Non-stationary Influence Research Articles

Nonlinear and nonstationary influences of geomagnetic activity on the winter North Atlantic Oscillation

[1] The relationship between the geomagnetic aa index and the winter North Atlantic Oscillation (NAO) has previously been found to be nonstationary, being weakly negative during the early 20th century and significantly positive since the 1970s. The study reported here applies a statistical method called the generalized additive modeling (GAM) to elucidate the underlying physical reasons. We find that the relationship between aa index and the NAO during the Northern Hemispheric winter is generally nonlinear and can be described by a concave shape with a negative relation for small to medium aa and a positive relation for medium to large aa. The nonstationary character of the aa-NAO relationship may be ascribed to two factors. First, it is modulated by the multidecadal variation of solar activity. This solar modulation is indicated by significant change points of the trends of solar indices around the beginning of solar cycle 14, 20, and 22 (i.e., ∼1902/1903, ∼1962/1963, and ∼1995/1996). Coherent changes of the trend in the winter time NAO followed the solar trend changes a few years later. Second, the aa-NAO relationship is dominated by the aa data from the declining phase of even-numbered solar cycles, implying that the 27 day recurrent solar wind streams may be responsible for the observed aa-NAO relationship. It is possible that an increase of long-duration recurrent solar wind streams from high-latitude coronal holes during solar cycles 20 and 22 may partially account for the significant positive aa-NAO relationship during the last 30 years of the 20th century.

Non-stationary influence of El Niño-Southern Oscillation and winter temperature on oak latewood growth in NW Iberian Peninsula

The properties of El Niño-Southern Oscillation (ENSO), such as period, amplitude, and teleconnection strength to extratropical regions, have changed since the mid-1970s. ENSO affects the regional climatic regime in SW Europe, thus tree performance in the Iberian Peninsula could be affected by recent ENSO dynamics. We established four Quercus robur chronologies of earlywood and latewood widths in the NW Iberian Peninsula. The relationship between tree growth and the Southern Oscillation Index (SOI), the atmospheric expression of ENSO, showed that only latewood growth was correlated negatively with the SOI of the previous summer-autumn-winter. This relationship was non-stationary, with significant correlations only during the period 1952-1980; and also non-linear, with enhanced latewood growth only in La Niña years, i.e. years with a negative SOI index for the previous autumn. Non-linear relationship between latewood and SOI indicates an asymmetric influence of ENSO on tree performance, biassed towards negative SOI phases. During La Niña years, climate in the study area was warmer and wetter than during positive years, but only for 1952-1980. Winter temperatures became the most limiting factor for latewood growth since 1980, when mean regional temperatures increased by 1°C in comparison to previous periods. As a result, higher winter respiration rates, and the extension of the growing season, would probably cause an additional consumption of stored carbohydrates. The influence of ENSO and winter temperatures proved to be of great importance for tree growth, even at lower altitudes and under mild Atlantic climate in the NW Iberian Peninsula.

Nonstationary influence of the North Atlantic Oscillation on European precipitation

This paper analyzes the changing role of the North Atlantic Oscillation (NAO) in European precipitation in relation to interdecadal shifts in the atmospheric dynamic over the North Atlantic region. For this purpose, moving window correlations between precipitation and NAO have been related to the interdecadal variability of atmospheric pressure patterns. Analyses have been replicated using observed climate data for the twentieth century, paleoclimate reconstructions since 1785, and physically modeled climate by AOGCMs (CGCM3.1(T63), 20C3M). Results evidence that nonstationary relationship of the NAO across Europe is linked to interdecadal shifts in the location of the positions of the NAO pressure centers, which have been occurred at least since the last 2 centuries. Such displacement of the pressure NAO centers points to possible existence of different NAO patterns, which may help to explain why studies relating NAO to European climate behavior have not shown systematically strong correlations in the course of the twentieth century. A general trend toward a strengthening of the NAO‐precipitation relationship over most of Europe has been detected for the twentieth century.

