Crosswavelet and Wavelet Coherence

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Background: Consultation rates of influenza-like illness (ILI) in an outpatient setting have been regarded as a good indicator of influenza virus activity in the community. As ILI-like symptoms may be caused by etiologies other than influenza, and influenza virus activity in the tropics and subtropics is less predictable than in temperate regions, the correlation between of ILI and influenza virus activity in tropical and subtropical regions is less well defined. Methodology and Principal Findings: In this study, we used wavelet analysis to investigate the relationship between seasonality of influenza virus activity and consultation rates of ILI reported separately by General Out-patient Clinics (GOPC) and General Practitioners (GP). During the periods 1998-2000 and 2002-2003, influenza virus activity exhibited both annual and semiannual cycles, with one peak in the winter and another in late spring or early summer. But during 2001 and 2004-2006, only annual cycles could be clearly identified. ILI consultation rates in both GOPC and GP settings share a similar non-stationary seasonal pattern. We found high coherence between ILI in GOPC and influenza virus activity for the annual cycle but this was only significant (P<0.05) during the periods 1998-1999 and 2002-2006. For the semiannual cycle high coherence (p<0.05) was also found significant during the period 1998-1999 and year 2003 when two peaks of influenza were evident. Similarly, ILI in GP setting is also associated with influenza virus activity for both the annual and semiannual cycles. On average, oscillation of ILI in GP and of ILI in GOPC preceded influenza virus isolation by approximately four and two weeks, respectively. Conclusions: Our findings suggest that consultation rates of ILI precede the oscillations of laboratory surveillance by at least two weeks and can be used as a predictor for influenza epidemics in Hong Kong. The validity of our model for other tropical regions needs to be explored. © 2008 Yang et al.

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Seasonal and interannual variability of remotely sensed by Sea-viewing Wide Field-of-View Sensor (SeaWiFS, aboard OrbView-2 satellite) and MODerate Resolution Imaging Spectroradiometer (MODIS-A, aboard Aqua satellite) surface chlorophyll a (CHL) in the Black Sea was analyzed since the start of SeaWiFS mission in 1997 till the spring of 2006. The spatio-temporal patterns analyzed by the empirical orthogonal functions (EOF) method revealed four main regions where CHL dynamics was correlated: northwest shelf (NW); southwest coastal region (SW); eastern region (E); and central region(C). Seasonal variability in the NW region had evident maximum in late spring resulting from the maximum of nutrient-rich Danube discharge. In three deep regions (SW, E, and C) the seasonal cycles were characterized by summer minimum and autumn-winter maximum. This pattern is typical to subtropical ocean areas where phytoplankton growth is nutrient-limited as a result of water column stratification. Danube discharge (correlated in 1990s-2000s with ENSO climatic cycle) looks like a most important factor regulating chlorophyll concentration on the northwestern shelf and, after a time lag of 2-2.5 years, in the deep Black Sea regions. © 2006 Springer-Verlag Berlin Heidelberg.

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In this work we make an analysis of significant periodicities shown by phenomena linked to solar activity such as coronal hole area, radio emission in the 10.7 cm band and sunspots. We use the wavelet method that gives information in the frequency and time domains. Of particular interest are the mid-term periodicities (1-2 yrs). Over the whole period, coronal holes and radio variations show an important annual variation and a quasi-biannual periodicity. The increase in these variations is most important around the years of maximum solar activity. When the time series are separated in low and high frequencies, the latter are modulated by the general solar cycle. Although somewhat shifted in frequency, these periodicities might well correspond with those found in cosmic ray intensity, solar magnetic flux and other terrestrial and interplanetary phenomena as a wavelet coherence analysis of these series with the solar magnetic flux reveals. © 2007 COSPAR.

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Bursts of upwelling-favorable winds lasting 4-20 days occur year-round south of Cape Blanco, a major headland on the Oregon coast. The ocean's response to these events was studied using moored current, temperature, and salinity data; satellite SST data; and a few across-shelf sections near the mooring site. The mooring was located at 42°26.49'N, 124°34.47'W, 6 n mi off Gold Beach, Oregon, from May 2000 to October 2003. After the spring transition but before upwelling jet separation, equatorward wind stress produced a steady upwelling response much the same as a long, straight coast. Currents and winds had similar spectral characteristics with a peak near 15 days. After jet separation, upwelling-favorable winds forced a much more variable current consisting of a series of thin equatorward jets that evolved and moved offshore across the mooring, with shorter time scales than the wind stress forcing. During autumn, the equatorward wind stress weakened slightly and a transition period occurred, with the flow often poleward along the bottom. During winter, the water column was unstratified during poleward winds and currents with little variation in SST across the shelf. Winter upwelling restratified the water column from the bottom up by drawing cold, salty water onshore along the bottom, with little or no change in SST. This scenario was modulated by strong intraseasonal and interannual variability in the ocean and atmosphere. A wavelet transform analysis of alongshore wind stress and the first empirical orthogonal mode of the alongshore currents revealed strong energy peaks in the 30-70-day band. These signals were particularly clear in the ocean and were not coherent with the local wind stress, suggesting they were due to Kelvin waves of equatorial origin. The shift toward longer (40-45-60 days) periods from 2000 to 2003 was consistent with decreasing (warming) northern oscillation index, suggesting that the period as well as the energy of the intraseasonal waves may be important in transmitting heat poleward during warm years.

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Aims. The sunspot activity on both solar hemispheres is known to show time intervals of different phase relationships. In this letter, we introduce an advanced statistical framework that allows an appropriate analysis of these relationships. Methods. We combine wavelet analysis with statistical measures of phase coherence to obtain well-defined phase differences in dependence on the selected oscillation period. Results. Our approach characterises the temporally varying phase difference of the hemispheric sunspot activity over the past 130 years, which is a major contribution to the north-south asymmetry. Coherent phase variables are found to exist for a frequency band with periodicities around the dominating 11-year cycle. We observe significant continuous long-term variability of interhemispheric phase shifts, which is not reproduced by solar activity records from previous centuries. © ESO 2007.

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Using monthly independently reconstructed gridded European fields for the 500 hPa geopotential height, temperature, and precipitation covering the last 235 years we investigate the temporal and spatial evolution of these key climate variables and assess the leading combined patterns of climate variability. Seasonal European temperatures show a positive trend mainly over the last 40 years with absolute highest values since 1766. Precipitation indicates no clear trend. Spatial correlation technique reveals that winter, spring, and autumn covariability between European temperature and precipitation is mainly influenced by advective processes, whereas during summer convection plays the dominant role. Empirical Orthogonal Function analysis is applied to the combined fields of pressure, temperature, and precipitation. The dominant patterns of climate variability for winter, spring, and autumn resemble the North Atlantic Oscillation and show a distinct positive trend during the past 40 years for winter and spring. A positive trend is also detected for summer pattern 2, which reflects an increased influence of the Azores High towards central Europe and the Mediterranean coinciding with warm and dry conditions. The question to which extent these recent trends in European climate patterns can be explained by internal variability or are a result of radiative forcing is answered using cross wavelets on an annual basis. Natural radiative forcing (solar and volcanic) has no imprint on annual European climate patterns. Connections to CO2 forcing are only detected at the margins of the wavelets where edge effects are apparent and hence one has to be cautious in a further interpretation. © Springer-Verlag 2007.

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The authors examine relationships between the East Asian winter monsoon and the ENSO, particularly on the interdecadal timescales. Based on the analyses of SLP data from 1899 to 1997, the East-Asian winter monsoon index (WMI) is defined as the zonal difference of SLP between ~120° E and ~160° E. It is found that 18 out of 28 strong winter monsoon years are either before the development of an El Nin~o or during the decaying La Nin~a event, 12 out of 28 weak winter monsoon are before the development of a La Nin~a or during the decaying El Nin~o event. There is a significant positive correlation coefficient value of about 0.49 between the normalized 11-yr running mean of WMI and ENSO index, however, the WMI-ENSO relationship is not consistently highly correlated. The temporal evolution of correlation between WMI and ENSO indices in both 11-yr and 21-yr moving window shows that the WMI-ENSO relationship clearly undergo low-frequency oscillation. Obviously, both observational and IPSL air-sea coupled modeling WMI index has a near-decadal peak with PDO timescales and internal peaks with ENSO timescales by applying the Multitaper method. Moreover, the cross wavelet and wavelet coherence analysis of WMI/ENSO indicate that there is a larger significant sections with an in phase behavior between WMI and ENSO at period of 20-30 yrs, suggesting that the interdecadal variation of the WMI-ENSO relationship might exist. © Springer-Verlag 2007.

