Dissertations / Theses on the topic 'CO2 Air-Sea fluxes'

Les plateaux continentaux sont des régions très productives et pourraient constituer de régions de puits significatif de CO2 pour l’atmosphère. De 2000 à 2006, vingt-deux sections océanographiques sur le plateau continental de la Mer de Patagonie (Projets ARGAU et GEF PATAGONIA) ont permis d'étudier la variabilité saisonnière des différences de pression partielle de CO2 (pCO2) et des flux de CO2 (FCO2) entre la mer et l'atmosphère. Ce travail présente une analyse de l'influence des différentes variables environnementales et des processus physiques et biologiques sur les flux de CO2 à l’interface Océan-Atmosphère en mer de Patagonie. Malgré une variabilité saisonnière importante de pCO2 dans les eaux de surface, la mer de Patagonie constitue un puits pendant toutes les saisons. Dans ce plateau continental, le puits de CO2 est dû à des processus dynamiques (stratification, mélange vertical et fronts) et est intensifié par la pompe biologique. Il est montré que les diatomées sont beaucoup plus efficaces pour le pompage du CO2 que les dinoflagellées. Le premier bilan du CO2 (naturel et anthropique) pour l’Argentine montre que la mer de Patagonie capture une quantité de CO2 similaire aux émissions dues à la consommation d’énergie domestique.

Carbon dioxide (CO2) is an important greenhouse gas, and the atmospheric concentration of CO2 has increased by more than 100 ppm since prior to the industrial revolution.  The global oceans are considered an important sink of atmospheric CO2, since approximately one third of the anthropogenic emissions are absorbed by the oceans. To be able to model the global carbon cycle and the future climate, it is important to have knowledge of the processes controlling the air-sea exchange of CO2. In this thesis, measurements as well as a model is used in order to increase the knowledge of the exchange processes. The air-sea flux of CO2 is estimated from high frequency measurements using three methods; one empirical method, and two methods with a solid theoretical foundation. The methods are modified to be applicable for various atmospheric stratifications, and the agreement between methods is good in average. A new parameterization of the transfer velocity (the rate of transfer across the air-sea interface), is implemented in a Baltic Sea model. The new parameterization includes also the mechanism of water-side convection. The impact of including the new parameterization is relatively small due to feedback processes in the model. The new parameterization is however more representative for flux calculations using in-situ measurement or remote sensing products. When removing the feedback to the model, the monthly average flux increases by up to 20% in some months, compared to when water-side convection is not included. The Baltic Sea carbon budget was estimated using the Baltic Sea model, and the Baltic Sea was found to be a net sink of CO2. This is consistent with some previous studies, while contradictory to others. The dissimilarity between studies indicates the difficulty in estimating the carbon budget mainly due to variations of the CO2 uptake/release in time and space. Local variations not captured by the model, such as coastal upwelling, give uncertainties to the model. Coastal upwelling can alter the uptake/release of CO2 in a region by up to 250%. If upwelling would be included in the model, the Baltic Sea might be considered a smaller sink of CO2.

The Carbon dioxide (CO2) concentration in the atmosphere has increased dramatically since the start of the industrialisation. The effects that the increase of CO2 has on the future climate are still not fully investigated. CO2 in the atmosphere contributes to the, for all life on earth, necessary greenhouse effect. It is confirmed that higher CO2 concentration in the atmosphere increases the green house effect, which results in higher temperature. The main source to the increase of CO2 is burning of fossil fuels. The change in land use is also a contribution to the increase of the CO2 concentration in the atmosphere. The largest sinks of CO2 are organic consumption and oceanic uptake. The organic consumption of CO2 varies a lot at higher latitudes due to the difference in vegetation between the seasons. During the warmer seasons the consumption of CO2 is large and during the winters the consumptions of CO2 is practically zero. The ocean uptake of CO2 varies also a lot during the year because the CO2 dissolves more easily in cold water. The purpose of this study is to analyse CO2 concentration and air-sea fluxes of CO2 measured at Östergarnsholm, a small flat island east of Gotland in the Baltic Sea, and compare the results to previous studies. The CO2 concentration data was collected between 1997 – 1999 and 2001 – 2003. The CO2 flux data was collected between 2001 and 2003. The analysis of the CO2 concentration showed that for the period 1997 to 1999, the CO2 concentration at Östergarnsholm was lower than for the reference series from a Polish site in the Baltic Sea. A correction was made by adding 27 ppm to the Östergarnsholm series. The annual fluctuations of CO2 concentration at Östergarnsholm are significant (about 40 ppm). During the summer 1998, the expected decrease was not as large as it should be because of the El Niño outbreak 97/98 and the locally cold and rainy summer. The direct measured CO2 fluxes were corrected with the well known Webb correction before they were analysed. The CO2 fluxes are wind dependant – higher wind speed give higher CO2 flux. The CO2 fluxes are also dependant of the difference in partial pressure between the air and the water. Parameterised CO2fluxes were calculated and compared to the direct measured CO2 fluxes. The parameterisations use a quadratic as well as a cubic wind dependency. To calculate the parameterised CO2 fluxes, a fixed value of the difference in partial pressure between the air and the water was used because the CO2 in the water was not measured. The parameterised CO2 fluxes wind dependency agreed with the direct measured CO2 fluxes.
Koldioxid(CO2)-koncentrationen i atmosfären har ökat stadigt sen början av industrialiseringen. Effekten som de ökade CO2-halterna kommer ha på framtidens klimat är ännu inte helt utrett. CO2 bidrar till den livsviktiga växthuseffekten. Det är en ökning av växthusgaser, bland annat CO2, som leder till en ökning av växthuseffekten. Ökad växthuseffekt leder till högre temperatur på jorden. Den största ökningen av CO2 i atmosfären beror på förbränning av fossila bränslen. Även förändringen i markanvändning leder till ökade halter av CO2. De största sänkorna av CO2 är den organiska konsumtionen av CO2 och havens upptag av CO2. Den organiska konsumtionen av CO2 varierar mycket under året och är som störst under de varmare månaderna. Havens upptag av CO2 varierar också mycket under året eftersom havens förmåga att lösa CO2 beror på vattnets temperatur. Syftet med den här studien är att analysera CO2-koncentrationen och CO2-flödena mellan hav och luft på Östergarnsholm, en liten, låg ö öster om Gotland. Resultaten jämförs med tidigare studier. CO2-koncentrationsdata samlades in mellan 1997 – 1999 och 2001 – 2003. CO2-flödesdata samlades in mellan 2001 och 2003. Analysen av CO2-koncentrationen visar att under perioden 1997 till 1999 är CO2- halterna för låga på Östergarnsholm. En korrektion gjordes genom att lägga till 27 ppm till de uppmätta CO2-halterna. Årsvariationerna av CO2-halterna är mycket tydliga men sommaren 1998 sjunker inte CO2-halten till så låga värden som de borde vara. Att CO2-halterna inte sjönk mer beror dels på El Niño-utbrottet 97/98 och dels på den lokalt kalla och regniga sommaren. De direkt mätta CO2-flödena korrigerades med hjälp av den välkända Webbkorrektionen innan de analyserades. CO2-flödena är beroende av vindhastigheten – högre vindhastighet ger högre CO2-flöden. CO2-flödena beror också på skillnaden i CO2-halt mellan luften och havet. Parameteriserade CO2-flöden beräknades och jämfördes med de direkt mätta CO2-flödena. De parameteriserade CO2-flödena beräknas antingen med kvadratiskt eller kubiskt vindberoende. För att beräkna parameteriserade CO2-flöden användes ett fast värde på skillnaden i CO2-halt mellan luften och vattnet eftersom CO2-halten i vattnet inte mäts. De parameteriserade CO2- flödenas vindberoende stämde överrens med de direkt mätta CO2-flödena.

