In the context of increasingly frequent extreme summer events, it has become essential to develop new hypoxia alert tools in order to improve the management of shellfish farming areas and fish nursery habitats. Indicators need to be developed not only in areas where persistent hypoxia are observed for long periods but also in areas subjected to intermittent (a few hours) or episodic (a few days to a few weeks) hypoxia. Repeated short-term hypoxic events can be more detrimental to benthic communities than a single extended period of anoxia. Our study explores the emergence of episodic hypoxic events in a shallow coastal ecosystem directly influenced by nutrient-rich river plumes. This ecosystem is located in the Bay of Vilaine (French Atlantic coast), one of France's main shellfish-growing regions, which has experienced recurrent hypoxic events since the 1980s. Using high-frequency measurements of temperature, salinity, dissolved oxygen and chlorophyll-a concentrations, we examine the interactions between surface phytoplankton blooms, stratification and bottom oxygen depletion events. We also highlight the environmental conditions that promote the formation of stratification and thus hypoxia. Finally, we use a three-dimensional model simulating extreme event scenarios to assess the increased risk of hypoxia in different parts of the bay.

Abstract. The offshore wind energy sector in France is poised for significant growth, with ambitious targets to install 18 GW of offshore capacity by 2035 and 45 GW by 2050. This expansion is crucial for achieving France’s goal of generating 20 % of its electricity from offshore wind by 2050. The 2C NOW project, led by France Energies Marines, investigates the impact of climate change on metocean conditions along France’s three maritime fronts (two representative points for each seafront). This is critical in the context of sector development, which aims to support long-term energy transition goals and the sustainability of wind farms over several decades. Using global climate models from the CMIP6 project (IPCC), a statistical downscaling of the datasets is performed using the CDF-t method (Michelangeli et al., 2009) based on the best available numerical reanalyses (selected using extensive in-situ data). The mean conditions show a general decrease in wind speed and wave height (multi-model average) for all SSP scenarios and for all seafronts in continental France. This downward trend is more significant for the long-term future period (horizon 2100), on the Atlantic and Mediterranean coasts. These average trends are nevertheless accompanied by strong model uncertainties. Regarding extreme conditions, an increase in extreme values of significant wave height is observed for the future climate scenarios, while there is less consensus on the wind speed. Water levels show significant increasing trends, regardless of the seafront or the conditions concerned.

Coastal ecosystems are under climate and anthropogenic pressures. Extreme events as Marine Heatwaves (MHW) directly impact environmental conditions necessary to sustain biodiversity in coastal oceans. Recent results showed increasing occurrence and intensity of MHW in the Bay of Biscay and the English Channel based on surface temperature observations. Combining satellite and in situ observations with recent high resolution numerical simulations, the impact of MHW on the whole water column is investigated to evaluate potential impacts on pelagic and benthic ecosystems. Special attention is given to the last two years, 2022 and 2023, as unprecedent warm years. The last two decades have been observed (in situ and from satellites) and simulated (using CROCO coastal ocean model with 1km resolution) allowing to identify and characterize MHW (occurrence, duration, intensity). The propagation in the water column of observed heating is investigated with regard to the local dynamics (e.g. tides, constrained shallow waters, river plume dynamics). Depending on hydrodynamical conditions, impacts of MHW on the water column are contrasted and sensitive to characteristics from this type of extreme events. Those first results are designed to pave the way for an assessment of the MHW impact on the coastal ecosystem during the last two decades with a focus on 2022 and 2023.

The latest Intergovernmental Panel on Climate Change report of 2023 alerts about an increase in the occurrence and intensity of extreme hydro-meteorological events such as storms and extreme river flows, i.e. drought and floods. Investigating the occurrence of these extreme events in the past 15 years and their impacts on sediment dynamics will provide crucial knowledge for anticipating future trajectories of coastal ecosystems. Time series from in situ observations are analyzed to identify extreme events of river flows and waves and examine their impact on Suspended Particulate Matter (SPM) dynamics in a highly turbid coastal area equipped with a high frequency in situ monitoring station at the interface between the Seine Estuary and the Bay of Seine (northern coast of France). Extreme river flow and wave orbital velocity events are investigated because high river discharge contributes to deliver large amounts of SPM concentration to the bay and strong wave action within the bay can lead to erosion and resuspension of bottom sediments. An original detection method is proposed, based on high frequency in situ observations combined with satellite and model data from 2006 to 2022. Extreme forcings are examined through their specific characteristics (high intensity, long duration, season of occurrence, succession of events), their impact on SPM concentration in the coastal environment and the comparison to mean seasonal dynamics. A positive relationship exists between SPM concentration and high SPM spatial extent and forcing intensity. Extremes are more intense in winter for both forcings and generate larger SPM concentration anomalies. However, extreme events during late spring/summer, periods or mean low forcing intensity, are demonstrated to generate SPM concentration anomalies up to 4 times larger than the monthly mean value, hence possibly strongly impacting the system during these atypical periods. This is particularly important as analyzing the distribution of extreme river flow events over the last 60 years indicated an increase in their occurrence and more important the progressive occurrence of high intensity extreme events during spring/summer periods.

