Trend analysis of precipitation time series in Greece and their relationship with circulation using surface and satellite data: 1955–2001

Summary In this study, the trends of annual and seasonal precipitation time series were examined on the basis of measurements of 22 surface stations in Greece for the period 1955–2001, and satellite data during the period 1980–2001. For this purpose, two statistical tests based on the least square method and one based on the Mann-Kendall test, which is also capable of detecting the starting year of possible climatic discontinuities or changes, are applied. Greece, in general, presents a clear significant downward trend in annual precipitation for the period 1955–2001, which is determined by the respective decreasing trend in winter precipitation. Both winter and annual series exhibit a downward trend with a starting year being 1984. Satellite-derived precipitation time series could be an alternative means for diagnosing the variability of precipitation in Greece and detecting trends provided that they have been adjusted by surface measurements in the wider area of interest. The relationship between precipitation variability in Greece and atmospheric circulation was also examined using correlation analysis with three circulation indices: the well-known North Atlantic Oscillation Index (NAOI), a Mediterranean Oscillation Index (MOI) and a new Mediterranean Circulation Index (MCI). NAOI is the index that presented the most interesting correlation with winter, summer and annual precipitation in Greece, whereas the MOI and MCI were found to explain a significant proportion of annual and summer precipitation variability, respectively. The observed downward trend in winter and annual precipitation in Greece is linked mainly to a rising trend in the hemispheric circulation modes of the NAO, which are connected with the Mediterranean Oscillation Index.     

Long-term variability of temperature and precipitation in the Czech Lands: an attribution analysis

Among the key problems associated with the study of climate variability and its evolution are identification of the factors responsible for observed changes and quantification of their effects. Here, correlation and regression analysis are employed to detect the imprints of selected natural forcings (solar and volcanic activity) and anthropogenic influences (amounts of greenhouse gases—GHGs—and atmospheric aerosols), as well as prominent climatic oscillations (Southern Oscillation—SO, North Atlantic Oscillation—NAO, Atlantic Multidecadal Oscillation—AMO) in the Czech annual and monthly temperature and precipitation series for the 1866–2010 period. We show that the long-term evolution of Czech temperature change is dominated by the influence of an increasing concentration of anthropogenic GHGs (explaining most of the observed warming), combined with substantially lower, and generally statistically insignificant, contributions from the sulphate aerosols (mild cooling) and variations in solar activity (mild warming), but with no distinct imprint from major volcanic eruptions. A significant portion of the observed short-term temperature variability can be linked to the influence of NAO. The contributions from SO and AMO are substantially weaker in magnitude. Aside from NAO, no major influence from the explanatory variables was found in the precipitation series. Nonlinear forms of regression were used to test for nonlinear interactions between the predictors and temperature/precipitation; the nonlinearities disclosed were, however, very weak, or not detectable at all. In addition to the outcomes of the attribution analysis for the Czech series, results for European and global land temperatures are also shown and discussed.     

Annual and seasonal variability of precipitation in Vojvodina,Serbia

Annual and seasonal variability of precipitation observed at 92 stations in Vojvodina (Serbia) were analyzed during the period 1946–2006. The rainfall series were examined by means of the empirical orthogonal functions (EOF). The first set of singular vectors explains from 68.8 % (in summer) to 81.8 % (in winter) of the total variance. The temporal variability of the time series associated with the main EOF configurations (the principal components, PCs) was examined using the Mann–Kendall test and the spectral analysis. The time series of PC1 revealed decreasing trend in the winter and spring precipitation and increasing trend in the autumn, summer, and annual precipitation. The relationships between the first PC and circulation patterns, such as the North Atlantic Oscillation (NAO), the East Atlantic (EA) pattern, and East Atlantic/West Russia pattern, were also investigated. The PC1, displaying temporal behavior of the first mode, demonstrated evident correspondence with the NAO index in analysis of the annual, winter, and autumn precipitation. Power spectra of the PC1 show statistically significant oscillations of about 3.3 years for the spring precipitation and about 8 and 15 years for the winter precipitation. Comparisons with spectral analysis of authors for some regions in Europe, most of them in the Mediterranean domain, show that similar periodicities are detected.     