L'augmentation continue des concentrations atmosphériques de CO2 due aux activités anthropogéniques est un des principaux facteurs responsable du changement climatique. De par leur forte propension à stocker ce CO2 anthropogénique, les océans jouent un rôle essentiel dans le cycle global du carbone. La quantification des échanges air-mer de CO2 et de leur variabilité à diverses échelles spatio-temporelles représentent encore aujourd'hui un défi majeur dans l'étude du cycle global du carbone. Alors que les flux air-mer de CO2 sont relativement bien quantifiés en milieu océanique, les études réalisées en milieu marin côtier demeurent insuffisantes au regard de l'importante variabilité spatio-temporelle de ces échanges et de la diversité de ces écosystèmes. L'objectif de cette thèse est de mener une étude approfondie de la dynamique du système des carbonates et des échanges air-mer de CO2 à de multiples échelles spatio-temporelles au sein des écosystèmes contrastés du plateau continental nord-ouest européen. Ces systèmes particulièrement dynamiques d'un point de vue biogéochimique présentent l'avantage d'être représentatifs des principales structures hydrographiques des marges continentales tempérés. A ce jour, les études portant sur la dynamique du CO2 dans les eaux de la partie occidentale du plateau continental nord-ouest européen restent peu nombreuses. Du cycle diurne à une échelle multi-annuelle, d'une station fixe au large de Roscoff au plateau continental nord-ouest européen, et d'échantillons d'eau de mer à des données satellitaires, cette thèse offre un aperçu exhaustif de la complexité de la dynamique du système des carbonates et des flux air-mer de CO2 en milieu côtier
The raise of atmospheric CO2 due to anthropogenic activities is a major driver of the climate change. The ocean plays a key role in the uptake of this anthropogenic CO2. The constraint of air–sea CO2 fluxes and their variability at various time and spatial levels remain a central task in global carbon cycle and climate studies. The contribution of open ocean to this uptake is presently rather well quantified, whereas the role of the coastal ocean to this process remains ambiguous due to the diversity and the high spatio-temporal variability of the CO2 system and air-sea CO2 fluxes in these ecosystems. This PhD thesis investigated the spatial and temporal variability of the CO2 system and air-sea CO2 fluxes in contrasted ecosystems of the north-west European continental shelf. These highly dynamic biogeochemical ecosystems host numerous key hydrographical structures (permanently well-mixed, seasonally stratified, frontal structures, estuarine) of temperate zones, in which the dynamic of the CO2 system were poorly documented. From tidal to multi-annual variability, from a fixed station off Roscoff to the north-west European continental shelf and from seawater samples to satellite data, this PhD thesis provides an integrative overview of the complexity of the CO2 system dynamics in coastal seas and the ongoing challenges to achieve

L'augmentation continue des concentrations atmosphériques de CO2 due aux activités anthropogéniques est un des principaux facteurs responsable du changement climatique. De par leur forte propension à stocker ce CO2 anthropogénique, les océans jouent un rôle essentiel dans le cycle global du carbone. La quantification des échanges air-mer de CO2 et de leur variabilité à diverses échelles spatio-temporelles représentent encore aujourd'hui un défi majeur dans l'étude du cycle global du carbone. Alors que les flux air-mer de CO2 sont relativement bien quantifiés en milieu océanique, les études réalisées en milieu marin côtier demeurent insuffisantes au regard de l'importante variabilité spatio-temporelle de ces échanges et de la diversité de ces écosystèmes. L'objectif de cette thèse est de mener une étude approfondie de la dynamique du système des carbonates et des échanges air-mer de CO2 à de multiples échelles spatio-temporelles au sein des écosystèmes contrastés du plateau continental nord-ouest européen. Ces systèmes particulièrement dynamiques d'un point de vue biogéochimique présentent l'avantage d'être représentatifs des principales structures hydrographiques des marges continentales tempérés. A ce jour, les études portant sur la dynamique du CO2 dans les eaux de la partie occidentale du plateau continental nord-ouest européen restent peu nombreuses. Du cycle diurne à une échelle multi-annuelle, d'une station fixe au large de Roscoff au plateau continental nord-ouest européen, et d'échantillons d'eau de mer à des données satellitaires, cette thèse offre un aperçu exhaustif de la complexité de la dynamique du système des carbonates et des flux air-mer de CO2 en milieu côtier
The raise of atmospheric CO2 due to anthropogenic activities is a major driver of the climate change. The ocean plays a key role in the uptake of this anthropogenic CO2. The constraint of air–sea CO2 fluxes and their variability at various time and spatial levels remain a central task in global carbon cycle and climate studies. The contribution of open ocean to this uptake is presently rather well quantified, whereas the role of the coastal ocean to this process remains ambiguous due to the diversity and the high spatio-temporal variability of the CO2 system and air-sea CO2 fluxes in these ecosystems. This PhD thesis investigated the spatial and temporal variability of the CO2 system and air-sea CO2 fluxes in contrasted ecosystems of the north-west European continental shelf. These highly dynamic biogeochemical ecosystems host numerous key hydrographical structures (permanently well-mixed, seasonally stratified, frontal structures, estuarine) of temperate zones, in which the dynamic of the CO2 system were poorly documented. From tidal to multi-annual variability, from a fixed station off Roscoff to the north-west European continental shelf and from seawater samples to satellite data, this PhD thesis provides an integrative overview of the complexity of the CO2 system dynamics in coastal seas and the ongoing challenges to achieve

L'océan Austral joue un rôle crucial dans la régulation du système climatique en absorbant de grandes quantités de CO2 atmosphérique. Toutefois de nombreuses incertitudes demeurent quant à l'évolution récente du puits de carbone austral notamment en raison du manque d'observations et des lacunes des modèles océaniques dans la représentation de processus dynamiques comme les tourbillons. Depuis quelques décennies notamment, l'efficacité du puits de carbone austral diminuerait en raison d'une intensification des vents liée à une tendance positive du Mode Annulaire Austral (SAM). L'objectif de ces travaux de thèse est de décrire et comprendre la variabilité spatiale et temporelle récente des flux air-mer de CO2 dans l'océan Austral. Pour cela, des simulations de sensibilité aux phases positives du SAM sont réalisées dans une configuration régionale de l'océan Austral (sud de 30°S), basée sur un modèle couplé dynamique-biogéochimie forcé par l'atmosphère et résolvant partiellement la méso-échelle océanique. Dans l'océan Austral, la réponse des flux de CO2 au SAM correspond à un dégazage intense de CO2 dans la zone antarctique dû à une augmentation des concentrations de surface de carbone inorganique dissous (DIC). Cette augmentation est pilotée par la dynamique de la couche de mélange et alimentée par un transport méridien de DIC qui résulte essentiellement de la compétition entre circulation induite par les vents et par les méandres stationnaires. Ces travaux montrent l'apport d'une augmentation de la résolution numérique des modèles pour la simulation des flux de CO2
By taking up large amounts of atmospheric CO2, the Southern Ocean helps to regulate the climate system. Southern Ocean carbon sink is poorly constrained, in part because data coverage is sparse and also because ocean models that have been used in such assessments fail to explicitly resolve key physical features such as mesoscale eddies. In recent decades, the growth of the Southern Ocean carbon sink may have been partly counteracted due to a loss of natural CO2 from the ocean driven by an intensification of westerlies, related to a positive trend in the Southern Annular Mode (SAM). This thesis focuses on documenting and understanding recent spatial and temporal variability of air-sea CO2 fluxes in the Southern Ocean. Sensitivity to positive phases of the SAM are tested by making simulations with a regional model of the Southern Ocean (south of 30°S) that couples biogeochemistry to the dynamics, is forced by atmosphere reanalysis data, and partially resolves the mesoscale. The resulting response of Southern Ocean CO2 fluxes to the SAM is dominated by a strong CO2 efflux to the atmosphere from the Antarctic Zone due to an increase in surface dissolved inorganic carbon (DIC). This increase is driven by the mixed-layer dynamics and is supplied by a meridional transport of DIC, a competition between the wind-driven circulation and the standing eddy-induced circulation. This work discusses the effect of increasing model resolution on simulated air-sea CO2 fluxes

