The Soil Moisture and Ocean Salinity (SMOS) mission is an Earth Explorer Opportunity mission from the European Space Agency (ESA). It was a direct response to the global observations of soil moisture and ocean salinity. Its goal is to produce global of these parameters using a dual-polarization L-band interferometric radiometer the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS). This instrument is a new polarimetric two-dimensional (2-D) Y-shaped synthetic aperture interferometric radiometer based on the techniques used in radio-astronomy to obtain high resolution avoiding large antenna structures. MIRAS measures remotely the brightness temperature (TB) emitted by the Earth's surface, which is not isotropic, since it depends on the incidence angle and polarization, the Soil Moisture (SM) or the Sea Surface (SSS), the surface roughness etc. among others. The scope of this doctoral thesis is the study of some potential improvements could eventually be implemented in future interferometric radiometers. To validate improvements a ground-based instrument concept demonstrator the Passive Advanced Unit Synthetic Aperture or (PAU-SA) has being designed and implemented. Both MIRAS and PAU-SA are Y-shaped array, but the receiver topology and the processing unit are different. This Ph.D. thesis has been developed in the frame of The European Investigator Awards (EURYI) 2004 project entitled "Passive Advanced Unit (PAU): Hybrid L-band Radiometer, GNSS Refectometer and IR-Radiometer for Passive Sensing of the Ocean", and supported by the European Science Foundation (ESF).

The upper ocean mediates the transfer of heat and carbon between the atmosphere and ocean interior. The study of this dynamic environment, made possible in part by long-term time series gathered from oceanographic moorings, is therefore crucial to our understanding of Earth’s climate. In this thesis, we use moored datasets from the Southeast Pacific and Southern Oceans to explore two upper-ocean processes relevant to the transfer and eventual sequestration of atmospheric heat and carbon into the deep ocean: wind-, wave-, and buoyancy-forced turbulence and the release of brine in Antarctic polynyas that drives the formation of Antarctic Bottom Water (AABW). In Chapter 1, we use measurements of turbulence kinetic energy (TKE) dissipation rate (ε) collected at 8.4 m depth on the long-established Stratus Mooring in the Southeast Pacific (20° S, 85° W) to assess the applicability of Monin-Obukhov similarity theory (MOST), Law of the Wall (LOW), and other boundary layer similarity scalings to turbulence in the upper ocean. TKE facilitates the mixing of heat, momentum, and solutes within and between the ocean and atmosphere and is generated in the upper ocean primarily by wind, waves, and buoyancy fluxes. Its production can generally be assumed to equal its dissipation, and measurements of ε therefore serve as a means for quantifying turbulence in a system. We present 9 months of ε measurements, a remarkably long time series made possible by the use of a moored pulse-coherent Acoustic Doppler Current Profiler (ADCP), a new methodology for measuring ε that uniquely allows for concurrent surface flux and wave measurements across an extensive length of time and range of conditions. We find that turbulence regimes are quantified similarly using the classic Obukhov length scale (L_M=(u_*³)/(κ𝐵ₒ), where u_* is ocean-side friction velocity, κ is the von Kármán constant, and B_0 is surface buoyancy flux) and the newer Langmuir stability length scale (L_L=(〖u_s u〗_*²)/B_0 , where u_s is surface Stokes drift velocity), suggesting that u_* implicitly captures the influence of Langmuir turbulence at this site. This is consistent with the strong correlation observed between u_s and u_*, likely promoted by the steady southeast trade winds, and suggests that classic wind and buoyancy-based boundary layer scalings sufficiently describe turbulence in this this region. Accordingly, we find the LOW (ε=(u_*³)/κz, where z is instrument depth) and surface buoyancy scaling (ε=B_0, where B_0 is destabilizing surface buoyancy flux) used in classic turbulence scaling studies, such as Lombardo and Gregg (1989), to describe our measurements well, and a newer scaling for Langmuir turbulence scaling based on u_s and u_* to scale ε well at times but to be overall less consistent than (u_*³)/κz. The performance of MOST relationships from prior studies in a variety of aquatic and atmospheric settings are also examined, and we find them to largely agree with our data in conditions where both convection and wind-driven current shear act as significant sources of TKE (-1<z/L_M <0). The apparent redundancy of Langmuir turbulence scaling and the sufficiency of LOW and MOST observed in this study may help inform the development of general circulation models (GCMs), which rely on boundary layer scaling to parametrize turbulent mixing in the upper ocean. In Chapters 2 and 3, we focus on the Terra Nova Bay Polynya in the western Ross Sea of Antarctica, where High Salinity Shelf Water (HSSW) forms as a result of the cooling and salinification of the surface ocean by an intense katabatic wind regime and its associated ice production. HSSW is a precursor to AABW, a vital water mass that feeds the bottom limb of the meridional overturning circulation (MOC) and facilitates the sequestration of atmospheric heat and carbon into the abyss. A decades-long freshening trend in the salinity of Ross Sea HSSW resulting from increased glacial meltwater fluxes, and more recently, its abrupt reversal associated with the occurrence of a climate anomaly, have highlighted the complexity of this system and its sensitivity to changes in climate. Because the density of HSSW has a direct impact on the density of downstream AABW, and therefore the strength of the MOC, it is imperative to better understand the variability and mechanisms of HSSW formation. However, inhospitable wintertime conditions in this region severely restrict the collection of in-situ data in the presence of active brine rejection and HSSW formation. Here, we present an unprecedented set of upper-ocean salinity, temperature, turbulence, current velocity, and acoustic surface tracking time series collected from a mooring in Terra Nova Bay during austral winter 2017. One poorly constrained aspect of HSSW in Terra Nova Bay is its rate of production, and in Chapter 2 we endeavor