TopArctic – Topografisk kontroll av havsirkulasjonen i ArktisOcean

• 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), s. 1–13. doi: 10.1088/2515-7620/ae68e7. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Hordoir, Robinson; Jahanmard, Vahidreza; Isachsen, Pål Erik; Löptien, Ulrike; Dietze, Heiner & Sandø, Anne Britt [Vis alle 7 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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), s. 1–20. doi: 10.1029/2024JC021713. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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), s. 401–418. doi: 10.5194/os-21-401-2025. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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), s. 206–226. doi: 10.16993/tellusa.4072. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Hörstmann, Cora; Hattermann, Tore; Thomé, Pauline C.; Buttigieg, Pier Luigi; Morel, Isidora & Waite, Anya M. [Vis alle 7 forfattere av denne artikkelen] (2024). Biogeographic gradients of picoplankton diversity indicate increasing dominance of prokaryotes in warmer Arctic fjords. Communications Biology. 7. doi: 10.1038/s42003-024-05946-8. Fulltekst i vitenarkiv Vis sammendrag

  Climate change is opening the Arctic Ocean to increasing human impact and ecosystem changes. Arctic fjords, the region’s most productive ecosystems, are sustained by a diverse microbial community at the base of the food web. Here we show that Arctic fjords become more prokaryotic in the picoplankton (0.2–3 µm) with increasing water temperatures. Across 21 fjords, we found that Arctic fjords had proportionally more trophically diverse (autotrophic, mixotrophic, and heterotrophic) picoeukaryotes, while subarctic and temperate fjords had relatively more diverse prokaryotic trophic groups. Modeled oceanographic connectivity between fjords suggested that transport alone would create a smooth gradient in beta diversity largely following the North Atlantic Current and East Greenland Current. Deviations from this suggested that picoeukaryotes had some strong regional patterns in beta diversity that reduced the effect of oceanographic connectivity, while prokaryotes were mainly stopped in their dispersal if strong temperature differences between sites were present. Fjords located in high Arctic regions also generally had very low prokaryotic alpha diversity. Ultimately, warming of Arctic fjords could induce a fundamental shift from more trophic diverse eukaryotic- to prokaryotic-dominated communities, with profound implications for Arctic ecosystem dynamics including their productivity patterns

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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), s. 1089–1103. doi: 10.1175/JPO-D-23-0142.1. Fulltekst i vitenarkiv Vis sammendrag

  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

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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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