Anna Lina Petruseviciute Sjur

• 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), s. 3055–3078. doi: 10.5194/os-22-3055-2026. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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.

• 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.

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• Sjur, Anna Lina Petruseviciute (2026). From Winds to Whirls: Dynamics that shape Arctic Ocean circulation. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  As the ocean moves, it transports heat, salt, nutrients, and pollution over vast distances. The ocean currents thus have the power to melt sea ice, support ecosystems, and shape both local and global climates. In her doctoral research, Anna Lina P. Sjur investigates how winds and rotating water masses, known as whirls or eddies, work together to shape the currents around the deep ocean basins of the Arctic Ocean. Currents at high latitudes, such as in the Arctic Ocean, are particularly influenced by the shape of the seafloor. Here, the current systems closely follow the continental slopes and underwater mountain ridges, forming closed loops around the basins. Existing theory predicts that as winds blowing over the ocean increase in strength, these currents speed up until bottom friction balances the wind's contribution. This doctoral work by Anna Lina P. Sjur highlights that while this theory often provide a good framework, it does not capture the full picture of the currents dynamics. Whirls interacting with the uneven seafloor can also play a significant role in shaping the currents according to findings in this doctoral work. The impact of the interaction is to drive the flow in an counterclockwise direction. As sea ice is rapidly retreating in the Arctic, stronger wind influences and more whirls are expected to come in the future. This underscores the need to better understand how winds and whirls influence ocean currents. Such knowledge is crucial for monitoring the impacts of these currents on the climate system and ecosystems, as well as for improving ocean predictions.

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