Research interests
- Ocean dynamics at high latitudes
- Baroclinic instability and eddy dynamics
- Numerical ocean modeling
Teaching
Tags:
Oceanography
Publications
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Sjur, Anna Lina Petruseviciute; Isachsen, Pål Erik; Nilsson, Johan & Allen, Susan Elizabeth
(2026).
Nonlinear dynamics of time-variable slope circulation.
Ocean Science.
ISSN 1812-0784.
22(5),
p. 3055–3078.
doi:
10.5194/os-22-3055-2026.
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Bottom topography strongly constrains ocean circulation in the Arctic, and both theory and numerical modeling suggest that nonlinear flow–topography interactions influence slope-following currents. Yet, how such interactions modify the circulation response to time-variable surface forcing remains poorly understood. Using idealized shallow-water simulations of flow over a corrugated slope in a re-entrant channel, we investigate how nonlinear features arise and evolve under oscillatory forcing. We observe both a prograde flow bias (aligned in the direction of topographic Rossby wave propagation) relative to linear estimates, and retrograde flow (opposing wave propagation) exhibiting flow strength saturation once the flow reaches sufficiently strong velocities. To identify the mechanisms responsible for these behaviors, we evaluate integrated momentum budgets. Which terms appear as dynamically relevant, in addition to linear surface and bottom frictional stresses, depends on the choice of integration path: when integrated along isobaths, the nonlinear dynamics appear as a cross-slope relative vorticity flux, whereas integration along straight transects instead highlights momentum flux convergence and topographic form stress. These perspectives can be unified under quasi-geostrophic scaling as describing a flux of potential vorticity (PV). This PV flux is predominantly down-slope and strongest during retrograde phases, resulting in the prograde bias. When retrograde velocities approach the arrest speed of topographic Rossby waves with wavelengths comparable to the corrugation wavelength, the flux increases sharply, halting further acceleration and producing the observed asymmetry. The interplay between forcing timescale and frictional timescale shapes how these nonlinear effects manifest: when the forcing period is comparable to the dampening timescale, the response is strongly low-pass filtered, whereas for much longer forcing periods the flow has time to adjust toward distinct prograde and retrograde states. These results show how flow–topography interactions shape time-variable slope circulation, biasing the flow toward prograde states and limiting retrograde flow strength. Such effects are likely under-represented in coarse-resolution numerical simulations, and highlight the need for improved representations of unresolved topographic interactions.
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Klocker, Andreas; Isachsen, Pål Erik; Munday, D R; Sauermilch, I & Whittaker, J M
(2026).
Eddy-topography interactions drive ocean ventilation across the Eocene–Oligocene transition: insights from a hierarchy of ocean model simulations.
Environmental Research Communications (ERC).
8(5),
p. 1–13.
doi:
10.1088/2515-7620/ae68e7.
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Abstract The Eocene–Oligocene transition (EOT), approximately 34 million years ago, represents one of the most profound climate shifts in Earth’s history—a rapid cooling that marked the transition from the warm Eocene greenhouse to the Cenozoic icehouse. The mechanisms driving this transition remain debated: proposed causes include a decline in atmospheric CO 2 concentrations and tectonic opening of Southern Ocean gateways, namely Drake Passage and the Tasman Gateway, which altered ocean circulation and poleward heat transport. Here we use ocean model simulations spanning a wide range of spatial resolutions—from a coarse-resolution, eddy-parameterised configuration to high-resolution, eddy-resolving simulations—to investigate how the representation of ocean eddies affects the simulated circulation response to gateway opening. Our results show that the establishment of a proto Antarctic Circumpolar Current (ACC), even if much weaker than today’s current, led to the onset of coastal upwelling along the Antarctic continental slope in the Australian Antarctic Basin (AAB). This upwelling has previously been linked to the onset of biogenic blooms and associated carbon drawdown in the Southern Ocean. The simulations suggest that the proto-ACC followed the Antarctic continental slope in the AAB and that upwelling was driven by either bottom Ekman transport beneath this slope current or, as indicated by the highest-resolution simulations, eddy-topography interactions—a process in which mesoscale eddies interacting with rough and steeply sloping bottom topography drive a net onshore transport of dense waters. Critically, neither a distinct continental slope current nor the associated upwelling is reproduced in coarse-resolution models using existing eddy parameterisations, implying that the climatic impact of Southern Ocean gateway opening during the EOT has been substantially underestimated in previous modelling studies.
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Hordoir, Robinson; Jahanmard, Vahidreza; Isachsen, Pål Erik; Löptien, Ulrike; Dietze, Heiner & Sandø, Anne Britt
[Show all 7 contributors for this article]
(2026).
Barents Sea atlantification driven by a shift in atmospheric synoptic timescale.
Nature Climate Change.
ISSN 1758-678X.
doi:
10.1038/s41558-025-02535-3.
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Abstract Climate change impinges on the Arctic Ocean, leading to sea-ice loss and potentially drastic cascading ecosystem changes. A recent process is atlantification, the growing influence of warm and salty waters from the Atlantic on the Arctic with increasing ocean volume transport from the Nordic Seas to the Barents Sea playing a key role. Despite its importance and a multitude of hypotheses that have been tested, this trend remains mainly unexplained. Here we explore nonlinear effects and successfully link the flow trend through the Barents Sea Opening to a frequency shift of atmospheric synoptic. We show that a part of the flow through Barents Sea Opening is driven by topographic Rossby waves, and that they have a very sensitive response to atmospheric frequency over the Nordic Seas. These findings highlight how anthropogenic changes to the atmosphere are altering ocean processes, with implications for sea-ice extent and ecosystems in the Arctic.
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Kallmyr, Jan‐Adrian Henriksen; Nilsson, Johan; Chafik, Léon & Isachsen, Pål Erik
(2025).
The Time‐Mean Arctic Ocean Circulation as Seen Through Satellite Altimetry and Hydrography.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
130(11).
doi:
10.1029/2024jc022203.
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As the number of in situ measurements of ocean currents in the central Arctic Ocean remains very limited, much of our understanding of the Arctic Ocean circulation is based on idealized wind‐driven models. In this paper, we make use of the latest available hydrography and Mean Dynamic Topography to study the Arctic Ocean time‐mean circulation. Key concepts such as to what degree the flow is steered by bathymetry, equivalent barotropic, and consistent in direction along isobaths are evaluated. Comparing along‐ and cross‐isobath velocities, we find that while the former generally has a larger magnitude, they are locally comparable in many regions. In these regions, the estimated cross‐isobath velocities imply vertical velocities that can be orders of magnitude larger than typical surface Ekman pumping velocities. Even so, we find that the surface and bottom flow is generally well‐aligned along closed ambient potential vorticity contours spanning local basins as well as the entire Arctic Ocean. In some regions of the central Arctic Ocean, where the hydrographic coverage is relatively sparse, bathymetric alignment is stronger in the surface. Despite this, our results suggest that the circulation is essentially equivalent barotropic and even barotropic in some regions. At some locations where water enters or exits the deep Arctic Basin, along‐isobath flow reversals are observed at the surface as well as the bottom. Finally, the direction and magnitude of the bottom flow in regions of anti‐cyclonic surface circulation are found to be sensitive to the choice of data set.