L'océan Atlantique tropical contrôle les échanges d'un hémisphère à l'autre et est un lieu de fortes interactions avec l’atmosphère. Cinq des plus grands fleuves du monde s'écoulent dans cet océan et la zone de convergence intertropicale (ITCZ) y est une source d’intenses précipitations. Cela induit une grande variabilité de la salinité et des flux air-mer de CO2. Alors que l'océan global est un fort puits de CO2, cette région est une source importante de CO2 en raison des eaux profondes riches en carbone inorganique qui remontent à la surface au niveau de l'équateur. Cependant, ce phénomène est atténué par le panache de l’Amazone, dont les eaux douces sont pauvres en carbone inorganique, riches en nutriments et favorisent le développement du phytoplancton. C'est dans ce cadre que se propagent les ondes tropicales d’instabilité (TIW) et les anneaux du courant Nord Brésil (NBC), les deux formes de mésoéchelle dominantes de l’Atlantique tropical. L'objectif de ce travail est de décrire et de comprendre la variabilité de la salinité et des flux air-mer de CO2 associée à la mésoéchelle. Pour cela, des observations in-situ sont couplées à des données satellitaires de salinité, température et chlorophylle-a de surface. Dans l'Atlantique équatorial, le gradient de salinité entre l'eau douce provenant des précipitations sous l’ITCZ et l'eau salée de l'upwelling équatorial est très fort en mai-juin. Les TIW déforment ce gradient, et l’observation de leur signature en salinité fournit des informations sur leur variabilité saisonnière et interannuelle complémentaires à celles de la température de surface. La salinité couplée à la température détermine les contrastes de densité de surface, ce qui influence l’énergie associée aux TIW. Le gradient horizontal de salinité contribue à la moitié de l’énergie potentielle générée par la déformation du gradient horizontal de densité. Ainsi, les TIW modifient et sont modifiés par les contrastes de salinité dans l'Atlantique équatorial. Sur le bord ouest du bassin, le panache de l'Amazone induit une variabilité de la salinité encore plus importante que celle observée dans l'Atlantique équatorial. Les anneaux du NBC, tourbillons de 200 km de diamètre, sont des structures très contrastées. Ils piègent les eaux salées et riches en CO2 du NBC, mais leur rotation advecte l’eau peu salée et appauvrie en CO2 du panache de l'Amazone. L'eau du panache renforce donc les échanges de CO2 et de chaleur entre l’océan et l'atmosphère. En février 2020, l'Atlantique tropical nord-ouest est un puits de carbone 10 fois plus fort qu'anticipé, et cela est dû à plus de 40% à l'effet des tourbillons. Leur rôle est double, d'une part ils entrainent le panache qui devient un fort puits de carbone, et d'autre part, ils ne gardent pas la signature de surface riche en CO2 des eaux qu'ils piègent. La situation en été est très différente de celle en hiver. Le NBC change son orientation de 90° et au lieu de suivre la côte sud-américaine, il s’écoule vers l'Afrique. Il passe au large de l’embouchure du fleuve Amazone, qui a alors un fort débit, et devrait entraîner le panache vers l'est. Cependant, les anneaux du NBC et les vents modifient ce schéma. La formation et la propagation de tourbillons interrompent cette circulation, et les vents favorisent un transport d'eau douce vers le nord-ouest. Ainsi, en août-septembre, alors qu'une part du panache est entrainée vers l'est, une autre part est advectée vers les Petites Antilles. L'été 2021 présente des exemples particulièrement forts de ce phénomène. Ces travaux montrent l'importance de la méso-échelle océanique pour la compréhension de phénomènes clés, comme la propagation des ondes tropicales d’instabilité, du panache de l'Amazone et le flux de CO2 dans l’océan Atlantique tropical
The tropical Atlantic Ocean (TAO) controls exchanges from one hemisphere to the other and is a place of strong interactions with the atmosphere. The TAO is home to five of the world's largest rivers as well as intense rainfall in the intertropical convergence zone (ITCZ). This induces large spatial variability of salinity and of air-sea CO2 flux. While the global ocean is a strong CO2 sink, the TAO is a strong source of CO2 to the atmosphere due to the deep waters rich in inorganic carbon upwelled to the surface at the equator. However, this source is mitigated by the low CO2 concentrations in the Amazon River plume whose freshwater is low in inorganic carbon and favours phytoplankton blooms. It is in this context that propagate the tropical instability waves (TIWs) and the North Brazil current (NBC) rings, the two dominant mesoscale forms in the TAO. The objective of this work is to describe and understand the variability of the surface salinity and CO2 fluxes associated with the mesoscale. In-situ observations collected during cruises and Argo floats are coupled to surface satellite salinity, temperature and chlorophyll-a. In the equatorial Atlantic the salinity gradient between the fresh water from rainfall under the ITCZ and the salty water of the equatorial upwelling is very strong in May-June. The TIWs strongly distort this gradient, and are therefore particularly well observed in surface salinity. The observation of TIWs in salinity provides complementary information to their observation in surface temperature on their seasonal and interannual variability. Furthermore, salinity does not only play the role of a passive tracer, as together with temperature, it determines the seawater surface density. This affects the energy that allows TIWs to develop and propagate. One of the energy sources is the potential energy generated by the deformation of the density gradient. The effect of salinity on this energy is as strong as that of temperature, which means that by adding the contribution of salinity, the potential energy is doubled. TIWs modify and are modified by the salinity in the equatorial Atlantic. On the western edge of the basin, the Amazon plume results in even more salinity variability than in the equatorial Atlantic. The NBC rings, eddies that are 200 km in diameter, are highly contrasted structures. They trap the salty, CO2-rich waters of the NBC, but their rotation stirs water from the Amazon plume. The fresh water of the plume enhances the exchanges of CO2 and heat with the atmosphere. The northwestern TA in February 2020 was found to be a CO2 sink 10 times stronger than expected, and more than 40% of this flux is due to the effect of eddies. Their role is twofold, on the one hand they stir the plume which becomes a strong carbon sink, but also, they do not retain the CO2-rich surface signature of the waters they trap, and instead often stir freshwater filaments. The situation in summer is very different from the one in winter. The NBC changes its orientation by 90° and instead of following the South American coastline, it flows towards Africa. The NBC passes the mouth of the Amazon that is close to its maximum discharge and advects the plume eastwards. However, the NBC rings and the winds change this classical pattern. The formation and propagation of eddies make the plume discontinuous, and the winds favour a northwestward transport of fresh water. Thus, in August -September, whereas part of the plume indeed flows eastwards, another part is advected towards the Lesser Antilles. Particularly strong examples of this were observed in late summer 2021. This work shows the importance of the oceanic mesoscale for understanding key phenomena, such as the propagation of the TIWs and of the Amazon plume and the TAO carbon budget