中国三峡地区汛期降水量的正态性研究

为了分析三峡地区降水量序列的正态性和谱结构,对降水量的常见各种变换进行试验性研究。试验包括单站降水量、降水量的平方根、立方根和Γ分布概率值等非线性变换,以及级别变换。研究区域多站平均降水量、区域降水量的主分量、区域降水量的非线性变换后的主分量和区域降水量的级别变换后的主分量等序列。研究发现三峡地区单站降水量的各种变换不改变序列原始谱结构,仅影响概率分布的偏度和峰度,使其较好地遵从正态分布,其中以Γ分布的变换以及级别变换在分布的偏度上为最好。 区域降水量的各种变换的综合指数(区域平均值和主分量)正态性及谱结构分析表明,除区域平均值变换后不改变原单站序列的谱结构外,主分量的综合指数能改变原单站序列的谱结构,同时也影响概率分布的偏度和峰度,使其能较好地遵从正态分布。其中以降水量的立方根和Γ分布概率变换以及级别变换,在分布的偏度上有较好的效果。     

基于CART算法的夏季干旱预测模型研究及应用

利用标准化降水蒸散指数(SPEI)作为判断干旱是否发生的标准,基于数据挖掘方法中的CART算法探究夏季西太平洋副高北界、夏季西太平洋副高强度指数、准两年振荡(QBO)、东亚夏季风指数、夏季北大西洋涛动(NAO)、夏季太平洋年代际振荡(PDO)、厄尔尼诺指数等多项气候因子与干旱的关系,构造分类决策树,得到干旱预报规则集,从而建立干旱的预报模型。预报模型以各项气候因子为输入变量,是否干旱为目标变量。根据1955—2012年商丘月平均气温和月总降水资料计算出商丘夏季58 a的SPEI指数作为干旱判定指标;以同期的多项气候因子数据作为输入变量,随机选取46 a的数据得到7条分类规则集,分类准确率为86.96%。使用剩余12 a的数据验证,准确率高达91.67%。结果有力地证明了基于CART算法建立干旱预报模型的可行性、科学性、有效性以及与干旱研究理论的一致性,为干旱模型的研究及季节性预测提供了科学有效的新思路。     

Improving the methodology for spectral analysis of climatic time series

The paper discusses a methodology able to estimate both the discrete and continuous spectra without any assumption on the shape of spectral densities. The approach to estimate the spectral density is based on a robust smoothing of the periodogram. Bandwidth, a quantity similar to the width of spectral windows traditionally used in spectral analysis, is estimated locally in contrast to intuitively chosen global window lengths. Detection and estimation of frequencies forming discrete spectra are also addressed. The procedure is applied to Central England temperature (CEt), North Atlantic Oscillation (NAO) index and Oxygen Isotope of North Greenland Ice Core Project (δ¹⁸O of NGRIP) data. Annual and half annual cycles were detected in CEt data, whilst 118.2- and 41.7-ky cycles were found in δ¹⁸O of NGRIP. This latter periodicity is almost as intense as the dominant longer cycle. Several local peaks of spectral densities were recognised in each time series that mostly cover earlier results. However, a few previous findings at low frequencies have not been reinforced by the present method. Identification of modest local peaks or discrete amplitudes at low frequencies is an extremely challenging task as climatic data generally have spectral densities rising to low frequencies.     

Interannual variability of winter precipitation over northwest India and adjoining region: impact of global forcings

The role of El Niño/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.     

Deficit and surplus of precipitation as a continuous function of time

The quantitative measure of drought is a complex problem. A common factor in all types of drought is that they originate from a deficiency of precipitation. The difference between the real and potentially expected daily precipitation amount is what we have used to define the continuous function of daily deficit and surplus of precipitation (DSP). We have also defined the monthly and yearly DSP. We tested the precipitation data series for the normal climatic period 1961–1990 observed at the Meteorological Observatory of Belgrade. The DSP considers that the intensity of drought during the current period is dependent upon the current precipitation plus the cumulative precipitation of the previous period. When applied to longer periods, this function denotes dry and wet periods.     

Long-term precipitation variability in the Ionian Islands, Greece (Central Mediterranean): climatic signal analysis and future projections

The present study examines the variability of the precipitation regime across the Ionian Islands complex, Greece (Central Mediterranean), for a period spanning more than one century. Significant negative long-term linear trends in the annual precipitation totals are observed, more pronounced in the southern parts of the studied area, while a climatic discontinuity possibly occurred during the 1970s, manifested first in the southern Ionian. Statistically significant nonlinear trends and subdecadal intermittent oscillations were detected using Monte Carlo singular spectral analysis. The correlation of the winter precipitation variability at Ionian complex with the North Atlantic Oscillation anomalies was also investigated and extended in the frequency domain. Finally, future projections were performed using an ensemble of Regional Climatic Models. Model simulations suggested a decrease of the order of ～20% or more in the mean annual precipitation of the area by the end of the century.     

