MATNOC – Et forvaltningsverktøy for lakseoppdrett basert på simuleringer av kystnær havsirkulasjon

• 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.
• 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
• Espenes, Håvard; Isachsen, Pål Erik & Nøst, Ole Anders (2023). Observations and modeling of tidally generated high-frequency velocity fluctuations downstream of a channel constriction. Ocean Science. ISSN 1812-0784. 19(6), s. 1633–1648. doi: 10.5194/os-19-1633-2023. Fulltekst i vitenarkiv Vis sammendrag

  We investigate data from an acoustic Doppler current profiler deployed in a constricted ocean channel showing a tidally dominated flow with intermittent velocity extrema during outflow from the constriction but not during inflow. A 2D numerical ocean model forced by tides is used to examine the spatial flow structure and underlying dynamical processes. We find that flow-separation eddies generated near the tightest constriction point form a dipole pair which propagates downstream and drives the observed intermittent flow variability. The eddies, which are generated by an along-channel adverse pressure gradient, spin up for some time near the constriction until they develop local low pressures in their centers that are strong enough to modify the background along-channel pressure gradient significantly. When the dipole has propagated some distance away from the constriction, the conditions for flow separation are recovered, and new eddies are formed.

• 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.

• Nøst, Ole Anders & Børve, Eli (2021). Flow separation, dipole formation, and water exchange through tidal straits. Ocean Science. ISSN 1812-0784. 17(5), s. 1403–1420. doi: 10.5194/os-17-1403-2021. Fulltekst i vitenarkiv Vis sammendrag

  We investigate the formation and evolution of dipole vortices and their contribution to water exchange through idealized tidal straits. Self-propagating dipoles are important for transporting and exchanging water properties through straits and inlets in coastal regions. In order to obtain a robust dataset to evaluate flow separation, dipole formation and evolution, and the effect on water exchange, we conduct 164 numerical simulations, varying the width and length of the straits as well as the tidal forcing. We show that dipoles form and start propagating at the time of flow separation, and their vorticity originates in the velocity front formed by the separation. We find that the dipole propagation velocity is proportional to the tidal velocity amplitude and twice as large as the dipole velocity derived for a dipole consisting of two point vortices. We analyze the processes creating a net water exchange through the straits and derive a kinematic model dependent on dimensionless parameters representing strait length, dipole travel distance, and dipole size. The net tracer transport resulting from the kinematic model agrees closely with the numerical simulations and provides an understanding of the processes controlling net water exchange.

• 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. 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.

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