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The assessment of the uncertainty of the hydrodynamical SYMPHONIE2015 model and its implications for Lagrangian dispersal studies
Abstract
To give sound management advice, the connectivity in coastal areas must be thoroughly understood. The red thread throughout this PhD is analysing the uncertainty of the SYMPHONIE2015 model and its effect on larval dispersal simulations. In the first chapter, the robustness of the model to assumption violation was tested. This was done by calculating six relative and absolute statistical indicators during and outside of wind, wave and stratification events. The results showed that the model’s performance is not affected by these events. In the second chapter, the instant error was calculated. Then, the cumulative error distributions were compared to each other in space and time. In time, the intraseasonal differences in error distributions were smaller than the interseasonal ones. In space, eight groups of error distributions could be formed. No link was found between the model’s performance and stratification, water depth, resolution and bathymetry slope. However, a strong correlation between the current speed and the error distributions was found. In chapter three, the instant error was added as noise to the Lagrangian dispersal simulations and compared to the original run to assess the effect of the models’ error on connectivity. The median difference in transfer rate between the runs with and without noise around zero. However, the relative difference in transfer rate can vary from -100% to 100%. Knowing the uncertainties in dispersal simulations can aid in using them for management advice.
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Knowledge about migration potential is key to forecasting species distributions in changing environments. For many marine benthic invertebrates, migration happens during reproduction because of larval dispersal. The present study aims to test whether larval size can be used as a surrogate for migration potential arising from larval longevity, competence, sinking, or swimming behavior. The hypothesis was tested using larvae of three sympatric gorgonian species that release brooded lecithotrophic larvae in the same season: Paramuricea clavata, Corallium rubrum and Eunicella singularis. Despite different fecundities and larval sizes, the median larval longevity was
similar among the three species. Free-fall speed increased with larval size. Nevertheless, the only net sinkers were the P. clavata larvae, as swimming was more common than free fall in the other two species with larger larvae. For the other two species, swimming activity frequency decreased as larval size increased. Interestingly, maximum larval longevity was lowest for the most active but intermediately sized larvae. Larval size did not covary consistently with any larval traits of the three species when considered individually. We thus advise not using larval size as a surrogate for migration potential in distribution models. The three species exemplified that different mechanisms, i.e., swimming activity or larval longevity, resulting from a trade-off in the use of energy reserves can facilitate migration, regardless of life history strategy.
The recent integration of Acoustic Doppler Current Profilers (ADCPs) onto underwater gliders changes the way current and sediment dynamics in the coastal zone can be monitored. Their endurance and ability to measure in all weather conditions increases the probability of capturing sporadic meteorological events, such as storms and floods, which are key elements of sediment dynamics. We used a Slocum glider equipped with a CTD (Conductivity, Temperature, Depth), an optical payload, and an RDI 600 kHz phased array ADCP. Two deployments were carried out during two contrasting periods of the year in the Rhone River region of freshwater influence (ROFI). Coastal absolute currents were reconstructed using the shear method and bottom tracking measurements, and generally appear to be in geostrophic balance. The responses of the acoustic backscatter index and optical turbidity signals appear to be linked to changes of the particle size distribution in the water column. Significantly, this study shows the interest of using a glider-ADCP for coastal zone monitoring. However, the comparison between suspended particulate matter dynamics from satellites and gliders also suggests that a synoptic view of the processes involved requires a multiplatform approach, especially in systems with high spatial and temporal variability, such as the Rhone ROFI area.
Zooplankton can be detected by using acoustic Doppler current profiler (ADCP) instruments through acquiring the mean volume backscattering strength (MVBS) data. However, the precision of the backscattered signal measured by single ADCP measurement has a limitation in the MVBS variation of zooplankton. The objectives of this study were to analyze the MVBS and vertical velocity from ADCPs at the same time and location for zooplankton’s daily vertical migration (DVM) observation. Measurements were conducted in Lembeh Strait, North Sulawesi, Indonesia. Instruments used included a moored ADCP 750 kHz and a mobile ADCP 307.2 kHz. High MVBS value was found at 11.5–16 m depths and was identified as the sound scattering layer (SSL). The DVM patterns in the SSL displayed significant differences over time and had good relationships with the diurnal cycle. Theoretical target strength (TS) from the scattering models based on a distorted-wave Born approximation (DWBA) was estimated for Oithona sp. and Paracalanus sp.; the two dominant species found in the observed area. However, ΔMVBS and ΔTS proved that the dominant zooplankton species were not the main scatterers. The strong signal in SSL was instead caused by the schools of various zooplankton species.
The United Nations’ target for global ocean protection is 10% of the ocean in Marine Protected Areas (MPAs) by 2020. There has been remarkable progress in the last decade, and some organizations claim that 7% of the ocean is already protected and that we will exceed the 10% target by 2020. However, currently only 3.6% of the ocean is in implemented MPAs, and only 2% is in implemented strongly or fully protected areas. Here we argue that current protection has been overestimated because it includes areas that are not yet protected, and that areas that allow significant extractive activities such as fishing should not count as ‘protected.’ The most rigorous projections suggest that we will not achieve the 10% target in truly protected areas by 2020. Strongly or fully protected areas are the only ones achieving the goal of protecting biodiversity; hence they should be the MPA of choice to achieve global ocean conservation targets.