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Heart rate variability (HRV) is often used to study disturbances in the autonomic nervous system. Respiratory related HRV is seen as an indicator of a working autonomic mechanism. However, sometimes a high HRV may be caused by non-autonomic mechanisms. This study investigated if the wavelet coherence could be used to study respiratory related fluctuations in heart rate. The wavelet coherence method was applied to two Familial amyloidotic polyneuropathy patients and one healthy control. The results showed that wavelet coherence is a promising method for studying respiratory related fluctuations in heart rate. © 2007 IEEE.

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A network of spinal neurons known as central pattern generator (CPG) produces the rhythmic motor patterns required for coordinated swimming, walking, and running in mammals. Because the output of this network varies with time, its analysis cannot be performed by statistical methods that assume data stationarity. The present work uses short-time Fourier (STFT) and wavelet-transform (WT) algorithms to analyze the non-stationary rhythmic signals produced in isolated spinal cords of neonatal rats during activation of the CPGs. The STFT algorithm divides the time series into consecutive overlapping or nonoverlapping windows and repeatedly applies the Fourier transform across the signal. The WT algorithm decomposes the signal using a family of wavelets varying in scale, resulting in a set of wavelet coefficients presented onto a continuous frequency range over time. Our studies revealed that a Morlet WT algorithm was the tool of choice for analyzing the CPG output. Cross-WT and wavelet coherence were used to determine interrelations between pairs of time series in time and frequency domain, while determining the critical values for statistical significance of the coherence spectra using Monte Carlo simulations of white-noise series. The ability of the cross-Morlet WT and cross-WT coherence algorithms to efficiently extract the rhythmic parameters of complex nonstationary output of spinal pattern generators over a wide range of frequencies with time is demonstrated in this work under different experimental conditions. This ability can be exploited to create a quantitative dynamic portrait of experimental and clinical data under various physiological and pathological conditions. Copyright © 2007 The American Physiological Society.

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The Late Triassic was a time of major environmental change, yet the precise chronology of events is poorly constrained owing to the nonmarine nature of most preserved Upper Triassic strata and the difficulty in correlating sections. St. Audrie's Bay, southwest England, has been the focus of many studies on this interval of time and is one of the proposed sections for the base Jurassic global stratotype section and point (GSSP). In this study, lacustrine deposits exposed at St. Audrie's Bay have been used to construct a floating astronomical time scale for ~3.7 m.y. of the Late Triassic based on the recognition of ~100 k.y. eccentricity cycles in rock color. In addition, we have correlated this time scale with an existing magnetostratigraphy through the same succession and produced an astronomically calibrated record of geomagnetic polarity. Using a novel statistical procedure, we have determined the correlation between this succession and the Late Triassic geomagnetic polarity time scale of the Newark Basin, North America, on which the current (2004) geological time scale is based. Our results show unequivocally that the studied St. Audrie's Bay succession represents part of the Norian and we demonstrate for the first time that cyclostratigraphy can be used in the correlation of Mesozoic strata between North America and Europe. © 2007 The Geological Society of America.

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Precise and reproducible spike timing is one of the alternatives of the sensory stimulus encoding. We test coherence (repeatability) of the response patterns elicited in projecting gracile neurons by tactile stimulation and its modulation provoked by electrical stimulation of the corticofugal feedback from the somatosensory (SI) cortex. To gain the temporal structure we adopt the wavelet-based approach for quantification of the functional stimulus-neural response coupling. We show that the spontaneous firing patterns (when they exist) are essentially random. Tactile stimulation of the neuron receptive field strongly increases the spectral power in the stimulus and 5- to 15-Hz frequency bands. However, the functional coupling (coherence) between the sensory stimulus and the neural response exhibits ultraslow oscillation (0.07 Hz). During this oscillation the stimulus coherence can temporarily fall below the statistically significant level, i.e., the functional stimulus-response coupling may be temporarily lost for a single neuron. We further demonstrate that electrical stimulation of the SI cortex increases the stimulus coherence for about 60% of cells. We find no significant correlation between the increment of the firing rate and the stimulus coherence, but we show that there is a positive correlation with the amplitude of the peristimulus time histogram. The latter argues that the observed facilitation of the neural response by the corticofugal pathway, at least in part, may be mediated through an appropriate ordering of the stimulus-evoked firing pattern, and the coherence enhancement is more relevant in gracilis nucleus than an increase of the number of spikes elicited by the tactile stimulus. Copyright © 2007 The American Physiological Society.

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A voltammetric analyzer was used to control an observatory mooring in the Delaware Bay at the same time oxygen data were collected with solid-state gold-amalgam (Au/Hg) electrodes. Oxygen, temperature and salinity data were collected for 8 days and sent wirelessly back to the shore-based laboratory. Observed oxygen saturations ranged from 70 to 171%, with a mean of 107%. The highest oxygen concentrations were observed when there were low tides during the solar maximum, indicating higher rates of production within the Delaware Bay than in coastal Atlantic Ocean waters which passed the mooring at high tides, as expected based on previous shipboard studies. Lower percent saturations indicated respiration, the decomposition of organic matter by oxygen, as the dominant process and corresponded to night periods. Upon recovery the electrodes were free of bio-fouling. This study demonstrates that voltammetric techniques and equipment can be used in observatory and mooring systems. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

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Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process. However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of the Earth using data from the four Cluster spacecraft. With di and de designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be 0.7±0.3d(i) and an associated oscillation is identified as a lower-hybrid wave with wavelength d(e). This insitu evidence of the full 3D reconnection geometry and associated dynamics provides an important step towards establishing an observational framework of 3D reconnection. © 2007 Nature Publishing Group.

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In the current context of global infectious disease risks, a better understanding of the dynamics of major epidemics is urgently needed. Time-series analysis has appeared as an interesting approach to explore the dynamics of numerous diseases. Classical time-series methods can only be used for stationary time-series (in which the statistical properties do not vary with time). However, epidemiological time-series are typically noisy, complex and strongly non-stationary. Given this specific nature, wavelet analysis appears particularly attractive because it is well suited to the analysis of non-stationary signals. Here, we review the basic properties of the wavelet approach as an appropriate and elegant method for time-series analysis in epidemiological studies. The wavelet decomposition offers several advantages that are discussed in this paper based on epidemiological examples. In particular, the wavelet approach permits analysis of transient relationships between two signals and is especially suitable for gradual change in force by exogenous variables. © 2007 The Royal Society.

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Vector autoregressive (VAR) modelling is one of the most popular approaches in multivariate time series analysis. The parameters interpretation is simple, and provide an intuitive identification of relationships and Granger causality among time series. However, the VAR modelling requires stationarity conditions which could not be valid in many practical applications. Locally stationary or time dependent modelling seem attractive generalizations, and several univariate approaches have already been proposed. In this paper we propose an estimation procedure for time-varying vector autoregressive processes, based on wavelet expansions of autoregressive coefficients. The asymptotic properties of the estimator are derived and illustrated by computer intensive simulations. We also present an application to brain connectivity identification using functional magnetic resonance imaging (fMRI) data sets. © 2006 Elsevier B.V. All rights reserved.

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Transient neural assemblies mediated by synchrony in particular frequency ranges are thought to underlie cognition. We propose a new approach to their detection, using empirical mode decomposition (EMD), a data-driven approach removing the need for arbitrary bandpass filter cut-offs. Phase locking is sought between modes. We explore the features of EMD, including making a quantitative assessment of its ability to preserve phase content of signals, and proceed to develop a statistical framework with which to assess synchrony episodes. Furthermore, we propose a new approach to ensure signal decomposition using EMD. We adapt the Hilbert spectrum to a time-frequency representation of phase locking and are able to locate synchrony successfully in time and frequency between synthetic signals reminiscent of EEG. We compare our approach, which we call EMD phase locking analysis (EMDPL) with existing methods and show it to offer improved time-frequency localisation of synchrony. © Springer Science+Business Media, LLC 2007.

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Theta increases with workload and is associated with numerous processes including working memory, problem solving, encoding, or self monitoring. These processes, in turn, involve numerous structures of the brain. However, the relationship between regional brain activity and the occurrence of theta remains unclear. In the present study, simultaneous EEG-fMRI recordings were used to investigate the functional topography of theta. EEG-theta was enhanced by mental arithmetic-induced workload. For the EEG-constrained fMRI analysis, theta-reference time-series were extracted from the EEG, reflecting the strength of theta occurrence during the time course of the experiment. Theta occurrence was mainly associated with activation of the insular cortex, hippocampus, superior temporal areas, cingulate cortex, superior parietal, and frontal areas. Though observation of temporal and insular activation is in accord with the theory that theta specifically reflects encoding processes, the involvement of several other brain regions implies that surface-recorded theta represents comprehensive functional brain states rather than specific processes in the brain. The results provide further evidence for the concept that emergent theta band oscillations represent dynamic functional binding of widely distributed cortical assemblies, essential for cognitive processing. This binding process may form the source of surface-recorded EEG theta. © 2006 Wiley-Liss, Inc.

