Publicaciones

Currents, particle fluxes and ecology were studied in the Palamós submarine canyon (also known as the Fonera canyon), located in the northwestern Mediterranean. Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope. Additionally, local and regional hydrographic cruises were carried out. Current data showed that mean near surface and mid-depth currents were oriented along the mean flow direction (NE-SW), although at 400 and 1200 m depth within the canyon current reversals were significant, indicating a more closed circulation inside the canyon. Mean near-bottom currents were constrained by the local bathymetry, especially at the canyon head. The most significant frequency at all levels was the inertial frequency. A second frequency of about three days, attributed to a topographic wave, was observed at all depths, suggesting that this wave was probably not trapped near the bottom. The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow. In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher up-canyon from about 1200 m depth than offshore, defining, along with the different

Keywords: NW Mediterranean; Catalan continental shelf; Mesoscale; Anticyclone; Remote sensing.

Through the analysis of satellite thermal images, mesoscale anticyclonic eddies have been observed to recurrently drift along the NW Mediterranean coasts. The presence of these eddies is known to cause transient "inversions" of the shelf-slope current. By means of an exhaustive analysis of sea surface temperature images a group of anticyclonic structures were monitored in September 2001. One of these eddy-like features was tracked and intensely surveyed by means of conductivity-temperature-depth (CTD) casts and repeated fast surveys with Nm-Shuttle (oscillating CTD) and Acoustic Doppler Current Profiler (ADCP) measurements during the ET2001 oceanographic cruise. A well developed anticyclonic eddy was surveyed over the Catalan continental shelf, 60 km north of Barcelona. In situ velocity data revealed that the anticyclonic motion was characterized by surface velocities of 50 cm/s and a diameter of about 45 km. The signal of the eddy was found down to 100 m depth, well below the seasonal thermocline. The passage of the eddy modified the local flow, involving advection and subduction of surrounding waters.

Keywords: Economics; Business and financial markets; Structures and organization in complex systems; Fractals.

It has been repeatedly reported that time series of returns in stock markets are of multifractal (multiscaling) character. Recently,a direct geometrical framework,much more revealing about the underlying dynamics than usual statistical approaches,has been introduced. In this paper we use this geometrical method to undercover several aspects that concern the dynamics of stock market time series. We introduce and discuss a new,powerful processing tool,namely the computation of sources. With the aid of the source field,we will separate the fast,chaotic dynamics defined by the multifractal structure from a new, so-far unknown slow dynamics which concerns long cycles in the series. We discuss the results on the perspective of detection of sharp dynamic changes and forecasting.

Keywords: Edge detection, fractal, multifractal, multiscale processing, rain detection.

It is thought that satellite thermal infrared (IR) images can aid to the detection of precipitation, an interesting possibility due to the existence of geostationary satellites with thermal IR sensors which would enable a good spatial and temporal tracking of rain and storms. In this letter, we explore the application of multiscale/multifractal techniques in the design of new methods for the assessment and tracking of pluviometry. We first identify the main streamlines by a singularity analysis of the wavelet projections of the IR record. From the streamlines, we derive a proxy scalar image that represents the result of pure horizontal advection. From the comparison of original and proxy we localize the places at which horizontal advection fails, which we identify with convection places. We illustrate our methodology with thermal IR images from Metosat acquired during heavy tropical rainfall, and compare the results with some data from the Tropical Rainfall Measuring Mission satellite.

Multifractal or multiaffine analysis is a promising new branch of methods in nonlinear physics for the study of turbulent flows and turbulentlike systems. In this Letter we present a new method based on themultifractal singularity extraction technique, the maximum singular stream-function method (MSSM), which provides a first order approximation to the stream function from experimental data in 2D turbulent systems. The essence of MSSM relies in relating statistical properties associated with the energy cascade in flows with geometrical properties. MSSM is a valuable tool to process sparse collections of data and to obtain instant estimates of the velocity field. We show an application of MSSM to oceanography as a way to obtain the current field from sea surface temperature satellite images; we validate the result with independent dynamical information obtained from sea level measurements

A new derivation and interpretation of the semigeostrophic (SG) material invariant in the theory of geophysical flows is introduced. First, a generalized three dimensional equation of the SG dynamics is established and the generalized equations for the rate of change of vorticity and for the rate of change of the velocity gradient cofactor tensor are obtained. Next, a conservation equation for the vorticity-velocity gradient cofactor tensor (denoted $\tilde{\Xi}$) is derived. The specific potential $\tilde{\Xi}$, that is, $\tilde{\Xi}$ in the reference configuration per unit of mass, is defined and an expression for its rate of change is obtained. The SG invariant is interpreted as the vertical component of the specific potential $\tilde{\Xi}$. Under the SG assumptions (advection of the geostrophic velocity, hydrostatic, and f-plane approximations) this vertical component is materially conserved in the SG flow. The generalized SG invariant (i.e., the specific potential $\tilde{\Xi}$) differs conceptually from the Beltrami-Rossby-Ertel specific potential vorticity. Its conservation in the SG flow seems to be highly dependent on the SG assumptions, especially on the f-plane approximation and on the horizontal nature of the geostrophic velocity.

