Kai Håkon Christensen

• Collins, Clarence O.; Amador, Andre; Babanin, Alexander; Behrens, James; Benetazzo, Alvise & Bergamasco, Filippo [Vis alle 51 forfattere av denne artikkelen] (2026). Measuring Ocean Surface Waves. Reviews of Geophysics. ISSN 8755-1209. 64(3), s. 1–128. doi: 10.1029/2025rg000888. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Propagating waves on the ocean surface can be represented as a stochastic process whose statistics are characterized by a spectrum. This paper reviews methods for measuring the wave spectrum and related quantities. Observations begin by sensing fluid dynamical properties of the sea surface over space and/or time. Visual observations, collected routinely since the mid‐18th century, comprise the longest‐running wave record. Nearshore measurement methods continue to advance, including traditional pressure and acoustic sensing as well as newer technologies like distributed acoustic sensing and LiDAR. Detailed small‐scale wave physics can now be explored with measurement techniques using light, including stereo‐imaging and polarimetry. Reductions in the size, cost, and power consumption of microelectronics have propagated through ocean wave instrumentation, most notably in wave buoys. Global networks of freely drifting miniature wave buoys offer novel observational capabilities. Remote sensing techniques based on radar and LiDAR continue to evolve and are widely deployed from land, ships, aircraft, autonomous vehicles, and satellites. Spaceborne altimeters form one of the most important records of wave height, and new spaceborne sensors now observe directional spectra globally with sampling akin to traditional altimetry. Aircraft and autonomous systems provide strategic sampling capabilities for detailed process studies and access to extreme storm environments. The quality and quantity of ocean wave measurements have never been greater. This review aims to help make sense of it all.

• Elyouncha, Anis; Brostrøm, Gøran & Christensen, Kai Håkon (2026). A simple method to reconstruct coastal surface currents from single SAR Doppler observations using the continuity equation. Ocean Dynamics. ISSN 1616-7341. 76(7). doi: 10.1007/s10236-026-01826-3. Fulltekst i vitenarkiv
• Weber, Jan Erik H. & Christensen, Kai Håkon (2026). Mean particle drift in gravity-modified acoustic waves over a compressible seabed. Theoretical and Computational Fluid Dynamics. ISSN 0935-4964. 40(4). doi: 10.1007/s00162-026-00792-2. Fulltekst i vitenarkiv
• Christensen, Kai Håkon; Albretsen, Jon; Asplin, Lars; Frøysa, Håvard Guldbrandsen; Gusdal, Yvonne & Iversen, Silje Christine [Vis alle 15 forfattere av denne artikkelen] (2026). “Norkyst” version 3: the coastal ocean forecasting system for Norway. Geoscientific Model Development. ISSN 1991-959X. 19(7), s. 2785–2798. doi: 10.5194/gmd-19-2785-2026. Fulltekst i vitenarkiv Vis sammendrag

  We describe the operational forecasting system “Norkyst”, now in version 3, which is used for predicting the ocean circulation along the coast of mainland Norway and in the fjords. The forecasting system is based on the Regional Ocean Modeling System (ROMS), and has sub-kilometric horizontal resolution to resolve mesoscale features. Here we describe the basic configuration and report verification statistics of unconstrained model runs. The main features of the circulation and hydrography, including seasonal variations, are well represented in Norkyst v.3, making the forecast system suitable for its intended use as an open service for users in public or private sectors such as aquaculture, fishery, shipping, research, consulting, environmental management, and others who needs detailed predictions of the physical state of the Norwegian coastal ocean.

• Iversen, Silje Christine; Sperrevik, Ann Kristin & Christensen, Kai Håkon (2026). How does assimilating satellite SSTs affected by strong diurnal warming impact higher latitude ocean forecasts? Ocean Modelling. ISSN 1463-5003. 200, s. 1–14. doi: 10.1016/j.ocemod.2025.102676. Fulltekst i vitenarkiv Vis sammendrag

  Infrared satellite sea surface temperature (SST) observations capture the strong warming signal at the surface of the ocean during diurnal warming events. Assimilating these observations into ocean forecast models might introduce forecast errors, as the models do not fully resolve the strong diurnal cycle captured by SST observations. A standard approach is to remove affected SST observations from the assimilated dataset using a simplistic filter. However, this approach also removes observations not affected by diurnal warming. For high northern latitudes, where diurnal warming events are less frequent than at lower latitudes, it is unclear whether observations affected by strong warming negatively impact the forecasts, which would justify applying the filter. Here, we explore the consequences of both assimilating diurnal warming-affected SSTs and applying the filter using an ocean forecast model covering the seas off Norway. Through data assimilation experiments assimilating synthetic SSTs, we find that the strong warming signal in observations impacts forecasts in undesirable ways. The surface warming spreads below the mixed layer, decreasing its depth, and changes made to the model below the mixed layer persist into subsequent forecasts. Applying the filter reduces assimilated observations by 46 % without degrading forecasts, suggesting a redundancy in satellite SSTs.

• Østenstad, Petter & Christensen, Kai Håkon (2025). Effect of Assimilation on Oceanographic Profile Prediction in the Frontal Zone North of Svalbard. OCEANS. ISSN 0197-7385. doi: 10.1109/OCEANS58557.2025.11104785. Fulltekst i vitenarkiv
• Rabault, Jean; Halsne, Trygve; Carrasco, Ana; Korosov, Anton; Voermans, Joey & Bohlinger, Patrik [Vis alle 28 forfattere av denne artikkelen] (2025). Buoy measurements of strong waves in ice amplitude modulation: a signature of the impact of sea ice closedness on waves in ice attenuation. The Cryosphere. ISSN 1994-0416. 19(11), s. 6229–6260. doi: 10.5194/tc-19-6229-2025. Fulltekst i vitenarkiv Vis sammendrag

  The Marginal Ice Zone (MIZ) forms a critical transition region between the ocean and sea ice cover, as it protects the close ice further in from the effect of the steepest and most energetic open ocean waves. As waves propagate through the MIZ, they become exponentially attenuated. Unfortunately, the associated attenuation coefficient is difficult to accurately estimate and model, and there are still large uncertainties around which attenuation mechanisms dominate depending on the conditions. This makes it difficult to predict waves in ice attenuation, as well as sea ice breakup and dynamics. Here, we report in situ observations of strongly modulated waves in ice amplitude, with a modulation period of around 12 h. We show that simple explanations, such as changes in the incoming open water waves or the direct effect of tides and currents and bathymetry on the propagating waves, cannot explain the observed modulation. Therefore, the wave height modulation observed in the ice comes from a modulation of the waves in ice attenuation coefficient. We gather evidence that sea ice convergence and divergence is likely the factor driving this modulation in the attenuation coefficient, through its influence on the ice “closedness”. This implies that the level of sea ice “closedness” needs to be taken into account by future dissipation parameterizations.

• Vikebø, Frode Bendiksen; Nepstad, Raymond; Matuszak, Mateusz; Rikardsen, Edel S. U.; Laurel, Benjamin J. & Meier, Sonnich [Vis alle 12 forfattere av denne artikkelen] (2025). Polar cod early life stage exposure to potential oil spills in the Arctic. Aquatic Toxicology. ISSN 0166-445X. 281, s. 1–10. doi: 10.1016/j.aquatox.2025.107293. Fulltekst i vitenarkiv Vis sammendrag

  Arctic amplification of climate change is causing sea ice to retreat at unprecedented rates, potentially opening up large vulnerable Arctic areas for oil and gas exploration and new shipping routes. This rapid warming marginalizes sympagic species habitats making them more sensitive to other anthropogenic pressures. Here, we assess potential impacts of hypothetic oil spills from the northernmost licensed oil field Wisting and additional neighbouring spill sites in areas currently not open to oil exploitation on the key ice-associated Arctic fish species polar cod (Boreogadus saida). We do this by developing and running combined data-driven models for the ocean, oil spill dispersal and fate, and the early life stages of polar cod. Sea ice and the Polar Front act as natural barriers limiting the exchange of polar cod eggs and larvae and oil spill between Atlantic and Polar Water. However, both barriers vary seasonally so that the sea ice retreats and the Polar Front weakens towards summer causing significant increases in oil exposure to early life stages of polar cod under varying oil spill scenarios investigated here. Previous literature emphasizes that fall feeding conditions must be sufficient for juvenile polar cod to allocate lipids and survive their first winter. Here, we show that less than half the exposed individuals experience these suitable feeding conditions in the fall. The seasonal exposure intensity suggests a need for petroleum regulations with temporal and spatial limitations varying through the year. However, even with these seasonal dynamic regulations in place, climate change induced by the use of fossil fuel will likely reduce these natural barriers through continued sea ice retreat and a weakening of the Polar Front thereby reducing their barrier effects. Risk assessments of anthropogenic impacts on key Arctic ecosystem components in the vicinity of the ice edge zone and the Polar Front will therefore have to be updated to account for these major changes.

• Ølberg, Judith Thu; Bohlinger, Patrik; Breivik, Øyvind; Christensen, Kai Håkon; Furevik, Birgitte Rugaard & Hole, Lars Robert [Vis alle 11 forfattere av denne artikkelen] (2024). Wave measurements using open source ship mounted ultrasonic altimeter and motion correction system during the one ocean circumnavigation. Ocean Engineering. ISSN 0029-8018. 292. doi: 10.1016/j.oceaneng.2023.116586. Fulltekst i vitenarkiv Vis sammendrag

  This study reviews the design and signal processing of ship borne ultrasonic altimeter wave measurements. The system combines a downward facing ultrasonic altimeter to capture the sea surface elevation as a time series, and an inertial measurement unit to compensate for the ship’s motion. The methodology is cost-effective, open source, and adaptable to various ships and platforms. The system was installed on the barque Statsraad Lehmkuhl and recorded data continuously during the 20-month One Ocean Expedition. Results from 1-month crossing of the Tropical Atlantic are presented here. The one-dimensional wave spectrum and associated wave parameters are obtained from the sea surface elevation time series. The observed significant wave height agrees well with satellite altimetry and a spectral wave model. The agreement between observations and the spectral wave model is better for the mean wave period than the peak period. We perform Doppler shift corrections to improve wave period estimates by accounting for the speed of the ship relative to the waves. This correction enhances the accuracy of the mean period, but not the peak period. We suggest that the Doppler correction could be improved by complementing the data sources with directional wave measurements from a marine X-band radar.