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Eight weeks of daily (low tide) time exposure images of the Gold Coast (Australia) were used to analyze the development of three-dimensional morphological patterns in a double sandbar system. Under continued low-energy wave conditions following a morphological reset (storm) event, the patterns in both the inner and outer bar evolved, albeit at different spatial and temporal scales, through the accretionary sequence commonly observed at single barred beaches. Continuous wavelet transforms (CWT) of the inner bar and outer bar breaker lines show that after a storm the patterns developed at a wide range of alongshore wavelengths and that variations in wavelength over space and time were common. With the bivariate extension of CWT, continuous cross-wavelet transforms, it is found that the initial inner bar and outer bar three-dimensional patterns were spatially noncoupled. However, over time the inner-bar morphology appeared to couple with that of the outer bar, with the inner bar patterns developing in response to the increasingly three-dimensional, onshore propagating outer bar. The wavelet analysis suggests that the outer bar geometry and the distance between the inner and outer bars are critical parameters governing the morphological evolution of the composite double sandbar system. Copyright 2007 by the American Geophysical Union.

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A wavelet analysis of the flare-intensity variations has been carried out for a solar flare observed with the Nobeyama Radioheliograph at 17 and 34 GHz and the spectrometer of Purple Mountain Observatory at 4.5-7.5 GHz. The signals contain a well-pronounced periodicity with a period of P = 14-17 s and stable, coherent behavior at frequencies higher and lower than the peak frequency. We simulated the modulation of the gyrosynchrotron emission by fast sausage magnetoacoustic oscillations for the cases of low and high plasma density in the radio source. The synchronism of the pulsations at high and low frequencies can be realized only in the case of high plasma density, when the low-frequency turnover of the microwave spectrum is due to the Razin effect, not self-absorption. © 2007 Pleiades Publishing, Ltd.

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Using unprecedented catalogues of past severe drought data for the Yucatan Peninsula between 1502 and 1900 coming from historical written documentation, we identified five conspicuous time lapses with no droughts between 1577-1647, 1662-1724, 1728-1764, 1774-1799 and 1855-1880, as well as time epochs with most frequent droughts between 1800 and 1850. Moreover, the most prominent periodicity of the historical drought time series was that of ~40 years. Using the Palmer Drought Severity Index for the Yucatan Peninsula for the period 1921-1987 we found prominent negative phases between ~1942-1946 and 1949-1952, 1923-1924, 1928-1929, 1935-1936, 1962-1963, 1971-1972 and 1986-1987. Two prominent periodicities clearly appear at ~5 and 10 years. Most modern and historical severe droughts lasted 1 year, and share a quasi-decadal frequency. Also, in the first 66 years of the twentieth century the frequency of occurrence of severe drought has been lower compared with the nineteenth century. Some of the major effects and impacts of the most severe droughts in the Yucatan region are examined. We also studied the relation between historical and modern droughts and several large scale climate phenomena represented by the Atlantic Multidecadal Oscillation (AMO) and the Southern Oscillation Index (SOI). Our results indicate that historical droughts and the cold phase of the AMO coincide, while the influence of the SOI is less clear. The strongest coherence between historical droughts and AMO occurred at periodicities of ~40 years. For modern droughts the coherence of a drought indicator (the Palmer Drought Severity Index) is similar with AMO and SOI, although it seems more sustained with the AMO. They are strongest at ~10 years and very clearly with the AMO cold phase. Concerning the solar activity proxies and historical droughts, the coherence with a record of beryllium isotope Be 10, which is a good proxy of cosmic rays, is higher than with Total Solar Irradiance. We notice that the strongest coherence between historical droughts and Be10 occurs at periods ~60-64 years. When studying modern droughts and solar activity, frequencies of ~8 years appear, and the coherences are similar for both sunspots and cosmic rays. Comparing natural terrestrial and solar phenomena, we found that the most sustained and strongest modulation of historical drought occurrence is at ~60-64 years and is between the historical drought series and the solar proxy Be10. For modern droughts we notice that the coherence is similar among AMO, SOI and the solar indices. We can conclude that the sea surface temperatures (AMO) and solar activity leave their signal in terms of severe droughts in the Maya lands, however in the long term, the influence of the SOI on this type of phenomenon is less clear. © 2007 Springer Science+Business Media, B.V.

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Historical sunspot records were analyzed by means of nonlinear tools to find synchronization phenomena at different time scales on the Sun. Using cross-recurrence plots it is shown that the north - south sunspot synchronization demonstrates a set of distinct periodic oscillations - 43.29, 18.52, and 7.63 years. Also we have traced the sunspot synchronization on shorter time scales. Very rare and episodic synchronization within half of the Carrington rotation rate was detected. By using the empirical mode decomposition technique the north - south sunspot time series were decomposed into intrinsic oscillatory modes. To determine which modes of the signal are responsible for synchronization we separated them into high- and low-frequency parts. It is shown that phase synchronization is detected only in the low-frequency modes. The high-frequency component demonstrates noisy behavior with amplitude synchronization and strong phase mixing. © 2007 Springer.

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Energetic electron and ion (electrons: 30 keV to 500 keV, protons: 39 keV to 1.5 MeV) flux variations associated with ultralow frequency (ULF) waves in the dayside magnetosphere were observed during the CLUSTER's perigee pass near 0900 MLT on Oct. 31, 2003. The ULF modulation terminated where higher frequency fluctuations appeared, as the CLUSTER spacecraft entered the plasmasphere boundary layer (PBL) where the plasma ion density was elevated. In the region from L ~ 5.0 to 10, the periods of the ion flux modulation and the electron flux modulation are same but out-of-phase. The observed magnetic ULF pulsations are dominated by the toroidal mode, along with a relatively weaker poloidal wave. A 90° phase shift between the radial electric field and the azimuthal magnetic field indicates that dominating toroidal standing waves observed at the southern hemisphere are a fundamental harmonic. This study shows that the modulation of the electron flux is dominated by the toroidal mode in the region of L > 7.5. The observations made in this analysis suggest the excitation of the energetic electron drift resonance at around 127 keV. Copyright 2007 by the American Geophysical Union.

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Flood/drought series during the past 1000 yrs in the Yangtze Delta, China, was reconstructed based on historical documents and local chronologies. Continuous wavelet transform was applied to detect the periodicity and variability of the flood/drought series. Research results indicate that: (1) Larger fluctuations of climatic changes in the Tibetan Plateau result in higher wavelet variance of flood/drought in the Yangtze Delta, for example, during 1400-1700, the proxy indicators indicate that the annual temperature in Tibet experienced larger variability and that this time interval exactly corresponds to the time when the higher and significant wavelet variance occurred; (2) Periods featured by colder temperature in the Tibetan Plateau usually correspond to periods characterized by higher wetness with higher probability of flood events; (3) Variability of heating features of the Tibetan Plateau exerted great influences on intensity and onset of Indian monsoon and south Asian summer monsoon, and these atmospheric activities are in direct connection with precipitation in Eastern China. Current global warming may alter the snow mass of Tibetan Plateau and then alters the heating features of Tibetan Plateau, which may in turn impact flood/drought conditions in the Yangtze Delta. © 2006 Elsevier B.V. All rights reserved.

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Soil N2O emission measurements have high-spatial and temporal variability that results in poor field scale predictive relationships, and this leads to high uncertainty in estimates of flux. Correlation between variables may be location and scale dependent. This needs to be understood to identify at what scales or locations predictive relationships may be used to reduce uncertainty in estimates. The purpose of this study was to describe the scale and location dependency of N2O emission on soil water and temperature using wavelet coherency and their change through time. Measurements of N2O flux, soil water content, and temperature were made multiple times from a 128-point transect on a hummocky, agricultural landscape in the Dark Brown soil zone of Saskatchewan, Canada. Using three sampling dates in the spring of 2004, the local wavelet, cross-wavelet, and wavelet coherency spectra were generated. In late March, the spatial pattern of wavelet coherency between soil N2O flux and temperature was scale dependent. At the end of April, there was no significant relationship between N2O and temperature. Instead, a location-dependent spatial pattern existed between flux and water-filled pore space. By late June, there were no significant scale- or location-dependent relationships. The results indicate that N2O emission models for this landscape cannot rely on a single predictive relationship, but may have to adjust between scale- and location-dependent relationships at different times of the year. Copyright 2007 by the American Geophysical Union.

