Abstract The Arctic is undergoing rapid and disproportionate climate change, driven by tightly coupled interactions among the ocean, sea ice, and atmosphere. This review synthesizes current understanding of historical and projected changes in the Arctic ocean–ice system, emphasizing the role of variability across timescales—from seasonal to multidecadal—in shaping observed trends. We highlight the interconnected nature of Arctic system components, focusing on feedbacks—including emerging coupled ocean–ice processes—pathways, and mechanisms that link variability and long‐term change. Particular attention is given to state‐dependent and potentially nonlinear responses, as well as to the roles of remote forcing and increasing connectivity with sub‐Arctic regions. We assess the relative contributions of internal variability and anthropogenic forcing, highlighting key challenges in attribution, and identify priorities for improving future projections. This synthesis provides a process‐based framework for understanding Arctic change and its growing influence on the global climate system.

The Arctic Ocean is undergoing rapid change, with freshwater playing a central role in shaping stratification, vertical heat exchange, and sea-ice loss. Using long-term observations from the Nansen and Amundsen Basins Observational System (NABOS), we document a strong freshening event in the eastern Eurasian Basin between late 2015 and early 2017. During this period, salinity in the upper 175 m decreased by ∼ 0.5 psu, equivalent to an additional ∼ 0.6 m of freshwater, relative to the preceding (2013–2015) and following (2017–2018) years. The anomaly originated on the Kara Sea shelves in 2014–2015, when exceptional Yenisey and Ob discharge provided a combined freshwater surplus of ∼ 0.78 m, sufficient to explain the observed freshening. Trajectory analysis traced the freshwater anomaly to the Kara Sea, with transport times of 8–9 months to the shelf and 22–23 months to offshore. The resulting enhanced stratification suppressed upper-ocean currents by ∼ 22 % and vertical shear by ∼ 50 %. These changes enabled thicker sea ice to persist through the summers of 2016–2017, in contrast to near-ice-free conditions in adjacent years. While wind anomalies aided the retention of freshwater along the slope, anomalous river discharge was the dominant driver of the event. Overall, the 2015–2017 event demonstrates how episodic river discharge events can restructure upper-ocean stratification, reduce oceanic heat fluxes, and lead to delayed melt and increased summer sea ice, highlighting the sensitivity of upper-ocean processes and sea ice to episodic freshwater forcing in the Arctic.

The Siberian Arctic Ocean links river runoff, sea ice, and Pacific-Atlantic exchanges, yet the drivers of its circulation variability remain poorly constrained. Using multi-decadal satellite altimetry, ocean reanalysis products, and in situ observations with cyclostationary empirical orthogonal function analysis, we show that shelf and slope currents are governed by distinct mechanisms across timescales. Seasonally, the Eastern Siberian Shelf Current is primarily regulated by salinity-driven sea-surface-height (SSH) gradients, with winds secondary, whereas the narrow Siberian Coastal Current is buoyancy-driven and strongly enhanced by summer winds. Interannually, the Siberian Slope Current captures a recent atmospheric transition from the Arctic Oscillation to the Arctic Dipole. We further identify a Siberia-Alaska sea-level-pressure dipole that modulates SSH gradients and regulates Pacific Water inflow through the Bering Strait, providing a physically based inflow index. Overall, SSH integrates buoyancy forcing, wind-driven circulation, and basin-scale atmospheric variability, identifying the Siberian Arctic Ocean as a key region for Arctic Ocean circulation and climate variability.

Extratropical cyclones are major drivers of variability in the Arctic, influencing sea ice, upper-ocean conditions, and atmosphere–ocean interactions. Understanding their impacts is critical because they can alter the state of the upper ocean, modulate surface heat fluxes, and affect sea ice dynamics, all of which have implications for the regional climate system and future Arctic environmental change. This study quantifies the effects of cyclones on atmosphere–ice–ocean interactions in the Siberian Arctic Ocean using 2021–2023 observations from seven moorings, complemented by atmospheric reanalysis data and satellite measurements of sea ice. Synoptic-scale anomalies in ocean temperature, salinity, current speed, sea ice concentration (SIC), surface air temperature, 10 m wind speed, and surface heat flux were analyzed to assess changes associated with cyclone events. Results indicate that cyclones strongly amplify air–ice–ocean interactions. The magnitude of ocean temperature, salinity, and current speed anomalies in the upper ocean increased by ∼20% in the presence of nearby cyclones. SIC, surface heat flux, salinity, and near-surface temperature show the strongest cyclone-related responses, with the number of high correlations among the parameters approximately doubling during cyclone events. Cyclones produce coherent oceanic and atmospheric responses across the eastern, western, and shallow regions of the Siberian Arctic, with no clear regional preference. Winter case studies show that cyclones can reduce SIC values by 15%–30%, triggering extreme anomalies in surface heat flux, ocean temperature, and salinity—demonstrating their impact on ice and upper-ocean conditions even outside of the melt season. These findings suggest that cyclones exert substantial influence on Arctic Ocean cold and warm season weather and climate variability by enhancing the connectivity between the atmosphere, sea ice, and upper ocean.

