Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin

Transport and mixing properties of surface currents can be detected from altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have the advantage of exploiting both spatial and temporal variability of the velocity field and are in principle able to unveil subgrid filaments generated by chaotic stirring. However, one may wonder whether this theoretical advantage is of practical interest in real-data, mesoscale and submesoscale analysis, because of the uncertainties and resolution of altimetric products, and the non-passive nature of biogeochemical tracers. Here we compare the ability of standard Eulerian diagnostics and the finite-size Lyapunov exponent in detecting instantaneaous and climatological transport and mixing properties. By comparing with sea-surface temperature patterns, we find that the two diagnostics provide similar results for slowly evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary process occuring along the Algerian current and above the Balearic Abyssal Plain. Such filaments are also observed, with some mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov exponents do not show any compact relation with other Eulerian diagnostics, unveiling a different structure even at the basin scale. We conclude that filamentation dynamics can be detected by reprocessing available altimetric data with Lagrangian tools, giving insight into (sub-)mesoscale stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics

Vortices of the Mediterranean Sea: An Altimetric Perspective

17 pages, 12 figures, 2 tablesThe presence of coherent vortices makes observed mesoscale fields of the ocean resemble two-dimensional 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 < −0.2σW, where σW 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 2σW, 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 patternsThis is a contribution to the IMAGEN project funded by the Spanish R+D Plan (REN2001-0802-C02-02) and MERSEA project funded by the European Union 6th Framework Program (AIP3-CT-2003-502885). Jordi Isern-Fontanet has been partially supported by contracts from IMAGEN and MERSEA projects. Altimetric maps for the period analyzed were elaborated and provided by CLS (Toulouse, France) under contract of the MATER project funded by the European Union MAST Program (MAS3-CT96-0051)Peer reviewe

Limitaciones de los parámetros de las funciones de predación. Implicaciones para la iniciación de las proliferaciones de fitoplancton

Phytoplankton blooms are events of production and accumulation of phytoplankton biomass that influence ecosystem dynamics and may also have effects on socio-economic activities. Among the biological factors that affect bloom dynamics, prey selection by zooplankton may play an important role. Here we consider the initial state of development of an algal bloom and analyse how a reduced grazing pressure can allow an algal species with a lower intrinsic growth rate than a competitor to become dominant. We use a simple model with two microalgal species and one zooplankton grazer to derive general relationships between phytoplankton growth and zooplankton grazing. These relationships are applied to two common grazing response functions in order to deduce the mathematical constraints that the parameters of these functions must obey to allow the dominance of the lower growth rate competitor. To assess the usefulness of the deduced relationships in a more general framework, the results are applied in the context of a multispecies ecosystem model (ERSEM).Las proliferaciones de fitoplancton son eventos de producción y acumulación de biomasa de fitoplancton que tienen una fuerte influencia tanto en la dinámica del ecosistema como en actividades socioeconómicas. Entre los factores biológicos que afectan la dinámica de las proliferaciones fitoplanctónicas, la selección de presas por el zooplancton puede jugar un papel importante. En este trabajo consideramos el estado inicial de desarrollo de una proliferación algal considerando que la presión de la predación puede permitir a una especie de algas con una menor tasa de crecimiento intrínseco que un competidor, ser dominante. Utilizamos un modelo sencillo con dos especies de microalgas y un herbívoro (zooplancton) para deducir relaciones generales entre el crecimiento del fitoplancton y la predación. Estas relaciones se aplican a dos funciones de respuesta de predación para deducir las limitaciones matemáticas que los parámetros de estas funciones deben obedecer. Para evaluar la utilidad de las relaciones deducidas en un marco más general, los resultados se aplican a un modelo de múltiples especies del ecosistema marino (ERSEM)

Multifractal Method for the Instantaneous Evaluation of the Stream Function in Geophysical Flows

4 pages, 3 figuresMultifractal 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 the multifractal 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. © 2005 The American Physical SocietyThis is a contribution to IMAGEN (Spanish R+D Plan: REN2001-0802-C02-02) and MERSEA projects (EU AIP3-CT-2003-502885). A. Turiel is contracted under the Ramon y Cajal program by the Spanish Ministry of Education. J. Isern-Fontanet is funded by the IMAGEN and MERSEA projectsPeer Reviewe

Modelling the deep-chlorophyll maximum: A coupled physical-biological approach

The Deep Chlorophyll Maximum (DCM) is simulated in two oligotrophic regions (SW Sargasso Sea and NW Mediterranean) using a physical/biological model that couples an upper ocean turbulent model to a nutrient/phytoplankton model. The biological model considers two types of primary producers, heterotrophs and atmospheric in addition to internal nitrate inputs. Model results appear to adequately reproduce the DCM structure in those regions. The DCM depth and magnitude is mainly determined by the vertical eddy diffusion and light extinction. The grazing parameters mainly affect the intensity of the DCM. This suggest the DCM is primarily the result of a balance between upward nutrient flux and light field characteristics. Consequently, the regenerated production only plays a secondary role