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Studying temporal patterns of hydrology and water quality can assist in understanding hydrological processes, improving hydrological modeling, and water quality monitoring. Using wavelet analysis, we analyzed temporal patterns of three hydrological signals (precipitation, stream flow, and well water level) for three periods (15 years, 3 year, and a hydrological year). For three unevenly sampled water quality signals (nitrate, chloride, and sodium), the weighted wavelet Z-transform (WWZ) method was employed. The results showed that the wavelet analysis of hydrological signals showed advantages of detecting detailed temporal patterns compared to the classical Fourier analysis. No strong temporal pattern of precipitation was found for all three periods. For the 15 years' continual monitoring datasets, strong consistent annual temporal pattern of well water level and an intermittent annual temporal pattern of stream flow were observed. For the relative short-time periods (three years and a hydrological year), strong seasonal patterns of stream flow and well water level were noticed. Using the WWZ method, seasonal patterns of the three stream water quality indicators can be associated with their seasonal shifts. Nitrate concentration showed stronger temporal patterns than chloride and sodium over longer time periods (15 and 3 years). In a hydrological year, temporal patterns of nitrate, chloride, and sodium shared some similarities as well as dissimilarities, as can be explained by their different transport pathways and sources. Sodium and chloride peak concentrations in the winter season were captured by the wavelet analysis. It is concluded that wavelet analysis can be a useful tool to analyze detailed temporal patterns of non-stationary hydrological and water quality signals over different temporal scales. © 2007 Elsevier B.V. All rights reserved.

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A method of single-trial coherence analysis is presented, through the application of continuous multiwavelets. Multiwavelets allow the construction of spectra and bivariate statistics such as coherence within single trials. Spectral estimates are made consistent through optimal time-frequency localization and smoothing. The use of multiwavelets is considered along with an alternative single-trial method prevalent in the literature, with the focus being on statistical, interpretive and computational aspects. The multiwavelet approach is shown to possess many desirable properties, including optimal conditioning, statistical descriptions and computational efficiency. The methods are then applied to bivariate surrogate and neurophysiological data for calibration and comparative study. Neurophysiological data were recorded intracellularly from two spinal motoneurones innervating the posterior biceps muscle during fictive locomotion in the decerebrated cat. © 2007 IEEE.

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Effective detection of unknown, transient oscillations in strong, colored, time varying noise and clutter has remained a signal processing challenge. Recently we presented a self-normalized wavelet detector for this purpose. Here, we revisit this detection problem for the case of a tapered, complex, transient oscillation observed coherently in two time series. We develop and compare three detectors based on the following: 1) the self-normalized Morlet cross wavelet spectrum (NCM); 2) the normalized cross discrete wavelet transforms (DWT), extended from Wang and Willet's DWT power law detector (NCDWT3); and 3) wavelet coherence (WCOH). Each detector normalizes the cross wavelet spectrum to perform a binary hypothesis test for noise only. Simulated receiver operating curves for complex tapered transient oscillations in colored, slowly time varying, Gaussian noise and 60 dB low frequency clutter are included for the three detector types and for one based on the fast Fourier transform (FFT) (NCFFT3) for comparison. Without clutter, detection rates at 0.1% false alarm rates for a (signal on) signal to noise ratio of 0.5 are: 99.4%, 99.1%, 97.0% and 96.3% for the NCFFT3, NCM, WCOH, and NCDWT3 detectors, respectively. When 60 dB clutter is present, the NCM and WCOH detection rates were unchanged, but the NCFFT3 and NCDWT3 detectors became ineffective. © 2007 IEEE.

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Based on magnetic field measurements from the satellite CHAMP, a detailed picture could be obtained of the upstream wave (UW) distribution in the topside ionosphere. The low, near-polar orbit of CHAMP, covering all local times, allows the global distribution of this type of pulsation to be revealed. The observations from space are compared to recordings of the ground-based MM100 meridional array covering the latitude range 66° to 42° in magnetic coordinates. UWs show up very clearly in the compressional component of the satellite magnetic field data, whereas on the ground, their signature is found in the H component, but it is mixed with oscillations from field line resonant pulsations. Here we first introduce a procedure for an automated detection of UW signatures, both in ground and space data. Then a statistical analysis is presented of UW pulsations recorded during a 132-day period, centred on the autumn 2001 equinox. Observations in the top-side ionosphere reveal a clear latitudinal distribution of the amplitudes. Largest signals are observed at the equator. Minima show up at about 40° latitude. The coherence between ground and satellite wave signatures is high over wide latitude and longitude ranges. We make suggestions about the entry mechanism of UWs from the foreshock region into the magnetosphere. The clear UW signature in satellite recordings between −60° and 60° latitude allows for detailed investigations of the dependence on solar wind conditions. We test the control of solar wind speed, interplanetary magnetic field strength and cone angle on UWs. For the first time, it is possible to derive details of the Doppler-shift effect by modifying the UW frequency from direct observations. The results reconcile foreshock wave generation predictions with near-Earth observations.

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A number of contemporary dendroecological studies from northwestern North America have highlighted a divergence in growth trends during recent decades. These studies suggest that warmer temperatures are now exceeding the physiological threshold of some northern tree species, or perhaps are contributing to increased drought stress as current precipitation is insufficient to offset increasing water demands under warmer conditions. Here we document additional evidence of these diverging growth trends from the Mackenzie Delta region of Canada and show they are anomalous to the twentieth century. Using wavelet coherency analyses we demonstrate that our white spruce tree ring chronologies exhibit little divergence from one another during the past four centuries, but coherency of the data-sets rapidly break down after the 1930s. Copyright 2007 by the American Geophysical Union.

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We present observational evidence from coastal tide station and bottom pressure data that basin-mode oscillations are frequently excited in the Japan/East Sea (JES). The fundamental basin-mode is a Kelvin-wave-like oscillation consisting of a single amphidromic system around which the high water propagates counter-clockwise. Its period is about 6.7 hours and its coastal wavelength is equivalent to the circumference of the JES. The relative amplitudes of the observed oscillations agree with Rikiishi's 1986 model results except for stations near the Korea Strait where the closed boundary in the model produces unrealistically high amplitudes. The basin oscillation amplitude varies on synoptic time scales (2-17 days) and exhibits seasonal variations. The optimal wind direction to generate basin-mode oscillations is along 60°/240°T. Copyright 2007 by the American Geophysical Union.

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This study proposes and justifies a Bayesian approach to modeling wavelet coefficients and finding statistically significant features in wavelet power spectra. The approach utilizes ideas elaborated in scale-space smoothing methods and wavelet data analysis. We treat each scale of the discrete wavelet decomposition as a sequence of independent random variables and then apply Bayes' rule for constructing the posterior distribution of the smoothed wavelet coefficients. Samples drawn from the posterior are subsequently used for finding the estimate of the true wavelet spectrum at each scale. The method offers two different significance testing procedures for wavelet spectra. A traditional approach assesses the statistical significance against a red noise background. The second procedure tests for homoscedasticity of the wavelet power assessing whether the spectrum derivative significantly differs from zero at each particular point of the spectrum. Case studies with simulated data and climatic time-series prove the method to be a potentially useful tool in data analysis.

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Variability and possible teleconnections between annual maximum streamflow from the lower, the middle and the upper Yangtze River basin and El Nin~o/Southern Oscillation (ENSO) are detected by continuous wavelet transform (CWT), cross-wavelet and wavelet coherence methods. The results show that: (1) different phase relations are found between annual maximum streamflow of the Yangtze River and El Nin~o/Southern Oscillation (ENSO) in the lower, the middle and the upper Yangtze River basin. In-phase relations are detected between annual maximum streamflow of the lower Yangtze River and anti-phase relations are found in the upper Yangtze River. But ambiguous phase relations occur in the middle Yangtze River, showing that the middle Yangtze River basin is a transition zone. Different climatic systems control the upper and the lower Yangtze River. The upper Yangtze River is mainly influenced by the Indian summer monsoon and the lower Yangtze is mainly influenced by the East Asian summer monsoon; (2) as for the individual stations, different phase relations are found in the longer and the shorter periods, respectively. In the longer periods, the annual maximum streamflow is more influenced by climatic variabilities, while in the shorter periods, it is influenced by other factors, e.g. human activities. The results of the study provide valuable information for improving the long-term forecasting of the streamflow using its relationship with ENSO and the Indian Monsoon. © 2006 Elsevier B.V. All rights reserved.