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Sjur, Anna Lina Petruseviciute; Isachsen, Pål Erik; Johan, Nilsson; Lacasce, Joseph Henry & Ryseth, Magnus Dyrmose
(2025).
The Wind-Driven Time-Variable Circulation in the Arctic Mediterranean.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
130(4),
p. 1–20.
doi:
10.1029/2024JC021713.
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The Arctic Ocean is a key component of Earth's climate system, and an understanding of ocean dynamics in this region is central for predicting how the Arctic is responding to a changing climate. In this study, we examine the ocean circulation in a high-resolution numerical model of the Arctic Ocean and Nordic Seas. Based on what is observed in this simulation, we reexamine an existing idealized linear model estimating the time-variable large-scale circulation in ocean basins, and test it against the highly nonlinear numerical model. The idealized model is an integral relation derived from the linear momentum equations and assumes that the circulation around a closed depth contour is driven by surface stresses and regulated by bottom friction. We show that the idealized model estimates agree very well with the numerical simulations. This indicates that much of the variability of the large-scale circulation can be explained by linear processes. In particular, a correct description of the net surface stress over partially ice-covered areas improves the correlation between linear model and numerical simulations significantly in the Arctic Ocean compared to a previous study. However, undetected in that previous study, we now find that the linear model might be lacking a cyclonic tendency.
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Børve, Eli; Isachsen, Pål Erik; Nøst, Ole Anders; Ghaffari, Peygham & Falk-Petersen, Stig
(2025).
Tidal effects on transport and dispersion of particles, cod eggs and larvae in the Lofoten and Vesterålen region, Norway.
Frontiers in Marine Science.
12.
doi:
10.3389/fmars.2025.1541652.
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Lofoten and Vesterålen region in Northern Norway contains the main spawning areas for the Northeast Arctic (NEA) cod. A large embayment, partially sheltered from the continental slope and open ocean by the Lofoten-Vesterålen archipelago, called Vestfjorden contains approximately 60 % of the NEA cod spawning in this region. The dynamical ocean processes that control transport paths and transport times of cod eggs and larvae out of this embayment are of major importance, not only for the fish stock, but in general for the marine ecosystem in the region. This study investigates the net impact of nonlinear tidal dynamics on transport and dispersion of particles, resembling cod eggs and larvae, out of Vestfjorden. The coastal ocean circulation in and around Vestfjorden is simulated with a variable mesh model, both with and without tides. The two different flow fields are used to advect passive particles seeded near Henningsværstraumen, a key spawning location within the embayment. A comparison of the two transport calculations reveals that nonlinear tidal dynamics clearly impact the particle drift in the region. When including tides in the model simulation, transport through the various straits that cut through the Lofoten-Vesterålen archipelago becomes more important, causing the total particle drift out of Vestfjorden to increase by about 10%. One strait in particular, Moskstraumen, contributes with a significant transport (∼ 30%) in the model simulation with tides included, but other straits contribute as well. For comparison, in the simulation where tides are excluded, almost 90% of the particles are transported out of the embayment around the southern tip of the archipelago and only 6% through Moskstraumen. A key implication of the tidal transport through the straits is that the journey for a large fraction of the cod eggs/larvae from Vestfjorden to the shelf is considerably shortened.
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Sterl, Miriam F.; Palóczy, André; Groeskamp, Sjoerd; Baatsen, Michiel; Lacasce, Joseph Henry & Isachsen, Pål Erik
(2025).
The joint effects of planetary β, topography and friction on baroclinic instability in a two-layer QG model.
Journal of Fluid Mechanics.
ISSN 0022-1120.
1012.
doi:
10.1017/jfm.2025.10172.
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The quasi-geostrophic two-layer model is a widely used tool to study baroclinic instability in the ocean. One instability criterion for the inviscid two-layer model is that the potential vorticity (PV) gradient must change sign between the layers. This has a well-known implication if the model includes a linear bottom slope: for sufficiently steep retrograde slopes, instability is suppressed for a flow parallel to the isobaths. This changes in the presence of bottom friction as well as when the PV gradients in the layers are not aligned. We derive the generalised instability condition for the two-layer model with non-zero friction and arbitrary mean flow orientation. This condition involves neither the friction coefficient nor the bottom slope; even infinitesimally weak bottom friction destabilises the system regardless of the bottom slope. We then examine the instability characteristics as a function of varying slope orientation and magnitude. The system is stable across all wavenumbers only if friction is absent and if the planetary, topographic and stretching PV gradients are aligned. Strong bottom friction decreases the growth rates but also alters the dependence on bottom slope. In conclusion, the often mentioned stabilisation by steep bottom slopes in the two-layer model holds only in very specific circumstances, thus probably plays only a limited role in the ocean.
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Johnston, Shaun; Sjur, Anna Lina Petruseviciute; Isachsen, Pål Erik & Lacasce, Joseph Henry
(2025).
Eddy- and wind-driven circulation in the enclosed
basins of the Norwegian Sea evaluated using a model
and absolute geostrophic flow from Argo.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
130(7).
doi:
10.1029/2024JC021990.
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Cyclonic flow in enclosed basins arises from cyclonic vorticity input from either winds or eddies. Previous observations show winds account for about half of the circulation (integral of parallel velocity around a closed isobath) in the Lofoten Basin of the Norwegian Sea, implying a comparable contribution from eddies. In contrast, in the Norwegian Basin, winds explain much of the circulation. We use Argo data from 2012 to 2022, a linear wind-forced and bottom friction-regulated model, and a primitive equation model to investigate how winds and eddies drive the circulation. While Argo cannot resolve individual eddies, it does resolve mesoscale spatial variability. Absolute geostrophic velocities to 2,000 m are calculated by referencing geostrophic shear from objective maps of the profile data to the trajectories at 1,000 m. The time-mean shear is anticyclonic. However, the flow is mostly cyclonic, bottom-intensified, and isobath following. The linear model explains less than half of the observed circulation, suggesting a contribution from an unobserved eddy flux. This interpretation is supported by a numerical model with 800-m horizontal resolution, which resolves the 10–15-km deformation radius. In this model, the difference between the full and linear response comes from an eddy-driven vorticity flux, which produces a largely cyclonic circulation. This result is consistent with the observed global prevalence of positive topostrophy (anticyclonic flow around elevations and cyclonic flow around basins). The observations also show a seasonal shift in the eddy flux, which maybe related to a decadal-scale fresh inflow that alters the density gradient and influences eddy production.
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Matuszak, Mateusz; Röhrs, Johannes; Isachsen, Pål Erik & Idzanovic, Martina
(2025).
Uncertainties in the finite-time Lyapunov exponent in an ocean ensemble prediction model.
