Investigations Exploring the Use of an Unstructured-Grid, Finite-Volume Modelling Approach to Simulate Coastal Circulation in Remote Island Settings—Case Study Region, Vanuatu/New Caledonia

Understanding coastal circulation and how it may alter in the future is important in island settings, especially in the South West Pacific, where communities rely heavily upon marine resources, and where sea level rise (SLR) is higher than the global average. In this study we explore the use of an unstructured-mesh finite-volume modelling approach to assist in filling the knowledge gaps with respect to coastal circulation in remote island locations—selecting the Vanuatu and New Caledonia archipelagos as our example study site. Past limited observations and modelling studies are leveraged to construct and verify a regional/coastal ocean model based on the Finite-Volume Community Ocean Model (FVCOM). Following verification with respect to tidal behaviour, we investigate how changes in wind speed and direction, and SLR, alter coastal water levels and coastal currents. Results showed tidal residual circulation was typically associated with flow separation at headlands and islands. Trade winds had negligible effect on water levels at the coast, however, wind-residual circulation was sensitive to both wind speed and direction. Wind-residual currents were typically strongest close to coastlines. Wind residual circulation patterns were strongly influenced by Ekman flow, while island blocking, topographic steering and geostrophic currents also appear to influence current patterns. Tidal amplitudes and phases were unchanged due to SLR of up to 2 m, while maximum current speeds altered by as much as 20 cm/s within some coastal embayments. Non-linear relationships between SLR and maximum current speeds were seen at some coastal reef platform sites. Under higher sea levels, tidal residual currents altered by less than ±2 cm/s which is relatively significant given maximum tidal residual current speeds are typically below 10 cm/s. Our findings indicate that under higher sea levels, coastal processes governing sediment transport, pollutant dispersal and larval transport are likely to alter, which may have implications for coastal environments and ecosystems. Given winds influence coastal circulation and subsequent coastal processes, changes in trade winds due to climate change may act to further alter coastal processes. It is felt that the current modelling approach can be applied to other regions to help fill critical knowledge gaps.

The Use of Unmanned Aerial Vehicles to Assess the Intensity of Manifestation of Dangerous Coastal Processes in the Water Protection Zone of the Tsimlyansk Reservoir

The water protection zone of the Tsimlyansk reservoir was chosen as the object of research. The subject of the study was the assessment of the intensity of manifestation of dangerous coastal processes of the reservoir: abrasion-latency, erosion activity. A method for monitoring erosion processes in water protection zones of water bodies using a software and hardware complex based on unmanned aerial vehicles and GIS technologies has been developed and tested. The optimal type of digital elevation models has been determined for assessing the density of the erosion net-work, determining the types of banks and the intensity of manifestation of abrasion and landslide processes, measuring the morphometric characteristics of erosional landforms. The types of erosional landforms were determined and the zoning of the territory of the water protection zone of the Tsimlyansk reservoir was carried out according to the density of the erosional dissection of the relief. Studies have shown that the predominant erosional forms of the relief of the water protection zone of the Tsimlyansk reservoir are ravines and gullies. The maximum average value of the density of the erosional network of the relief within the boundaries of the administrative districts of the reservoir coast falls on the Surovikinsky district. In the Kalachevsky district, the maximum value of the density of the erosion network is noted within the water protection zone of the Tsimlyansk reservoir.

Global Analysis of Coastal Gradients of Sea Surface Salinity

Sea surface salinity (SSS) is a key variable for ocean–atmosphere interactions and the water cycle. Due to its climatic importance, increasing efforts have been made for its global in situ observation, and dedicated satellite missions have been launched more recently to allow homogeneous coverage at higher resolution. Cross-shore SSS gradients can bear the signature of different coastal processes such as river plumes, upwelling or boundary currents, as we illustrate in a few regions. However, satellites performances are questionable in coastal regions. Here, we assess the skill of four gridded products derived from the Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites and the GLORYS global model reanalysis at capturing cross-shore SSS gradients in coastal bands up to 300 km wide. These products are compared with thermosalinography (TSG) measurements, which provide continuous data from the open ocean to the coast along ship tracks. The comparison shows various skills from one product to the other, decreasing as the coast gets closer. The bias in reproducing coastal SSS gradients is unrelated to how the SSS biases evolve with the distance to the coast. Despite limited skill, satellite products generally agree better with collocated TSG data than a global reanalysis and show a large range of coastal SSS gradients with different signs. Moreover, satellites reveal a global dominance of coastal freshening, primarily related to river runoff over shelves. This work shows a great potential of SSS remote sensing to monitor coastal processes, which would, however, require a jump in the resolution of future SSS satellite missions to be fully exploited.

