We investigate the turbulence kinetic energy (TKE) budget of the upper ocean and its response to wind and wave forcing in the Southern Ocean, using realistically forced large-eddy simulations (LESs) and in situ microstructure shear observations. The widely used law of the wall similarity scaling assumes that shear production of TKE is balanced by its dissipation. However, our findings reiterate that this assumption is violated under wave forcing: Dissipation is primarily balanced by local Stokes shear production, augmented by almost equal contributions from local Eulerian shear production and nonlocal convergence of TKE transport. Despite this, the canonical law of the wall scaling reasonably describes the vertical distribution of dissipation rates in the boundary layer in both realistically forced LES and observations, even though the underlying physical reasoning does not hold. We propose a modified scaling to account for the nonlocal component of the TKE budget that yields accurate predictions of the TKE budget as well as an interpretation accounting for wave effects. These insights have important implications for interpreting turbulence dissipation rate observations. SIGNIFICANCE STATEMENT: This study provides and evaluates a physical model of turbulence in the near-surface layer of the Southern Ocean under strong winds and waves. This turbulence model quantifies how the vertical transport and dissipation of turbulence relate to the production of turbulence due to winds and waves. This model of turbulence is valuable because the Southern Ocean plays a key role in regulating climate through the uptake of heat and carbon from the atmosphere, which is mediated by small-scale turbulent motions that generally must be estimated without turbulence observations in global ocean and climate simulations. In addition, this model provides insight into dissipation rate observations, which are generally collected without observations of the corresponding turbulence production and transport.

On the continental slope north of Svalbard, the boundary current carrying Atlantic Water (AW) loses heat as it flows eastward. This cooling cannot be fully attributed to atmospheric heat loss or turbulent mixing. Lateral exchange, potentially linked to mesoscale activity, has previously been proposed as a contributing factor, based on limited observations of eddies. Here, we analyse a year-long dataset of hydrography and velocity observations from two mooring arrays, supplemented by output from an eddy-resolving ocean model, to quantify the seasonal variability of eddy kinetic energy (EKE) and baroclinic and barotropic energy conversion rates over time-scales from days to months. Both EKE and conversion rates peak in autumn and winter, coinciding with the strongest boundary current and the warmest AW. Local EKE variability, however, is only weakly associated with conversion rates, suggesting advection from upstream generation sites or unresolved variability from limited measurements. Conversion is mainly baroclinic, through boundary current instability, providing conditions favourable for offshore propagation of warm-core eddies. Modelled conversion rates have a complex spatial structure with substantial values on the offshore, deeper side of the boundary current with comparable contributions from baroclinic and barotropic processes. Resulting mesoscale activity enhances lateral stirring and heat loss from the boundary current, particularly in winter and spring, contributing to the along-stream cooling of AW.

Abstract Sea ice is a crucial component of polar climate systems and is undergoing substantial changes in both hemispheres due to evolving climatic conditions. Arctic sea ice is transitioning from perennial to seasonal cover, and the Southern Ocean sea ice is exhibiting recent minima and enhanced seasonality. As global warming continues, the role of sea ice in polar climate systems is expected to transform further. However, many theoretical frameworks and parameterizations in current sea‐ice models are based on observations from an earlier era dominated by thicker multi‐year ice. Here, we synthesize the physical processes governing the dynamics and thermodynamics of drift ice—the mobile pack ice—and its coupling with the atmospheric and oceanic boundary layers. Our goal is to provide a coherent theoretical framework of the sea‐ice evolution equations and to summarize parameterizations of sub‐grid processes used across models of varying complexity. These include representations of momentum and scalar fluxes, ice‐thickness distribution and redistribution, snow and melt‐pond processes, wave–ice interactions, and physical–biogeochemical feedbacks. We also examine how sea ice impacts ocean stratification and mixing, as well as the atmospheric boundary layer and clouds. Finally, we highlight recent observational findings and outline priorities for improving the representation of drift‐ice processes, particularly in light of the changing climate and ice state.

