Tesis doctorales realizadas en el departamento

The Global Ocean is the largest Earth compartment holding carbon and nutrients that reaches the upper-ocean at temporal scales ranging from months to 10 kyr. The availability of these nutrients is fundamental to sustain primary production and the concentration of dissolved inorganic carbon (DIC) in surface waters controls the glacial-interglacial changes in atmospheric. One process that influences both nutrients and carbon availability is the Microbial Carbon Pump (MCP), which refers to the production of refractory dissolved organic carbon (RDOC) compounds via heterotrophic microbial activity. Variations in the RDOC pool affect long-term carbon storage in the ocean, hence influencing the carbon cycle and climate. The general objective of this thesis is to expand our understanding of the connections between RDOC production by MCP and the ocean metabolism (understood as the upper-ocean net autotrophic community production), paying special attention to the role of the marine microbial processes in the glacial-interglacial transitions of the Earth system. The RDOC production by MCP is inferred through the lineal dependence of fluorescent dissolved organic matter (FDOM) with apparent oxygen utilization (AOU) and nutrients. This relationship, however, depends on the preformed content in the water masses. In this thesis, a valuable dataset, obtained from a high-resolution spatial sampling along 7.5ºN in the equatorial Atlantic Ocean, is used to distinguish the variability of FDOM distribution associated with in situ production from that related to the water properties at origin. A simple objective nonlinear-global methodology for resolving the non-conservative fraction of biogeochemical variables distribution is presented. The approach focuses on fitting high-order polynomial models over the entire temperature-salinity space. The differences between the modelled values and the observations are identified as biogeochemical anomalies. The goodness of the method is compared, for each water stratum, with the traditional approach, which is based on the local linear mixing of a maximum of three source water masses. The new methodology has good skill at distinguishing between the conservative and non-conservative contributions to biogeochemical variables, lending information about biogeochemical processes, stoichiometric ratios and patterns of connectivity within a certain region. For the first time, a general relationship between humic-like FDOM and AOU in the dark equatorial Atlantic Ocean is formulated, irrespective of the water masses. The results endorse the idea that FDOM is mostly produced in situ in the dark ocean. In the second part of the thesis, the role of RDOC pool in quaternary climate transitions is explored. The glacial-interglacial transitions are considered as functional states of the complex Earth system, with different energetic conditions in terms of solar energy conversion through marine photosynthesis. However, the oceanic system capacity to capture and transform the incident solar radiation depends on the availability of DIC and nutrients to the productive upper ocean. The supply of DIC and nutrients by the Meridional Overturning Circulation (MOC) and the DOM pool are evaluated through a simple two-box and two-state relaxation-type model for the DIC and nutrients in the upper ocean. The model, inspired on physiological concepts, considers the upper ocean to switch between basal (glacial) and enhanced (interglacial) metabolic states. The model reproduces well the atmospheric time series for the last 420 kyr, providing a solution for the size and temporal dependence of the MOC and setting global constraints on primary production and remineralization in the upper ocean. The RDOC accumulates during the glacial period and its availability at the end of this cycle sets the metabolic intensity of the subsequent interglacial, in what constitutes a central component of the Earth’s pulsating homeostatic organization

The Southwest Atlantic Ocean is a key component of the global climate system. It holds the Confluence of the Brazil and Malvinas Currents (BMC), one of the highest frontal systems of the world ocean. The BMC is the encounter point of subtropical origin warm and salty waters, transported southward within the Brazil Current, and subantarctic waters flowing equa- torward along the Malvinas Current. The intense mixing and cross-frontal exchanges highly transform the water masses at this region, which will finally flow southward, eastward or will be subducted into the subtropical thermocline. This PhD dissertation aims at extending our knowledge on the circulation and dynamics in the BMC, placing these results in perspective from the small to the global scales. The intense property contrast found in this region resemble the intense temperature gradients between low and high latitudes. We first analyse the changes in the heat content of the atmosphere and upper ocean compartments for the last 450 kyr and analyse the sensitivity of the system to changes in albedo, cloud cover and atmospheric and oceanic heat transports. We then describe the hydrographic conditions found at the BMC during and early fall cruise (TIC-MOC cruise). These reveal the presence of brackish river water on top the frontal system. The salinity anomaly at the surface correlates with the presence of large ageostrophic velocities along the frontal jet. In addition, the rapid evolution of these waters impinges on the ther-mohaline variability in the proximity of the front. The comparison of surface overview during the cruise is well represented by the high-resolution reanalysis, although at depth it misses the thermohaline intrusions developed both sides the front. On the other hand, climatological data, as it shows the monthly fields, is not able to reproduce the numerous mesoscale features. [...]

In January 1969, Jack F. Paris published a comprehensive review of all the knowledge about microwave physics, engineering and its applications to meteorology and oceanography. In this 241 pages-long review, he pointed out that radiometers operating between 1.0 GHz and 5.4 GHz could be used to survey remotely both temperature and salinit along coastal and river regions in the world. [...]

The Mediterranean Outflow Water (MOW) is a dense (¿>1028.5 kg/m3), saline (38.5 g/kg) ocean stream originated in the evaporative Mediterranean basin flowing westward past Espartel Sill as a fast (>1 m/s) and often unstable (as indicated by its gradient Richardson number) gravity current. During its descense into the Gulf of Cadiz, the MOW entrains the overlying North Atlantic Central Water (NACW), until the density difference between both water masses vanishes, and reaches its equilibrium depth. Inertia, Coriolis and frictional forces (both internal and with the bottom), as well as pressure gradients (associated with both seafloor slope and density gradients), play varying roles throughout the MOW’s trajectory. In particular, bathymetric steering due to contouritic, turbiditic and diapiric structures controls the early trajectory of the outflow. Using both historic and current hydrographic data, and a new high-resolution bathymetry of a critical region west of Espartel Sill, we examine the hydrographic characteristics of the MOW and the mechanisms that set its pathway. Our study is complemented with the proposal of simple models that help explain pbservations of vertical mixing in the Strait of Gibraltar and the behavior of the MOW as a gravity current within the Gulf of Cádiz, as well as with a new tool, based on water tributaries methods, that assesses the relative impact of both seafloor slope and Coriolis force in steering the outflow.

