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The purpose of this study was to analyze the influence of solar activity described by the sunspot number (SSN) on certain terrestrial variables that might impact the Southeastern European climate at different spatio-temporal scales (the North Atlantic Oscillation Index, NAOI, and the Greenland–Balkan Oscillation Index, GBOI—on a large scale; the Palmer Hydrological Drought Index, PHDI—on a regional scale; the Danube discharge at the Orsova (lower basin), Q, representative of the Southeastern European climate—on a local scale). The investigations were carried out for the 20th century using the annual and seasonal averages. To find the connections between terrestrial (atmospheric and hydrological) parameters and SSN, the wavelet coherence were used both globally and in the time–frequency domain. The analyses were carried out for the time series and considered simultaneously (in the same year or season), as well as with lags from 1 to 5 years between the analyzed variables. For the annual values, the type of correlation (linear/non-linear) was also tested using elements from information theory. The results clearly revealed non-linear links between the SSN and the terrestrial variables, even for the annual average values. By applying the wavelet transform to test the solar influence on the terrestrial variables, it was shown that the connections depend on both the terrestrial variable, as well as on the considered lags. Since, in the present study, they were analyzed using wavelet coherence, but only the cases in which the coherence was significant for almost the entire analyzed time interval (1901–2000) and the terrestrial variables were in phase or antiphase with the SSN were considered. Relatively few results had a high level of significance. The analysis of seasonal averages revealed significant information, in addition to the analysis of annual averages. Thus, for the climatic indices, the GBOI and NAOI, a significant coherence (>95%) with the solar activity, associated with the 22-year (Hale) solar cycle, was found for the autumn season for lag = 0 and 1 year. The Hale solar cycle, in the case of the PHDI, was present in the annual and summer season averages, more clearly at lag = 0. For the Danube discharge at Orsova, the most significant SSN signature (~95%) was observed at periods of 33 years (Brüuckner cycle) in the autumn season for lags from 0 to 3 years. An analysis of the redundancy–synergy index was also carried out on the combination of the terrestrial variables with the solar variable in order to find the best synergistic combination for estimating the Danube discharge in the lower basin. The results differed depending on the timescale and the solar activity. For the average annual values, the most significant synergistic index was obtained for the combination of the GBOI, PHDI, and SSN, considered 3 years before Q. [ABSTRACT FROM AUTHOR]

The present study aims to investigate the possible influence of solar/geomagnetic forcing on climate variables, such as the drought index, Danube discharge and large-scale atmospheric indices. Our analysis was performed separately for each season for two time periods, 1901–2000 and 1948–2000. The relationship between terrestrial variables and external indices was established based on the application of (1) information theory elements, namely, synergy, redundancy, total correlation, transfer entropy and (2) wavelet coherence analysis. Bandpass filtering has also been applied. The most significant signature of the solar/geomagnetic forcing in the climate variables was obtained for the data smoothed by the bandpass filter. According to our results, significant solar/geomagnetic forcing appears in the terrestrial variables with a delay of 2–3 years. [ABSTRACT FROM AUTHOR]

Declination annual mean time series longer than 1 century provided by 24 geomagnetic observatories worldwide, together with 5 Western European reconstructed declination series over the last 4 centuries, have been analyzed in terms of the frequency constituents of the secular variation at inter-decadal and sub-centennial timescales of 20-35 and 70-90 years. Observatory and reconstructed time series have been processed by several types of filtering, namely Hodrick-Prescott, running averages, and Butterworth. The Hodrick-Prescott filtering allows us to separate a quasi-oscillation at a decadal timescale, which is assumed to be related to external variations and called the "11-year constituent", from a long-term trend. The latter has been decomposed into two other oscillations called "inter-decadal" and "sub-centennial" constituents by applying a Butterworth filtering with cutoffs at 30 and 73 years, respectively. The analysis shows that the generally accepted geomagnetic jerks occur around extrema in the time derivative of the trend and coincide with extrema in the time derivative of the 11-year constituent. The sub-centennial constituent is traced back to 1600 in the five 400-year-long time series and seems to be a major constituent of the secular variation, geomagnetic jerks included. [ABSTRACT FROM AUTHOR]

This paper presents the extent to which the combination of extra-atmospheric and hydroclimatic factors can be deciphered to record their contribution to the evolution and forecasting of the Danube discharge (Q) in the lower basin. A combination of methods such as wavelet filtering and deep learning (DL) constitutes the basic method for discriminating the external factors (solar activity through Wolf numbers) that significantly contribute to the evolution and prediction of the lower Danube discharge. An ensemble of some of the most important factors, namely, those representing the atmospheric components, i.e., the Greenland-Balkan Oscillation Index (GBOI) and the North Atlantic Oscillation Index (NAOI); the hydroclimatic indicator, the Palmer Hydrological Drought Index (PHDI); and the extra-atmospheric factor, constitutes the set of predictors by means of which the predictand, Q, in the summer season, is estimated. The external factor has to be discriminated in the Schwabe and Hale spectra to make its convolutional contribution to the Q estimation in the lower Danube basin. An interesting finding is that adding two solar predictors (associated with the Schwabe and Hale cycles) to the terrestrial ones give a better estimation of the Danube discharge in summer, compared to using only terrestrial predictors. Based on the Nash–Sutcliffe (NS) index, a measure of performance given by the extreme learning machine (ELM), it is shown that, in association with certain terrestrial predictors, the contribution of the Hale cycle is more significant than the contribution of the Schwabe cycle to the estimation of the Danube discharge in the lower basin. [ABSTRACT FROM AUTHOR]