We present transport estimates of Atlantic Water (AW) and Recirculating Atlantic Water (RAW) across a zonal transect at 77 degrees 15 ' N using repeated ocean glider observations. Over three missions during autumn and winter of 2020 to 2022, 22 high-resolution sections were collected, enabling detailed characterization of circulation branches and volume transport. On average, the West Spitsbergen Current (WSC) and the Front Current each transport approximately 2.5Sv of AW (Theta > 2 degrees C, S-A > 35.06g kg(-1)) northward, yielding a combined net flux of about 5Sv toward the Arctic. Variability in transport and current structure is substantial and appears linked to atmospheric forcing. Case studies reveal that anomalous northward wind stress coincides with peak AW transport, roughly twice the seasonal mean, consistent with Ekman dynamics and elevated sea surface height along the coast. Conversely, strong southward wind stress weakens the WSC and nearly eliminates the Front Current. Transport of RAW (Theta > 0 degrees C, S-A > 35.06g kg(-1)) west of the Front Current is estimated to be about 1Sv, but this does not capture the expected stronger recirculation transport further west, beyond the glider's target transect. These results highlight the capability of gliders to resolve spatial variability in boundary currents that mooring arrays cannot capture. Extended seasonal coverage, including summer, is needed to assess transport variability under peak wind forcing.

Warm, saline Atlantic waters and fresher Arctic-origin waters converge in the central Nordic Seas, creating strong mesoscale and submesoscale variability that influences both hydrography and sound propagation. During the Northern Ocean Rapid Surface Evolution 2022 experiment, high-resolution temperature and salinity measurements were collected using shipboard profiling and drifting Wirewalker systems, alongside acoustic transmissions in the 500-1,500 Hz band. Water-mass structure was characterized using two-dimensional temperature-salinity histograms, which revealed distinct surface and intermediate water types and their spatial and temporal evolution. Two processes dominated variability: topographically trapped internal tides over the East Jan Mayen Ridge and a propagating anticyclonic eddy composed of modified Atlantic water. Both features produced measurable shifts in the depth and thickness of the intermediate Atlantic water layer, which forms a regional sound channel. These hydrographic changes led to predictable modulation of waterborne acoustic arrival patterns and, in the case of the eddy, downward refraction strong enough to eliminate the waterborne path through bathymetric blocking. The results demonstrate how evolving water-mass structure in high-latitude frontal systems directly governs mid-frequency acoustic propagation, with implications for acoustic observing, environmental prediction, and interpretation of variability in Arctic-influenced basins.

Nitrogen constrains biomass across the Arctic Ocean, with nitrate (NO3) supply to the surface waters fuelling new primary production and net carbon drawdown. In this Review, we explore the physical mechanisms driving NO3 fluxes to the euphotic zone across the Arctic Ocean and how biological processes respond. The volume and inflow depth of Atlantic and Pacific Ocean waters, together with sea ice and halocline dynamics, govern internal physical mixing of NO3. Respectively, these inflows supply ~34 ± 5 kmol NO3 s−1 and 9 ± 1 kmol NO3 s−1, spreading at mid-depth. NO3 from below the euphotic zone is mixed upwards via several mechanisms. Overall, NO3 fluxes associated with diffusive and turbulent mixing, submesoscale fronts and cyclonic mesoscale eddies are relatively low (on the order of ~0.1–0.7 mmol m−2 per day) but cover a large area, with peaks associated with wind events or individual strong eddies. By comparison, upwelling-driven fluxes are much stronger (on the order of ~1 mmol m−2 per day) but are more localized. Near-inertial and tidal mixing over the Arctic Ocean’s complex bathymetry drives perhaps the strongest NO3 fluxes, for example, reaching 4.5 mmol m−2 per day in the Barents Sea. Comparing these fluxes with observed biological NO3 uptake rates indicates that the internal physical supply of NO3 only limits primary productivity in 9 of the 17 cases considered. Thereafter, light limitation and lagged growth responses can result in excess NO3 remaining in the surface waters. Future research should prioritize linking NO3 supply and uptake at corresponding spatiotemporal scales. Vertical nitrate fluxes to sunlit surface waters have an important role in supporting primary production in the Arctic Ocean. This Review explores the contributions of various physical mixing mechanisms in supplying nitrate to the Arctic euphotic zone and compares them to the biological uptake rates.