During a three year occupation of Station Mike (66°N 2°E), the Norwegian Ocean Weather Ship Polarfront was equipped with a range of meteorological and seastate measuring instruments, including the autonomous air-sea flux system “AutoFlux” (Yelland et al., 2009) and an underway ΔpCO2 system. An extensive set of direct, eddy covariance measurements of momentum, latent heat, sensible heat and CO2 flux was obtained over a wide range of open ocean conditions. The maximum recorded 20-minute mean wind speed was 25 m.s-1. The maximum significant wave height was 11 m. The initial CO2 flux results were subject to a large, commonly observed humidity cross-sensitivity error. A novel iterative correction procedure was developed, tested against an independent data set and proved to be robust (Prytherch et al., 2010a). Open-path sensors may now be used for air-sea CO2 flux measurement, greatly increasing the number of measurements available for analysis. There are large differences between existing gas transfer to wind speed relationships, particularly at high wind speeds, and there is significant uncertainty over the form (quadratic or cubic) of the relationship. From the 3938 direct CO2 flux measurements made onboard Polarfront, a new relationship between gas transfer velocity, k660 , and wind speed, U10n has been obtained: k660 = −0.51+ 0.095U10n 2.7 0 ≤U10n ≤ 20 m.s-1 The motion corrected fluxes were found to have a large signal at frequencies associated with platform motion. This signal is also apparent in results from previous air-sea experiments from both fixed and moving platforms. The cause of this signal, whether error or real wind-wave nteraction, remains unknown. The gas transfer relationship obtained after removal of this signal is: k660 = −0.09 + 0.02U10n 3.1 2 ≤U10n ≤ 20 m.s-1 demonstrating that the observed near cubic dependence on wind speed, also reported in some previous experiments over a more limited wind speed range (McGillis et al., 2001a), is a robust result. This suggests a significant role for wave breaking and bubble-mediated exchange in air-sea gas transfer.

Carbon plays a major role in physical and biogeochemical processes in the atmosphere, the biosphere, and the ocean. CO2 and CH4 are two of the most common carbon-containing compounds in the atmosphere, also recognized as major greenhouse gases. The exchange of CO2 and CH4 between the ocean and the atmosphere is an essential part of the global carbon cycle. The exchange is controlled by the air–sea concentration gradient and by the efficiency of the transfer processes. The lack of knowledge about the forcing mechanisms affecting the exchange of these climate-relevant gases is a major source of uncertainty in the estimation of the global oceanic contributions. Quantifying and understanding the air–sea exchange processes is essential to constrain the estimates and to improve our knowledge about the current and future climate. In this thesis, the mechanisms controlling the air–sea gas exchange in the Baltic Sea are investigated. The viability of micrometeorological techniques for CH4 monitoring in a coastal environment is evaluated. One year of semi-continuous measurements of air–sea CH4 fluxes using eddy covariance measurements suggests that the method is useful for CH4 flux estimations in marine environments. The measurements allow long-term monitoring at high frequency rates, thus, capturing the temporal variability of the flux. The region off Gotland is a net source of CH4, with both the air–sea concentration gradient and the wind as controlling mechanisms. A sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms controlling the air–sea CO2 exchange in the Baltic Sea. This analysis shows that the spatio-temporal variability of CO2 fluxes is strongly modulated by water-side convection, precipitation, and surfactants. The effect of these factors is relevant both at regional and global scales, as they are not included in the current budget estimates.

World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained. Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur. During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7  m s-1.  The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2.  The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

The Southern Ocean forms a vital component of the earth system as a sink of CO2 and heat, taking over 40% of the annual oceanic CO2 uptake (75% of global heat uptake), slowing down the accumulation of CO2 in the atmosphere and thus the rate of climate change. However, recent studies based on the Coupled Model Intercomparison Project version 5 (CMIP5) Earth System Models (ESMs) show that CMIP5 ESMs disagree on the phasing of the seasonal cycle of the CO2 flux (FCO2) and compare poorly with available observation estimates in the Southern Ocean. Notwithstanding these differences, the seasonal cycle is a dominant mode of CO2 variability in the Southern Ocean, and hence this is an important bias. Previous studies suggest that these biases of FCO2 in ESMs might be a significant limitation to the long-term simulation of CO2 characteristics in the Southern Ocean. Consequently, this study has three primary objectives: first, to develop a process-based diagnostic method to analyze and isolate key biases and their underlaying mechanisms in the model-observations seasonal cycle of FCO2 differences for forced ocean models and ESMs. Second, to use this framework to examine sources of biases responsible for the limited skill of CMIP5 models in simulating the seasonal cycle of FCO2 with respect to observed estimates. Thirdly, to investigate how these present-day biases in the seasonality and drivers of CO2 in CMIP5 ESMs affect modelled longterm changes in the mechanisms of CO2 uptake in the Southern Ocean. In the first part of the dissertation, an objective diagnostic framework was established to analyze model-observation biases in the seasonal scale of FCO2 using the NEMO PISCES ORCA2LP model output, and Takahashi et al. (2009) observed estimates. The diagnostic framework focuses on examining the relative contributions of the competing drivers (SST and DIC) and related processes (solubility, biological and mixing) to instantaneous monthly changes in surface pCO2 (and FCO2) at the seasonal scale. In the second part of the dissertation, this approach is applied to 10 CMIP5 models in the Southern Ocean, to investigate the mechanistic basis for the seasonal cycle of FCO2 biases. It was found that FCO2 biases in CMIP5 models can be grouped into two main categories, i.e. group-SST and group-DIC. Group-SST models are characterized by an exaggeration of the seasonal rates of change of Sea Surface Temperature (SST) in autumn and spring during the cooling and warming peaks, respectively. These faster-than-observed rates of change of SST tip the control of the seasonal cycle of pCO2 and FCO2 towards SST and result in divergence between the observed and modelled seasonal cycles, particularly in the Sub-Antarctic Zone. While almost all analyzed models show these SST-driven biases, 3 out of 10 (namely NorESM1-ME, HadGEM2-ES and MPI-ESM, collectively the group-DIC models) compensate the solubility bias because of their exaggerated primary production, such that biologically-driven DIC changes become the regulators of the seasonal cycle of FCO2. It was also found that despite significant differences in the spatial characteristics of the mean annual fluxes, CMIP5 models show a zonal homogeneity in the seasonal cycle of FCO2 at the basin-scale in contrast to observed estimates. In the final third of the dissertation, using five CMIP5 ESMs from the RCP8.5 scenario, it was found that CMIP5 models present climate biases in the seasonality and drivers of FCO2 are fundamental to how models simulate long-term changes in the mechanisms of CO2 uptake in the Southern Ocean. Although all five analyzed models show an increased annual mean CO2 uptake by the end of the century, they show significant differences in the mechanisms. The present-day temperature biased models (group-SST) generally maintain the dominance of the temperature driver in the seasonal variability of FCO2 to end of the century. But show enhanced CO2 uptake due to increased anthropogenic atmospheric CO2 and decreased surface CO2 buffering capacity but they display a weak to null role of biological activity in the increased CO2 sink. On the other hand, the increased CO2 uptake at the end of the century in group-DIC models is explained increased biological driven CO2 uptake in spring, linked to increased Revelle factor and solubility driven CO2 uptake in winter. Increased Revelle factor at the end of the century enhance pCO2 changes for even smaller DIC changes.