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Sea level observed by altimeter during the 1993-2004 period, thermosteric sea level from 1945 through 2004, and tide gauge records are analyzed to investigate the interannual variability of sea level in the South China Sea (SCS) and its relationship with ENSO (El Nin~o and Southern Oscillation). Both the interannual variations of the observed sea level and the thermosteric sea level are closely related to ENSO. An 'enigma' that the SST and sea level in the SCS have inverse response to ENSO is revealed. It is found that the thermosteric sea level has an excellent correspondence to seawater temperature at 100 m depth, and their variations are unsynchronized to SST. Detailed analysis denotes that the warming of seawater occurs only in the upper 75 m during and after the mature phase of El Nin~o, while the cooling appears in the layers deeper than 75 m during El Nin~o years. The volume transports between the SCS and the adjacent oceans and the anomalous Ekman pumping contribute a lot for the sea level fall in the developing stage of El Nin~o, while the mass exchange, which is dominated by precipitation, plays a more significant role in the following continuous negative sea level anomalies. © 2006.

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Wavelet cross-correlation (WCC) is used to analyse the relationship between low-frequency oscillations in near-infrared spectroscopy (NIRS) measured cerebral oxyhaemoglobin (O2Hb) and mean arterial blood pressure (MAP) in patients suffering from autonomic failure and age-matched controls. Statistically significant differences are found in the wavelet scale of maximum cross-correlation upon posture change in patients, but not in controls. We propose that WCC analysis of the relationship between O2Hb and MAP provides a useful method of investigating the dynamics of cerebral autoregulation using the spontaneous low-frequency oscillations that are typically observed in both variables without having to make the assumption of stationarity of the time series. It is suggested that for a short-duration clinical test previous transfer-function-based approaches to analyse this relationship may suffer due to the inherent nonstationarity of low-frequency oscillations that are observed in the resting brain. © 2007 IOP Publishing Ltd.

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This paper investigates the relationship between the onset date of the South China Sea summer monsoon (SCSSM) and the El Nin~o/Southern Oscillation (ENSO). The monsoon onset date (MOD) is defined on the basis of the switch of the 850-hPa zonal winds over the South China Sea (SCS) from easterly to westerly for two consecutive pentads. The ENSO signal is represented by the ocean heat content (OHC), which is proportional to the depth of the 20°C isotherm. It is found that, in years associated with a warm (cold) ENSO event or the year after, the monsoon tends to have a late (an early) onset and the intensity of the SCSSM also tends to be weaker (stronger). During a 2-year period prior to the onset, anomalies of OHC have an obvious eastward propagation. The 850-hPa flow east of the Philippines, specifically the strength of the subtropical high, is also found to be critical in determining the MOD. The link between these two results appears to be the propagation of cold (warm) subsurface water into the western North Pacific (WNP), which strengthens (weakens) the subtropical high, and hence a late (an early) SCSSM onset. Copyright © 2006 Royal Meteorological Society.

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The total precipitation of the highest 1 day, 3 day, 5 day and 7 day precipitation amount (R1D, R3D, R5D and R7D) in the Yangtze River basin was analyzed with the help of linear trend analysis and continuous wavelet transform method. The research results indicated that: 1) Spatial distribution of R1D is similar in comparison with that of R3D, R5D and R7D. The Jialingjiang and Hanjiang river basins are dominated by decreasing trend, which is significant at >95% confidence level in Jialingjiang River basin and insignificant at >95% confidence level in Hanjiang River basin. The southern part of the Yangtze River basin and the western part of the upper Yangtze River basin are dominated by significant increasing trend of R1D extreme precipitation at >95% confidence level. 2) As for the R3D, R5D and R7D, the western part of the upper Yangtze River basin is dominated by significant increasing trend at >95% confidence level. The eastern part of the upper Yangtze River basin is dominated by decreasing trend, but is insignificant at >95% confidence level. The middle and lower Yangtze River basin is dominated by increasing trend, but insignificant at >95% confidence level. 3) The frequency and intensity of extreme precipitation events are intensified over time. Precipitation anomalies indicated that the southeastern part, southern part and southwestern part of the Yangtze River basin are dominated by positive extreme precipitation anomalies between 1993-2002 and 1961-1992. The research results of this text indicate that the occurrence probability of flash flood is higher in the western part of the upper Yangtze River basin and the middle and lower Yangtze River basin, esp. in the southwestern and southeastern parts of the Yangtze River basin. © Science in China Press 2007.

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The paper focuses on the investigation of time series of paleoclimatic proxy records of Finland's temperature and the ratio of oxygen isotope in ice cores of Greenland. Joint study of these records should improve the understanding of the climatic variability over the entire North Atlantic sector. Paleoclimatic proxy records are analyzed with the help of multifractal formalism, wavelet analysis and topological dynamics methods to reveal scaling features as well as their nonlinear dynamics and interrelationship. © 2006 Elsevier B.V. All rights reserved.

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Insight into the spatial variation of an ecosystem can provide better understanding of ecological processes and patterns in different scales. Detecting these multiple scales of spatial variation in grassland landscapes is valuable for determining management options, designing proper sampling regimes, and selecting suitable resolutions of remote sensing products. The objective of this study is to examine how environmental factors affect spatial variation of biophysical properties in mixed grassland ecosystems. Field leaf area index (LAI), soil moisture, and topographical parameters (relative elevation, upslope length, and a wetness index) were obtained in three parallel transects of a grassland ecosystem in Saskatchewan, Canada in 2004. One 20-m resolution SPOT 4 (HRVIR) image was acquired at the same period of the growing season but in the following year. Normalized difference vegetation index (NDVI) was calculated from the satellite image of the centre 381-m transect and two extensive 2560-m perpendicular transects. A wavelet approach was used to identify the scales of variations. Statistical results showed that LAI is significantly correlated to the wetness index (r2=0.37) and soil moisture (r2=0.43). The wetness index is better than relative elevation and upslope length in demonstrating the effect of topography on grassland vegetation. The variation of soil moisture is significant at two small scales of about 20 m and 40 m, and that of the wetness index is at the large scale of 120 m. The variation of grassland LAI is significant at three scales (20 m, 40 m, and 120 m), which indicates that the spatial variation of LAI might be controlled by both topography and soil moisture, though the 120 m is the dominant scale of variation in LAI. NDVI significantly correlated with grassland LAI along the centre transect. The effect of topography on grassland LAI is also proven by the significant relationships between NDVI and the wetness index. The wavelet analysis identifies the variation of two extensive transects at the scale of about 120 m, which is similar to the dominant variation scale of grassland LAI. These results confirmed that the effect of topography on spatial variation can be identified from the appropriate satellite image. This study suggests that the spatial scales of soil and topographic data aid in the selection of appropriate satellite image resolution for monitoring and managing ecosystems.

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Monthly data of Self-Calibrated Palmer Drought Severity Index (PDSI) from 1951 to 2000 are calculated using historical precipitation and temperature data for Chinese 160 stations. Temporal and spatial patterns of the first empirical orthogonal function (EOF) of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature, which is similar to the linear PDSI trend during 1951-2000 calculated using all monthly data. The EOF analysis also reveals that the leading mode correlates significantly with ENSO events in time and space. The ENSO EOF shows that during the typical warm phase of ENSO, surface conditions are drier in most regions of China, especially North China, but wetter than normal in the southern regions of Changjiang River, and Northwest China. During the typical cold phase of ENSO, these anomalies reverse sign. From 1951 to 2000, there are large multi-year to decadal variations in droughts and wet spells over China, which are all closely related to strong El Nin~o events. In other words, when one strong El Nin~o event happens, there is a possible big variability in droughts and wet spells over China on the multi-year or decadal scale. Studies also suggest that during the last 2-3 decades climate changes over China, especially North China's drying and northwest China's wetting, are closely related to the shift in ENSO towards warmer events and global warming since the late 1970s. The instability of the relationship is also studied. It is revealed that there is a good correlation between ENSO and Chinese variations in droughts and wet spells in the 3-8-year band, but the correlation between ENSO and Chinese variations in droughts and wet spells is instable. Studies suggest that there are decadal changes in the correlation: the wavelet coherency between ENSO and Chinese variations in droughts and wet spells is high during 1951-1962 and 1976-1991, but low during 1963-1975 and 1992-2000. © Science in China Press 2007.