During Arctic summer, meltwater inputs and a fragmented ice cover impede quantifying the role of boundary stress for turbulent mixing in the ice-ocean boundary layer. Here, we show that less than two-thirds of the turbulent kinetic energy (TKE) generated from mean flow shear under drifting sea ice is dissipated, and the remainder can be attributed to balancing stabilizing buoyancy fluxes. We deployed a high-resolution acoustic Doppler current profiler under an ice floe to estimate Reynolds stress, shear production, and dissipation rate of TKE. At 0.75 m below the interface, dissipation rates from 1.5 x 10(-9) to 4.2 x 10(-7) m(2) s(-3) and shear production from 6.9 x 10(-10) to 7.7 x 10(-7) m(2) s(-3) were measured (5%-95% percentiles), with shear production exceeding dissipation on average. The turbulent stress was largest during an event with similar to 9.2-h-period oscillations in the upper ocean, consistent with tidally forced lee waves generated near steep topography. An overall estimate of the quadratic skin drag coefficient representative of the ice floe is CD0=7.0x10-4 . We further identified three qualitative regimes of atmosphere-ice-ocean coupling in our observations: a high-frequency range [>4 cycles per day (cpd)] in which the ice acts like a rigid lid atop the ocean, an intermediate range, and a low-frequency range (<0.8 cpd), where wind-driven ice drift determines the under-ice current. As the latter only contained half of the variance of the ice-relative flow, we emphasize that resolving subdaily time scales is crucial in observing and modeling atmosphere-ice-ocean coupling.

Tides are an important factor shaping the sea ice system in the Arctic Ocean, by altering vertical heat fluxes and advection patterns. Unfortunately, observations are sparse and the analysis of tides is complicated by the proximity of wind-driven inertial oscillations to the semi-diurnal frequencies. Furthermore, computational costs typically prohibit the inclusion of tides in ocean models, leaving a significant gap in our understanding. Motivated by summer observations showing elevated downward surface heat fluxes in the presence of tides, we analyzed simulations carried out with an eddy-permitting coupled ice-ocean model to quantify the impact of tidal effects on Arctic sea ice. In line with previous studies, we find an overall decrease in sea ice volume when tides are included in the simulations, associated with increased vertical mixing and the upward flux of heat from deeper layers of the Arctic Ocean, but this sea ice volume decrease is less pronounced than previously thought. Surprisingly, our simulations suggest that in summer, Arctic sea ice area is larger, by up to 1.5\%, when tides are included in the simulations. This effect is partly caused by an increased downward surface heat flux and a consequently lower sea surface temperature, delaying sea ice melting predominantly in the Siberian Seas, where tides are moderately strong and the warm Atlantic Water core is located relatively deep and does not encroach on the wide continental shelf. Here, tidally enhanced downward heat flux from the surface in summer can dominate over the increased upward heat flux from the warm Atlantic Water layer.

