,分别与其后的冬季和夏季的大气环流显著相关。欧亚大陆土壤湿度异常超前大气环流的信号,将有助于改善冬季和夏季北半球季节气候预报能力。     

Numerical simulation of Eurasian teleconnection pattern in atmospheric circulation during the Northern Hemisphere winter

In this paper, the anomaly of disturbance height field over Northern Hemisphere due to SST anomaly in the tropical Atlantic Ocean is simulated by using the general circulation model of IAP. A quasi-geostrophic, 34-level spherical coordinate model is also used to compute the distribution of atmospheric circulation anomaly when there is an anomaly of heat source over the tropical Atlantic. The computed results show that, owing to the heat source anomaly over the tropical Atlantic, the EU-pattern anomaly in the winter circulation may be caused. Namely, a ridge will be enhanced over the northwest Europe, a trough will be deepened over Siberia, but a positive anomaly of disturbance height field will be formed over the northeast China, Japan and other areas of East Asia. Moreover, the numerically simulated results show that the above-mentioned EU-pattern anomalies of circulation are due to the propagations of planetary wave train. About 15 days after an anomaly of the heat source over the tropical Atlantic is injected, the EU-pattern anomaly of atmospheric circulation is formed. This is in good agreement with the results analysed theoretically. On the leave from Geophysical Institute, Faculty of Science in Tokyo University, Japan.     

ENSO与北半球冬季大气环流异常年代际关系的数值模拟

利用NCAR/CCM3大气环流模式,模拟不同海温背景场强迫下,北半球冬季大气环流对ENSO事件的响应。结果表明,模式能够模拟出不同海温背景场下,北半球冬季大气环流对ENSO事件的不同响应,模拟结果与诊断结果基本一致。在20世纪70年代末之后的平均海温背景场强迫下,El Nio对阿留申低压的加强作用比70年代末之前更明显,而ENSO暖位相对蒙古高压的作用则由70年代末之前的轻微减弱变为明显加强。虽然70年代末以后ENSO事件的强度比70年代末之前有所增大,但ENSO对东亚大槽和东亚冬季风的减弱作用反而不再那么明显;同时在高空风场中,由ENSO激发出的PNA型异常环流在70年代末以后明显加强。     

Large-scale atmospheric circulation forms and their impact on air temperature in Europe and northern Asia

Air temperature variations in Europe and northern Asia are strongly affected by atmospheric circulation. A large-scale study of temperature signals is presented, using a newly available global gridded daily temperature dataset. Major types of European Grosswetterlagen (large-scale weather patterns) and the Russian Vangengeim–Girs classification are compared in their spatial applicability to air temperatures within the past 110 years (1901–2010). The consistency of spatial patterns in the three most recent decades (1981–2010) is investigated, and temperature changes are interpreted against the backdrop of changes in character and frequency of circulation patterns. Both classifications largely explain the observed temperature variability. Spatial patterns are large-scale and strong in both regions, especially in winter. Both spatial extent and signal magnitude show a distinct seasonality with maximum values in winter and minimum ones in summer. Spatial patterns show little changes in Europe; yet the ability to explain temperature variability in northern Asia decreased within 1981–2010. European winter warming corresponds to increased maritime and to decreased continental air mass inflow, superimposed on the general warming trend. Northern Asian winter warming is partly explainable by circulation changes in January and February, but to a lesser extend in December. These results may be used to advance input variables of global climate models and to improve their performance in the European–Northern Asian area.     

北方涛动同北半球温带大气环流的遥相关(一)——基本结构

本文利用1951—1980年逐季的平均值资料(共120个季)讨论了北方涛动和与其相联系的北太平洋海温与北半球海平面气压场、500hPa位势高度场遥相关的基本结构,并与南方涛动和赤道东太平洋海温的结果进行了对比分析.发现北太平洋Namias海区和加利福尼亚海流区海温的变化与北方涛动具有很密切的联系;北方涛动和这两个海区的海温同北半球中高纬度大气环流特别是PNA型和NAO型环流异常存在明显的遥相关关系;南方涛动和赤道太平洋海温同WP型或NPO型环流异常关系比较密切,而与PNA型和NAO型的关系不如北方涛动和Namias海区及加利福尼亚海流区海温的显著.     