Scaling in non-stationary time series. (I)

Most data processing techniques, applied to biomedical and sociological time series, are only valid for random fluctuations that are stationary in time. Unfortunately, these data are often non-stationary and the use of techniques of analysis resting on the stationary assumption can produce a wrong information on the scaling, and so on the complexity of the process under study. Herein, we test and compare two techniques for removing the non-stationary influences from computer generated time series, consisting of the superposition of a slow signal and a random fluctuation. The former is based on the method of wavelet decomposition, and the latter is a proposal of this paper, denoted by us as step detrending technique. We focus our attention on two cases, when the slow signal is a periodic function mimicking the influence of seasons, and when it is an aperiodic signal mimicking the influence of a population change (increase or decrease). For the purpose of computational simplicity the random fluctuation is taken to be uncorrelated. However, the detrending techniques here illustrated work also in the case when the random component is correlated. This expectation is fully confirmed by the sociological applications made in the companion paper. We also illustrate a new procedure to assess the existence of a genuine scaling, based on the adoption of diffusion entropy, multiscaling analysis and the direct assessment of scaling. Using artificial sequences, we show that the joint use of all these techniques yield the detection of the real scaling, and that this is independent of the technique used to detrend the original signal.

Nonstationary Robust Control Method Taking Account of Nonstationary Influence of Uncertainty. Its Application to Positioning Control of Flexible Structures.

In positioning control problem, generally, the influence of uncertainty in a controlled object model increases at the period of stoppage. That is, it possesses a nonstationary characteristic. For such a problem, this study presents a nonstationary robust control method (NRCM) taking account of nonstationary influence of uncertainty. In this method, we define a frequency-shaped weight and a time-varying coefficient on unstructured uncertainty. Moreover, we introduce the generalized plant including them. The nonstationary controller is derived from the combination of the following two criteria. The first one is a time-varying game-type criterion function. The second one is the criterion function on the error between the responses of the nominal system and the uncertain system with the controller derived from the first one. Therefore, NRCM corresponds to the optimization problem based upon the coupled criteria. From its application to a positioning control problem of a flexible structure, the usefulness of the method is verified numerically and experimentally.

Nonlinear Interaction of Light with Transversely Moving Medium.

Interaction of a light beam with matter reveals extreme sensitivity to transverse motions. Spatio-temporal delocalization of interaction and diffusive transport of light-induced perturbations is revealed through a new family of diffraction patterns. The response of the medium to the transverse motion of the light enhances near the second order phase transition point. The phenomenon has large application potential for the study of motions that are beyond the power of Doppler-effect-based methods, and suggests fundamental problems about critical processes with nonstationary influences.

Application of Measurement Models for Analysis of Impedance Spectra

Abstract Measurement models have been applied to electrochemical impedance data collected at the corrosion potential for a copper disk rotating in alkaline- and acid-aerated 0.5 M chloride solutions. Several applications of measurement models for the interpretation of experimental impedance data are demonstrated. Corrosion is a nonstationary process, and nonstationary behavior associated with corrosion frequently influences impedance data. If data can be shown to be consistent with the Kramers-Kronig relations, nonstationary influences can be neglected. Measurement models provide a quantitative method for identifying the portion of an impedance spectrum that is consistent with the Kramers-Kronig relations and, therefore, can be interpreted in terms of pseudo-steady-state models. Measurement models also provide a very sensitive means of interpreting impedance spectra in terms of the effects of physical parameters, such as immersion time and disk rotation speed.

A Simple Optimization Technique of Vibration Controller for Mechanical Structures Subjected to Nonstationary Input.

In this study, the optimization problem of dynamical characteristic of vibration controllers for mechanical structures subjected to such a nonstationary input as seismic wave is dealt with. A simple optimization technique based upon the stationary design and the modal analysis is presented. In order to take into account the nonstationary influence in the response, the stationary criterion is modified in each mode by the ratio of the response variance of each mode to the square of the response spectrum: By applying this technique to the optimization problem of the allocation and the dynamical characteristics of a dynamical vibration absorber for a piping system, its usefulness is verified.