• Beiser, Florian; Holm, Håvard Heitlo; Sætra, Martin Lilleeng; Kristensen, Nils Melsom & Christensen, Kai Håkon (2024). Combining barotropic and baroclinic simplified models for drift trajectory predictions. Journal of operational oceanography. Publisher: The Institute of Marine Engineering, Science & Technology. ISSN 1755-876X. 17(3), s. 187–206. doi: 10.1080/1755876X.2024.2364975. Fulltekst i vitenarkiv Vis sammendrag

  The shallow-water equations are often used as a classical simplified ocean model for barotropic ocean dynamics. The same equations can also be used to model simplified baroclinic dynamics through the reduced-gravity model. Herein, we propose to utilise a GPU-accelerated shallow-water simulation framework for representing two decoupled simplified ocean models for each of the barotropic and baroclinic dynamics, and use these models for ensemble prediction of short-term drift trajectories in coastal domains. This system can be used complementary to current operational systems as a lightweight tool for uncertainty quantification and in the future for ensemble-based data assimilation of in-situ observations. We show the relevance of our approach by demonstrating how the barotropic and baroclinic dynamical components are of varying importance at different geographical locations, and that one of the models can be used alone or in combination with the other. We benchmark our approach by showing the resulting ensemble trajectories in reference to deterministic trajectories produced by operational models for three Norwegian coastal regions.

• Espenes, Håvard; Carrasco, Ana; Dagestad, Knut-Frode; Christensen, Kai Håkon; Drivdal, Magnus Bjørn & Isachsen, Pål Erik (2024). Stokes drift in crossing windsea and swell, and its effect on near-shore particle transport in Lofoten, Northern Norway. Ocean Modelling. ISSN 1463-5003. 191. doi: 10.1016/j.ocemod.2024.102407. Fulltekst i vitenarkiv Vis sammendrag

  Transport assessments near the coast are often related to particles drifting near the surface. Such “particles” may be salmon lice, cod eggs, macro plastics or ship debris. Their drift depends on the Eulerian currents and the Stokes drift associated with the wind-generated surface wave field. The Stokes drift must be parameterized, and in doing so, one will inevitably make bulk estimates of the direction and speed of the drift for the wave spectrum. This paper implements a recently proposed parameterization of the Stokes drift vertical profile, which accounts for the effect of swell and windsea that propagate in different directions, to the open-source Lagrangian particle tracking model OpenDrift. We investigate how particles drifting in the Lofoten archipelago in Northern Norway depend on the Stokes drift and how we parameterize it. The parameterization accounting for crossing windsea and swell leads to lower residence time near the coastline than other popular parameterizations in the domain we studied.

• Halsne, Trygve; Benetazzo, Alvise; Barbariol, Francesco; Christensen, Kai Håkon; Carrasco, Ana & Breivik, Øyvind (2024). Wave Modulation in a Strong Tidal Current and Its Impact on Extreme Waves. Journal of Physical Oceanography. ISSN 0022-3670. 54(1), s. 131–151. doi: 10.1175/JPO-D-23-0051.1. Fulltekst i vitenarkiv Vis sammendrag

  Accurate estimates of extreme waves are central for maritime activities, and stochastic wave models are the best option available for practical applications. However, the way currents influence the statistics of space–time extremes in spectral wave models has not been properly assessed. Here we demonstrate impacts of the wave modulation caused by one of the world’s strongest open ocean tidal currents, which reaches speeds of at least 3 m s−1. For a bimodal swell and wind sea state, we find that most intense interactions occur when the wind sea opposes the tidal current, with an increase in significant wave height and spectral steepness up to 45% and 167%, respectively. The steepness modulation strengthens the second-order Stokes contribution for the normalized extreme crests, which increases between 5% and 14% during opposing wind sea and current. The normalized extreme wave heights have a strong dependence on the narrow-bandedness parameter, which is sensitive to the variance distribution in the bimodal spectrum, and we find an increase up to 12% with currents opposing the wind sea. In another case of swell opposing a tidal jet, we find the spectral steepness to exceed the increase predicted by a simplified modulation model. We find support in single-point observations that using tidal currents as forcing in wave models improves the representation of the expected maximum waves, but that action must be taken to close the gap of measurements in strong currents.

• Halsne, Trygve; Christensen, Kai Håkon; Hope, Gaute & Breivik, Øyvind (2023). Ocean wave tracing v.1: a numerical solver of the wave ray equations for ocean waves on variable currents at arbitrary depths. Geoscientific Model Development. ISSN 1991-959X. 16(22), s. 6515–6530. doi: 10.5194/gmd-16-6515-2023. Fulltekst i vitenarkiv Vis sammendrag

  Lateral changes in the group velocity of waves propagating in oceanic or coastal waters cause a deflection in their propagation path. Such refractive effects can be computed given knowledge of the ambient current field and/or the bathymetry. We present an open-source module for solving the wave ray equations by means of numerical integration in Python v3. The solver is implemented for waves on variable currents and arbitrary depths following the Wentzel–Kramers–Brillouin (WKB) approximation. The ray tracing module is implemented in a class structure, and the output is verified against analytical solutions and tested for numerical convergence. The solver is accompanied by a set of ancillary functions such as retrieval of ambient conditions using OPeNDAP, transformation of geographical coordinates, and structuring of data using community standards. A number of use examples are also provided.

• Weber, Jan Erik H.; Whitchelo, Yiyi; Pirolt, Jon Alexander; Christensen, Kai Håkon; Rabault, Jean & Jensen, Atle (2023). Observations of transient wave-induced mean drift profiles caused by virtual wave stresses in a two-layer system. Journal of Fluid Mechanics. ISSN 0022-1120. 975( A12). doi: 10.1017/jfm.2023.864. Fulltekst i vitenarkiv Vis sammendrag

  An experimental study of long interfacial gravity waves was conducted in a closed wave tank containing two layers of viscous immiscible fluids. The study focuses on the development in time of the mean particle drift that occurs close to the interface where the two fluids meet. From a theoretical analysis by Weber & Christensen ( Eur. J. Mech.- B/ Fluids , vol. 77, 2019, pp. 162–170) it is predicted that the growing drift in this region is associated with the action of the virtual wave stress. This effect has not been explored experimentally before. Interfacial waves of different amplitudes and frequencies were produced by a D-shaped paddle. Particle tracking velocimetry was used to track the time development of the Lagrangian mean drift. The finite geometry of the wave tank causes a mean return flow that is resolved by mass transport considerations. The measurements clearly demonstrate the importance of the virtual wave stress mechanism in generating wave drift currents near the interface.

• Christensen, Kai Håkon & Weber, Jan Erik H. (2023). Application of a Robin boundary condition to surface waves. Frontiers in Marine Science. 10. doi: 10.3389/fmars.2023.1129643. Fulltekst i vitenarkiv Vis sammendrag

  Surface effects on deep-water gravity waves are investigated theoretically by the application of a Robin boundary condition with a complex Robin parameter, R=Rr+iRi. The Robin condition combines the shear stress and the horizontal velocity at the ocean surface. We show that this condition describes wave damping related to surface phenomena like elastic films or thin viscous fluid layers. It may also model wave generation by oscillating surface stresses depending on the signs and magnitudes of Rr and Ri.

• Röhrs, Johannes; Halsne, Trygve; Sutherland, Graig; Dagestad, Knut-Frode; Hole, Lars Robert & Brostrøm, Gøran [Vis alle 7 forfattere av denne artikkelen] (2023). Current shear and turbulence during a near-inertial wave. Frontiers in Marine Science. 10. doi: 10.3389/fmars.2023.1115986. Fulltekst i vitenarkiv Vis sammendrag

  Surface currents and turbulent mixing were observed during a near-inertial wave (NIW) using an accousting doppler current profiler (ADCP) and satellite-tracked drifters. Drifter trajectories sampled at three depth levels show characteristics of an Ekman solution superposed with the NIW. Velocity and dissipation estimates from the ADCP reveal strong shear with a distinct constant flux layer in between the roughness length and a critical depth at 4m. Below, a shear free slab layer performing an inertial oscillation is observed. Dissipation, as estimated from the vertical beam of the ADCP, peaks in the wave-enhanced friction layer when the current opposes the wind and wave direction. Below the constant flux layer, maximum turbulence is observed when the NIW is in a phase that is in opposite direction to the time-averaged current. During this phase, currents at various depths rapidly realign in the entire boundary layer.

• Rabault, Jean; Müller, Malte; Voermans, Joey; Brazhnikov, Dmitry; Turnbull, Ian & Marchenko, Aleksey [Vis alle 28 forfattere av denne artikkelen] (2023). A dataset of direct observations of sea ice drift and waves in ice. Scientific Data. 10. doi: 10.1038/s41597-023-02160-9. Fulltekst i vitenarkiv Vis sammendrag

  Variability in sea ice conditions, combined with strong couplings to the atmosphere and the ocean, lead to a broad range of complex sea ice dynamics. More in-situ measurements are needed to better identify the phenomena and mechanisms that govern sea ice growth, drift, and breakup. To this end, we have gathered a dataset of in-situ observations of sea ice drift and waves in ice. A total of 15 deployments were performed over a period of 5 years in both the Arctic and Antarctic, involving 72 instruments. These provide both GPS drift tracks, and measurements of waves in ice. The data can, in turn, be used for tuning sea ice drift models, investigating waves damping by sea ice, and helping calibrate other sea ice measurement techniques, such as satellite based observations.

• Röhrs, Johannes; Gusdal, Yvonne; Rikardsen, Edel S. U.; Moro, Marina Durán; Brændshøi, Jostein & Kristensen, Nils Melsom [Vis alle 13 forfattere av denne artikkelen] (2023). Barents-2.5km v2.0: an operational data-assimilative coupled ocean and sea ice ensemble prediction model for the Barents Sea and Svalbard. Geoscientific Model Development. ISSN 1991-959X. doi: 10.5194/gmd-16-5401-2023. Fulltekst i vitenarkiv Vis sammendrag

  An operational ocean and sea ice forecast model, Barents-2.5, is implemented for short-term forecasting at the coast off northern Norway, the Barents Sea, and the waters around Svalbard. Primary forecast parameters are sea ice concentration (SIC), sea surface temperature (SST), and ocean currents. The model also provides input data for drift modeling of pollutants, icebergs, and search-and-rescue applications in the Arctic domain. Barents-2.5 has recently been upgraded to include an ensemble prediction system with 24 daily realizations of the model state. SIC, SST, and in situ hydrography are constrained through the ensemble Kalman filter (EnKF) data assimilation scheme executed in daily forecast cycles with a lead time up to 66 h. Here, we present the model setup and validation in terms of SIC, SST, in situ hydrography, and ocean and ice velocities. In addition to the model's forecast capabilities for SIC and SST, the performance of the ensemble in representing the model's uncertainty and the performance of the EnKF in constraining the model state are discussed.