Highlights: [•] Decomposition of the main geomagnetic field variation in multi-decadal ingredients. [•] Signals at two time scales (22- and ∼80-year) are relevant. [•] Geomagnetic jerks actually result from the combination of two ingredients. [•] 11-year external signal significantly contributes to jerks sharp appearance. [•] The ∼80-year variation was tracked back to ∼1600. [Copyright &y& Elsevier]

Abstract: Annual means of measured and reconstructed solar, heliospheric, and magnetospheric parameters are used to infer solar activity signatures at the Hale and Gleissberg cycles timescales. Available open solar flux, modulation strength, cosmic ray flux, total solar irradiance data, reconstructed back to 1700, solar wind parameters (speed and density) and the magnitude of the heliospheric magnetic field at 1AU, reconstructed back to 1870, as well as the time series of geomagnetic activity indices (aa, IDV, IHV), going back to 1870, have been considered. Simple filtering procedures (successive 11-, 22-, and 88-year running averages and differences between them) and scaling by the standard deviation from the average value for the common interval covered by the data show that the long-discussed variation in the 20th century (a pronounced increase since ∼1900, followed by a depression in the ‘60s and a new, slower, increase) seen in the 11-year averages of parameters such as geomagnetic activity indices and reconstructed heliospheric magnetic field strength, solar wind speed, open solar flux, is a result of the superposition in data of solar activity signatures at Hale and Gleissberg cycles timescales. The Hale and Gleissberg signals were characterized and similarities and differences in the temporal behavior of the analyzed parameters at these timescales are discussed. The similarities in the studied parameters point to a common pacing source, the solar dynamo. [ABSTRACT FROM AUTHOR]

The purpose of this study was to obtain synergistic information and details in the time–frequency domain of the relationships between the Palmer drought indices in the upper and middle Danube River basin and the discharge (Q) in the lower basin. Four indices were considered: the Palmer drought severity index (PDSI), Palmer hydrological drought index (PHDI), weighted PDSI (WPLM) and Palmer Z-index (ZIND). These indices were quantified through the first principal component (PC1) analysis of empirical orthogonal function (EOF) decomposition, which was obtained from hydro-meteorological parameters at 15 stations located along the Danube River basin. The influences of these indices on the Danube discharge were tested, both simultaneously and with certain lags, via linear and nonlinear methods applying the elements of information theory. Linear connections were generally obtained for synchronous links in the same season, and nonlinear ones for the predictors considered with certain lags (in advance) compared to the discharge predictand. The redundancy–synergy index was also considered to eliminate redundant predictors. Few cases were obtained in which all four predictors could be considered together to establish a significant information base for the discharge evolution. In the fall season, nonstationarity was tested through wavelet analysis applied for the multivariate case, using partial wavelet coherence (pwc). The results differed, depending on the predictor kept in pwc, and on those excluded. [ABSTRACT FROM AUTHOR]

The Earth's surface magnetic field and its secular variation (SV) are composed of several constituents, at sub-centennial (60–90 years) and inter-decadal (20–30 years) timescales, of internal origin, and at decadal (~11 years) timescale, of external origin, superimposed on a so-called inter-centennial constituent (timescale of about 400 years or larger). In this study we explore the appearance, structure, and dynamics of the SV foci from the perspective of the constituents with internal sources. Data from long time-span geomagnetic models, such as gufm1 (1590–1990) and COV-OBS (1840–2010), have been used to separate various constituents by Hodrick-Prescott (HP) and Butterworth filtering. Time-Longitude/Latitude plots, based on main field models time series on a grid of 2.5°x2.5° were constructed for various longitudes/latitudes respectively, on which a Radon transform was applied to get information on displacement in time of the constituents and of the SV foci, in the form of Latitude-Azimuthal-Speed and Longitude-Meridional-Speed power plots. The Earths surface field variability for the 400 years time-span is analyzed for the constituents of the field. Assessment of the geomagnetic field evolution at the Earth's surface and its characteristics, by our approach, revealed: (1) tendency of the westward drift of the inter-centennial constituent to the southern hemisphere, (2) absent meridional displacement of the inter-centennial constituent, (3) westward drift of the higher frequency constituents in a ± 20° equatorial band. • We decompose geomagnetic secular variation (SV) foci to sub-centennial, inter-decadal and inter-centennial constituents. • Time-Longitude/Latitude plots and Radon transform applied to constituents of the radial field at the Earth's surface. • Equatorial band westward drift of 0.25º/year, Southern Hemisphere displacements between 0.1 and 0.4º/year, were found. [ABSTRACT FROM AUTHOR]

