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Dinâmica de vórtices na Corrente dos Açores sobre a Crista Média Atlântica
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Resumo(s)
A Corrente dos Açores é a estrutura de circulação mais importante no Atlântico Norte Subtropical. A evolução dos meandros associados à corrente origina a formação de vórtices, tendo estes uma importância elevada nos fluxos de momento e calor no oceano, bem como no transporte de nutrientes ao longo de grandes distâncias. Ao propagarem-se no oceano para além da interação com outros vórtices, estas estruturas podem interagir com a topografia, alterando significativamente o seu trajeto ou mesmo causando a sua erosão total. A Crista Média Atlântica (MAR) possui uma batimetria bastante pronunciada com profundidades aproximadas das da própria corrente dos Açores. No entanto, a influência da topografia na corrente média e nas estruturas turbulentas na zona da corrente dos Açores é praticamente desconhecida. Neste estudo é feita uma caracterização das estruturas de mesoescala na zona da corrente dos Açores tendo como base o estudo da interação destas estruturas com a topografia, em particular com a MAR. Utilizando altimetria de satélite e grandezas derivadas desta, foi possível estudar algumas questões associadas a esta dinâmica/interação, abrangendo pela primeira vez um período de 19 anos de dados. Os vórtices que interagem com a crista fazem-no de duas formas distintas: cruzando-a ou propagando-se ao longo do seu flanco Este, existindo por vezes uma combinação das duas. Conclui-se neste trabalho que existem quatro formas básicas de passagem sobre a crista: como estrutura coerente, através de divisão do vórtice, da coalescência de vórtices ou através de um processo de streamer. Dada a grande dinâmica da região, cujos valores de energia cinética turbulenta (EKE) excedem os 200cm2/s2 na zona da crista, ocorre muitas vezes uma conjugação das diferentes formas de passagem. No total, apenas 15% dos vórtices com inicio de deteção na região associada ao eixo principal da corrente, interagem com a crista. A migração destas estruturas sobre a crista pode ultrapassar os 6 meses, pelo que se pode concluir que os vórtices desta região são profundos o suficiente de modo a serem influenciados pela topografia. Uma descoberta interessante neste trabalho foi a constatação da existência de vórtices que se propagam ao longo do flanco Oeste da crista, ao contrário do esperado tendo em conta o efeito do β-topográfico.
The Azores Current is the most important circulation structure in the Subtropical North Atlantic. The evolution of the meanders associated with the current causes the formation of vortices, which have a high importance in the flows of momentum and heat in the ocean, as well as in the transportation of nutrients over great distances. As these structures propagate in the ocean beyond interaction with other vortices, they may also interact with the topography, significantly altering its path or even causing its total erosion. The Atlantic Mean Ridge (MAR) has a very pronounced bathymetry with depths close to those of the Azores current. However, the influence of the topography on the average current and turbulent structures in the Azores current zone is practically unknown. In this study a characterization of the mesoscale structures in the zone of the Azores current is made based on the study of the interaction of these structures with the topography, in particular with MAR. Using satellite altimetry and physical values derived from it, it was possible to study some issues associated with this dynamics / interaction, covering for the first time a period of 19 years of data. The vortices interacting with the MAR do it in two distinct ways: crossing it or propagating along its Eastern flank, and sometimes combining these two. It is concluded in this work that there are four basic forms of passage over the crest: as a coherent structure, through vortex division, vortex coalescence or through a streamer process. Given the great dynamics of the region, whose turbulent kinetic energy (EKE) values exceed 200 cm2/s2 in the ridge zone, a combination of the different forms of passage often occurs. In total, only 15% of the vortices whose detection starts in the region associated to the main axis of the current, interact with the crest. The migration of these structures on the crest can exceed 6 months, so it can be concluded that the vortices of this region are deep enough to be influenced by the topography. An interesting finding in this work was the existence of vortices that propagate along the Western flank of the ridge, contrary to what is expected, taking into account the β-topographic effect.
The Azores Current is the most important circulation structure in the Subtropical North Atlantic. The evolution of the meanders associated with the current causes the formation of vortices, which have a high importance in the flows of momentum and heat in the ocean, as well as in the transportation of nutrients over great distances. As these structures propagate in the ocean beyond interaction with other vortices, they may also interact with the topography, significantly altering its path or even causing its total erosion. The Atlantic Mean Ridge (MAR) has a very pronounced bathymetry with depths close to those of the Azores current. However, the influence of the topography on the average current and turbulent structures in the Azores current zone is practically unknown. In this study a characterization of the mesoscale structures in the zone of the Azores current is made based on the study of the interaction of these structures with the topography, in particular with MAR. Using satellite altimetry and physical values derived from it, it was possible to study some issues associated with this dynamics / interaction, covering for the first time a period of 19 years of data. The vortices interacting with the MAR do it in two distinct ways: crossing it or propagating along its Eastern flank, and sometimes combining these two. It is concluded in this work that there are four basic forms of passage over the crest: as a coherent structure, through vortex division, vortex coalescence or through a streamer process. Given the great dynamics of the region, whose turbulent kinetic energy (EKE) values exceed 200 cm2/s2 in the ridge zone, a combination of the different forms of passage often occurs. In total, only 15% of the vortices whose detection starts in the region associated to the main axis of the current, interact with the crest. The migration of these structures on the crest can exceed 6 months, so it can be concluded that the vortices of this region are deep enough to be influenced by the topography. An interesting finding in this work was the existence of vortices that propagate along the Western flank of the ridge, contrary to what is expected, taking into account the β-topographic effect.
Descrição
Tese de mestrado em Ciências Geofísicas, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2017
Palavras-chave
Corrente dos Açores Vórtices Crista Média Atlântica Teses de mestrado - 2017