Suspended particulate matter (SPM) dynamics and exchange fluxes at the interface between a macrotidal estuary and its adjacent coastal sea were investigated from long-term high-frequency in situ observations. Optical and acoustic measurements were coupled to calculate SPM concentration over the whole water column using an existing acoustic inversion algorithm. A method was developed to distribute over the water column the surface and bottom calibrated equivalent particle diameters based on complementary ship-based surveys. Surface and bottom SPM show similar patterns in response to main forcings (tide, river discharge and waves), but present significantly higher concentrations near the bed. Increasing tidal ranges were responsible for higher tidal-median SPM concentrations, with spring/neap SPMC ratio varying from 2 to 3. This increase is driven by local resuspension during flood phase at the bottom, and low salinity turbid water flushed out from the estuary from mid-ebb to low tide at the surface. Increasing river discharge implies a downward shift of the estuarine turbidity maximum from the inner estuary to the mouth, and yields a 2-fold increase in both surface and bottom tidal-median concentration. Waves generated strong resuspension, with the highest SPM concentration recorded both at the surface and near the bed. Analysing SPM residual fluxes highlighted large up-estuary fluxes from low to moderate tidal ranges (below 6 m), and exporting seaward fluxes for higher tidal ranges, due to stronger mixing during spring tides. High river discharge enhanced stratification at the mouth and strengthened density circulation and up-estuary residual bottom circulation, resulting in larger up-estuary fluxes for all tidal ranges. Larger SPM concentrations along the tidal cycle during wave events yield high exporting fluxes.

The latest Intergovernmental Panel on Climate Change (IPCC) report describes an increase in the number and intensity of marine heatwaves (MHWs) and a decrease in marine cold spells (MCSs) in the global ocean. However, these reported changes are not uniform on a regional to local basis, and it remains unknown if coastal areas follow the open-ocean trends. Surface ocean temperature measurements collected by satellites (from 1982–2022) and 13 coastal buoys (from 1990–2022) are analyzed in the northeastern Atlantic and three subregions: the English Channel, Bay of Brest and Bay of Biscay. The activity metric, combining the number of events, intensity, duration and spatial extent, is used to evaluate the magnitude of these extreme events. The results from in situ and satellite datasets for each of the studied regions are quite in agreement, although the satellite dataset underestimates the amplitude of activity for both MHWs and MCSs. This supports the applicability of the method to both in situ and satellite data, albeit with caution on the amplitude of these events. Also, this localized study in European coastal northeastern Atlantic water highlights that similar changes are being seen in coastal and open oceans regarding extreme events of temperature, with MHWs being more frequent and longer and extending over larger areas, while the opposite is seen for MCSs. These trends can be explained by changes in both the mean of and variance in sea-surface temperature. In addition, the pace of evolution and dynamics of marine extreme events differ among the subregions. Among the three studied subregions, the English Channel is the region experiencing the strongest increase in summer MHW activity over the last 4 decades. Summer MHWs were very active in the English Channel in 2022 due to long events, in the Bay of Biscay in 2018 due to intense events and in the Bay of Brest in 2017 due to a high occurrence of events. Winter MCSs were the largest in 1987 and 1986 due to long and intense events in the English Channel. Finally, our findings suggest that at an interannual timescale, the positive North Atlantic Oscillation favors the generation of strong summer MHWs in the northeastern Atlantic, while low-pressure conditions over northern Europe and a high off the Iberian Peninsula in winter dominate for MCSs. A preliminary analysis of air–sea heat fluxes suggests that, in this region, reduced cloud coverage is a key parameter for the generation of summer MHWs, while strong winds and increased cloud coverage are important for the generation of winter MCSs.