Since the beginning of the industrial revolution, a large amount of greenhouse gases such as carbon dioxide (CO2) have been emitted into the atmosphere due to human activities. One of the main consequences of these emissions is a rapid increase in atmospheric CO2 concentration and a profound modification of the Earth's climate system. The ocean plays an important role in the Earth radiative balance since it acts as an important CO2 sink for the atmosphere. By currently absorbing about 25 % of the CO2 emitted by humans it considerably slows down climate change. Understanding the present-day spatial and temporal dynamics of the air-sea CO2 exchange and the different processes that govern this exchange is of critical importance to anticipate the evolution of the oceanic CO2 sink in the future.This thesis was realized in this context and focused on an improved quantification of the exchange of CO2 through the air-sea interface (FCO2) of the global ocean, embracing open ocean waters and coastal regions. The main objective was to fill knowledge gaps in our understanding of the processes that govern the spatial and temporal distribution of FCO2. This objective was mainly achieved through observational approaches and addressed three main aspects: a quantification of the different sources of FCO2 uncertainties at the global scale, an analysis of spatial distribution of the oceanic CO2 exchange with a strong focus on the coastal ocean and a first assessment of the coastal seasonal FCO2 dynamics and its underlying drivers. The latter relied on a data-model fusion approach allowing to decompose the FCO2 seasonality into its main physical and biogeochemical drivers. The quantification of the oceanic FCO2 from observations consists in calculating an air-sea partial pressure CO2 gradient (ΔpCO2) between the atmosphere and the sea surface. Global monthly continuous partial pressure of CO2 (pCO2) products can for example be derived from observational pCO2 databases and statistical interpolation methods. This ΔpCO2 is then multiplied by a gas exchange transfer rate coefficient (k), which depends on wind speed. However, the parametrization of k is still entailed with poorly quantified uncertainties. From a literature review of all k parameterizations available in the literature over the past 25 years, I first quantified the FCO2 uncertainties associated with k globally and regionally for the open ocean. I also quantified the uncertainties associated with the choice of a wind product over another. Our results show that the range of global FCO2, calculated with these k relationships, diverge by 12 % when using CCMP, ERA or NCEP1. Regional discrepancies in FCO2 are more pronounced than global. These global and regional differences significantly increase when using NCEP2 or other k formulations. To minimize uncertainties associated with the choice of wind product, it is possible to recalculate the parametrization of k globally for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. In a second step, we improved the quantification and analysis of the dominant patterns and drivers of the FCO2 spatial distribution for the coastal ocean worldwide. This analysis was performed globally (at 0.25° spatial resolution), using a regional segmentation of the coastal ocean, and latitudinally. I found that coastal regions at high latitudes act as a CO2 sink while tropical regions and along the equator tend to act as an atmospheric CO2 source. Globally integrated, I quantified that the coastal seas act currently as a CO2 sink with a value of -0.20 ± 0.02 Pg C yr-1. For the first time, I also compared the spatial patterns of coastal FCO2 to that of the adjacent open ocean, globally. With the exception of some regions such as those dominated by riverine inputs, I demonstrated that they present similar latitudinal distribution of their FCO2 density per unit of surface area, suggesting analogous responses to increasing atmospheric CO2. I also reevaluated the global ocean CO2 budget and estimated a global anthropogenic CO2 uptake ranging between -2.6 ± 0.4 Pg C yr-1 and -2.9 ± 0.5 Pg C yr-1 for the 1998-2015 period. In a third step, I contributed to the first continuous observational pCO2 data product merging the coastal and open ocean in a consistent manner. This study showed that difference between open ocean and coastal ocean estimates along the overlap area increases with latitude but remains close to 0 µatm globally. Stronger discrepancies, however, exist on the regional level resulting in differences that exceed 10 % of the climatological mean pCO2, particularly in regions constrained by fewer observations, paired with biogeochemical complexity, such as the Peruvian upwelling system and ice covered regions.In a fourth step, a temporal analysis of the FCO2 seasonality was performed for the coastal ocean based on an observational approach. I analyzed and quantified the FCO2 seasonal dynamics globally and for different latitudinal bands. Globally, coastal regions act as a CO2 sink with a more intense uptake occurring in summer (-21 Tg C month-1) because of the disproportionate influence of high latitude shelves in the Northern Hemisphere. I also estimated the contribution of different drivers (sea-ice coverage, wind speed, and ΔpCO2 change) to the FCO2 seasonal amplitude. This data-driven approach allowed me to conclude that the ΔpCO2 is the main driver of the FCO2 variability at the seasonal timescale. I then used a global oceanic biogeochemical model to decompose the seasonal coastal pCO2 variability further into its driving physical and biological processes. From a first qualitative assessment, I concluded that the thermal effect associated to sea surface temperature changes is the main effect governing the coastal seasonal pCO2 variability except at high latitudes where the non-thermal effect associated to changes in biology, circulation, fresh water and the air-sea CO2 exchange itself dominate. I also found that, overall, the thermal effect alone should lead to larger seasonal fluctuations, but its influence is partly offset by the non-thermal effect. Throughout this thesis, I also evaluated the extent to which the continuous observational pCO2 products derived from an artificial neuronal network approach and from the global ocean biogeochemical model MOM6-COBALT could reproduce the raw pCO2 fields extracted from global databases. Overall, I showed that at the regional scale, the two products are in relatively good agreement compared to observations. I also identified regions where discrepancies are the largest and where future observational data are needed in the future, as well as regions where agreement is the most satisfactory and, thus, most suitable for further process-based analyses.
Depuis le début de la révolution industrielle, une grande quantité de gaz à effet de serre tels que le dioxyde de carbone (CO2) a été émise dans l'atmosphère en raison des activités humaines. L'une des principales conséquences de ces émissions est une augmentation rapide de la concentration en CO2 atmosphérique et une modification profonde du système climatique de la Terre. L'océan joue un rôle important dans l'équilibre radiatif de la Terre car il agit comme un important puits de CO2 pour l'atmosphère. En absorbant actuellement environ 25 % du CO2 émis par l'homme, il ralentit considérablement le changement climatique. Comprendre la dynamique spatiale et temporelle actuelle de l'échange de CO2 air-mer et les différents processus qui régissent cet échange est d'une importance cruciale pour anticiper l'évolution du puits océanique de CO2 à l'avenir.Cette thèse a été réalisée dans ce contexte et s'est concentrée sur une meilleure quantification de l'échange de CO2 à travers l'interface air-mer (FCO2) de l'océan global, considérant à la fois l’océan ouvert et les régions côtières. L'objectif principal était de combler les lacunes dans notre compréhension des processus qui régissent la distribution spatiale et temporelle du FCO2. Cet objectif a été principalement atteint grâce à des approches observationnelles et a abordé trois aspects principaux: une quantification des différentes sources d'incertitudes du FCO2 à l'échelle globale, une analyse de la distribution spatiale de l'échange de CO2 océanique avec un fort accent sur l'océan côtier et une première évaluation de la dynamique saisonnière du FCO2 côtier et de ses moteurs sous-jacents. Ce dernier s'est appuyé sur une approche de fusion de modèles et d’approches observationnelles permettant de décomposer la saisonnalité du FCO2 en ses principaux moteurs physiques et biogéochimiques.La quantification du FCO2 océanique à partir d’observations consiste à calculer un gradient de pression partielle air-mer de CO2 (ΔpCO2) entre l'atmosphère et la surface de la mer. Des produits globaux continus mensuels de la pression partielle de CO2 (pCO2) peuvent par exemple être dérivés à partir de bases de données observationnelles de pCO2 et de méthodes d'interpolation statistique. ΔpCO2 est ensuite multiplié par un coefficient de vitesse de transfert d'échange gazeux (k), qui dépend de la vitesse du vent. Cependant, la paramétrisation de k est sujette à de larges incertitudes et mal quantifiées. À partir d'une synthèse de la littérature de toutes les paramétrisations de k disponibles dans la littérature au cours des 25 dernières années, j'ai d'abord quantifié les incertitudes sur FCO2 associées à k à l'échelle globale et régionale pour l'océan ouvert. J'ai également quantifié les incertitudes associées au choix d'un produit éolien par rapport à un autre. Nos résultats montrent que la gamme du FCO2 global, calculée avec ces différentes paramétrisations de k, diverge de 12 % lors de l'utilisation de CCMP, ERA ou NCEP1. En raison des différences dans les pattern de vent régionaux, les différences régionales sur le FCO2 sont plus prononcés que globalement. Ces différences globales et régionales augmentent de manière significative lors de l'utilisation de NCEP2 ou d'autres formulations de k. Afin de réduire les incertitudes associées au choix du produit de vent, il est possible de recalculer la paramétrisation de k pour un produit de vent donné et à une résolution spatio temporelle.Dans un deuxième temps, nous avons amélioré la quantification et l'analyse des principaux pattern et des différents processus sur la distribution spatiale du FCO2 pour l’ensemble des régions côtières. Cette analyse a été réalisée à l'échelle globale (à une résolution spatiale de 0.25°), en utilisant une segmentation régionale de l'océan côtier, et latitudinalement. J'ai trouvé que les régions côtières aux hautes latitudes agissent comme un puits de CO2 tandis que les régions côtières tropicales et le long de l'équateur ont tendance à agir comme une source de CO2 atmosphérique. Globalement, j'ai quantifié que les régions côtières agissent actuellement en tant que puits de CO2 avec une valeur de -0.20 ± 0.02 Pg C an-1. Pour la première fois, j'ai également comparé la distribution spatiale du FCO2 côtier à celle de l'océan ouvert adjacent, à l'échelle globale. À l'exception de certaines régions telles que celles dominées par les apports fluviaux, j'ai démontré que les régions côtières et l’océan ouvert adjacent présentaient une distribution latitudinale similaire sur leur densité de FCO2 par unité de surface, suggérant des réponses analogues à l'augmentation du CO2 atmosphérique. J'ai également réévalué le budget mondial de CO2 de l'océan et estimé une absorption mondiale de CO2 anthropique comprise entre -2.6 ± 0.4 Pg C an-1 et -2.9 ± 0.5 Pg C an-1 pour la période 1998-2015. Dans un troisième temps, j'ai contribué à la création du premier produit continu de pCO2 observationnelles fusionnant le domaine côtier et l'océan ouvert de manière cohérente. Cette étude a montré que la différence entre les estimations provenant du produit de pCO2 de l’océan ouvert à celles dérivant du produit de pCO2 de l’océan côtier le long de leur zone de chevauchement augmente avec la latitude mais reste proche de 0 µatm globallement. Des divergences plus fortes existent cependant au niveau régional, entraînant des différences qui dépassent 10 % sur la moyenne climatologique de pCO2, en particulier dans les régions contraintes par moins d'observations, associées à une complexité biogéochimique, comme le système d'upwelling péruvien et les régions couvertes de glace.Dans une quatrième étape, une analyse temporelle de la saisonnalité du FCO2 a été réalisée pour l'océan côtier sur la base d'une approche observationnelle. J'ai analysé et quantifié la dynamique saisonnière du FCO2 à l'échelle globale et pour différentes bandes latitudinales. À l'échelle globale, les régions côtières agissent comme un puits de CO2 avec une absorption plus intense se produisant en été (-21 Tg C mois-1) en raison de l'influence disproportionnée des régions côtières des hautes latitudes dans l'hémisphère Nord. J'ai également estimé la contribution de différents processus (couverture de glace de mer, vitesse du vent et changement de ΔpCO2) à l'amplitude saisonnière du FCO2. Cette approche basée sur les données observationnelles m'a permis de conclure que ΔpCO2 est le principal moteur de la variabilité du FCO2 à l'échelle saisonnière. J'ai ensuite utilisé un modèle biogéochimique océanique global pour décomposer davantage la variabilité saisonnière du pCO2 côtier en ses processus physiques et biologiques. À partir d'une première évaluation qualitative, j'ai conclu que l'effet thermique associé aux changements de température de la surface de la mer est le principal effet régissant la variabilité côtière saisonnière du pCO2 sauf aux hautes latitudes où l'effet non thermique associé aux changements de biologie, de circulation, d'eau douce et de l’échange de CO2 air-mer domine. J'ai également constaté que, globalement, l'effet thermique à lui seul devrait entraîner des fluctuations saisonnières plus importantes, mais son influence est en partie compensée par l'effet non thermique.Tout au long de cette thèse, j'ai également évalué dans quelle mesure les produits continus de pCO2 observationnelles dérivés d'une approche de réseau de neurones artificiels et du modèle biogéochimique océanique global MOM6-COBALT pourraient reproduire les champs de pCO2 bruts extraits des bases de données globale. Dans l'ensemble, j'ai montré qu'à l'échelle régionale, les deux produits sont relativement en bon accord par rapport aux observations. J'ai également identifié les régions où les différences sont les plus importantes et où de futures données observationnelles sont nécessaires à l'avenir, ainsi que les régions où les deux produits présentent un accord le plus satisfaisant et, par conséquent, le plus approprié pour de futures analyses de compréhension des différents processus.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