• Halsne, Trygve; Bohlinger, Patrik; Christensen, Kai Håkon; Carrasco, Ana & Breivik, Øyvind (2022). Resolving regions known for intense wave–current interaction using spectral wave models: A case study in the energetic flow fields of Northern Norway. Ocean Modelling. ISSN 1463-5003. 176. doi: 10.1016/j.ocemod.2022.102071. Fulltekst i vitenarkiv Vis sammendrag

  Oceanic current forcing in spectral wave models have recently been demonstrated to have a large impact on wave heights at scales between one and up to several hundred kilometers. Here we investigate the impact of such forcing on open-ocean wave heights in Northern Norway using a high-resolution spectral wave model with currents from an ocean circulation model of similar resolution. We find that the wave model, to a large extent, resolves regions identified in the Norwegian Pilot Guide for maritime navigation as having dangerous sea states due to wave–current interaction. This is in contrast to a wave model forced with surface wind fields only. We present a novel diagnostic method to map the spatio-temporal scales associated with the wave height modulation between the two wave model predictions. The method is employed to map areas where significant wave–current interaction can be expected. In many cases, we are also able to confirm the physical mechanisms reported in the Pilot Guide, which are leading to an increase in wave energy due to currents. The largest wave height differences between the two models occur when waves and currents are opposing each other. In such situations, refraction and wave blocking are the dominating effects for the swell and wind sea parts of the spectrum, respectively. Furthermore, including current forcing significantly improves the agreement with in situ observations in strong tidal currents. Here, we see an increase in significant wave height of up to 50%. Even larger relative differences, exceeding 100%, are found in sheltered areas, with one specific region showing a reduction in model errors of 18% due to refraction and advection of wave action.

• Rabault, Jean; Nose, Takehiko; Hope, Gaute; Müller, Malte; Breivik, Øyvind & Voermans, Joey [Vis alle 20 forfattere av denne artikkelen] (2022). OpenMetBuoy-v2021: An Easy-to-Build, Affordable, Customizable, Open-Source Instrument for Oceanographic Measurements of Drift and Waves in Sea Ice and the Open Ocean. Geosciences. 12(3). doi: 10.3390/geosciences12030110. Fulltekst i vitenarkiv Vis sammendrag

  There is a wide consensus within the polar science, meteorology, and oceanography communities that more in situ observations of the ocean, atmosphere, and sea ice are required to further improve operational forecasting model skills. Traditionally, the volume of such measurements has been limited by the high cost of commercially available instruments. An increasingly attractive solution to this cost issue is to use instruments produced in-house from open-source hardware, firmware, and postprocessing building blocks. In the present work, we release the next iteration of the open-source drifter and wave-monitoring instrument of Rabault et al. (see “An open source, versatile, affordable waves in ice instrument for scientific measurements in the Polar Regions”, Cold Regions Science and Technology, 2020), which follows these solution aspects. The new design is significantly less expensive (typically by a factor of 5 compared with our previous, already cost-effective instrument), much easier to build and assemble for people without specific microelectronics and programming competence, more easily extendable and customizable, and two orders of magnitude more power-efficient (to the point where solar panels are no longer needed even for long-term deployments). Improving performance and reducing noise levels and costs compared with our previous generation of instruments is possible in large part thanks to progress from the electronics component industry. As a result, we believe that this will allow scientists in geosciences to increase by an order of magnitude the amount of in situ data they can collect under a constant instrumentation budget. In the following, we offer (1) a detailed overview of our hardware and software solution, (2) in situ validation and benchmarking of our instrument, (3) a fully open-source release of both hardware and software blueprints. We hope that this work, and the associated open-source release, will be a milestone that will allow our scientific fields to transition towards open-source, community-driven instrumentation. We believe that this could have a considerable impact on many fields by making in situ instrumentation at least an order of magnitude less expensive and more customizable than it has been for the last 50 years, marking the start of a new paradigm in oceanography and polar science, where instrumentation is an inexpensive commodity and in situ data are easier and less expensive to collect.

• Sætra, Øyvind; Halsne, Trygve; Carrasco, Ana; Breivik, Øyvind; Pedersen, Torstein & Christensen, Kai Håkon (2021). Intense Interactions between Ocean Waves and Currents Observed in the Lofoten Maelstrom. Journal of Physical Oceanography. ISSN 0022-3670. 51(11), s. 3461–3476. doi: 10.1175/JPO-D-20-0290.1. Fulltekst i vitenarkiv Vis sammendrag

  The Lofoten Maelstrom has been known for centuries as one of the strongest open-ocean tidal currents in the world, estimated to reach 3 m s−1, and by some estimates as much as 5 m s−1. The strong current gives rise to choppy seas when waves enter the Moskenes Sound, making the area extremely difficult to navigate. Despite its reputation, few studies of its strength exist, and no stationary in situ measurements for longer time periods have been made due to the challenging conditions. By deploying for the first time in situ wave and current instruments, we confirm some previous estimates of the strength of the current. We also show that its strength is strongly connected with wave breaking. From a consideration of specific forcing terms in the dynamical energy balance equation for waves on a variable current, we assess the impact of the underlying current using a convenient metric formulated as a function of the horizontal current gradients. We find that the horizontal gradients are a likely explanation for the observed enhanced wave breaking during strong currents at a rising tide.

• Camus, Lionel; Rodriguez, Hector Antonio Andrade; Aniceto, Ana Sofia; Aune, Magnus; Bandara, Kanchana & Basedow, Sünnje Linnéa [Vis alle 30 forfattere av denne artikkelen] (2021). Autonomous Surface and Underwater Vehicles as Effective Ecosystem Monitoring and Research Platforms in the Arctic - The Glider Project. Sensors. 21(20). doi: 10.3390/s21206752. Fulltekst i vitenarkiv Vis sammendrag

  Effective ocean management requires integrated and sustainable ocean observing systems enabling us to map and understand ecosystem properties and the effects of human activities. Autonomous subsurface and surface vehicles, here collectively referred to as “gliders”, are part of such ocean observing systems providing high spatiotemporal resolution. In this paper, we present some of the results achieved through the project “Unmanned ocean vehicles, a flexible and cost-efficient offshore monitoring and data management approach—GLIDER”. In this project, three autonomous surface and underwater vehicles were deployed along the Lofoten–Vesterålen (LoVe) shelf-slope-oceanic system, in Arctic Norway. The aim of this effort was to test whether gliders equipped with novel sensors could effectively perform ecosystem surveys by recording physical, biogeochemical, and biological data simultaneously. From March to September 2018, a period of high biological activity in the area, the gliders were able to record a set of environmental parameters, including temperature, salinity, and oxygen, map the spatiotemporal distribution of zooplankton, and record cetacean vocalizations and anthropogenic noise. A subset of these parameters was effectively employed in near-real-time data assimilative ocean circulation models, improving their local predictive skills. The results presented here demonstrate that autonomous gliders can be effective long-term, remote, noninvasive ecosystem monitoring and research platforms capable of operating in high-latitude marine ecosystems. Accordingly, these platforms can record high-quality baseline environmental data in areas where extractive activities are planned and provide much-needed information for operational and management purposes.

• Weber, Jan Erik H. & Christensen, Kai Håkon (2021). On the analogy between rigid plate motion and mean momentum transfer in surface gravity waves. AIP Advances. 11(4). doi: 10.1063/5.0043751. Fulltekst i vitenarkiv Vis sammendrag

  The transfer of mean momentum from surface gravity waves to Eulerian mean fluxes is investigated by modeling the total wave momentum (the Stokes flux) as a thin surface layer moving with the phase speed c of the waves and with a thickness Z derived from the divergence effect. The bulk of the fluid receiving momentum from the waves is modeled by a rigid lower layer acted upon by the virtual wave stress, where the latter is related to the form stress from the wind and the time rate of change in Z through −cdZ/dt. In addition, the effects of surface wind and bottom stress are included in the model. Through the combination with measurements, this robust model may yield information about the Eulerian fluxes when the ocean surface is covered by a combination of ice and oil, and the constitutive equations for the surface cover are not easily obtained.

• Weber, Jan Erik H. & Christensen, Kai Håkon (2021). On the singular behavior of the Stokes drift in layered miscible fluids. Wave motion. ISSN 0165-2125. 102, s. 1–8. doi: 10.1016/j.wavemoti.2021.102712. Fulltekst i vitenarkiv Vis sammendrag

  Gravity waves of the Stokes-type in a system of two horizontal layers of inviscid, stratified and miscible fluids are studied by applying a Lagrangian description of fluid motion. It is shown that for the total vertically-integrated Stokes drift (the Stokes flux) to become zero, the density must be continuous at the interface. This is not the case in a system of two immiscible homogeneous layers of different densities. For miscible fluids, however, one can extend the density of the lower layer continuously to that of the upper layer through an infinitely thin interface. This introduces a negative Stokes drift with a delta-function behavior. The corresponding finite negative Stokes flux obtained by integrating the Stokes drift across this infinitely thin layer adds to the original positive Stokes fluxes in the upper and lower layer, yielding the required vanishing total Stokes flux between the bounding planes of the model. The delta-function behavior of the Stokes drift at the interface also induces additional Lagrangian mean displacements in the vertical so that the position of the Lagrangian mean interfacial level is the same as in the absence of waves.

• Løken, Trygve Kvåle; Rabault, Jean; Thomas, Erin E.; Müller, Malte; Christensen, Kai Håkon & Sutherland, Graig [Vis alle 7 forfattere av denne artikkelen] (2020). A comparison of wave observations in the Arctic marginal ice zone with spectral models. Proceedings of the IAHR International Symposium on Ice. s. 499–508. Fulltekst i vitenarkiv
• Løken, Trygve Kvåle; Rabault, Jean; Jensen, Atle; Sutherland, Graig; Christensen, Kai Håkon & Müller, Malte (2020). Wave measurements from ship mounted sensors in the Arctic marginal ice zone. Cold Regions Science and Technology. ISSN 0165-232X. 182. doi: 10.1016/j.coldregions.2020.103207. Fulltekst i vitenarkiv Vis sammendrag

  This study presents wave measurements in the Marginal Ice Zone (MIZ) obtained from ship mounted sensors. The system combines altimeter readings from the ship bow with ship motion correction data to provide estimated single point ocean surface elevation. Significant wave height and mean wave period, as well as one-dimensional wave spectra are derived from the combined measurements. The results are compared with integrated parameters from two spectral wave models over a period of eight days in the open ocean, and with spectra and integrated parameters derived from motion detecting instruments placed on ice floes inside the MIZ. Mean absolute errors of the integrated parameters are in the range 13.4-29.9% when comparing with the spectral wave models and 1.0-9.6% when comparing with valid motion detecting instruments. The spatial wave damping coefficient is estimated by looking at the change in spectral wave amplitude found at discrete frequency values as the ship was moving along the longitudinal direction of the MIZ within time intervals where the wave field is found to be approximately constant in time. As expected from theory, high frequency waves are effectively dampened by the presence of sea ice. The observed wave attenuation rates compare favourably with a two-layer dissipation model. Our methodology can be regarded as a simple and reliable way to collect more waves-in-ice data as it can be easily added to any ship participating to ice expeditions, at little extra cost.

