Ocean acidification is punctuated by episodic extremes of low pH and saturation state with regard to aragonite (ΩA). Here, we use a hindcast simulation from 1984 to 2019 with a high‐resolution regional ocean model (ROMS‐BEC) to identify and track ocean acidification extremes (OAX) in the northeast Pacific and the California current system (CCS). In the first step, we identify all grid‐cells whose pH and ΩA are simultaneously below their first percentile over the analysis period (1984–2019). In the second step, we aggregate all neighboring cells with extreme conditions into three‐dimensional time evolving events, permitting us to track them in a Lagrangian manner over their lifetime. We detect more than 22 thousand events that occur at least once in the upper 100 m during their lifetime, with broad distributions in terms of size, duration, volume, and intensity, and with 26% of them harboring corrosive conditions (ΩA < 1). By clustering the OAXs, we find three types of extremes in the CCS. Near the coast, intense, shallow, and short‐lasting OAXs dominate, caused by strong upwelling. A second type consists of large and long‐lasting OAX events that are associated with westward propagating cyclonic eddies. They account for only 3% of all extremes, but are the most severe events. The third type is small extremes at depth arising from pycnocline heave. OAXs potentially have deleterious effects on marine life. Marine calcifiers, such as pteropods, might be especially impacted by the long‐lasting events with corrosive conditions.

While climate change driven long term ocean deoxygenation induces profound changes in marine ecosystems (

Abstract. The acidification of the ocean (OA) increases the frequency and intensity of ocean acidity extreme events (OAXs), but this increase is not occurring homogeneously in time and space. Here we use daily output from a hindcast simulation with a high-resolution regional ocean model coupled to a biogeochemical-ecosystem model (ROMS-BEC) to investigate this heterogeneity in the progression of OAX in the upper 250 m of the Northeast Pacific from 1984 to 2019. We focus on the temporal and spatial changes in OAX using a relative threshold approach and using a fixed baseline reflecting the initial conditions. Concretely, conditions are considered extreme when the local hydrogen ion concentration ([H+]) exceeds the 99th percentile of the [H+] distribution of the baseline simulation where atmospheric CO2 was held at its 1979 level. Within the 36 years of our hindcast simulation, the increase in atmospheric CO2 causes a strong increase in OAX throughout the upper 250 m, but most accentuated near the surface. On average across the entire Northeast Pacific, for every additional 10 μatm of CO2 in the atmosphere, OAXs occupy an additional 6.3 % of the upper 250 m depth, last 7.6 days longer, and are 0.18 nmol L−1 (~ −0.006 pH units) more intense. This causes the OAXs to occupy at the end of the simulation a more than 10-times larger volume. The more than 11-fold increase in length, and the strong increase in the number of extreme days per year causes 88 % of the surface area in 2019 to experience near permanent extreme conditions. Finally, the model simulates a more than 6-fold intensification of the OAXs, causing also the intensity of the events with return periods of 10 years or more to increase by more than 80 %. Superimposed on these overall trends are very substantial spatial and temporal differences in these changes. The fraction of the volume identified as extreme across the top 250 m increases in the Central Northeast Pacific up to 160-times, while the deeper layers of the nearshore regions experience "only" a 4-fold increase. Throughout the upper 50 m of the Northeast Pacific, OAXs increase relatively linearly with time, but sudden rapid increases in yearly extreme days and OAX duration are simulated to occur in the thermocline of the Central Northeast Pacific. These differences largely emerge from the large spatial differences in the magnitude and nature of variability in [H+], with the transition between the rather variable thermocline waters of the Offshore Northeast Pacific and the very stable waters of the Central Northeast Pacific causing a very sharp transition in the occurrence of OAX. This transition is caused by the limited offshore reach of offshore propagating eddies that are the dominant driver of OAX in the Northeast Pacific. As the OAXs become more extreme, more of them also become undersaturated with respect to aragonite (ΩA < 1), i.e., become corrosive. In the final year of our hindcast, we find that below 100 m OAXs are characterized by corrosive conditions across a wide stretch of the region offshore of the U.S. and Canadian Coasts. The spatially and temporal heterogeneous increases in OAX, including the abrupt appearance of extremes, likely have negative effects on the ability of marine organisms to adapt to the progression of OA and its associated extremes.