Ocean Science.
ISSN 1812-0784.
21(1),
p. 401–418.
doi:
10.5194/os-21-401-2025.
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Lagrangian coherent structures (LCSs) are transient features in the ocean circulation that describe particle transport, revealing information about transport barriers and accumulation or dispersion regions. The method of finite-time Lyapunov exponents (FTLEs) uses Lagrangian data to approximate LCSs under certain conditions. In this study FTLEs are used to characterize flow field features in a high-resolution regional ocean forecast system. Generally, trajectory simulations, such as Lagrangian trajectories, inherit uncertainty from the underlying ocean model, bearing substantial uncertainties as a result of chaotic and turbulent flow fields. As the FTLE characterizes the flow, which may impact particle transport, we aim to investigate the uncertainty of FTLE fields at any given time using an ensemble prediction system (EPS) to propagate velocity field uncertainty into the FTLE analysis. In addition, velocity fields often evolve rapidly in time, and we therefore also evaluate the time variability of FTLE fields. We find that averaging over ensemble members can reveal robust FTLE ridges, i.e., FTLE ridges that exist across ensemble realizations. Likewise, time averaging can reveal persistent FTLE ridges, i.e., ridges that occur over extended periods of time. In addition, large-scale FTLE ridges are more robust and persistent than small-scale ridges. Averaging of FTLE fields is thus effective at removing short-lived and unpredictable structures and may provide the means to employ FTLE analysis in forecasting applications that require the ability to separate uncertain from certain flow features.
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Nilsson, Johan; Kallmyr, Jan-Adrian H. & Isachsen, Pål Erik
(2024).
Topographic Steering of the Upper Arctic Ocean Circulation by Deep Flows.
Tellus A: Dynamic Meteorology and Oceanography.
ISSN 0280-6495.
76(1),
p. 206–226.
doi:
10.16993/tellusa.4072.
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Dynamically, the Arctic Ocean is characterised by the presence of closed f/H contours, where f is the Coriolis parameter and H the depth. On closed f/H contours, a net integrated surface wind stress can theoretically drive relatively strong near-bottom flows. Nevertheless, the Rossby number of the large-scale time-mean flow in the Arctic Ocean is estimated to be small, implying that the near-bottom flow should essentially be aligned with the f/H contours. Observations indicate that the time-mean surface flow also tends to follow the f/H contours, which in the Arctic are essentially controlled by H. To examine mechanisms that can organise the Arctic Ocean surface flow along the topography, we use a two-layer large-scale geostrophic model on an f-plane (exploiting that f/H variations are dominated by depth variations). The effect of time-dependent baroclinic eddies is represented as an eddy diffusion of the upper-layer thickness. We study how wind forcing, stratification, eddy diffusivity and bottom friction affect the topographic steering of the time-mean surface flow, introducing relevant non-dimensional parameters. The analyses suggest that the Arctic Ocean is in a regime where strong along-isobath near-bottom flows can align the buoyancy field and, thereby, the surface currents with the topography. We then discuss the model results in relation to satellite-derived surface currents in the Arctic Ocean and briefly consider additional mechanisms that can align surface flows with the topography.
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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.
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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.
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Nummelin, Aleksi & Isachsen, Pål Erik
(2024).
Parameterizing Mesoscale Eddy Buoyancy Transport Over Sloping Topography.
Journal of Advances in Modeling Earth Systems.
16(3).
doi:
10.1029/2023MS003806.
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Most of the ocean's kinetic energy is contained within the mesoscale eddy field. Models that do not resolve these eddies tend to parameterize their impacts such that the parameterized transport of buoyancy and tracers reduces the large-scale available potential energy and spreads tracers. However, the parameterizations used in the ocean components of current generation Earth System Models rely on an assumption of a flat ocean floor even though observations and high-resolution modeling show that eddy transport is sensitive to the potential vorticity gradients associated with a sloping seafloor. We show that buoyancy transport coefficient diagnosed from idealized eddy-resolving simulations is indeed reduced over both prograde and retrograde bottom slopes (topographic wave propagation along or against the mean flow, respectively) and that the reduction can be skilfully captured by a mixing length parameterization by introducing the topographic Rhines scale as a length scale. This modified “GM” parameterization enhances the strength of thermal wind currents over the slopes in coarse-resolution, non-eddying, simulations. We find that in realistic global coarse-resolution simulations the impact of topography is most pronounced at high latitudes, enhancing the mean flow strength and reducing temperature and salinity biases. Reducing the buoyancy transport coefficient further with a mean-flow dependent eddy efficiency factor, has notable effects also at lower latitudes and leads to reduction of global mean biases.
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Isachsen, Pål Erik; Vogt-Vincent, Noam S.; Johnson, Helen L. & Nilsson, Johan
(2024).
Instability and Mesoscale Eddy Fluxes in an Idealized 3-Layer Beaufort Gyre.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
129(8).
doi:
10.1029/2023JC020757.
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We study the impacts of a continental slope on instability and mesoscale eddy fluxes in idealized 3-layer numerical model simulations. The simulations are inspired by and mimic the situation in the Arctic Ocean's Beaufort Gyre, where anti-cyclonic winds drive anti-cyclonic currents that are guided by the continental slope. The forcing and currents are retrograde with respect to topographic Rossby waves. The focus of the analysis is on eddy potential vorticity (PV) fluxes and eddy-mean flow interactions under the Transformed Eulerian Mean framework. Eddy lateral vorticity fluxes dominate over the continental slope where eddy form stress, that is, vertical momentum flux, is suppressed due to the topographic PV gradient. The diagnosis also shows that while eddy momentum fluxes are up-gradient over parts of the slope, the total quasi-geostrophic PV flux is down-gradient everywhere. We then calculate the linearly unstable modes of the time-mean state and find that the most unstable mode contains several key features of the observed finite-amplitude fluxes over the slope, including down-gradient PV fluxes. When accounting for additional unstable modes, more qualitative features of the observed eddy fluxes in the numerical model are reproduced.
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Sterl, Miriam F.; Lacasce, Joseph Henry; Groeskamp, Sjoerd; Nummelin, Aleksi Henrynpoika; Isachsen, Pål Erik & Baatsen, Michiel L. J.
(2024).
Suppression of Mesoscale Eddy Mixing by Topographic PV Gradients.
Journal of Physical Oceanography.
ISSN 0022-3670.
54(5),
p. 1089–1103.
doi:
10.1175/JPO-D-23-0142.1.
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Oceanic mesoscale eddy mixing plays a crucial role in Earth’s climate system by redistributing heat, salt, and carbon. For many ocean and climate models, mesoscale eddies still need to be parameterized. This is often done via an eddy diffusivity
K
, which sets the strength of turbulent downgradient tracer fluxes. A well-known effect is the modulation of
K
in the presence of background potential vorticity (PV) gradients, which suppresses mixing in the direction of the PV gradients. Topographic slopes can induce such suppression through topographic PV gradients. However, this effect has received little attention, and topographic effects are often not included in parameterizations for
K
. In this study, we show that it is possible to describe the effect of topography on
K
analytically in a barotropic framework, using a simple stochastic representation of eddy–eddy interactions. We obtain an analytical expression for the depth-averaged
K
as a function of the bottom slope, which we validate against diagnos
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Espenes, Håvard; Isachsen, Pål Erik & Drivdal, Magnus Bjørn
(2024).
