Moderate to heavy cold-weather precipitation occurrences in Tehran and the associated circulation types

Large-scale atmospheric circulations associated with 133 moderate to heavy cold-weather precipitation events recorded at Mehrabad station in Tehran, Iran, during the period 1951–2013 are analysed. To this end, the performance of un-rotated, orthogonally rotated and obliquely rotated solutions of T-mode principal component analysis (PCA) is examined in classifying the atmospheric circulations into a few representative circulation types (CTs). The T-mode PCAs were applied to the 500-hPa geopotential height for the events in a domain from 10^°E to 70^°E and from 20^°N to 50^°N. The first six leading principal components were retained and then orthogonally and obliquely rotated using varimax and promax solutions, respectively. Statistical inter-comparison of the CTs obtained using the three solutions suggests that the obliquely rotated solution is the better choice for circulation classification in the present study. The six CTs obtained using the oblique rotation were then linked to the daily total precipitation and daily mean temperature variability at Tehran station as well as to the standardized anomalies of the daily total precipitation and mean daily temperature of a dense network of stations distributed across Iran. It is found that the CTs identified, though generally comparable in producing significant precipitation in Tehran, vary in their potential to bring cold weather and generate snowfall in Tehran specifically and in the country in general. While the first three CTs give rise to regional patterns of standardized precipitation anomalies centred in Tehran, the next three CTs leave a pronounced precipitation signature almost across the whole country. As regards the standardized temperature anomalies, with the exception of one CT that causes deep and widespread negative standardized anomalies over most parts of the country, the other CTs are characterized with a dipolar structure of a deep intrusion of cold weather to the west and prevailing warm weather to the east of the country.     

Arctic or North Atlantic Oscillation? Arguments based on the principal component analysis methodology

Summary The Arctic Oscillation (AO) appears as the leading unrotated mode of principal component analysis (PCA) of monthly mean sea level pressure anomalies, whereas the North Atlantic Oscillation (NAO) results from rotated PCA, regardless of the number of PCs rotated. Three criteria are employed to decide whether the interpretation in terms of the NAO or AO should be preferred: the degree of simple structure, the similarity between the PC loadings and correlation/covariance maps, and the sensitivity to spatial subsampling. All these criteria favour, to a different extent, the interpretation in terms of the NAO. This is further supported by more general arguments. Therefore, the statistical arguments suggest that in interpreting the Northern Hemisphere circulation variability, the sectorial view, i.e. the NAO, should be preferred to the hemispheric view, i.e. the AO. Our analysis supports the idea expressed in other studies that the AO is rather a statistical artifact.     

Application of Non-hierarchically Clustered Circulation Components to Surface Weather Conditions: Lake Superior Basin Winter Temperatures

Summary  An S-mode principal component analysis was applied to the correlation and covariance matrices of a 700 hPa height window centered over eastern North America. The first five components were retained and rotated. K-means clustering algorithm was applied to the four solutions to generate 22 circulation types for each. Regression models were built, relating monthly mean temperature anomalies to monthly circulation type frequencies. The results were compared to those obtained in an earlier study using a correlation-based approach to classify the same data set. It was found that i) the amount of within-type variability is comparable for the two and that ii) the difference between cold and warm days of a circulation type is for both classification approaches an anomaly located to the east of the lake. A detailed analysis of this difference indicated that some information is contained in the scores for the first five principal components (and could therefore be used to improve the regression results) but that some is contained in the higher order components and was thus lost. Differences in the PC scores between warm and cold days were used to divide the circulation types into warm and cold subtypes. This improved the regression results but the best results were still inferior to those that had been obtained with the correlation-based classification. Received August 25, 1998 Revised January, 5, 1999     

A circulation classification scheme applicable in GCM studies

Summary  The goal of the paper is to present and examine the method of classification of daily circulation patterns that allows (i) a fair comparison of groupings among different datasets (typically representing the observed climate and that simulated by a general circulation model, GCM) and (ii) huge datasets, common in GCM studies based on daily values, to be classified. The circulation classification method is shown to be a useful tool in GCM validation and analysis of climate change response, particularly in comparisons of (i) shapes of the mean type patterns, (ii) the frequency and persistence of the types, (iii) the probability of transitions from one type to another, and (iv) conditional surface temperature distributions. It is also shown that a simultaneous examination of multiple classifications is beneficial in eliminating subjectivity of any single classification and allowing a detailed inspection of differences between climates. The classification method is a modification of the T-mode principal component analysis (PCA). The T-mode refers to the input data matrix where gridpoint values are arranged in rows and daily patterns in columns. The classification procedure is applied to observed daily 500 hPa geopotential height patterns and those simulated by the control and ECHAM3 GCM runs. Received November 1, 1999/Revised May 8, 2000     