Atlantification has altered the hydrography of both the Barents Sea (BS) and Eurasian Basin (EB), yet their dynamical connection remains poorly quantified. Using three decades of ship-based CTD observations, eastern EB mooring records, and ORAS5 reanalysis, we investigate the role of the BS in modulating eastern EB halocline variability. Isopycnal backtracking simulations identify the winter marginal ice zone in the central and northeastern BS as the primary source of upper- and lower-halocline waters reaching the eastern EB within ~2.5 years. Lagged correlations show that salinity anomalies generated in this region propagate downstream through the St. Anna Trough and along the continental slope. Negative correlations between sea-ice thickness and salinity indicate that sea-ice variability regulates upper-ocean salinity in the BS, linking sea-ice and shelf processes to the transformation of Atlantic-derived waters and their subsequent influence on Atlantification of the eastern EB on decadal timescales.

The Siberian Arctic Ocean (SAO) is the largest integrator and redistributor of Siberian freshwater resources and acts to significantly influence the Arctic climate system. Moreover, the SAO is experiencing some of the most notable climate changes in the Arctic, and advection of anomalous Atlantic- (atlantification) and Pacific-origin (pacification) inflow waters and biota continue to play a major role in reshaping the SAO in recent decades. In this study, we use a large collection of mooring data to create a coherent picture of the spatiotemporal patterns and variability of currents and shear in the upper SAO during the past decade. Although there was no noticeable trend in the upper SAO's current speed and shear from 2013 to 2023, their seasonal cycle has significantly strengthened. The cycle reveals a strong relationship between upper ocean currents and their shear with sea ice conditions – particularly during transitional seasons – evidenced by a strong negative correlation (−0.94) between seasonal sea ice concentration and current shear. In the shallow (< 20–30 m) summer surface mixed layer, currents have increased because strong stratification prevents wind energy from propagating into the deeper layers. In this case, strong near-inertial currents account for more than half of the summertime current speed and shear. In the winter, a thicker surface layer is created by deep upper SAO ventilation due to atlantification, which distributes wind energy to far deeper (> 100 m) layers. These findings are critical to understanding the ramifications for mixing and halocline weakening, as well as the rate of atlantification in the region.

Atlantification-the northward inflow of anomalous waters and biota from the Atlantic into the polar basins-has wide-ranging climatological ramifications. We present previously unknown observational evidence that the atlantification processes are strengthening in the eastern Eurasian Basin. The primary example is the diminishing sea ice, which is related to a powerful ocean-heat/ice-albedo feedback, which accelerates sea-ice losses. Furthermore, we observe that atlantification is extending far beyond the Lomonosov Ridge into the Makarov Basin of the Arctic Ocean where upper ocean ventilation creates a new and unique ecological environment. The eastern part of the Siberian Arctic Ocean is still strongly stratified, but the atlantification-driven shoaling of warm, salty, and nutrient-rich intermediate waters already has important ecological consequences there. Disentangling the role of atlantification in multiple and complex high-latitude changes should be a priority in future modeling and observational efforts.

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.

Previous case studies have linked cyclone-induced atmospheric forcing and/or upper-ocean processes to notable Arctic sea ice loss events in the summers of 2012 and 2016. This study examines a more recent and noteworthy case in late summer 2021 in which substantial sea ice loss followed a period of surface meteorological extremes in the Beaufort Sea region of the Arctic. We focus on the period from mid-August to mid-September 2021 that coincided with the Office of Naval Research THINICE Pilot Field Campaign and investigate stormy and windy conditions with respect to air-sea processes impacting sea ice conditions. We find that during the stormy first half of the campaign, cyclone-induced energy fluxes into the marginal ice zone and surrounding waters preconditioned the ice pack for more rapid melt later in the campaign. The second half of the campaign, in contrast, was marked by non-cyclone wind events that enhanced turbulent (namely sensible) heat fluxes into the ice and upper ocean that increased melt. Moreover, this latter period had enhanced advection of the Beaufort Sea ice pack into above-freezing waters, increasing bottom melt to >1 cm d ^−1 over the remainder of the campaign. While findings are shown to vary by surface type and at relatively small (i.e. ice-floe) scales, insights are offered on the roles of late summer coupled processes on rapid ice loss events in today’s Arctic environment.