冬季北半球风暴轴的协同变化及其与大气环流的相互作用

基于1971—2016年NCEP/NCAR的逐日、逐月再分析资料,研究冬季北半球西伯利亚风暴轴(Siberian Storm Track,SIST)、北太平洋风暴轴(Pacific Storm Track,PST)和北大西洋风暴轴(Atlantic Storm Track,AST)的协同变化特征及其与大气环流的关系。结果表明:(1)三大风暴轴不仅各自的位置与强度变化存在显著相关性,风暴轴之间也存在一定的协同变化且年代际尺度上比年际尺度上更紧密。年际尺度上,SIST与AST的经度变化呈显著负相关,而PST和AST的协同性较差;年代际尺度上,SIST与PST的经、纬度变化均呈弱的负相关,SIST与AST的经度和强度变化均呈显著正相关,PST与AST的经、纬度变化均呈显著负相关。(2)由联合EOF分析得到北半球风暴轴的协同变化时空特征:在年际尺度上,第一模态主要表现为SIST偏弱(强),PST主体偏弱(强)、东南偏强(弱),AST略偏北(南)偏强(弱)但不显著的协同变化。PC1为正位相时,对应的大气环流异常为:500 hPa高度场上为太平洋北美(Pacific North America,PNA)型和欧亚(Eurasian,EU)型的正位相,东亚急流偏强且偏南;第二模态主要表现为SIST偏强(弱)且偏东(西),PST中东部偏南(北)、西部强度偏强(弱),AST偏强(弱)的协同变化。PC2为正位相时,对应的大气环流异常为:500 hPa高度场上为PNA型和大西洋东部(East Atlantic,EA)型的正位相,北美急流减弱;在年代际尺度上,第一模态主要表现为SIST偏西(东)且偏弱(强),PST偏东(西)且偏弱(强),AST偏西(东)且偏弱(强)的协同变化。PC1为正位相时,对应的大气环流异常为:500 hPa高度场上为西大西洋(West Atlantic,WA)型和EU型的正位相。第二模态主要表现为SIST偏强(弱)且偏北(南),PST偏南(北)且偏弱(强),AST北抬(南压)的协同变化。PC2为正位相时,对应的大气环流异常为:500 hPa高度场上为EU型和WA型的正位相,东亚急流强度加强且偏南,北美急流强度减弱。     

北方涛动同北半球温带大气环流的遥相关(二)——季节变化

在以前工作的基础上,本文进一步分析了各个季节北方涛动和与其相联系的北太平洋海温同北半球温带大气环流遥相关的特征,并与南方涛动和赤道太平洋海温的结果进行了比较,发现它们同北半球温带大气环流的遥相关,不仅具有很强的地区性,而且具有明显的季节变化。 冬季,北方涛动和北太平洋海温与PNA型相关非常明显,且比南方涛动和赤道太平洋海温的更密切。除PNA型外,北方涛动还同500hPa高度的亚洲-北美(ANA)流型有联系。 夏季,PNA遥相关型的表现仍然非常清楚,但位置较冬季偏酉和偏北,并在北美西海岸具有特殊的分叉现象。     

Sunshine duration and its variability in the main ridge of the Karkonosze Mountains in relation to with atmospheric circulation

Theoretical and Applied Climatology - Sunshine duration analysis was based on a series of measurements spanning the period from 1901 to 2014 for Śnieżka (1603 m a.s.l.)...     

Warm spells on the East African plateau and impacts in the White Nile basin

Theoretical and Applied Climatology - This study analyzes warm spells in the dry season over the East African plateau. Lake levels were observed to decline in the period July 2004 to January 2007....     