WeatherVolume 78, Issue 11 p. 321-323 Spotlight The marine heatwave west of Ireland in June 2023 Gerard D. McCarthy, Corresponding Author Gerard D. McCarthy [email protected] orcid.org/0000-0002-2363-0561 ICARUS: Irish Climate Research Unit, Department of Geography, Maynooth University, Ireland Correspondence to: G. D. McCarthy [email protected] Contribution: Conceptualization, Writing - original draft, Writing - review & editingSearch for more papers by this authorSandra Plecha, Sandra Plecha Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorGuillaume Charria, Guillaume Charria Ifremer, Univ. Brest, CNRS, IRD, Laboratory for Ocean Physics and Satellite Remote Sensing (LOPS), IUEM, Brest, France Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorAmélie Simon, Amélie Simon Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorColine Poppeschi, Coline Poppeschi orcid.org/0000-0002-0386-0378 Ifremer, Univ. Brest, CNRS, IRD, Laboratory for Ocean Physics and Satellite Remote Sensing (LOPS), IUEM, Brest, France Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorAna Russo, Ana Russo Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this author Gerard D. McCarthy, Corresponding Author Gerard D. McCarthy [email protected] orcid.org/0000-0002-2363-0561 ICARUS: Irish Climate Research Unit, Department of Geography, Maynooth University, Ireland Correspondence to: G. D. McCarthy [email protected] Contribution: Conceptualization, Writing - original draft, Writing - review & editingSearch for more papers by this authorSandra Plecha, Sandra Plecha Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorGuillaume Charria, Guillaume Charria Ifremer, Univ. Brest, CNRS, IRD, Laboratory for Ocean Physics and Satellite Remote Sensing (LOPS), IUEM, Brest, France Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorAmélie Simon, Amélie Simon Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorColine Poppeschi, Coline Poppeschi orcid.org/0000-0002-0386-0378 Ifremer, Univ. Brest, CNRS, IRD, Laboratory for Ocean Physics and Satellite Remote Sensing (LOPS), IUEM, Brest, France Contribution: Formal analysis, Writing - review & editingSearch for more papers by this authorAna Russo, Ana Russo Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz (IDL), Lisboa, Portugal Contribution: Formal analysis, Writing - review & editingSearch for more papers by this author First published: 15 November 2023 https://doi.org/10.1002/wea.4498Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Caesar L, Rahmstorf S, Robinson A et al. 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556: 191. Frölicher TL, Laufkötter C. 2018. Emerging risks from marine heat waves. Nat. Commun. 9(1): 650. Hobday AJ, Alexander LV, Perkins SE et al. 2016. A hierarchical approach to defining marine heatwaves. Prog. Oceanogr. 141: 227–238. https://doi.org/10.1016/j.pocean.2015.12.014 Huang B, Liu C, Banzon V et al. 2020. Improvements of the daily optimum interpolation sea surface temperature (DOISST) version 2.1. J. Clim. 34: 2923–2939. https://doi.org/10.1175/JCLI-D-20-0166 IPCC. 2021. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. V Masson-Delmotte, P Zhai, A Pirani et al. (eds). Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA. McCarthy GD, Caesar L, Ulthaman A et al. 2023. Chapter 03: physical oceanography. In: Irish Ocean Climate & Ecosystem Status Report. G Nolan, C Cusack, D Fitzhenry (eds). Marine Institute: Galway, Ireland, pp 25–35. Met Éireann. 2023. Marine heat wave 2023 – a warning for the future. 3–7. https://www.met.ie/marine-heat-wave-2023-a-warning-for-the-future [accessed 6 October 2023]. NASA Earth Observatory. 2023. Exceptionally low Antarctic sea ice. https://earthobservatory.nasa.gov/images/151692/exceptionally-low-antarctic-sea-ice [accessed 6 October 2023]. NOAA. 2023. Record shattering: Earth had its hottest July in 174 years. https://www.noaa.gov/news/record-shattering-earth-had-its-hottest-july-in-174-years [accessed 6 October 2023]. Oliver ECJ, Donat MG, Burrows MT et al. 2018. Longer and more frequent marine heatwaves over the past century. Nat. Commun. 9: 1–12. https://doi.org/10.1038/s41467-018-03732-9 Simon A, Plecha SM, Russo A et al. 2022. Hot and cold marine extreme events in the Mediterranean over the period 1982–2021. Front. Mar. Sci. 9: 1–12. https://doi.org/10.3389/fmars.2022.892201 Vaughan L, Minto C, Reid D et al. 2023. Chapter 06: commercial fisheries. In: Irish Ocean Climate & Ecosystem Status Report. G Nolan, C Cusack, D Fitzhenry (eds). Marine Institute: Galway, Ireland, pp 75–91. WorldWeatherAttribution. 2023. Extreme heat in North America, Europe and China in July 2023 made much more likely by climate change. https://www.worldweatherattribution.org/extreme-heat-in-north-america-europe-and-china-in-july-2023-made-much-more-likely-by-climate-change/ [accessed 6 October 2023]. Volume78, Issue11November 2023Pages 321-323 ReferencesRelatedInformation