Precipitation is one of the most important, and also difficult to predict, elements of climate. This difficulty is associated with the transport of moisture through weather fronts that change their pathway, shape and intensity at different spatial and temporal scales. Despite this difficulty, a proper rainfall prediction is necessary both to quantify the resources and infrastructures necessary to bring water to farm fields and cities and to anticipate the occurrence of extreme precipitation episodes that may cause human death and huge economic losses. The main objective of this dissertation is to investigate the influence of weather systems on the patterns and amount of precipitation over Iraq, from its variability at different temporal scales (daily to inter-decadal) to the contribution of severe precipitation events to total rainfall and the mechanisms underlying these extreme episodes. With this objective, we use monthly (1938-2016) and daily (2005-2016) precipitation records from a meteorological station in the city of Baghdad, ran by the Iraqi Meteorological and Seismology Organization. Additionally, we use the temperature, humidity, geopotential height, horizontal and vertical velocity, horizontal divergence, relative vorticity and potential vorticity fields from the ERA-Interim global atmospheric reanalysis, available four times a day on a 0.75º × 0.75º latitude-longitude grid from the European Centre for Medium-Range Weather Forecasts (ECMWF). These data, downloaded for a domain extending from 0°E to 60°E and from 10°N to 60°N, are then used to calculate the equivalent potential temperature, precipitable water, moisture flux, moisture flux convergence, Q-vector divergence, relative vorticity and potential vorticity advection, as well as several instability indices (K, CAPE, SWEAT and LI). In Chapter 2 we focus on the analysis of the monthly rainfall data for the 1938-2016 period. The mean annual precipitation is 135.8 ± 61.9 mm/yr and the mean monthly precipitation is 11.3 ± 18.7 mm. Despite the linear trend for the entire period is small, there are decades (1950s and 1960s) of relatively high mean-annual precipitation and other ones (1990s and 2000s) with substantially lower values. The monthly rainfall time series also shows the existence of substantial inter-annual variations, with annual precipitation values ranging between 29.3 and 307.7 mm (2012 and 1974, respectively). The seasonal cycle changes substantially between arid and wet years, with December-February receiving most during the arid years, and November-April collecting most precipitation during the wet years. The inter-annual changes in precipitation show no correlation with a global index for El Niño-Southern Oscillation. In Chapter 3 we analyse a 12-year time series of daily data (2005-2016) from the Baghdad meteorological station. We identify the contribution of cut-off lows to precipitation: 38 events contributed to 43.4% of the total precipitation over Baghdad. Cut-off lows occur all year long but those happening between October and December account for one-third of the total annual precipitation. In Chapter 4 we have considered the dynamics behind the 18-20 November 2013 extreme rainfall episode, the largest one since 1980. The middle and upper atmospheric conditions resulted from the development of an omega block into a Rex block. The eastern part of the cyclonic structure of the Rex block was experiencing horizontal divergence and the associated upward motions reached all the way from 1000 to 250 hPa. Simultaneously, the lower atmosphere (1000 to 700 hPa) exhibited intense southerly cool winds that brought large amounts of moisture into central Iraq. Finally, in Chapter 5 we again combine the daily Baghdad rainfall data with the ERA-Interim analysis to explore the weather patterns associated with the 20 extreme precipitation events for the 2005-2016 period. These events brought 805.2 mm (51.3%) of the total precipitation to the Baghdad station.

There is a growing concern for protection and conservation of coastal ecosystems, which require a proper understanding of ecosystem dynamics and the ability to detect and predict changes in the ecosystem state. For this purpose, there is general agreement on the requirement of multidisciplinary and multiscale observing systems, due to the complex physical and biogeochemical processes occurring in these environments at a wide range of time-space scales. [...]

There is a lack on ocean dynamics understanding, and that lead oceanographers to the need of acquiring more reliable data to study ocean characteristics. Oceanographic measurements are difficult and expensive but essential for effective study oceanic and atmospheric systems. Despite rapid advances in ocean sampling capabilities, the number of disciplinary variables that are necessary to solve oceanographic problems is large. In addition, the time scales of important processes span over ten orders of magnitude, and due to technology limitations, there are important spectral gaps in the sampling methods obtained in the last decades. Thus, the main limitation to understand these dynamics is an inaccurate measurement of the process due to undersampling. But fortunately, recent advances in ocean platforms and in situ autonomous sampling systems and satellite sensors are enabling unprecedented rates of data acquisition as well as the expansion of temporal and spatial coverage. Many advances in technologies involving different areas such as computing, nanotechnology, robotics, molecular biology, etc. are being developed. There exist the effort that these advantages could be applied to ocean sciences and will prove to extremely beneficial for oceanographers in the next few decades. Autonomous underwater vehicles, in situ automatic sampling devices, high spectral resolution optical and chemical sensors are some of the new advances that are being utilized by a limited number of oceanographers, and in a few years are expected to be widely used. Thanks to new technologies and, for instance, utilization of data assimilation models coupled with autonomous sampling platforms can increase temporal and spatial sampling capabilities. For instance, studies of phytoplankton dynamics in the water column, or the transportation and aggregation of organisms need a high rate of sampling because of their rapid evolution, that is why new strategies and technologies to increase sampling rate and coverage would be really useful. However, other challenges come up when increasing the variety and quantity of ocean measurements. For instance, number of measurements are limited by costs of instruments and their deployment, as well as data processing and production of useful data products and visualizations. In some studies, there exists the necessity to discriminate and detect different phytoplankton species present in sea water, and even track their evolution. The use of their optical properties is one of the approximations used by some of them. Acquiring optical properties is a non-invasive and non-destructive method to study phytoplankton communities. Phytoplankton species are then organized thanks to presenting similar optical characteristics. Fluorescence spectroscopy has been used and found as a really potential technique for this goal, although passive optical techniques such as the study of the absorption can be also useful, or even their combination can be studied. Specifically speaking about fluorescence, the majority of the studies have centered their effort in discriminating phytoplankton groups using their excitation spectra because the emission spectra contains less information. The inconvenient of using this kind of information, is that the acquisition is not instantaneous and it is necessary to spend some time (over a second) exciting the sample at different wavelengths sequentially. In contrast, the whole emission spectra can be acquired instantaneously. Therefore, the aim of this thesis is to explore new and powerful signal processing techniques able to discriminate between different phytoplankton groups from their emission fluorescence spectra. This document presents important results that demonstrate the capabilities of these methods.

Let me start with a fundamental question: why should we study climate? Climate affects our daily life in many ways: on the food we eat, on the houses we live in, on our work, on how we travel... Even affects our culture, our spare time or our health. But all of those are only locally important. What we need to see here is the big picture: climate affects the way all living species have adapted to the biosphere, it is key for our survival, but we are drastically influencing it and we cannot predict the consequences of our actions. It is, then, fundamental to broaden our knowledge on the climate system of Earth, the only home we have ever known, to study the past climatic variability on various time scales to obtain clues that will help society face future climate change. As Carl Sagan foreseen on Cosmos, one of his most known books (Sagan [1980]) [...]

