C. López

Institute for Cross-Disciplinary Physics and Complex Systems, Palma, Balearic Islands, Spain

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Publications (14)16.7 Total impact

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    ABSTRACT: The minimal ecological requirements for formation of regular vegetation patterns in semiarid systems have been recently questioned. Against the general belief that a combination of facilitative and competitive interactions is necessary, recent theoretical studies suggest that, under broad conditions, nonlocal competition among plants alone may induce patterns. In this paper, we review results along this line, presenting a series of models that yield spatial patterns when finite-range competition is the only driving force. A preliminary derivation of this type of model from a more detailed one that considers water-biomass dynamics is also presented. Keywords: Vegetation patterns, nonlocal interactions
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 02/2014; To appear.. · 2.86 Impact Factor
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    ABSTRACT: Population dynamics of individuals undergoing birth and death and diffusing by short- or long-range two-dimensional spatial excursions (Gaussian jumps or Lévy flights) is studied. Competitive interactions are considered in a global case, in which birth and death rates are influenced by all individuals in the system, and in a nonlocal but finite-range case in which interaction affects individuals in a neighborhood (we also address the noninteracting case). In the global case one single or few-cluster configurations are achieved with the spatial distribution of the bugs tied to the type of diffusion. In the Lévy case long tails appear for some properties characterizing the shape and dynamics of clusters. Under nonlocal finite-range interactions periodic patterns appear with periodicity set by the interaction range. This length acts as a cutoff limiting the influence of the long Lévy jumps, so that spatial configurations under the two types of diffusion become more similar. By dividing initially everyone into different families and following their descent it is possible to show that mixing of families and their competition is greatly influenced by the spatial dynamics.
    Physical Review E 04/2012; 85(4 Pt 1):041105. · 2.31 Impact Factor
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    ABSTRACT: We study three dimensional oceanic Lagrangian Coherent Structures (LCSs) in the Benguela region, as obtained from an output from the ROMS model. To this end we first compute Finite-Size Lyapunov exponent (FSLE) fields in the region volume, characterizing mesoscale stirring and mixing there. LCSs are extracted as ridges of the calculated FSLE fields. They present a "curtain-like" geometry in which the strongest attracting and repelling structures appear as quasivertical surfaces. LCSs around a particular cyclonic eddy, pinched off from the upwelling front are also calculated. The LCSs are confirmed to provide pathways and barriers to transport in and out of the eddy.
    04/2012;
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    ABSTRACT: In recent years it appears that meso- and submesoscale features (fronts, eddies, filaments) in surface ocean flow have a crucial influence on marine ecosystems. Their dynamics partly control the foraging behaviour and the movements of marine top. One of the challenges in ecology is to define critical habitats and understand the rules of habitat selection. Recently new tools for detection of coherent structures at submesoscale open the way for new studies never investigated before in marine ecology. Through two examples we highlight novel research on the importance of submesoscale structures for the spatial distribution of marine top predators. We studied two seabird populations with contrasting characteristics: Frigatebirds in the Mozambique Channel, and Cape gannets in the Benguela upwelling off southern Africa. Frigatebirds are mainly offshore birds while Cape gannets do not venture beyond the continental shelf. For these two studies, we used products derived from remote sensing data, to describe submesoscale coherent structures (<10km). In the first example, using Finite-Size Lyapunov Exponents (FSLE), we have identified Lagrangian coherent structures (LCSs) present in the surface flow of the Mozambique Channel resulting from an intense mesoscale activity. By comparing seabird satellite positions with LCSs locations, we demonstrate that frigatebirds track precisely these structures in the Mozambique Channel, providing the first evidence that a top predator is able to track these FSLE ridges to locate food patches. Although many questions remain unanswered, this work remains a pioneering on this topic. Despite the interest of FSLE, they are limited to offshore areas due to altimetry products limitation on continental shelves. However, many seabirds operate in coastal areas undergoing stronger anthropogenic pressures, such as Cape gannets off South Africa. The Benguela system is characterized by an upwelling inhabited by numerous fronts and filaments that very likely have a strong impact on seabird spatial distribution, and it was not possible to implement FSLE in this area. Thus, we used a newly developed method based on the singularity exponents calculated from remotely sensed data of ocean colour and SST, which delivers a precise picture of meso and submeso-scale structures off the coast. Preliminary results on understanding the rules of habitat selection by Cape gannets, and contribution to the conservation of this endangered species will be presented.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: A biological competition model where the individuals of the same species perform a two-dimensional Markovian continuous-time random walk and undergo reproduction and death is studied. The competition is introduced through the assumption that the reproduction rate depends on the crowding in the neighborhood. The spatial dynamics corresponds either to normal diffusion characterized by Gaussian jumps or to superdiffusion characterized by L\'evy flights. It is observed that in both cases periodic patterns occur for appropriate parameters of the model, indicating that the general macroscopic collective behavior of the system is more strongly influenced by the competition for the resources than by the type of spatial dynamics. However, some differences arise that are discussed.