VILLELA, FRANCO N. J. Análise decadal do fluxo de CO2 entre o oceano e a atmosfera na passagem de Drake, Oceano Austral. 2011. 148 f. Dissertação (mestrado) Programa de Pós-Graduação em Ciência Ambiental (PROCAM), Universidade de São Paulo, São Paulo, 2011. Para a área delimitada pelos paralelos 60ºS e 62,5ºS e pelos meridianos 60ºW e 65ºW, localizada no sul da Passagem de Drake, no Oceano Austral, próximo à Península Antártica, foram calculadas as distribuições médias de 2000 a 2009, sazonais e anual, do fluxo de CO2 na interface oceano-atmosfera e de suas variáveis associadas: a pressão parcial de CO2 na superfície marinha (PCO2sw), a pressão parcial de CO2 na atmosfera (PCO2ar), a diferença da pressão parcial de CO2 entre o oceano e a atmosfera (PCO2) e a taxa de transferência gasosa (TR), que é produto do coeficiente solubilidade do CO2 na água do mar pela velocidade de transferência gasosa. A parametrização utilizada no cálculo dos fluxos foi a de Takahashi et al. (2009) com TR dependente da velocidade do vento ao quadrado multiplicada por um fator de escala 0,26. A área de estudo tem cerca de 75 mil km2 e foi dividida em uma grade espacial de 0,5º x 0,5º, resultando em 50 quadrículas. Foram utilizados mais de 46 mil medições de PCO2sw, que na média espacial variou de 362,7 ±11,2 a 371,9 ±17,5 µatm, no verão e primavera respectivamente. A PCO2 variou de -0,4 a 5,7 µatm no outono e primavera, respectivamente. A TR variou de 0,065 ±0,04 a 0,088 ±0,002 gC.mês-1.m-2.µatm-1, no verão e inverno, respectivamente. O fluxo líquido, se tomando a concentração de gelo como negligenciável, variou de -0,039 ±0,865 a 0,456 ±1,221 gC.m-2.mês-1, no outono e inverno, respectivamente. O fluxo total anual de carbono, estimado através da média espacial por quadrícula, foi de 95 GgC.ano-1. Dessa maneira, na estimativa anual, a superfície do mar se comporta como fonte de CO2 para a atmosfera, principalmente devido à região da plataforma continental com PCO2sw consideravelmente maior que o da atmosfera. Sazonalmente sugere-se que no verão a maior disponibilidade de radiação solar, a temperatura da superfície do mar (TSM) mais elevada e os ventos mais fracos favorecem a produção de biomassa fitoplanctônica, fazendo com que a bomba biológica seja o processo dominante na diminuição da PCO2sw e na absorção de CO2 atmosférico pela superfície marinha. Já no inverno, os ventos se intensificam e, associados com o forte resfriamento da TSM, promovem a mistura com águas profundas ricas em carbono inorgânico dissolvido, levando a superfície marinha a um estado de supersaturação de CO2 em relação à atmosfera. Ventos circumpolares de oeste mais intensos e deslocados para sul tem sido apontados como a causa do aumento da PCO2sw em igual ou maior taxa do que ocorre na atmosfera. Na área de estudo foi levantada uma tendência média da intensidade do vento de 0,23 ±0,03 m.s-1.década-1 e um aumento na freqüência da componente zonal de oeste (positiva) de 1,47 ± 1,13 % .década-1. Sugere-se que estas tendências estejam relacionadas com o Modo Anular Austral (SAM). Entretanto, a tendência decadal estimada para a PCO2sw foi menor que para a atmosfera, apesar de ambas indicarem tendência de aumento. Acredita-se que a grande variabilidade e distribuição esparsa de dados tenham mascarado a magnitude da estimativa da tendência de PCO2sw.
VILLELA, FRANCO N. J. Decadal analysis of the CO2 sea-air flux in the Drake Passage, Southern Ocean 2011. 148 f. Dissertação (mestrado) Programa de Pós-Graduação em Ciência Ambiental (PROCAM), Universidade de São Paulo, São Paulo, 2011. For the area bounded by parallels 60°S and 62.5°S and meridians 60°W and 65°W, located in the southern Drake Passage in the Southern Ocean, near the Antarctic Peninsula, mean seasonal and annual distributions of CO2 flux at the ocean-atmosphere interface, from 2000 to 2009, have been computed, as well as their associated variables: the CO2 partial pressure at sea surface (PCO2sw), the CO2 partial pressure in atmosphere (PCO2ar), the CO2 pressure difference between ocean and atmosphere (PCO2), and the gas transfer rate (TR), which is the product of the CO2 solubility coefficient in sea water by the gas transfer velocity. The parameterization used to calculate fluxes was that of Takahashi et al. (2009) with TR depending on the squared wind speed multiplied by a scale factor 0.26. The study area has about 75,000 km2 and was divided into a grid of 0.5° x 0.5°, resulting in 50 area boxes. Over 46,000 PCO2sw measurements were used, which in the spatial mean varied from 362.7±11.2 to 371.9±17.5 µatm, in summer and spring, respectively. The PCO2 varied from 0.4 to 5.7 µatm in autumn and spring, respectively. TR varied from 0.065±0.04 to 0,088±0.002 gC.month-1.m-2.µatm-1, in summer and winter, respectively. The net flux, taking ice concentration as negligible, varied from 0.039±0.865 to 0.456±1.221 gC.month-1.m-2, in autumn and winter, respectively. The total annual carbon flux, estimated through the spatial mean per square, was 95 GgC.y-1. Thus, in the annual estimate the region acts as a source to the atmosphere, mainly due to the continental shelf having PCO2sw considerably greater than that of the atmosphere. Seasonally, it is suggested that in summer the greater availability of solar radiation, warmer sea surface temperature (SST), and weaker winds favor the production of phytoplanktonic mass, making the biological pump the dominating process in lowering the PCO2sw and the absorption of atmospheric CO2 by the sea surface. On the other hand, in winter winds intensify and, in association with the strong cooling of the SST, promote mixing with deep waters rich in dissolved inorganic carbon, leading the sea surface to a state of supersaturation in CO2 relative to the atmosphere. Stronger circumpolar west winds and displaced to the south have been pointed as the cause for the increase of PCO2sw at a rate equal to or greater than that occurring in the atmosphere. In the study area it has been detected a mean trend of wind intensity 0.23±0.03 m.s-1.decade-1 and an increase in the western zonal component of 1.47±1.3%.decade-1. It is suggested that these trends are related to the Southern Annular Mode (SAM). However, the decadal trend estimated for the PCO2sw was smaller than for the atmosphere, in spite of both indicating increasing tendencies. It is believed that the great variability and scatter distribution of the data have masked the magnitude of the PCO2SW trend estimate.