The recent sea‐ice retreat in the western Nordic Seas has exposed the ocean to the atmosphere in winter, thereby facilitating dense‐water formation. Here, we present a 2‐year long record (2016–2018) of ocean stratification and currents from the northwestern Iceland Sea, which was obtained from a mooring deployed in Eggvin Offset, an approximately 1,500 m deep passage between the Greenland and Iceland Seas. The trajectory of an Argo float deployed in winter 2017/2018 indicates a connection between Eggvin Offset and the slope north of Iceland, where the boundary currents that supply the overflows east and west of Iceland originate. However, the low transport of potential overflow water ( 27.8 kg m −3 ) through Eggvin Offset demonstrates that it is not a major passage for the exchange of dense water. Dense‐water formation occurred during both winter 2016/2017 and winter 2017/2018; the mixed layer cooled and deepened mainly as an integrated response to a succession of cold‐air outbreaks. Greater turbulent heat fluxes and a more even distribution of cold‐air outbreaks at the mooring site in winter 2016/2017 resulted in mixed‐layer depths reaching a maximum of approximately 450 m, compared to 350 m the following winter. The water formed in Eggvin Offset those recent winters attained densities similar to those of water formed in the central Iceland Sea four decades ago. This supports the notion that in a warming climate, the locus of dense‐water formation has shifted from the interior basin to the western Iceland Sea.

The warm and saline Atlantic Water has long been recognized as being subjected to substantial heat loss during its transit towards the polar regions. In particular, the Lofoten Basin, a subpolar sea with energetic eddy activity and strong air-sea interactions, plays a crucial role in the transformation of Atlantic Water. Vertical heat transport at submesoscales (0.1-10 km) in the Lofoten Basin is potentially a key link in the heat transfer to the atmosphere. Here, based on multi-year Seaglider observations augmented by satellite altimeters, radiometers, and high-resolution numerical model results, we evaluate the oceanic vertical heat transport in the Lofoten Basin and demonstrate how geostrophic strain enhances heat transport. The enhancement is found to be associated with submesoscale ageostrophic motions along the mesoscale eddy edges, occurring on spatial scales smaller than 10 km and below the mixed layer depth. These strain-induced submesoscale vertical motions transport heat from the ocean interior to the surface, leading to a 0.4 °C increase in sea surface temperature and the formation of "warm ring" structures in both cyclones and anticyclones. The dominant role of submesoscale heat transport likely represents the primary mechanism for substantial heat loss from Atlantic Water in the Lofoten Basin.

The Lofoten Vortex (LV) is an intense, apparently permanent anticyclone in the Lofoten Basin of the Norwegian Sea. It is characterised by a 1200 m thick core of Atlantic Water, with a radius of 15-20 km, in nearly solid-body rotation, reaching speeds up to 0.8 ms-1. Potential vorticity in the core is nearly 2 orders of magnitude lower than the surroundings, creating a barrier to lateral mixing. It has previously been postulated that anticyclonic eddies in the Lofoten Basin, shed from the eastern branch of the Norwegian Atlantic Current along the Lofoten Escarpment, merge into the LV, contributing to maintaining its large heat and salt content and energetics, but such merging events have proven to be difficult to observe directly due to their transient and unpredictable nature. In April 2023, an eddy merger event was successfully observed using a combination of in situ data from an autonomous ocean glider and absolute dynamic topography (and derived velocities) from the fast sampling calibration phase of the Surface Water Ocean Topography (SWOT) satellite altimeter. During the observed merging process, an incoming eddy gradually approaches the LV and then elongates as the two begin to co-rotate and then merge, with a corresponding spin-up of vorticity and eddy kinetic energy and possible ejection of water of low potential vorticity from the merged LV core. The incoming eddy had a smaller radius and higher Rossby number than the LV. It has a similar density range to the LV, and, therefore, a double-core vertical structure did not form after the merger. During the observed period, merging eddies were the dominant process affecting the evolution of the LV, clearly outweighing vertical 1D processes due to atmospheric forcing and lateral mixing between the LV core and the outer rim. Through the influx of buoyant waters, spin-up of eddy kinetic energy and increasingly anticyclonic vorticity, eddy mergers contribute to the longevity of the LV.