Coastal processes modify projections of some climate-driven stressors in the California Current System

Global projections for ocean conditions in 2100 predict that the North Pacific will experience some of the largest changes. Coastal processes that drive variability in the region can alter these projected changes but are poorly resolved by global coarse-resolution models. We quantify the degree to which local processes modify biogeochemical changes in the eastern boundary California Current System (CCS) using multi-model regionally downscaled climate projections of multiple climate-associated stressors (temperature, O2, pH, saturation state (Ω), and CO2). The downscaled projections predict changes consistent with the directional change from the global projections for the same emissions scenario. However, the magnitude and spatial variability of projected changes are modified in the downscaled projections for carbon variables. Future changes in pCO2 and surface Ω are amplified, while changes in pH and upper 200 m Ω are dampened relative to the projected change in global models. Surface carbon variable changes are highly correlated to changes in dissolved inorganic carbon (DIC), pCO2 changes over the upper 200 m are correlated to total alkalinity (TA), and changes at the bottom are correlated to DIC and nutrient changes. The correlations in these latter two regions suggest that future changes in carbon variables are influenced by nutrient cycling, changes in benthic–pelagic coupling, and TA resolved by the downscaled projections. Within the CCS, differences in global and downscaled climate stressors are spatially variable, and the northern CCS experiences the most intense modification. These projected changes are consistent with the continued reduction in source water oxygen; increase in source water nutrients; and, combined with solubility-driven changes, altered future upwelled source waters in the CCS. The results presented here suggest that projections that resolve coastal processes are necessary for adequate representation of the magnitude of projected change in carbon stressors in the CCS.

Projected changes in wind wave directional spectra and their impact on coastal processes

<p>The assessment of the projected changes in wave climate due to climate change has been subject of study during the last two decades (Morim et al., 2018), largely due to the severe impacts these changes may have on coastal processes such as flooding and erosion. The wind wave climate is fully described by the sea surface elevation spectrum, which represents the distribution of energy resulting from the contributions of several superimposed waves with different periods and directions. Nevertheless, to this day the standard approach to address the future behavior of wind waves is based on the use of integrated wave parameters (e.g. significant wave height, mean wave period, mean wave direction) as a representation of the full spectrum. In this study, we analyze the changes in wave energy from directional spectra discretized in 24 directions and 32 frequencies in a number of locations distributed across all ocean basins, shedding light on the added value that an assessment based on the full spectrum offers with respect to the standard approach. In addition, the ESTELA method (Pérez et al., 2014) is applied to ease the understanding of the changes obtained in wave energy at the locations of study.</p><p>The spectral approach helps to assess the projected change in the energy of each wave system that reach a specific location. Results demonstrate that the use of integrated wave parameters can mask important information about the sign, magnitude and uncertainty of the actual projected changes in mean wave climate due to the offset of the expected variations in the different wave systems that integrate the spectrum. It is especially relevant at locations where an increase in the wave period or wave energy is hidden by the application of the standard approach, as these parameters are proven to play a key role in coastal processes. In addition, we reach relevant conclusions about the future behavior of swell systems. For instance, a robust increase in the energy carried by swells generated below 40°S can be observed in every ocean basin and both hemispheres, even beyond 30°N. Similarly, a decrease in the energy carried by northern swells can be observed close to the equator.</p>

High-resolution regional projections of sea level changes along Western European coasts during the 21st century

<p>Coastal regions are subject to an increasing anthropogenic pressure. Projections of coastal sea level changes are of great interest for coastal risk assessment and decision-making processes. Sea level projections are typically produced using global climate models. However, their coarse resolution limits the realism of the representation of coastal dynamical processes influencing sea level changes at the coast, potentially leading to substantial biases. Dynamical downscaling methods can be used to refine projections at regional scales by increasing the model spatial resolution and by explicitly including more processes. Such methods rely on the implementation of a high-resolution regional climate model (RCM). </p><p>In this work, we developed the IBI-CCS regional ocean model based on a 1/12° North Eastern Atlantic NEMO ocean model configuration. IBI-CCS includes coastal processes such as tides and atmospheric pressure forcing in addition to the ocean general circulation (dynamic sea level). This RCM is used to perform a dynamical downscaling of CNRM-CM6-1-HR, a global climate model (GCM) developed by the Centre National de Recherches Météorologiques (CNRM) with a 1/4° resolution over the ocean. CNRM-CM6-1-HR contributes to the Coupled Model Intercomparison Project 6th Phase (CMIP6). IBI-CCS is thus forced by the GCM ocean and atmospheric outputs at the lateral and air-sea boundaries. Several corrections were applied to the GCM forcings to avoid the propagation of climate drifts and biases into the regional simulations. The computations are performed over the 1950 to 2100 period for several CMIP6 climate change scenarios.</p><p>In order to validate the dynamical downscaling method, the regionally downscaled (IBI-CCS) and GCM (CNRM-CM6-1-HR) simulations are compared to reanalyses and observational datasets over the 1993-2014 period. These comparisons are performed at different time scales for a selection of ocean variables including sea level. The results show that large scale performances of IBI-CCS are better than those of the GCM thanks to the corrections applied. In addition, high frequency diagnostics are carried out and highlight for example that IBI-CCS sea level extreme events are similar to those of a reference regional ocean reanalysis. In a second phase, the RCM and GCM sea level rise projections are compared over the 21<sup>st</sup> century. These comparisons allow to investigate the impact of the model resolution and of a more complete representation of coastal processes for the simulation of projected sea level changes. </p>