A complex catalogue of high speed streams produced by coronal holes and their effects in the terrestrial magnetosphere – the so-called geomagnetic storms – for Solar Cycle 24 (2009 to 2019) is presented here. This catalogue is structured in three parts describing the high-speed stream characteristics, the interplanetary magnetic field state, and the properties of the associated geomagnetic storms. The catalogue is available online at http://www.geodin.ro/varsiti/. [ABSTRACT FROM AUTHOR]

The aims of this study are to investigate the influence of large-scale atmospheric circulation quantified by indices such as the North Atlantic Oscillation index (NAOI), the Greenland-Balkan Oscillation index (GBOI) and blocking-type indices on the Lower Danube discharge. We separately analysed each season for the 1948–2000 period. In addition to the statistical linear procedure, we applied methods to quantify nonlinear connections between variables, as mutual information between predictors and predictand, using Shannon's information entropy theory. The nonlinear correlation information between climate indices and discharge is higher than that obtained from the linear measure, providing more insight into real connections. Also, the non-stationarity of the link between variables is highlighted by spectral coherence based on wavelet analysis. For the physical interpretation, we analyse composite maps over the Atlantic-European region. The most significant influence on the discharge of the Lower Danube Basin is given by the GBOI and blocking-type atmospheric circulation over Europe. [ABSTRACT FROM AUTHOR]

The aim of this research was to determine relationships between solar activity and variability of discharges of three Central European rivers: the Vistula, Odra and Warta in Poland in the multi-annual period of 1901–2020. Changes in precipitation and air temperature at Poznań meteorological station in the same period were also analyzed. The long-term variations in river runoff were investigated both from the point of view of temporal variability in relation to climate variations in the study area, and from the point of view of linear/non-linear links to solar activity, as described by the Wolf sunspot number. The wavelet transform analysis was used to highlight the frequency-time distribution of the coherences between solar and discharge variability. It was found that most of the links between solar activity and discharges were non-linear. [ABSTRACT FROM AUTHOR]

This study addresses the causal links between external factors and the main hydro-climatic variables by using a chain of methods to unravel the complexity of the direct sun–climate link. There is a gap in the literature on the description of a complete chain in addressing the structures of direct causal links of solar activity on terrestrial variables. This is why the present study uses the extensive facilities of the application of information theory in view of recent advances in different fields. Additionally, by other methods (e.g., neural networks) we first tested the existent non-linear links of solar–terrestrial influences on the hydro-climate system. The results related to the solar impact on terrestrial phenomena are promising, which is discriminant in the space-time domain. The implications prove robust for determining the causal measure of climate variables under direct solar impact, which makes it easier to consider solar activity in climate models by appropriate parametrizations. This study found that hydro-climatic variables are sensitive to solar impact only for certain frequencies (periods) and have a coherence with the Solar Flux only for some lags of the Solar Flux (in advance). [ABSTRACT FROM AUTHOR]

In this study, nonlinear connections between large scale atmospheric circulation patterns described by climate indices (North Atlantic Oscillation Index, NAOI; Greenland-Balkan Oscillating Index, GBOI; East Atlantic, EA; East Atlantic/Western Russia, EAWRUS; Arctic Oscillation, AO; Atlantic Multidecadal Oscillation, AMO; Scandinavian Pattern, SP; Southern Oscillation, SO) and the precipitation in the Danube basin, were tested based on the mutual information (MI). The results obtained by MI were compared with the Pearson correlation as a linear association metric. The analysis was separately performed for each season.During the spring season, the both linear and nonlinear relationships between precipitation and climate indices had a low confidence level (CL), except for the GBOI for which the link is significant, but the non-linear relation is not statistically higher than the linear one.For the summer, the GBOI, AO, EA and NAOI present a stronger significant non-linear connection with precipitation, while in case of the GBOI the linear relation is also significant. In autumn, the AO, EAWRUS and GBOI have significant connections with the precipitation, but the non-linear measure is close to the linear one.During the winter, the AO, EAWRUS and GBOI show significant connections (CL> 99%) with the precipitation, both for the nonlinear and linear measures. The NAOI has only a significant linear link with precipitation.For the other climate indices considered in this study, namely the AMO, SP and SO, no statistically significant relationship with the precipitation in the Danube basin was obtained. The degree of non-linearity of the connection between the Danube basin precipitation and the climate indices, depends on the season and on the intrinsic characteristics of the circulation patterns. The results of this study revealed that, to better describe the relationship between hydroclimatic phenomena, the nonlinear approaches are required. [ABSTRACT FROM AUTHOR]