Independently of its phase, NC leads to a spectrum of cognitive abnormalities, ranging from impairment in a single domain, to CIND and, occasionally, to dementia. These findings are more conspicuous during active vesicular phase and less prominent in calcified stages.
Infering causal relationships from observed time series has attracted much recent attention. In cases of nonlinear coupling, adequate inference is often hindered by the need to specify coupling details that call for many parameters and global minimization of nonconvex functions. In this paper we use an example to investigate a new concept, termed here running entropy mapping, whereby time series are mapped onto other entropy related time sequences whose analysis via a linear parametric time series methods, such as partial directed coherence, is able to expose the presence of formerly linearly undetectable causal relationships.
Abstract. The Madden–Julian oscillation (MJO) is the main controller of the weather in
the tropics on intraseasonal timescales, and recent research provides
evidence that the quasi-biennial oscillation (QBO) influences the MJO
interannual variability. However, the physical mechanisms behind this
interaction are not completely understood. Recent studies on the normal-mode
structure of the MJO indicate the contribution of global-scale Kelvin and
Rossby waves. In this study we test whether these MJO-related normal modes are
affected by the QBO and stratospheric ozone. The partial directed coherence
method was used and enabled us to probe the direction and frequency of the
interactions. It was found that equatorial stratospheric ozone and
stratospheric zonal winds are connected with the MJO at periods of 1–2 months
and 1.5–2.5 years. We explore the role of normal-mode interactions behind the
stratosphere–troposphere coupling by performing a linear regression between
the MJO–QBO indices and the amplitudes of the normal modes of the atmosphere
obtained by projections on a normal-mode basis using ERA-Interim reanalysis
data. The MJO is dominated by symmetric Rossby modes but is also influenced by
Kelvin and asymmetric Rossby modes. The QBO is mostly explained by westward-propagating inertio-gravity waves and asymmetric Rossby waves. We explore the
previous results by identifying interactions between those modes and between
the modes and the ozone concentration. In particular, westward inertio-gravity
waves, associated with the QBO, influence the MJO on interannual
timescales. MJO-related modes, such as Kelvin waves and Rossby waves
with a symmetric wind structure with respect to the Equator, are shown to have
significantly different dynamics during MJO events depending on the phase of
the QBO.
Here we investigate a new concept, kernel-nonlinear-Partial Directed Coherence, whereby a kernel feature space representation of the data allows detecting nonlinear causal links that are otherwise undetectable through linear modeling. We show that adequate connectivity detection is achievable by applying asympotic decision criteria similar to the ones developed for linear models.
The geomagnetic field's dipole undergoes polarity reversals in irregular time intervals. Particularly long periods (of the order of 10 7 yrs) without reversals, named superchrons, have occurred at least three times in history. We provide observational evidence for high non-Gaussianity in the vicinity of a transition to and from a geomagnetic superchron, consisting of a sharp increase in high-order moments (skewness and kurtosis) of the dipole's distribution. Such increase in the moments is a universal feature of crisis-induced intermittency in low-dimensional dynamical systems undergoing global bifurcations. This suggests temporal variation of the underlying parameters of the physical system. Through a low dimensional system that models the geomagnetic reversals we show that the increase in the high-order moments during transitions to geomagnetic superchrons is caused by the progressive destruction of global periodic orbits exhibiting both polarities as the system approaches a merging bifurcation. We argue that the non-gaussianity in this system is caused by the redistribution of the attractor around local cycles as global ones are destroyed.
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