A historical set of expendable bathythermograph (XBT) and Pegasus sections across the Gulf Stream in natural coordinates is examined to investigate the isopycnic structure of the current off Cape Hatteras. In isopycnic-natural coordinates, the axis of the Stream remains vertically aligned, in contrast to its well-known offshore tilting when plotted as a function of depth. These results are confirmed using the geostrophic velocities obtained from a synthetic temperature field for the Gulf Stream. We prove that a baroclinic current aligned with density cannot be aligned with depth, and vice versa, and we show that the density alignment of the Gulf Stream results from the distortion of the density field and has negligible dependence on the choice of reference level. The invariable character of intense geophysical jets is supported through analogous representations for the upper level atmospheric jet stream in isentropic coordinates. These show that the atmospheric jet, when plotted on to a section normal to the direction of its maximum velocity core, is vertically aligned with potential temperature.

This synthesis, initiated during the meeting, was consolidated therafter by inputs received from the workshop participants, and in particular from John Allen, Isabel Ambar, Isabelle Taupier Letage, Marlon Lewis, Ananda Pascual and Volker Strass. Jordi Font and Frederic Briand, assisted by Carolyn Scheurle and Paula Moschella, took care of the final editing, while Valérie Gollino did oversee the physical production process

Keywords: Hydrographic cruises; Canary Current; Cape Ghir region.

We present hydrographic data for several sections located along the African coastline and off Cape Ghir, carried out at times of weak surface winds (October 1995 and September 1997). The main sections are near the continental slope, at mean distances between 100 and 150 km from the coastline. North of Cape Ghir (318N) the geostrophic transport (referenced to 650 m) of North Atlantic Central Water through these sections is 3.7 and 2.0 Sv for 1995 and 1997, respectively. This confirms that a major fraction of the water transport by the Canary Current flows east, into the continental slope off northwest Africa, at latitudes above Cape Ghir. Most of this flow continues south past Cape Ghir, along the coast and probably through the eastern passages of the Canary Archipelago. A significant fraction, however, may escape through surface Ekman transport (0.3-0.5 Sv during the early fall season) and by offshore flow at Cape Ghir (1.1 Sv in September 1997, referenced to 650 m). Despite the weak winds the Cape Ghir filament was clearly visible, characterized by localized coastal upwelling associated to a cyclonic shallow structure and cold (and fresh) waters stretching offshore as a very shallow feature (50-100 m deep). The satellite images show that the surface temperature field is highly variable, in rapid response to the surface winds, always with a core region of relatively cold water and commonly with one or two associated eddies. Our results support the existence of two recirculation cells in the area: a horizontal one that connects the interior eastern boundary currents with the coastal region and a vertical one related to both wind-induced and filament upwelling. The data also show a salinity subsurface maximum at the root of the filament, linked to water inflow from northern latitudes, and a subsurface anticyclonic eddy over the Agadir canyon, likely related to the poleward slope undercurrent.

Keywords: Coastal upwelling; Canary Current; Water recirculation; Organic matter.

The surface and upper-thermocline waters of the Canary Basin are characterised by very strong coupling between the open ocean and the coastal upwelling region. Such coupling has its origin in water inflow into the upwelling region north of the Canary Islands and its recirculation south along the continental slope, which is the true Canary Current. A portion of this recirculating water is intermittently exported offshore through surface filaments. During late fall, a major diversion takes place at Cape Ghir, allowing the presence of northward flow from Cape Blanc till Cape Yubi. The fraction of water that flows through the Canary Archipelago is the origin of intense mesoscale variability south of the Canary Archipelago, which interacts strongly with the coastal region. These physical characteristics are responsible of intense alongshore and vertical fluxes of nutrients and dissolved inorganic carbon within the upwelling region. Coastal filaments and cyclonic eddies cause localised offshore export of nutrients and organic matter, making possible that respiration be several times larger than production in the open ocean. A major characteristic of the ecosystem comes from the seasonal variation in the current pattern, allowing coastal convergence and intense transfer of coastal properties to the open ocean during late fall.

Keywords: image processing, shape characterisation, shape descriptors.