Regions isolated from the wind-driven circulation are found in the eastern margins of the world’s tropical oceans. The weak and stagnant circulation of these so-called ‘shadow zones’, in combination with the intense respiration of organic matter provided by the overlying highly productive waters, promotes the existence of large oceanic volumes with very low dissolved oxygen. These volumes are known as oxygen minimum zones (OMZs). The last decades of observations have revealed that OMZs are in general expanding, and thus potentially threatening the surrounding rich marine ecosystems. However, it is not clear whether this change is based on natural variability or it has an anthropogenic origin. Furthermore, under a global warming scenario, the future evolution of the OMZs is uncertain due to the complex interaction between the physical and biochemical processes that interplay in the OMZs dynamics. This dissertation seeks to unravel the key elements of the circulation in the North Atlantic shadow zone, aiming to provide a deeper understanding of the physical components that rule the dynamics of the North Atlantic OMZ (naOMZ) – this being the less intense OMZ of the world ocean but the one that has experienced the largest expansion. A comprehensive description of the North Atlantic shadow zone circulation is presented from novel (CANOA08 cruise) and historic observations, including numerical outputs from the assimilative ECCO2 circulation model. The main outcome of our work is that two markedly distinct regimes of circulation exist in the thermocline layers of the naOMZ, above and below σθ=26.8 kg m-3. In the upper layer, within the upper Central Water (uCW), the circulation is governed by the cyclonic regime of the subtropical cell. This stratum is characterized by relatively high oxygenation, with a predominance of South Atlantic Central Water (SACW). The lower layer, within the lower Central Water (lCW), presents a drastic decrease in the oxygen content due to its sluggish circulation; in contrast with the uCW, a mean anticyclonic circulation leads to a marked increase of North Atlantic Central Water (NACW). This result implies an equatorward transfer of mass from the subtropical gyre to the shadow zone, providing a previously unaccounted supply of oxygen from the well-ventilated subtropical thermocline. In fact, at the core of the naOMZ, the contribution of NACW is 50%. Such a mixture of SACW and NACW, with very low oxygen A broad band of eastward flows is located between 10ºN and 20ºN, just south of the westward flowing North Equatorial Current. These flows, here referred as the Cape Verde Current system, emerge as the major contributor in he water mass supply to the naOMZ. Lagrangian simulation reveals that while in the uCW, most of the water supply occurs south of 10ºN, in the 1CM, more than two thirds of the total water supply takes place north of 10ºN, through the Cape Verde Current system with a high contribution of water directly recirculated from the subtropical gyre. The accurate numerical reproduction of the water mass composition within that naOMZ thermocline, as directly deduced from particle-track Lagrangian simulations, supports the goodness of the ECCO2 velocity field. The uCW and lCW strata not only exhibit opposite circulation patterns, they also present opposite large-scale vertical motions with predominant upwelling in the uCW as part of the subtropical cell regime while the lCW presents a broad downwelling pattern. We propose the existence of an inverse subtropical cell within the lCW dynamically coupled to the uCW regime. The major role these cells play in the circulation of the North Atlantic shadow zone, together with their substantial natural inter annual and decadal variability, makes them major players on the oxygen anomalies observed during the last decades in the world OMZs.

Remote sensing imagery of the ocean surface provides a synoptic view of the complex geometry of ocean circulation, which is dominated by mesoscale variability. The signature of filaments and vortices is present in different ocean scalars advected by the oceanic flow. The most probable origin of the observed structures is the turbulent character of ocean currents, and those signatures are persistent over time scales compatible with ocean mesoscale dynamics. At spatial scales of kilometers or more, turbulence is mainly 2D, and a complex geometry, full of filaments and eddies of different sizes, emerges in remote sensing images of surface chlorophyll-a concentration and surface salinity, as well as in other scalars acquired with higher quality such as surface temperature and absolute dynamic topography. The aim of this thesis is to explore and apply mapping methodologies to improve the quality of remote sensing maps in general, but focusing in the case of remotely sensed sea surface salinity (SSS) data. The different methodologies studied in this thesis have been applied with the specific goal of improving surface salinity maps generated from data acquired by the European Space Agency’s mission SMOS, the first satellite able to measure soil moisture and ocean salinity from space at a global scale. The first part of this thesis will introduce the characteristics of the operational SMOS Level 2 (L2) SSS products and the classical approaches to produce the best possible SSS maps at Level 3 (L3), namely data filtering, weighted average and Optimal Interpolation. In the course of our research we will obtain a set of recommendations about how to process SMOS data starting from L2 data. A fusion technique designed to exploit the common turbulent signatures between different ocean variables is also explored in this thesis, in what represents a step forward from L3 to Level 4 (L4). This fusion technique is theoretically based on the geometrical properties of advected tracers (Turiel et al., 2005a). Due to the effect of the strong shear in turbulent flows, the spatial structure of tracers inherit some properties of the underlying flow and, in particular, its geometrical arrangement. As a consequence, different ocean variables exhibit scaling properties, similar to the turbulent energy cascade (Seuront and Schmitt, 2005; Nieves et al., 2007; Nieves and Turiel, 2009; Isern-Fontanet et al., 2007). The fusion method takes a signal affected by noise, data gaps and/or low resolution, and improves it in a geophysically meaningful way. This signal improvement is achieved by using an appropriate data, which is another ocean variable acquired with higher quality, greater spatial coverage and/or finer resolution. A key point in this approach is the assumption of the existence of a multifractal structure in ocean images (Lovejoy et al., 2001b), and that singularity lines of the different ocean variables coincide. Under these assumptions, the horizontal gradients of both variables, signal and template, can be related by a smooth matrix. The first, simplest approach to exploit such an hypothesis assumes that the relating matrix is proportional to the identity, leading to a local regression scheme. As shown in the thesis, this simple approach allows reducing the error and improving the coverage of the resulting Level 4 product; Moreover, information about the statistical relationship between the two fields is obtained since the functional dependence between signal and template is determined at each point.

Ocean currents are a key component to understanding many oceanic and climatic phenomena and knowledge of them is crucial for both navigation and operational applications. Therefore, a key problem in oceanography is the estimation of the synoptic velocity field. Currently, global ocean surface velocities are routinely estimated from Sea Surface Height (SSH) measurements provided by altimeters. However, the separation between passes, as well as and the limited number of available altimeters leads to errors in the accurate location of oceanic currents when these measurements are used exclusively. Contrarily, satellite images of Sea Surface Temperature (SST) provide a good qualitative view of the location of ocean patterns, which has encouraged the investigation of alternative methodologies to reconstruct the velocity field based on these observations. This Ph.D. thesis has assessed the capability of SST microwave radiometers observations to retrieve ocean surface currents. The reconstruction of the ocean surface currents from SST observations can be expressed in terms of a transfer function notation, that allows to convert SST maps into SSH, and thus into currents. Because under geostrophic balance, the slope of SSH is proportional to ocean surface currents. This transfer function can be theoretically derived using the Surface Quasi-Geostrophic equations (SQG). Two different approaches were analyzed at a global scale: on one side, the analysis of the validity of the SQG approach has been performed, and on the other, an approach based on the synergetic properties between simultaneous SST and SSH observations has been analyzed. Both approaches have been compared with ocean surface currents retrieved from merged altimetric observations. The study has been focused on the period from October 2002 to May 2005, since during that period there were available four different altimeters, and the quality of the merged altimetric observations was enhanced. [...]

