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Antarctic sea ice has exhibited significant variability over the satellite record, including a period of prolonged and gradual expansion, as well as a period of sudden decline. A number of mechanisms have been proposed to explain this variability, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatiotemporal structure of observed Antarctic sea ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low and accounts for most of the pan-Antarctic sea ice variability and almost all of the sea ice variability in the Weddell Sea. The third mode is also related to periods of abrupt Antarctic sea ice decline that are associated with a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat transport. Broadly, these results suggest that climate model biases in long-term Antarctic sea ice and large-scale sea surface temperature trends are related to each other and that eastern Pacific ENSO variability is a key ingredient for abrupt Antarctic sea ice changes. [ABSTRACT FROM AUTHOR]

Eddy stirring at mesoscale oceanic fronts generates finescale filaments, visible in submesoscale-resolving model simulations and high-resolution satellite images of sea surface temperature, ocean color, and sea ice. Submesoscale filaments have widths of O(1–10) km and evolve on time scales of hours to days, making them extremely challenging to observe. Despite their relatively small scale, submesoscale processes play a key role in the climate system by providing a route to dissipation; altering the stratification of the ocean interior; and generating strong vertical velocities that exchange heat, carbon, nutrients, and oxygen between the mixed layer and the ocean interior. We present a unique set of in situ and satellite observations in a standing meander region of the Antarctic Circumpolar Current (ACC) that supports the theory of cold filamentary intensification—revealing enhanced vertical velocities and evidence of subduction and ventilation associated with finescale cold filaments. We show that these processes are not confined to the mixed layer; EM-APEX floats reveal enhanced downward velocities (>100 m day−1) and evidence of ageostrophic motion extending as deep as 1600 dbar, associated with a ∼20-km-wide cold filament. A finer-scale (∼5 km wide) cold filament crossed by a towed Triaxus is associated with anomalous chlorophyll and oxygen values extending at least 100–200 dbar below the base of the mixed layer, implying recent subduction and ventilation. Energetic standing meanders within the weakly stratified ACC provide an environment conductive to the generation of finescale filaments that can transport water mass properties across mesoscale fronts and deep into the ocean interior. [ABSTRACT FROM AUTHOR]

Ocean heat exchanges at the marginal ice zone (MIZ) play an important role in melting sea ice. Mixed‐layer eddies transport heat and ice floes across the MIZ, facilitating the pack's access to warm waters. This study explores these frontal dynamics using disk‐shaped floes coupled to an upper‐ocean model simulating the sea ice edge. Numerical experiments reveal that small floes respond more strongly to fine‐scale ocean currents, which favors higher dispersion rates and weakens sea ice drag onto the underlying ocean. Floes with radii smaller than resolved turbulent filaments (∼2–4 km) result in a wider and more energetic MIZ, by a factor of 70% each, compared to larger floes. We hypothesize that this floe size dependency may affect sea ice break‐up by controlling oceanic energy propagation into the MIZ and modulate the sea ice pack's melt rate by regulating lateral heat transport toward the sea ice cover. Plain Language Summary: Sea ice forms as a thin layer of frozen ocean waters, which breaks into individual floes due to the action of waves, ocean currents, and atmospheric winds. At the edge of the pack, these floes are vulnerable to warm waters in the open ocean, which can favor the melt of sea ice. The transport of heat from the open ocean into ice‐covered regions is not well represented in existing climate models, notably due to their poor spatial resolution and their inability to represent the dynamics of individual floes. In this study, we use a regional numerical model to investigate how fine‐scale ocean currents (2–30 km) can help transport heat toward a sea ice pack composed of broken‐up floes. We find that this heat transport is most efficient when floes are small, because they cannot efficiently damp the mechanical energy from the surface currents and they are easily transported into the open ocean by these currents. These two processes combined may lead to sea ice melt feedbacks that are currently not captured by coarser climate models. Key Points: The sensitivity of sea ice dispersion and upper‐ocean energetics to floe size is considered at a mesoscale frontFloes smaller than turbulent filaments disperse more strongly and lead to a wider marginal ice zoneThese small floes allow for stronger eddy kinetic energy propagation and heat transport into the pack [ABSTRACT FROM AUTHOR]

In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss-a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air-sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. [ABSTRACT FROM AUTHOR]

Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind‐driven variations in warm water transport toward ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the warm water layer at the ice‐shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal‐scale thermocline depth variations, even when the supply of warm water from the shelf‐break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near‐coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind‐driven theories. Plain Language Summary: Ice loss from the West Antarctic Ice Sheet contributes significantly to current and projected rates of global sea‐level rise. The ice sheet is primarily losing mass via glaciers that flow from the Antarctic continent into the Amundsen Sea, where floating ice shelves are exposed to much warmer ocean waters than elsewhere around Antarctica. In this work we present a simplified mathematical model for the volume of warm water at Amundsen Sea ice shelf fronts that reproduces observed patterns of warm water variability. The modeled variability relies on interactions between ice shelf melt and coastal polynyas, regions where enhanced wintertime sea‐ice production can trigger mixing that diverts heat carried by warm waters away from the ice shelf and into the atmosphere. Higher melt rates inhibit polynya convection, allowing more warm water into the ice shelf cavity and reinforcing a high melt state, whilst lower melt rates facilitate polynya convection, diverting heat away from the ice shelf and reinforcing a low melt state. Interannual variations in polynya sea‐ice production trigger shifts between these reinforcing states. Our results promote the importance of coastal processes in explaining observed variations in Amundsen Sea ice shelf melt, which have previously been attributed to remote wind patterns. Key Points: Rates of ocean‐driven Amundsen Sea ice shelf melt respond to variations in warm water transport to the coast and modification at the coastA simple Amundsen Sea continental shelf overturning model, based on water mass transformation, reveals bistable high and low melt regimesFeedbacks between glacial melt and polynya convection are central to the bistability and produce variability consistent with observations [ABSTRACT FROM AUTHOR]

Meltwater content and pathways determine the impact of Antarctica's melting ice shelves on ocean circulation and climate. Using ocean glider observations, we quantify meltwater distribution and transport within the Bellingshausen Sea's Belgica Trough. Meltwater is present at different densities and with different turbidities: both are indicative of a layer's ice shelf of origin. To investigate how ice‐shelf origin separates meltwater into different export pathways, we compare these observations with high‐resolution tracer‐release model simulations. Meltwater filaments branch off the Antarctic Coastal Current into the southwestern trough. Meltwater also enters the Belgica Trough in the northwest via an extended western pathway, hence the greater observed southward (0.50 mSv) than northward (0.17 mSv) meltwater transport. Together, the observations and simulations reveal meltwater retention within a cyclonic in‐trough gyre, which has the potential to promote climactically important feedbacks on circulation and future melting. Plain Language Summary: Recent research has advanced our understanding of interactions between warm ocean waters and the underside of Antarctica's floating ice shelves. But what happens to meltwater that these ice shelves release? Meltwater is fresh: it reduces the salinity of sea water and upsets the delicate balance between salty and fresh water that drives many polar ocean processes ‐ for example, the sinking of cold, salty water to the deepest regions of the ocean, which is of global climatic importance. We use high‐resolution observations and state‐of‐the‐art model simulations to determine the pathways that meltwater takes as it flows in and around the Belgica Trough in the central Bellingshausen Sea, Antarctica. Meltwater is principally confined to the edges of the trough's clockwise gyre circulation. Meltwater from eastern ice shelves is generally found at shallower depths than meltwater from southern ice shelves; this explains evidence of two meltwater layers found in our observations. In addition, meltwater from eastern ice shelves is turbid, while meltwater from southern ice shelves is clear. These results will help us to better predict how Antarctica's coastal seas will respond to a warming climate as ever more meltwater is released into the ocean. Key Points: Meltwater pathways in the Bellingshausen Sea, Antarctica, are explored using observations from gliders and a high‐resolution regional modelMeltwater observed at different densities originates from different ice shelves: lighter meltwater layers originate from eastern ice shelvesMeltwater from different ice shelves is distinguished by different turbidity signatures, suggestive of different biogeochemical properties [ABSTRACT FROM AUTHOR]

Antarctic sea ice gradually increased from the late 1970s until 2016, when it experienced an abrupt decline. A number of mechanisms have been proposed for both the gradual increase and abrupt decline of Antarctic sea ice, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatial-temporal structure of observed Antarctic sea-ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea-surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode, and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low, and accounts for most of the pan-Antarctic sea-ice variability and almost all of the sea ice variability in the Weddell Sea. This mode is associated with periods of abrupt Antarctic sea-ice decline and is related to a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat convergence. Broadly, these results suggest that climate model biases in long-term Antarctic sea-ice and global sea-surface temperature trends are related to each other and that eastern Pacific ENSO variability causes abrupt sea ice changes. [ABSTRACT FROM AUTHOR]