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Large lightning wildfires in Canada and Alaska account for most of the area burnt and are main determiners of the age mosaic of the landscape. Such fires occur when positive midtroposphere height anomalies persist > 10 days during the fire season. Midtroposphere anomalies are part of teleconnections which are created by atmospheric and coupled sea/air dynamics. Large lightning fire occurrence and area burnt data were used to define eight centers of large wildfire variability in Canada and Alaska during 1959-1999. Preferred positions of persistent positive midtroposphere anomalies correlated with the Fire Regions during large fire events. Active fire weather showed strong relations with Pacific Decadal Oscillation (PDO) at interdecadal timescales and with El Nin~o Southern Oscillation (ENSO) and Arctic Oscillation (AO) mostly at interannual (2 to 6 years) timescales. PDO and ENSO (AO) related large fires were more frequent in the western (eastern) regions. The mountain ranges in western Canada play a major role in the large-scale patterns of large fire occurrence through retention of PDO-related Pacific Ocean moisture, causing the dynamics of large fires each side of the ranges to be mostly in antiphase. The PDO/ENSO regime shift of 1976/1977, together with the strong and persistent positive phase of AO during the late 1980s and 1990s contributed to the increase in area burned in the study area except in British Columbia and Alaska. PDO-ENSO-AO interactions with active fire weather provide an explanation for changes in large fire occurrence frequency during the last centuries in the area. Copyright 2006 by the American Geophysical Union.

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[No abstract available]

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It has been proposed that solar cycle irradiance variations may affect the whole planet's climate via the stratosphere, the Quasi-Biennial Oscillation (QBO) and Arctic Oscillation (AO). We test this hypothesis by examining causal links between time series of sunspot number and indices of QBO, AO and ENSO activity. We use various methods: wavelet coherence, average mutual information, and mean phase coherence to study the phase dynamics of weakly interacting oscillating systems. All methods clearly show a cause and effect link between Southern Oscillation Index (SOI) and AO, but no link between AO and QBO or solar cycle over all scales from biannual to decadal. We conclude that the 11-year cycle sometimes seen in climate proxy records is unlikely to be driven by solar forcing, and most likely reflects other natural cycles of the climate system such as the 14-year cycle, or a harmonic combination of multi-year cycles. Copyright 2006 by the American Geophysical Union.

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Pattern-process analysis is one of the main threads in landscape ecological research. It aims at understanding the complex relationships between ecological processes and landscape patterns, identifying the underlying mechanisms and deriving valid predictions for scenarios of landscape change and its consequences. Today, various studies cope with these tasks through so called "landscape modelling" approaches. They integrate different aspects of heterogeneous and dynamic landscapes and model different driving forces, often using both statistical and process-oriented techniques. We identify two main approaches to deal with the analysis of pattern-process interactions: the first starts with pattern detection, pattern description and pattern analysis, the second with process description, simulation and pattern generation. Focussing on the interplay between these two approaches, landscape analysis and landscape modelling will improve our understanding of pattern-process interactions. The comparison of simulated and observed pattern is a prerequisite for both approaches. Therefore, we identify a set of quantitative, robust, and reproducible methods for the analysis of spatiotemporal patterns that is a starting point for a standard toolbox for ecologists as major future challenge and suggest necessary further methodological developments. © 2006 Elsevier B.V. All rights reserved.

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The importance of the substantia nigra pars reticulata (SNPr), striatum (STR) and superior colicullus (SC) in the blockade of experimental seizures is well known. But, in audiogenic seizures (brainstem tonic-clonic seizures), the anticonvulsant activity of these nuclei is still controversial. In the present study we aimed to analyze the STR-SNPr-CS circuitry in the audiogenic seizures of Wistar audiogenic rat (WAR). Behavioral and electroencephalographic (EEG) data were collected from WARs under no treatment or injection with systemic (phenobarbital) or intracerebral (intranigral) drugs (muscimol and phenobarbital). The main EEG frequency oscillation of STR, SNPr and SC seen before, during and after audiogenic seizures or during seizure protection, was determinated with wavelet spectral analyses. This method allows the association between behavior and EEG (video-EEG). Audiogenic seizures last only for half a minute in average, suggesting that the interruptions of seizures are probably not due to exhaustion. Systemic phenobarbital caused an acute and dose-dependent behavioral and EEGraphic anticonvulsant effect both in WARs. The dose of phenobarbital 15 mg/kg protected animals almost completely, without side effects such as ataxia and sedation. In our data, this endogenous "natural" seizure blockade (or termination) seems to be similar to the "forced" seizure abolition, like the one caused by a systemic non-ataxic phenobarbital dose, because in both cases an intense decrease in the EEG main frequency oscillation can be seen in SNPr and SC. Intranigral phenobarbital or muscimol did not protect animals, and actually induced an increase in the main EEG frequency oscillation in SC. The main finding of the present study is that, in contrast to what is well believed about the incapacity to control audiogenic seizures by the striato-nigro-tectal circuitry, we collected here evidences that these nuclei are involved in the ability to block these seizures. However, the striato-nigro-tectal circuitry in WARs, a genetically developed strain, seems to have different functional mechanisms when compared with normal rats. © 2006 Elsevier B.V. All rights reserved.

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Among the various frameworks in which Eleetroencephalographic (EEG) signal synchronization has been traditionally formulated, the most widely studied and used is the coherence that is entirely based on frequency analysis. However, at present time it is possible to capture information about the temporal profile of coherence, which is particularly helpful in studying non-stationary time-varying brain dynamics, like the Wavelet Coherence (WC). In this paper we propose a new approach of studying brain synchronization dynamics by extending the use of WC to include certain statistically significant (in terms of signal coherence) time segments, to study and characterize any disturbances present in the functional connectivity network of schizophrenia patients. Graph theoretical measures and visualization provide the tools to study the "disconnection syndrome" as proposed for schizophrenia. Specifically, we analyzed multichannel EEG data from twenty stabilized patients with schizophrenia and controls in an experiment of working memory (WM) using the gamma band (i.e., the EEG frequency of ca. 40 Hz), which is activated during the connecting activity (i.e., the "binding" of the neurons). The results are in accordance with the disturbance of connections between the neurons giving additional information related to the localization of most prominent disconnection. © 2006 IEEE.

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We studied the turbulent interactions among vertical wind velocity and temperature time-series measured in the Amazonian forest, during the wet season campaign of Large Biophere-Atmosphere Experiment in Amazonia (LBA) in 1999. The approach is based on the estimation of the correlation coefficient between the different scales in turbulent fields and Cross Wavelet Power (XWP). The results suggest that the correlations among scales of the vertical wind velocity are due to the Coherent Structures (CS), a large scale signature in the thermal profile. These coherent structures, kind of ramps, promoted an increase in the interaction among both variables, vertical wind velocity and temperature, and also depends on the atmospheric stability conditions. Furthermore, these coherent structures may explain the higher values of the correlation coefficient found in the large scales during the diurnal period compared with the nocturnal period, for the vertical wind velocity.

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Wavelet transform and cross wavelet transform were applied for analyzing long time series of sea level and alongshore wind stress to identify intraseasonal variability off western South America and the relations with remote and local forcings. Hydrographic data were used to estimate properties of coastal trapped waves with a theoretical model. For El Nin~o years, we found the existence of intraseasonal oscillations with periods 20-90 days, between 2S and 27S. At the peak of 91-92 and 97-98 EL Nin~os, we found perturbations in the northern region, probably associated with remotely forced internal Kelvin waves, with periods 6-11 days and phase velocities 160-260 km/day. Between 12S and 15S, during two El Nin~o events, our calculations show perturbations which appear to be barotropic shelf waves propagating southward with velocities between 110 and 150 km/day and periods between 30 and 50 days. Copyright 2006 by the American Geophysical Union.

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An efficient class of nonlinear models, constructed using cardinal B-spline (CBS) basis functions, are proposed for high tide forecasts at the Venice lagoon. Accurate short term predictions of high tides in the lagoon can easily be calculated using the proposed CBS models.