A numerical study of rectification driven by a tidal jet.
Estuarine, Coastal and Shelf Science.
ISSN 0272-7714.
302.
doi:
10.1016/j.ecss.2024.108774.
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The barotropic tide in fjords is often assumed to play a negligible role in driving the circulation over timescales greater than the tide itself, the residual circulation. However, this is not necessarily true when the tide interacts with the fjords’ topography. Of particular interest are narrow channels, where the tide can excite non-linear flow features such as flow separation eddies and establish a tidal jet. The jet and associated eddies are known to influence transport through the channel and dominate local dispersion. We investigate how the jet can influence the residual circulation in Kvænangen, a Northern Norwegian fjord. We model the flow dynamics using an unstructured grid ocean model, solving the shallow water equations. The model results show that the jet can drive residual currents around the fjords basin if the jet forces a vorticity flux across closed bathymetric contours. We demonstrate that the rectification is sensitive to how the channel is represented in the ocean model. With a wide channel, the fjord-scale residual circulation diminishes. Our work thus highlights the importance of accurately resolving the actual geometry of the coastline, since small-scale flow phenomena generated in narrow channels can force a far-field circulation response and consequently influence the drift of tracers in the fjord.
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Weber, Jan Erik H. & Isachsen, Pål Erik
(2023).
Energy transfer from sub-inertial Kelvin waves to continental shelf waves at a transverse bottom escarpment.
Continental Shelf Research.
ISSN 0278-4343.
258.
doi:
10.1016/j.csr.2023.104985.
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We consider barotropic coastally-trapped Kelvin waves of tidal origin along a straight coast that propagate from deeper water across a transverse bottom escarpment into a more shallow region. The component, which has a frequency above the inertial frequency, is modified by crossing the escarpment, but continues basically as a Kelvin wave into the shallow region. For tidal components that have frequencies smaller than, like, some of the energy in the Kelvin wave is converted into a continental shelf wave (CSW) that follows the escarpment with shallow water to the right (in the northern hemisphere). The theory is applied to the shelf north-west of Norway where the deep Norwegian Sea is separated from the much shallower Barents Sea by an escarpment reaching from the north-Norwegian coast towards Spitsbergen. By introducing an idealized bottom topography and coastal geometry, analytical calculations and numerical modeling are used to investigate the energy flux in CSWs towards Spitsbergen caused by sub-inertial Kelvin wave motion along the west coast of Norway. Both analytical and numerical calculations indicate that the CSW energy flux towards Spitsbergen is about eighty percent of the incident sub-inertial Kelvin wave energy flux, and that only twenty percent of the energy flux is transmitted into the Barents Sea in the form of sub-inertial Kelvin waves.
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Brown, Nicola; Mauritzen, Cecilie; Li, Camille; Madonna, Erica; Isachsen, Pål Erik & Lacasce, Joseph Henry
(2023).
Rapid Response of the Norwegian Atlantic Slope Current to Wind Forcing.
Journal of Physical Oceanography.
ISSN 0022-3670.
53(2),
p. 389–408.
doi:
10.1175/JPO-D-22-0014.1.
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We explore drivers of variability in the Norwegian Atlantic Slope Current, which carries relatively warm Atlantic Water toward the Barents Sea and Arctic Ocean, using Copernicus Marine Environment Monitoring Service (CMEMS) satellite altimetry data and TOPAZ4 ocean reanalysis data. Previous studies have pointed to a variety of causes, on a variety of time scales. We use data with daily resolution to investigate day-to-day changes in ocean transport across three sections crossing the shelf-slope of Norway (Svinøy, Gimsøy, and the Barents Sea Opening). The highest (lowest) extremes in transport at all sections develop over two days as a cyclonic (anticyclonic) atmospheric pressure system approaches from the southwest, piling up (extracting) water at the coast of Norway. The actual peak is reached when the pressure system passes the site of measurement, and the transport then relaxes for the next two days as the system continues northward along the coast. Other sources of short-term variability, such as propagating continental shelf waves and baroclinic instability, are unlikely to yield covariability over large separations. Monthly variability in the current can also be explained by passing weather systems since their numbers and intensity vary greatly from month to month. Many studies of longer-term variability, especially in the Barents Sea Opening, have pointed to the North Atlantic Oscillation (NAO) as the main cause of variability. Our results show that passing weather systems offer a better explanation of month-to-month variability.
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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),
p. 1633–1648.
doi:
10.5194/os-19-1633-2023.
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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.
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Dugstad, Johannes Sandanger; Isachsen, Pål Erik & Fer, Ilker
(2021).
The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations.
Ocean Science.
ISSN 1812-0784.
17(3),
p. 651–674.
doi:
10.5194/os-17-651-2021.
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Warm Atlantic-origin waters are modified in the Lofoten Basin in the Nordic Seas on their way toward the Arctic. An energetic eddy field redistributes these waters in the basin. Retained for extended periods, the warm waters result in large surface heat losses to the atmosphere and have an impact on fisheries and regional climate. Here, we describe the eddy field in the Lofoten Basin by analyzing Lagrangian simulations forced by a high-resolution numerical model. We obtain trajectories of particles seeded at three levels – near the surface, at 200 m and at 500 m depth – using 2D and 3D velocity fields. About 200 000 particle trajectories are analyzed from each level and each simulation. Using multivariate wavelet ridge analysis, we identify coherent cyclonic and anticyclonic vortices in the trajectories and describe their characteristics. We then compare the evolution of water properties inside cyclones and anticyclones as well as in the ambient flow outside vortices. As measured from Lagrangian particles, anticyclones have longer lifetimes than cyclones (16–24 d compared to 13–19 d), a larger radius (20–22 km compared to 17–19 km) and a more circular shape (ellipse linearity of 0.45–0.50 compared to 0.51–0.57). The angular frequencies for cyclones and anticyclones have similar magnitudes (absolute values of about 0.05f). The anticyclones are characterized by warm temperature anomalies, whereas cyclones are colder than the background state. Along their path, water parcels in anticyclones cool at a rate of 0.02–0.04 ∘Cd−1, while those in cyclones warm at a rate of 0.01–0.02 ∘Cd−1. Water parcels experience a net downward motion in anticyclones and upward motion in cyclones, often found to be related to changes in temperature and density. The along-path changes in temperature, density and depth are smaller for particles in the ambient flow. An analysis of the net temperature and vorticity fluxes into the Lofoten Basin shows that while vortices contribute significantly to the heat and vorticity budgets, they only cover a small fraction of the domain area (about 6 %). The ambient flow, including filaments and other non-coherent variability undetected by the ridge analysis, hence plays a major role in closing the budgets of the basin.