Spatial modelling of air temperature and precipitation for Andorra (Pyrenees) from daily circulation patterns

Based on the daily sea level pressure (SLP) circulation catalogue obtained by Esteban, Martin-Vide and Mases, Int J Climatol 26:1501–1515, (2006) for Western Europe, high-resolution maps of daily maximum and minimum temperature, mean daily precipitation and daily precipitation probability have been obtained for Andorra (Pyrenees). The 20 daily-circulation patterns cover the period 1960–2001 and were generated using new approaches based on the rotated principal component analysis (PCA) and clustering technique. The final maps of Andorra associated with each circulation pattern have been constructed using altitude, latitude, continentality and solar radiation as multiple regression predictors (Ninyerola, Pons and Roure, Int J Climatol 20:1823–1841, 2000). The daily temperature and rainfall series used from Andorran, French and Catalan/Spanish weather stations have been checked for data quality. The results confirm the complexity of the spatial distribution of meteorological phenomena over mountainous areas such as in Andorra, and show the importance of the Mediterranean and Atlantic influence upon the climate of this country of the Pyrenees. On the other hand, different tests have been made showing that the classification results could improve the resulting interpolated climate maps by the use of the circulation-pattern frequencies.     

Winter and autumn daily precipitation patterns in Catalonia,Spain

Summary A principal component analysis (PCA), based on a network consisting of 60 pluviometric gauges and their daily precipitation data, is attempted in order to describe the main winter and autumn patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation correlation matrix and rotated component loadings are then deduced and extensively interpreted. The PCA results are then used in a clustering process (Average Linkage), leading to two rainfall divisions, one for each season, which are then compared.With 5 Figures     

Surface Synoptic Circulation and Daily Precipitation in Catalonia

Summary Catalonian daily precipitation patterns were obtained by means of S-mode Principal Component Analysis (PCA) applied to subsets built up according to synoptic surface airflow. Daily data from November to April 1990–1994 from 45 raingauge sites in Catalonia without missing data were used. Eight circulation classes for days in this period were considered. A Precipitation Activity Index (PAI) was calculated for each station and class, leading to the determination of the highest activity regions for each subset. In addition, we determined which circulation type produced most precipitation at each station of the network. Separate PCAs were performed for each of the eight classes considered and the first four Varimax rotated solutions were interpreted for each circulation type. The physical meaning of the rest of the factors were omitted as they were associated with local behaviour. Finally, some concluding remarks on the cyclogenic properties of the Western Mediterranean Basin and the diversity of the extracted patterns are presented and supported by the authors experience in forecasting in Catalonia. Received February, 11, 1997 Revised May 23, 1997     

我国夏季气温、降水场的时空特征分析

本文用主分量及转动主分量分析方法对我国夏季气温及降水量场(1951—1985年期间)的时空特征进行了研究。从所提取的空间模式中发现气温的解释方差较降水的大,且具有较好的均匀性;从对应的时间分量分析发现气温与降水均具有2—3年的主要变化周期,与东亚大气环流的关系分析中以气温表现较为密切。 对气温和降水场时空特征的稳定性分析表明,无论在持续性、周期性及与东亚大气环流的关系上,进入70年代后均有较明显的变化。比较表明气温场的稳定性较降水为好。 气温场与降水场相互关系分析发现它们有显著的反相关,表现显著的地区为长江中下游、华南及华北等。     

An empirical classification of weather types in the Mediterranean Basin and their interrelation with rainfall

Summary  This paper presents a classification of weather types in the Mediterranean Basin based on cluster analysis of the daily occurrences of several surface pressure centers and the subjective identification of 500 hPa trough axis positions (1992–1996). The procedure results in 20 types that explain 69% of overall pressure center variance and which are consistent with the seasonal succession of regional circulation. The development of weather types in winter is primarily controlled by the eastward propagation of barotropic waves while departures from the zonal flow pattern in summer tend to be linked to blocked stationary pools. H1-types with anticyclonic circulation in the Western Mediterranean and cyclonic flow in the eastern part are well interrelated with zonal and anticyclonic general weather types in Central Europe. H2-types featuring a weak Azores Anticyclone interrelate with a variety of meridional circulation types after the Hess and Brezowski (1969) classification. The 20 types explain rainfall variance in the core Mediterranean regions (as defined by principal components) to a high degree while rainfall variance in marginal regions is influenced by circulation patterns not being typical for the Mediterranean Basin. Received January 29, 1999 Revised March 28, 2000     

Temporal instability of temperature singularities in a long-term series at Prague-Klementinum