    EPL (Europhysics Letters) 08/2010; 92(4). · 2.27 Impact Factor
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    ABSTRACT: Nonlinear phenomena are essential ingredients in many oceanic and atmospheric processes, and successful understanding of them benefits from multidisciplinary collaboration between oceanographers, meteorologists, physicists and mathematicians. The present Special Issue on ``Nonlinear Processes in Oceanic and Atmospheric Flows'' contains selected contributions from attendants to the workshop which, in the above spirit, was held in Castro Urdiales, Spain, in July 2008. Here we summarize the Special Issue contributions, which include papers on the characterization of ocean transport in the Lagrangian and in the Eulerian frameworks, generation and variability of jets and waves, interactions of fluid flow with plankton dynamics or heavy drops, scaling in meteorological fields, and statistical properties of El Ni\~no Southern Oscillation. Comment: This is the introductory article to a Special Issue on "Nonlinear Processes in Oceanic and Atmospheric Flows'', published in the journal Nonlinear Processes in Geophysics, where the different contributions are summarized. The Special Issue itself is freely available from http://www.nonlin-processes-geophys.net/special_issue103.html
    06/2010;
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    ABSTRACT: The Lagrangian description of fluid transport has been largely enriched with the introduction of stretching quantifiers such as the different types of Lyapunov exponents. Here we will focus on the finite-size Lyapunov exponent case, as applied to horizontal ocean flows estimated from satellite altimetry. The influence on biological organisms of the structures revealed by the Lyapunov analysis will be illustrated with examples from the bottom and from the top of marine ecosystems: the distribution of phytoplankton in ocean upwelling areas, and the trajectories of frigatebirds in the West Indian Ocean, which fly on top of the ridges of Lyapunov structures. The convenience of moving from a two-dimensional to a fully three-dimensional description is highlighted.
    Proceedings of the 3rd Conference on Nonlinear Science and Complexity, Ankara, July 2010; 01/2010
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    ABSTRACT: Eastern Boundary Upwelling Systems (EBUS) are characterized by a high productivity of plankton associated with large commercial fisheries, thus playing key biological and socio-economical roles. The aim of this work is to make a comparative study of these four upwelling systems focussing on their surface stirring, using the Finite Size Lyapunov Exponents (FSLEs), and their biological activity, based on satellite data. First, the spatial distribution of horizontal mixing is analysed from time averages and from probability density functions of FSLEs. Then we studied the temporal variability of surface stirring focussing on the annual and seasonal cycle. There is a global negative correlation between surface horizontal mixing and chlorophyll standing stocks over the four areas. To try to better understand this inverse relationship, we consider the vertical dimension by looking at the Ekman-transport and vertical velocities. We suggest the possibility of a changing response of the phytoplankton to sub/mesoscale turbulence, from a negative effect in the very productive coastal areas to a positive one in the open ocean. Comment: 12 pages. NPG Special Issue on "Nonlinear processes in oceanic and atmospheric flows". Open Access paper, available also at the publisher site: http://www.nonlin-processes-geophys.net/16/557/2009/
    Nonlinear Processes in Geophysics 09/2009; · 1.69 Impact Factor
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    ABSTRACT: The oceanic submeso and mesoscale circulation and its eddies, filaments, meanders play a major role in marine ecosystems dynamics from the lower trophic levels to the marine top predators. We study here the interplay between turbulence in fluid dynamics on these scales and biological activity at different trophic levels using two cases study. The first example focuses on the four eastern boundary upwelling zones, the Canary, Benguela, California and Humboldt upwelling systems which constitute the largest contribution to the world ocean productivity. These areas are spatially heterogeneous, populated with a large variety of mesoscale and sub-mesoscale structures such as filaments, plumes and eddies, which control exchange processes between the shelf and open ocean and play a major role in modulating the biomass, rates and structure of marine ecosystems. We will present here results from a lagrangian approach based on Finite Size Lyapunov Exponents (FSLE) using altimetric and scatterometric data to