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Meneghello, Gianluca; Marshall, John; Lique, Camille; Isachsen, Pål Erik; Doddridge, Edward & Campin, Jean-Michel
[Show all 8 contributors for this article]
(2021).
Genesis and decay of mesoscale baroclinic eddies in the seasonally ice-covered Arctic Ocean.
Journal of Physical Oceanography.
ISSN 0022-3670.
51(1),
p. 115–129.
doi:
10.1175/JPO-D-20-0054.1.
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Abstract Observations of ocean currents in the Arctic interior show a curious, and hitherto unexplained, vertical and temporal distribution of mesoscale activity. A marked seasonal cycle is found close to the surface: strong eddy activity during summer, observed from both satellites and moorings, is followed by very quiet winters. In contrast, subsurface eddies persist all year long within the deeper halocline and below. Informed by baroclinic instability analysis, we explore the origin and evolution of mesoscale eddies in the seasonally ice-covered interior Arctic Ocean. We find that the surface seasonal cycle is controlled by friction with sea ice, dissipating existing eddies and preventing the growth of new ones. In contrast, subsurface eddies, enabled by interior potential vorticity gradients and shielded by a strong stratification at a depth of approximately 50 m, can grow independently of the presence of sea ice. A high-resolution pan-Arctic ocean model confirms that the interior Arctic basin is baroclinically unstable all year long at depth. We address possible implications for the transport of water masses between the margins and the interior of the Arctic basin, and for climate models’ ability to capture the fundamental difference in mesoscale activity between ice-covered and ice-free regions.
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Børve, Eli; Isachsen, Pål Erik & Nøst, Ole Anders
(2021).
Rectified tidal transport in Lofoten–Vesterålen, northern Norway.
Ocean Science.
ISSN 1812-0784.
17(6),
p. 1753–1773.
doi:
10.5194/os-17-1753-2021.
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Vestfjorden in northern Norway, a major spawning ground for the northeast Arctic cod, is sheltered from the continental shelf and open ocean by the Lofoten–Vesterålen archipelago. The archipelago, however, is well known for hosting strong and vigorous tidal currents in its many straits, currents that can produce significant time-mean tracer transport from Vestfjorden to the shelf outside. We use a purely tidally driven unstructured-grid ocean model to look into non-linear tidal dynamics and the associated tracer transport through the archipelago. Of particular interest are two processes: tidal pumping through the straits and tidal rectification around islands. The most prominent tracer transport is caused by tidal pumping through the short and strongly non-linear straits Nordlandsflaget and Moskstraumen near the southern tip of the archipelago. Here, tracers from Vestfjorden are transported tens of kilometers westward out on the outer shelf. Further north, weaker yet notable tidal pumping also takes place through the longer straits Nappstraumen and Gimsøystraumen. The other main transport route out of Vestfjorden is south of the island of Røst. Here, the transport is primarily due to tracer advection by rectified anticyclonic currents around the island. There is also an anticyclonic circulation cell around the island group Mosken–Værøy, and both cells have flow speeds up to 0.2 m s−1, magnitudes similar to the observed background currents in the region. These high-resolution simulations thus emphasize the importance of non-linear tidal dynamics for transport of floating particles, like cod eggs and larvae, in the region.
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Heiskanen, Tuomas Ilkka Henrikki; Graversen, Rune Grand; Rydsaa, Johanne Hope & Isachsen, Pål Erik
(2020).
Comparing wavelet and Fourier perspectives on the decomposition of meridional energy transport into synoptic and planetary components.
Quarterly Journal of the Royal Meteorological Society.
ISSN 0035-9009.
146(731 Part B),
p. 2717–2730.
doi:
10.1002/qj.3813.
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The Arctic region shows some of the world's most significant signs of climate change; for instance, a negative trend in summer sea‐ice cover of around 15% per decade and Arctic amplified surface‐air warming that is three times the global average. The atmospheric energy transport plays an important role in the Arctic climate. Recently a Fourier‐based method for studying the atmospheric energy transport contribution by planetary‐ and synoptic‐scale waves has been proposed. Recent studies based on this method show that planetary waves contribute more than synoptic waves to the atmospheric energy transport into the Arctic. However, this Fourier method suffers from being incapable of resolving spatially localized systems such as cyclones. Here an attempt to evaluate this problem is presented by applying the method on synthetic and reanalysis data. In addition, an alternative method based on a wavelet decomposition is proposed and compared with the Fourier‐based method. The wavelet method is based on localized basis functions which should be capable of resolving these localized systems. The wavelet method shows an impact of synoptic‐scale transport on Arctic temperatures which is not captured by the Fourier method, whilst the planetary‐scale effect of both methods appears similar.
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Trodahl, Marta; Isachsen, Pål Erik; Lilly, Jonathan M.; Nilsson, Johan & Kristensen, Nils Melsom
(2020).
The regeneration of the lofoten vortex through vertical alignment.
Journal of Physical Oceanography.
ISSN 0022-3670.
50(9),
p. 2689–2711.
doi:
10.1175/JPO-D-20-0029.1.
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Manucharyan, Georgy & Isachsen, Pål Erik
(2019).
Critical role of continental slopes in halocline and eddy dynamics of the Ekman-driven Beaufort Gyre.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
124(4),
p. 2679–2696.
doi:
10.1029/2018JC014624.
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The Beaufort Gyre (BG) is a large‐scale bathymetrically constrained circulation driven by a surface Ekman convergence that creates a bowl‐shaped halocline and stores a significant portion of the Arctic Ocean's freshwater. Theoretical studies suggest that in the gyre interior, the halocline is equilibrated by a balance between Ekman pumping and counteracting mesoscale eddy transport energized by baroclinic instability. However, the strongest anticyclonic flows occur over steep continental slopes, and, despite bathymetric slopes being known to influence baroclinic instability, their large‐scale impacts on BG halocline remain unexplored. Here we use an idealized eddy‐resolving BG model to demonstrate that the existence of continental slopes dramatically affects key gyre characteristics leading to deeper halocline, stronger anticyclonic circulation, and prolonged equilibration. Over continental slopes, the magnitude of the Eulerian mean circulation is dramatically reduced due to the Ekman overturning being compensated by the eddy momentum‐driven overturning. The eddy thickness flux overturning associated with lateral salt transport is also weakened over the slopes, indicating a reduction of eddy thickness diffusivity despite the isopycnal slopes being largest there. Using a theoretical halocline model, we demonstrate that it is the localized reduction in eddy diffusivity over continental slopes that is critical in explaining the halocline deepening and prolonged equilibration time. Our results emphasize the need for observational studies of eddy overturning dynamics over continental slopes and the development of slope‐aware mesoscale eddy parameterizations for low‐resolution climate models.
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Dugstad, Johannes Sandanger; Kozalka, Inga Monika; Isachsen, Pål Erik; Dagestad, Knut-Frode & Fer, Ilker
(2019).