Located at the confluence of the Arctic and North Pacific and with Alaska at its heart, the Pacific Arctic Region (PAR) is a unique and interconnected regional climate system. Significant climatic changes in the PAR are described by a novel, mobile monthly Alaska Arctic Front (AAF) index, which is defined by sea level pressure differences between the migratory cores of the Beaufort High and Aleutian Low. Regional climate variability associated with the AAF shows prominent decadal signatures that are driven by the opposing effects of the North Pacific and the Arctic atmospheric pressure fields. Low AAF (negative phase) is dominated by North Pacific forcing, whereas high AAF (positive phase) is dominated by Arctic atmospheric processes. The recent (2011-2021) negative AAF phase, which is associated with the westward displacement of Aleutian Low explaining stronger northward winds and enhanced water transport northward through Bering Strait, is conducive to increased oceanic heat and freshwater content, reduced regional sea ice cover in the PAR, and to the expansion of Pacific species into the Arctic. These factors are all indicators of the Pacification of the Arctic Ocean, a key feature of climate change related to progression of anomalous Pacific water masses and biota into the polar basins. It is not yet clear if or when the recent phase of decadal variability will change and alter the rate of Pacification of the Arctic climate system.

This study evaluates the impact of increasing resolution on Arctic Ocean simulations using five pairs of matched low- and high-resolution models within the OMIP-2 (Ocean Model Intercomparison Project phase 2) framework. The primary objective is to assess whether a higher resolution can mitigate typical biases in low-resolution models and improve the representation of key climate-relevant variables. We reveal that increasing the horizontal resolution contributes to a reduction in biases in mean temperature and salinity and improves the simulation of the Atlantic water layer and its decadal warming events. A higher resolution also leads to better agreement with observed surface mixed-layer depth, cold halocline base depth and Arctic gateway transports in the Fram and Davis straits. However, the simulation of the mean state and temporal changes in Arctic freshwater content does not show improvement with increased resolution. Not all models achieve improvements for all analyzed ocean variables when spatial resolution is increased so it is crucial to recognize that model numerics and parameterizations also play an important role in faithful simulations. Overall, a higher resolution shows promise in improving the simulation of key Arctic Ocean features and processes, but efforts in model development are required to achieve more accurate representations across all climate-relevant variables.

Strengthened by polar amplification, Arctic warming provides direct evidence for global climate change. This analysis shows how Arctic surface air temperature (SAT) extremes have changed throughout time. Using ERA5, we demonstrate a pan-Arctic (>60 degrees N) significant upward SAT trend of +0.62 degrees C decade(-1) since 1979. Due to this warming, the warmest days of each month in the 1980s to 1990s would be considered average today, while the present coldest days would be regarded as normal in the 1980s to 1990s. Over 1979-2021, there was a 2 degrees C (or 7%) reduction of pan-Arctic SAT seasonal cycle, which resulted in warming of the cold SAT extremes by a factor of 2 relative to the SAT trend and dampened trends of the warm SAT extremes by roughly 25%. Since 1979, autumn has seen the strongest increasing trends in daily maximum and minimum temperatures, as well as counts of days with SAT above the 90th percentile and decreasing trends in counts of days with SAT below the 10th percentile, consistent with rapid Arctic sea ice decline and enhanced air-ocean heat fluxes. The modulated SAT seasonal signal has a significant impact on the timing of extremely strong monthly cold and warm spells. The dampening of the SAT seasonal fluctuations is likely to continue to increase as more sea ice melts and upper-ocean warming persists. As a result, the Arctic winter cold SAT extremes may continue to exhibit a faster rate of change than that of the summer warm SAT extremes as the Arctic continues to warm.