The present work has been developed in the frame of the MOC2 project (Ocean Climate Memory: fluxes of intermediate waters in the Southern Atlantic and their transformation into surface waters in the equatorial Atlantic, reference number CTM2008-06438), which was carried out between January 2009 and December 2011 with funding from the Ministerio de Educaci´on y Ciencia of the Spanish Government. The project consisted of two subprojects, being the present PhD thesis part of the first subproject: Ocean Climate Memory: mechanisms and paths of surface water formation in the equatorial Atlantic (reference number CTM2008-06438-C02-01). The main objective of this subproject was to study the processes and velocity of incorporation of the Intermediate Waters towards the superficial ocean in the equatorial Atlantic. The project was funded by the Spanish Government, and carried out at the Physical Oceanography Department of the ICM-CSIC (Institut de Ciències del Mar de Barcelona). Josep Lluís Pelegrí was the project coordinator and PI of the first subproject. He and Jerome Gourrion are the directors of this PhD thesis. The original aim of this PhD thesis was to explore and develop a novel method to study the paths and rates of transformation of Antarctic Intermediate Water (AAIW) in the equatorial Atlantic Ocean by using the information contained in the floats of the Argo program. In particular, the ocean velocities at the sea surface and the parking depth are inferred from the Argo float trajectories; when combined with the salinity and temperature data, this gives information not only on the paths of the water parcels but also on their transformations. During the last years other authors have explored how to extract the velocity field from the Argo float trajectories and have produced velocity data sets that are regularly updated. Having our own method, however, lets us to control the parameters required by the method to produce the velocity fields, hence allowing us to carry numerous sensitivity studies. Other advantages are the generation of the velocity fields with all the available Argo data and the possibility of setting particular configurations of the method for regional studies. Finally, future tests and developments of the method can be planned and executed with relatively minor efforts. In this dissertation the method is first presented, and it is later used to examine the statistics of the ocean horizontal velocities and to explore the dynamics of the currents south of Australia and in the equatorial Atlantic Ocean. The dissertation ends up with a description of several ongoing developments that should lead to a future improvement of the method and its applications

The eastern South Pacific (ESP) is a key region of the world ocean with direct influence on the global climate. Two clear examples are the El Niño-Southern Oscillation (ENSO), which occurs in the Pacific but influences the weather worldwide, and the observed slowdown in the rise of global air temperatures since 2001, recently explained in terms of a more vigorous upwelling of cold waters in the ESP and in the Equatorial Pacific. Over the last decade, the oceanographic scientific community has turned its attention to the oxygen minimum zones (OMZs) found in the world oceans, with one of the most intense being located in the ESP. OMZs are apparently expanding under the ongoing global warming, threatening productive marine habitats and weakening the ocean’s ability to sequester atmospheric carbon dioxide by promoting nitrogen loss. Inside suboxic OMZs, the oceanic-fixed nitrogen is released to the atmosphere, partially as nitrous oxide, a powerful greenhouse gas that further disrupts the atmospheric radiative balance. This dissertation aims at extending our knowledge of the ESP OMZ through two main approaches. The first approach is based on tracer analysis and inverse modeling techniques. Such techniques are applied to hydrographic datasets in order to examine the water mass structure, its variability and its role on the ventilation of the ESP OMZ. The second approach brings a more dynamic perspective to the study of the ventilation of the ESP OMZ, with the assessment of the annual-mean advective and turbulent oxygen transports into the ESP OMZ from Argo and dissolved oxygen datasets. We describe the distribution of water masses offshore Chile, Peru and Ecuador and discuss their spreading pathways. A thorough characterization of the water masses has been accomplished, resulting in the first set of ESP water types accounting for inorganic nutrients and dissolved oxygen. The low oxygen waters that compose the ESP OMZ are mainly Equatorial Subsurface Water (ESSW). The ESP OMZ is ventilated from the south by the Antarctic Intermediate Water (AAIW) and by the shallower Subantarctic Water (SAAW). These water masses are transported into this region by the Peru Chile Current (PCC) or by the adjacent flow that forms part of the subtropical gyre. We also examine the changes induced by two opposite phases of ENSO in water mass distribution and biogeochemical activity. During La Niña, vigorous upwelling promotes the rise in depth of the upper part of the ESP OMZ and denitrification strengthens in the subsurface layer. Furthermore, the upward displacement of isopycnals induced by La Niña favors the ventilation of a shallower depth range of the OMZ by the upper portion of the AAIW. The opposite occurs during El Niño conditions. We find that, with a larger oxygen supply, respiration increases balancing most of the extra oxygen gain. This suggests that there is an excess of organic matter waiting to be remineralized whenever oxygen supply increases slightly and this situation favors the maintenance of the ESP OMZ. Furthermore, our results show that AAIW flowed along shallower isopycnals in 2009 than in 1993. Such shoaling is not caused by the ENSO phenomenon and changes the way AAIW ventilates the ESP OMZ. This finding might explain why an increase in oxygen content has been reported between 200 and 700 m off Chile. A global picture of the main processes and predominant paths of oxygen supply into ESP OMZ is provided here for the first time. Two main advective routes are found, the traditional equatorial pathway and a previously unreported subtropical pathway. Remarkably, the subtropical pathway provides more net oxygen gain than the equatorial pathway at the core of the ESP OMZ. This finding challenges the common assumption that the ESP OMZ is only ventilated by the eastward flowing zonal currents of the Equatorial Current System. This result is endorsed by the independent water mass analyses accomplished in the first part of this dissertation where the fingerprint of AAIW and SAAW is clearly found in the ESP OMZ. In addition, an unreported eastward zonal current, located at intermediate depths between 12-15ºS, is found flowing all the way from the central Pacific into the ESP OMZ and its oxygen supply is quantified. Mean advection dominates oxygen supply in the upper layers due to the large contribution by the eastward flowing equatorial zonal currents. However, epineutral turbulent diffusion becomes the dominant term at levels deeper than γn = 26.75 kg m-3. The annual mean oxygen budget for the whole volume of the ESP OMZ is unveiled. Epineutral turbulent diffusion provides a net oxygen supply of 417.4 ± 43.0 kmol s-1, advection supplies 292.7 ± 25.2 kmol s-1 and dianeutral turbulent diffusion provides 85.8 ± 17.6 kmol s-1. The mean biological consumption of oxygen required to close the budget is 795.9 ± 195.0 kmol s-1.