Vertical structure and seasonal variability of the inflow to the Lofoten Basin inferred from high resolution Lagrangian simulations.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
124(12),
p. 9384–9403.
doi:
10.1029/2019JC015474.
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The Lofoten Basin in the eastern Nordic Seas plays a central role in modifying the warm Atlantic Water inflow toward the Arctic Ocean. Here, the Atlantic Water experiences increased residence times, cooling, and substantial transformation. In this study, we investigate the Atlantic Water inflow pathways to the Lofoten Basin and their vertical and seasonal variations using 2‐D and 3‐D Lagrangian simulations forced by a high‐resolution ocean model. Atlantic Water enters the basin from all directions, but we find two main inflow pathways at all vertical levels, one close to the Lofoten Escarpment in the southeast, associated with the Slope Current, and another close to the Helgeland Ridge in the southwest, associated with the Front Current. The surface inflow exhibits a stronger seasonal forcing than the inflow at depth as well as a stronger heat loss that is dominated by water masses entering the basin from the south. At deeper levels, the warm inflow from the east cools, while the relatively colder inflow from the west warms. The 2‐D and 3‐D synthetic trajectories show similar pathways. However, they are affected differently by the seasonal signal, giving different heat exchange patterns. Our results have implications for how results from Lagrangian observations in the region should be interpreted.
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Waldman, Robin; Somot, Samuel; Herrmann, Marine; Sevault, Florence & Isachsen, Pål Erik
(2018).
On the chaotic variability of deep convection in the Mediterranean Sea.
Geophysical Research Letters.
ISSN 0094-8276.
45(5),
p. 2433–2443.
doi:
10.1002/2017GL076319.
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Chaotic intrinsic variability is a fundamental driver of the oceanic variability. Its understanding is key to interpret observations, evaluate numerical models, and predict the future ocean and climate. Here we study intrinsic variability of deep convection in the northwestern Mediterranean Sea using an ensemble eddy‐resolving hindcast simulation over the period 1979–2013. We find that the variability of deep convection is mostly forced but also, to a considerable extent, intrinsic. The intrinsic variability can dominate the total convection variability locally and over a single winter. It also makes up a significant fraction of its interannual variability but has only modest impacts on the long‐term mean state. We find that the occurrence of deep convection is random 18% of years at the basin scale, and 29% locally at the LION observational site. Spatially, the intrinsic variability is highest far from the continental shelf. We relate this pattern to baroclinic instability theory that takes bottom stabilization into account.
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Trodahl, Marta & Isachsen, Pål Erik
(2018).
Topographic influence on baroclinic instability and the mesoscale eddy field in the northern North Atlantic Ocean and the Nordic Seas.
Journal of Physical Oceanography.
ISSN 0022-3670.
48(11),
p. 2593–2607.
doi:
10.1175/JPO-D-17-0220.1.
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Weakly stratified layers over sloping topography can support a submesoscale baroclinic instability mode, a bottom boundary layer counterpart to surface mixed layer instabilities. The instability results from the release of available potential energy, which can be generated because of the observed bottom intensification of turbulent mixing in the deep ocean, or the Ekman adjustment of a current on a slope. Linear stability analysis suggests that the growth rates of bottom boundary layer baroclinic instabilities can be comparable to those of the surface mixed layer mode and are relatively insensitive to topographic slope angle, implying the instability is robust and potentially active in many areas of the global oceans. The solutions of two separate one-dimensional theories of the bottom boundary layer are both demonstrated to be linearly unstable to baroclinic instability, and results from an example nonlinear simulation are shown. Implications of these findings for understanding bottom boundary layer dynamics and processes are discussed.
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Ghaffari, Peygham; Isachsen, Pål Erik; Nøst, Ole Anders & Weber, Jan Erik H.
(2018).
The influence of topography on the stability of the Norwegian Atlantic Current off Northern Norway.
Journal of Physical Oceanography.
ISSN 0022-3670.
48(11),
p. 2761–2777.
doi:
10.1175/JPO-D-17-0235.1.
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The steep continental slope off the Lofoten–Vesterålen islands of northern Norway appears to be the source of the most intense mesoscale eddy field in all of the Nordic Seas. Here we use linearized two-layer shallow-water equations to study the stability of the Norwegian Atlantic Current in this region. The study extends previous works that used linearized quasigeostrophic vertical mode equations to examine the effects of bottom topography on baroclinic instability here. We find evidence of baroclinic instability in the stacked shallow-water model but also of barotropic instability that is associated with the upper-layer lateral shear. The calculations give an indication that growth rates of barotropic instability may be comparable to or larger than those of baroclinic instability over the steepest parts of the continental slope.
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Bashmachnikov, I. L.; Sokolovskiy, M. A.; Belonenko, T. V.; Volkov, D. L.; Isachsen, P. E. & Carton, X.
(2017).
On the vertical structure and stability of the Lofoten vortex in the Norwegian Sea.
Deep Sea Research Part I: Oceanographic Research Papers.
ISSN 0967-0637.
128,
p. 1–27.
doi:
10.1016/j.dsr.2017.08.001.
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Hattermann, Tore; Isachsen, Pål Erik; Appen, Wilken-Jon von; Albretsen, Jon & Sundfjord, Arild
(2016).
Eddy-driven recirculation of Atlantic Water in Fram Strait.
Geophysical Research Letters.
ISSN 0094-8276.
doi:
10.1002/2016GL068323.
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Hattermann, Tore; Isachsen, Pål Erik; Appen, Wilken-Jon von; Albretsen, Jon & Sundfjord, Arild
(2016).
Eddy-driven recirculation of Atlantic Water in Fram Strait.
Geophysical Research Letters.
ISSN 0094-8276.
43(7),
p. 3406–3414.
doi:
10.1002/2016GL068323.
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Eddy‐resolving regional ocean model results in conjunction with synthetic float trajectories and observations provide new insights into the recirculation of the Atlantic Water (AW) in Fram Strait that significantly impacts the redistribution of oceanic heat between the Nordic Seas and the Arctic Ocean. The simulations confirm the existence of a cyclonic gyre around the Molloy Hole near 80°N, suggesting that most of the AW within the West Spitsbergen Current recirculates there, while colder AW recirculates in a westward mean flow south of 79°N that primarily relates to the eastern rim of the Greenland Sea Gyre. The fraction of waters recirculating in the northern branch roughly doubles during winter, coinciding with a seasonal increase of eddy activity along the Yermak Plateau slope that also facilitates subduction of AW beneath the ice edge in this area.
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Isachsen, Pål Erik
(2015).
Baroclinic instability and the mesoscale eddy field around the Lofoten Basin.
Journal of Geophysical Research (JGR): Biogeosciences.
ISSN 2169-8953.
120(4),
p. 2884–2903.
doi:
10.1002/2014JC010448.
