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- Kinematics of surface currents at the northern margin of the Gulf of CádizPublication . De Oliveira Júnior, Luciano; Relvas, Paulo; Garel, ErwanThe 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.
- The structure of incipient coastal counter currents in South Portugal as indicator of their forcing agentsPublication . De Oliveira Júnior, Luciano; Garel, Erwan; Relvas, PauloThe alongshore subtidal water circulation along the South Portugal inner shelf is characterized by the temporal alternation of equatorward (i.e., broadly eastward) flows related to coastal upwelling processes and poleward (i.e., broadly westward) Coastal Counter Currents (CCCs). The objective of this study is to get insights about the main drivers of CCCs based on kinematic parameters describing the structure of the flow at the moment it changes direction. The parameters are derived from an extensive bottom-mounted ADCP dataset (16 deployments; 34,121 hourly records) collected at a single mooring (23 m water depth). Results show that the so-called incipient flows present contrasted general patterns whether they turn from equatorward to poleward or the opposite. Complementary observations at a nearby station indicate that these characteristics are spatially consistent along the studied area. Although 70% of CCCs are generated under favourable wind conditions (Levanter), these flows generally develop through the bed layer, in particular in summer. Hence, the Levanter wind - expected to promote flow setup through the surface layer - is not the main driver of CCCs in most cases. The general structure of incipient CCCs strongly suggests that the dominant force competing with the wind stress is an alongshore pressure gradient (APG). Furthermore, the maximum equatorward flow magnitude before CCCs setup is significantly correlated with the following (poleward) acceleration of incipient CCCs near the bed. Such relation is consistent with the development of CCCs due to the unbalance of an APG (produced during active upwelling) when wind relaxes. This process is further supported by an analysis of the depth-averaged momentum equation which suggests that the coastal circulation is mainly driven by linear dynamics in the region.
- Coastal counter-currents setup patterns in the Gulf of Cadiz: an indication of their forcing factorsPublication . De Oliveira Júnior, Luciano; Garel, Erwan; Relvas, PauloAlongshore coastal counter-currents (CCCs) are frequent features of Eastern Boundary Upwelling Systems, where they temporally alternate with upwelling driven jets of opposite direction. Along the northern margin of the Gulf of Cadiz inner shelf, these CCCs are oriented poleward (eastward) and are responsible for sharp temperature increases during the upwelling season, along with potential decline in water quality at the coast. This research is based on a multi-year ADCP velocity time-series (2008-2017), recorded at a single location (23 m water depth) over 13 deployments up to 3 months-long. The analysis focuses on the water column alongshore velocities during current inversions (i.e., the transition from equatorward upwelling jets to poleward CCCs). A set of parameters were derived from the flow structure to identify distinct types of inversions and to hypothesize about their driving mechanisms. Results showed that 77% of the inversions start near the bed, propagating then to the upper layers. The bottom layer also changes direction before the surface layer for most events (71%). The vertical shear in this case is one order of magnitude greater than in the (less frequent) opposite situation. No seasonal variability was observed in the CCCs occurrences. However, the parameters analysed in this study suggest different types of inversion between winter and summer. In winter, inversions are well defined (low variability), with similar patterns near the surface and bed layers as a result of a strong barotropic component. In summer, the inversion patterns are more variable. In particular, the upper and bed layers are often importantly decoupled during inversions, indicating the strengthening of baroclinicity. A categorization of inversions events is proposed based on the correlation of the characteristics between the developed parameters. Various types of inversion were obtained, suggesting that CCCs are driven by different forcings that may act separately or jointly.
- Relatório setor zonas costeiras e mar: vulnerabilidades atuais e futurasPublication . Moura, Delminda; Garel, Erwan; Martins, Flávio; Mendes, Isabel; Janeiro, João; de Oliveira Júnior, Luciano; Carrasco, A. Rita; Sampath, Ruwan; Costas, Susana; C. Veiga-Pires, C.O litoral algarvio estende-se desde a foz da Ribeira de Odeceixe na Costa Oeste (37° 26’ 38,35’’ N; 008° 48’ 50,00’’ O) à foz do Rio Guadiana na costa Sul (37° 10’ 02,10’’ N; 007° 24’ 07’’ O) num total de cerca de 210 km. Este extenso litoral é constituído por morfologias costeiras muito diversas e talhado em formações com idades e litologias muito variadas. De um modo geral, a costa do Algarve pode ser dividida em i) litoral de arribas rochosas, ii) litoral de arribas arenosas e iii) litoral baixo, que se diferenciam entre si pelo substrato físico que os suporta e pelos processos dinâmicos responsáveis pela sua evolução.
- Upwelling processes variability and water circulation along the northern margin of the Gulf of CadizPublication . de Oliveira Júnior, Luciano; Relvas, Paulo; Garel, ErwanThe 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.
- Ocean noise field-calibration constraints for deep sea miningPublication . de Oliveira Júnior, Luciano; Rodríguez, Orlando; Jesus, SergioThe EU Horizon project TRIDENT aims at developing a comprehensive study to improve the understanding of the activities related to deep sea mining as well as to develop observation methodologies, technologies required for impact forecast, and possible mitigation measures. In the framework of TRIDENT, the impact of the acoustic field will be assessed by deploying a fixed array of acoustic recorders in the surrounding of the mining activity on top of the Tropic Seamount to monitor the near field, while acoustic gliders equipped with hydrophones will provide the observations from the far field. In complement, noise models will be routinely used for predicting sound distribution requiring frequent calibration with field data with a suitable spatial coverage. This work proposes a methodology and tests the calibration capabilities for ocean noise modelling in the context of DSM activities, where the main objective is to define the best sampling strategy for a glider to record data for calibrating acoustic propagation models. The analysis of the broadband transmission loss from contrasting oceanographic conditions allowed the identification of regions of interest assuring the best use of the acoustic glider in the upcoming sea trial.
- Meagre (Argyrosomus regius) move westward with favourable flows along the south coast of PortugalPublication . Garel, Erwan; Winkler, Alexander; Gunasekara, S.S.; Abecasis, David; de Oliveira Júnior, LucianoBased 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.