The Barents Sea, an important component of the Arctic Ocean, is experiencing changes in its ocean currents, stratification, sea ice variability, and marine ecosystems. Inflowing Atlantic Water (AW) is a key driver of these changes. As AW predominantly enters the Barents Sea via the Barents Sea Opening, other pathways remain relatively unexplored. Comparisons of summer climatology fields of temperature from the last century with those from 2000–2019 indicate warming in the Storfjordrenna trough and along two shallow banks, Hopenbanken and Storfjordbanken, within the Svalbard Archipelago. Additionally, they indicate shoaling of AW that extends further into the “channel” between the islands of Edgeøya and Hopen. This region emerges as a pathway enabling AW to enter the northwestern Barents Sea. Moreover, 1-year-long records from a mooring deployed between September 2018 and November 2019 at the saddle of this channel show the flow of Atlantic-origin waters into the Arctic domain of the northwestern Barents Sea. The average current is directed eastwards into the Barents Sea and exhibits significant variability throughout the year. Here, we investigate this variability on timescales ranging from hours to months. Wind forcing mediates currents, water exchange, and heat exchange through the channel by driving geostrophic adjustment to Ekman transport. The main drivers of the warm-water inflow and across-saddle transport of positive temperature anomalies include persistently strong semidiurnal tidal currents, intermittent wind-forced events, and wintertime warm-water intrusions forced by upstream conditions. We propose that similar topographic constraints near AW pathways may become more important in the future. Ongoing warming and shoaling of AW, coupled with changes in large-scale weather patterns, are likely to increase warm-water inflow and heat transport through the processes identified in this study.

This paper presents a methodological tool for dynamic reconstruction of the state of the ocean, based, as an example, on observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) experiment. The data used in this study were collected in the Amundsen Basin between October 2019 and January 2020. Analysing observational data to assess tracer field and upper-ocean dynamics is highly challenging when measurement platforms drift with the ice pack due to continuous drift speed and direction changes. We have equipped the new version of the coastal branch of the global Finite-volumE sea ice–Ocean Model (FESOM-C) with a nudging method. Model nudging was carried out assuming a quasi-steady state. Overall, the model can reproduce the lateral and vertical structure of the temperature, salinity, and density fields, which allows for projecting dynamically consistent features of these fields onto a regular grid. We identify two separate depth ranges of enhanced eddy kinetic energy located around two maxima in buoyancy frequency: the depth of the upper halocline and the depth of the warm (modified) Atlantic Water. Simulations reveal a notable decrease in surface layer salinity and density in the Amundsen Basin towards the north but no significant gradient from east to west. However, we find a mixed-layer deepening from east to west, with a 0.084 m km−1 gradient at 0.6 m km−1 standard deviation, compared to a weak deepening from south to north. The model resolves several stationary eddies in the warm Atlantic Water and provides insights into the associated dynamics. The model output can be used to further analyse the thermohaline structure and related dynamics associated with mesoscale and submesoscale processes in the central Arctic, such as estimates of heat fluxes or mass transport. The developed nudging method can be utilized to incorporate observational data from a diverse set of instruments and for further analysis of data from the MOSAiC expedition.

Sea ice cover influences the generation of surface ocean turbulence in ways that sometimes enhance, but mostly inhibit air-water gas exchange. Inhibition happens as ice cover reduces wind fetch, enhancement occurs when haline convection or sea ice drift creates additional surface turbulence. We used the bulk turbulence relationships within the Wave Age Gas Transfer model to estimate air-sea gas transfer velocity (k(WAGT)), based on sea ice cover and turbulence conditions in the ice-ocean boundary layer, throughout a year-long (2019-2020) ice drift campaign in the central Arctic Ocean. During the drift, sea ice cover averaged >97%, with a minimum of 58%, and boundary layer shear played a dominant role in the turbulence budget. Modeled turbulent kinetic energy dissipation was compared against 167 in-situ profiles of ocean dissipation to evaluate model performance and explore related processes. The modeled dissipation and observed dissipation profiles, averaged over 0-4 m depth, agreed within 1% of each other, with a mean dissipation of 5.8 x 10(-7) W kg(-1). Examining individual dissipation estimates by surface conditions, however, revealed poorest agreement in leads, especially leads covered by thin ice, which the model cannot detect. Dissipation from the model was used to produce a time series of k(WAGT), revealing an average velocity of 0.034 m d(-1) or 1% of the global average for the open ocean. Comparison with a widely used wind speed parameterization for gas exchange showed that wind speed scaling would overestimate k during 92% of the drift by 3.5 times on average, demonstrating how fetch limitation can suppress gas exchange, even as open water increases. These results suggest that photic zone processes, under-ice blooms, and attendant cycling of CO2 and O-2 as well as CH4 can remain isolated from the atmosphere for an entire annual cycle in the central Arctic.