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Isachsen, Pål Erik; Sørlie, Signe Røysland; Mauritzen, Cecilie; Lydersen, Christian; Dodd, Paul & Kovacs, Kit
(2014).
Upper-ocean hydrography of the Nordic Seas during the International Polar Year (2007-2008) as observed by instrumented seals and Argo floats.
Deep Sea Research Part I: Oceanographic Research Papers.
ISSN 0967-0637.
93,
p. 41–59.
doi:
10.1016/j.dsr.2014.06.012.
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Carrasco, Ana; Semedo, Alvaro; Isachsen, Pål Erik; Christensen, Kai H. & Saetra, Øyvind
(2014).
Global surface wave drift climate from ERA-40: The contributions from wind-sea and swell.
Ocean Dynamics.
ISSN 1616-7341.
64(12),
p. 1815–1829.
doi:
10.1007/s10236-014-0783-9.
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Ghaffari, Peygham; Isachsen, Pål Erik & Lacasce, Joseph Henry
(2013).
Topographic effects on current variability in the Caspian Sea.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
118(12),
p. 7107–7116.
doi:
10.1002/2013JC009128.
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Wang, Keguang; Debernard, Jens; Sperrevik, Ann Kristin; Isachsen, Pål Erik & Lavergne, Thomas
(2013).
A combined optimal interpolation and nudging scheme to assimilate OSISAF sea-ice concentration into ROMS.
Annals of Glaciology.
ISSN 0260-3055.
54(62),
p. 8–12.
doi:
10.3189/2013AoG62A138.
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Isachsen, Pål Erik; Drivdal, Magnus; Eastwood, Steinar; Gusdal, Yvonne; Noer, Gunnar & Saetra, Øyvind
(2013).
Observations of the ocean response to cold air outbreaks and polar lows over the Nordic Seas.
Geophysical Research Letters.
ISSN 0094-8276.
40(14),
p. 3667–3671.
doi:
10.1002/grl.50705.
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Isachsen, Pål Erik; Koszalka, I. & Lacasce, Joseph Henry
(2012).
Observed and modeled surface eddy heat fluxes in the eastern Nordic Seas.
Journal of Geophysical Research (JGR): Oceans.
ISSN 2169-9275.
117(C8).
doi:
10.1029/2012JC007935.
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Isachsen, Pål Erik
(2011).
Baroclinic instability and eddy tracer transport across sloping bottom topography: How well does a modified Eady model do in primitive equation simulations?
Ocean Modelling.
ISSN 1463-5003.
39,
p. 183–199.
doi:
10.1016/j.ocemod.2010.09.007.
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Bekkeby, Trine; Nilsson, H. C.; Olsgard, F.; Rygg, B.; Isachsen, Pål Erik & Isæus, Martin
(2008).
Identifying soft sediments at sea using GIS-modelled predictor variables and Sediment Profile Image (SPI)
measured response variables.
Estuarine, Coastal and Shelf Science.
ISSN 0272-7714.
79,
p. 631–636.
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Bekkeby, Trine; Isachsen, Pål Erik; Isæus, Martin & Bakkestuen, Vegar
(2008).
GIS modeling of wave exposure at the seabed: A depth-attenuated wave exposure model.
Marine Geodesy.
ISSN 0149-0419.
31,
p. 117–127.
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Isachsen, Pål Erik; Mauritzen, Cecilie & Svendsen, Harald
(2007).
Dense water formation in the Nordic Seas diagnosed from sea
surface buoyancy fluxes.
Deep Sea Research Part I: Oceanographic Research Papers.
ISSN 0967-0637.
54(1),
p. 22–41.
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Isachsen, Pål Erik; Lacasce, Joseph Henry & Pedlosky, Joseph
(2007).
Rossby wave instability and apparent phase speeds in large ocean basins.
Journal of Physical Oceanography.
ISSN 0022-3670.
37,
p. 1177–1191.
doi:
10.1175/JPO3054.1.
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Lacasce, Joseph Henry & Isachsen, Pål Erik
(2007).
On Sverdrup discontinuities and vortices in the Southwest Indian Ocean.
Journal of Physical Oceanography.
ISSN 0022-3670.
37,
p. 2940–2950.
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Nøst, Ole Anders & Isachsen, Pål Erik
(2003).
The large-scale time-mean ocean circulation in the Nordic Seas
and Arctic Ocean estimated from simplified dynamics.
Journal of Marine Research.
ISSN 0022-2402.
61(2),
p. 175–210.
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Stevens, Craig L.; Hurd, Catriona L. & Isachsen, Pål Erik
(2003).
Modeling Diffusion Boundary-Layers in Subtidal Macroalgal Canopies: The Response to Waves and Currents.
Aquatic Sciences.
ISSN 1015-1621.
65(1),
p. 81–91.
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Isachsen, Pål Erik; LaCasce, Joe H; Mauritzen, Cecilie & Hakkinen, Sirpa
(2003).
Wind-driven variability of the large-scale recirculating flow in the Nordic seas and Arctic Ocean.
Journal of Physical Oceanography.
ISSN 0022-3670.
33(12),
p. 2534–2550.
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Winther, Jan-Gunnar; Godtliebsen, Fred; Gerland, Sebastian & Isachsen, Pål Erik
(2002).
Surface albedo in Ny-Ålesund, Svalbard: variability and trends during 1981--1997.
Global and Planetary Change.
ISSN 0921-8181.
32,
p. 127–139.
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Isachsen, Pål Erik & Pond, Stephen
(2000).
The influence of the spring-neap tidal cycle on currents and
density in Burrard Inlet, British Columbia, Canada.
Estuarine, Coastal and Shelf Science.
ISSN 0272-7714.
51,
p. 317–330.
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Nummelin, Aleksi Henrynpoika & Isachsen, Pål Erik
(2023).
Improving simulations of the Arctic Ocean with a topography aware mesoscale eddy parameterization.
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Dugstad, Johannes Sandanger; Koszalka, I.; Dagestad, Knut-Frode; Fer, Ilker & Isachsen, Pål Erik
(2020).
Mass exchange of Atlantic Water with the Lofoten Basin derived from high-resolution Lagrangian simulations.
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Stige, Leif Christian; Kløver, Geir Olav; Dupont, Nicolas; Borgå, Katrine; Stenseth, Nils Christian & Isachsen, Pål Erik
[Show all 7 contributors for this article]
(2019).
Kunnskap fra isødet.
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Åpent arrangement med disse foredragene: "Kunnskapsinnhøsting i Arktis ombord i Fram 1893-1896" (Kløver); "Arven etter Nansen" (Stige); "Hva skjer med økosystemet når sjøisen forsvinner?" (Dupont); "Miljøgifter i Arktis" (Borgå); "Havstrømmene i Arktis – hvorfor er de spesielle?" (Isachsen); "En djerv polarforsker: Hanna Resvoll-Holmsen (1873-1943)" (Sandberg). Ordstyrere: Stige og Stenseth.