Decadal time series of chlorophyll a concentrations sampled at high and low frequencies are explored to study climate-induced impacts on the processes inducing interannual variations in the initiation of the phytoplankton growing period (IPGP) in early spring. We specifically detail the IPGP in two contrasting coastal temperate ecosystems under the influence of rivers highly rich in nutrients: the Bay of Brest and the Bay of Vilaine. In both coastal ecosystems, we observed a large interannual variation in the IPGP influenced by sea temperature, river inputs, light availability (modulated by solar radiation and water turbidity), and turbulent mixing generated by tidal currents, wind stress, and river runoff. We show that the IPGP is delayed by around 30 d in 2019 in comparison with 2010. In situ observations and a one-dimensional vertical model coupling hydrodynamics, biogeochemistry, and sediment dynamics show that the IPGP generally does not depend on one specific environmental factor but on the interaction between several environmental factors. In these two bays, we demonstrate that the IPGP is mainly caused by sea surface temperature and available light conditions, mostly controlled by the turbidity of the system before first blooms. While both bays are hydrodynamically contrasted, the processes that modulate the IPGP are similar. In both bays, the IPGP can be delayed by cold spells and flood events at the end of winter, provided that these extreme events last several days.

Extreme weather events affect coastal marine ecosystems. The increase in intensity and occurrence of such events drive modifications in coastal hydrology and hydrodynamics. Here, focusing on the winter period (from December to March), we investigated multi-decade (2000–2018) changes in the hydrological properties of the Bay of Brest (French Atlantic coast) as an example of the response of a semi-enclosed bay to extreme weather episodes and large-scale atmospheric circulation patterns. The relationships between extreme weather events and severe low salinity conditions (as a proxy for changes in water density) were investigated using high-frequency in situ observations and high-resolution numerical simulations. The identification of intense episodes was based on the timing, duration, and annual occurrence of extreme events. By examining the interannual variability of extreme low salinity events, we detect a patent influence of local and regional weather conditions on atmospheric and oceanic circulation patterns, precipitation, and river runoff. We revealed that low salinity events in Brittany were controlled by large-scale forcings: they prevailed during the positive phase of the North Atlantic Oscillation and periods of low occurrences of the Atlantic Ridge weather regime. The increase in severe storms observed in western France since 2010 has led to a doubling of the occurrence and duration of extreme low salinity events in Brittany.

The Bay of Biscay and the English Channel, in the North-eastern Atlantic, are considered as a natural laboratory to explore the coastal dynamics at different spatial and temporal scales. In those regions, the coastal circulation is constrained by a complex topography (e.g. varying width of the continental shelf, canyons), river runoffs, strong tides and a seasonally contrasted wind-driven circulation. Based on different numerical model experiments (from 400m to 4km spatial resolution, from 40 to 100 sigma vertical layers using 3D primitive equation ocean models), different features of the Bay of Biscay and English Channel circulation are assessed and explored. Both spatial (submesoscale and mesoscale) and temporal (from hourly to monthly) scales are considered. Modelled spatial scales, with a specific focus on the variability of fine scale features (e.g. fronts, filaments, eddies), are compared with remotely sensed observations (i.e. Sea Surface Temperature). Different methodologies as singularity and Lyapunov exponents allow describing fine scales features and are applied on both modelled and observed datasets. For temporal scales, in situ high frequency surface temperature measurements from coastal moorings (from COAST-HF observing network) provide a reference for the temporal variability to be modelled. Exploring differences in the temporal scales (from an Empirical Mode Decomposition) advises on the efficiency of our coastal modelling approach. This result overview in the Bay of Biscay and the English Channel aims illustrating the input of coastal modelling activities in understanding multi-scale interactions (spatial and temporal).