Marine larval dispersal is a complex biophysical process that depends on the effects of species biology and oceanography, leading to logistical difficulties in estimating connectivity among populations of marine animals with biphasic life cycles. To address this challenge, the application of multiple methodological approaches has been advocated, in order to increase confidence in estimates of population connectivity. However, studies seldom account for sources of uncertainty associated with each method, which undermines a direct comparative approach. In the present study we explicitly account for the statistical uncertainty in observed connectivity matrices derived from elemental chemistry of larval mussel shells, and compare these to predictions from a biophysical model of dispersal. To do this we manipulate the observed connectivity matrix by applying different confidence levels to the assignment of recruits to source populations, while concurrently modelling the intrinsic misclassification rate of larvae to known sources. We demonstrate that the correlation between the observed and modelled matrices increases as the number of observed recruits classified as unknowns approximates the observed larval misclassification rate. Using this approach, we show that unprecedented levels of concordance in connectivity estimates (r = 0.96) can be achieved, and at spatial scales (20-40 km) that are ecologically relevant.
During winter 2012-2013, open-ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean, occurred in the Gulf of Lions (Northwestern Mediterranean Sea) and has been thoroughly documented thanks in particular to the deployment of several gliders, Argo profiling floats, several dedicated ship cruises, and a mooring array during a period of about a year. Thanks to these intense observational efforts, we show that deep convection reached the bottom in winter early in February 2013 in a area of maximum 28±3 109m9. We present new quantitative results with estimates of heat and salt content at the sub-basin scale at different time scales (on the seasonal scale to a ten days basis) through optimal interpolation techniques, and robust estimates of the deep water formation rate of 2.0 ± 0.2Sν. We provide an overview of the spatio-temporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue. They concern key circulation features for the deep convection and the subsequent bloom such as Submesoscale Coherent Vortices (SCVs), the plumes and symmetric instability at the edge of the deep convection area.
In the north-western Mediterranean, the strong, dry, cold winds, the Tramontane and Mistral, produce intense heat and moisture exchange at the interface between the ocean and the atmosphere leading to the formation of deep dense waters, a process that occurs only in certain regions of the world. The purpose of this study is to demonstrate the ability of a new coupled ocean–atmosphere modelling system based on MESONH-SURFEX-SYMPHONIE to simulate a deep-water formation event in real conditions. The study focuses on summer 2012 to spring 2013, a favourable period that is well documented by previous studies and for which many observations are available. Model results are assessed through detailed comparisons with different observation data sets, including measurements from buoys, moorings and floats. The good overall agreement between observations and model results shows that the new coupled system satisfactorily simulates the formation of deep dense water and can be used with confidence to study ocean–atmosphere coupling in the north-western Mediterranean. In addition, to evaluate the uncertainty associated with the representation of turbulent fluxes in strong wind conditions, several simulations were carried out based on different parameterizations of the flux bulk formulas. The results point out that the choice of turbulent flux parameterization strongly influences the simulation of the deep-water convection and can modify the volume of the newly formed deep water by a factor of 2.
Over the last decades, nutrients loading to the sea have significantly increased due to the growing anthropogenic pressure in the river watersheds. Some areas of the English Channel and Southern Bight of the North Sea are particularly affected by the resulting eutrophication nuisances. Establishing the link between these nuisances and anthropogenic activities requires (1) the identification of the major nutrient sources and (2) the assessment of the ecosystem response to these nutrient alterations. A nutrient tracking approach has been implemented in the marine ecological model MIRO&CO to allow tracing marine nitrogen back to its continental sources over 2000–2010. On average, nitrogen atmospheric deposition contributes between 1 and 10 mmol N/m³ to marine nutrients concentrations in the English Channel and Southern Bight of the North Sea. This corresponds to relative contributions between 10 and 30%. River contributions remained localized except for the Seine and small French rivers. Different geographical patterns of sources contribution were found for wet and dry periods. Results also showed different contribution systems between offshore and coastal areas. Relative contributions from nitrogen sources to nutrients and phytoplankton biomass are similar. This study provides useful information to help identifying the causes of marine eutrophication and mitigating its nuisances.
Lagrangian Flow Network (LFN) is a modelling framework in which ocean sub-areas are represented as nodes in a network interconnected by links representing transport of propagules (eggs and larvae) by currents. We asses the sensitivity and robustness of four LFN-derived connectivity metrics measuring retention and exchange. The most relevant parameters are tested over large ranges and a wide region with contrasting hydrodynamics: density of released particles, node size (spatial scale of discretization), Pelagic Larval Duration (PLD) and spawning modality. We find a minimum density of released particles that guarantees reliable values for most of the metrics examined. We also find that node size has a nontrivial influence on them. Connectivity estimates for long PLDs are more robust against biological uncertainties (PLD and spawning date) than for short PLDs. For mass-spawners releasing propagules over short periods (≈ 2-10 days), daily release must be simulated to properly consider connectivity fluctuations due to variable currents. In contrast, average connectivity estimates for species that spawn repeatedly over longer durations (few weeks to few months) remain robust even using longer periodicity (5-10 days). Our results have implications to design connectivity experiments with particle-tracking models and to evaluate the reliability of their results.