Variability in oceanic conditions directly impacts ice loss from marine outlet glaciers in Greenland, influencing the ice sheet mass balance. Oceanic conditions are available from Atmosphere‐Ocean Global Climate Model (AOGCM) output, but these models require extensive computational resources and lack the fine resolution needed to simulate ocean dynamics on the Greenland continental shelf and close to glacier marine termini. Here, we develop a statistical approach to generate ocean forcing for ice sheet model simulations, which incorporates natural spatiotemporal variability and anthropogenic changes. Starting from raw AOGCM ocean heat content, we apply: (a) a bias‐correction using ocean reanalysis, (b) an extrapolation accounting for on‐shelf ocean dynamics, and (c) stochastic time series models to generate realizations of natural variability. The bias‐correction reduces model errors by ∼25% when compared to independent in‐situ measurements. The bias‐corrected time series are subsequently extrapolated to fjord mouth locations using relations constrained from available high‐resolution regional ocean model results. The stochastic time series models reproduce the spatial correlation, characteristic timescales, and the amplitude of natural variability of bias‐corrected AOGCMs, but at negligible computational expense. We demonstrate the efficiency of this method by generating >6,000 time series of ocean forcing for >200 Greenland marine‐terminating glacier locations until 2100. As our method is computationally efficient and adaptable to any ocean model output and reanalysis product, it provides flexibility in exploring sensitivity to ocean conditions in Greenland ice sheet model simulations. We provide the output and workflow in an open‐source repository, and discuss advantages and future developments for our method. Plain Language Summary: Model simulations of the Greenland ice sheet (GrIS) require knowledge of ocean conditions. The evolution of ocean conditions has a strong impact on ice sheet model predictions, as there are more than 200 glaciers in Greenland flowing directly into the ocean. However, modeling oceanic forcing is difficult. The state‐of‐the‐art approach is to use output from Atmosphere‐Ocean Global Climate Models (AOGCMs). But these models cannot accurately capture the ocean dynamics on the Greenland shelf, and they can show strong biases compared to observations. Furthermore, AOGCMs are computationally expensive, meaning that it is impossible to thoroughly characterize the uncertainty associated with the chaotic nature of climate. Here, we propose a procedure to bias‐correct and extrapolate oceanic output from AOGCMs. Our method exploits observational datasets, as well as available high‐resolution ocean model results. Using statistical models, we reproduce patterns of spatiotemporal ocean variability at low computational expense, and represent internal climate variability and global warming trends. The goal is to provide a scalable procedure to generate ocean forcing for long‐term GrIS model predictions. Key Points: We develop a statistical method to generate ocean forcing boundary conditions for Greenland ice sheet model simulationsThe method bias‐corrects and extrapolates global climate model output using reanalysis products and high‐resolution model resultsStochastic time series models reproduce the spatiotemporal variability of ocean conditions at negligible computational expense [ABSTRACT FROM AUTHOR]

We investigate statistical properties of surface currents as well as coherent mesoscale eddies in the seasonally ice‐covered Southern Ocean. Based on a recent regional Sea Level Anomaly satellite altimetry data set, we compute Eddy Kinetic Energy (EKE) and detect mesoscale eddies. EKE is about one order of magnitude higher in the northern sector of the subpolar basin and over the continental slope, as compared to the middle of the subpolar gyres. An eddy detection methodology reveals that eddies are distributed evenly in the subpolar Southern Ocean, and their amplitude follows the spatial pattern of EKE. In addition to regional circulation variations, sea ice concentration arises as an important driver of eddy properties. Eddies have low amplitude and density in the pack ice, in particular in the middle of the gyres where the background circulation is unfavorable for instabilities. In contrast, the northern part of the Marginal Ice Zone is favorable for mesoscale eddies, especially cyclonic. There, eddies are stronger and their density is higher than in any other region of the ice‐covered or ice‐free subpolar Southern Ocean. This region is expected to be a site of frontogenesis due to sea ice melt and upwelling generated from interactions between the wind and the sea ice. While many mesoscale eddies will fall below detection level due to the small Rossby radius at high latitudes, these results contribute to understanding the interactions between mesoscale eddies, sea ice, and the background circulation in the subpolar region. Plain Language Summary: Ocean eddies are spatially coherent vortices, ubiquitous in the global oceans, and are central structures contributing to shaping the global ocean circulation. In the Southern Ocean, they have been shown to play a potentially important role in the cross‐front transport of heat and nutrients at mid and high latitudes. Recent improvements in satellite altimetry capabilities in the ice‐covered regions allow for a first description of mesoscale eddies in the ice‐covered Southern Ocean, and the investigation of their interactions with sea ice. The location of the detected eddies is consistent with the main large‐scale circulation features. In addition to the background circulation, the sea ice cover has a strong impact on the size of eddies. While weaker eddies are found in the highly concentrated sea ice areas, the marginal ice zone seems to be a region with enhanced eddy activity, with more and stronger eddies detected. Key Points: A new satellite altimetry data set allows the detection of mesoscale eddies in the ice‐covered Southern OceanEddy generation and strength are primarily impacted by sea ice concentration and the background circulationWhile the eddy strength is damped under high sea‐ice concentration, the northern edge of the sea‐ice region is a hotspot of large eddies [ABSTRACT FROM AUTHOR]