Extreme event occurrences and impacts in coastal waters of western Europe Coline Poppeschi1, Maximilian Unterberger1, Guillaume Charria1, Peggy Rimmelin-Maury2, Eric Goberville3, Nicolas Barrier5, Emilie Grossteffan2, Michel Repecaud6, Loïc Quemener6, Sébastien Theetten1, Sébastien Petton7, Jean-François Le Roux1, Paul Tréguer4 1 Ifremer, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, 29280 Brest, France. 2 OSU-Institut Universitaire Européen de la Mer (IUEM), UMS3113, F-29280, Plouzané, France. 3 Muséum National d’Histoire Naturelle, UMR 7208 BOREA, Sorbonne Université, CNRS, UCN, UA, IRD, Paris, France. 4 IUEM, UMR-CNRS 6539 Laboratoire de l’Environnement Marin (LEMAR), OSU IUEM, F-29280, Plouzané, France. 5 MARBEC, Université de Montpellier, Centre National de la Recherche Scientifique (CNRS), Ifremer, Institut de Recherche pour le Développement (IRD), F-34203 Sète, France. 6 Ifremer, Centre de Brest, REM/RDT/DCM, F-29280, Plouzané, France. 7 Ifremer, Centre de Brest, RBE/PFOM/LPI, F-29840, Argenton en Landunvez, France. Abstract The occurrence and the impact of the atmospheric extreme events in coastal waters of western Europe is evolving. Responses of the coastal environment to those events and evolutions need to be explored and explained. In this framework, the hydrodynamical and biogeochemical processes driven by extreme events in the bay of Brest are studied to better estimate their impacts on the local ecosystem. We are analyzing long-term in situ observations (since 2000), sampled at high and low frequencies, from the COAST-HF and SOMLIT network sites, located at the entrance to the bay of Brest. This study is divided into two main parts: the detection and characterization of extreme events, followed by the analysis of a realistic numerical simulation of these events to understand the underlying ocean processes. We focus on freshwater events during the winter months (December, January, February and March), considering the season with most of extreme event occurrence. The relationship between local extreme events and variability at larger scales, considering climate indices such as the North Atlantic Oscillation (NAO), is detailed. A comparison between the low frequency data from the SOMLIT network and the high frequency data from the COAST-HF network is carried out, highlighting the potential of high frequency measurements for the detection of extreme events. A comparison between in situ data and two numerical simulations of different resolutions is also performed over salinity time series. The interannual variability of extreme event occurrences and features in a context of climate change is also discussed. The link between these extreme low salinity events and the winter nitrate levels in the bay of Brest is shown. Then, we investigate the relationship between extreme events and biology in the coastal environment. Keywords In-situ observations, High and low frequency measurements, Extreme events, Numerical simulations, Bay of Brest, Weather regimes.

A dataset of 15 geo-referenced orthomosaics photos was generated to address long-term shoreline change along approximately 270 km of high-energy sandy coast in SW France between 1950 and 2014. The coast consists of sandy beaches backed by coastal dunes, which are only disrupted by two wide tidal inlets (Arcachon and Maumusson), a wide estuary mouth (Gironde) and a few small wave-dominated inlets and coastal towns. A time and spatially averaged erosion trend of 1.12 m/year is found over 1950-2014, with a local maximum of approximately 11 m/year and a maximum local accretion of approximately 6 m/year, respectively. Maximum shoreline evolutions are observed along coasts adjacent to the inlets and to the estuary mouth, with erosion and accretion alternating over time on the timescale of decades. The two inlet-sandspit systems of Arcachon and Maumusson show a quasi-synchronous behaviour with the two updrift coasts accreting until the 1970s and subsequently eroding since then, which suggests that shoreline change at these locations is controlled by allocyclic mechanisms. Despite sea level rise and the well-established increase in winter wave height over the last decades, there is no capture of significant increase in mean erosion rate. This is hypothesized to be partly the result of relevant coastal dune management works from the 1960s to the 1980s after a long period of coastal dune disrepair during and after the Second World War. This study suggests that long-term shoreline change of high-energy sandy coasts disrupted by inlets and/or estuaries is complex and needs to consider a wide range of parameters including, non-extensively, waves, tides, inlet dynamics, sea level rise, coastal dune management and coastal defences, which challenges the development of reliable long-term coastal evolution numerical models. (C) 2017 Elsevier Ltd. All rights reserved.