The main objective of the LAgrangian Transport EXperiment (LATEX) project was to study the influence of coastal mesoscale and submesoscale physical processes on circulation dynamics, cross-shelf exchanges, and biogeochemistry in the western continental shelf of the Gulf of Lion, Northwestern Mediterranean Sea. LATEX was a five-year multidisciplinary project based on the combined analysis of numerical model simulations and multi-platform field experiments. The model component included a ten-year realistic 3D numerical simulation, with a 1 km horizontal resolution over the gulf, nested in a coarser 3 km resolution model. The in situ component involved four cruises, including a large-scale multidisciplinary campaign with two research vessels in 2010. This review concentrates on the physics results of LATEX, addressing three main subjects: (1) the investigation of the mesoscale to submesoscale processes. The eddies are elliptic, baroclinic, and anticyclonic; the strong thermal and saline front is density compensated. Their generation processes are studied; (2) the development of sampling strategies for their direct observations. LATEX has implemented an adaptive strategy Lagrangian tool, with a reference software available on the web, to perform offshore campaigns in a Lagrangian framework; (3) the quantification of horizontal mixing and cross-shelf exchanges. Lateral diffusivity coefficients, calculated in various ways including a novel technique, are in the range classically encountered for their associated scales. Cross-shelf fluxes have been calculated, after retrieving the near-inertial oscillation contribution. Further perspectives are discussed, especially for the ongoing challenge of studying submesoscale features remotely and from in situ data.
Feedback between the Mediterranean Sea and the atmosphere on various temporal and spatial scales plays a major role in the regional climate system. We studied the impact of horizontal atmospheric grid resolution (grid-spacing of ~9 vs. ~50 km) and dynamic ocean coupling (the ocean model NEMOMED12) in simulations with the regional climate model COSMO-CLM. The evaluation focused on sea surface heat fluxes, 10-m wind speed, and sea surface temperature (SST) parameters on both seasonal and annual timescales. The finer grid improved the wind speed (particularly near coastal areas) and subsequently the turbulent heat flux simulations. Both parameters were better simulated with the interactive ocean model NEMOMED12 than with prescribed daily ocean SSTs (using near-observation ERA-Interim reanalysis based SSTs), but coupling introduced a warm SST bias in winter. Radiation fluxes were slightly better represented in coarse-grid simulations. Still, only the higher-resolution coupled simulations could reproduce the observed net outgoing total heat flux over the Mediterranean Sea. Investigation of the impact of sub-diurnal SST variations showed a strong effect on sub-daily heat fluxes and wind speed but minor effects at longer time scales. Therefore, a coupled atmosphere–ocean climate model should be preferred for studying the Mediterranean Sea climate system. Higher-resolution models should be preferred, but they are not yet able to perform better than their coarse-resolution predecessors in all aspects.
We present here a unique oceanographic and meteorological dataset focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multi-platform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high-frequency potential temperature, salinity and current measurements from the mooring LION located within the convection region.
From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly-formed deep water. Each deep convection winter formed a new warmer and saltier '“vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3+/-0.2 *10−3/yr) and potential temperature (3.2+/-0.5 *10−3°C/yr) observed from 2009 to 2013 for the 600-2300m layer.
For the first time, the overlapping of the 3 “phases” of deep convection can be observed with secondary vertical mixing events (2-4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event.
The evolution of the stratification of the north-western Mediterranean between summer 2012 and the end of winter 2013 was simulated and compared with different sets of observations. A summer cruise and profiler observations were used to improve the initial conditions of the simulation. This improvement was crucial to simulate winter convection. Variations of some parameters involved in air - sea exchanges (wind, coefficient of transfer used in the latent heat flux formulation, and constant additive heat flux) showed that the characteristics of water masses and the volume of dense water formed during convection cannot be simply related to the time-integrated buoyancy budget over the autumn - winter period. The volume of dense water formed in winter was estimated to be about 50,000 km3 with a density anomaly larger than 29.113 kg m−3. The effect of advection and air/sea fluxes on the heat and salt budget of the convection zone was quantified during the preconditioning phase and the mixing period. Destratification of the surface layer in autumn occurs through an interaction of surface and Ekman buoyancy fluxes associated with displacements of the North Balearic front bounding the convection zone to the south. During winter convection, advection stratifies the convection zone: from December to March, the absolute value of advection represents 58% of the effect of surface buoyancy fluxes. This article is protected by copyright. All rights reserved.