As a part of the Scientific Committee on Oceanographic Research (SCOR) Working Group #160 “Analyzing ocean turbulence observations to quantify mixing” (ATOMIX), we have developed recommendations on best practices for estimating the rate of dissipation of kinetic energy, ε, from measurements of turbulence shear using shear probes. The recommendations provided here are platform-independent and cover the conceivable range of dissipation rates in the ocean, seas, and other natural waters. They are applicable to commonly deployed platforms that include vertical profilers, fixed and moored instruments, towed profilers, submarines, self-propelled ocean gliders, and other autonomous underwater vehicles. The procedure for preparing the shear data for spectral estimation is discussed in detail, as are the quality control metrics that should accompany each estimate of ε. The methods are illustrated using a high-quality ‘benchmark’ dataset, while potential pitfalls are demonstrated with a second dataset containing common faults.

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, 2019--2020), a year-long drift with the Arctic sea ice, has provided the scientific community with an unprecedented, multidisciplinary dataset from the Eurasian Arctic Ocean, covering high atmosphere to deep ocean across all seasons. However, the heterogeneity of data and the superposition of spatial and temporal variability, intrinsic to a drift campaign, complicate the interpretation of observations. In this study, we have compiled a quality-controlled physical hydrographic dataset with best spatio-temporal coverage and derived core parameters, including the mixed layer depth, heat fluxes over key layers, and friction velocity. We provide a comprehensive and accessible overview of the ocean conditions encountered along the MOSAiC drift, discuss their interdisciplinary implications, and compare common ocean climatologies to these new data. Our results indicate that, for the most part, ocean variability was dominated by regional rather than seasonal signals, carrying potentially strong implications for ocean biogeochemistry, ecology, sea ice, and even atmospheric conditions. Near-surface ocean properties were strongly influenced by the relative position of sampling, within or outside the river-water influenced Transpolar Drift, and seasonal warming and meltwater input. Ventilation down to the Atlantic Water layer in the Nansen Basin allowed for a stronger connectivity between subsurface heat and the sea ice and surface ocean via elevated upward heat fluxes. The Yermak Plateau and Fram Strait regions were characterized by heterogeneous water mass distributions, energetic ocean currents, and stronger lateral gradients in surface water properties in frontal regions. Together with the presented results and core parameters, we offer context for interdisciplinary research, fostering an improved understanding of the complex, coupled Arctic System.

Glaciers in the Arctic have lost considerable mass during the last two decades. About a third of the glaciers by area drains into the ocean, yet the mechanisms and drivers governing mass loss at glacier calving fronts are poorly constrained in part due to few long-term glacier-ocean observations. Here, we combine a detailed satellite-based record of calving front ablation for Austfonna, the largest ice cap on Svalbard, with in-situ ocean records from an offshore mooring and modelled freshwater runoff for the period 2018-2022. We show that submarine melting and calving occur almost exclusively in autumn for all types of outlet glaciers, even for the surging and fast-flowing glacier Storisstraumen. Ocean temperature controls the observed frontal ablation, whereas subglacial runoff of surface meltwater appears to have little direct impact on the total ablation. The seasonal warming of the offshore waters varies both in magnitude, depth and timing, suggesting a complex interplay between inflowing Atlantic-influenced water at depth and seasonally warmed surface water in the Barents Sea. The immediate response of frontal ablation to seasonal ocean warming suggests that marine-terminating glaciers in high Arctic regions exposed to Atlantification are prone to rapid changes that should be accounted for in future glacier projections.