Recent efforts in understanding the variability inherent in coastal and offshore waters have highlighted the need for higher resolution sampling at finer spatial and temporal resolutions. Gliders are increasingly used in these transitional waters due to their ability to provide these finer resolution data sets in areas where satellite coverage may be poor, ship-based surveys may be impractical, and important processes may occur below the surface. Since no single instrument platform provides coverage across all needed spatial and temporal scales, Ocean Observation systems are using multiple types of instrument platforms for data collection. However, this results in increasingly large volumes of data that need to be processed and analyzed and there is no current “best practice” methodology for combining these instrument platforms. In this study, high resolution glider data, High Frequency Radar (HFR), and satellite-derived data products (MERRA_2 and ARMOR3D NRT Eddy Tracking) were used to quantify: 1) dominant scales of variability of the central Red Sea, 2) determine the minimum sampling frequency required to adequately characterize the central Red Sea, 3) discriminate whether the fine scale persistency of oceanographic variables determined from the glider data are comparable to those identified using HFR and satellite-derived data products, and 4) determine additional descriptive information regarding eddy occurrence and strength in the Red Sea from 2018-2019. Both Integral Time Scale and Characteristic Length Scale analysis show that the persistence time frame from glider data for temperature, salinity, chlorophyll-α, and dissolved oxygen is 2-4 weeks and that these temporal scales match for HFR and MERRA_2 data, matching a similar description of a ”weather-band” level of temporal variability. Additionally, the description of eddy activity in the Red Sea also supports this 2-4-week time frame, with the average duration of cyclonic and anticyclonic eddies from 2018-2019 being 22 and 27 days, respectively. Adoption of scale-based methods across multiple ocean observation areas can help define “best practice” methodologies for combining glider, HFR, and satellite-derived data to better understand the naturally occurring variability and improve resource allocation.

The influence of mesoscale ocean eddies on near-surface ocean temperature, surface stress and phytoplankton communities is investigated by collocating numerous satellite measurements along with vertical profiles of oceanic temperature and salinity to the interiors of eddies identified and tracked in altimetric sea surface height maps. The surface currents associated with mesoscale ocean eddies impart a curl of the surface stress from the relative motion between surface air and water. This stress curl has a polarity opposite that of the vorticity of the eddy, thus attenuating the eddies by generating Ekman upwelling in the cores of anticyclones and downwelling in the cores of cyclones. Ekman pumping also arises from eddy-induced spatial variability of the sea surface temperature (SST) field that generates a wind stress curl in regions of crosswind SST gradients through a response of surface winds to SST-induced surface heating variations. SST-induced Ekman pumping is shown to be secondary to surface current-induced pumping in most regions of the World Ocean. Eddy-induced Ekman pumping resulting from the combination of surface current effects and air-sea interaction represents an order 1 perturbation of the background, basin-scale Ekman pumping velocities from the large-scale wind fields. In western boundary currents and equatorward-flowing eastern boundary currents, cyclonic eddies preferentially entrain water from the coastal side of the boundary current, which primes the interiors of cyclones to have phytoplankton concentrations that are elevated relative to the background. In contrast, anticyclones formed in these regions contain locally depressed phytoplankton concentrations from the offshore waters. While eddy pumping from vertical displacements of isopycnals during eddy formation can affect the biology in the interiors of cyclones during the transient stage of their development, this ecosystem response cannot be sustained because of the persistent eddy-induced Ekman downwelling throughout the rest of the eddy lifetimes. Likewise, the persistent eddy-induced Ekman upwelling in anticyclones is of little benefit because of their low phytoplankton content at the time of formation. A