The UN's globally adopted Convention on Biological Diversity coverage target for marine protected areas (MPAs) is ≥10% by 2020. In 2014 the World Parks Congress recommended increasing this to ≥30%. We reviewed 144 studies to assess whether the UN target is adequate to achieve, maximise or optimise six environmental and/or socio-economic objectives. Results consistently indicate that protecting several tens-of-percent of the sea is required to meet goals (average 37%, median 35%, modal group 21–30%), greatly exceeding the 2.18% currently protected and the 10% target. The objectives we examined were met in 3% of studies with ≤10% MPA coverage, 44% with ≤30% coverage and 81% with more than half the sea protected. The UN's 10% target appears insufficient to protect biodiversity, preserve ecosystem services and achieve socio-economic priorities. As MPA coverages generated from theoretical studies inherently depend on scenario(s) considered, our findings do not represent explicit recommendations but rather provide perspective on policy goals. This article is protected by copyright. All rights reserved
Mature science reveals opportunities for policy progress.
Biodiversity loss can affect ecosystem functions and services. Individual ecosystem functions generally show a positive asymptotic relationship with increasing biodiversity, suggesting that some species are redundant. However, ecosystems are managed and conserved for multiple functions, which may require greater biodiversity. Here we present an analysis of published data from grassland biodiversity experiments, and show that ecosystem multifunctionality does require greater numbers of species. We analysed each ecosystem function alone to identify species with desirable effects. We then calculated the number of species with positive effects for all possible combinations of functions. Our results show appreciable differences in the sets of species influencing different ecosystem functions, with average proportional overlap of about 0.2 to 0.5. Consequently, as more ecosystem processes were included in our analysis, more species were found to affect overall functioning. Specifically, for all of the analysed experiments, there was a positive saturating relationship between the number of ecosystem processes considered and the number of species influencing overall functioning. We conclude that because different species often influence different functions, studies focusing on individual processes in isolation will underestimate levels of biodiversity required to maintain multifunctional ecosystems.
Quantifying larval retention and connectivity remains a major hurdle in the development of realistic spatially-explicit population models in marine systems. This lack of knowledge is primarily due to the difficulty of conducting mark-recapture studies in species that are characterized by the production of large numbers of small pelagic offspring that suffer high initial mortality rates. Advances in artificial and natural tagging methodologies have, however, significantly increased the ability of marine ecologists to track larvae throughout the pelagic larval phase and subsequent recruitment into benthic populations. Many of these empirical approaches are now possible with the development of DNA sequencing and mass spectrometric instrumentation in the last decade. The presence of artificial tags in recaptured individuals remains the only unequivocal method for marking marine larvae. However, the difficulties of tagging sufficient numbers of larvae, with negligible handling effects, are formidable. Natural tags, including genetic markers and geochemical signatures in calcified tissues, are rarely unique indicators of source location, but have a significant advantage because all larvae released from an area are indelibly tagged. Given the strengths and limitations of the techniques, an approach that combines two or more techniques will likely be necessary to quantify larval retention and connectivity over appropriate spatio-temporal scales. Where possible, such a multi-technique strategy should include both artificial and natural tags.
Many species have multi-stage life cycles in which the youngest stages (e.g., larvae) are small, dispersive, and abundant, whereas later stages are sessile or sedentary. Quantifying survival throughout such early stages is critical for understanding dispersal, population dynamics, and life history evolution. However, dispersive stages can be very difficult to sample in situ, and estimates of survival through the entire duration of these stages are typically poor. Here we describe how demographic information from juveniles and adults can be used to estimate survival throughout a dispersive larval stage that was not sampled directly. Using field measurements of demography, we show that detailed information on post-settlement growth, survival, and reproduction can be used to estimate average larval survivorship under the assumption that a typical individual replaces itself over its lifetime. Applying this approach to a common coral reef fish (bicolor damselfish, Stegastes partitus), we estimated average larval survivorship to be 0.108 % (95 % CI 0.025-0.484). We next compared this demography-based estimate to an expected value derived from published estimates of larval mortality rates. Our estimate of larval survivorship for bicolor damselfish was approximately two orders of magnitude greater than what would be expected if larval mortality of this species followed the average, size-dependent pattern of mortality inferred from a published sample of marine fishes. Our results highlight the importance of understanding mortality during the earliest phases of larval life, which are typically not sampled, as well as the need to understand the details of how larval mortality scales with body size.
Population connectivity, which is essential for the persistence of benthic marine metapopulations, depends on how life history traits and the environment interact to influence larval production, dispersal and survival. Although we have made significant advances in our understanding of the spatial and temporal dynamics of these individual processes, developing an approach that integrates the entire population connectivity process from reproduction, through dispersal, and to the recruitment of individuals has been difficult. We present a population connectivity modelling framework and diagnostic approach for quantifying the impact of i) life histories, ii) demographics, iii) larval dispersal, and iv) the physical seascape, on the structure of connectivity and metapopulation dynamics. We illustrate this approach using the subtidal rocky reef ecosystem of Port Phillip Bay, were we provide a broadly-applicable framework of population connectivity and quantitative methodology for evaluating the relative importance of individual factors in determining local and system outcomes.
The spatial characteristics of marine population connectivity are primarily influenced by larval mortality, the duration of the pelagic larval stage, and the settlement competency characteristics, with significant variability imposed by the geographic setting and the timing of larval release. The relative influence and the direction and strength of the main effects were strongly consistent among 10 connectivity-based metrics.