Teleconnections of Inter-Annual Streamflow Fluctuation in Slovakia with Arctic Oscillation, North Atlantic Oscillation, Southern Oscillation, and Quasi-Biennial Oscillation Phenomena

The aim of the paper is to analyze a possible teleconnection of Quasi-Biennial Oscillation （QBO）, Southern Oscillation （SO）, North Atlantic Oscillation （NAO）, and Arctic Oscillation （AO） phenomena with longterm streamflow fluctuation of the Bela River （1895-2004） and Cierny Hron River （1931-2004） （central Slovakia）. Homogeneity, long-term trends, as well as inter-annual dry and wet cycles were analyzed for the entire 1895-2004 time series of the Bela River and for the 1931-2004 time series of the Cierny Hron River. Inter-annual fluctuation of the wet and dry periods was identified using spectral analysis. The most significant period is that of 3.6 years. Other significant periods are those of 2.35 years, 13.5 years, and 21 years. Since these periods were found in other rivers of the world, as well as in SO, NAO, and AO phenomena, they can be considered as relating to the general regularity of the Earth.     

Extreme daily precipitation in Belgrade and their links with the prevailing directions of the air trajectories

The main goal of this study was to present the statistical analysis of the daily precipitation exceeding 20 mm in Belgrade and their links with the prevailing directions of the air trajectories at 500, 1,500 and 5,000 m. For the extreme precipitation analysis, the generalised extreme value (GEV) distribution and generalised Pareto distribution (GPD) were used. The estimated return levels for 100- and 10-year return periods using GEV and GPD were obtained. Four-day backward trajectory simulations were conducted for days with precipitation exceeding 20 mm to investigate the regional transport of the air moisture towards Belgrade using the hybrid single-particle Lagrangian integrated trajectory model. The air trajectories were classified into 13 trajectory categories by the origin and direction of their approach to Belgrade. Three of the most frequent categories of air flow from south-west, south-east and north-west contributed to more than a half of the observed precipitation. Almost 74.5 % of precipitation totals in Belgrade fell during the warmer part of the year. These were directly connected with the intensive convection of colder and humid, usually maritime, air masses.     

Influence of the North Atlantic Oscillation on winter rainfall in Calabria (southern Italy)

Seasonal rainfall amounts, directly responsible for availability of water resources on a specified area, are strongly dependent on the climate system. In order to highlight some features of such dependence, generally circulation indexes based on the difference in the sea level pressure between two geographic areas are taken into account. In the present study, the relationships between winter rainfall series observed in the Calabria region (southern Italy) and the North Atlantic Oscillation Index (NAOI) have been analysed. Firstly, a correlation analysis between precipitation and the NAOI was performed. Subsequently, the influence of the different phases of the NAO on the winter precipitation has been detected by a composite analysis, and by identifying changes in the behaviour of the probability density functions (gamma distribution) fitted on monthly rainfall. The results evidence a clear link existing between the phases of the climatic index and the amount of winter rainfall.     

淮河流域夏季降水异常与若干气候因子的关系

基于旋转经验正交函数分解 (REOF) 方法探讨淮河流域1961—2010年夏季降水与厄尔尼诺/南方涛动 (ENSO)、北大西洋涛动 (NAO)、印度洋偶极子 (IOD)、太平洋年代际振荡 (PDO) 之间的关系,并进一步分析各气候因子不同位相单独以及联合对淮河流域夏季降水的影响。结果表明:淮河流域夏季降水与ENSO,PDO,NAO,IOD等气候因子具有较稳定的相关性,其中,PDO和IOD是影响淮河流域夏季降水的关键因子,且PDO与夏季降水呈显著负相关关系;各气候因子的冷暖位相单独及联合对淮河流域夏季降水的影响不同,PDO的冷期以及NAO,IOD冷位相使流域北部的夏季降水量呈显著增加趋势,PDO分别联合ENSO,NAO和IOD的冷、暖位相对流域北部地区和淮河上游地区的夏季降水影响显著。     

Witnessing North Atlantic westerlies variability from ships’ logbooks (1685–2008)