• Fritzner, Sindre Markus; Graversen, Rune & Christensen, Kai Håkon (2020). Assessment of High‐Resolution Dynamical and Machine Learning Models for Prediction of Sea Ice Concentration in a Regional Application. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 125(11). doi: 10.1029/2020JC016277. Fulltekst i vitenarkiv Vis sammendrag

  In this study, the potential for sea ice concentration prediction using machine‐learning methods is investigated. Three different sea ice prediction models are compared: one high‐resolution dynamical assimilative model and two statistical machine‐learning models. The properties of all three models are explored, and the quality of their forecasts is compared. The dynamical model is a state‐of‐the‐art coupled ocean and sea ice ensemble‐prediction system with assimilation. The observations assimilated are high‐resolution sea ice concentration from synthetic aperture radar (SAR) and sea surface temperature from infrared instruments. The machine‐learning prediction models are a fully convolutional network and a <i>k</i>‐nearest neighbors method. These methods use several variables as input for the prediction: sea ice concentration, sea surface temperature, and 2‐m air temperature. Earlier studies have applied machine‐learning approaches primarily for seasonal ice forecast. Here we focus on short‐term predictions with a length of 1–4 weeks, which are of high interest for marine operations. The goal is to predict the future state of the sea ice using the same categories as traditional ice charts. The machine‐learning forecasts were compared to persistence, which is the assumption that the sea ice does not change over the forecasting period. The machine‐learning forecasts were found to improve upon persistence in periods of substantial change. In addition, compared to the dynamical model, the <i>k</i>‐nearest neighbor algorithm was found to improve upon the 7‐day forecast during a period of small sea ice variations. The fully convolutional network provided similar quality as the dynamical forecast. The study shows that there is a potential for sea ice predictions using machine‐learning methods.

• Sutherland, Graig; Soontiens, Nancy; Davidson, Fraser; Smith, Gregory C.; Bernier, Natacha & Blanken, Hauke [Vis alle 12 forfattere av denne artikkelen] (2020). Evaluating the leeway coefficient of ocean drifters using operational marine environmental prediction systems. Journal of Atmospheric and Oceanic Technology. ISSN 0739-0572. 37(11), s. 1943–1954. doi: 10.1175/JTECH-D-20-0013.1. Fulltekst i vitenarkiv Vis sammendrag

  The water following characteristics of six different drifter types are investigated using two different operational marine environmental prediction systems: one produced by Environment and Climate Change Canada (ECCC) and the other produced by MET Norway (METNO). These marine prediction systems include ocean circulation models, atmospheric models, and surface wave models. Two leeway models are tested for use in drift object prediction: an implicit leeway model where the Stokes drift is implicit in the leeway coefficient, and an explicit leeway model where the Stokes drift is provided by the wave model. Both leeway coefficients are allowed to vary in direction and time in order to perfectly reproduce the observed drifter trajectory. This creates a time series of the leeway coefficients that exactly reproduce the observed drifter trajectories. Mean values for the leeway coefficients are consistent with previous studies that utilized direct observations of the leeway. For all drifters and models, the largest source of variance in the leeway coefficient occurs at the inertial frequency and the evidence suggests it is related to uncertainties in the ocean inertial currents.

• Holm, Håvard Heitlo; Brodtkorb, André R.; Brostrøm, Gøran; Christensen, Kai Håkon & Sætra, Martin Lilleeng (2020). Evaluation of selected finite-difference and finite-volume approaches to rotational shallow-water flow. Communications in Computational Physics. ISSN 1815-2406. 27, s. 1234–1274. doi: 10.4208/cicp.OA-2019-0033. Fulltekst i vitenarkiv Vis sammendrag

  The shallow-water equations in a rotating frame of reference are important for capturing geophysical flows in the ocean. In this paper, we examine and compare two traditional finite-difference schemes and two modern finite-volume schemes for simulating these equations. We evaluate how well they capture the relevant physics for problems such as storm surge and drift trajectory modelling, and the schemes are put through a set of six test cases. The results are presented in a systematic manner through several tables, and we compare the qualitative and quantitative performance from a cost-benefit perspective. Of the four schemes, one of the traditional finitedifference schemes performs best in cases dominated by geostrophic balance, and one of the modern finite-volume schemes is superior for capturing gravity-driven motion. The traditional finite-difference schemes are significantly faster computationally than the modern finite-volume schemes.

• Strand, Kjersti Opstad; Breivik, Øyvind; Pedersen, Geir; Vikebø, Frode Bendiksen; Sundby, Svein & Christensen, Kai Håkon (2020). Long-Term Statistics of Observed Bubble Depth Versus Modeled Wave Dissipation. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 125(2). doi: 10.1029/2019JC015906. Fulltekst i vitenarkiv Vis sammendrag

  Down‐mixed air bubbles measured with a bottom‐based echosounder provide an opportunity to observe upper ocean processes continuously through all types of weather. Here we investigate the correlation between air bubble depth and modeled energy flux from breaking waves. We investigate 1 year of data available from an offshore location in Northern Norway. Results show that the hourly mean and maximum bubble depths are highly variable, reaching 18 and 38 m, respectively, and strongly correlated with wind and sea state. The bubble depth is shallowest during summer, following the seasonal variations in wind and wave height. We find that the bubble depth is not limited by the depth of the mixed layer. A relationship between bubble depth and energy flux is found, similar to the relationship between wind and energy flux. This is important for prediction of both dispersal of particles (such as oil spills and marine plankton) and transfer of gases from the atmosphere to the ocean.

• Breivik, Øyvind & Christensen, Kai Håkon (2020). A combined stokes drift profile under swell and wind sea. Journal of Physical Oceanography. ISSN 0022-3670. 50(10), s. 2819–2833. doi: 10.1175/JPO-D-20-0087.1. Fulltekst i vitenarkiv Vis sammendrag

  A combined directional Stokes drift profile for swell and wind sea is presented. The profile can be used to calculate the shear under crossing seas and as such is relevant for Langmuir turbulence and Stokes–Coriolis forcing, but also for material advection. The swell is represented as either a monochromatic wave or as a Phillips spectrum, while the wind sea is represented as a Phillips spectrum. The profile is found to compare well against the full directional Stokes drift calculated from the 2D spectrum of ERA-Interim in an open-ocean location in the North Atlantic. The error compared to a Phillips-type unidirectional Stokes drift profile is markedly lower for a combined profile with a monochromatic swell Stokes profile. However, representing the swell as a Phillips-type Stokes drift profile yields even better results. The combined profile relies on integrated wave parameters readily available from wave models and can be calculated at low cost. The global Stokes drift climate is investigated using ERA-Interim reanalysis data with the intention of identifying regions dominated by crossing Stokes drift. We find that the eastern equatorial Pacific Ocean probably experiences the greatest degree of crossing Stokes drift, and the entire subtropical band 20°–30°S/N exhibits a significant degree of crossing Stokes drift and swell dominance over the Stokes drift.

• Rabault, Jean; Sutherland, Graig; Jensen, Atle; Christensen, Kai Håkon & Marchenko, Aleksey (2019). Experiments on wave propagation in grease ice: combined wave gauges and particle image velocimetry measurements. Journal of Fluid Mechanics. ISSN 0022-1120. 864, s. 876–898. doi: 10.1017/jfm.2019.16. Fulltekst i vitenarkiv Vis sammendrag

  Water wave attenuation by grease ice is a key mechanism for the polar regions, as waves in ice influence many phenomena such as ice drift, ice breaking and ice formation. However, the models presented so far in the literature are limited in a number of regards, and more insights are required from either laboratory experiments or fieldwork for these models to be validated and improved. Unfortunately, performing detailed measurements of wave propagation in grease ice, either in the field or in the laboratory, is challenging. As a consequence, laboratory data are relatively scarce, and often consist of only a couple of wave elevation measurements along the length of the wave tank. We present combined measurements of wave elevation using an array of ultrasonic probes, and water kinematics using particle image velocimetry (PIV), in a small-scale wave tank experiment. Experiments are performed over a wider frequency range than has been previously investigated. The wave elevation measurements are used to compute the wavenumber and exponential damping coefficient. In contrast to a previous study in grease ice, we find that the wavenumber is consistent with the mass loading model, i.e. it increases compared with the open water case. Wave attenuation is compared with a series of one-layer models, and we show that they satisfactorily describe the viscous damping occurring. PIV data are also consistent with exponential wave amplitude attenuation, and a proper orthogonal decomposition analysis reveals the existence of mean flows under the ice that are a consequence of the displacement and packing of the ice induced by the gradient in the wave-induced stress. Finally, we show that the dynamics of grease ice can generate eddy structures that inject eddy viscosity into the water under the grease ice, which would lead to enhanced mixing and participating in energy dissipation.