to produce the first production rate estimates to be based on in-situ salinity data. We find an average production rate of ~0.6 Sverdrups (10⁶ m³ s⁻¹), which allows us to improve on and validate an existing approach for estimating rates using parametrized net surface heat fluxes out of the polynya. We use this approach to examine interannual variability in production across the decade and find estimates of HSSW production in Terra Nova Bay to be largely increasing from 2015 onward. As higher production rates of Terra Nova Bay HSSW, the saltiest variety of HSSW across Antarctica, could increase the salinity of downstream AABW, this apparent increase may have played a previously unrecognized role in the recently observed recovery of AABW salinity in this region. In Chapter 3, we examine a number of interconnected processes surrounding HSSW formation, including the coupling of salinity to winds, the breakdown of summer stratification that primes the water column for HSSW formation in the winter, wind-driven turbulence that facilitates the breakdown of stratification and mixing of HSSW to depth, and potential circulation pathways for HSSW formed at the mooring site. We find that salinity at the shallowest depth on the mooring line, 47 m, couples strongly to wind speeds measured at the nearby Automatic Weather Station (AWS) Manuela from April onward, demonstrating the dependence of polynya formation, ice production, and brine rejection on winds at the mooring site. Salinity at the deepest depth on the mooring line, 360 m, couples to salinity at 47 m beginning in June, following the progressive breakdown of lingering summertime water column stratification that previous studies have established as a prerequisite for HSSW formation in the winter. We incorporate concepts from Chapter 1 to explore the scaling of turbulence in a polynya environment, finding that daily-averages of ε are sufficiently approximated according to the classic LOW scaling, despite visible evidence of Langmuir circulation in the polynya. To the best of our knowledge, this represents the first examination of turbulence scaling using in-situ time series measurements in an Antarctic polynya, an environment that connects the turbulent mixing of heat and solutes in the upper ocean to the properties of the deepest layer of the ocean. Lastly, we infer from current velocities and a late-winter coupling of salinity measured at our mooring to that measured by a second mooring within the Drygalski Basin that HSSW may travel one of two pathways following its formation at our mooring site: Directly southeastward into the Drygalski Basin or northeastward along with the cyclonic gyre of Terra Nova Bay. More mooring deployments across space and time within the bay are needed in order to further elucidate the variability and mechanisms surrounding HSSW formation, critical foci of study in the context of a rapidly changing Antarctic environment.

The composition of a community and the environmental conditions in which they exist fundamentally influence productivity and responses of systems to change. In the Northern Gulf of Mexico (NGoM), the relationships between nutrients, salinity, and phytoplankton populations are complex and have been changing over time. This work focuses on describing and analyzing: 1) a case study of diversity and recommendations for change within an academic institution; 2) spatial and temporal trends in surface dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the NGoM over 35 years; 3) nutrient addition experiments (NAEs) to determine prevailing NGoM surface slope region nutrient limitation; and 4) NGoM surface continental slope phytoplankton community composition and dynamics. Over time, academic institutions have not made progress toward increasing diversity, equity, and inclusion (DEI) in the geosciences. The first chapter of this work serves as a roadmap for other institutions to make progress toward ingraining DEI frameworks into the foundations of our institutional systems. Toward explaining trends in nutrients from 1985 to 2019, I compiled the largest data set of NGoM surface dissolved nutrient concentrations to date and analyzed it to delineate spatiotemporal trends and identify potential drivers of nutrient change. DIP concentrations in both the Mississippi-Atchafalaya River system (MAR) and in the NGoM increased over time, but the increase of NGoM DIP exceeded the DIP loads coming from only the MAR, suggesting additional sources of P to the NGoM. To determine nutrient controls on surface slope NGoM phytoplankton growth and populations, we calculated growth rates and pigment composition using redundancy analyses and a variety of nutrient limitation criteria for each nutrient amendment over 48 hours. Nutrient limitation criteria concluded predominant NP limitation in the NGoM, though single N and P limitation and nutrient replete conditions were also present. In individual NAEs with N and NP amendments, phytoplankton pigment changes were driven by the growth of diatoms and Synechococcus (Syn). Though release from nutrient limitation stimulated responses in some phytoplankton groups, nutrient limitation of phytoplankton growth could not fully be predicted by the criteria and response thresholds evaluated in this study. Additionally, an analysis of environmental variables and phytoplankton pigments was conducted for the surface slope region of the NGoM to determine how phytoplankton community composition varies spatially with the influence of the MAR plume using group-specific chlorophyll a (Chl-a) calculations, bivariate linear regression, multivariate redundancy analysis, and cluster analysis. The largest proportion of Chl-a occurred in the nano/microphytoplankton group, followed by Syn, with both peaking at the high and low ends of the salinity gradient. Redundancy and cluster analyses showed that nutrients and salinity alone cannot predict or subdivide phytoplankton community composition; however, with the addition of pigments, we can characterize specific regions based on shared environmental variables (i.e., low salinity, high biomass) and pigment abundance. In sum, this work produced a straightforward and reproducible guide to leading a DEI task force, the largest NGoM surface nutrient data set to date, and characterizations of NGoM continental slope nutrient limitation and pigment composition and their relation to environmental variables.