Variabilidad de las masas de agua a pequeña escala en un cañón submarino (cañón del Besòs) en el NO del mar Mediterráneo

In this work we report short-term measurements of the thermohaline structure and velocity field inside a narrow submarine canyon by means of a yo-yo–like profiler. An Aqualog profiler was deployed inside the Besòs Canyon in the northwestern Mediterranean continental margin, providing a unique data set on the vertical evolution of water column characteristics with unprecedented fine-scale spatial and temporal resolution. The observations reported here show a very dynamic transient short-term response with a complex vertical structure not observed previously in any submarine canyon of this region. The vertical distribution of water masses was characteristic of the western Mediterranean basin with Atlantic waters (AW) at the surface, Western Intermediate waters (WIW) in the middle and Levantine Intermediate (LIW) waters below. Turner angle and empirical orthogonal functions show that double-diffusive and isopycnal mixing are the main dominant processes at small scales. The interfaces of the three layers exhibit highly vertical excursions in relatively short times. At the surface, deepening of AW was observed, associated with flow intensification events. Deeper in the water column, within the submarine canyon confinement, the WIW-LIW interface uplifts about 100-150 m. These motions are associated with relatively up- and down-canyon–enhanced current events (up to 15-20 cm s-1 at 500 and 800 m depths) along the canyon axis. The time scales of the vertical variability were concentrated in a broad band around the semi-diurnal and local inertial frequencies within the WIW and LIW layers.En este trabajo se muestran medidas a corto plazo de la estructura termohalina y del campo de velocidad dentro de un estrecho cañón submarino a través de un perfilador tipo yo-yo. Este perfilador Aqualog se desplegó dentro del cañón del Besòs en el noroeste del margen continental del Mediterráneo, proporcionando un conjunto de datos único sobre la evolución vertical de las características de la columna de agua, con una resolución espacial y temporal a escala fina sin precedentes. Las observaciones aportadas aquí muestran una respuesta de un periodo transitorio muy dinámico con una estructura vertical compleja no observada anteriormente en ningún cañón submarino de esta región. La distribución vertical de las masas de agua es característica de la cuenca mediterránea occidental con aguas del Atlántico (AW) en la superficie, aguas occidentales Intermedias (WIW) debajo de estas y aguas de Levante Intermedio (LIW) por debajo. Los ángulos de Turner calculados y sus EOFs muestran que la estabilidad a la difusión y mezcla isopicna son los principales procesos dominantes a escalas pequeñas. Las interfaces de las tres capas exhiben excursiones notablemente verticales en tiempos relativamente cortos. En la superficie, la profundización de AW se observó asociada a eventos de intensificación del flujo. A más profundidad en la columna de agua, dentro del régimen de cañón submarino, la interfase WIW-LIW se eleva unos 100-150 m. Tales movimientos se asocian al flujo de agua hacia arriba y hacia abajo del cañón (hasta 15-20 cm s-1 a 500 y 800 m de profundidad) a lo largo del eje del cañón. Las escalas de tiempo de variabilidad vertical aparecen concentradas en una amplia banda de frecuencias alrededor de los periodos inerciales semidiurnos y locales dentro de las capas WIW y LIW

Satellite image georegistration from coast-line codification

This paper presents a contour-based approach for automatic image registration in satellite oceanography. Accurate image georegistration is an essential step to increase the eff ectiveness of all the image processing methods that aggregate information from diff erent sources, i.e. applying data fusion techniques. In our approach the images description is based on main contours extracted from coast-line. Each contour is codifi ed by a modifi ed chain-code, and the result is a discrete value sequence. The classical registration techniques were area-based, and the registration was done in a 2D domain (spatial and/or transformed); this approach is feature-based, and the registration is done in a 1D domain (discrete sequences). This new technique improves the registration results. It allows the registration of multimodal images, and the registration when there are occlusions and gaps in the images (i.e. due to clouds), or the registration on images with moderate perspective changes. Finally, it has to be pointed out that the proposed contour-matching technique assumes that a reference image, containing the coastlines of the input image geographical area, is available

Satellite image georegistration from coast-line codification

Martech 2007 International Workshop on Marine Technology, 15-16 november 2007, Vilanova i la Geltrú, Spain.-- 2 pages, 3 figuresThis paper presents a contour-based approach for automatic image registration in satellite oceanography. Accurate image georegistration is an essential step to increase the eff ectiveness of all the image processing methods that aggregate information from diff erent sources, i.e. applying data fusion techniques. In our approach the images description is based on main contours extracted from coast-line. Each contour is codifi ed by a modifi ed chain-code, and the result is a discrete value sequence. The classical registration techniques were area-based, and the registration was done in a 2D domain (spatial and/or transformed); this approach is feature-based, and the registration is done in a 1D domain (discrete sequences). This new technique improves the registration results. It allows the registration of multimodal images, and the registration when there are occlusions and gaps in the images (i.e. due to clouds), or the registration on images with moderate perspective changes. Finally, it has to be pointed out that the proposed contour-matching technique assumes that a reference image, containing the coastlines of the input image geographical area, is availablePeer reviewe