This thesis investigates physical-ecological and vortex-wave interactions through potential vorticity (PV) considering a stratified and oligotrophic ocean. To this end, a NPZ (Nutrients-Phytoplankton-Zooplankton) model is coupled to a physical one that conserves PV explicitly on isopycnals. The physical-ecological coupled model is initialized using stationary NPZ solutions numerically stable with the fluid at rest. These solutions are implemented homogeneous both on horizontal and isopycnals levels to quantify the effect of horizontal and vertical advection caused by mesoscale and submesoscale vortex structures, and isopycnal mixing. At the interior of the vortex separatrix, plankton and PV distributions translate in phase at vortex propagation speed. Within cyclones, isopycnal doming enhances plankton biomass at the vortex center in different trophic conditions. Furthermore, isopycnal mixing associated to small-scale motions maximizes the phytoplankton (P) biomass in cyclones through a resonant response between P and diffusive timescales. This P increase is significant in mesotrophic conditions and occurs where the vertical displacement of isopycnals is maximum, and hence where vertical gradients of PV are large. At the separatrix outer, horizontal and vertical advection are of the same order of magnitude than the ecological forcing and enhance P through different mechanisms. Firstly, vertical velocity w uplifts nutrients and P to better lit levels. P responds with some time lag to this perturbation and the associated increase in biomass occurs far from the upwelling location due to the action of horizontal advection. As a result, P correlates with w, and thus with horizontal gradients of PV, only at initial times. In the particular case of translating cyclones, this mechanism explains the development of a P trail at their wake. And secondly, the horizontal advection of a surface ecosystem patch by subsurface vortices decreases P self-shading at the patch front in benefit of P growth. Finally, interactions between vortex structures and pure inertial and gravity large amplitude waves are investigated. The advection of PV by waves causes vortices to be unsteady and modifies the upper and lower bounds of the wave frequency band. The advection of waves by vortices Doppler shifts the local wave frequency. When inertial waves are involved, a near-inertial right-handed helical wave is developed due to a non-linear interaction. As a result, total w increases one order of magnitude and correlates with horizontal gradients of PV. These results aim to shed further light on the ecological impact of long-lived coherent vortices in the open ocean.

During the last two decades the scientific community has recognized the importance of the tropical Atlantic Ocean and the upwelling regions on the Earth’s climate. This recognition has opened new questions such as: ¿What are the mechanisms for the ocean to adjust to variations in atmospheric forcing?, ¿Is there any indirect relation between the atmospheric seasonal cycle and the response of the surface ocean?, ¿How are the meridional boundary flows connected with the zonal jets in the interior ocean?, ¿What is the relevance of these processes in the redistribution of properties such as water mass, heat and fresh water? [..]

The variability of soil moisture and ocean salinity controls the continuous exchange of water between the oceans, the atmosphere and the land. Therefore, the accurate and periodic measurements of these geophysical variables are paramount to improve the climate change prediction and extreme-event forecasting. However, until very recently, global measurements of these parameters with a suitable spatial and temporal resolution have not been available. Real aperture radiometers have been frequently used for Earth observation applications. Nevertheless, for space-borne sensors at a low Earth orbit, the requirements on spatial resolution and coverage, at the operating frequencies (L-band), would require an unfeasibly large antenna. Conversely, synthetic aperture radiometry achieves high resolution using an array of small antennas, becoming a sound alternative to real aperture radiometry at low microwave frequencies. The ESA’s SMOS (Soil Moisture and Ocean Salinity) mission, successfully launched on November 2009, is the first mission ever attempted to frequently and globally measure soil moisture over the continents and sea surface salinity over the oceans. The single payload of the mission, the MIRAS (Microwave Imaging Radiometer by Aperture Synthesis) instrument, is the first space-borne L-band two dimensional synthetic aperture radiometer. This completely new type of instrument implies a technological challenge, for which the development of a detailed error model definition, dedicated calibration and image reconstruction algorithms have been needed. The calibration of MIRAS tackles all activities devoted to retrieve the SMOS scientific products from raw data measurements with the accuracy required by the scientific community. Characterization activities, mainly performed prior to the beginning of the in-orbit operation, have been required to develop and test the calibration activities. Within the framework of the SMOS mission, this Ph.D. Thesis is focused on the characterization of the interferometric radiometers devoted to Earth observation. The main contributions of this Thesis, which are directly related to the MIRAS payload performance assessment, are: (i) the definition of tests for the characterization campaigns, data processing methods and success criteria and (ii) the development of calibration algorithms and tools to fine-tune the instrument in order to fully achieve the system requirements and therefore the scientific requirements of the mission. Most of the work has been done in the framework of the MIRAS/SMOS Pre-Commissioning Phase activities and it has been completed in the framework of the Commissioning Phase preparatory work. Calibration tools and techniques developed for the MIRAS ground characterization have been adapted to fulfill in-orbit instrument characterization during the first months of the Commissioning Phase and contributed to the development and consolidation of the SMOS operational level-1 processing.

The world ocean heat content has increased between mid 1950’s and mid 1990’s, highlighting the key role of the ocean in the climate system. A set of ship-based observations have evidenced substantial temperature and salinity changes in the Atlantic thermocline, although they are neither uniform nor monotonic. In this thesis we evaluate the recent variability of the tropical and subtropical North Atlantic Ocean over different spatial scales, and explore the possible controlling mechanisms. […]

Coastal upwelling in the eastern margin and offshore curl-driven upwelling in the southeastern margin, make the subtropical Northeast Atlantic a region of major primary productivity. When examining a broad zonal area, from the coast to 40º W, we find that the upward transport of nutrients due to offshore curl-driven upwelling becomes the main control on productivity. Nevertheless, despite its relatively small zonal extension of about 100 km, coastal upwelling extends its impact towards the open ocean through offshore Ekman transport and convergence of the meridional flow at Cape Blanc (21º N). Analysis of hydrographic data from spring 1973 and fall 1975 shows an export from the coast to the open ocean of 2.9 Sv of water mass and 53 kmol s-1 of nitrate during spring and 0.6 Sv and 3 kmol nitrate s-1 during fall in the area south of Cape Blanc. It is fundamental to improve our understanding of the distribution of the different water masses of the upper thermocline, as they carry the nutrients that will reach the euphotic layer and sustain primary production. In the study region, central waters of northern and southern origin meet at the Cape Verde frontal zone. Northern waters are nutrient-poor and oxygen-rich while local southern waters are nutrient-rich and oxygen-poor. Here, intense double diffusive mixing enhances horizontal heat transfer, thus the front appears as a smooth feature in terms of temperature, but resembles a barrier in terms of other properties such as salt or nutrients. The application of an Optimum Multiparameter analysis to hydrographic data collected during November 2007 and November 2008 shows a sharp front separating the central waters of northern and southern origin. In contrast, at intermediate layers, the transition between Mediterranean Water and Antarctic Intermediate Water is smoother.