SMOS (Soil Moisture and Ocean Salinity) is ESA's Earth Explorer series satellite carrying the novel MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) interferometric synthetic aperture radar. Its objective is monitoring and studying the planet's water cycle by following the changes in soil moisture levels and ocean surface salt concentrations on a global scale. The success of the mission calls for nearly uninterrupted operation of the science payload. However, the instrument experiences sporadically problems with its hardware, which cause losses of scientific data and may require intervention from ground to resolve. The geographical areas in which most of these anomalies occur, polar regions and the South-Atlantic anomaly, give cause to assume these problems are caused by charged particles in the planet's ionosphere. In this thesis, methods of predicting occurrence of hardware anomalies from indicators of Earth radiation environment are investigated.

Ce travail a porté sur la variabilité de la salinité de surface (SSS) de l'océan dans le gyre subtropical Nord Atlantique. J'ai étudié la variabilité saisonnière de la SSS en lien avec les flux d'eau douce échangés avec l'atmosphère et la circulation océanique à méso échelle, au cours de plus de deux ans, d'août 2012 à décembre 2014. Les produits issus de la mission satellitaire soil moisture and ocean salinity (SMOS) corrigés de biais systématiques aux grandes échelles ont été testés et utilisés pour restituer la variabilité méso-échelle de SSS. Nous avons de surcroit utilisé les simulations numériques à haute résolution PSY2V4R2-R4 de Mercator. Les champs issus de SMOS et des simulations ont été comparés aux données in situ de bouées dérivantes et de thermosalinographes recueillies pendant l'expérience SPURS, avec des résultats satisfaisants, en particulier en hiver, et des écarts-type de différences typiques de l'ordre de 0.15 pss. Le flux d’eau douce échangé avec l’atmosphère est le terme dominant dans le bilan saisonnier de la SSS. Ce sont des termes associés à la dynamique océanique qui le compensent partiellement. En particulier, l’entrainement des eaux sous-jacentes contribue fortement en début d’hiver. Il agit d’ordinaire à réduire la SSS, à l’exception de la région au sud du maximum de SSS, où c’est au contraire une augmentation qu’il induit. L’advection est une seconde contribution importante à la variabilité de la SSS. Elle transfert ainsi vers le nord les eaux ‘salinisées’ plus au sud dans la région du maximum de perte d’eau douce vers l’atmosphère. La contribution d'advection est fortement dépend du type de données utilisées et leur résolution spatiale. / The focus of this work is on sea surface salinity (SSS) variability in the North Atlantic subtropical gyre. We study seasonal SSS variability and its link to the atmospheric freshwater flux at the ocean surface and to ocean dynamics at meso-scales for the period August 2012 – December 2014. The products from the soil moisture and ocean salinity (SMOS) satellite mission corrected from large scale systematic errors are tested and used to retrieve meso-scale salinity features. Furthermore, the PSY2V4R2-R4 simulation produced by Mercator with a high spatial resolution is also used. The comparison of corrected SMOS SSS data and Mercator simulation with drifter's in situ and TSG measurements from the SPURS experiment shows a reasonable agreement with RMS differences on the order of 0.15 pss.The freshwater seasonal flux is the leading term in the SSS seasonal budget. To balance its effect the ocean dynamics strongly contribute. The entrainment of deeper water is strong during the winter time. It usually acts to lower SSS, except in the south of the SSS–max region where it contributes to increase salinity. Advection is the second important component responsible for the SSS variability. It transfers further north the salty water from the evaporation maximum region. The contribution of advertion term is strongly dependent on the type of data used and their spatial resolution.