碩士
中興大學
環境工程學系所
99
Open path CO2 analyzer has been used in CO2 flux measurements over ocean recently. CO2 flux has been estimated by close path CO2 analyzer for decades. To compare the CO2 flux of open path and close path methods, three experiments were set in the study: (1) using open path eddy covariance system (ECS) and close path Underway pCO2 system to measure CO2 fluxes over pool in Black-faced Spoonbill Conservation Association in Cigu Dist. The results showed that the CO2 flux of open path CO2 analyzer is up to 100 times larger than that of close path, but the direction of flux is opposite; (2) in the second case, the third method, profile method was added to estimate CO2 flux. The result of profile method showed that the tendency of flux is the same as ECS, but the flux of profile method is about 8 times larger than open path, however, direction of flux was still inconsistent with Underway pCO2 system; (3) the results of the experiment in a fish pond which has stronger ecosphere activities showed that the flux of open path is positive (sink), and it’s negative (source) for close path method. In addition, the diurnal cycle of CO2 concentration is obvious where it is lower in the daytime and higher at night. The PKT Method was used to calibrate the effect between relative humidity and CO2 concentration in case (1) and case (3). In both cases, PKT Method can correct the biases resulted from the relative humidity after WPL correction, the CO2 flux after calibrating by PKT Method was smaller than WPL calibration method, and the flux in May 14th is similar to the flux of Underway pCO2 system, but the direction of flux is still opposite. After adding pH parameter and CO2 atmospheric concentration measured at 1.5 m into the CO2 flux calculation of Underway pCO2 system, the results showed that the flux and the direction of flux will be consistent with ECS.

碩士
國立臺灣大學
海洋研究所
102
To accurately investigate air-sea CO2 exchange in the coastal waters, especially for the East China Sea (ECS), is challenging because of the environment complexities and diversity of the shelf seas, easily affected by human activities and climate changes. Reliable assessments of air-sea CO2 exchange fluxes in the ECS are additionally limited by inadequately spatiotemporal coverage and shortage of manpower resources. Here, we explore seasonally representative CO2 uptakes by the whole ECS by combining the remote sensing data and field observations. We firstly evaluated the results of Tseng et al. (2014) and further demonstrated the reliability and representativeness of Tseng’s empirical algorithm for computing pCO2 by using remote sensing data including SST, Chlorophyll a (Chl-a) and wind speed. Secondly, we demonstrated the satellite wind speed data are higher than those collected in land weather station (field wind speed = 0.8 × satellite wind speed), in order to re-evaluate the CO2 fluxes in the ECS. The average annual flux between 2003 and 2010 was constrained to -1.1 mol C m-2 y-1 as a net sink of atmospheric CO2 with the seasonal mean fluxes of -2.1 (Mar.-May), -0.3 (June-Aug.), -0.2 (Sep.-Nov.) and -1.9(Dec.-Jan.), respectively. The flux seasonality showed a strong sink in spring and winter, a sink-to-source transition during late summer – mid-fall period and a source-to-sink transition in late fall. Finally, the annual mean CO2 flux estimated in this study was nearly one half of those reported previously, indicating the importance of wind effect regarding spatial variability and reliability of wind field. Especially in some severe weather events, the more spatial gradients of wind speed would make more significant impact on the air-sea exchange flux of CO2 in continental margins.