The Bellingshausen Sea hosts heat transport onto the continental shelf, potentially enhancing ice shelf basal melt. Here, we use the GLORYS12V1 1993–2018 reanalysis to identify physical processes that set seasonal and interannual variability of water mass properties in the Eltanin and Latady Bays on the southern Bellingshausen Sea continental shelf. Annual means of potential temperature from 300 m to the seabed reveal interannual variability and allow separation into warm and cold regimes. The Amundsen Sea Low (ASL) is more intense and extends further east during the warm regime than the cold regime. In the warm regime, a wind‐induced reduction of sea ice concentration near the coast increases surface heat loss, convection, and formation of cold dense water in winter, associated with a decrease in heat content of the southern Bellingshausen Sea over time and a net northward heat transport. In contrast, in the cold regime, increased sea ice concentration reduces surface heat loss and thus formation of cold, dense water. Combined with an increase in heat content over time and a net southward heat transport, this results in a warming of the southern Bellingshausen Sea. This suggests that variability in the deep water temperature in the southern Bellingshausen Sea is primarily due to local surface heat fluxes above the shelf. The variability of surface heat fluxes is related to the variability of the ASL and its influence on sea ice extent and local formation of cold, dense water in winter. Plain Language Summary: The stability of the West Antarctic Ice Sheet is a key factor in uncertainty in global sea level predictions on decadal to centennial time scales. Today, melt of West Antarctic ice shelves is dominated by relatively warm (above freezing) ocean waters, and the warmest waters around Antarctica are found in the Bellingshausen Sea. There is a lack of long‐term observations in this inaccessible region so we use a model product to investigate variability of deep water temperatures, identifying warm and cold years. We find a link between sea ice concentration, heat loss to the atmosphere and cold water formation and the wind fields associated with a weather system, the Amundsen Sea Low (ASL). Winds in the southern Bellingshausen Sea are stronger in warm years than in cold years and blow the ice away from the coast more rapidly, allowing more heat loss to the atmosphere, cooling the ocean in winter. Conversely, in cold years increased sea ice cover reduces heat loss to the atmosphere, less cold water is formed, and there is increased southward transport of warm water. We highlight the importance of the ASL's location and intensity for cold water formation and its impacts on temperature. Key Points: Intensity and location of Amundsen Sea Low (ASL) affect ocean properties of southern Bellingshausen Sea continental shelf in 26‐year reanalysisThe ASL modifies the surface heat flux, warming Bellingshausen Sea shelf when it shifts westward and weakens, and vice versaThe southern Bellingshausen Sea temperature depends more on surface heat fluxes above the continental shelf than southward heat transport [ABSTRACT FROM AUTHOR]

