We introduce a continuous Galerkin finite element discretization of the non-hydrostatic Boussinesq approximation of the Navier-Stokes equations, suitable for various applications such as coastal ocean dynamics and ice-ocean interactions, among others. In particular, we introduce a consistent modification of the gravity force term which enhances conservation properties for Galerkin methods without strictly enforcing the divergence-free condition. We show that this modification results in a sharp energy estimate, including both kinetic and potential energy. Additionally, we propose a new, symmetric, tensor-based viscosity operator that is especially suitable for modeling turbulence in stratified flow. The viscosity coefficients are constructed using a residual-based shock-capturing method and the method conserves angular momentum and dissipates kinetic energy. We validate our proposed method through numerical tests and use it to model the ocean circulation and basal melting beneath the ice tongue of the Ryder Glacier and the adjacent Sherard Osborn Fjord in two dimensions on a fully unstructured mesh. Our results compare favorably with a standard numerical ocean model, showing better resolved turbulent flow features and reduced artificial diffusion.

Submarine glacier melt rates of the Greenland Ice Sheet remain a major uncertainty in climate model projections of future sea level rise. Development of submarine melt parameterizations have to a high degree relied on ocean circulation modelling of glacial fjords, designed to quantify effects such as ocean thermal forcing and fjordglacier geometry. Greenlandic fjords are relatively narrow, and it is frequently assumed that across-fjord flow variations are small enough to allow marine melt to be quantified with two-dimensional ocean–circulation models. Here, we present three–dimensional model simulations showing that the interplay between fjord–glacier geometry, side wall friction, and Earth’s rotation makes the circulation in ice–shelf cavities three–dimensional even in narrow fjords. Remarkably, we find that Earth’s rotation changes the flow pattern in the cavity below the ice shelf leading to a decrease in the marine melt on a 10 km wide ice shelf by a factor of five compared to a non–rotating simulation. Our study prompts using three–dimensional model configurations of Greenlandic fjords.

The expansion of transient and permanent coastal benthic anoxia is one of the most severe problems for the coastal ocean globally. We report frequent, hidden hypoxia in the bottom 5 cm of the water column of a coastal site in the central Baltic Sea by continuous high-resolution profiling of oxygen (O₂) directly above the sediment surface. This hypoxia stood in stark contrast to 30-yr O₂ monitoring records at this site that suggest apparent continuous well-oxygenated conditions. In situ measurements showed highly dynamic conditions in the bottom 30 cm recording frequent gradual and abrupt changes between normoxic (> 63 μmol L⁻¹) and hypoxic (< 63 μmol L⁻¹) conditions that would remain undetectable by conventional bottom water O₂ monitoring. The temporal variability of these “hidden” hypoxia is tied to the dynamic current field and to changes in O₂ consumption following resuspension events. Our observations suggest that transient benthic hypoxia is much more common than routine monitoring data indicate.

The oceanic forcing of basal melt under floating ice shelves in Greenland and Antarctica is one of the major sources of uncertainty in climate icesheet modelling. We use a high-resolution, nonhydrostatic configuration of the Massachusetts Institute of Technology general circulation model(MITgcm) to investigate basal melt rates and melt-driven circulation in the Sherard Osborn Fjord under the floating tongue of Ryder Glacier,northwestern Greenland. The control model configuration, based on the first-ever observational survey by Ryder 2019 Expedition, yieldedmelt rates consistent with independent satellite estimates. A protocol of model sensitivity experiments quantified the response to oceanic thermalforcing due to warming Atlantic Water and to the buoyancy input from the subglacial discharge of surface fresh water. We found that the averagebasal melt rates show a nonlinear response to oceanic forcing in the lower range of ocean temperatures, while the response becomes indistinguishablefrom linear for higher ocean temperatures, which unifies the results from previous modelling studies of other marine-terminating glaciers. The meltrate response to subglacial discharge is sublinear, consistent with other studies. The melt rates and circulation below the ice tongue exhibit aspatial pattern that is determined by the ambient density stratification.

Forecasting and early warning systems are important investments to protect lives, properties, and livelihood. While early warning systems are frequently used to predict the magnitude, location, and timing of potentially damaging events, these systems rarely provide impact estimates, such as the expected amount and distribution of physical damage, human consequences, disruption of services, or financial loss. Complementing early warning systems with impact forecasts has a twofold advantage: It would provide decision makers with richer information to take informed decisions about emergency measures and focus the attention of different disciplines on a common target. This would allow capitalizing on synergies between different disciplines and boosting the development of multihazard early warning systems. This review discusses the state of the art in impact forecasting for a wide range of natural hazards. We outline the added value of impact-based warnings compared to hazard forecasting for the emergency phase, indicate challenges and pitfalls, and synthesize the review results across hazard types most relevant for Europe.

