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

Meagre (Argyrosomus regius) move westward with favourable flows along the south coast of Portugal

Publication . Garel, Erwan; Winkler, Alexander; Gunasekara, S.S.; Abecasis, David; de Oliveira Júnior, Luciano

Based on acoustic detections from a network of five stations along the south coast of Portugal, 25 transits of tagged meagre were identified between 2019 and 2020, mostly in summer. Comparisons with hourly current measurements from a moored Acoustic Doppler current Profiler and an High Frequency Radar system indicate that meagre move westward with favourable alongshore flows at a confidence level > 90% (while eastward transits observations were too few to be significant). Several similar transits of different individuals occurred within two days suggesting a species response to environmental changes. The analysis of surface seawater temperature from ERA5 shows that meagre always swam westward towards colder water, despite their warm water affinities. As such, they reached food-rich upwelling areas when the water temperature was higher than usual (due to the westward advection of warm water), probably optimizing their feeding ability. The demonstrated alongshore meagre movements in response to temperature variations induced by the mesoscale coastal circulation is particularly relevant for the management of this high economic value species in southern Portugal.

2024-07Journal article

Open access

Understanding planktonic diatoms to inform site selection for shellfish offshore aquaculture systems

Publication . Araújo Freitas, Carla Alexandra; Goela, Priscila; Icely, John; Dias Duarte Fragoso, Bruno; de Oliveira Júnior, Luciano; Cristina, Sónia; Newton, Alice

Sustainable bivalve aquaculture offers a solution to rising food demand. This study aimed to help identify suitable offshore shellfish aquaculture sites in coastal upwelling regions. Here, planktonic diatom abundance, crucial for bivalve growth, is a key success factor. A successful bivalve production area was analysed to understand how diatom dynamics and upwelling influence a suitable location for shellfish offshore aquaculture. A diatom-based criterion for site selection is proposed. Surface water samples were collected over two years in southwest Iberia, near a major upwelling centre. Phytoplankton abundance and composition were assessed along with chlorophyll-a concentration and wind-stress upwelling indices. Episodes of high diatom concentrations were frequent and distributed throughout the year. Most diatom blooms were dominated by Pseudo-nitzschia, particularly the delicatissima group. Although this genus is potentially toxic, the levels of amnesic shellfish poisoning (ASP) toxin-the domoic acid (DA) in bivalves were mostly low or undetectable, even during periods of high Pseudo-nitzschia abundances. Conversely, the less abundant Pseudo-nitzschia seriata group presence often coincided with detectable DA in bivalves. These findings suggest that high Pseudo-nitzschia abundance does not hinder aquaculture and may even benefit it when the delicatissima group prevails in high numbers. Focusing on the Pseudo-nitzschia seriata group in predictive models may better predict DA levels for harvest closures, minimising economic losses and health risks. This is particularly true for locations where the seriata group is the one linked to DA production. Additionally, we identified oceanographic conditions coinciding with DA levels exceeding regulatory limit.

2025-08Journal article

Open access