The recent study of meteorological singularities in the Czech Republic—?ezní?ková et al. (Theor Appl Climatol 88:179–192, 2007)—is supplemented with respect to findings concerning temporal stability of air temperature singularities. A long-term daily series at Prague-Klementinum is examined over 1881–2000, adopting the same statistical background for the detection of singularities. A procedure that reduces the dependence of results on the choice of a reference period is proposed. It is shown that nearly all temperature singularities found over the period since 1961 and/or the whole period of 1881–2000 are temporally unstable, and a lot of them should be treated as statistical properties of particular data samples (time periods) reflecting sampling variability rather than stable and recurrent anomalies related to distinct circulation patterns. The most important singularities are identified, occurring on 4 days in year only (around St. Médard’s Day, 8 June), and they are linked to intraseasonal variations in the occurrence of circulation patterns conducive to warm and cold temperature anomalies. A sharp increase (decrease) in the frequency of north (south) circulation types after 8 June leads to the most robust irregularity in the mean annual cycle of temperature.     

Pattern characteristics of Indian monsoon rainfall using principal component analysis (PCA)

In the present study the Principal Component Analysis (PCA) is used to determine the dominant rainfall patterns from rainfall records over India. Pattern characteristics of seasonal monsoon rainfall (June–September) over India for the period 1940 to 1990 are studied for 68 stations. The stations have been chosen on the basis of their correlation with all India seasonal rainfall after taking the ‘t’ Student distribution test (5% level). The PCA is carried out on the rainfall data to find out the nature of rainfall distribution and percentage of variance is estimated. The first principal component explains 55.50% of the variance and exhibits factor of one positive value throughout the Indian subcontinent. It is characterized by an area of large positive variation between 10°N and 20°N extending through west coast of India. These types of patterns mostly occur due to the monsoon depression in the head Bay of Bengal and mid-tropospheric low over west coast of India. The analysis identifies the spatial and temporal characteristics of possible physical significance. The first eight principal component patterns explain for 96.70% of the total variance.     

中国日降雨量变化特征分析

将旋转主成分分析和复变量莫莱特小波分析应用于14a中国境内实测与ECMWF再分析日降雨量资料,研究其年际、季际与季节内变化的时空分布特征和再分析日降雨量资料的有效性。结果表明:从实测资料中分析的降水特征,都可以由再分析资料再现出来。除周内振荡大有出入外,其余频率的振荡二者大致相符或基本吻合。因此,周内振荡还不能被再分析数据较好地刻划,但是与旱涝灾害和季风爆发联系密切的周际以上振荡,在再分析日降雨量中基本上能被正确地表现出来。     

Catalonian precipitation patterns: Principal component analysis and automated regionalization

Summary A principal component analysis (PCA) based on a network including 179 pluviometric gauges is attempted in order to describe the main patterns governing precipitation in Catalonia (NE Spain). This PCA procedure is applied to the interstation covariance matrix and component loadings are then deduced from it and extensively interpreted. PCA results are used immediately after in a Davis and Kalkstein clustering process, leading to a rainfall division. The homogeneity of the different groups obtained is tested by means of the analysis of their spatial and temporal within-group variances.With 8 Figures     

平流层下部气温年际异常的旋转主成分分析

利用旋转主成分方法分析了北半球冬季平流层下部月平均气温距平，得到的4个主要的空间分布型，分别表示中纬、低纬的纬向对称异常、太平洋北部暖中心和极区冷中心的异常。还指出中纬异常的时间演变最单调，整个冬季有持续性，年际变化呈现明显的准两年振荡周期(QBD)。     

Comparison of the MSL synoptic pressure patterns of the Arctic as observed and simulated by the GISS General Circulation Model

Summary The observed daily pressure fields over the Arctic are compared with those obtained from a control run of the Goddard Institute for Space Studies (GISS) General Circulation Model using a classification typing procedure developed by Kirchhofer (1973) and a rotated principal components analysis for both data sets. The simulated patterns appear quite realistic in both frequency and geographical characteristics, although the model data show greater extremes and more closed cells. The procedure described provides a potentially useful method for evaluating the synoptic representativeness of GCM simulations.With 9 Figures     

Interannual horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean: assimilation versus TAO analysis

This study is motivated by an interest in obtaining a new automated classification scheme of daily circulation types suitable for use throughout Europe. The classification scheme is performed on 500 hPa geopotential height anomalies (NCEP Reanalysis data, 2.5°×2.5°). Nine grid points represent the study area. Five anticyclonic types (Anw, Ane, A, Asw and Ase) and seven cyclonic types (C, Cnnw, Cwnw, Cwsw, Cssw, Cse, Cne) are defined. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The classification scheme is applied to three different case studies in the Mediterranean Basin: Greece, Cyprus and central Italy. The precipitation percentage of the cyclonic type and the mean seasonal correlation coefficients for all circulation types are the two criteria used to evaluate the performance of the classification scheme. The ability of the HadAM3P general circulation model to reproduce the mean pattern and frequency of circulation types at the 500 hPa level in comparison to the NCEP dataset for the period 1960–1990 is also evaluated. The percentage of rainfall that corresponds to the cyclonic circulation types is greater than 85% for the three study regions. Furthermore, the correlation coefficients for the three classifications are very encouraging, for nearly all days of the study period. Compared to observations, the GCM is able to capture the mean patterns but not able to replicate exactly the observed variability of the circulation types over the three study regions.     

