Rhombic arrays were obtained by sidewall forcing during Turing pattern formation. Locking between the frequency of forcing and the wave length between blobs in accordance with the Farey sequence was obtained. This locking being represented by a perfect rhombic array oriented in the direction of the imposed forcing. For a constant forcing in duration and amplitude, the following scheme of bifurcation was observed: parallel stripes i—> rhombic array i—> domains of hexagons and rhombi separated by "penta-hepta" defects. Symmetry considerations based on a non-uniform stretching along the x-axis was used to describe these transitions. Unstable "varicose-vein" stripes were observed to evolve during the temporal evolution of rhombic arrays. Permanent address: Dept. of Electrical Engineering and Computer Sciences. Univ. of California at Berkeley, CA 94720, USA.

Classical Faraday experiments were conducted on the oscillatory chemical Belousov-Zhabotinsky (BZ) reaction. The vertical periodic modulation of the acceleration induces flows in the system that change the BZ dynamics, and thus the patterns exhibited. The resulting reaction-diffusion-advection system exhibits four different types of pattern for increasing stirring amplitude: deformed targets and spiral waves, filamentary patterns arranged in large-scale vortices, advection phase waves, and finally front annihilation where the medium becomes homogeneous. A wave period analysis of the forced system has been carried out. Contrary to what is expected, i.e., a continuous increase of the wave period with increasing forcing, the period changes dramatically at the boundaries between pattern domains.

The role of spatially correlated stochastic perturbations on a Morris-Lecar neural network subject to an aperiodic subthreshold signal is analyzed in detail. Our results suggest that optimum signal-to-noise ratios can be obtained for two critical noise intensities due to the interplay of the subthreshold Poisson process and the correlated Gaussian forcing. For the second peak, most of the cells are periodically excited, the information transfer is enhanced, and a collective behavior develops measured in terms of the averaged activity of the network. The maximum signal-to-noise ratio increases with the correlation length, although it saturates for global coupling. It was found that there is a range of mean frequencies of the subthreshold signal that increases the signal-to-noise ratio output.

The study of the spatiotemporal response of pattern forming systems to spatially resonant external forcing has unveiled striking new phenomena which challenge the understanding of self-organization in nonlinear, nonequilibrium systems. Here we show that a simple spatiotemporal two-dimensional forcing of a system supporting an intrinsic wavelength but no intrinsic frequency, under conditions of spatial resonance, may induce complex and entirely new spatiotemporal behaviors which do not reflect in any simple way the structure of the imposed forcing. We demonstrate this phenomenon in the Turing regime of the (photosensitive) CDIMA reaction by projecting a traveling stripe light pattern onto the reactor. By controlling the velocity of the forcing we induce distinct dynamical regimes that express the externally imposed frequency in new and intriguing ways. A detailed analysis of the experimental relevant parameters is presented.

A network of coupled neural oscillators can present different types of spatiotemporal behaviors depending on the coupling coefficients. However, small perturbations in the system could modify the expected behavior of the coupled system. We study the effect of time-correlated stochastic perturbations on the neural network activity when they are applied to the external current. Depending on the diffusion coefficient and the time correlation of the applied noise, an aperiodic synchronization among the cells of the network is achieved for a specific value of the noise intensity.

In this paper the effect of noise on activity of a neural network of diffusively coupled excitatory-inhibitory cells with time delay is analyzed. We distinguish between global and local noise studying the different types of spatiotemporal behaviors observed when a space-time correlated noise is applied to the system. The results show a synchronization of the network in aperiodic behavior with the help of tiny noisy perturbations on the external signal depending on the spatial correlation of noise and the coupling coefficients.

The effect of a periodic signal and a time correlated Gaussian noise on the modified Morris-Lecar model of the CA3 region of the hippocampus is analyzed. Spatially correlated and uncorrelated forcings are used to investigate the possibility to ”anticontrol” the synchronized behavior, typical of epileptic seizures, in order to lead the system dynamics to a disordered pattern characteristic of normal brain functioning.

The behavior of a system of coupled ordinary differential equations is studied in order to characterize the CA3 region of the hippocampus. Clustering and synchronization behavior in a one-dimensional array of cells modeled by a modified Morris–Lecar model is analyzed in terms of a time delay included in the model. The random formation of phase dislocations whose number increases with the time delay seems to be responsible for complex spatiotemporal patterns that have been observed. Alterations to the transmission time between cells have been simulated by adding some noise to the system.

The Ria de Pontevedra is one of the Galician Rias Baixas, partially mixed estuaries on the north-west coast of the Iberian peninsula. The hydrodynamics of the rias is far from being fully understood. In this application, the hydrodynamics of the Ria de Pontevedra is studied by means of a 3-D baroclinic model. Our aim was to establish the circulation pattern driven by tide and the horizontal density gradient between river and shelf waters under estuarine conditions. The spatial variability of tidal velocities and salinities is studied. The residual component of the flow, related to transport, is described and discussed. As expected in a partially mixed estuary, a double-layered residual pattern is observed, with water flowing seaward in a surface layer and upstream in a bottom layer. A cross-channel residual flow, not described up to date, is presented.