&lt;p&gt;Superimposed on long-term ocean acidification are ocean acidity extremes (OAXs), i.e., periods of unusual acidity in the marine environment. Such extremes form through the interplay of the various spatio-temporal scales of variability associated with the ocean&amp;#8217;s carbonate system, ranging from multi-decadal trends to subseasonal dynamics. Using a high-resolution regional ocean model coupled to a biogeochemical-ecosystem model (ROMS-BEC) we assess the role of five mechanisms associated with different scales of variability &amp;#8211; namely atmospheric CO&lt;sub&gt;2&lt;/sub&gt; rise, the decadal North Pacific Gyre Oscillation (NPGO), the interannual El-Ni&amp;#241;o-Southern Oscillation (ENSO), seasonal upwelling, and mesoscale eddies &amp;#8211; in the occurrence of OAXs in a 300 km nearshore band along the U.S. West Coast and the Alaskan coast over the period 1984 to 2019. We find that the annual fraction of the upper 250 m depth that is hit by OAXs increases at a rate of 0.16 % units.&amp;#956;atm&lt;sup&gt;-1&lt;/sup&gt; driven by the increase in atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentration. In addition, our analysis reveals that Pacific climate variability substantially modulates OAXs occurrence on interannual timescales. The fraction of the upper 250 m depth that is hit by OAXs increases by 0.53 % units per unit decrease in the ENSO index, and 0.39 % units per unit increase in the NPGO index. Last but not least, we find that seasonal upwelling and mesoscale cyclonic eddies are key regional drivers of OAXs along the U.S. West Coast. Our results show that coastal upwelling forms intense and shallow OAXs near the coast, while mesoscale cyclonic eddies drive large and long-lasting OAXs that propagate over hundreds of kilometers from the coast to the offshore. Altogether, our results quantify the respective imprint of five mechanisms associated with different scales of variability on the occurrence of OAXs in coastal regions. This knowledge opens new perspectives for improving the predictability of OAXs in the highly productive coastal regions of the northeast Pacific. &amp;#160;&lt;/p&gt;

Les structures protectrices dissipant la houle qui arrive sur la côte sont diverses et communément constituées de milieux poreux comme les digues en enrochements. Afin d'évaluer l'efficacité de telles structures, il est important de quantifier le taux de dissipation à l'aide des coefficients de transmission et de réflexion. Dans ce projet, un canal à houle avec un milieu poreux est reproduit en utilisant le logiciel open source OpenFOAM. Les phénomènes de transfert entre énergie cinétique et énergie potentielle et de dissipation visqueuse sont modélisés par une méthode RANS (Reynolds-Averaged Navier-Stokes) en VOF (Volume of Fluid). La confrontation de mesures physiques obtenues antérieurement aux résultats du modèle numérique permet de valider ce dernier sans recours à un calage spécifique. Le modèle numérique complète les études expérimentales car il présente quelques avantages : temps de mise en oeuvre et coût moins élevés, pas de problème d'échelle réduite pouvant induire des soucis délicats de similitude pour ces phénomènes d'ondes gravitaires avec dissipation visqueuse non négligeable, précision satisfaisante sans étalonnage préalable, pas de contrainte d'appareil de mesures. L'étude de l'importance de l'influence de la surface spécifique sur la dissipation est poursuivie grâce à ce moyen numérique. Comme pour l'étude précédente en canal physique, un milieu poreux composé d'un réseau de cylindres est disposé dans un canal "numérique". On présente les résultats de l'étude portant sur l'impact des paramètres de forme et d'arrangement, que l'on rapproche de l'influence constatée de la surface spécifique.