The air-sea interface is a key gateway in the Earth system. It is where the atmosphere sets the ocean in motion, climate/weather-relevant air-sea processes occur, and pollutants (i.e., plastic, anthropogenic carbon dioxide, radioactive/chemical waste) enter the sea. Hence, accurate estimates and forecasts of physical and biogeochemical processes at this interface are critical for sustainable blue economy planning, growth, and disaster mitigation. Such estimates and forecasts rely on accurate and integrated in situ and satellite surface observations. High-impact uses of ocean surface observations of essential ocean/climate variables (EOVs/ECVs) include (1) assimilation into/validation of weather, ocean, and climate forecast models to improve their skill, impact, and value; (2) ocean physics studies (i.e., heat, momentum, freshwater, and biogeochemical air-sea fluxes) to further our understanding and parameterization of air-sea processes; and (3) calibration and validation of satellite ocean products (i.e., currents, temperature, salinity, sea level, ocean color, wind, and waves). We review strengths and limitations, impacts, and sustainability of in situ ocean surface observations of several ECVs and EOVs. We draw a 10-year vision of the global ocean surface observing network for improved synergy and integration with other observing systems (e.g., satellites), for modeling/forecast efforts, and for a better ocean observing governance. The context is both the applications listed above and the guidelines of frameworks such as the Global Ocean Observing System (GOOS) and Global Climate Observing System (GCOS) (both co-sponsoredby the Intergovernmental Oceanographic Commission of UNESCO, IOC-UNESCO; the World Meteorological Organization, WMO; the United Nations Environment Programme, UNEP; and the International Science Council, ISC). Networks of multiparametric platforms, such as the global drifter array, offer opportunities for new and improved in situ observations. Advances in sensor technology (e.g., low-cost wave sensors), high-throughput communications, evolving cyberinfrastructures, and data information systems with potential to improve the scope, efficiency, integration, and sustainability of the ocean surface observing system are explored.

The Lofoten Basin in the eastern Nordic Seas plays a central role in modifying the warm Atlantic Water inflow toward the Arctic Ocean. Here, the Atlantic Water experiences increased residence times, cooling, and substantial transformation. In this study, we investigate the Atlantic Water inflow pathways to the Lofoten Basin and their vertical and seasonal variations using 2-D and 3-D Lagrangian simulations forced by a high-resolution ocean model. Atlantic Water enters the basin from all directions, but we find two main inflow pathways at all vertical levels, one close to the Lofoten Escarpment in the southeast, associated with the Slope Current, and another close to the Helgeland Ridge in the southwest, associated with the Front Current. The surface inflow exhibits a stronger seasonal forcing than the inflow at depth as well as a stronger heat loss that is dominated by water masses entering the basin from the south. At deeper levels, the warm inflow from the east cools, while the relatively colder inflow from the west warms. The 2-D and 3-D synthetic trajectories show similar pathways. However, they are affected differently by the seasonal signal, giving different heat exchange patterns. Our results have implications for how results from Lagrangian observations in the region should be interpreted.

Bottom-water oxygen possibly constitutes the strongest regulating factor on ecosystem function of the seafloor environment. The conventional view of bottom-water oxygen regulation in temperate regions is a strong seasonal variability imposed by deposition of fresh, labile organic carbon following the spring and fall plankton bloom generating changing sediment oxygen demand. The supply of oxygen is considered to be mainly regulated by seasonally-varying thermal stratification or large-scale lateral advection of water masses. 

However, benthic oxygen variability also occurs over much shorter timescales, but the spatiotemporal regulation of this O2 variability is not well resolved. We conducted a 10-month-long in-situ study (2022–2023) of bottom-water O2 dynamics at a 40-m-deep coastal site in the Baltic Sea. Following the spring bloom and the onset of thermal stratification, the gradual decline in O2 was regularly interrupted by rapid (<24h) oxygenation events in which O2 levels spiked, subsided, but remained elevated relative to conditions before the event. Without these events, extrapolation of the observed O2 decline implied hypoxic or anoxic bottom-water conditions by mid-May.  Similarly, reoxygenation of the lower water column in the late fall was controlled by few rapid, and strong reoxygenation events. 

These events are mostly likely associated with regional-scale ocean circulation and mixing processes associated with downwelling events and/or coastal trapped waves. We suggest that rapid benthic oxygenation events are likely a common characteristic of coastal seafloor and critical for benthic ecosystem functioning. We describe these events using in-situ measurements in the benthic boundary layer in combination with the regional-scale hydrographic variability.