These important intrinsic factors (mortality, length of the pelagic larval stage, and the extent of the precompetency window) and the spatial and temporal variability represent key research priorities for advancing our understanding of the connectivity process and metapopulation outcomes.
Coastal zones are exposed to a range of coastal hazards including sea-level rise with its related effects. At the same time, they are more densely populated than the hinterland and exhibit higher rates of population growth and urbanisation. As this trend is expected to continue into the future, we investigate how coastal populations will be affected by such impacts at global and regional scales by the years 2030 and 2060. Starting from baseline population estimates for the year 2000, we assess future population change in the low-elevation coastal zone and trends in exposure to 100-year coastal floods based on four different sea-level and socio-economic scenarios. Our method accounts for differential growth of coastal areas against the land-locked hinterland and for trends of urbanisation and expansive urban growth, as currently observed, but does not explicitly consider possible displacement or out-migration due to factors such as sea-level rise. We combine spatially explicit estimates of the baseline population with demographic data in order to derive scenario-driven projections of coastal population development. Our scenarios show that the number of people living in the low-elevation coastal zone, as well as the number of people exposed to flooding from 1-in-100 year storm surge events, is highest in Asia. China, India, Bangladesh, Indonesia and Viet Nam are estimated to have the highest total coastal population exposure in the baseline year and this ranking is expected to remain largely unchanged in the future. However, Africa is expected to experience the highest rates of population growth and urbanisation in the coastal zone, particularly in Egypt and sub-Saharan countries in Western and Eastern Africa. The results highlight countries and regions with a high degree of exposure to coastal flooding and help identifying regions where policies and adaptive planning for building resilient coastal communities are not only desirable but essential. Furthermore, we identify needs for further research and scope for improvement in this kind of scenario-based exposure analysis.
Abstract We present a Mediterranean climatology (1º x 1º x 12 months) of the mixed layer and of the seasonal thermocline, based on a comprehensive collection of temperature profiles spanning 44 years (1969-2012). The database includes more than 190,000 profiles, merging CTD, MBT/XBT, profiling floats, and gliders observations. This data set is first used to describe the seasonal cycle of the mixed layer depth and temperature, together with the seasonal thermocline depth and averaged temperature, on the whole Mediterranean on a monthly climatological basis. Our analysis discriminates several regions with coherent behaviors, in particular the deep water formation sites, characterized by significant differences in the winter mixing intensity. Heat Storage Rate (HSR) is calculated as the time rate of change of the heat content due to variations in the temperature integrated from the surface down to the base of the seasonal thermocline. For the first time the quantification of heat storage rate in the upper-ocean, based only on in situ oceanographic data, is made for the whole Mediterranean. The spatial and temporal variability of the HSR in the Mediterranean Sea and its link with dynamic structures like oceanic gyres is also discussed.
GUSTAFSSON ET AL 2833 SKELUFTÏJl; HOBRTÀU€< STRÖMSTAD ^»GÖTEBORG GDANÔC FIG. 7. The 31 thermodynamic regions of the BOBA-PROBE ocean model system.
The coastal areas of the North-Western Mediterranean Sea are one of the
most challenging places for ocean forecasting. This region is exposed to
severe storms events that are of short duration. During these events,
significant air-sea interactions, strong winds and large sea-state can
have catastrophic consequences in the coastal areas. To investigate
these air-sea interactions and the oceanic response to such events, we
implemented the Coupled Ocean-Atmosphere-Wave-Sediment Transport
Modeling System simulating a severe storm in the Mediterranean Sea that
occurred in May 2010. During this event, wind speed reached up to 25
m.s-1 inducing significant sea surface cooling (up to
2°C) over the Gulf of Lion (GoL) and along the storm track, and
generating surface waves with a significant height of 6 m. It is shown
that the event, associated with a cyclogenesis between the Balearic
Islands and the GoL, is relatively well reproduced by the coupled
system. A surface heat budget analysis showed that ocean vertical mixing
was a major contributor to the cooling tendency along the storm track
and in the GoL where turbulent heat fluxes also played an important
role. Sensitivity experiments on the ocean-atmosphere coupling suggested
that the coupled system is sensitive to the momentum flux
parameterization as well as air-sea and air-wave coupling. Comparisons
with available atmospheric and oceanic observations showed that the use
of the fully coupled system provides the most skillful simulation,
illustrating the benefit of using a fully coupled
ocean-atmosphere-wave model for the assessment of these storm
events.
The subpolar North Atlantic represents a key region for global climate, but most numerical models still have well-described limitations in correctly simulating the local circulation patterns. Here, we present the analysis of a 30-year run with a global eddy-resolving (1/12°) version of the NEMO ocean model. Compared to the 1° and 1/4° degree equivalent versions, this simulation more realistically represents the shape of the Subpolar Gyre, the position of the North Atlantic Current, and the Gulf Stream separation. Other key improvements are found in the representation of boundary currents, multi-year variability of temperature and depth of winter mixing in the Labrador Sea, and the transport of overflows at the Greenland-Scotland Ridge. However, the salinity, stratification and mean depth of winter mixing in the Labrador Sea, and
the density and depth of overflow water south of the sill, still present challenges to the model. This simulation also provides further insight into the spatio-temporal development of the warming event observed in the Subpolar Gyre in the mid 1990s, which appears to coincide with a phase of increased eddy activity in the southernmost part of the gyre. This may have provided a gateway through which heat would have propagated into the gyre's interior.