The European Space Agency\'s Soil Moisture and Ocean Salinity (SMOS) satellite was launched on November, 2, 2009 from the Russian cosmodrome of Plesetsk. Its objective is to globally and regularly collect measurements of Sea Surface Salinity (SSS). To do that, a pioneering instrument has been developed: the Microwave Imaging Radiometer by Aperture Synthesis (MI-RAS), the rst space-borne, 2-D interferometric radiometer ever built; it operates at L-band, with a central frequency of 1.4135 GHz, and consists of 69 antennas arranged in a Y shape array. MIRAS\' output are brightness temperature maps, from which SSS can be derived through an iterative algorithm, and using auxiliary information. For each overpass of the satel-lite an SSS map is produced, with an estimated accuracy of 1 psu (rmse). According to the Global Ocean Data Assimilation Experiment (GODAE) the mission requirement is instead specied as 0.1 psu after averaging in a 10-day and 2º x 2º spatio-temporal boxes. In previuos works ((Sabia et al., 2010), or more extensively in Dr. Sabia\'s Ph.D. thesis (Sabia, 2008)) the main error sources in retrieving SSS from SMOS measurements were determined as: 1. Scene-dependent bias in the simulated measurements, 2. L-band forward modeling denition, 3. Radiometric sensitivity and accuracy, 4. Constraints in the cost function, and 5. Spatio-temporal averaging. This Ph.D. thesis, is an attempt of reducing part of the aforementioned errors (the relative to the one-overpass SSS (1 - 4)) by a more sophisticated data processing. Firstly, quasi-realistic brightness temperatures have been simulated using the SMOS End-to-end Performance Simulator (SEPS) in its full mode and an ocean model, as provider for geophysical parameters. Using this data set the External Brightness Temperature Calibration technique has been tested to mitigate the scene-dependent bias, while the error introduced by inaccuracies in the L-band forward models has been accounted for by the application of the External Sea Surface Salinity Calibration. Apart from simulated brightness temperatures, both External Brightness Temperature Calibration and External Sea Surface Salinity Calibration have been tested using real synthetic-aperture brightness temperatures, collected by the Helsinki University of Technology HUT-2D radiometer during the SMOS Calibration and Valdation Rehearsal Campaign in August 2007 and on one week of data (in Dual polarization mode) acquired by the SMOS satellite between march 3 and 9, 2010. Finally, a study of the cost function used to derive SSS has been performed: The correlation between measurement mists has been estimated and the efect of including it in the processing have been assessed. As an outcome of a 3-month internship at the Laboratoire LOCEAN in Paris, a theoretical review of the eect of the rain on the very top SSS vertical prole has been carried out and is presented as Appendix.

Soil moisture is a key state variable of the Earth’s system; it is the main variable that links the Earth’s water, energy and carbon cycles. Soil moisture variations affect the evolution of weather and climate over continental regions, and accurate observations of the Earth’s changing soil moisture are needed to achieve sustainable land and water management, and to enhance weather and climate forecasting skill, flood prediction and drought monitoring. This Ph.D. Thesis focuses on measuring the Earth’s surface soil moisture from space at a global and regional scale. [...]

Complex systems are abundant in our natural environment. In linear systems, the equations of their dynamics can be very difficult to solve, but if they cannot be described with a single characteristic scale, at least they can be described by a set of few characteristic scales that are totally decoupled from each other. However, this takes on a completely different flavour in non-linear systems, where scales are coupled and appropriate multiscale analysis is in order. This is the case of complex systems and, more particularly, scale invariant systems. In these, the approach to their solution is different, and it usually involves a multiscale basis. In this context, wavelets are one of the most used representation paradigms. The research context of complex systems and, particularly, scale invariant systems and multifractals has been in constant evolution over the last few years. Theoretical advances, either statistical (stochastic processes and probability distributions) or geometrical (function analysis and measure theory), along with fancy signal-processing algorithms suited to scale invariant data (and additionally handling aliasing, discretization and other artefacts of experimental data), have originated new tools for multifractal characterization of systems. While ten years ago the only methods available were statistical, by the start of this thesis project, development of geometrical methods had begun (most notably, the microcanonical multifractal formalism (MMF)). Geometrical methods have a clear advantage over statistical methods: they characterize each point of the system and thus they permit new applications such as reconstruction and prediction of signals, i.e., not only statistical characterization. Additionally, geometrical methods provide statistical characterization with much less need of data than statistical methods. In the present thesis, we have worked on the generalization and improvement of MMF, as well as its applications to the inference and forecasting of systems that follow a cascade process. In particular, we have described applications to two very different systems: stock-market series and ocean turbulence. The representation of the signal as a microcanonical cascade plays a crucial role in these applications. This representation can be achieved with one particular wavelet called optimal wavelet. The most relevant theoretical achievements are the regularization of diverging multifractal measures, the establishment of the bridge between multiplicative variables in microcanonical cascade processes and local singularity exponents, and the design of accurate and robust measure of wavelet optimality for a given dataset. To achieve this, we have introduced a new formalism, that of microcanonical cascades, that marries the cascade formalisms with MMF. Regarding the developed applications, on stock-market time series, we have inferred the distribution of future returns conditioned by the cascade and we have shown that a prediction based on this inference improves that of an ARIMA model. From the distribution of future returns, future volatility and value-at-risk can be reliably forecasted. On ocean data we have characterized dynamical aspects from optimal wavelet cascade analysis. In particular, we have observed that anomalies in the cascade of sea surface temperature show particular points of heat transfer between structures at different scales in the zones of wind-driven currents, also in the gyres. Both understanding -- combined with appropriate modelling -- of dynamics and design of inference/forecasting algorithms have crucial importance for the anticipation of changes in natural phenomena. In this context, the chain formed by the three steps followed during the thesis, namely multifractal characterization first, then obtaining of the optimal wavelet and finally design of inference algorithms, summarizes the direction we have followed to tackle the study of econometric time series and ocean maps.

Turbulent ows are of major interest for scientists and engineers, playing an essential role in uid dynamics. Even though a precise denition of turbulence does not exist,it is generally assumed that turbulence is a ow regime characterized by instabilities at large Reynolds numbers. The Reynolds number is a measure of the ratio of inertial forces to viscous forces. In the ocean, Reynolds number are of the order of 106, and so ocean dynamics is strongly nonlinear involving a large spectra of processes across all space and time scales. However, the ocean seems to be very active at around 30-300 Km, which is known as mesoscale. A schematic diagram of the spatial and temporal scales of various oceanic phenomena is shown in Figure 1.1. Instabilities in the ocean explain the meandering nature of oceanic currents, which can isolate and shed eddies when the meanders reach large amplitudes [3]. Examples of such ows are high- and low-pressure systems that are formed in the Gulf Streamarea (see Figure 1.2). These systems have been related to coherent structures, which are a combination of the geometrical and dynamical properties of the ow, i.e., regions containing most of the surviving vorticity [56, 47]. As it occurs in the ocean, a turbulent ow self-organizes into a collection of coherent structures [91, 141]. The smaller eddiesare exposed to the strain-rate eld of these coherent structures [124]. In recent studies, coherent structures not only have been identied with vortices, but also their presence has been connected with non-Gaussian Probability Density Functions (PDFs) of velocity elds from the ocean [17, 40, 54, 51, 140]. In addition to this characteristic, a near Gaussian component is expected for the background ow. These statistical distributions of ocean velocity elds have similar characteristics to the ones of numerical simulations of bidimensional turbulence.

