Emilio García-Ladona

Identification of Marine Eddies from Altimetric Maps

A procedure is presented to detect eddy cores from sea level anomaly (SLA) maps obtained from altimetric measurements. The method is based on finding the sign of Q, which is an invariant of the velocity gradient tensor (=u). This parameter, commonly used in studies of two-dimensional turbulence, measures the relative contribution of deformation and vorticity. Vortex cores are associated with regions with large and positive values of Q. It is shown that this parameter is adequate to detect marine eddies in altimetric maps and, when the geometry of streamline contours in SLA maps is unclear, the Q . 0 criterion appears to work more consistently. The performance of the methodology is applied, as an example, to altimetric maps of the Algerian Basin in the Western Mediterranean Sea, where two long-lived eddies are tracked. The observed trajectories are in good agreement with Advanced Very High Resolution Radiometer (AVHRR) images.

Vortices of the Mediterranean Sea: An Altimetric Perspective

The presence of coherent vortices makes observed mesoscale fields of the ocean resemble twodimensional turbulence. Using this analogy, a common definition of a coherent structure has been used to study the statistical properties of Mediterranean Sea vortices observed by satellite altimeters over a 7-yr period. A vortex has been defined as the simply connected region with values of the Okubo–Weiss parameter W 0.2W, where W is the spatial standard deviation of W, and the same sign of vorticity. This definition is shown to be appropriate to detect and characterize, statistically, properties such as size, mean kinetic energy, and amplitude of vortices in the Mediterranean basin from sea level anomaly maps corresponding to the period from October 1992 to October 1999. The distribution of such properties for the Mediterranean vortices suggests a heuristic criterion to extract and select very coherent and long-lived vortices from the whole set of structures identified in altimetric maps. Such coherent vortices appear to be selected for amplitudes greater than 2W, where the amplitude has been defined in terms of the Okubo– Weiss parameter rather than vorticity, and strongly correspond to those reported from observations with independent data. Systematic locating and tracking of such vortices provide, for the first time, a general picture of their preferential paths in the Mediterranean basin, which are characterized by complex but rather well defined patterns.

Otolith shape contour analysis using affine transformation invariant wavelet transforms and curvature scale space representation

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 represent otolith images, since it can reconstruct a version of the contour that is close to the original by choosing a reduced set of harmonic terms. However, it is difficult to locate the contours 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 noise. The results are interpreted and discussed in relation to traditional morphology studies.

Microcanonical multifractal formalism: Application to the estimation of ocean surface velocities

In this paper we investigate the validity of the multifractal formalism to study sea surface temperature (SST). It is shown that SST patterns observed in moderate resolution SST images have anomalous scaling properties characteristic of a multifractal structure. The most probable origin of the observed structures is the turbulent character of the oceanic flow as they evolve slowly and are very persistent in times compatible with ocean mesoscale dynamics (several days). The spectrum of singularity exponents indicates that the dynamics of the processes leading to the geometrical arrangement of the SST patterns is quite general over the available range of scales. As a consequence, multifractal techniques can be used to extract properties of the underlying flow. In particular, the geometry of the SST multifractal components is closely linked with the ocean flow, which allows to build a reasonable guess of the streamfunction (defined as the maximum singular streamfunction (MSS)) from a single SST image. Thus the ocean surface velocity field can be easily inferred, with some limitations. As multifractal analysis is in essence a geometrical approach, the method is able to retrieve a high resolution velocity field, well localized in space, but with some indetermination on the modulus and sense of velocity vectors. To solve this, a general framework for the integration of extra information is proposed, what is illustrated with an example merging MSS with altimetric data.