Les plateaux continentaux constituent l'exutoire des fleuves et de leur charge sédimentaire provenant de l'érosion des continents. En raison de l'anthropisation des bassins versants, les fleuves charrient également de nombreux contaminants qui se trouvent majoritairement sous forme particulaire, étroitement liés aux particules sédimentaires fines. La dynamique de la matière d'origine fluviale est mal connue et fortement dépendante des caractéristiques morphologiques, hydrodynamiques et du régime fluvial du site considéré. Le plateau continental du Golfe du Lion est une zone dont la dynamique des apports fluviaux a été intensément étudiée depuis plusieurs décennies. Les tempêtes d'est ont été reconnues comme une composante essentielle du transport à travers le plateau. Elles induisent en effet de forts courants dirigés vers le sud-ouest qui exportent les eaux côtières hors du Golfe du Lion vers la marge continentale espagnole, mais aussi des vagues qui remettent en suspension les sédiments. Cette thèse a pour premier objectif d'étudier le devenir des apports du Rhône sur le plateau continental du Golfe du Lion et plus précisément d'évaluer les temps de résidence de l'eau fluviale sur le plateau en réponse aux forçages physiques. Le second objectif est de caractériser la dynamique des bilans sédimentaires du Golfe, en matière de stockage et d'érosion pour différentes régions du plateau, mais aussi de transfert entre ces régions, et finalement d'export vers la pente et la marge continentale espagnole. La méthodologie utilisée repose sur la modélisation numérique hydrosédimentaire de l'automne 2010 à l'hiver 2011 et s'appuie sur des jeux d'observations innovants. Le dispositif numérique se base sur le couplage du modèle 3D SYMPHONIE avec le modèle d'état de mer WW3 et le modèle de transport sédimentaire MUSTANG. Les observations sont issues de mouillages permanents et de la campagne CASCADE (mars 2011) durant laquelle de nombreux instruments ont été déployés. La thèse est organisée en deux parties. La première partie porte sur les temps de résidence de l'eau d'origine Rhodanienne sur le plateau du Golfe du Lion et sur son export hors du Golfe. Les résultats montrent que les temps de résidence, moyennés sur 3 mois, peuvent varier de 30 à 55 jours avec un rôle important des vents forts pour réduire ces temps de résidence. Les voies d'export au sud-ouest du Golfe du Lion ont ensuite été examinées. Dans cet objectif, l'extrémité du plateau continental, qui ouvre vers le plateau continental espagnol, et la pente continentale, qui mène vers le domaine profond, ont été séparées. En raison de la douceur de l'hiver étudié, l'export se fait à 70% vers le plateau espagnol. Ce résultat contraste avec des études antérieures montrant, pour un hiver froid, qu'une telle proportion transitait vers la pente continentale. La seconde partie concerne la dynamique des sédiments, avec pour première région étudiée le prodelta du Rhône. Sur les 8 mois considérés, 20% des apports sédimentaires du Rhône sont stockés sur 140 km2 devant l'embouchure. Le dépôt de sable est presque exclusivement limité à la zone 0-20 m tandis qu'au-delà le dépôt est majoritairement composé de vase. Sur ces zones plus profondes, les tempêtes ont pour effet d'éroder une part relativement faible des dépôts de vase et d'y déposer du sable. La seconde région étudiée est l'ensemble du plateau continental sur lequel différentes zones ont été mises en évidence. Certaines sont dominées par l'érosion des sédiments (e.g. au nord du Cap d'Agde) et d'autres, au contraire, pour lesquelles le dépôt de matière sédimentaire est dominant (e.g. vasière entre Narbonne et Leucate). Finalement, sur l'ensemble de la période étudiée, le plateau continental du Golfe du Lion a connu un flux de matière sortant de l'ordre de 5.7 Mt pour un total d'apport fluvial de 3.2 Mt.
A three-dimensional unstructured-grid hydrodynamic and water quality model (SCHISM-ICM) is applied successfully for Chesapeake Bay. The model is validated with observations of salinity, chlorophyll-a, dissolved oxygen, nutrients, and phytoplankton productions from the year 1991 to 1995 for the mainstem and some major tributaries, based on multiple model skill scores. Model experiments are conducted to test the importance of having 1) an accurate representation of bathymetry in order to correctly predict hypoxia and other processes, and 2) a high-resolution model grid for tributaries to correctly simulate water quality variables. Comparison with the model experiment results with bathymetry smoothing indicates that bathymetry smoothing, as commonly used for many systems, changes the stratification and lateral circulation pattern, resulting in more salt intrusion into shallow water regions, and an increase of the freshwater age. Consequently, a model with bathymetry smoothing can lead to an unrealistic prediction of the distribution of hypoxia and phytoplankton production. Local grid refinement shows significant improvement of model simulations on local stratification and water quality variables. Overall, the use of high-resolution unstructured grid model leads to a faithful representation of the complex geometry, and thus a seamless cross-scale capability for simulating water quality processes in the Bay including tributaries and tidal creeks.