Aquesta tesi es una investigació de la generació espontània i propagació d’ones inercials-gravitatòries (OIGs) de petita escala en fluxos geofísics inicialment en balanç. La investigació inclou les fonts d’ones (flux en balanç) i les OIGs (flux que no està en balanç). La velocitat vertical de mesoescala w s’obté a partir de dades experimentals en el ‘Western Alboran Gyre’ (WAG) resolent una aproximació de l’equació ω generalitzada (equació ω), i la circulació ageostròfica tridimensional (3D) es simula numèricament en un dipol baroclí ‘quasi en balanç’ a partir d’un model de Bousinessq no-hidrostàtic. Aquests fluxos són inercialment estables amb nombres de Rossby màxims de Rmax≈0.9 i 0.7, respectivament. La velocitat vertical es relaciona, en ambdós fluxos, amb l’advecció de la component vertical de la vorticitat per la cisalla vertical. Les diferències entre w i la velocitat vertical quasigeostròfica en el WAG es deuen a la inclusió de la part ageostròfica del vector Q en l’equació ω. La trajectòria del dipol es relaciona amb el màxim de l’anomalia de vorticitat potencial (VP) i la grandària dels remolins. El flux ageostròfic del dipol depèn de la distància entre els remolins d0. Per a petites d0, la w té una distribució 3D octupolar i el dipol és compacte. Si augmentem d0, la velocitat del dipol i el màxim de |w| disminueixen, i les oscil·lacions internes o ‘heading’ trenquen ocasionalment l’estructura octupolar de w. La velocitat horitzontal ageostròfica s’explica per l’acceleració advectiva, concretament per l’acceleració centrípeta. Les OIGs es detecten en el flux total a partir de sèries temporals i distribucions horitzontals de w i de la seva derivada vertical. La generació espontània d'OIGs es mostra en diferents fluxos geofísics: un anticicló, un corrent de doll, un dipol, un tripol, i una col·isió entre dipols. Les OIGs són extretes, durant la interacció de dos anticiclons baroclins, amb el mètode ‘Optimal potential vorticity balance.’ Els paquets d’OIGs que es propaguen lliurement en el camp llunyà tenen fronts d’ones en forma d’espiral mentre que els quals es propaguen en l’interior del flux vortical poden ser atrapats i els fronts d’ona formar distribucions “frontals” o anulars. L’augment de la VP i de la isotropía del remolí incrementa l’emissió espontània d'OIGs. Es consideren tres indicadors possibles de la generació espontània d’OIGs: (i) la advecció de la VP, (ii) el residu de l’equació del balanç no lineal, i (iii) l’extensió 3D de l’analogia de la generació d'OIGs, en aigües succintes, de la teoria de radiació del so de Lighthill. L’anàlisi numèrica de les OIGs extretes suggereix la coexistència de dos mecanismes de generació. El primer es relaciona amb l’advecció de la VP, i el segon amb la interacció coherent d’OIGs preexistents que, ocasionalment, cooperen per a produir divergència horitzontal de major escala, l’ascens de les isopicnes, i amb això l’emissió d’un nou paquet d'OIGs de major amplitud.

Interest on the study of algal blooms is increasing, especially because some of them (known as harmful algal blooms or HAB) may have deletereous effects for socio-economic activities or for human health. Here, we will refer in general to ”Exceptional algal blooms” or EAB, without assuming noxious effects for human interests, to deal with the rapid proliferation of a phytoplankton population, leading to cell densities above a certain reference level that can be operatively defined. EAB are controlled by both physical (such as advection and dispersion) and biological factors (growth rate, predation and other biological interactions). This thesis examines the potential influence of toxic effects of the alga that causes the bloom towards other organisms of the ecosystem (not necessarily implying effects on human health or interests). Assuming favourable physico-chemical conditions for the success of a bloom, mathematical modelling and numerical simulations have been used to study: (i) the effect of a toxin produced by a microalga towards microalgal competitors (allelopathy) and (ii) the effect of this toxin on potential predators, causing feeding avoidance of the toxin producer and increasing predation pressure on other non-toxic components of the microalgal community (feeding avoidance). The overall objective of this mathematical approach is to open new perspectives in the evaluation of the role of biological interactions on EAB (and HAB) development, within a general ecological context. Chapter 3 presents the adjustment of experimental data to a mathematical model of allelopathy and the estimation of values for the relevant parameters. A mathematical model involving feeding avoidance is examined in chapter 4. The simultaneous effect of both interactions is addressed in chapter 5. Assuming pre-bloom conditions and parameter values similar to those derived from the laboratory experiments, allelopathy does not appear to be a significant cause of HAB initiation, although it must be taken into account when the bloom is already well developed (population density > 100000 cells/l). Therefore, this interaction has been included in the multispecies model (chapter 5) that simulates algal bloom and post-bloom phases. On the other hand, the mathematical model indicates that feeding avoidance of a toxic alga may have an important influence on the development of an EAB of the toxin producer. A relationship involving the degree of feeding avoidance of the toxic alga and other parameters of the algal and predator populations can be used to determine a critical threshold value for the development of a bloom. Based on this relationship, an analogy has been made with critically self-organized systems (such as phase transitions), which can be characterized by internal and control parameters. Examination of the effects of allelopathy and feeding avoidance in a multispecies model indicated that allelopathy was not significant for the concentrations and parameter ranges considered. However, feeding avoidance caused by a predation repellent toxin had important consequences for the behaviour of the system. The effects of this interaction on the functional groups included in the model were both direct and indirect and were more important for microzooplankton than for mesozooplankton predators. Above a certain threshold value for feeding avoidance effects, the behaviour of the system showed notable changes

This PhD thesis has been done in the framework of the SMOS (Soil Moisture and Ocean Salinity) mission, from the European Space Agency. This satellite will be launched in February 2007 and will provide global sea surface salinity and soil moisture maps, variables that never have been measured before from space. The payload instrument (MIRAS) is an L-band interferometric radiometer. This will be the first time an instrument with this characteristics is put in orbit. However, there are still a lot of issues that need to be solved. This thesis is focused on some open questions of the salinity retrieval process from SMOS measurements. [...]