definitive response to eddy pumping is therefore difficult to detect from satellite observations alone. Eddies formed in regions where anticyclones preferentially entrain water with elevated phytoplankton concentrations, such as the South Indian Ocean, or in some mid-ocean gyre regions where small-amplitude eddies form (e.g., the oligotrophic South Pacific), an ecosystem response to eddy-induced Ekman pumping is observed. Conversely, cyclones in these regions entrain water that is low in chlorophyll, resulting in negative chlorophyll anomalies that are sustained by Ekman downwelling throughout the eddy lifetimes. The phytoplankton response to eddy-induced Ekman upwelling in anticyclones is seasonal, occurring only during the winter. It is proposed that the mechanism for the lack of ecosystem response to eddy-induced Ekman upwelling during the summer is the decoupling of the mixed layer from the nutricline. The observations presented in this dissertation provide a baseline from which coupled ocean circulation and biogeochemical models can be assessed. If coupled models are able to reproduce correctly the observed influence of mesoscale eddies on photoautotrophic communities, further insight into the mechanisms for this variability could be gained from the model output using the methodologies developed in this dissertation together with investigation of subsurface variability in the models below the depth to which chlorophyll can be inferred from the satellite observations. / Graduation date: 2013

By collocating 10 years (1999-2009) of remotely sensed surface turbulent heat fluxes with satellite altimetry data, we investigate the impact of ocean mesoscale eddies on the latent and sensible heat fluxes in the South Atlantic ocean. Eddies were identified using the method proposed by Chaigneau et al. (2009), which is based on closed contours of sea level anomaly. Most of the identified eddies had a radius of ~70 km and amplitude of ~5 cm. On average, in the South Atlantic, eddies play a minor role on the ocean-atmosphere heat exchange. However, in strongly energetic regions such as the Brazil-Malvinas confluence or Agulhas Current retroflection regions, eddies can account up to 20-30% of the total variance of the surface turbulent heat fluxes with averaged anomalies of ±10-20 W/m2 for both heat flux components. Cyclonic (anticyclonic) eddies, associated with negative (positive) heat fluxes anomalies tend to cool (warm) the overlying atmosphere. A composite analysis of the turbulent heat fluxes anomalies within the eddies reveals a direct relationship between the eddy amplitude and the intensity of the latent and sensible fluxes anomalies, such that large-amplitude eddies have a stronger signature in the turbulent surface heat fluxes. Heat fluxes anomalies are also much stronger near the eddy centers and decay radially to reach minimum values outside the eddies. / Uma combinação de 10 anos (1999-2009) de fluxos turbulentos de calor pela superfície, medidos a partir de satélites, e dados altimétricos de anomalia da altura da superfície do mar, foram o utilizados com objetivo de investigar o impacto de vórtices de meso-escala nos fluxos de calor sensível e latente na bacia do Atlântico Sul. Para a detecção dos vórtices foi aplicado o método proposto por Chaigneau et al. (2009), que baseia-se em contornos fechados de anomalia da altura da superfície do mar. A maior parte dos vórtices identificados possui raio de ~70 km e amplitude de ~5 cm. Em média, no Atlântico Sul, o impacto dos vórtices para as trocas de calor entre oceano e atmosfera é relativamente fraco. Entretanto, em regiões de alta variabilidade energética como na Confluência Brasil- Malvinas e na retroflecção da Corrente das Agulhas, vórtices de meso-escala podem contribuir com anomalias médias de até ±10-20 W/m2 nos fluxos turbulentos. Vórtices ciclônicos (anti-ciclônicos), associados com anomalias negativas (positivas) de fluxos de calor, tendem a esfriar (esquentar) a atmosfera adjacente. Mapas composite foram analisados para milhares de vórtices, mostrando um relação direta entre a magnitude das anomalias dos fluxos e a amplitude dos vórtices, de tal modo que vórtices de maior amplitude contribuem com maiores anomalias de calor latente e sensível. Além disso, os padrões espaciais dos composites médios revelam que as anomalias são significativamente maiores próximo ao centro dos vórtices e decaem radialmente até atingirem valores absolutos mínimos fora dos contornos dos vórtices.