Changes of atmospheric circulation in central Europe and their influence on climatic trends in the Czech Republic

This work deals with the influence of changes of atmospheric circulation on observed trends of 11 climatic elements at 21 stations in the Czech Republic in the period 1961–1998. Atmospheric circulation in central Europe is described by the German (Hess-Brezowsky) and Czech-Slovak (Brádka’s) subjective catalogues of synoptic types. In the study period there is a strong downward trend in the occurrence of anticyclonic types in Brádka’s catalogue in all seasons, this trend being most prominent in autumn. Westerly and northwesterly types become more frequent in autumn and winter, less frequent in spring and summer under both classifications. In the Hess-Brezowsky catalogue, the occurrence of anticyclonic types increases in winter, spring, and summer. To assess the effect of circulation changes on observed climate trends we have used the method of “hypothetical” seasonal trends that are calculated from a daily series, constructed by assigning the long-term monthly average of the given climatic element under a specific circulation type to each day classified with this type. The ratio of these circulation-conditioned trends and observed seasonal trends shows that changes in atmospheric circulation are the primary cause of massive winter warming and autumn cooling, which is connected with increasing precipitation and humidity. Summer climate trends are unrelated to changes in atmospheric circulation. Simultaneous use of more circulation classifications for the detection of climatic changes is highly recommended, as the long-term circulation trends depend on the catalogue applied.     

中国冬季气温月际变化特征及其对大气环流异常的响应

基于1951—2014年中国160站月气温和NCEP/NCAR再分析资料,利用季节的经验正交函数分解（S-EOF）等方法,研究了中国冬季气温月际变化的时、空演变特征及其对大气环流异常的响应。结果表明,中国冬季气温在月尺度上常常出现前、后冬相反甚至冷暖交替的现象。中国冬季气温月际变化存在3个主模态:全冬一致型、前后反相型和冷暖交替型。当西伯利亚高压冬季一致偏强（偏弱）时,冬季一致冷（暖）;当海陆热力差异由强变弱、西伯利亚高压强度由强变弱,东亚西风急流比较稳定,强度偏强,位置由南向北移动时,冬季前冷后暖;当大气环流发生突变,尤其是海平面气压场和500 hPa位势高度场上大气活动中心的频繁调整,西伯利亚高压强度在月时间尺度上强弱交替时,冬季气温呈冷—暖—冷交替变化。      

Autoregressive logistic regression applied to atmospheric circulation patterns

Autoregressive logistic regression models have been successfully applied in medical and pharmacology research fields, and in simple models to analyze weather types. The main purpose of this paper is to introduce a general framework to study atmospheric circulation patterns capable of dealing simultaneously with: seasonality, interannual variability, long-term trends, and autocorrelation of different orders. To show its effectiveness on modeling performance, daily atmospheric circulation patterns identified from observed sea level pressure fields over the Northeastern Atlantic, have been analyzed using this framework. Model predictions are compared with probabilities from the historical database, showing very good fitting diagnostics. In addition, the fitted model is used to simulate the evolution over time of atmospheric circulation patterns using Monte Carlo method. Simulation results are statistically consistent with respect to the historical sequence in terms of (1) probability of occurrence of the different weather types, (2) transition probabilities and (3) persistence. The proposed model constitutes an easy-to-use and powerful tool for a better understanding of the climate system.     

Improved atmospheric circulation over Europe by the new generation of CMIP6 earth system models

Global Climate Models (GCMs) generally exhibit significant biases in the representation of large-scale atmospheric circulation. Even after a sensible bias adjustment these errors remain and are inherited to some extent by the derived downscaling products, impairing the credibility of future regional projections. In this study we perform a process-based evaluation of state-of-the-art GCMs from CMIP5 and CMIP6, with a focus on the simulation of the synoptic climatological patterns having a most prominent effect on the European climate. To this aim, we use the Lamb Weather Type Classification (LWT, Lamb British isles weather types and a register of the daily sequence 736 of circulation patterns 1861-1971. METEOROL OFF, GEOPHYS MEM; 737 GB; DA 1972; NO 116; PP 1-85; BIBL 2P1/2, 1972), a subjective classification of circulation weather types constructed upon historical simulations of daily mean sea level pressure. Observational uncertainty has been taken into account by considering four different reanalysis products of varying characteristics. Our evaluation unveils an overall improvement of salient atmospheric circulation features consistent across observational references, although this is uneven across models and large frequency biases still remain for the main LWTs. Some CMIP6 models attain similar or even worse results than their CMIP5 counterparts, although in most cases consistent improvements have been found, demonstrating the ability of the new models to better capture key synoptic conditions. In light of the large differences found across models, we advocate for a careful selection of driving GCMs in downscaling experiments with a special focus on large-scale atmospheric circulation aspects.