| Nome: | Descrição: | Tamanho: | Formato: | |
|---|---|---|---|---|
| 173.32 MB | Adobe PDF |
Orientador(es)
Resumo(s)
As Zonas de Oxigénio Mínimo (ZOM) são regiões oceânicas onde as concentrações de oxigénio (O2) dissolvido são suficientemente baixas para impactar os ecossistemas marinhos e o ciclo biogeoquímico. No Atlântico Norte Tropical Oriental (ANTO), encontra-se uma das mais extensas ZOM do mundo, que se distingue pela presença de duas camadas hipóxicas: uma profunda, centrada a cerca de 400 m, associada à fraca ventilação, e outra pouco profunda (ZOMpp), com núcleo a ≈100 m, associada ao consumo de oxigénio decorrente da remineralização de matéria orgânica exportada pela elevada produtividade primária à superfície. A ZOMpp do ANTO é particularmente dinâmica e heterogénea. Apesar da sua importância, os estudos sobre a sua formação são escassos. A investigação existente associa o seu desenvolvimento a vórtices oceânicos altamente produtivas, mas com concentrações hipóxicas de oxigénio na sub-superficie (O2 < 40 µmol L-1). Contudo, a maioria destes estudos baseia-se num número limitado de vórtices, deixando lacunas significativas na compreensão da evolução espacial e temporal de eventos hipóxicos, especialmente no exterior dos vórtices. Por exemplo, embora seja provável que condições hipóxicas no interior dos vórtices persistam para além da sua dissipação, a evolução e impacto desses eventos permanece inexplorada. Para além dos vórtices, foram observadas condições hipóxicas na plataforma e talude continental da Mauritânia e Senegal, atribuídas a uma combinação de elevada remineralização de matéria orgânica, fraca ventilação e variabilidade do afloramento costeiro. Dado que muitos vórtices são gerados perto da margem continental, levanta-se a hipótese de que possam aprisionar condições hipóxicas preexistentes. Contudo, permanece por investigar se os vórtices podem formar-se já com O2 baixo ou se estas condições apenas se desenvolvem posteriormente. Esta tese investiga os mecanismos físicos e biogeoquímicos que controlam a formação, manutenção e variabilidade de eventos hipóxicos que contribuem para a ZOMpp no ANTO. Para isso, recorre-se a uma simulação climatológica de cinco anos com um modelo físicobiogeoquímico de alta resolução (≈3 km), acoplando o PISCES (”Pelagic Interactions Scheme for Carbon and Ecosystem Studies”) ao modelo CROCO (”Coastal and Regional Ocean COmmunity model”). Embora o domínio cubra todo o Atlântico Tropical – permitindo uma representação realista dos jatos zonais que desempenham um papel crucial na delimitação da ZOM – a análise centra-se no setor nordeste do domínio (10–22∘N, 15–27∘O), englobando o sistema de afloramento Mauritânia-Senegal e incluindo a região adjacente ao arquipélago de Cabo Verde. A metodologia combina análises Eulerianas com diagnósticos lagrangianos para rastrear eventos e massas de água hipóxicas no espaço e no tempo.
The Eastern Tropical North Atlantic (ETNA) hosts a highly heterogeneous shallow Oxygen Minimum Zone (sOMZ), largely sustained by the remineralization of organic matter exported from productive surface waters. Despite its ecological importance, the spatial distribution, drivers, and evolution of hypoxic events remain poorly understood. This thesis investigates the physical and biogeochemical mechanisms forming the sOMZ in the ETNA using a high resolution climatological simulation with CROCO-PISCES. A combined Eulerian-Lagrangian framework is employed to characterize the structure, dynamics, and water mass composition of hypoxic events. Persistent hypoxia is found along the continental margin, modulated by seasonal variability in the Poleward Undercurrent (PUC), while offshore hypoxia is more intermittent and associated with mesoscale eddies. Coastal hypoxia intensifies in summer, when a shoaling PUC reduces ventilation of upper central waters. Coastal-generated eddies play a key role in the recirculation and mixing of coastal waters through horizontal steering. Those formed in northern latitudes–where oxygen concentrations are lowest–often trap pre-existing hypoxic waters, intensifying them via internal remineralization and core isolation as they propagate westward. Anticyclonic Modewater Eddies (ACMEs) are the most susceptible to hypoxia, yet most hypoxic conditions develop independently due to their initial water mass composition delivered by the PUC. Mixed layer deepening appears to transform ACMEs into surface-intensified anticyclones, while simultaneously eroding their oxygen-poor, nutrient-rich cores–injecting nutrients into the euphotic zone and sustaining primary productivity. Cyclonic and anticyclonic eddies also contribute to offshore hypoxia via remineralization, driven by eddy-induced upwelling, and by the combined effects of nutrient trapping and eddy-induced Ekman pumping, respectively. Overall, sOMZ variability in the ETNA results from the interplay of coastal and offshore processes. Given the region’s sensitivity to changes in ventilation and upwelling, climate-driven shifts are likely to increase the frequency and severity of hypoxic events, with significant implications for marine ecosystems and fisheries.
The Eastern Tropical North Atlantic (ETNA) hosts a highly heterogeneous shallow Oxygen Minimum Zone (sOMZ), largely sustained by the remineralization of organic matter exported from productive surface waters. Despite its ecological importance, the spatial distribution, drivers, and evolution of hypoxic events remain poorly understood. This thesis investigates the physical and biogeochemical mechanisms forming the sOMZ in the ETNA using a high resolution climatological simulation with CROCO-PISCES. A combined Eulerian-Lagrangian framework is employed to characterize the structure, dynamics, and water mass composition of hypoxic events. Persistent hypoxia is found along the continental margin, modulated by seasonal variability in the Poleward Undercurrent (PUC), while offshore hypoxia is more intermittent and associated with mesoscale eddies. Coastal hypoxia intensifies in summer, when a shoaling PUC reduces ventilation of upper central waters. Coastal-generated eddies play a key role in the recirculation and mixing of coastal waters through horizontal steering. Those formed in northern latitudes–where oxygen concentrations are lowest–often trap pre-existing hypoxic waters, intensifying them via internal remineralization and core isolation as they propagate westward. Anticyclonic Modewater Eddies (ACMEs) are the most susceptible to hypoxia, yet most hypoxic conditions develop independently due to their initial water mass composition delivered by the PUC. Mixed layer deepening appears to transform ACMEs into surface-intensified anticyclones, while simultaneously eroding their oxygen-poor, nutrient-rich cores–injecting nutrients into the euphotic zone and sustaining primary productivity. Cyclonic and anticyclonic eddies also contribute to offshore hypoxia via remineralization, driven by eddy-induced upwelling, and by the combined effects of nutrient trapping and eddy-induced Ekman pumping, respectively. Overall, sOMZ variability in the ETNA results from the interplay of coastal and offshore processes. Given the region’s sensitivity to changes in ventilation and upwelling, climate-driven shifts are likely to increase the frequency and severity of hypoxic events, with significant implications for marine ecosystems and fisheries.
Descrição
Tese de doutoramento em Ciências Geofísicas e da Geoinformação (Oceanografia), Universidade de Lisboa, Faculdade de Ciências, 2025.
Palavras-chave
Shallow Oxygen Minimum Zone Eastern Tropical North Atlantic Eddies Poleward Undercurrent Physical-biogeochemical modeling Zona de Oxigénio Mínimo pouco profunda Atlântico Norte Tropical Oriental Vórtices Ocêanicos Corrente Subsuperficial para o Pólo Modelação Física-Biogeoquímica