MARS3D Model Research Articles

Trends and inter-annual variability of altimetry-based coastal sea level in the Mediterranean Sea: Comparison with tide gauges and models

In the framework of the Coastal sea level project of the ESA Climate Change Initiative, a new coastal sea level product from the Jason-1, 2 and 3 missions over 2002–2018 has been made recently available. This product consists of along-track, high-resolution (20 Hz, i.e., ~350 m) sea level anomalies and coastal sea level trends derived from a complete reprocessing of the Jason altimetry data, including a retracking of the radar waveforms using the ALES (Adaptive Leading Edge Sub-waveform) retracker, in several coastal regions worldwide. In this study, we extend the assessment of this coastal sea level product in the Mediterranean Sea region by comparing with tide gauge data where available. We selected a set of 14 coastal sites where the distance between the Jason track at the coast is less than 30 km from a tide gauge for which the in-situ record at least partly covers the 2002-present time span. In a first part of this study, we compared the interannual variability and trends of the coastal sea level anomalies and of the tide gauge data. A good agreement is found between the altimetry-based sea level time series and the tide gauge data at interannual time scales. In terms of trends, the comparison also shows general good agreement within the respective uncertainties. In a second part of this study, we focus on the Senetosa site (south Corsica) where a significant altimetry-based sea level trend increase is observed in the last 3–4 km to the coast, and investigate which physical process could explain this trend behavior. We analyzed temperature and salinity data of the high-resolution (400 m) MARS3D model, available around Corsica over the January 2014 - December 2019 time span, and computed the steric sea level component and its trend along the Jason track. A clear steric sea level trend increase is found in the last 3–4 km to the coast. Although the MARS3D model does not provide outputs prior to 2014, we suggest that if the steric sea level trend increase found over 2014–2019 is a long-life time feature, it has the right amplitude to explain the altimetry-based coastal sea level trend increase observed at Senetosa.

Mapping the Intertidal Microphytobenthos Gross Primary Production, Part II: Merging Remote Sensing and Physical-Biological Coupled Modeling

Microphytobenthos (MPB) at the sediment surface of intertidal mudflats are known to show a high spatial and temporal variability in response to the biotic and abiotic conditions prevailing at the mud surface. It makes long-term and large-scale monitoring of MPB Gross Primary Production (GPP) difficult to set up. In this study, we developed the first 3D physical-biological coupled model (MARS-3D) that explicitly simulates GPP of intertidal MPB at the mudflat scale, and we compared the outputs with in situ and space remote sensing MPB GPP data. We discuss the sources of discrepancies between the modeling and the remote sensing approach in the light of future developments to be done. For instance, the remote sensing algorithm provides a very synoptic view of the mudflat GPP. It is well-suited to achieve diagnostic estimates of MPB GPP at the synoptic spatial and temporal scale. By contrast, the MARS-3D model provides a more dynamic representation of the MPB activity and prognostic estimates of MPB GPP over the mudflat. It is very relevant to resolve the seasonal and inter-annual dynamics of MPB. Getting comparable GPP estimates derived from the remote sensing algorithm and 3D physical-biological coupled model will further require a better convergence in terms of equations structure, biological constants parameterization, and source data used (i.e., vegetation index vs. chlorophyll a). Setting a common parameterization in both the numerical model and remote sensing algorithm might be challenging in a perspective of mapping MPB PP over large mudflats from a synoptic to inter-annual time scale, but it could open the door to a new way of quantifying MPB GPP over large intertidal mudflats.

Swinging boat moorings: Spatial heterogeneous damage to eelgrass beds in a tidal ecosystem

Seagrass meadows are currently known to be subjected to huge physical disturbances including boat moorings in shallow bays. We aimed to identify the impact of permanent swing mooring on the fast-growing seagrass Zostera marina in a mega-tidal area. Coupling the hydrodynamic MARS3D model to simulate mooring chain movements and in situ measurements of plant traits, we analyzed the structural responses of the eelgrass bed to scraping disturbance in the western English Channel (France). A comparison of the results with a reference site without any permanent swing boat mooring showed a significant impact on eelgrass structure (shoot density, leaf size, leaf dry weight), depending on the direction and distance from the mooring. Zostera marina was absent close to the mooring fixation point in three out of the four directions we evaluated. Beyond 5 m, the canopy height remained lower than in the reference site, most likely due to regular disturbances by mooring chains. Conversely, shoot density beyond 5 m was higher than in the reference site. This adaptive response counter-balanced the decrease in canopy height at these distances. The fluctuations of the structure of the eelgrass cover (number of shoots, leaf length) at a small spatial scale was clearly in accordance with the scraping intensity simulated by the MARS3D model. The tidal currents coupled to tidal amplitude variability imply a small-scale heterogeneous effect of permanent mooring on the benthic compartment, previously undetected by an aerial survey. The present results highlight the interest of coupling approaches so as to understand how physical pressure influences fast-growing species traits. The resulting important modifications could imply a more functional impact such as biodiversity loss and carbon sequestration, which is beyond the scope of the present paper.

Occurrence of perfluoroalkyl substances in the Bay of Marseille (NW Mediterranean Sea) and the Rhône River.