• Ali, Alfatih; Christensen, Kai Håkon; Breivik, Øyvind; Bertino, Laurent; Malila, Mika Petteri & Raj, Roshin Pappukutty [Vis alle 8 forfattere av denne artikkelen] (2019). A comparison of Langmuir turbulence parameterizations and key wave effects in a numerical model of the North Atlantic and Arctic Oceans. Ocean Modelling. ISSN 1463-5003. 137, s. 76–97. doi: 10.1016/j.ocemod.2019.02.005. Fulltekst i vitenarkiv Vis sammendrag

  Five different parameterizations of Langmuir turbulence (LT) effect are investigated in a realistic model of the North Atlantic and Arctic using realistic wave forcing from a global wave hindcast. The parameterizations mainly apply an enhancement to the turbulence velocity scale, and/or to the entrainment buoyancy flux in the surface boundary layer. An additional run is also performed with other wave effects to assess the relative importance of Langmuir turbulence, namely the Coriolis-Stokes forcing, Stokes tracer advection and wave-modified momentum fluxes. The default model (without wave effects) underestimates the mixed layer depth in summer and overestimates it at high latitudes in the winter. The results show that adding LT mixing reduces shallow mixed layer depth (MLD) biases, particularly in the subtropics all year-around, and in the Nordic Seas in summer. There is overall a stronger relative impact on the MLD during winter than during summer. In particular, the parameterization with the most vigorous LT effect causes winter MLD increases by more than 50% relative to a control run without Langmuir mixing. On the contrary, the parameterization which assumes LT effects on the entrainment buoyancy flux and accounts for the Stokes penetration depth is able to enhance the mixing in summer more than in winter. This parameterization is also distinct from the others because it restrains the LT mixing in regions of deep MLD biases, so it is the preferred choice for our purpose. The different parameterizations do not change the amplitude or phase of the seasonal cycle of heat content but do influence its long-term trend, which means that the LT can influence the drift of ocean models. The combined impact on water mass properties from the Coriolis-Stokes force, the Stokes drift tracer advection, and the wave-dependent momentum fluxes is negligible compared to the effect from the parameterized Langmuir turbulence.

• Sutherland, Graig; Rabault, Jean; Christensen, Kai Håkon & Jensen, Atle (2019). A two layer model for wave dissipation in sea ice. Applied Ocean Research. ISSN 0141-1187. 88, s. 111–118. doi: 10.1016/j.apor.2019.03.023. Fulltekst i vitenarkiv Vis sammendrag

  Sea ice is highly complex due to the inhomogeneity of the physical properties (e.g. temperature and salinity) as well as the permeability and mixture of water and a matrix of sea ice and/or sea ice crystals. Such complexity has proven itself to be difficult to parameterize in operational wave models. Instead, we assume that there exists a self-similarity scaling law which captures the first order properties. Using dimensional analysis, an equation for the kinematic viscosity is derived, which is proportional to the wave frequency and the ice thickness squared. In addition, the model allows for a two-layer structure where the oscillating pressure gradient due to wave propagation only exists in a fraction of the total ice thickness. These two assumptions lead to a spatial dissipation rate that is a function of ice thickness and wavenumber. The derived dissipation rate compares favourably with available field and laboratory observations.

• Hoguane, Antonio Mubango; Gammelsrød, Tor; Christensen, Kai Håkon; Furaca, Noca Bernardo; Nharreluga, Bilardo António Da Silva & Poio, Manuel Victor (2019). Nearshore currents and safety to swimmers in Xai-Xai Beach. Journal of Integrated Coastal Zone Management. doi: 10.5894/RGCI-N148. Fulltekst i vitenarkiv
• Fritzner, Sindre Markus; Graversen, Rune; Christensen, Kai Håkon; Rostosky, Philip & Wang, Keguang (2019). Impact of assimilating sea ice concentration, sea ice thickness and snow depth in a coupled ocean-sea ice modelling system. The Cryosphere. ISSN 1994-0416. 13(2), s. 491–509. doi: 10.5194/tc-13-491-2019. Fulltekst i vitenarkiv Vis sammendrag

  The accuracy of the initial state is very important for the quality of a forecast, and data assimilation is crucial for obtaining the best-possible initial state. For many years, sea-ice concentration was the only parameter used for assimilation into numerical sea-ice models. Sea-ice concentration can easily be observed by satellites, and satellite observations provide a full Arctic coverage. During the last decade, an increasing number of sea-ice related variables have become available, which include sea-ice thickness and snow depth, which are both important parameters in the numerical sea-ice models. In the present study, a coupled ocean–sea-ice model is used to assess the assimilation impact of sea-ice thickness and snow depth on the model. The model system with the assimilation of these parameters is verified by comparison with a system assimilating only ice concentration and a system having no assimilation. The observations assimilated are sea ice concentration from the Ocean and Sea Ice Satellite Application Facility, thin sea ice from the European Space Agency's (ESA) Soil Moisture and Ocean Salinity mission, thick sea ice from ESA's CryoSat-2 satellite, and a new snow-depth product derived from the National Space Agency's Advanced Microwave Scanning Radiometer (AMSR-E/AMSR-2) satellites. The model results are verified by comparing assimilated observations and independent observations of ice concentration from AMSR-E/AMSR-2, and ice thickness and snow depth from the IceBridge campaign. It is found that the assimilation of ice thickness strongly improves ice concentration, ice thickness and snow depth, while the snow observations have a smaller but still positive short-term effect on snow depth and sea-ice concentration. In our study, the seasonal forecast showed that assimilating snow depth led to a less accurate long-term estimation of sea-ice extent compared to the other assimilation systems. The other three gave similar results. The improvements due to assimilation were found to last for at least 3–4 months, but possibly even longer.

• Weber, Jan Erik H. & Christensen, Kai Håkon (2019). Virtual wave stress and transient mean drift in spatially damped long interfacial waves. European journal of mechanics. B, Fluids. ISSN 0997-7546. 77, s. 162–170. doi: 10.1016/j.euromechflu.2019.04.004. Fulltekst i vitenarkiv Vis sammendrag

  The mean drift in spatially damped long gravity waves at the boundary between two layers of immiscible viscous fluids is investigated theoretically by applying a Lagrangian description of motion. The focus of the paper is on the development of the drift near the interface. The initial drift (inviscid Stokes drift + viscous boundary-layer terms) associated with the instantaneously imposed wave field does not generally fulfill the conditions at the common boundary between the layers. Hence, transient Eulerian mean currents develop on both sides of the interface to ensure continuity of velocities and viscous stresses. The development of strong jet-like Eulerian currents increasing with time in this problem is related to the action of the virtual wave stress (VWS). Very soon (after a few wave periods) the transient Eulerian part dominates in the Lagrangian mean current. This effect is similar to that found for the drift in short gravity waves with a film-covered surface. A new relation is derived showing that the difference between the VWS’s at the interface is given by the divergence of the total horizontal wave momentum flux in a two-layer system. Our analysis with spatially damped waves also yields the Lagrangian change of the mean surface level and mean interfacial level (the divergence effect) due to periodic baroclinic wave motion.

• Esters, Leonie; Breivik, Øyvind; Landwehr, Sebastian; Doeschate, Anneke ten; Sutherland, Graig & Christensen, Kai Håkon [Vis alle 8 forfattere av denne artikkelen] (2018). Turbulence Scaling Comparisons in the Ocean Surface Boundary Layer. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 123(3), s. 2172–2191. doi: 10.1002/2017JC013525. Fulltekst i vitenarkiv
• Fritzner, Sindre Markus; Graversen, Rune; Wang, Keguang & Christensen, Kai Håkon (2018). Comparison between a multi-variate nudging method and the ensemble Kalman filter for sea-ice data assimilation. Journal of Glaciology. ISSN 0022-1430. 64(245), s. 387–396. doi: 10.1017/jog.2018.33. Fulltekst i vitenarkiv Vis sammendrag

  Increasing ship traffic and human activity in the Arctic has led to a growing demand for accurate Arctic weather forecast. High-quality forecasts obtained by models are dependent on accurate initial states achieved by assimilation of observations. In this study, a multi-variate nudging (MVN) method for assimilation of sea-ice variables is introduced. The MVN assimilation method includes procedures for multivariate update of sea-ice volume and concentration, and for extrapolation of observational information spatially. The MVN assimilation scheme is compared with the Ensemble Kalman Filter (EnKF) using the Los Alamos Sea Ice Model. Two multi-variate experiments are conducted: in the first experiment, sea-ice thickness from the European Space Agency’s Soil Moisture and Ocean Salinity mission is assimilated, and in the second experiment, sea-ice concentration from the ocean and Sea Ice Satellite Application Facility is assimilated. The multivariate effects are cross-validated by comparing the model with non-assimilated observations. It is found that the simple and computationally cheap MVN method shows comparable skills to the more complicated and expensive EnKF method for multivariate update. In addition, we show that when few observations are available, the MVN method is a significant model improvement compared to the version based on one-dimensional sea-ice concentration assimilation.

• Christensen, Kai Håkon; Sperrevik, Ann Kristin & Brostrøm, Gøran (2018). On the Variability in the Onset of the Norwegian Coastal Current. Journal of Physical Oceanography. ISSN 0022-3670. 48(3), s. 723–738. doi: 10.1175/JPO-D-17-0117.1. Fulltekst i vitenarkiv
• Christensen, Kai Håkon; Breivik, Øyvind; Dagestad, Knut-Frode; Röhrs, Johannes & Ward, Brian (2018). Short-term predictions of: Oceanic drift. Oceanography. ISSN 1042-8275. 31(3), s. 59–67. doi: 10.5670/OCEANOG.2018.310. Fulltekst i vitenarkiv
• Christensen, Kai Håkon; Carrasco, Ana; Bidlot, Jean-Raymond & Breivik, Øyvind (2017). The "shallow-waterness" of the wave climate in European coastal regions. Ocean Science. ISSN 1812-0784. 13, s. 589–597. doi: 10.5194/os-13-589-2017. Fulltekst i vitenarkiv Vis sammendrag

  In contrast to deep water waves, shallow water waves are influenced by bottom topography, which has consequences for the propagation of wave energy as well as for the energy and momentum exchange between the waves and the mean flow. The ERA-Interim reanalysis is used to assess the fraction of wave energy associated with shallow water waves in coastal regions in Europe. We show maps of the distribution of this fraction as well as time series statistics from eight selected stations. There is a strong seasonal dependence and high values are typically associated with winter storms, indicating that shallow water wave effects can occasionally be important even in the deeper parts of the shelf seas otherwise dominated by deep water waves.