An isothermal reaction-diffusion system is considered in a two-dimensional fluid medium within a gravitational field. Inhomogeneities in the concentration field of the species give rise to a fluid flow due to buoyancy forces. A two-dimensional reaction-diffusion-convection model of an excitable medium is presented. The influence of hydrodynamics on spiral wave dynamics is systematically studied. A kinematic model is also introduced to better understand the mechanisms involved here.

The present paper studies circulation in the Ria of Vigo (northwest Spain) by means of a 3-D baroclinic model coupled with a Lagrangian model of dispersion. The model is examined to obtain the residual currents’ underlying periodic tidal movements. These currents are related to the quality of water because they determine the residence time. The model is forced at the mouth of the estuary with the most important tidal harmonics, and at the innermost zone with the freshwater discharge due to the presence of the Oitaven River. Results show that the area under study has a two-layered circulation, where most of water enters the estuary through the southern mouth and leaves it through the northern one.

In order to ameliorate the dearth of existing scientific knowledge concerning the hydrography of the Pontevedra Ria, a systematic investigation was carried out between October 1997–98. Salinity variations were closely related to river discharge whereas bottom waters presented oceanic characteristics over the whole year. Current was controlled by tide, river discharge, and wind in the internal ria where the highest velocities were directed along the ria channel with a low transverse component. Favorable atmospheric conditions in spring induced coastal upwelling up the continental shelf. In May the upwelling was sufficiently strong to be detected in the inner ria and intensified in July and August, cooling the ria water to 12°–14°C. Upwelling ceased in September, and from November to March seawater transported by the poleward current (35.9; 15°C) was detected on the shelf. From January until March, unanticipated favorable upwelling conditions provoked an influx of poleward inside the ria. Ria intrusion of poleward water and association with occasional winter upwelling conditions has not been observed previously. Isopycnic three‐dimensional (3‐D) surface and 2‐D isopycnal maps show that with high river runoff or intense upwelling, lower‐salinity water leaves the ria near the northern margin in the surface layer. Under negative upwelling conditions, the water is partially dammed inside the ria and exits the ria when the wind speed falls. During upwelling events, ENACW penetrated the ria, especially near the southern shore. Arrival of ENACW at the northern entrance impedes the outward water flow through this mouth.

A synchronization scheme through compound chaotic signal is analyzed both numerically and experimentally to study the performance of this scheme, in a noisy environment, considering its possibilities for communication systems with masking technique. The influence of noise has been analyzed in terms of the Bit Error Rate. An important factor of this method is the use of encryption keys which add more robustness and complexity to the system.

Two different experimental methods to terminate a reentrant activity were studied within the framework of nonlinear electronic circuits, which mimic the discrete nature of cardiac tissue at a microscopic scale. It was shown that trains of low voltage (with infra- and supra-threshold stimulating amplitude) and low frequency (period larger than that of reentry) pulses can be used efficiently to terminate a pinned reentry when applied near to the reentry.

The wind effect on water circulation was described by means of hydrodynamical and thermohaline measurements carried out in the ria of Pontevedra (NW Spain) from February to July 1998. The samples were measured both at an anchored station situated in the inner part of the ria and at other stations located in the main channel. In this way, temporal and spatial events were related. Thermohaline variables (salinity and temperature) were used to clarify the current patterns measured at the anchored station. It has been observed that there is a clear predominance of wind in the main estuary axis direction. The wind direction is strongly dependent on topography. Wind speeds higher than 4 ms−1are able to dominate the current at surface layers, even against tidal effect. Easterly winds force water to leave the estuary at surface layers while westerly winds force water to enter it. Diurnal breezes associated with westerly winds were rarely recorded. Bottom layers were controlled by tide, except during an upwelling event.

Spiral chemical waves subjected to a spatiotemporal random excitability are experimentally and numerically investigated in relation to the light-sensitive Belousov-Zhabotinsky reaction. Brownian motion is identified and characterized by an effective diffusion coefficient which shows a rather complex dependence on the time and length scales of the noise relative to those of the spiral. A kinematically based model is proposed whose results are in good qualitative agreement with experiments and numerics.

Phase synchronization is shown to occur between opposite cells of a ring consisting of chaotic Lorenz oscillators coupled unidirectionally through driving. As the coupling strength is diminished, full phase synchronization cannot be achieved due to random generation of phase jumps. The Brownian dynamics underlying this process is studied in terms of a stochastic diffusion model of a particle in a one-dimensional medium.

A simple model of macroparasitic infections has been used to evaluate the potential use of parasites as biological tags of fish populations. In the model, the parasite-host interaction is regulated by a birth-death process, and parasites can only be acquired by the non-specific migratory host population in a particular area of the space domain. In this case, we show that parasites can be succesfully used for stocks identification and to describe the migratory routes taken by some marine fish species.

In real electrophysiological experiments, irregularities in the extracellular excitation spread are believed to depend on cardiac tissue microstructure. An electronic hardware model was developed to analyze this dependence by placing some inhomogeneities (slow propagation areas) in the medium. The position of such inhomogeneities is correlated with abnormal delays and irregularities measured in signal propagation.