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The study of transitory signals and local variability structures in both/ either time and space and their role as sources of climatic memory, is an important but often neglected topic in climate research despite its obvious importance and extensive coverage in the literature. Transitory signals arise either from non-linearities, in the climate system, transitory atmosphere-ocean couplings, and other processes in the climate system evolving after a critical threshold is crossed. These temporary interactions that, though intense, may not last long, can be responsible for a large amount of unexplained variability but are normally considered of limited relevance and often, discarded. With most of the current techniques at hand these typology of signatures are difficult to isolate because the low signal-to-noise ratio in midlatitudes, the limited recurrence of the transitory signals during a customary interval of data considered. Also, there is often a serious problem arising from the smoothing of local or transitory processes if statistical techniques are applied, that consider all the length of data available, rather than taking into account the size of the specific variability structure under investigation. Scale-dependent correlation (SDC) analysis is a new statistical method capable of highlighting the presence of transitory processes, these former being understood as temporary significant lag-dependent autocovariance in a single series, or covariance structures between two series. This approach, therefore, complements other approaches such as those resulting from the families of wavelet analysis, singular-spectrum analysis and recurrence plots. A main feature of SDC is its high-performance for short time series, its ability to characterize phase-relationships and thresholds in the bivariate domain. Ultimately, SDC helps tracking short-lagged relationships among processes that locally or temporarily couple and uncouple. The use of SDC is illustrated in the present paper by means of some synthetic time-series examples of increasing complexity, and it is compared with wavelet analysis in order to provide a well-known reference of its capabilities. A comparison between SDC and companion techniques is also addressed and results are exemplified for the specific case of some relevant El Nin~o-Southern Oscillation teleconnections. © Springer-Verlag 2006.

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On the basis of the nonlinear techniques for the estimation of coupling between oscillatory systems from time series, we investigate the dynamics of climatic modes characterizing global and Northern Hemisphere (NH) processes. The North Atlantic Oscillation (NAO) and Arctic Oscillation indices and the El Nin~o-Southern Oscillation (ENSO) indices are analyzed in terms of the most reliable data from 1950 through 2004 and earlier data since the 19th century. These indices characterize changes in NH atmospheric pressure (specifically, sea-level pressure in the North Atlantic and NH extratropical latitudes as a whole) and in equatorial Pacific sea-surface temperature and sea-level pressure to which the strongest variations of global surface temperature and global climate on interannual time scales and of regional climatic anomalies in the NH are linked. The methods used are based on phase-dynamics modeling and nonlinear prediction models (a nonlinear version of Granger causality). From both methods and various ENSO indices, the inference about the ENSO effect on the NAO during the latter half of the 20th century and in the early 21st century is made with confidence probability of at least 0.95. The influence is characterized by a time delay of about two years. No inverse influence is found with a similar degree of reliability. Results of estimating the coupling between the ENSO and the NAO depend on the type of index that is used to describe the NAO. The ENSO effect on the NAO is detected with sufficient confidence when the NAO index is chosen to be a larger scale characteristic. However, when a more local index of the NAO is used, no statistically significant coupling to the ENSO is found. Increasing the length of the analyzed ENSO and NAO series (over more than 100 yr) does not lead to any more reliable detection of coupling. Analysis of the data for different time intervals during 1950-2004 has revealed a strengthening of the ENSO effect on the NAO, although this inference is not reliable. © Pleiades Publishing, Inc. 2006.

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A simple model is developed to describe the significant land-atmosphere interaction processes in the warm climate. It includes bulk soil hydrology, dynamic vegetation, and simple land-atmosphere interaction processes. The model can simulate the basic features of land surface control on evapotranspiration (ET) and exhibits a multiequilibrium behavior similar to that of some more complex models. In order to study the role of land surface processes in climate variability on monthly to seasonal timescales, a series of experiments are performed with the model over different land covers and at different external forcings. The major findings are: 1 The maximum soil wetness memory and precipitation predictability tend to occur at a sparser (denser) vegetation cover with the weakening (strengthening) of external forcing. 2 For vegetated region, the soil moisture memory and precipitation persistence will be underestimated if vegetation is not interactive, and the percentage of underestimation is larger over denser vegetation covers. 3 Interactive vegetation can enhance the low-frequency coherency between soil wetness and precipitation, but its influence on high-frequency coherency is small. 4 Large coherencies between soil wetness and precipitation in the time-frequency domain correspond to strong wavelet power of external forcing in the same domain. These findings provide guidance for the development of and study with more complex models. Copyright 2006 by the American Geophysical Union.

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Area burned variability in the province of Ontario, Canada, was inferred from 25 tree ring width chronologies covering A.D. 1781-1982 and distributed largely across the Boreal Shield. The area burned estimates account for 39.5% of the variance in the actual area burned recorded from 1917 to 1981 and were verified using a split sample calibration-verification scheme. The reconstruction showed that a positive trend in area burned from circa 1970-1981 was preceded by three decades during which area burned was amongst the lowest during the past 200 years. The area burned exhibited a trend toward increasing variance during the past century, recently reaching magnitudes similar to those seen prior to 1850. Signal analyses further identified the presence of two prominent periodic components in area burned that related to decade-to-decade variations. This will help to place the recent increase in area burned in a context relative to the long-term history of the province.

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We analyze the Permanent Service for Mean Sea Level (PSMSL) database of sea level time series using a method based on Monte Carlo Singular Spectrum Analysis (MC-SSA). We remove 2-30 year quasi-periodic oscillations and determine the nonlinear long-term trends for 12 large ocean regions. Our global sea level trend estimate of 2.4 ±1.0 mm /yr for the period from 1993 to 2000 is comparable with the 2.6 ± 0.7 mm/yr sea level rise calculated from TOPEX/Poseidon altimeter measurements. However, we show that over the last 100 years the rate of 2.5 ± 1.0 mm/yr occurred between 1920 and 1945, is likely to be as large as the 1900s, and resulted in a mean sea level rise of 48 mm. We evaluate errors in sea level using two independent approaches, the robust bi-weight mean and variance, and a novel "virtual station" approach that utilizes geographic locations of stations. Results suggest that a region cannot be adequately represented by a simple mean curve with standard error, assuming all stations are independent, as multiyear cycles within regions are very significant. Additionally, much of the between-region mismatch errors are due to multiyear cycles in the global sea level that limit the ability of simple means to capture sea level accurately. We demonstrate that variability in sea level records over periods 2-30 years has increased during the past 50 years in most ocean basins. Copyright 2006 by the American Geophysical Union.

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The 800-year sulfate record from Lomonosovfonna was analyzed by a novel multiple linear regression algorithm that attempts to explain sulfate variability in terms of other chemical species measured in the core and sulfur emission inventories. We use three statistical approaches to determine sulfate sources. We examine trends using singular spectrum analysis with confidence intervals, finding clear evidence that anthropogenic sources are important but not dominant; we use cross-wavelet coherence to examine significant multidecadal covariance in terrestrial sulfate; but our main tool is multiple regression analysis of the sulfate dependency on other ions and anthropogenic emission inventories. Models are fitted in a moving time window of typically 50 years length, explaining 80% of the sulfate variance. A suite of model predictors are examined, and the variation in relative magnitudes of the model coefficients along the core can be used to infer variations in the strength of various sulfate sources. We observe large changes in sulfate sources at the end of the Little Ice Age associated with changes in Barents Sea marine productivity, changes in North Atlantic sulfate input and a long-lasting period of disturbance caused by the large Laki volcanic eruption. Modeling sulfur emission inventories shows that western Europe contributes about 15% of the sulfate budget, with essentially no input from other regions, in contrast with predictions from global circulation models incorporating sulfur chemistry. Multidecadal cycles are mainly confined to the Little Ice Age and most likely associated with increased storminess and enhanced deposition of both marine aerosol and biogenic sulfate from the Atlantic Ocean. The model residuals do not show a normal distribution but display very large spikes. Unexpectedly, those significant at the 99% level can be closely matched to major volcanic eruptions using independent dating methods. The 20th century sulfate in the core is inventoried as sea salt (15%), terrestrial (10%), volcanic (5-10%), western European anthropogenic (10-25%), Barents Sea biogenic (20-40%), and Atlantic biogenic (10-15%). Copyright 2006 by the American Geophysical Union.

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Using data from the Carte Synoptique solar filaments archive, we investigate whether there is a time lag between high-latitude solar activity and low-latitude solar activity. The cross-correlation analysis of the number of high-latitude filaments per Carrington rotation (NHF) and that of low-latitude filaments per Carrington rotation (NLF) shows, although inconclusively, that NLF possibly lags behind NHF. The periodic characteristics of both NHF and NLF clearly indicate that the activity of high-latitude filaments is evidently leading the activity of low-latitude filaments. Thus, the present study suggests that high-latitude solar activity leads low-latitude solar activity in time phase. © 2006. The American Astronomical Society. All rights reserved.

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Using the extension of Monte Carlo Singular System Analysis (MC SSA), based on evaluating and testing the regularity of the dynamics of the SSA modes against the colored noise null hypothesis, we demonstrate detection of oscillatory modes with period of about 27 months in records of monthly average near-surface air temperature from several European locations, as well as in the monthly North Atlantic Oscillation index. According to their period, the detected modes can be attributed to the quasi-biennial oscillations (QBO). The QBO modes extracted from the temperature and from the NAO index underwent synchronization analysis and their phase synchronization has been confirmed with high statistical significance.