• Esters, Leonie; Landwher, Sebastian; Sutherland, Graigory John; Bell, Tom G.; Christensen, Kai Håkon & Saltzman, Eric S. [Vis alle 8 forfattere av denne artikkelen] (2017). Parameterizing air-sea gas transfer velocity with dissipation. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 122(4), s. 3041–3056. doi: 10.1002/2016JC012088. Fulltekst i vitenarkiv Vis sammendrag

  The air-sea gas transfer velocity k is frequently estimated as an empirical function of wind speed. However, it is widely recognized that k depends on processes other than wind speed alone. The small-eddy model, which describes periodic events of small eddies disturbing the sea surface with water from below, suggests a direct relation between k and the dissipation rate of turbulent kinetic energy at the air-sea interface. This relation has been proven both in laboratories and in the field in various freshwater and coastal environments, but to date has not been verified in open ocean conditions. Here, concurrent North Atlantic field observations of and eddy covariance measurements of DMS and CO2 air-sea gas flux are presented. Using measurements, we compare the small-eddy model at various depths to previously published observations. Extrapolating the measured profiles to the thickness of the viscous sublayer allows us to formulate a function of k that depends solely on the water side friction velocity uw , which can be inferred from direct eddy covariance measurements of the air-side friction velocity ua . These field observations are generally consistent with the theoretical small-eddy model. Utilizing a variable Schmidt number exponent in the model, rather than a constant value of 1 2 yields improved agreement between model and observations. This research was originally published in Journal of Geophysical Research: Oceans. © 2017 Wiley

• Sperrevik, Ann Kristin; Röhrs, Johannes & Christensen, Kai Håkon (2017). Impact of data assimilation on Eulerian versus Lagrangian estimates of upper ocean transport. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 122(7), s. 5445–5457. doi: 10.1002/2016JC012640. Fulltekst i vitenarkiv Vis sammendrag

  Using four-dimensional variational analysis, we produce an estimate of the state of a coastal region in Northern Norway during the late winter and spring in 1984. We use satellite sea surface temperature and in situ observations from a series of intensive field campaigns, and obtain a more realistic distribution of water masses both in the horizontal and the vertical than a pure downscaling approach can achieve. Although the distribution of Eulerian surface current speeds are similar, we find that they are more variable and less dependent on model bathymetry in our reanalysis compared to a hindcast produced using the same modeling system. Lagrangian drift currents on the other hand are significantly changed, with overall higher kinetic energy levels in the reanalysis than in the hindcast, particularly in the superinertial frequency band.

• Sutherland, Graigory John; Marié, Louis; Reverdin, Gilles; Christensen, Kai Håkon; Brostrøm, Gøran & Ward, Brian (2016). Enhanced turbulence associated with the diurnal jet in the ocean surface boundary layer. Journal of Physical Oceanography. ISSN 0022-3670. 46(10), s. 3051–3067. doi: 10.1175/JPO-D-15-0172.1. Fulltekst i vitenarkiv Vis sammendrag

  Detailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate ε below the depth of the diurnal jet, there was an observed increase in ε within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of ε. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diurnal heating limits the depth of the momentum flux due to the wind, shear instability provides an additional source of turbulence that interacts with the enhanced shear of the diurnal jet to increase ε within this shallow layer. © 2016 American Meteorological Society

• Rabault, Jean; Sutherland, Graigory John; Ward, Brian; Christensen, Kai Håkon; Halsne, Trygve & Jensen, Atle (2016). Measurements of Waves in Landfast Ice Using Inertial Motion Units. IEEE Transactions on Geoscience and Remote Sensing. ISSN 0196-2892. 54(11), s. 6399–6408. doi: 10.1109/TGRS.2016.2584182. Fulltekst i vitenarkiv
• Jones, Cathleen Elaine; Dagestad, Knut-Frode; Breivik, Øyvind; Holt, Benjamin; Röhrs, Johannes & Christensen, Kai Håkon [Vis alle 9 forfattere av denne artikkelen] (2016). Measurement and modeling of oil slick transport. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 121(10), s. 7759–7775. doi: 10.1002/2016JC012113. Fulltekst i vitenarkiv
• Esters, Leonie; Landwehr, Sebastian; Sutherland, Graigory John; Bell, Thomas G.; Saltzman, Eric S. & Christensen, Kai Håkon [Vis alle 8 forfattere av denne artikkelen] (2016). The relationship between ocean surface turbulence and air-sea gas transfer velocity: An in-situ evaluation. IOP Conference Series: Earth and Environmental Science (EES). ISSN 1755-1307. 35(1). doi: 10.1088/1755-1315/35/1/012005. Fulltekst i vitenarkiv
• Röhrs, Johannes; Sperrevik, Ann Kristin; Christensen, Kai Håkon; Broström, Göran & Breivik, Øyvind (2015). Comparison of HF radar measurements with Eulerian and Lagrangian surface currents. Ocean Dynamics. ISSN 1616-7341. 65(5), s. 679–690. doi: 10.1007/s10236-015-0828-8. Fulltekst i vitenarkiv
• Röhrs, Johannes & Christensen, Kai Håkon (2015). Drift in the uppermost part of the ocean. Geophysical Research Letters. ISSN 0094-8276. 42(23), s. 10349–10356. doi: 10.1002/2015GL066733. Fulltekst i vitenarkiv
• Sperrevik, Ann Kristin; Christensen, Kai Håkon & Röhrs, Johannes (2015). Constraining energetic slope currents through assimilation of high-frequency radar observations. Ocean Science. ISSN 1812-0784. 11(2), s. 237–249. doi: 10.5194/os-11-237-2015. Fulltekst i vitenarkiv Vis sammendrag

  Assimilation of high-frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in northern Norway. In the realistic case, the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about 1 day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography. However, combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

• Weber, Jan Erik H.; Drivdal, Magnus Bjørn; Christensen, Kai Håkon & Brostrøm, Gøran (2015). Some aspects of the Coriolis-Stokes forcing in the oceanic momentum and energy budgets. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 120(8), s. 5589–5596. doi: 10.1002/2015JC010717. Fulltekst i vitenarkiv
• Taskjelle, Torbjørn; Barthel, Knut; Christensen, Kai Håkon; Furaca, Noca; Gammelsrød, Tor & Hoguane, Antonio Mubango [Vis alle 7 forfattere av denne artikkelen] (2014). Modelling alongshore flow in a semi-enclosed lagoon strongly forced by tides and waves. Estuarine, Coastal and Shelf Science. ISSN 0272-7714. 149, s. 294–301. doi: 10.1016/j.ecss.2014.09.008. Fulltekst i vitenarkiv Vis sammendrag

  Alongshore flows strongly driven by tides and waves is studied in the context of a one-dimensional numerical model. Observations from field surveys performed in a semi-enclosed lagoon (1.7 km×0.2 km) outside Xai-Xai, Mozambique, are used to validate the model results. The model is able to capture most of the observed temporal variability of the current, but sea surface height tends to be overestimated at high tide, especially during high wave events. Inside the lagoon we observed a mainly uni-directional alongshore current, with speeds up to 1 ms−1. The current varies primarily with the tide, being close to zero near low tide, generally increasing during flood and decreasing during ebb. The observations revealed a local minimum in the alongshore flow at high tide, which the model was successful in reproducing. Residence times in the lagoon were calculated to be less than one hour with wave forcing dominating the flushing. At this beach a high number of drowning casualties have occurred, but no connection was found between them and strong current events in a simulation covering the period 2011–2012.

• Drivdal, Magnus Bjørn; Broström, Göran & Christensen, Kai Håkon (2014). Wave-induced mixing and transport of buoyant particles: Application to the Statfjord a oil spill. Ocean Science. ISSN 1812-0784. 10(6), s. 977–991. doi: 10.5194/os-10-977-2014. Fulltekst i vitenarkiv
• Weber, Jan Erik H.; Christensen, Kai Håkon & Broström, Göran (2014). Stokes drift in internal equatorial Kelvin waves; continuous stratification versus two-layer models. Journal of Physical Oceanography. ISSN 0022-3670. 44(2), s. 591–599. doi: 10.1175/JPO-D-13-0135.1. Fulltekst i vitenarkiv
• Broström, Göran; Christensen, Kai Håkon; Drivdal, Magnus & Weber, Jan Erik H. (2014). Note on Coriolis-Stokes force and energy. Ocean Dynamics. ISSN 1616-7341. 64(7), s. 1039–1045. doi: 10.1007/s10236-014-0723-8. Fulltekst i vitenarkiv

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• Ali, Alfatih; Bentsen, Mats; Brevik, Øyvind Jakobsen; Carrasco, Ana; Christensen, Kai Håkon & Debernard, Jens Boldingh [Vis alle 9 forfattere av denne artikkelen] (2026). Wave-ocean coupling experiments in the Norwegian Earth Model System (NorESM). Fulltekst i vitenarkiv Vis sammendrag

  With the purpose of understanding the climate impact of surface wind waves, we add an ocean surface wave model, WAVEWATCH III (WW3), in the Norwegian Earth Model System (NorESM) and include parametrisations for wave-mediated processes that modify air-sea fluxes, ocean surface mixing, and sea-ice drift. NorESM is a derivative of the Community Earth System Model (CESM), with a different ocean model (Bergen Layered Ocean Model, BLOM) and modified atmosphere (CAM-Nor) and sea-ice (CICE6) components. Here we present the analysis of results from a set of simulations under contemporary (“present day”) forcings. Integrations where WW3 is coupled one-way only (i.e. forced without feedbacks) are compared with runs where WW3 is two-way coupled with either of CAM-Nor, BLOM, and CICE6—and all components together. The wave-to-ocean coupling consists of a Langmuir turbulence parametrization, the wave-to-atmosphere coupling in a sea-state dependent Charnock parameter, and the wave-to-ice coupling in a wave radiation stress added to sea-ice momentum. The wave-ocean coupling causes the most robust climatic impact in our experiments. In general, wave-ocean coupling causes a deepening of the mixed layer changes. The effect of the deeper marine boundary layers on SSTs then acts on the atmosphere and sea-ice and waves.. The climatological effects of other couplings are, by contrast, strongly dependent on the base-line climatology itself, and by implication on model biases.

• Halvorsen, Daniel Ørnes; Christensen, Kai Håkon; Drivdal, Magnus Bjørn; Ellingsen, Ingrid Helene; Frøysa, Håvard Guldbrandsen & Jakobsson, Øyvind [Vis alle 9 forfattere av denne artikkelen] (2026). Modeling Connectivity–Derived Infection Networks to Inform Area–Organization in Norway’s Salmon Aquaculture Industry. Fulltekst i vitenarkiv
• Ali, Alfatih; Bentsen, Mats; Breivik, Øyvind; Carrasco, Ana; Christensen, Kai Håkon & Debernard, Jens Boldingh [Vis alle 9 forfattere av denne artikkelen] (2025). Wave coupling experiments in the Norwegian Earth Model System (NorESM). Fulltekst i vitenarkiv Vis sammendrag

  The North Atlantic–Arctic Ocean is a key region for Northern Hemisphere storm tracks, characterized by strong air-sea exchange and associated meridional fluxes of momentum, enthalpy and water vapour. Regionally these determine much of the climate of western Eurasia and of the Nordic Seas, with strong effects on the Arctic sea-ice cover. The atmosphere-ocean state in particular is affected by the effects of strong diabatic and mechanical air-sea fluxes, which are notably influenced by the large activity of surface waves generated by storms. With the purpose of understanding the climatological role of surface wind waves, we add a wave model, WAVEWATCH III (WW3), to the Norwegian Earth Model System (NorESM). NorESM is a derivative of the Community Earth System Model (CESM), with a different ocean model (Bergen Layered Ocean Model, BLOM), interactive ocean biogeochemistry module (iHAMOCC) and modified atmosphere (CAM-Nor) and sea-ice (CICE6) components. We run WW3 coupled one-way with atmospheric winds only, and interactively coupled, two-ways, with CAM-Nor, BLOM, and CICE6. The wave-ocean coupling consists of a Langmuir turbulence parametrization whereas wave-atmosphere coupling is in the form of the sea-state dependent Charnock parameter. Finally the wave-ice coupling is done by including the wave to ice stress. Preliminary results indicate systematic changes to the simulated climatology of the North Atlantic associated with air-sea fluxes during high-wind conditions.

