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  • Piece‐wise constant cluster modelling of dynamics of upwelling patterns
    Publication . Nascimento, Susana; Martins, Alexandre; Relvas, Paulo; Luis, Joaquim; Mirkin, Boris
    A comprehensive approach is presented to analyse season's coastal upwelling represented by weekly sea surface temperature (SST) image grids. Our three-stage data recovery clustering method assumes that the season's upwelling can be divided into shorter periods of stability, ranges, each to be represented by a constant core and variable shell parts. Corresponding clustering algorithms parameters are automatically derived by using the least-squares clustering criterion. The approach has been successfully applied to real-world SST data covering two distinct regions: Portuguese coast and Morocco coast, for 16 years each.
  • The mediterranean overflow in the Gulf of Cadiz: a rugged journey
    Publication . Sanchez-Leal, Ricardo F.; Jesus Bellanco, Maria; Miguel Fernandez-Salas, Luis; Garcia-Lafuente, Jesus; Gasser-Rubinat, Marc; Gonzalez-Pola, Cesar; Hernandez-Molina, Francisco J.; Pelegri, Josep L.; Peliz, Alvaro; Relvas, Paulo; Roque, David; Ruiz-Villarreal, Manuel; Sammartino, Simone; Carlos Sanchez-Garrido, Jose
    The pathways and transformations of dense water overflows, which depend on small-scale interactions between flow dynamics and erosional-depositional processes, are a central piece in the ocean's large-scale circulation. A novel, high-resolution current and hydrographic data set highlights the intricate pathway travelled by the saline Mediterranean Overflow as it enters the Atlantic. Interaction with the topography constraints its spreading. Over the initial 200 km west of the Gibraltar gateway, distinct channels separate the initial gravity current into several plunging branches depth-sorted by density. Shallow branches follow the upper slope and eventually detach as buoyant plumes. Deeper branches occupy mid slope channels and coalesce upon reaching a diapiric ridge. A still deeper branch, guided by a lower channel wall marked by transverse furrows, experiences small-scale overflows which travel downslope to settle at mid-depths. The Mediterranean salt flux into the Atlantic has implications for the buoyancy balance in the North Atlantic. Observations on how this flux enters at different depth levels are key to accurately measuring and understanding the role of Mediterranean Outflow in future climate scenarios.
  • 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.
  • Acoustic inversion of the cold water filaments off the Southwest coast of Portugal
    Publication . Felisberto, P.; Jesus, S. M.; Relvas, Paulo
    Cold water filaments have important implications in the biological and chemical exchanges between the coastal and offshore ocean. The Cape São Vicente area in the Southwest coast of Portugal is a well know region where such phenomenon is observed. In October 2004, the multidisciplinary project ATOMS, involving oceanographers and acousticians, was conducted with the objective to complement the sea surface temperature (SST) satellite observation with a full water column characterization. Due to weather and technical conditions during the project sea trial, only CTD measurements in upper layers of the water column were performed. These at sea collected data together with archival data from the NODC database, allowed to establish realistic scenario of the 3D temperature distribution in the area, including deeper water layers. Archival data of temperature profiles suggest the occurrence of other important oceanic phenomena such as the subduction of warm Mediterrenean water, that should also influence the acoustic propagation. With the help of forward acoustic modelling the significance and signature of the individual oceanographic phenomena on the acoustic propagation, regarding different sampling strategies of the area by acoustic means is investigated. These investigations allowed to develop strategies to settle the main problem addressed by this work: invert the cold water filament structure by acoustic means in a complex environment where acoustic propagation is affected also by other important oceanic and bathymetric features. Since, the objective of this work, is to evaluate the ability to perform a 3D characterization of vertical structure of the ocean, a minimal transmit-receive acquisition composed of a suspended source from a ship and a drifting vertical array, is assumed. The spatial structure is obtained by a combination of inversions for ”mean” sound speed/temperature perturbations obtained for source-array cross-sections covering the area of interest. Matched-field and ray tracing based tomography techniques are used in the inversion for the ”mean” perturbations. The planned sampling strategies and necessary acoustic equipment to resolve such oceanic features is discussed having in mind future sea trials.