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A catalog containing an unprecedented amount of historical data in the southeastern part of Mexico covering almost four centuries (1502-1899) is used to construct a drought time series. The catalog records information of agricultural disasters and includes events associated with hydrometeorological phenomena or hazards whose effects were mainly felt in the agricultural sector, such as droughts. An analysis of the historical series of droughts in southeastern Mexico for the period 1502-1899 is performed. The highest drought frequency occurred around the years 1650, 1782, and 1884; no droughts were reported around 1540, between 1630 and 1640, along the largest time lapse of 1672-1714, and between 1740 and 1760. From 1760 until the end of the period of study droughts definitively occur more often than they did from ~1550 to 1760. In addition, most droughts lasted for 1-2 yr. Analyzing the frequencies of the drought time series it is found that the most conspicuous cycles are ~3-4 and 7 yr, although cycles of ~12, 20, 43, and 70 yr are also evident. The relation between droughts and El Nin~o events indicates that 38% of droughts are associated with El Nin~o. Sea surface temperature changes, the Southern Oscillation index, and solar activity leave their signals in the southeastern part of Mexico, with the signs in Oaxaca clearer than in the Yucatan Peninsula. However, the dominance of some phenomena over others depends on the time scales considered. © 2006 American Meteorological Society.

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This letter reports on wavelet spectral analysis of collocated time-series of streamwise wind speed and aeolian sand transport collected at 0.04 m above the bed in a field experiment on top of a large sand mound in a desert environment. Results suggest three distinct spatio-temporal scales of interaction between boundary layer turbulence and sediment transport: a long-term scale associated with the regional mixing layer, an integral time-scale associated with the internal boundary layer, and a streamer scale associated with the inertial subrange (eddy cascade) and saltation inertia. Copyright 2006 by the American Geophysical Union.

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Very few in situ measurements so far show the details of the interactions between CME and magnetosphere. Here we report a fortuitous observation that the Cluster spacecraft were located at the dusk-side magnetopause, meandering back and forth four times between the ICME and the magnetosphere, and observed part process of the interaction between the CME and the magnetosphere. The primarily analysis showed that: (1) a fast ion beam other than the ambient plasma was observed almost perpendicular to the magnetic field; (2) The left-hand-polarized kinetic Alfve´n waves excitation by the ion beam; and (3) the resonances between the kinetic Alfve´n waves and part of the beam particles lead to the pitch angle diffusion of resonant particles and formating of a monospheric distribution with nearly constant kinetic energy. The wave-particle resonance may pay an important role in the energy transfer and particle exchange across the magnetopause between the magnetosheath and magnetosphere during the CME impulsion. © 2006 International Astronomical Union.

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In this paper we report an analysis of the solar magnetic fluxes estimated in the period 1971-1998. We applied the wavelet technique to find the significant periodicities of these series. We concentrate particularly in the mid-term quasi-periodicities (1-2 years). The mid-term periodicity of 1.7 year is the dominant fluctuation for all the types of fluxes analyzed (total, closed, open, low and high latitude open fluxes) and has a strong tendency to appear during the descending phase of solar activity. The mid-term fluctuation of 1 year is significantly present in total and closed fluxes, but it is less important in open fluxes. It is recognizable in the high latitude open flux, but it is absent in the low latitude open flux. Due to the uncertainties involved in estimating the exact period of the quasi-annual peak, this component may not be different from the previously-reported 1.3 year periodicity. The high frequency fluctuations of all the fluxes but the high latitude open flux are in phase with the 11 years solar cycle. The high latitude flux tends to be present all the time, showing that along the cycle both the low latitude bipolar active regions and the polar coronal holes regulate this flux. These findings rule out the possibility of a more basic periodicity different from the 11 years cycle. © Springer 2006.

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Three minimum extreme temperature series from meteorological stations located in or near the Mexican Pacific coast, Acapulco, Comita´n and Manzanillo, are spectrally analyzed. The series cover a period from 1941 to 1981. The spectral analysis indicates that a significant quasiquinquenial periodicity (~5 yrs) is present in Comita´n and Manzanillo, even more, Acapulco also shows a frequency ~5yr if the uncertainties are taken into account. This spectral peak can be considered either as related to solar activity or to strong El Nin~o events. The remaining periodicities can be associated to meteorological phenomena like El Nin~o and the quasi-biennial oscillation or to some solar activity phenomena. Furthermore, the behavior deduced from a coherence spectra analysis between the temperature and the Southern Oscillation Index, considered as a proxy of El Nin~o, and sunspot number, considered as a proxy of solar activity, indicates that the stations closer to the ocean might be more influenced by El Nin~o than by solar activity, while the station inland has both influences.

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The Arctic Oscillation (AO) has a major impact on climate variability in the extra tropical regions of the Northern Hemisphere. In this study, we show that specific alignments between the Sun, the Moon and the Earth known as the Luni-Solar Oscillation (LSO) that occur at frequencies of nearly 9 and 18 years are unambiguously correlated with the AO since the mid-1960's. The occurrence of the LSO peaks is predictable and, as a result, it improves climate predictability. Furthermore, we hypothesize that the recent observed increase in the AO amplitude maybe due to ice melting in the Arctic and SST increases at northern latitudes combined with the recent LSO peaks. Copyright 2005 by the American Geophysical Union.

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[1] A soil property may be related to another and the relationships may change depending on the scale and location. Understanding these scale- and location-dependent relationships is important for prediction of one soil property based on another. The objective of this study is to use wavelet coherency analysis to examine whether the relationship between hydraulic properties and soil physical properties are scale- and location-dependent. Undisturbed cores were collected along a transect from the sandy loam soil of a farm field in northern Saskatchewan, Canada. Saturated hydraulic conductivity (Ks), sand content, and organic carbon content (OC) were measured on these cores, and their relationships as a function of scale and location were analyzed using wavelets. Results indicated that the wavelet coherency between Ks and sand content is only significantly different from that of red noises at the scales around 48 m. The cross-wavelet spectrum and wavelet coherency are predominantly in phase, suggesting a positive correlation between Ks and sand. For Ks and OC, significant coherency exists at scales from 30 to 48 and around 80 m. At the scales of 30-48 and around 80 m the relationships are predominantly out of phase, suggesting negative correlation. Therefore relationships between Ks and sand or K s and OC are not only scale-dependent but also location-dependent. Scale and location dependence have an important implication for understanding the scaling relationships between Ks and sand and OC and for the prediction of Ks from sand and OC. Copyright 2005 by the American Geophysical Union.

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The evolution of radiation-active atmospheric components and climatic characteristics derived from Vostok Antarctic ice-core data is analyzed over 420000 years with a time resolution of 500 years. These data include the temperature variations ?T (determined from the deuterium content dDi in the ice core) and the contents of carbon dioxide q(CO2) and methane q(CH4) in the atmosphere. Cross correlation analysis shows that, on the whole, the variations in q(CO2) and q(CH4) are lagged with respect to the dDi and temperature variations on time scales from millennia to several hundred thousand years. The characteristic time lag ?t of the variations in q(CO2) and q(CH4) relative to the variations in ?T over the entire analyzed period is about 1000 and 500 years, respectively. At the same time, in individual 100 000-yr intervals, the value of ?t reaches 1500 years for q(CO2) and 1000 years for q(CH4). For the earliest 100 000-yr interval, the temperature variations lag behind the variations in the atmospheric greenhouse gas contents. Cross-spectral analysis suggests that the variations in the carbon dioxide and methane contents are generally lagged relative to the temperature variations at periods of approximately 20 000 years or longer except for periods of about 20 000 years for methane. The cross wavelet analysis of the data derived from the Vostok Antarctic ice core over the last 420 000 years reveals that the variations in q(CO2) and q(CH4) lag behind the temperature variations, including the modes characteristic of variations in the Earth's orbital parameters (Milankovitch cycles), while opposite phase lags are observed in individual time intervals and frequency ranges. The coevolution of ?T and q(CH4) is less conclusive than that of ?T and q(CO2). Over the last 100 000-yr period, the fundamental 100 000-yr mode of the deuterium content and temperature does not lead that of the greenhouse gas contents. An opposite effect is observed, with the temperature variations lagged more pronouncedly relative to the methane-content variations than to the carbon dioxide content variations. For the 100 000-yr mode, the variations in q(CH4) lead those in q(CO2) during the last period. Copyright © 2005 by MAIK "Nauka/Interperiodica" (Russia).

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