• Sperrevik, Ann Kristin; Christensen, Kai Håkon & Idzanovic, Martina (2024). Improving prediction of surface currents through assimilation of HF-radar observations. Fulltekst i vitenarkiv
• Litzler, Emma; Nepstad, Raymond; Nordam, Tor; Röhrs, Johannes; Christensen, Kai Håkon & Rikardsen, Edel (2024). Oil spill risk assessment based on ocean model ensemble prediction system. Fulltekst i vitenarkiv
• Carrasco, Ana; Rabault, Jean; Halsne, Trygve; Debernard, Jens Boldingh; Breivik, Øyvind & Müller, Malte [Vis alle 8 forfattere av denne artikkelen] (2023). Understanding the physical drivers behind observations of hs with a 12-hr modulation in the Arctic Marginal Ice Zone. Fulltekst i vitenarkiv
• Christensen, Kai Håkon (2023). Orientering om Arven etter Nansen - Presentasjon for Overvåkningsgruppen for norske havområder. Fulltekst i vitenarkiv
• Reigstad, Marit; Gerland, Sebastian; Eldevik, Tor; Kunisch, Erin; Wiedmann, Ingrid & Sundfjord, Arild [Vis alle 21 forfattere av denne artikkelen] (2023). The Nansen Legacy project invites you to its upcoming international symposium in November 2023. Fulltekst i vitenarkiv
• Rabault, Jean; Müller, Malte; Hope, Gaute; Breivik, Øyvind; Nose, Takehiko & Voermans, Joey [Vis alle 16 forfattere av denne artikkelen] (2023). Collecting Marginal Ice Zone ground truth observations: recent deployments, data use, and outstanding questions. Fulltekst i vitenarkiv
• Simonsen, Magne & Christensen, Kai Håkon (2023). Fjord transport and two-way nested models. Fulltekst i vitenarkiv
• Wallhead, Philip; Frigstad, Helene; Staalstrøm, André; Kristiansen, Trond; Gundersen, Hege & Sample, James Edward [Vis alle 17 forfattere av denne artikkelen] (2022). MARTINI koblet fysisk og biogeokjemisk modellering. Fulltekst i vitenarkiv
• Carrasco, Ana; Ali, Alfatih; Breivik, Øyvind; Debernard, Jens Boldingh; Christensen, Kai Håkon & Bentsen, Mats [Vis alle 7 forfattere av denne artikkelen] (2022). MakingWaves: Wave-mediated atmosphere-ocean-sea-ice interactions and their climatic impacts in the Nordic Seas and eastern Arctic. Fulltekst i vitenarkiv
• Svehagen, Mai-Linn. Finstad; Sætra, Martin Lilleeng; Brodtkorb, André R.; Christensen, Kai Håkon & Holm, Håvard Heitlo (2022). Spillteknologi gir nye og bedre modeller av havet. [Internet]. Forskning.no. Fulltekst i vitenarkiv
• Østenstad, Petter; Brændshøi, Jostein & Christensen, Kai Håkon (2021). Improved representation of vertical (T,S) profiles using real-time assimilation in Arctic waters. Fulltekst i vitenarkiv
• Løken, Trygve Kvåle; Rabault, Jean; Thomas, Erin E.; Müller, Malte; Christensen, Kai Håkon & Sutherland, Graig [Vis alle 7 forfattere av denne artikkelen] (2020). A comparison of wave observations in the Arctic marginal ice zone with spectral models. Fulltekst i vitenarkiv
• Breivik, Øyvind; Rabault, Jean; Hole, Lars R; Christensen, Kai Håkon; Müller, Malte & Jensen, Atle [Vis alle 7 forfattere av denne artikkelen] (2019). Bølger i isen påvirker Arktis. Å forstå isdønningene kan forbedre værvarsler og klimamodeller. Forskning.no. ISSN 1891-635X. Fulltekst i vitenarkiv
• Løken, Trygve Kvåle; Rabault, Jean; Jensen, Atle; Sutherland, Graig; Christensen, Kai Håkon & Müller, Malte (2019). Wave measurements from ship mounted sensors in the Arctic marginal ice zone. Fulltekst i vitenarkiv
• Fritzner, Sindre Markus; Christensen, Kai Håkon; Graversen, Rune Grand & Wang, Keguang (2019). Assimilation of high-resolution ice charts in a coupled ocean-sea-ice model. Proceedings IGARSS-97.. doi: 10.1109/IGARSS.2019.8898221. Fulltekst i vitenarkiv
• Fritzner, Sindre Markus; Christensen, Kai Håkon; Graversen, Rune & Wang, Keguang (2019). ASSIMILATION OF HIGH-RESOLUTION ICE CHARTS IN A COUPLED OCEAN-SEA-ICE MODEL. doi: 10.1109/IGARSS.2019.8898221. Fulltekst i vitenarkiv Vis sammendrag

  In this study, we show assimilation results from a coupled ocean sea-ice model. The model has a horizontal resolution of 2.5 km. In the assimilation system, we assimilate high-resolution ice charts, structured on a 1 km grid. We compare the assimilation of passive microwave observations with the assimilation of ice charts. It is shown that the ice charts have a larger impact on the assimilation system than the passive microwave observations. In addition, a few results from the assimilation system with ice charts are shown. These indicate improvements for the assimilation system assimilating ice charts compared to a free-run without assimilation and an assimilation system assimilating passive microwave observations.

• Løken, Trygve Kvåle; Rabault, Jean; Jensen, Atle; Christensen, Kai Håkon & Müller, Malte (2019). Wave statistics from ship mounted sensors in the marginal ice zone. Fulltekst i vitenarkiv
• Christensen, Kai Håkon (2018). Short term predictions of oceanic drift. Fulltekst i vitenarkiv
• Ali, Alfatih Omer Mohammed Ahmed; Christensen, Kai Håkon; Breivik, Øyvind; Bertino, Laurent & Raj, Roshin Pappukutty (2018). Assessing the impact of surface waves on the North Atlantic and Nordic Seas simulations using HYCOM. Fulltekst i vitenarkiv
• Christensen, Kai Håkon (2018). The importance of wave-current interactions in operational oceanography. Fulltekst i vitenarkiv Vis sammendrag

  The purpose of ocean circulation models in an operational setting is typically to provide support for oil spill mitigation, search-and-rescue, ship routing, offshore operations, and so on. Focus is therefore primarily on the dynamics of the upper ocean. Surface waves play an important role with regard to turbulent mixing and modification of upper ocean drift velocities through wave-current interactions. The latter is particularly important, and the wave-induced component of the drift of oil spills or other bouyant materials is often dominant. Furthermore, since the waves influence the air-sea fluxes of momentum and energy, the upper ocean response to transient atmospheric forcing is tightly coupled to the waves, and this coupling must be taken into account in the forecasting systems in order to correctly model the currents in the Ekman layer. We still lack observations to validate several aspects of wave-current interaction theory, but some progress has been made using autonomous instrument platforms and in dedicated campaigns where a wide range of sensors have been deployed. I will show some examples from the field, and also examples from modeling studies of oceanic drift in which wave effects have been taken into account. I will also comment on a few outstanding challenges from both an observational and modeling perspective.

• Christensen, Kai Håkon; Sperrevik, Ann Kristin & Brostrøm, Gøran (2017). The circulation in the Kattegat and Skagerrak. Fulltekst i vitenarkiv
• Christensen, Kai Håkon (2017). The impact of surfactants on surface waves. Fulltekst i vitenarkiv
• Röhrs, Johannes & Christensen, Kai Håkon (2017). Drift in the uppermost part of the ocean - 5th Norway-Scotland wave symposium 2017. Fulltekst i vitenarkiv
• Carrasco, Ana; Semedo, Alvaro; Behrens, Arno; Weisse, Ralf; Breivik, Øyvind & Sætra, Øyvind [Vis alle 7 forfattere av denne artikkelen] (2016). Projected Changes on the Global Surface Wave Drift Climate towards the END of the Twenty-First Century. Fulltekst i vitenarkiv
• Christensen, Kai Håkon; Breivik, Øyvind; Sutherland, Graigory John; Gargett, Ann; Ward, Brian & Belcher, Stephen (2016). A robust definition for the turbulent Langmuir number. Fulltekst i vitenarkiv
• Brodtkorb, André R.; Christensen, Kai Håkon; Røed, Lars Petter & Sætra, Martin Lilleeng (2016). Using the GPU to Predict Drift in the Ocean. Fulltekst i vitenarkiv
• Weber, Jan Erik H. & Christensen, Kai Håkon (2016). Mean particle drift in long gravity waves at the interface between immiscible viscous fluids. Fulltekst i vitenarkiv
• Röhrs, Johannes & Christensen, Kai Håkon (2016). Wind- and wave induced drift in the uppermost part of the ocean. Fulltekst i vitenarkiv
• Ward, Brian; Sutherland, Graigory John; Reverdin, Gilles; Marie, Luois; Christensen, Kai Håkon & Brostrøm, Gøran [Vis alle 8 forfattere av denne artikkelen] (2015). Ocean Surface Observations of the Diurnal Cycle of Turbulence with ASIP. Fulltekst i vitenarkiv
• Röhrs, Johannes; Sundby, Svein; Vikebø, Frode Bendiksen & Christensen, Kai Håkon (2014). SURFACE GRAVITY WAVES TRANSPORTING PELAGIC FISH EGGS: STOKES DRIFT AND VERTICAL MIXING. Fulltekst i vitenarkiv
• Brostrøm, Gøran; Drivdal, Magnus Bjørn; Carrasco, Ana; Christensen, Kai Håkon & Mattson, Johan (2014). The Golden Trader oil spill; evaluation of operational oil spill models. Fulltekst i vitenarkiv
• Sutherland, Graigory John; Ward, Brian & Christensen, Kai Håkon (2014). EVALUATING TURBULENCE REGIMES IN THE OCEAN SURFACE BOUNDARY LAYER: MEASUREMENTS FROM A VERTICALLY RISING MICROSTRUCTURE PROFILER. Fulltekst i vitenarkiv
• Drivdal, Magnus Bjørn; Brostrøm, Gøran & Christensen, Kai Håkon (2014). Wave induced transport and mixing of buoyant particles. Fulltekst i vitenarkiv
• Wallhead, Philip; Frigstad, Helene; Sample, James Edward; Jackson-Blake, Leah Amber; Staalstrøm, Andre & Kvile, Kristina Øie [Vis alle 17 forfattere av denne artikkelen] (2026). Modellering av Oslofjorden: scenarier med redusert menneskelig belastning for å oppnå god økologisk tilstand. Norsk institutt for vannforskning (NIVA). ISSN 9788257779238. Fulltekst i vitenarkiv Vis sammendrag