  • Characterisation of coastal counter-currents on the inner shelf of the Gulf of Cadiz
    Publication . Garel, Erwan; Laiz, I.; Drago, T.; Relvas, Paulo
    At the Gulf of Cadiz (GoC), poleward currents leaning along the coast alternate with coastal upwelling jets of opposite direction. Here the patterns of these coastal countercurrents (CCCs) are derived from ADCP data collected during 7 deployments at a single location on the inner shelf. The multiyear (2008–2014) time-series, constituting ~ 18 months of hourly records, are further analysed together with wind data from several sources representing local and basin-scale conditions. During one deployment, temperature sensors were also installed near the mooring site to examine the vertical thermal stratification associated with periods of poleward flow. These observations indicate that the coastal circulation is mainly alongshore and barotropic. However, a baroclinic flow is often observed shortly at the time of flow inversion to poleward. CCCs develop all year-round and exclusively control the occurrence of warm coastal water during the upwelling season. On average, one poleward flow lasting 3 days was observed every week, corresponding to CCCs during ~ 40% of the time without seasonal variability. Thus, the studied region is distinct from typical upwelling systems where equatorward coastal upwelling jets largely predominate. CCCs often start to develop near the bed and are frequently associated with 2-layer cross-shore flows characteristic of downwelling conditions (offshore near the bed). In general, the action of alongshore wind stress alone does not justify the development of CCCs. The coastal circulation is best correlated and shows the highest coherence with south-eastward wind in the basin that proceeds from the rotation of southward wind at the West coast of Portugal, hence suggesting a dominant control of large-scale wind conditions. In agreement, wavelet analyses indicate that CCCs are best correlated with alongshore wind occurring in a band period characteristic of the upwelling system (8–32 days). Furthermore, in the absence of wind coastal currents tend to be poleward during summer. This set of observations supports that CCCs develop in response to the unbalance of an alongshore pressure gradient during the relaxation of (system-scale) upwelling-favourable winds, oriented south-eastward in the basin. The relaxation periods defined based on this wind direction show a good correspondence with the periods of poleward flow.
  • Variability patterns and phenology of harmful phytoplankton blooms off southern Portugal: looking for region-specific environmental drivers and predictors
    Publication . Lima, M.J.; Relvas, Paulo; Barbosa, Ana
    Harmful algal blooms (HABs) negatively impact coastal ecosystems, fisheries, and human health, and their prediction has become imperative for effective coastal management. This study aimed to evaluate spatialtemporal variability patterns and phenology for key toxigenic phytoplankton species off southern Portugal, during a 6-year period, and identify region-specific environmental drivers and predictors. Total abundance of species responsible for amnesic shellfish poisoning (Pseudo-nitzschia spp.), diarrhetic shellfish poisoning (Dinophysis spp.), and paralytic shellfish poisoning (G. catenatum) were retrieved, from the National Bivalve Mollusk Monitoring System public database. Contemporaneous environmental variables were acquired from satellite remote sensing, model-derived data, and in situ observations, and generalized additive models (GAMs) were used to explore the functional relationships between HABs and environmental variables and identify regionspecific predictors. Pseudo-nitzschia spp. showed a bimodal annual cycle for most coastal production areas, with spring and summer maxima, reflecting the increase in light intensity during the mixed layer shoaling stage, and the later stimulatory effects of upwelling events, with a higher bloom frequency over coastal areas subjected to stronger upwelling intensity. Dinophysis spp. exhibited a unimodal annual cycle, with spring/summer maxima associated with stratified conditions, that typically promote dinoflagellates. Dinophysis spp. blooms were delayed with respect to Pseudo-nitzschia spp. spring blooms, and followed by Pseudo-nitzschia spp. summer blooms, probably reflecting upwelling-relaxation cycles. G. catenatum occurred occasionally, namely in areas more influenced by river discharges, under weaker upwelling. Statistical-empirical models (GAMs) explained 7-8%, and 21− 54% of the variability in Pseudo-nitzschia spp. and Dinophysis spp., respectively. Overall, a set of four easily accessible environmental variables, surface photosynthetically available radiation, mixed layer depth, sea surface temperature, and chlorophyll-a concentration, emerged as the most influential predictors. Additionally, over the coastal production areas along the south coast, river discharges exerted minor negative effects on both HAB groups. Despite evidence supporting the role of upwelling intensity as an environmental driver of Pseudonitzschia spp., it was not identified as a relevant model predictor. Future model developments, such as the inclusion of additional environmental variables, and the implementation of species- and period-specific, and hybrid modelling approaches, may further support HAB operational forecasting and managing over complex coastal domains.
  • Monitoring invasive macroalgae in southern Portugal: drivers and citizen science contribution
    Publication . Jiménez, Javier; Simes, Dina; Abecasis, Ana Rita Costa; Relvas, Paulo; Garel, Erwan; Martins, Paula Ventura; Santos, Rui
    Under certain environmental and oceanographic conditions, macroalgae can overgrow and accumulate in massive quantities on beaches, causing serious ecological and economic impacts. To address this problem, a citizen science monitoring platform was created to determine the spatial and temporal distribution of macroalgae accumulations along the beaches of Algarve in southern Portugal, with the aim to assess the extent of beach-cast events and their relationship with abiotic factors. A Redundancy Analysis (RDA) and a permutational analysis of variance (PERMANOVA) were carried out to explore the relationship between macroalgae accumulation level and the abiotic variables: sea surface temperature, wind speed, wind direction, currents, maximum sea level, significant wave height, salinity, nitrate, ammonium, phosphate, precipitation and radiation. The citizen science campaign showed great participation, resulting in 404 submissions between July 2021 and September 2023. The campaign revealed that three species of macroalgae accumulated on the beaches of Algarve, Ulva sp. (with the presence of Ectocarpales and Dyctiotales) along the sandy eastern coast, and the invasive species Asparagopsis armata and Rugulopteryx okamurae in the rocky central and western beaches, respectively. The accumulations of R. okamurae increased from 2021 to 2023, were registered throughout the year and were more abundant than those of Ulva sp. and A. armata, which were only observed in spring and summer. The highest levels of R. okamurae beach-cast depositions were related to strong wave conditions, and high sea surface temperature and salinity. The accumulation of Ulva sp. was related to high sea surface temperature and salinity whereas A. armata was also correlated with winds parallel to the shore (NW-W). PERMANOVA analysis revealed that sea surface temperature and wave conditions had a significant effect on the overall abundance of macroalgae beach-cast accumulations. Overall, our citizen science campaign effectively involved the public, leading to the collection of important data on monitoring macroalgae accumulations. Through these findings, we were able to pinpoint the environmental, atmospheric, and hydrodynamic factors that contribute to their development, movement, and buildup along the Algarve coastlines.