The present study reviewed some of the critical pre-processing stops required for otolith shape characterisation for automatic classification with heterogeneous distributed data. A common procedure for optimising automatic classification is to apply data pre-processing in order to reduce the dimension of vector inputs. One of the key aspects of these pre-processing methods is the type of codification method used for describing the otolith contour. Two types of codification methods (Cartesian and Polar) were evaluated, and the limitations (loss of information and the benefits (invariance to affine transformations) associated with each method were pointed out. The comparative study was developed using four types of shape descriptors ( morphological, statistical, spectral and multiscale), and focused on data codification techniques and their effects on extracting shape features for automatic classification. A new method derived from the Karhunen-Loève transformation was proposed as the main procedure for standardising the codification of the otolith contours.

Keywords: Marine algae; Toxicity; Allelopathy; Mixed cultures; Fitting.

Allelopathy among microalgae has been modelled in order to fit the dynamics of mixed cultures of Chrysocromulina polylepis and Heterocapsa triquetra, carried out with three different initial concentrations of the allelopathic species C. polylepis. The experimental data have been analysed with a simple Lotka-Volterra type model including an allelopathic term. Based on the population dynamics of the two species in unialgal and mixed cultures, the model has been simplified and solved analytically. The best fit of the allelopathy parameter (gamma ≈ 10-5 to 10-6) provides a good agreement between the theoretical curve and the experimental data for the two highest initial concentrations of C. polylepis but, interestingly, shows a less accurate fit for low initial concentrations of the toxic alga. Following the same procedures, a modified model, in which the allelopathic effect is dependent on the square of the concentration of the toxic alga, has been adjusted to the experimental data. This modified model presents a good fit for all the range of initial concentrations of the toxic alga. These results allow a quantification of the strength of the allelopathic interaction between two marine phytoplankton species. The growth curve of the non-toxic alga is significantly affected by the allelopathy term only after the toxic alga has reached relatively high concentrations. This supports field observations suggesting that, at least in the case of C. polylepis, allelopathy may be important once a bloom is well developed, but is not likely to be a key factor in initial phases of the proliferation.

Keywords: Spatio-temporal variation; Chlorophyll a fractionation; Picoplankton; Nanoplankton; Microplankton; Ebro river shelf; Northwestern Mediterranean..

The mesoscale distribution and seasonal variation of the size structure of phytoplankton biomass, as measured by chlorophyll a (chl a), was studied in the Ebro shelf area (NW Mediterranean) during three different seasons: autumn, winter and summer. In autumn and summer, when the water column was, respectively, slightly or strongly stratified and nutrient concentrations were low at surface, average total chl a values were 0.31 and 0.29 mg m-3, respectively. In winter, the intrusion of nutrients into the photic zone by intense vertical mixing and strong riverine inputs, produced an increase of the total autotrophic biomass (0.76 mg m-3). In the three seasons, the main contributor to total chl a was the picoplanktonic (<2 mi m) size fraction (42% in winter and around 60% in autumn and summer). The nanophytoplankton (2–20 mi m) contribution to total chl a showed the lowest variability amongst seasons (between 29% and 39%). The microplanktonic (>20 mi m) chl a size fraction was higher in winter (27%) than in the other seasons (less than 13%). The maximum total chl a concentrations were found at surface in winter, at depths of 40 m in autumn and between 50 and 80 m in summer. The relative contribution of the <2 mi m size fraction at these levels of the water column tended to be higher than at other depths in autumn and winter and lower in summer. In autumn and winter, nutrient inputs from Ebro river discharge and mixing processes resulted in an increase on the >2 mi m contribution to total chl a in the coastal zone near the Ebro Delta area. In summer, the contribution of the <2 and >2 mi m chl a size fractions was homogeneously distributed through the sampling area. In autumn and summer, when deep chl a maxima were observed, the total amount of the autotrophic biomass in the superficial waters (down to 10 m) of most offshore stations was less than 10% of the whole integrated chl a (down to 100 m or to the bottom). In winter, this percentage increased until 20% or 40%. The >2 mi m chl a increased linearly with total chl a values. However, the <2 mi m chl a showed a similar linear relationship only at total chl a values lower than 1mg m-3 (in autumn and summer) or 2 mg m-3 (winter). At higher values of total chl a, the contribution of the <2 mi m size fraction remained below an upper limit of roughly 0.5 mg m-3. Our results indicate that the picoplankton fraction of phytoplankton may show higher seasonal and mesoscale variability than is usually acknowledged.