Much research has been devoted to understanding the ocean carbon cycle because of its prominent role in controlling global climate. Coastal oceans remain a source of uncertainty in global ocean carbon budgets due to their individual characteristics and their high spatial and temporal variability. Recent attempts to establish general patterns suggest that temperate and high-latitude coastal oceans act as sinks for atmospheric carbon dioxide (CO2). In this thesis, carbon cycling in two Canadian coastal ocean regions is investigated, and the uptake of atmospheric CO2 is quantified. A combination of ship-board measurements and highly temporally resolved data from an autonomous mooring was used to quantify the seasonal to multi-annual variability in the inorganic carbon system in the Scotian Shelf region of the northwestern Atlantic for the first time. The Scotian Shelf, unlike other shelf seas at similar latitude, acts as a source of CO2 to the atmosphere, with fluxes varying over two orders of magnitude in space and time between 1999 and 2008. The first observations of the inorganic carbon system in the Amundsen Gulf region of the southern Beaufort Sea, covering the full annual cycle, are also presented. Air-sea CO2 fluxes are computed and a carbon budget is balanced. The Amundsen Gulf system acts as a moderate sink for atmospheric CO2; seasonal ice-cover limits winter CO2 uptake despite the continued undersaturation of the surface waters. Biological production precedes the ice break-up, and the growth of under-ice algae constitutes nearly 40% of the annual net community production. The Scotian Shelf may be described as an estuarine system with an outflow of surface water, and intrusion of carbon-rich subsurface water by a combination of wind-driven mixing, upwelling and convection, which fuels the CO2 release to the atmosphere. In contrast, Amundsen Gulf may be described as an anti-estuarine, or downwelling, system, with an inflow of surface waters and an outflow of subsurface waters. Wind-driven and convective mixing are inhibited by ice-cover and restrict the intrusion of carbon- and nutrient-rich waters from below, maintaining the CO2 uptake by the surface waters.
PhD Thesis

碩士
國立臺灣師範大學
海洋環境科技研究所
97
The distribution of CO2 in the surface water and the sea-air flux exchange in the sea areas around Taiwan are investigated in this study, and to discuss the reason of variation and the relationship with the distribution of water mass. Automated Underway pCO2 System are used to detect the seawater and air fCO2 during the late spring and early summer of 2008, from May 28 to July 13, including the South China Sea(SCS), the West Philippine Sea(WPS), the Western Taiwan Coast(WTC), and the East China Sea(ECS). The range of the atmospheric fCO2 is 367.4~402.2 μatm and the peaks are found near lands (Taiwan, China, and Luzon Island), the difference of concentration up to 35 μatm. The ranges of the surface water fCO2 are as follows: SCS: 352.3~415.6 μatm(Avg.= 389.3±16.5, n=1400), WPS: 346.9~399.0 μatm(Avg.= 377.6±5.8, n=840), TS: 370.5~407.3 μatm(Avg.= 389.2±4.8, n=836), ECS: 162~707 μatm(Avg.=378±69, n=1497); and ECS has the highest variation up to 545 μatm. The lowest and second lowest values of fCO2(217、162 μatm) are found in Changjiang Plume and Minjiang Plume, increasing from west to east with longitude and opposite to the concentration of chl-a. It’s quite obvious that the gradient of seawater fCO2 increase with the decrease of the biomass of plankton. The high values of fCO2(707、676 μatm) are found in Changjiang Upwelling and Coastal Upwelling which have low temperature. These areas also have very low transmittance(13.9 %) and very high nutrients(NO2+NO3) and Chl-a(32.2 μM, 106.7 mg/m3). It’s speculated that the high fCO2 may come from the bottom water of Changjiang Upwelling and Coastal Upwelling. Water masses in SCS and WPS are more stable and have the fCO2 gradient increase from shelf to offshore because the low temperature and rich of chl-a in nearshore seawater make the fCO2 of water decrease. In offshore, the fCO2 of SCS and WPS are high in daytime and low at night(△fCO2 =7.9), mainly reflecting the temperature difference between day and night(0.2~0.3℃) because of low biological effect. The surface water fCO2 of WTC has few variations but the atmospheric fCO2 has regional peaks because it is influenced by terrigenous matter. Data in this study suggests that the sea areas around Taiwan served as a source of atmospheric fCO2 during late spring and early summer, and the sea-to-air CO2 flux in SCS is +1.74±2.06 mol C/m2/yr, in WPS is +0.54±0.59, in WTC is +0.29±0.18, and in the ECS is +0.28±4.94.

碩士
國立臺灣海洋大學
海洋環境化學與生態研究所
101
Previous studies suggest that strong winds can largely enhance air-sea CO2 exchange flux during the passage of typhoon. Understanding how air-sea CO2 exchange flux would response to the passage of typhoon is therefore essential to better quantify the carbon budget in the subtropical oceans. However, limited to the rough sea conditions and cruise arrangement, the field data of air-sea CO2 exchange flux just before and after typhoon passage are scarce. In this study, two cruises (OR2 1893, 2012/7/26-30 and OR2 1894B, 2012/8/4-6) off the southeast Taiwan were fortuitously conducted just pre- and post-Typhoon Saola passage (2012/8/1-3) in summer 2012, which thus provide a unique opportunity to examine the response of air-sea CO2 exchange flux to typhoon passage in the subtropical ocean. The results show that ∆pCO2 (the difference of partial pressure of CO2 between the surface water and the air = pCO2sw – pCO2air) ranged between -37 to 41 and -36 to 38 atm before and after the typhoon passage, respectively. The source area is generally associated with the occurrence of high pCO2sw, while the sink area is coincident with the appearance of high pCO2air, which may be resulted from the air mass from land. The averaged air-sea CO2 exchange flux was -0.05 and 0.05 mmolC m-2 day-1 before and after the typhoon passage, respectively, suggesting that the study area turned from a sink of atmospheric CO2 pre-typhoon passage to a CO2 source post-typhoon passage. The wind speeds difference between the pre- and post-typhoon periods was not significant so that it only play a minor role on the variation of air-sea CO2 flux in response to the typhoon passage. Nonetheless, the elevated wind speed during the period of typhoon passage might lead to CO2 efflux increase by about 7 times (~0.48 mmolC m-2 day-1). Furthermore, the amount of CO2 uptake during the non-typhoon period in summer was estimated to be about 4.25 mmolm-2, while the amount of CO2 release during the typhoon period was about 4.32 mmolm-2. In other words, the CO2 uptake during the non-typhoon period was almost totally release back to the atmosphere during the typhoon period. As a result, the study area was nearly neutral to the atmospheric CO2 in summer 2012. Finally, our result generally confirms the previous findings that the passage of typhoon may enhance CO2 release from ocean to the atmosphere in the subtropical oceans, which may represent a positive feedback to global warming.