基于迁移CNN的江淮持续性强降水环流分型

利用新建的1981-2018年区域持续性强降水个例集、1981-2018年中国逐日降水量及NCEP/NCAR全球再分析资料,运用江淮地区持续性强降水典型模态个例样本及残差神经网络(CNN),通过迁移学习分步训练建立针对江淮强降水的环流客观分型模型;并运用该模型对1981-2015年全国持续性强降水个例的环流进行客观分型...     

Wind forcing of the ocean and the Atlantic meridional overturning circulation

Wind forcing of the oceans is analysed for a millennium-length period of a simulation with a global coupled model. The time mean value of the zonal wind energy input to the oceans was found to be 0.97?TW, similar to other estimates, with maximum inputs in the Southern Ocean and Equatorial Pacific Ocean. The meridional wind energy input was also evaluated. The time series of the zonal wind energy input consisted of white noise, with marked multiannual and multidecadal variability, and had a range of 1.6 between minimum and maximum values. Seasonal variations in the wind energy input were most marked in the Pacific Ocean. Composites of the various climatic terms involved in the wind energy input, for maximum and minimum values of the input time series, identified distinct differences in anomaly values, particularly over the Southern Ocean. The temporal variability of the Pacific equatorial wind energy input was clearly identified with ENSO events. The model reproduced the observed structure of the Atlantic meridional overturning circulation surprisingly well. The time series of this circulation exhibited interannual to centennial variability. Correlations, both instantaneous and lagged, failed to identify any meaningful relationship between the temporal variability of the circulation and the wind energy input to the Southern Ocean. The optimum correlation was found between time smoothed versions of the time series for this circulation and the heat input to the North Atlantic Ocean, implying, as noted elsewhere, that this heat input is the principal driver of the temporal variability of the circulation.     

On the separability of the North Atlantic oscillation and Arctic oscillation

We address the issue of whether the Arctic (AO), and North Atlantic oscillations (NAO) are inseparable, forming an annular mode in the Northern Hemisphere atmospheric circulation. This annular mode is the leading empirical orthogonal function of hemispheric sea level pressure (SLP) data, explaining the largest amount of its variability. We examine whether the NAO and AO are inseparable spatial modes of the atmospheric circulation using rotated principal component analysis (RPCA), a methodology that identifies simple and unique patterns of spatial dataset variability. RPCA of the spring, summer, and autumn SLP fields from 1946-1998 reveal NAO and AO-like patterns, occurring as separate regional teleconnections forming the first and second principal components respectively. The RPCA-based NAO dipole pattern is like that observed in many prior studies, while the AO-like pattern exhibits high SLP variability over the Kara and Laptev seas. During winter however, and in annual analyses, a distinct AO-like pattern is not obtained and the two patterns may be inseparable using commonly accepted RPCA methods. The RPCA-based AO-like mode is significantly linked to north-central Siberian seasonal air temperatures and to the prevailing direction of motion of the underlying Arctic Ocean in summer, suggesting that the non-winter AO-like pattern, as a stand alone teleconnection separate from the NAO, contributes significantly to high-latitude climate and ocean variability. The winter NAO/AO inseparability is discussed as a possible effect of a shared winter storm track between the northeastern Atlantic and the Arctic.     

Euro-Atlantic circulation types and modes of variability in winter

Extra-tropical atmospheric circulation variability is addressed in this study using two complementary approaches: circulation types and modes of variability. Principal component analysis (PCA) in T- and S-modes has been used to estimate the features. An objective synoptic classification of Euro-Atlantic atmospheric circulation is described. Eight circulation types have been identified and described in terms of their spatial features, mean frequency and lifetime, transitions and trends. The most persistent type is connected with the presence of a ridge over the British Isles and Iceland, while the less persistent type is related to a tilted ridge west of the continent. Increases in the persistence of some circulation types such as the Scandinavian and the Euro-Atlantic blockings and a ridge with axis over Western Europe have been found in the 1990s. Data from two independent re-analyses are used to test the robustness of the results. The main differences between the two datasets have been found in the distribution of events as a function of their duration. In a second step, the main modes of variability over the Euro-Atlantic area have been identified using daily data. An analysis of the relationship between these modes and the circulation types suggests that specific circulation types shift the phases of certain modes of variability.