  Formålet med prosjektet var å vurdere effektene av mulige forvaltningstiltak og nødvendige belastningsreduksjoner for å oppnå god økologisk tilstand i vannforekomstene i Osloforden, ved hjelp av numeriske modeller. Vi undersøkte to scenarier for forvaltningstiltak: (A) middels ambisjonsnivå og (B) høyt ambisjonsnivå. Simuleringene antydet at mange av fordelene ved scenario B – i form av økt oksygen i bunnvannet i Indre Osloford, bedre vannklarhet og redusert sedimentering – allerede ville oppnås med scenario A. Betydelige tilleggseffekter ble imidlertid oppnådd med scenario B når det gjelder næringsinnholdet i overflatevannet og potensielle økologiske fordeler knyttet til redusert vekst av opportunistiske algearter og «lurv», selv om omfanget av sistnevnte var usikkert. Avlastningsbehov for å oppnå god tilstand av biogeokjemiske kvalitetselementer ble vurdert til omtrent på nivå med scenario B for oppløst uorganisk nitrogen, men omtrent på nivå med scenario A for totalfosfor, partikler, og organisk materiale.

• Sagen, Hanne; Sandven, Stein; Hamre, Torill; Rønning, Bjørn; Tollefsen, Dag & Daase, Malin [Vis alle 14 forfattere av denne artikkelen] (2025). Technologies and collaboration for sustained Arctic Ocean Observing Systems (AOOS). Deliverable D4: User needs for in situ ocean observations in the Arctic. Zenodo. doi: 10.5281/zenodo.17224017. Fulltekst i vitenarkiv
• Skern-Mauritzen, Mette; Andersson, Ingvild; Arneberg, Per; Sanchez-Borque, Jorge; Christensen, Kai Håkon & Danielsen, Ida Kristin [Vis alle 37 forfattere av denne artikkelen] (2025). Revisjon av indikatorer for tilstandsvurdering av miljø og økosystem i norske havområder — Gruppen for overvåking av de marine økosystemene. Havforskningsinstituttet. Fulltekst i vitenarkiv
• Albretsen, Jon; Arneberg, Per; Assmann, Karen; Assmy, Philipp; Bohlin-Nizzetto, Pernilla & Børsheim, Knut Yngve [Vis alle 49 forfattere av denne artikkelen] (2023). Status for miljøet i norske havområder - Rapport fra Overvåkingsgruppen 2023. Havforskningsinstituttet. Fulltekst i vitenarkiv Vis sammendrag

  I denne rapporten gir Overvåkingsgruppen, for første gang, en felles vurdering av miljøtilstanden i Barentshavet og havområdene utenfor Lofoten, Norskehavet og Nordsjøen med Skagerrak. Det er også første rapport som bruker resultater fra det nylig utviklede fagsystemet for vurdering av økologisk tilstand. I denne rapporten dekkes to hovedtemaer: (1) Dominerende trekk i status og utvikling i økosystemet i alle tre havområdene, basert på vurderingene av økologisk tilstand, Overvåkingsgruppens rapport om forurensning fra 2022, indikatorer fra Overvåkingsgruppen som ikke er dekket under vurdering av økologisk tilstand, samt rapporter og annen relevant informasjon fra forskning, og (2) en vurdering av karbonbinding i marint plankton, marine vegetasjonstyper og marine sedimenter. I tillegg er det gitt en oppsummering for endringer i ytre påvirkning, vurdering av kunnskapsbehov samt en vurdering av indikatorverdier i forhold til referanseverdier og tiltaksgrenser. Vurderingen av dominerende trekk i utvikling og tilstand av miljøet som er gitt i kapittel 2, utgjør Overvåkingsgruppens bidrag til Faglig forums samlerapport om det faglige grunnlaget for revisjon og oppdatering av de helhetlige forvaltningsplanene for norske havområder.

• Rabault, Jean; Jensen, Atle; Sutherland, Graig & Christensen, Kai Håkon (2018). An investigation into the interaction between waves and ice. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  The polar regions are the focus of increased attention due to a combination of environmental, political and economic reasons. In order to help support human activities in the arctic and monitor the state of the ecosystem, detailed understanding and models of the state of sea ice are necessary. Sea ice is affected by incoming waves, and therefore studying wave-ice interaction is an important part of this effort. In my thesis work, I investigated how waves propagate through different types of ice covers. More specifically, two phenomena of practical importance were studied. First, I looked at how waves attenuate as they propagate through sea ice. This, in turn, determines how much ice can be broken by incoming waves and should be part of waves and ice forecasts. Second, I investigated the currents created under the ice as a consequence of the propagation of the waves. There it was shown experimentally that mean currents can arise, which are expected to play a role in the dispersion of nutriments and pollutants.

• Röhrs, Johannes; Sperrevik, Ann Kristin; Christensen, Kai Håkon & Breivik, Øyvind (2018). NorShelf: An ocean reanalysis and data-assimilative forecast model for the Norwegian Shelf Sea. Meteorologisk institutt. Fulltekst i vitenarkiv
• Sperrevik, Ann Kristin; Christensen, Kai Håkon; Johannessen, Johnny Andre & Gustafsson, Nils (2017). Modeling coastal circulation in Norway using a high-resolution 4D-Var ocean assimilation system. Universitetet i Bergen. Fulltekst i vitenarkiv Vis sammendrag

  The circulation along the Norwegian coast is characterized by many transient smallscale features such as eddies and meanders that are challenging to reproduce by means of numerical modeling. In this thesis I investigate the use of advanced data assimilation (DA) techniques in high-resolution coastal models to improve the circulation estimates. One particularly interesting observational platform for the coastal ocean is highfrequency (HF) radars, which measure surface currents in the coastal zone up to 200 km offshore. The suitability of such observations for use in high-resolution coastal DA systems is assessed by quantifying which components of the near-surface current field are observed by the HF radars. Our results show that there are no contributions from wave drift in the measurements, thus they are suitable for use in coastal DA. Assimilation of HF radar currents in a high-resolution model shows clear improvement in the circulation estimates. Further improvement is obtained when CTD profiles of temperature and salinity are included in the assimilated data set. A reanalysis of a period, during which in-situ observations were abundant in the study area, is used to assess how an observational network dense enough to constrain the water mass distribution affects the upper ocean circulation estimates. Our investigations of the results show a weakening of the topographically steered currents and, as the stratification increases the effective resolution of the model, more small-scale circulation features are developed. Such changes may have a significant effect on upper ocean transport. Finally, the potential of using high-resolution coastal reanalyses to study specific physical processes is demonstrated for the case of the mechanisms causing variability in the Norwegian Coastal Current.

• Röhrs, Johannes; Furevik, Birgitte Rugaard & Christensen, Kai Håkon (2014). Surface wave effects in the upper ocean and consequences for biological modeling. Universitetet i Bergen. ISSN 9788230829561. Fulltekst i vitenarkiv Vis sammendrag

  Effects of surface gravity waves on particle transport and currents in the upper ocean are studied using field observations and numerical models. Knowledge on upper ocean particle drift is needed for plankton transport studies, search- and rescue operations and oil drift modelling. Such applications depend on upper ocean currents on time scales from hours up to several months, and surface gravity waves affect the air-sea momentum exchange particularly on these time scales. Objects near the surface are also exposed to the wave-induced Stokes drift, which moves particles in the wave propagation direction at a speed with similar magnitude as the Eulerian current. Surface waves furthermore modify the Eulerian mean currents by momentum release during wave breaking and the dissipation of wave energy into oceanic turbulence. Measurements of upper ocean currents, surface drifters, and waves were taken during field campaigns in Northern Norway. Ocean currents from acoustic Doppler current profilers, high-frequency (HF) radars and satellite-tracked surface drifters were synthesized with wave measurements to separate the Eulerian currents from the Lagrangian, their difference is the Stokes drift. The contribution of waves to surface drifter velocities and observed HF radar currents was quantified and compared with theoretical concepts from the literature. The surface drifters followed the sum of Eulerian currents and Stokes drift with little wind drag. In contrast to previous assumptions, the observation showed that HF radar-derived currents do not include the Stokes drift. The observations were also used to quantify the momentum and energy fluxes from waves into the ocean during a wind event. The wave-induced drift of suspended plankton was investigated for the example of Northeast Arctic (NEA) cod eggs. Their physical properties are relatively well known, providing a practical example for biological models that depend on oceanic circulation. A combined ocean circulation model, wave model and particle tracking model is used to model pathways of cod eggs from their spawning grounds towards nursery grounds. The cod eggs are affected by (i) the Stokes drift; (ii) the wave-induced forcing on Eulerian mean currents; and (iii) the turbulent mixing induced by breaking waves. Accounting for wave effects in the model system yields more accurate drift velocities and vertical particle distributions compared with observations. Waves are shown to significantly affect the fate of NEA cod eggs: The Stokes drift causes a shoreward flux of particles and thereby reduces the alongshore transport causing retention of NEA cod eggs and larvae around the spawning grounds. The discussed wave effects generally apply to all plankton in the pelagic layer. The Stokes drift contributes significantly to the fate of drifting particles in the long term because it exhibits a horizontally more uniform direction than the Eulerian currents. The wave field varies on the scales from the atmospheric forcing, giving better predictability than ocean currents. Including wave effects in models therefore facilitates better forecast skills of surface drift applications.

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