Different works appearing during the last decade show that the western Mediterranean has suffered a temperature and salinity increase during the 20th century. Most of these works analyze long-term trends in Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW); those dealing with changes in shallow and coastal waters are very scarce. It is still an open question whether these changes are due to meteorological factors occurring in the western Mediterranean or if they are caused by the salinity increase of the water masses contributing to deep water formation. In this work we analyze the data obtained in the last decade of the 20th century (and longer time series in some cases) within the frame of six projects aimed at the systematic collection of hydrographic data at fixed stations in the northern sector of the western Mediterranean (mainly in coastal areas). We detect strong warming trends in those stations located in the continental shelf (and probably with no influence of the LIW). This result could indicate that changes observed in the WMDW cannot be explained only on the basis of changes imported from the eastern basin. Another striking result is that these trends are an order of magnitude higher than those reported for the rest of the century, indicating that the 1990s have been an exceptionally warm decade. On the other hand, time series affected by the LIW show a salinity increase, and in some cases this is not accompanied by the corresponding temperature increment, indicating that the LIW salinity increase could also be a factor to be taken into account.

We use the trajectory of three buoys dragged below the surface mixed layer, together with sea surface temperature imagery, to examine the evolution of an anticyclonic warm-core eddy since its generation by the Canary Islands. Two buoys remain within the eddy during some 100 days, and the third one remains almost 200 days, while drifting southwestward up to 500 km with the mean Canary Current. The eddy merges with several younger anticyclonic and cyclonic eddies, in each occasion, suffering substantial changes. The eddy core, defined as a region with near-solid-body-ype rotation and radial convergence, initially occupies the whole eddy. After interacting with another vortex the inner core markedly slows down, although it continues displaying radial convergence and relatively small radial oscillations, and an uncoupled outer ring is formed or enhanced, which revolves even more slowly and displays large radial fluctuations. The vortex extensive life is consistent with its inertially stable character and observations of radial convergence. A very simple model of vortex merging, where cylinders fuse conserving mass and angular momentum, gives fair results. The observations suggest that the eddy changes, as the result of its own slow evolution and sporadic mixing events, from a young stage, where the core retains its vorticity and occupies most of the eddy, through a mature stage, where the eddy has a reduced inner core and a slowly revolving outer ring, to a decay stage, where the vorticity maximum is substantially reduced.

Keywords: Curvature scale space, fish otolith, Fourier harmonic, shape análisis, wavelet transform.

Fish otolith morphology has been closely related to landmark selection in order to establish the most discriminating points that can help to differentiate or find common characteristics in sets of otolith images. Fourier analysis has traditionally been used to present otolith images, since it can reconstruct a version of the contour that in close to the original by choosing a reduced set of harmonic terms. However, it is difficult to locate the contour’s singularities from this spectrum. As an alternative, wavelet transform and curvature scale space representation allow us to quantify the irregularities of the contour and determine its precise position. These properties make these techniques suitable for pattern recognition purposes, ageing, stock determination and species identification Studies. In the present study both techniques are applied and used in an otolith classification system that shows robustness against affine image transformations, shears and the presence of Boise. The results are interpreted and discussed in relation to traditional morphology studies.

The last generation of satellites leads to the very high-resolution images which offer a high quality of detailed information about the Earth\'s surface. However, the exploitation of such images becomes more complicated and less efficient as a consequence of the great heterogeneity of the objects displayed. In this paper, we address the problem of edge-preserving smoothing of high-resolution satellite images. We introduce a novel approach as a preprocessing step for feature extraction and/or image segmentation. The method we propose is related with the idea of resolution reduction and is derived from the multifractal formalism used for image compression. First, a multifractal decomposition scheme allows to extract the most singular transitions of the image. Then, an entropy-based criterium enables to consider a particular manifold composed with the most, simultaneously, relevant and singular pixels. Finally, a reconstruction scheme performed over this manifold provides an approximation of the original image. Such an approach is ideal, as it assumes that objects can be reconstructed from their boundary information, and it provides presegmented images where the main structures are preserved

Per poder diagnosticar la situació en què es troben les poblacions explotades en una determinada regió cal disposar de la màxima informació possible dels patrons d’explotació: captures, esforç, arts emprats, etc., així com de la biologia de cadascuna de les espècies implicades: cicles de vida, època de reproducció, hàbitats, etc. Clàssicament, amb aquesta informació, actualitzada i obtinguda al llarg del temps (quan més llarga la sèrie, millor), es pot realitzar una avaluació raonable de la situació en què es troba el recurs, poder donar pautes d’explotació que siguin sostenibles i fer previsions de la seva evolució. Així doncs, el paper del científic en aquest procés s'ha adreçat essencialment a d’una banda ampliar el coneixement de la biologia de les espècies al mateix temps que s’analitzaven les sèries històriques de dades de l'explotació, per extreure’n el diagnòstic i facilitar al gestor les seves tasques reguladores que haurien de conduir cap a una explotació sostenible.