Statistical analysis of the surface circulation in the Algerian Current using Lagrangian buoys

Abstract The Algerian Current (AC) is one of the most energetic flows in the Mediterranean basin. A characteristic picture of this current is formed by a series of mesoscale eddies at different scales. Here, statistical analysis of 15 surface Argos buoy tracks in 1996–1997 provides a complete Lagrangian view of the AC. The buoys, released upstream and across a coastal meander between 0°E and 1°E longitude, were followed for 3 months. They travelled eastward at an average speed of 14 cm/s and showed high energetic fluctuations related to several mesoscale eddies. The characteristic integral time and space scales are highly anisotropic. For the zonal component, these are about 4 days and 66 km, and for the meridional component, about 2 days and 26 km. Representative values of effective diffusivities from single dispersion statistics are within 0.7–1.3×10 8 and 1.5–6.0×10 7 cm 2 /s for the zonal and meridional directions, respectively. A local analysis shows that mesoscale motions are particularly relevant in the region 1–3°E and 7–8°E, provided the considerably high values of eddy kinetic energy in comparison with the mean kinetic energy. Eddy–mean current interactions are evidenced by the significant changes of sign of horizontal covariance from west to east. Finally, an Eulerian picture of the AC is built, exhibiting similar trends than previous and recent field observations.

Otolith shape feature extraction oriented to automatic classification with open distributed data

The present study reviewed some of the critical pre-processing steps required for otolith shape char- acterisation 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 infor- mation) 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-Loeve transformation was proposed as the main procedure for standardising the codification of the otolith contours.

Non-Gaussian Velocity Probability Density Functions: An Altimetric Perspective of the Mediterranean Sea

Abstract Velocity probability density functions (PDFs) are a key tool to study complex flows and are of great importance to model particle dispersion. The PDFs of geostrophic velocities derived from sea level anomalies maps for the Mediterranean Sea have been computed and analyzed, guided by recent results found in studies of two-dimensional and geostrophic turbulence. At the basin scale results show that the geostrophic velocity PDF derived from SLA maps is non-Gaussian. To understand the origin of this non-Gaussianity, a topological partition of the flow based on the Okubo–Weiss parameter is applied to separate the contribution of coherent vortices from the background field. After such separation the non-Gaussian part of the PDF appears mostly associated with the presence of such structures. Only about 20% of the vortices identified in the dataset are mainly responsible for this deviation. These vortices, called intense vortices in previous works, are those vortices with values of the amplitude larger t...

The use of surface drifting floats in the monitoring of oil spills. The Prestige case

Comunicacion presentada en VERTIMAR-2005, Simposio sobre Vertidos Accidentales de Petroleo en el Medio Marino = Symposium on Marine Accidental Oil Spills, celebrada del 13 al 16 de julio de 2005 en Vigo (Espana)

Wavelet Filtering to Extract Coherent Vortices from Altimetric Data

Abstract Because of the optimal features of wavelet processing, the use of wavelets for describing and analyzing signals in 2D turbulence has been generalized since a decade ago. In spite of the close analogy between 2D turbulence and geophysical fluid dynamics, few works have tried to generalize the rich framework of wavelet techniques to the study of experimental signals in oceanography. In this paper, the authors extend a prominent wavelet technique designed for the study of direct numerical simulations (DNSs) on 2D turbulence, the coherent vortex simulation, and analyze with it ocean velocity fields obtained from sea surface height maps derived from satellite altimetry. The authors demonstrate the pertinence of this technique to describe altimetry data, resulting in a description of oceanic flows with a reduced number of degrees of freedom. In particular, it is shown that the western Mediterranean circulation is well approximated by a field of extracted coherent vortices when an appropriate wavelet ba...

Detection of wave fronts in the Indian Ocean from geostationary sunglint satellite imagery

We demonstrate the potential of monitoring coherent wave fronts on the tropical ocean surface using an advanced detection algorithm applied to visible sunglint‐affected imagery from geostationary satellites. Data from MeteoSat are processed with the advanced analysis technique to reveal transient wave activity near and to the west of the Mascarene Ridge of the western Indian Ocean. The technique is based on a wavelet‐based singularity analysis which enhances the subtle but highly coherent wave patterns in the meteorological satellite data. It is likely that the patterns represent internal ocean waves, a pathway to ocean mixing and climate. The prospect for using this detection algorithm in greatly expanded satellite global surveys of wave activity is outlined.