Radium isotopes, which are sourced from sediments, are useful tools for studying potential climate-driven changes in the transfer of shelf-derived elements to the open Arctic Ocean. Here we present observations of radium-228 and radium-226 from the Siberian Arctic, focusing on the shelf-basin boundary north of the Laptev and East Siberian Seas. Water isotopes and nutrients are used to deconvolve the contributions from different water masses in the study region, and modeled currents and water parcel back-trajectories provide insights on water pathways and residence times. High radium levels and fractions of meteoric water, along with modeled water parcel back-trajectories, indicate that shelf- and river-influenced water left the East Siberian Shelf around 170 degrees E in 2021; this is likely where the Transpolar Drift was entering the central Arctic. A transect extending from the East Siberian Slope into the basin is used to estimate a radium-228 flux of 2.67 x 107 atoms m-2 d-1 (possible range of 1.23 x 107-1.04 x 108 atoms m-2 d-1) from slope sediments, which is comparable to slope fluxes in other regions of the world. A box model is used to determine that the flux of radium-228 from the Laptev and East Siberian Shelves is 9.03 x 107 atoms m-2 d-1 (possible range of 3.87 x 107-1.56 x 108 atoms m-2 d-1), similar to previously estimated fluxes from the Chukchi Shelf. These three shelves contribute a disproportionately high amount of radium to the Arctic, highlighting their importance in regulating the chemistry of Arctic surface waters. Half of the Arctic Ocean is composed of shallow seas that extend over the continental shelf, so understanding what controls the chemical composition of the seawater in these regions is imperative to predicting how the Arctic Ocean as a whole may respond to climate change. Radium isotopes are naturally occurring radioactive elements that are produced in seafloor sediments, and they can be used to study the transfer of elements from the continental shelf into the overlying water. Here we combine measurements of radium isotopes with other chemical tracers and physical oceanographic models to study the transport and chemical signature of the Laptev and East Siberian Seas into the open Arctic Ocean. We find that the transport of elements from shelf sediments into the ocean is particularly strong in these seas and in the neighboring Chukchi Sea. Our data indicates that this region has an impact on the chemistry of the broader Arctic Ocean and emphasizes the need to continue to monitor this area for potential climate-driven changes. In 2021, shelf- and river-influenced waters crossed the shelf-basin boundary near 170 degrees E, indicating the likely position of the Transpolar Drift originModels suggest a residence time of similar to 13 months in the Laptev and East Siberian Seas before surface water enters the central Arctic OceanThe Laptev, East Siberian, and Chukchi Shelves contribute a disproportionately high amount of radium to Arctic surface waters

Enhanced warm, salty subarctic inflows drive high-latitude atlantification, which weakens oceanic stratification, amplifies heat fluxes, and reduces sea ice. In this work, we show that the atmospheric Arctic Dipole (AD) associated with anticyclonic winds over North America and cyclonic winds over Eurasia modulates inflows from the North Atlantic across the Nordic Seas. The alternating AD phases create a “switchgear mechanism.” From 2007 to 2021, this switchgear mechanism weakened northward inflows and enhanced sea-ice export across Fram Strait and increased inflows throughout the Barents Sea. By favoring stronger Arctic Ocean circulation, transferring freshwater into the Amerasian Basin, boosting stratification, and lowering oceanic heat fluxes there after 2007, AD+ contributed to slowing sea-ice loss. A transition to an AD− phase may accelerate the Arctic sea-ice decline, which would further change the Arctic climate system.

The Arctic Ocean is strongly stratified by salinity in the uppermost layers. This stratification is a key attribute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water heat, nutrients, and CO2 between intermediate depths and the surface of the Eurasian and Amerasian basins (EB and AB). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening in the AB is linked to freshening and deepening of the halocline. In the EB, the weakened stratification is associated with salinification and shoaling of the halocline (Atlantification). Simulations from a suite of CMIP6 models project that, under a strong greenhouse-gas forcing scenario (ssp585), the overall surface freshening and warming continue in both basins, but there is a divergence in hydrographic trends in certain regions. Within the AB, there is agreement among the models that the upper layers will become more stratified. However, within the EB, models diverge regarding future stratification. This is due to different balances between trends at the surface and trends at depth, related to Fram Strait fluxes. The divergence affects projections of future state of Arctic sea ice, as models with the strongest Atlantification project the strongest decline in sea ice volume in the EB. From these simulations, one could conclude that Atlantificaton will not spread eastward into the AB; however, we need to improve models to simulate tendencies in a more delicately stratified EB correctly.

Arctic Ocean gateway fluxes play a crucial role in linking the Arctic with the global ocean and affecting climate and marine ecosystems. We reviewed past studies on Arctic–Subarctic ocean linkages and examined their changes and driving mechanisms. Our review highlights that radical changes occurred in the inflows and outflows of the Arctic Ocean during the 2010s. Specifically, the Pacific inflow temperature in the Bering Strait and Atlantic inflow temperature in the Fram Strait hit record highs, while the Pacific inflow salinity in the Bering Strait and Arctic outflow salinity in the Davis and Fram straits hit record lows. Both the ocean heat convergence from lower latitudes to the Arctic and the hydrological cycle connecting the Arctic with Subarctic seas were stronger in 2000–2020 than in 1980–2000. CMIP6 models project a continuing increase in poleward ocean heat convergence in the 21st century, mainly due to warming of inflow waters. They also predict an increase in freshwater input to the Arctic Ocean, with the largest increase in freshwater export expected to occur in the Fram Strait due to both increased ocean volume export and decreased salinity. Fram Strait sea ice volume export hit a record low in the 2010s and is projected to continue to decrease along with Arctic sea ice decline. We quantitatively attribute the variability of the volume, heat, and freshwater transports in the Arctic gateways to forcing within and outside the Arctic based on dedicated numerical simulations and emphasize the importance of both origins in driving the variability.