La mar Mediterrània es caracteritza per l'entrada d’aigües poc salades procedents de l'Atlàntic a través de l'estret de Gibraltar. El ux sovint es inestable i genera vòrtex coherents, amb longituds característiques compreses entre els 50 i 150 km i temps de vida de mesos fins i tot anys, que afecten fortament el transport a la conca. L'objectiu principal de la tesi es estudiar i caracteritzar les propietats dels vòrtex coherents a la Mediterrània. Des dels inicis dels anys 90 l’ús intensiu d'alt__metres a proporcionat dades sobre la superfície lliure de l'oceà, que, recentment, han estat assimilades a sistemes de predicció oceànica de la Mediterrània. Aquest fet proporciona un conjunt de dades de la superfície lliure forca gran. La presencia d'estructures coherents fa que les observacions de mesoescala de l'oceà s'assemblin a la turbulència bidimensional. Aquestes similituds suggereixen que els darrers avenços en turbulència bidimensional es podrien fer servir com a punt de partida per investigar els vòrtex Mediterranis. Un vòrtex coherent es defineix com la regió simplement connexa amb valors del paràmetre d'Okubo-Weiss W < 􀀀0:2_W, on _W _es la desviació estàndard espaial de W, i el mateix signe de la vorticitat. Quan un vòrtex es identificat les seves propietats tals com, mida, energia cinètica o amplitud es poden calcular. En aquest estudi, aquesta definició s'ha aplicat als mapes d'anomalies del nivell del mar (SLA) del període octubre 1992-setembre 1999 i a simulacions numèriques de la Mediterrània pel període setembre 1999 a abril 2003. Els resultats de l'altimetria mostren que la Mediterrània es caracteritza per una distribució de vòrtex aproximadament homogènia. De tota manera, algunes de les propietats dels vòrtex com l'energia o l'amplitud tenen distribucions irregulars amb valors més alts en regions on estaven documentada la presència de vòrtex de mesoescala. Aquest fet suggereix que una classificació basada en l'amplitud podria permetre separar aquests vòrtex de les altres estructures. L'anàlisi de la dependència de l'amplitud de la mida mitjana dels vòrtex mostra un comportament asimptòtic que tendeix cap als a radis d'uns 40 km. Aquests resultats suggereixen la classificació heur_stica de les estructures coherents en vòrtex intensos (caracteritzats per valors de l'amplitud menors de 􀀀2_w) que tenen la mida dels vòrtex de mesoescala, i vòrtex febles que es corresponen amb estructures sorolloses i estats poc energètics dels vòrtex de mesoescala. Aquesta separació d'estructures permet seguir amb facilitat de mapa a mapa els vòrtex i, per primera vegada, construir una imatge completa dels Camins preferencials que segueixen. Per altra banda també s'han analitzat les distribucions de probabilitat de la velocitat (PDF) derivades dels mapes de SLA. S'ha dividit la Mediterrània 7 regions depenent de la geometria i la distribució vòrtex intensos. Per cada regió s'han calculat les PDF de la velocitat geostròfica. Les formes de les distribucions observades es caracteritzen per tenir un nucli gaussià amb cues exponencials. De tota manera, la mida del nucli i les cues canvia d'una regió a una altra depenent de la distribució de vòrtex intensos. La descomposició del camp de velocitats en: un camp de fons, un camp induït pels vòrtex febles i un camp induït pels vòrtex intensos mostra que els dos primers es caracteritzen per tenir distribucions semblants a una gaussiana, mentre que el tercer té una distribució propera a una exponencial. L'aplicació a les simulacions numèriques mostra resultats equivalents amb la diferència que conté més estructures petites.

Most of the remote-sensing satellite radar systems can provide sea-surface wind field information, which in turn is very useful for a number of meteorological and oceanographic applications. This thesis reviews the wind retrieval procedures of such systems and explores fundamental methodology to overcome the up-to-date unresolved problems. [...]

El perfilador de corriente por efecto Doppler acoplado a un barco oceanográfico es un instrumento cada vez más utilizado para el estudio de las corrientes en el océano. Sin embargo, también es bien conocido la dificultad que implica el procesado de la señal acústica y los errores asociados que pueden llegar a tener las medidas de la velocidad en función de la metodología del procesado y de la precisión de los instrumentos auxiliares utilizados por el perfilador. En este trabajo se presenta una nueva metodología que permite mejorar notablemente la precisión de las medidas de velocidad gracias al uso de un nuevo sistema de posicionamiento tridimensional (3DF Ashtech) utilizado de forma auxiliar por el perfilador. La reducción del error asociado a las velocidades medidas se ha evaluado en un 25% comparado con el error inducido con otras metodologías anteriores. El esfuerzo en mejorar la precisión de las medidas de velocidad tiene un objetivo principal: Llevar a cabo un análisis multivariante en el que se combinan los datos hidrográficos registrados con un SeaSoar (CTD ondulante) y de velocidad obtenidos con el perfilador de corriente. En primer lugar el análisis multivariante ha mostrado ser muy superior al univariante ya que reduce en un 50% los errores estadísticos del análisis. La limitación principal de esta técnica es que es bidimensional y por tanto no aprovecha la coherencia vertical de los perfiles registrados. Por otro lado, en términos de diagnóstico de variables, tanto básicas (temperatura, salinidad, densidad y altura dinámica) como derivadas (vorticidad relativa y velocidad vertical), los resultados multivariantes son más consistentes que los univariantes. A partir del análisis multivariante se han diagnosticado velocidades verticales del orden de 45 m/día que pueden contribuir a la fertilización (entrada de nutrientes) de la capa superficial, aunque también hay que tener en cuenta que en la zona de estudio (mar de Alborán) se han medido velocidades horizontales muy elevadas (hasta 1.4 m/s en superficie) que inducen una importante advección horizontal. Por último, para aprovechar la coherencia vertical de los perfiles se ha implementado un esquema de análisis basado en funciones empíricas ortogonales (EOFs), donde a diferencia del método clásico, los modos dependen únicamente de la profundidad y las amplitudes de la dimensión horizontal. La principal novedad del método es la posibilidad de incluir en el mismo análisis los datos hidrográficos y de velocidad. El método de EOFs-multivariante ha resultado ser muy eficaz para el caso particular de estudio (remolino anticiclónico del mar de Alborán occidental), ya que el primer modo permite explicar mas del 99% de la varianza del campo de altura dinámica. Los resultados de las diferentes variables estimadas (vorticidad relativa, estabilidad, etc) a partir del análisis de EOFs permiten caracterizar la zona como un una región extremadamente energética, donde existe un importante gradiente vertical de la velocidad en los niveles superiores, pero a la vez también existe una gran estabilidad de la columna de agua que impide la mezcla turbulenta y por tanto la fertilización de la capa superficial del océano por dicho mecanismo de mezcla. Las velocidades verticales antes referidas, se consideran por tanto como el mecanismo principal responsable del transporte de nutrientes desde niveles inferiores (150-175 m) a los más superficiales del océano (0-50 m)

La séparation de sources est un problème central en traitement du signal. Il apparaît dès que plusieurs sources émettent des signaux qui sont reçus simultanément par un ensemble de capteurs. Ce problème a reçu un intérêt croissant ces quinze dernières années. Il intervient en effet dans une très grande variété d’applications. Parmi elles et pour n’en citer que quelques unes, les communications mobiles pour lesquelles il faut, au niveau des antennes, séparer les différents signaux associés à la parole, les électroencéphalogrammes et électrocardiogrammes qui peuvent recevoir diverses excitations électriques au niveau des capteurs (lorsqu’une femme est enceinte par exemple, certains capteurs d’un électrocar-diogramme reçoivent à la fois les signaux liés à la mère et ceux liés à l’enfant) ou encore les barrages sur lesquels on place plusieurs capteurs afin d’essayer de détecter d’éventuelles causes anormales de vieillissement. [...]