Basé sur l'analyse conjointe et complémentaire de ~10 ans de données altimétriques et de profils verticaux de température et de salinité acquis par les profileurs Argo, l'objectif principal de cette thèse est d'étudier en détail les caractéristiques de surface et la structure verticale des tourbillons dans les 4 grands systèmes d'upwelling mondiaux (EBUS Pérou-Chili, Californie, Canaries et Benguela), qui partagent une dynamique à grande échelle relativement similaire. Les résultats principaux montrent que l'ensemble des tourbillons détectés sur les cartes altimétriques dans les 4 EBUS ont des propriétés physiques relativement proches, avec une forte proportion de tourbillons de faibles dimensions ayant une durée de vie relativement courte. Au contraire, la faible partie des tourbillons échantillonnés par les profileurs Argo montre des dimensions bien plus grandes pour des durées de vie plus longues. La sur-représentation de ces grandes échelles dans le jeu des tourbillons échantillonnés par les profileurs Argo est donc le biais majeur des résultats obtenus sur la structure verticale des tourbillons. L'analyse des profils moyens d'anomalies de température et de salinité acquis par les profileurs Argo dans les tourbillons révèle une forte hétérogénéité entre les 4 EBUS, mais également au sein de chacun de ces systèmes d'upwelling. Les structures verticales des tourbillons sont fortement liées à l'hydrologie et à la dynamique locale. Par exemple, les tourbillons de subsurface du PCUS occupent plutôt la partie Sud du système, alors que les tourbillons intensifiés en surface sont présents à la limite Nord de la gyre subtropicale. Dans le CALUS, la présence de Cuddies se devine à la côte, les autres sous-régions contenant des tourbillons dont les anomalies sont majoritairement intensifiées à la base de la pycnocline, avec comme particularité un changement de signe lié à la salinité des couches superficielles. Le CANUS est peuplé de tourbillons très différents en fonction de leur position par rapport à la zone frontale du Cap Vert : au Nord, des tourbillons intenses en subsurface, avec la présence de quelques Meddies ; au Sud, des tourbillons très superficiels. De même dans le BENUS, la partie au Nord de 15°S contient plutôt des tourbillons intensifiés dans les couches de surface, alors qu'au Sud du front d'Angola-Benguela, les tourbillons présentent des anomalies fortes sur une grande partie de la colonne d'eau. L'extrême Sud de cet EBUS est également le lieu de passage des Anneaux des Aiguilles. Les contributions de l'advection isopycnale et du déplacement de la colonne d'eau sur la verticale nous permettent d'affiner la description de ces différentes structures. La présence de grands types de tourbillons, à la morphologie bien distincte (cœur de surface, de subsurface, grande extension verticale, tourbillons fortement intensifiés, etc.), est confirmée par l'étude des tourbillons spécifiquement générés dans la bande côtière. L'analyse Lagrangienne de ces tourbillons nous permet également de décrire l'évolution temporelle de leur structure verticale, qui montre une homogénéité temporelle inattendue. Cette thèse présente donc plusieurs outils facilement applicables dans différentes régions océaniques pour caractériser la structure thermohaline des tourbillons et fournit pour la première fois une description des grands types de tourbillons peuplant les EBUS, soulignant la grande diversité de la mésoéchelle. / Merging ~10 years of altimetry maps and vertical profiles provided by Argo floats, we aim to study in details the eddy's surface characteristics and vertical structure in the 4 major Eastern Boundary Upwelling Systems (EBUS : Peru-Chile, California, Canaries and Benguela), sharing similar large-scale dynamics. Our main results show that the eddies detected on altimetry maps in the 4 EBUS have close physical properties, with a lot of small-scale structures (radius < 40km, amplitude < 1cm and lifetime < 30 days). In contrast, the few eddies sampled by Argo floats have larger dimensions (radius of ~90-140 km, amplitude of ~3-7 cm) and longer lifetimes (6-10 months). The major bias with the analyzed vertical structure is the over representation of these large-scale eddies. The temperature and salinity anomaly mean profiles acquired by Argo floats surfacing within eddies reveals a strong heterogeneity between each of the 4 EBUS, but also within them. The eddies' vertical structure is strongly influence by the local hydrology and dynamics. For example, the subsurface-intensified eddies of the PCUS tend to be located in the Southern part of this EBUS, while the surface-intensified eddies are preferentially located near the Northern boundary of the subtropical gyre. In the CALUS, we can identify Cuddies in some coastal sub-regions, but in this EBUS, most of the eddies are intensified at the base of the pycnocline, with a reversal of the salinity anomaly compared to the surface layers. In the CANUS, the Cape Verde frontal zone separates distinct subsurface-intensified eddies and some Meddies in the North, from the Southern part, where eddies are surface-intensified. In the same way, the Angola-Benguela Front of the BENUS separates the surface-intensified eddies in the North from strong, deep-reaching anomalies in the South. The Southern-most part of the BENUS is also a preferential pathway for the large Agulhas Rings and their associated cyclones. The respective contributions of isopycnal advection and vertical displacement improve the description of these very diverse structures. The presence of several eddy-types, with distinct thermohaline properties (surface or subsurface-intensified, deep vertical extend, intense or not, etc.) is confirmed by the study of eddies generated in the coastal area of each EBUS. Their Lagrangian analysis allows us to describe the temporal evolution of their vertical structure, which shows an unexpected temporal homogeneity. This manuscript presents different efficient tools used to analyze the surface characteristics, the thermohaline properties and the temporal evolution of mesoscale eddies in the 4 major EBUS, highlighting their diversity.