A coupled ocean-wave model was used to investigate residence times of the water masses on the Gulf of Lion shelf and their export routes during autumn 2010 and winter 2010–2011. Particular attention is paid to the Cap de Creus region and submarine canyon, a key site for the export of water from the Gulf of Lion shelf. First, model results were compared to numerous observations taken during the same period. The timing of strong current pulses on the shelf and at 300 m depth within the Cap de Creus Canyon, linked to easterly winds during winter, were well reproduced by the model. Lagrangian particle trajectories were used to calculate residence times of water masses on the Gulf of Lion shelf. Those of waters located near the Rhône River ranged from 10 to 40 days for autumn 2010, a period which was dominated by frequent strong winds, to 40–60 days for winter 2010–2011, which has been linked to less frequent strong winds and a slope current flowing farther away from the shelf. In the Cap de Creus region the volumes of water exported were estimated at 747 km³ in autumn and 1513 km³ in winter. Results show that, in autumn, only 4% of the water was exported at depths below 200 m while, in winter, this percentage was 25% because it was related to coastal dense water cascades. Yet, this export remains low compared to other winters. It is likely that the low heat losses that characterized the second part of the winter were responsible for the shallow export depth through the Cap de Creus Canyon. These conditions favoured an export of water from the Gulf of Lion to the Spanish coastal zone that would represent 70% of the total exported volume. Interannual variability of the distribution of this export was investigated over the longer period of 2010–2017. Heat losses in February and March appear to be an indicator of dense shelf water cascading, suggesting about 3 or 4 out of 8 winters being affected by deep cascading (reaching 1000 m depth). Understanding the variability of exports of continental inputs to this region is essential to anticipate their potential impacts on ecosystems and human activities in environments as contrasting as the coastal zone and deep canyons.
• Larval transport by ocean circulation and its emerging property at the population level, i.e. connectivity, has received increasing attention thanks to the Aichi target 11 of protecting 10% of ocean surfaces through well‐connected marine protected area (MPA) networks. Furthermore, it is also important to investigate retention within a site, as it determines the self‐persistence of a population in an isolated MPA.
• Mediterranean rocky substrates host a conspicuous and diverse biota, which explains why MPA designations have targeted rocky habitat. Retention rates in the fragmented rocky habitat of the Gulf of Lion were established at two spatial scales (10 and 1 km²) using dispersal simulations. To this end we computed three‐dimensional flow simulations with high spatial resolution nearshore (80 m), combined with a high density of release spots (every 100 m).
• This study shows that among the six rocky 10‐km² patches, the four with highest average retention rates for pelagic larval duration (PLD), of up to 42 days, were designated MPAs. Furthermore, within each MPA, small zones where special protection measures are applied correspond to 1‐km² subpatches where the highest local retention rates were found. Yet the 2% most retentive subpatches of the rocky habitat do not exhibit retention rates large enough to ensure the local persistence of most species.
Coastal and marine environments can begin up to 100 kilometers inland, extend to the continental shelf, and include ocean systems with waters up to 50 meters in depth. The distinct marine ecosystems found in these environments include estuarine and coastal wetlands, such as marshes and mangroves, sand beaches and dunes, seagrass beds, and coral and oyster reefs.
An experiment was carried out in the Gulf of Lions (NW Mediterranean) in February 2014 to assess the temporal and spatial variability of the distribution and size of suspended particulate matter (SPM) in the Rhône Region of Freshwater Influence (ROFI). A set of observations from an autonomous underwater glider, satellite ocean color data, and meteorological and hydrological time-series data highlighted the high variability of the Rhône River surface turbid plume and presence of a bottom nepheloid layer (BNL) that depended on wind and river discharge conditions. While continental winds pushed the surface plume offshore, marine winds pressed the plume at the coast and favored the sedimentation of as well as nourishment of the BNL. Moderate storm events favored breakage of the plume stratification and along-shelf transport of Rhône River particles. The spectral slopes of glider and satellite-derived light backscattering coefficients, γ, were used as a proxies of the SPM size distribution. The results clearly showed that the change of the SPM size in the nepheloid layers was induced by the flocculation of fine sediments, which became finer seaward throughout the ROFI, as well as the effect of rough weather in the breakup of flocs.
The Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion.
We review what is known about the convec-tive process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convec-tive and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are de-scribed, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by labo-ratory and numerical experiments of rotating convec-tion, great progress in our description and understand-ing of the processes at work is being made.