Characterization of 3D coastal mesoscale structures and transports from multiplatform observations and a data reconstruction method

<p>The role of coastal mesoscale variability in the modulation of surface along-shelf and cross-shelf exchanges in the SE Bay of Biscay has been demonstrated by several works, from land-based and satellite observations, including high resolution current fields from high-frequency (HF) radars. However, the characterization of physical processes and associated transports at subsurface levels from observations remains a challenge since observations are often too scarce to offer the required spatio-temporal resolution and coverage. In addition to the numerical modelling, the use of methods to reconstruct three-dimensional (3D) current fields from the combination of multiplatform data offers an alternative approach for the study of 3D properties of mesoscale coastal processes, and an improved background to explore bio-physical interactions. Studying the physical properties of coastal mesoscale structures at subsurface levels, where primary production and plankton concentration peak, is key to understand the coupling between physical and biological processes. In this work, we use a previously validated data-reconstruction method and different CMEMS products (coastal simulations, observations from HF radar, satellite, mooring) and glider data, to better characterize the 3D structure of a coastal mode-water eddy and its associated water volume transport. Different Lagrangian properties (maps of particle density, residence times, Lagrangian eddy kinetic energy) obtained at surface and subsurface levels provide a new insight into the water volume transports associated with the main coastal processes in the area.</p>

Internal mouth-bar variability and differential preservation of coastal-process indicators in low-accommodation deltaic settings

<p>Mouth bars are fundamental architectural elements of deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial-, tide- and wave-dominated) is therefore paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low-accommodation systems because they are commonly characterized by a thin, condensed and top-truncated expression. In this study we analyze the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), which encompasses a fluvio-deltaic system along a ~450 km depositional dip-parallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ~25 km2 study area at the Tucumcari Basin margin. Analysis of facies distribution, depositional architecture and stratigraphic surfaces mapping reveals a 6–10-m-thick, sharp-based and sand-prone deltaic package, comprising several laterally-extensive (>1.4 km width) mouth bars. Within those, we distinguish four different along-strike sub-environments based on differences in grain size, sedimentary structures, bed thicknesses, and bioturbation indices; these are mouth bar axis, off-axis, lateral fringe to distal lateral fringe deposits, and overall reflect waning depositional energy with increasing distality from the distributary channel mouth. The interpreted mouth-bar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the lateral fringe and distal fringe environments. However, mouth-bar deposits amalgamate to form an extensive sand-rich sheet body throughout the study area, in which interflood mudstone to very-fine grained sandstone beds are nearly absent. This indicates a low accommodation/supply (A/S) setting, which promoted recurrent channel avulsion/bifurcation and thus reworking of mouth-bar fringe and distal-fringe sediments, where background coastal processes tend to be better recorded.</p><p>Trends in along-strike changes in sedimentary characteristics from axial to lateral environments are also recognized in other wave- and river-dominated deltaic settings as well, where axial components consist of higher energy facies associations resulting from high-density currents, whereas heterolithics become dominant towards the fringes, where there is an alternation of low- and high-density deposits combined with an increased recording of finer-grained facies associations. Complemented with our study, this suggests that internal hierarchy of mouth bars is evident and observed regardless of dominant coastal processes. Consequently, subdivision of mouth bars into different components can reduce complexity of models deriving from a myriad of facies subdivisions, and guide prediction of facies changes and sand distribution in future studies of proximal deltaic settings. Finally, results of this study evidence internal process-regime variability within mouth-bar components. This cautions against relying solely on the preserved deposits at one given location in a system to infer dominant and subordinate coastal processes (e.g. tidal indicators), with a consequent risk of underestimating the true mixed-influence nature of low-accommodation deltaic settings.</p>