By collocating 10 years (1999-2009) of remotely sensed surface turbulent heat fluxes with satellite altimetry data, we investigate the impact of ocean mesoscale eddies on the latent and sensible heat fluxes in the South Atlantic ocean. Eddies were identified using the method proposed by Chaigneau et al. (2009), which is based on closed contours of sea level anomaly. Most of the identified eddies had a radius of ~70 km and amplitude of ~5 cm. On average, in the South Atlantic, eddies play a minor role on the ocean-atmosphere heat exchange. However, in strongly energetic regions such as the Brazil-Malvinas confluence or Agulhas Current retroflection regions, eddies can account up to 20-30% of the total variance of the surface turbulent heat fluxes with averaged anomalies of ±10-20 W/m2 for both heat flux components. Cyclonic (anticyclonic) eddies, associated with negative (positive) heat fluxes anomalies tend to cool (warm) the overlying atmosphere. A composite analysis of the turbulent heat fluxes anomalies within the eddies reveals a direct relationship between the eddy amplitude and the intensity of the latent and sensible fluxes anomalies, such that large-amplitude eddies have a stronger signature in the turbulent surface heat fluxes. Heat fluxes anomalies are also much stronger near the eddy centers and decay radially to reach minimum values outside the eddies. / Uma combinação de 10 anos (1999-2009) de fluxos turbulentos de calor pela superfície, medidos a partir de satélites, e dados altimétricos de anomalia da altura da superfície do mar, foram o utilizados com objetivo de investigar o impacto de vórtices de meso-escala nos fluxos de calor sensível e latente na bacia do Atlântico Sul. Para a detecção dos vórtices foi aplicado o método proposto por Chaigneau et al. (2009), que baseia-se em contornos fechados de anomalia da altura da superfície do mar. A maior parte dos vórtices identificados possui raio de ~70 km e amplitude de ~5 cm. Em média, no Atlântico Sul, o impacto dos vórtices para as trocas de calor entre oceano e atmosfera é relativamente fraco. Entretanto, em regiões de alta variabilidade energética como na Confluência Brasil- Malvinas e na retroflecção da Corrente das Agulhas, vórtices de meso-escala podem contribuir com anomalias médias de até ±10-20 W/m2 nos fluxos turbulentos. Vórtices ciclônicos (anti-ciclônicos), associados com anomalias negativas (positivas) de fluxos de calor, tendem a esfriar (esquentar) a atmosfera adjacente. Mapas composite foram analisados para milhares de vórtices, mostrando um relação direta entre a magnitude das anomalias dos fluxos e a amplitude dos vórtices, de tal modo que vórtices de maior amplitude contribuem com maiores anomalias de calor latente e sensível. Além disso, os padrões espaciais dos composites médios revelam que as anomalias são significativamente maiores próximo ao centro dos vórtices e decaem radialmente até atingirem valores absolutos mínimos fora dos contornos dos vórtices.

L’étude des processus physiques d’un système d’upwelling est essentielle pour comprendre sa variabilité actuelle et ses changements passés et futurs. Cette thèse présente une étude interdisciplinaire du système d’upwelling côtier à partir de différentes données acquises par satellite, l’accent étant mis principalement sur le système d’upwelling d’Afrique du Nord-Ouest (NWA). Cette étude interdisciplinaire aborde (1) le problème de l’identification et de l’extraction automatiques du phénomène d’upwelling à partir d’observations satellitaires biologiques et physiques. (2) Une étude statistique de la variation spatio-temporelle de l’upwelling de la NWA tout au long de son extension et de ses différents indices d’upwelling. (3) Une étude des relations non linéaires entre le mélange de surface et l’activité biologique dans les régions d’upwelling. (4) études lagrangiennes de tourbillons cohérents; leurs propriétés physiques et identification automatique. (5) L’étude des transports effectués par les tourbillons lagrangiens de la NWA Upwelling et leur impact sur l’océan. / Studying physical processes of an upwelling system is essential to understand its present variability and its past and future changes. This thesis presents an interdisciplinary study of the coastal upwelling system from different satellite acquired data, with the main focus placed on the North West African (NWA) upwelling system. This interdisciplinary study covers (1) the problem of the automatic identification and extraction of the upwelling phenomenon from biological and physical satellite observations. (2) A statistical study of the spatio-temporal variation of the NWA upwelling throughout its extension and different upwelling indices. (3) A Study of the nonlinear relationships between the surface mixing and biological activity in the upwelling regions. (4) Lagrangian studies of coherent eddies; their physical properties and automatic identification. (5) The study of transport made by Lagrangian eddies off the NWA Upwelling and their impact on the open ocean. [...]