Coastal circulation along the northern shelf of the Gulf of Cadiz

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Kinematics of surface currents at the northern margin of the Gulf of Cádiz

Publication . De Oliveira Júnior, Luciano; Relvas, Paulo; Garel, Erwan

The subtidal surface water circulation at the northern margin of the Gulf of Cadiz, at the southern extremity of the Iberian upwelling system, is described based on validated hourly high-frequency radar measurements from 2016 to 2020. Statistical analyses (mean, standard deviation, eccentricity and empirical orthogonal functions) are applied to the dataset, which is completed with ADCP time series from multiple moorings at five inner-shelf stations and ERAS wind. Off the shelf, the main circulation pattern consists of a slope current, best developed in summer when north-westerlies dominate, in particular at the most exposed western region. Mechanisms other than upwelling must contribute to this flow in order to explain its seasonal persistence. The slope circulation reverses for regional wind events with an east component > 10 m s(-1), approximately. On the shelf, currents are mainly alongshore and balanced. The circulation is generally continuous along the coast, except for weak (< 0.1 m s(-1), broadly) poleward flows. In the latter case, the flow tends to remain equatorward near Cape Santa Maria. In winter, coastal poleward flows often extend over the entire margin and are mainly wind-driven. In summer, these flows generally consist of coastal counter currents (CCCs) with the poleward direction opposed to that of the slope current. The CCCs are associated with significant cyclonic recirculation, strongest to the west, where a transient eddy is shortly observed for weak wind stress. This circulation develops after periods of strong north-westerlies, supporting that CCCs result from the imbalance of a regional alongshore pressure gradient.

2022Journal article

Open Access

Water circulation along the northern margin of the Gulf of Cadiz

Publication . Júnior, Luciano de Oliveira; Garel, Erwan; Relvas, Paulo

Um conjunto inédito de observações que inclui perfis verticais de corrente medidos por um correntómetro acústico de efeito Doppler (ADCP) em 5 locais durante vários fundeamentos entre 2008 e 2019, e também 4,5 anos de velocidades superficiais coletadas por radares de alta frequência (HFR) são analisados na presente Tese juntamente com outras variáveis ambientais. O objetivo principal deste trabalho é atualizar o conhecimento sobre os padrões de circulação e explicar a dinâmica envolvida nos padrões observados. Ventos provenientes de um modelo de previsão de alta resolução foram usados para avaliar o efeito da interação atmosfera-oceano com o intuito de inferir sua relevância no desenvolvimento da circulação oceânica na área de estudo. Os resultados mostram que a variabilidade sazonal e espacial de diferentes mecanismos de afloramento está associada com a variabilidade do gradiente transversal de pressão, contribuindo assim para a posição meridional da Corrente do Golfo de Cádiz sobre o talude continental na região oeste. Além disso, uma depressão do nível do mar é frequentemente observada na mesma região onde se observam valores elevados de Ekman “pumping”. O efeito dessa depressão poderá contribuir significativamente para o desenvolvimento do vórtice ciclónico e de escoamentos para oeste ao longo da costa. Na plataforma, a circulação é polarizada, alinhada com a direção da costa. As principais direções, para oeste ou para leste, são observadas praticamente com a mesma frequência. Os escoamentos para leste estão associados com a ação direta do vento, enquanto os escoamentos para oeste estão associados a um gradiente horizontal de pressão ao longo da costa, possivelmente como resultado da intensidade variável do afloramento ao longo da costa. Por fim, a análise extensiva das observações provenientes dos HFR e ADCPs possibilitou o desenvolvimento de um modelo conceptual da circulação superficial para os períodos de primavera e verão.

2023-10-02Doctoral thesis

Open Access

Upwelling processes variability and water circulation along the northern margin of the Gulf of Cadiz

Publication . de Oliveira Júnior, Luciano; Relvas, Paulo; Garel, Erwan

The present study investigates the contributions of upwelling mechanisms (coastal upwelling from boundary divergence and Ekman pumping from spatially variable wind) at the northern margin of the Gulf of Cadiz (NMGoC) based on high-resolution wind. The effects of the spatiotemporal upwelling mechanisms variability are then explored using current observations along with sea surface temperature and sea level anomaly. Upwelling favourable conditions occur throughout the year along the NMGoC, with the strongest intensity near Cape Sao ˜ Vicente due to a persistent positive wind stress curl. In winter, the surface water divergence is restricted to the coastal boundary due to strong coastal upwelling events that result in a cross-shore sea-level gradient close to the coast. Towards the summer, the gradient increases and extends further offshore (over the slope) at the western region due to the intensification of the Ekman pumping. Therefore, the seasonal and spatial variability of coastal upwelling and Ekman pumping contributes significantly to the offshore position of the geostrophic Gulf of Cadiz Current over the western shelf slope. Furthermore, a permanent sea level depression corresponds to the location of the strongest Ekman pumping, near Cape Sao ˜ Vicente. The dynamic adjustment of this feature may drive the cyclonic cell and alongshore poleward currents often observed in the area.

2024-10Journal article

Open Access