Common turbulent signature in sea surface temperature and chlorophyll maps

5 pages, 3 figuresOceans and 2D turbulence present similar characteristics, as for instance the dominant role played by eddies in energy and matter transport. However, providing a complete justification of this analogy is difficult, as it requires knowledge of the ocean's dynamic state at different instants and over large scales. Recently, new techniques coming from the Microcanonical Multifractal Formalism have made it possible to infer the streamlines from the analysis of satellite images of some scalar variables. In this paper, we will show that this information is enough to characterize the scaling properties of the energy cascade, which is manifested as a multifractal signature; further, the multifractal signature is obtained at each location in a local basis. Different scalars obtained from satellite measurements such as Sea Surface Temperature or Surface Chlorophyll Concentration present essentially the same multifractal structure, which is interpreted as a consequence of the pervading character of the turbulent advection at the scales of observationA. Turiel is contracted under the Ramón y Cajal program by the Spanish Ministry of Education. V. Nieves is supported by a Ph.D. grant funded by MERSEA. C. Llebot is supported by a Ph.D. grant funded by CSIC, J. Solé is supported by a post-doc grant funded by EU Streps Project SEEDS. This work is a contribution to the MERSEA project. Partial support from the European Commission under contract SIP3-CT-2003-502885 is gratefully acknowledged. This work is also a contribution to the Spanish projects ESEOO (VEM2003- 20577-C14-10), MIDAS-4 (ESP2005-06823-C05-1) and OCEANTECH (PIF-2006)Peer reviewe

Marine heat waves detection in climate warming seas: their evolution in the NW Mediterranean sea

Ponencia presentada en: XII Congreso de la Asociación Española de Climatología celebrado en Santiago de Compostela entre el 19 y el 21 de octubre de 2022.[ES]El aumento extremo y prolongado de la temperatura del mar, una ola de calor marina (MHW), se detecta por comparación con los valores históricos en cada localización y época del año. Así, el correcto establecimiento de los valores de referencia es una tarea clave en la detección de MHW. Al comparar diferentes épocas es necesario considerar dos contribuciones a la evolución de los episodios extremos: la tendencia subyacente de la temperatura y los cambios súbitos de la misma. Siguiendo la definición de Hobday (2016), comparamos las MHW detectadas en caso de corregir o no la tendencia climatológica en la definición de los valores de referencia. Para ello empleamos 38 años de temperatura superficial del mar Mediterráneo proporcionados por el servicio Copernicus. El trabajo se centra alrededor de la reserva marina de las islas Columbretes, donde existe una estación costera de medición de la temperatura del mar integrada en la red T-MedNet. Esto permite vislumbrar el efecto a diferentes profundidades aunque con series temporales más cortas. El resultado es que no corregir la tendencia al definir la referencia subestima el número de eventos detectados en los primeros años de la serie y los sobreestima en los últimos, independientemente de la longitud de la serie. La influencia del cambio climático en la alteración de los ecosistemas marinos debido a las MHW no parece deberse al incremento en la frecuencia de las mismas, sino a que éstas tienen lugar en un mar cada vez más caliente, actuando sobre sistema biológicos con mayor estrés térmico.[EN]Extreme and prolonged increases in the sea temperature, a marine heatwave (MHW), are detected by comparison with historical values at each location and time of year. Thus, the correct estimation of reference values is key in detecting marine heatwaves. In temporally separated epochs comparison, it is necessary to consider two contributions to the evolution of extreme events: the underlying trend in temperature and its sudden changes. Following the definition of Hobday (2016), we compare the detected MHW in case of correcting or not the climatological trend in the definition of the reference values. We use 38 years of the surface temperature of the Mediterranean Sea provided by the Copernicus service. This work is centered around the Columbretes Islands marine reserve, where there is a coastal sea temperature measurement station integrated into the T-MedNet network. This allows us to have a glimpse of the effect at different depths but with shorter time series. The results show that if the long-term trend is not removed from the reference, then the number of events is underestimated in the first years of the series and overestimated in the last ones regardless of the length of the series. The influence of climate change on the alteration of marine ecosystems caused by the MHWs does not seem to be caused by the increased frequency of MHWs, but rather by the fact that the MHWs take place in an increasingly hot sea, acting on biological systems having greater thermal stress.This is a contribution made with the support of the MPA-Engage project (Grant:5216 | 5MED18_3.2_M23_007) and MINKE project (Grant: 101008724). The authors would like to thank the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)