Mesoscale eddies are relatively small structures that dominate the ocean variability and have large impact on large scale circulation, heat fluxes and biological processes. In the western Mediterranean Sea, a high number of eddies has been observed and studied in the past with in-situ observations. Yet, a systematic characterization of these eddies is still lacking due to the small scales involved in these processes in this region where the Rossby deformation radius that characterizes the horizontal scales of the eddies is small (10-15 km). The objective of this thesis is to perform a characterization of mesoscale eddies in the western Mediterranean. For this purpose, we propose to develop tools to study the fine scales of the basin. First, we develop an eddy resolving simulation of the region for the last 20 years. The performance of the simulation is evaluated with independent observations (drifters, satellites, hydrographic profiles) showing realistic behavior. This simulation shows that existing altimetry maps underestimate the mesoscale signal. Therefore, we attempt to improve existing satellite altimetry products to better resolve mesoscale eddies. We show that this improvement is possible but at the cost of the homogeneity of the fields; the resolution can only be improved at times and locations where altimetric observations are densely distributed. In a second part, we apply three different eddy detection and tracking methods to extract eddy characteristics from the outputs of the high-resolution simulation, a coarser simulation and altimetry maps. The results allow the determination of some characteristics of the detected eddies. The size of eddies can greatly vary but is around 25-30 km. About 30 eddies are detected per day in the region with a very heterogeneous spatial distribution. Unlike other areas of the open ocean, they are mainly advected by currents of the region. Eddies can be separated according to their lifespan. Long-lived eddies are larger in amplitude and scale and have a seasonal cycle with a peak in late summer, while short-lived eddies are smaller and more present in winter. The penetration depth of detected eddies has also a large variance but the mean depth is around 300 meters. Anticyclones extend deeper in the water column and have a more conic shape than cyclones.
Many coral reef populations exist as discrete habitat patches linked through larval dispersal into a larger network. On these reefs, organisms spawn periodically and release propagules over a range of frequencies. Biophysical models of larval transport examine marine networks, yet particle release frequency needs careful consideration. We describe the time between sequential spawning events as the release interval and define any linkage of modeled larvae between two habitat sites as a connection. We investigate how changing the release interval affects the connectivity networks of three Caribbean species with low- to high-dispersal potential and swimming behavior. We find that spawning periodicity controls the number and persistence of network connections. Further, larval vertical movement behavior stabilizes the network, significantly increasing connections and connection persistence. This work demonstrates the impact of release interval on connectivity networks and underscores including larval behavior with realistic spawning periodicity in biophysical models of larval transport.
Connectivity within marine species plays a fundamental role in population dynamics, genetic diversity, spread of disease, and resilience to human exploitation. However, for shellfish species that are sessile as adults, the larval-dispersal stage remains largely unresolved. Appreciation of larval connectivity is therefore crucial to population genetics and marine management. We describe a coupled three-dimensional hydrodynamic and Lagrangian particle tracking model used to simulate larval transport and show how temporal and spatial hydrodynamic changes, together with larval behavior, are likely to affect dispersal. A case study of Irish Sea (United Kingdom) shellfish populations incorporates a wide range of hydrodynamic environments that are prevalent in other marine systems around the world. Our simulations tested two main processes that control larval dispersal: hydrodynamics and vertical migration. Simulated larval cohorts were released from estuaries and soft sediment locations in regions that were oceanographically distinct. Larvae originating from exposed areas could migrate offshore (low retention and high connectivity) and disperse farther than larvae that remained in flood-dominant estuaries, which promote retention. Simulated self-recruitment and connectivity with neighboring populations (similar to 50 km apart) were generally high, although well-developed mesoscale residual currents were important, controlling dispersal pathways offshore. Vertical migration strategies, synchronized either with the tide (tidal stream transport) or with the Earth's rotation (diel transport), enabled more larvae to remain close to the coast, and simulations indicated higher retention than for passive larvae. However, the probability of connectivity with other populations and potential survivorship was greater for tidal strategies than for passive (although passive transport populated more distinct areas albeit in smaller proportions, as more larvae remained stranded offshore), or diel, where larvae remained close to their release location.
Temperature, salinity and other measurements taken from six research
vessels are discussed with regard to the formation of deep water.
The ecological consequences of biodiversity loss have aroused considerable interest and controversy during the past decade.
Major advances have been made in describing the relationship between species diversity and ecosystem processes, in identifying
functionally important species, and in revealing underlying mechanisms. There is, however, uncertainty as to how results obtained
in recent experiments scale up to landscape and regional levels and generalize across ecosystem types and processes. Larger
numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments. A major
future challenge is to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.
The Leap Frog time stepping scheme (hereafter LF) partly loses its conservation properties when a Robert–Asselin filter (hereafter RA) is used to damp the computational mode. The LF + RA scheme actually leads to a well-known long term attenuation of the physical mode. Besides, the stability of the LF, e.g. the maximum permitted time step, is lowered by the use of the RA. Several methods, derived from the Laplacian approach of Marsaleix et al. (2008), are presented as an alternative to the RA. It appears that the physical mode is eventually much less impacted by higher order time filters. However, in some cases, the stability of the time stepping scheme becomes worse than that of the LF + RA. A five points scheme finally appears to preserve both the amplitude of the physical mode and the stability of the time stepping scheme. The analysis of these filters is based on a triple approach: the kinetic energy balance, the amplification factors of the oscillation equation, numerical experiments performed with a 3D circulation ocean model.