A monthly index based on the persistence of the westerly winds over the English Chanel is constructed for 1685–2008 using daily data from ships’ logbooks and comprehensive marine meteorological datasets. The so-called Westerly Index (WI) provides the longest instrumental record of atmospheric circulation currently available. Anomalous WI values are associated with spatially coherent climatic signals in temperature and precipitation over large areas of Europe, which are stronger for precipitation than for temperature and in winter and summer than in transitional seasons. Overall, the WI series accord with the known European climatic history, and reveal that the frequency of the westerlies in the eastern Atlantic during the twentieth century and the Late Maunder Minimum was not exceptional in the context of the last three centuries. It is shown that the WI provides additional and complementary information to the North Atlantic Oscillation (NAO) indices. The analysis of WI series during the industrial era indicates an overall good agreement with the winter and high-summer NAO, with the exception of several multidecadal periods of weakened correlation. These decoupled periods between the frequency and the intensity of the zonal flow are interpreted on the basis of several sources of non-stationarity affecting the centres of the variability of the North Atlantic and their teleconnections. Comparisons with NAO reconstructions and long instrumental indices extending back to the seventeenth century suggest that similar situations have occurred in the past, which call for caution when reconstructing the past atmospheric circulation from climatic proxies. The robustness and extension of its climatic signal, the length of the series and its instrumental nature make the WI an excellent benchmark for proxy calibration in Europe and Greenland.     

The Medieval Warm Period, the Little Ice Age and simulated climatic variability

The CSIRO Mark 2 coupled global climatic model has been used to generate a 10,000-year simulation for ‘present’ climatic conditions. The model output has been analysed to identify sustained climatic fluctuations, such as those attributed to the Medieval Warm Period (MWP) and the Little Ice Age (LIA). Since no external forcing was permitted during the model run all such fluctuations are attributed to naturally occurring climatic variability associated with the nonlinear processes inherent in the climatic system. Comparison of simulated climatic time series for different geographical locations highlighted the lack of synchronicity between these series. The model was found to be able to simulate climatic extremes for selected observations for century timescales, as well as identifying the associated spatial characteristics. Other examples of time series simulated by the model for the USA and eastern Russia had similar characteristics to those attributed to the MWP and the LIA, but smaller amplitudes, and clearly defined spatial patterns. A search for the frequency of occurrence of specified surface temperature anomalies, defined via duration and mean value, revealed that these were primarily confined to polar regions and northern latitudes of Europe, Asia and North America. Over the majority of the oceans and southern hemisphere such climatic fluctuations could not be sustained, for reasons explained in the paper. Similarly, sustained sea–ice anomalies were mainly confined to the northern hemisphere. An examination of mechanisms associated with the sustained climatic fluctuations failed to identify a role for the North Atlantic Oscillation, the El Niño-Southern Oscillation or the Pacific Decadal Oscillation. It was therefore concluded that these fluctuations were generated by stochastic processes intrinsic to the nonlinear climatic system. While a number of characteristics of the MWP and the LIA could have been partially caused by natural processes within the climatic system, the inability of the model to reproduce the observed hemispheric mean temperature anomalies associated with these events indicates that external forcing must have been involved. Essentially the unforced climatic system is unable to sustain the generation of long-term climatic anomalies.     

A spectral analysis of Iberian Peninsula monthly rainfall

Summary A spectral analysis of Iberian Peninsula monthly rainfall series was carried out. The data set consists of monthly precipitation records from 40 meteorological observatories over 74 years (1919–1992). The stations are representative of most of the Iberian Peninsula. The rainfall series were analyzed spatially by means of Principal Component Analysis (PCA) and temporally by means of the Multi-Taper Method (MTM) of spectral analysis of by Monte-Carlo Singular Spectrum Analysis (MCSSA). The PCA gave six dominant modes of variation which explain 75% of the variance with each component affecting a different region of the Peninsula. The spectral analysis showed 7 year oscillations for the dominant pattern and 2.7 and 16 years for the third pattern. The 7-year oscillation seems to be related to other climatic oscillations recorded in the northern hemisphere while the 2.7-year oscillation could be related to the ENSO phenomenon. Received July 18, 2000 Revised April 19, 2001     

Wavelet analysis of a centennial (1895–1994) southern Brazil rainfall series (Pelotas, 31°46′19″S 52°20′ 33″W)