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Dugstad, Johannes Sandang; Koszalka, Inga; Isachsen, Pål Erik; Dagestad, Knut-Frode; Fer, Ilker & Lacasce, Joseph Henry
(2019).
Mass exchange of Atlantic Water with the Lofoten Basin derived from high resolution Lagragian simulations and Eulerian models.
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Simonsen, Magne; Isachsen, Pål Erik; Sætra, Øyvind; Lind, Ole Christian; Salbu, Brit & Skjerdal, Hilde Kristin
(2016).
Long-term dispersion of radioactive 99Tc from Sellafield - the impact of resolution.
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Røed, Lars Petter; Kristensen, Nils Melsom; Isachsen, Pål Erik & Sætra, Øyvind
(2013).
The triply nested Norwegian numerical ocean weather prediction system: Problems and possible solutions.
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Røed, Lars Petter; Brostrøm, Gøran; Isachsen, Pål Erik; Kristensen, Nils Melsom & Engedahl, Harald
(2011).
Numerical Ocean Weather Prediction: A quest for computer power and capacity.
META.
ISSN 1890-1956.
p. 28–31.
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To predict the drift and spreading of nutrients, algae blooms, oil and contaminants in the ocean, one of the key elements besides wind and waves is accurate predictions of ocean currents, temperature and salinity or “ocean weather”. The same is true regarding drift of floating objects such as ships, small crafts, containers, man overboard, icebergs and sea-ice. In addition such predictions are of crucial importance for the offshore industry, fisheries and for marine operations in general. The prediction of the ocean weather is, however, a continuous quest for more computer power and capacity.
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Simonsen, Magne; Lind, Ole Christian; Sætra, Øyvind; Isachsen, Pål Erik; Salbu, Brit & Strand, Per
(2019).
Marine transport modeling of radionuclides using a dynamic speciation approach.
Norges miljø- og biovitenskapelige universitet (NMBU).
ISSN 9788257516024.
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Det finnes mange kilder til antropogent utslipp av radionuklider til havet, slik som atmosfærisk nedfall, direkte utslipp fra atomkraftverk (enten via operasjonelle eller utilsiktede utslipp) og indirekte utslipp via elveavrenning gjennom estuarier. Videre blir radionuklidetransporten i havet påvirket av en rekke prosesser, som hydrodynamisk adveksjon, spredning og diffusjon, turbulent blanding, sedimentasjon og resuspensjon. Både transporten og toksisteten avhenger av fordelingen mellom de forskjellige tilstandsformene (speciene), og numeriske modeller bør derfor kunne beskrive de relevante formene og dynamiske overganger mellom disse for å forutsi hvordan radionuklider transporteres og hvordan de vil påvirke miljøet. Dette PhD-prosjektet tar opp temaer i krysningspunktet mellom fysisk oseanografi og radioøkologi. For å undersøke hvordan modellresultatene påvirkes av biogeokjemiske og geofysiske prosesser, ble et numerisk modellsystem utviklet og tatt i bruk. Et sett med casestudier ble gjennomført, både et historisk og et hypotetisk radionuklide-scenario og i tillegg et tilfelle med transport av et spormetall i et estuarie. Der det var mulig, ble modellresultatene sammenlignet med observasjonsdata. I Paper I ble det fokusert på hydrodynamiske prosesser, og det ble undersøkt hvordan inkludering av mesoskala virvler og tidevann påvirket modellens resultater i en langtids-simulering over 12 år av historiske utslipp av technetium-radionuklider fra Sellafield i Storbritannia. Sammenligning med observasjonsdata viste at økt horisontal oppløsning ga en klar forbedring av modellresultatene på grunn av at virvelfelt på mesoskala ble bedre oppløst. Påvirkningen fra tidevannsdrevne transportflukser ble undersøkt ved å sammenligne simuleringer hvor tidevannet var inkludert (1 times tidsoppløsning) med simuleringer der tidevannet var filtrert ut (døgnmidler). Sammenligningen viste en systematisk tidevannsdrevet drift av vannmassene i Irskesjøen, som også påvirket technetiumkonsentrasjonene over større avstander. Derfor ble det konkludert med at for å unngå systematiske feil, bør mesoskala virvler og tidevann også inkluderes i modellsimuleringer som skal beregne langtransport av radionuklider. Transportmodellen ble utvidet med modellkode som beskriver fordelingen og transformasjoner mellom radionuklidenes forskjellige tilstandsformer. Disse prosessene har vanligvis blitt neglisjert i tidligere modellstuder, men ved å inkludere disse ble det antatt at den totale usikkerheten ble redusert siden det gjorde modellen mer virkelighetsnær. Likevel vil slik modellutvikling også introdusere nye usikkerheter som er forbundet med nye parametere og algoritmer som trengs for å beskrive transformasjonsprosessene. Virkningen av sentrale prosesser ble undersøkt i et hypotetisk tilfelle med utslipp av 137Cs fra elver og spredning langs kysten (Paper II). Effekten av å inkludere interaksjoner med faste stoffer var betydelig, da forurensningsnivåene lokalt økte med opp mot en faktor 10. Transportberegningene viste seg å være sensitive overfor faktorer som påvirker partikkel-affiniteten, særlig fraksjonen som var ’sakte-reversibelt’ bundet til partikler men også til den antatte fordelingen av partikkelstørrelse, siden økt sedimentering også ga redusert transport vekk fra kildene og økt totaleksponering nær elvemunningene (opp til en faktor 10). Men i de fleste tilfeller er parameteriseringen av disse faktorene forbundet med stor usikkerhet. For å undersøke hvordan endrede eksterne miljøforhold påvirket specieringen, ble det utviklet et mer komplekst specieringsoppsett i Paper III. Her var overføringshastighetene avhengig av lokal salinitet i det omkringliggende vannet, kalibrert mot observert oppførsel av Al i Storelva og Sandnesfjorden. Modellen klarte fint å reprodusere de observerte trendene, både for totalkonsentrasjon av Al og for hver av de forskjellige tilstandsformene. Resultatene fra dette settet av casestudier viste at det fortsatt er stor usikkerhet knyttet til modellprognoser av radionuklidetransport. Dette skyldes blant annet manglende kunnskap om hvordan transformasjonsprosessene bør beskrives og vanskeligheter med å overføre målte verdier mellom forskjellige lokasjoner. Dette studiet har identifisert noen av de faktorene som bidrar mest til slike usikkerheter og hvordan disse påvirker transportestimatene. Ved å inkludere disse nøkkelfaktorene og -prosessene (tidevann, mesoskala virvler, speciering og transformasjoner) blir modellen forbedret. I de aktuelle tilfellene klarte modellen i all hovedsak å gjenskape de observerte konsentrasjonsnivåene. Videre kan dette modellsystemet videreutvikles og benyttes som et verktøy som vil være nyttig både til konsekvensutredninger, i nødsituasjoner og til forskningsformål.
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Published
Jan. 13, 2016 4:13 PM
- Last modified
Nov. 14, 2022 3:16 PM