Four perfluoroalkyl substances (PFAS) were analyzed in 62 duplicate surface water samples from the Rhône River and Marseille Bay (France; NW Mediterranean Sea). Perfluorooctane sulfonate (PFOS) was detected in all samples and exceeded the European Environmental Quality Standard (EQS) values in over 80% of the cases. The most contaminated samples were from the Rhône River (up to 200 ng L-1 ∑4 PFAS), as well as those collected near a wastewater treatment plant outlet in Marseille Bay (up to 9 ng L-1 ∑4 PFAS). While PFOS was the predominant PFAS in Marseille Bay, remarkably high concentrations of perfluorohexanoic acid (PFHxA) were measured in the Rhône River (8-193 ng L-1). The relative abundances of individual compounds differed thus significantly between the Rhône River and Marseille Bay, indicating different sources. A simulation made with the MARS3D model showed that PFOS inputs from the Rhône River can enter Marseille Bay at levels > EQS.

Numerical modelling of circulation and dispersion processes in Boulogne-sur-Mer harbour (Eastern English Channel): sensitivity to physical forcing and harbour design

The MARS-3D model in conjunction with the particle tracking module Ichthyop is used to study circulation and tracer dynamics under a variety of forcing conditions in the eastern English Channel, and in the Boulogne-sur-Mer harbour (referred to hereafter as BLH). Results of hydrodynamic modelling are validated against the tidal gauge data, VHF radar surface velocities and ADCP measurements. Lagrangian tracking experiments are performed with passive particles to study tracer dispersal along the northern French coast, with special emphasis on the BLH. Simulations revealed an anticyclonic eddy generated in the harbour at rising tide. Tracers, released during flood tide at the Liane river mouth, move northward with powerful clockwise rotating current. After the high water, the current direction changes to westward, and tracers leave the harbour through the open boundary. During ebb tide, currents convergence along the western open boundary but no eddy is formed, surface currents inside the harbour are much weaker and the tracer excursion length is small. After the current reversal at low water, particles are advected shoreward resulting in a significant increase of the residence time of tracers released during ebb tide. The effect of wind on particle dispersion was found to be particularly strong. Under strong SW wind, the residence time of particles released during flood tide increases from 1.5 to 6 days. For release during ebb tide, SW wind weakens the southward tidally induced drift and thus the residence time decreases. Similar effects are observed when the freshwater inflow to the harbour is increased from 2 to 10 m3/s during the ebb tide flow. For flood tide conditions, the effect of freshwater inflow is less significant. We also demonstrate an example of innovative coastal management targeted at the reduction of the residence time of the pathogenic material accidentally released in the harbour.

French

Un modèle numérique couplant la circulation tridimensionnelle, résolue par le code MARS3D, et l'état de la mer, calculé par le code WAVEWATCH III, est développé pour étudier l'hydrodynamique des zones côtières et littorales. Le principe général du modèle, faisant appel au coupleur PALM est décrit, ainsi que les aménagements apportés aux deux codes existants afin de rendre le couplage possible. Des premiers tests en deux dimensions horizontales ont permis de développer une première version. En trois dimensions, nous montrons que le modèle doit calculer la vitesse quasi-eulérienne, car il n'existe pas de forme analytique cohérente du forçage par les vagues pour la vitesse lagrangienne. Cet aspect est illustré par un résultat comportant de très fortes erreurs par rapport à la solution analytique, obtenu avec une formulation en vitesse lagrangienne. Abstract: A numerical model coupling the three-dimensional oceanic circulation, thanks to the MARS3D model, and the sea state computed by the WAVEWATCH III model, is developed in order to study the hydrodynamic of the nearshore zone. The main principle of the coupled model based on the coupler PALM and the necessary modifications made for the coupling procedure to both numerical codes are described. First tests in horizontal two-dimension are made and they allowed us to develop a primary version. For three-dimensional flow, we show that the coupled model must solve the quasi-Eulerian velocity. In fact, a coherent analytical form of the wave forcing for the Lagrangian velocity does not exist. This point is illustrated by one example with large errors in comparison with the analytical solution when solving for the Lagrangian mean flow. Keywords: Wave-current modeling; Nearshore zone; MARS3D; WAVEWATCH III.

On the coupling of wave and three-dimensional circulation models: Choice of theoretical framework, practical implementation and adiabatic tests

Many theoretical approaches and implementations have been proposed for the coupling of the three-dimensional ocean circulation with waves. The theoretical models are reviewed and it is shown that the formulation in terms of the quasi-Eulerian velocity circumvents the essential difficulty of alternative formulations for the Lagrangian mean velocity. Namely, models based on this Lagrangian velocity require an estimation of wave-induced motions to first order in the horizontal gradients of the wave field in order to estimate the vertical flux of wave pseudo-momentum. So far, only three-dimensional wave models have been able to provide these estimates, and all published theories based on the simpler Airy theory are not consistent at the leading order, because they ignore or incorrectly estimate the vertical momentum flux. With an adiabatic example on a sloping bottom it is shown that this inconsistency produces very large spurious velocities. These errors are independent of the slope for the inviscid case, and are still significant when a realistic vertical mixing is applied. A quick diagnostic of the potential accuracy of a theoretical model is the vertical profile of the wave-induced forcing terms: if it is not uniform over depth in adiabatic conditions then it will produce spurious artificial flow patterns in conditions with shoaling waves. Although conceptually more challenging, the quasi-Eulerian velocity theories only introduce minor modifications of the solution procedure for the standard primitive equations: a modification of the surface boundary condition for the mass conservation, the addition of the Stokes drift in the tracer advection equations, and sources of momentum and turbulent kinetic energy with associated surface and bottom fluxes. All the necessary modifications of primitive equation models are given in detail. This implementation is illustrated with the MARS3D model, which passes the test of the adiabatic shoaling waves.