  • The structure of incipient coastal counter currents in South Portugal as indicator of their forcing agents
    Publication . De Oliveira Júnior, Luciano; Garel, Erwan; Relvas, Paulo
    The 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.
  • Northerly wind trends along the Portuguese marine coast since 1950
    Publication . Leitão, Francisco; Relvas, Paulo; Canovas, Fernando; Baptista, Vânia; Teodosio, M. A.
    Wind is a marine coastal factor that is little understood but has a strong interaction with biological productivity. In this study, northerly wind trends in three regions of the Portuguese coast (Northwestern: NW, Southwestern: SW, and Southern: S) were analyzed. Two datasets with long-term (ICOADS: 1960-2010) and short-term data (Satellite: 1989-2010) were used to complement one another. The study revealed the northerly wind yearly data to be non-stationary and highly variable between years. Overall, the northerly wind intensity increased throughout the 1960s regardless of the area and dataset. Between 1960 and 2010, the northerly wind increased at a linear rate of 0.24, 0.09, and 0.15ms-1 per decade in the NW, SW, and S coastal regions, respectively. The rate was higher in recent decades (1988-2009), with the wind intensity increasing by 0.4, 0.3, and 0.3ms-1 per decade in the NW, SW, and S regions, respectively. Analyses of the sudden shifts showed significant increases in northerly wind intensities after 2003, 2004, and 1998 in the NW, SW, and S coast, respectively. Exceptions were found for autumn (September for short-term data), when a decrease in northerly winds was observed in recent decades, regardless of the area, and for summer, when no changes in wind trends were recorded in the NW and SW. The long-term data also showed a major increase in northerly winds in winter (January and February), which is the recruitment season for many small and medium-sized pelagic fish. The increase in the intensity of the northerly winds over the past two decades and the past half-century occurred at a higher rate than was estimated by the IPCC for the next century.
  • Erasmus experience between the University of Cadiz (Spain) and the University of Algarve (Portugal)
    Publication . Laiz, Irene; Relvas, Paulo; Plomaritis, Theocharis A.; Garel, Erwan
    A mobility program was carried out during the last two years between the Universities of Cadiz (Spain) and Algarve (Portugal) under the EU funded Erasmus+ Mobility for Teaching. The objective of the mobility was twofold: on one hand, it included the strengthening of the existing scientific cooperation between the University of Cadiz (home institution) and the University of Algarve (host institution) in the field of the Gulf of Cadiz Physical Oceanography; on the other hand, it pretended to improve the teaching quality, focusing on both the lecturers and the students. Both institutions have long ties of cooperation that have recently been intensified under the umbrella of the International Campus of Marine Excellence (CeiMar). Specific objectives oriented towards the lecturers included the exchange of teaching experiences among them as well as the comparison of teaching strategies and methodologies between the host and home institutions at the Master level in order to evaluate and enhance the best teaching practices with the aim of improving the students learning process. Specific objectives oriented towards the students included: (1) to provide local students that cannot afford studying a Master degree abroad with a foreign teacher in the discipline that will offer them different added expectations; (2) to teach students different subjects from those taught at the host institution, thus benefitting from new scientific knowledge and experiences. It must be pointed out that the subject taught by the home institution lecturer represents a competence lacking at the host institution, hence complementing the program of the discipline and providing an added value to the Master degree. Informal questionnaires carried out among students by the host institution revealed that they evaluated having a foreign teacher as a very positive experience. In terms of research, collaboration among both institutions is of great importance because they are both located within the same geographic region and hence, they share common interests. The mobility promoted finishing on-going collaborative publications as well as sharing new research experiences, data and knowledge, hence leading to an improvement of the Physical Oceanography state-of-the-art in the Gulf of Cadiz. In fact, two scientific papers on the Gulf of Cadiz circulation system and two on the storm climate along the Gulf of Cadiz and its relation with coastal hazards have been recently published as a direct result of the mobility program.