In this work we apply the wavelet transform to the Pelotas (southern Brazil) total annual rainfall series (1894–1995). Classical, wavelet and cross-wavelet analyses were performed in the El Niño Southern Oscillation (ENSO), Quasi-Biennial Oscillation (QBO), sunspot number (Rz) and Pelotas rainfall time series. Classical spectral analysis for Pelotas has shown a large number of short periods – between 2.2–5.6 years (yr) and periods at 8.9, 11.7 and 24.9 yr. Further, we have found that the Pelotas rainfall wavelet spectrum shows the most significant periodicities around 2–8 yr, but they have an intermittent character. Cross-wavelet spectrum showed that: rainfall and QBO series are correlated at 2–3 yr (QBO) scales and this cross-power is continuous along the time series interval; rainfall and SOI have higher cross-power around 4–8 yr, but this signal is sporadic; rainfall and sunspot number (Rz) showed higher cross-power around the 11-yr solar cycle period, but this cross-power is sporadically high and low; finally, the rainfall cross-spectrum with the double sunspot number (Rz22) revealed a high cross-power around 20–22 yr which is more persistent in duration, compared to the 11-yr period. These wavelet results are compared with classical spectral analysis and with previous work results. We concluded that the phenomenon that influences most of Pelotas rainfall variability is ENSO, but only a minor part of the variance (～30%) can be described by a simple multi-linear dependence on solar/ENSO/QBO phenomena, this result could imply that non-linear coupling among sun and internal climatic variability (QBO, ENSO) has an important role in the local/regional climate variations.     

Fractal analysis of climatic data: Annual precipitation records in Spain

Summary The rescaled range analysis was applied to the annual precipitation series from 10 weather stations in Spain for the period 1901–1989. The analysis reveals that the series of precipitations fits a fractal distribution, with a mean fractal dimension of 1.32 ± 0.01. This lies in the same order of magnitude as the fractal dimensions obtained from other macrometeorological and paleoclimatic registers. The favourable comparison between fractal dimensions of the variables on both small time scales and long-term time spans suggests that such values are characteristic of climatic change over the spectral range of 10 to 10⁶ years. The results contribute to the establishment of this assumption as a valid hypothesis for the interpolation of climatic change from one scale to the next, and also in applications such as hydrological design.With 2 Figures     

Temperature and precipitation fluctuations in the Czech Republic during the period of instrumental measurements

The history of early meteorological observations using instruments in the Czech Lands is described (the longest temperature series for Prague-Klementinum starts in 1775, precipitation series for Brno in 1803). Using the PRODIGE method, long-term monthly temperature and precipitation series from selected secular stations were homogenised (for 10 and 12 stations, respectively). All the seasonal and annual temperature series for the common period 1882–2010 show a significant positive linear trend with accelerated warming from the 1970s onwards. No significant linear trends were disclosed in the series of seasonal and annual precipitation totals. Correlation coefficients between the Czech series analysed decrease as distances between measuring stations increase. A sharper decrease of correlations for precipitation totals displays much weaker spatial relationships than those for mean temperatures. The highest correlations between all stations appeared in 1921–1950, the lowest in 1891–1920 (temperature) and 1981–2010 (precipitation). Wavelet analysis reveals that very distinct annual cycles as well as the slightly weaker semi-annual ones are better expressed for temperature series than for precipitation. Statistically significant cycles longer than 1?year are temporally unstable and sporadic for precipitation, while in the temperature series cycles of 7.4–7.7 and 17.9–18.4?years were recorded as significant by all stations in 1882–2010 (quasi-biennial cycle of 2.1–2.2?years for half the stations). Czech homogenous temperature series correlate best with those of the Northern Hemisphere for annual, spring and summer values (with significant correlation coefficients between 0.60 and 0.70), but this relation is temporally unstable. Circulation indices, such as the North Atlantic Oscillation Index (NAOI) and the Central European Zonal Index (CEZI), may explain the greater part of Czech temperature variability, especially from December to March and for the winter; however, this relationship is much weaker, or even random, for precipitation series. Further, relationships with the Southern Oscillation Index (SOI) are weak and random. Relatively weak coincidences exist between statistically significant cycles in the Czech series and those detected in NAOI, CEZI and SOI series.     

气象要素时间序列的演化建模分析与短期气候预测

以武汉站(5~9月)汛期降水量观测数据序列为例,将这类具有明显的不规则性(混沌特征)时间序列分解为宏观气候尺度周期的波动部分和迭加其上的微观气候尺度周期的波动部分,分别采用演化建模方法和自然基小波方法模拟逼近。特别强调由演化建模方法得到的非线性常微分方程较之传统的线性建模具有更好的分析预测能力。