La télédétection par satellite est aujourd'hui une composante à part entière de l'océanographie. Elle permet d'effectuer des mesures de vents, de température de surface (SST), de couleur de l'eau, de topographie, ... avec des couvertures spatiales et temporelles bien supérieures à celles obtenues par des méthodes in situ. Cependant, il n'existe pas à l'heure actuelle de mesure satellitaire de salinité de surface des océans (SSS), et celle-ci reste sous échantillonnée à la fois spatialement et temporellement. La salinité étant un paramètre important pour la circulation des masses d'eau océaniques, son observation globale et régulière constituerait un apport conséquent à l'océanographie physique. C'est pourquoi de nombreuses équipes scientifiques à travers le monde relèvent actuellement le défi technologique de la télédétection de la SSS par satellite, et particulièrement en Europe grâce à la mission de l'Agence Spatiale Européenne « Soil Moisture and Ocean Salinity » (SMOS). Au cours de ma thèse, j'ai étudié la faisabilité de la mesure de la SSS à l'aide d'un radiomètre hyperfréquence en bande L (i.e. fréquence = 1.4 GHz <=> longueur d'onde = 21 cm), en estimant les sources d'incertitude sur la SSS qui sera restituée dans le cadre de la mission SMOS. Pour cela, j'ai codé un modèle direct, qui simule les processus physiques intervenant depuis la surface océanique jusqu'à l'antenne du radiomètre. Ce modèle est constitué d'un modèle d'émissivité de la mer à « deux échelles » (i.e. on distingue les vagues selon qu'elles soient « grandes » ou « petites » par rapport à la longueur d'onde du radiomètre), et d'un modèle de transfert radiatif à travers l'atmosphère. Le modèle d'émissivité m'a permis d'estimer la sensibilité de la température de brillance (Tb) de l'océan aux paramètres géophysique océanique (i.e. SSS, SST, et rugosité de surface induite par le vent ou la houle), ainsi que l'incertitude sur cette sensibilité en comparant les résultats obtenus à partir de paramétrisations différentes. J'ai conclu de ces études que la sensibilité de la Tb à la SSS est relativement bien connue (de l'ordre de quelques dixièmes de Kelvin par psu) mais que l'effet de la rugosité est très incertain à cause de l'imprécision des modèles de spectre des vagues, alors que cet effet ne semble pas être négligeable (la sensibilité de la Tb au vent étant comprise entre 0.12 à 0.25 K/(m/s) selon le modèle de spectre). Le modèle de transfert radiatif m'a permis d'estimer les différentes contributions de l'atmosphère (atténuation des rayonnements la traversant et émission propre), ainsi que la sensibilité de ces contributions aux paramètres atmosphériques (i.e. profils de température, pression et humidité relative). En bande L, l'atmosphère est quasiment transparente (épaisseur optique ~ 0.01 néper) et sa température de brillance est de l'ordre de 2 K. Ces effets sont peu sensibles aux paramètres atmosphériques, particulièrement à la vapeur d'eau. Je présente aussi dans la thèse des comparaisons du modèle avec des mesures radiométriques en bande L récentes (campagnes WISE 2000, WISE 2001 et EuroSTARRS) ainsi que les conclusions sur la validité des différents modèles de spectre de mer étudiés.

Un regain d'intérêt pour la télédétection de la salinité de surface de l'océan (SSS) par radiométrie en bande-L (21cm) est apparu dans les années 1990 et a conduit au lancement des missions spatiales SMOS (nov. 2009) et Aquarius (juin 2011). Cependant, en raison du faible rapport signal sur bruit, l'inversion de la SSS à partir des mesures radiométriques en bande-L est très difficile. Ce travail porte sur l'étude de la signature radiométrique en bande-L des propriétés de la surface de l'océan (en particulier SSS et rugosité) à partir des mesures du radiomètre aéroporté en bande-L CAROLS, acquises dans le golfe de Gascogne en 2009 et 2010. Une première étude a montré que la SSS déduite des mesures du radiomètre CAROLS était précise à mieux que 0.3 pss dans une zone de forte variabilité spatio-temporelle avec une meilleure précision que les modèles océanographiques côtiers. La seconde étude qui combine les mesures passives (CAROLS) et active (diffusiomètre en bande-C STORM) a mis en évidence l'amélioration des nouveaux modèles de rugosité par rapport aux modèles pré-lancement satellitaires. Par ailleurs, l'étude a montré l'importance de la prise en compte des moyennes et grandes échelles de rugosité (> 20 cm) pour l'interprétation des mesures radiomé- triques loin du nadir.