estimate the spatial and temporal variations in the lateral stirring and mixing of tracers in the upper ocean within the four areas. When investigating links with chlorophyll a concentration as a proxy for biological activity in these upwelling systems, results show that surface horizontal stirring and mixing vary inversely with chlorophyll standing stocks. FSLEs lead to a clear clustering of the systems suggesting that one may use them as integrated and comparative indices for characterizing horizontal dynamical features in all eastern boundary upwellings. Then we investigate the role of submesoscale structures in the Mozambique Channel on the distribution of a top marine predator, the Great Frigatebird. Using similar dynamical concept, the FSLE, we have identified Lagrangian Coherent Structures (LCSs) present in the surface flow in the Channel. By comparing seabirds' satellite positions with LCSs locations, we demonstrate that frigatebirds track precisely these structures in the Mozambique Channel, providing the first evidence that a top predator is able to track these FSLE ridges to locate food patches. We propose several hypotheses to understand how frigatebirds can follow these LCSs. A thorough comprehension of their foraging behaviour and movement during the breeding season is crucial not only to seabirds' ecology but also to an appropriate ecosystemic approach of fisheries in the Channel.
    03/2009; 11:1260.
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    ABSTRACT: We have analyzed surface velocity data of the Mediterranean Sea as obtained from a primitive equation circulation model. We have computed the Finite Size Lyapunov Exponents (FSLEs) from this data set, which provides a measure of oceanic horizontal stirring. In order to study the intermittency and its impact in transport and mixing, the robustness and other statistical properties, the FSLEs are computed at different spatial resolutions, both above and below that of the velocity field. In particular we have analyzed: i) the robustness of the FSLEs by introducing a small error in the velocity data, ii) the influence of the eddy diffusivity in the filamental and vortex structures, and iii) the multifractal character of the spatial distribution of the FSLEs.
    03/2009; 11:13976.
  • Nonlinear Processes in Geophysics 01/2009; 16:557-568. · 1.41 Impact Factor
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    ABSTRACT: We summarize recent work on the location of Lagrangian structures in velocity fields obtained from realistic simulations and from satellite altimetry of the surface layers of the Mediterranean sea. Finite-size Lyapunov exponents are found to be useful quantities to characterize stretching and compressing structures, and their implications for mixing. Direct calculation of hyperbolic points and their stable and unstable manifolds identifies ocean structures, and allows the use of the tools of lobe dynamics to characterize fluid transport. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    PAMM 10/2008; 7(1):1101307 - 1101308.
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    ABSTRACT: We study the interplay of hydrodynamic mesoscale structures and the growth of plankton in the wake of an island, and its interaction with a coastal upwelling. Our focus is on a mechanism for the emergence of localized plankton blooms in vortices. Using a coupled system of a kinematic flow mimicking the mesoscale structures behind the island and a simple three component model for the marine ecosystem, we show that the long residence times of nutrients and plankton in the vicinity of the island and the confinement of plankton within vortices are key factors for the appearance of localized plankton blooms.
    Nonlinear Processes in Geophysics 03/2008; · 1.69 Impact Factor
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    ABSTRACT: We present a comparative study of the horizontal mixing properties, from satellite derived data of the surface velocity field, of the two eastern boundary Canary and Benguela upwelling systems, based on a Finite Size Lyapunov Exponents analysis. Each area can be subdivided into two subsystems attending to their mixing activity values. These coincide nicely with distinct biological signatures. When investigating links with chlorophyll as a proxy for biological activity in these two upwelling systems, results show that surface horizontal stirring and mixing vary inversely with chlorophyll standing stocks. Ekman-transport induced upwelling exhibits a positive correlation with chlorophyll. These two findings are complementary since spatial structure in plankton distributions results from both dynamics of the 3D turbulent medium and of the marine ecosystem.
    Geophysical Research Letters 01/2008; 35:L11602. · 4.46 Impact Factor