Marginal ice zones are composed of discrete sea‐ice floes, whose dynamics are not well captured by the continuum representation of sea ice in most climate models. This study makes use of an ocean large eddy simulation (LES) model, coupled to cylindrical sea‐ice floes, to investigate thermal and mechanical interactions between melt‐induced submesoscale features and sea‐ice floes, during summer conditions. We explore the sensitivity of sea‐ice melt rates and upper‐ocean turbulence properties to floe size, ice‐ocean drag, and surface winds. Under low wind conditions, upper ocean turbulence transports warm cyclonic filaments from the open ocean toward the center of the floes and enhances their basal melt. This heat transport is partially suppressed by trapping of ice within cold anticyclonic features. When winds are stronger, melt rates are enhanced by the decoupling of floes from the cold, melt‐induced lens underneath sea ice. Distinct dynamical regimes emerge in which the influence of warm filaments on sea‐ice melt is mitigated by the strength of ice‐ocean coupling and eddy size relative to floe size. Simple scaling laws, which may help parameterize these processes in coarse continuum‐based sea‐ice models, successfully capture floe melt rates under these limiting regimes. Plain Language Summary: At high latitudes, the ocean is often covered by a thin layer of sea ice, which forms in the winter due to the freezing of seawater. Sea ice is typically characterized by a collection of distinct pieces, known as ice floes, which range in size between several meters to tens of kilometers. In the summer, floes melt due to a warm atmosphere above, as well as a warm ocean below. The melting of sea‐ice floes often generates small‐scale (1–10 km) ocean currents at the surface, which in turn affect sea‐ice motion and melt. Here, we study these ice‐ocean interactions, using an idealized sea‐ice model coupled to a regional ocean model that resolves these fine‐scale dynamics. We find that small‐scale oceanic currents and surface winds accelerate the sea‐ice melt rate by allowing warm waters to move into colder regions under the ice floes. Simple relationships, or scalings, involving the floe size, wind speed, and ice‐ocean friction, are able to explain these varying ice floe melt rates across a range of environmental conditions. These scalings, when included in global climate models, may help to improve future predictions of sea‐ice changes by accounting for the impact of small‐scale ocean currents. Key Points: The relative size of sea‐ice floes and submesoscale eddies controls lateral heat transport to sea iceMechanical coupling shields floes from warm filaments but winds disrupt this coupling and advect ice into warm watersScaling laws capture sea‐ice melt rate dependence on floe size, ice‐ocean drag and wind speed [ABSTRACT FROM AUTHOR]

Arctic sea ice extent continues to decline at an unprecedented rate that is commonly underestimated by climate projection models. This disagreement may imply biases in the representation of processes that bring heat to the sea ice in these models. Here we reveal interactions between ocean-ice heat fluxes, sea ice cover, and upper-ocean eddies that constitute a positive feedback missing in climate models. Using an eddy-resolving global ocean model, we demonstrate that ocean-ice heat fluxes are predominantly induced by localized and intermittent ocean eddies, filaments, and internal waves that episodically advect warm subsurface waters into the mixed layer where they are in direct contact with sea ice. The energetics of near-surface eddies interacting with sea ice are modulated by frictional dissipation in ice-ocean boundary layers, being dominant under consolidated winter ice but substantially reduced under low-concentrated weak sea ice in marginal ice zones. Our results indicate that Arctic sea ice loss will reduce upper-ocean dissipation, which will produce more energetic eddies and amplified ocean-ice heat exchange. We thus emphasize the need for sea ice-aware parameterizations of eddy-induced ice-ocean heat fluxes in climate models. Arctic sea ice extent continues to decline at an unprecedented rate that climate projection models commonly underestimate. In this study, authors reveal a positive feedback between ocean-ice heat fluxes, sea ice cover, and upper-ocean vortices that is missing in coarse-resolution climate models. [ABSTRACT FROM AUTHOR]

Climate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate. Plain Language Summary: The Indonesian Throughflow (ITF) is a key component of the global ocean circulation. By exchanging water between the low‐latitude Indian Ocean and Pacific Ocean, the ITF has been suggested to play an important role in shaping global warming patterns in response to greenhouse gas forcing. Climate models consistently project the ITF strength to decline in the 21st century. Traditionally, changes in the strength of the ITF have been attributed to local processes, such as changes in precipitation and atmospheric winds. Here, we suggest that remote processes can also have a significant impact on ITF variability. In particular, we show that the projected weakening in the ITF during the 21st century could be tied to changes in the Atlantic meridional overturning circulation (AMOC). Through this transient version of the ocean's conveyor belt circulation, changes in the high‐latitude North Atlantic (e.g., Arctic sea ice melt) can affect the climate in the low‐latitude Indo‐Pacific Ocean. An intriguing corollary is the potential to use the ITF to monitor or interpret long‐term trends in the AMOC. Key Points: Basin‐scale transient responses of the global ocean overturning circulation are explored with a hierarchy of modelsChanges in AMOC strength can produce a response in ITF volume transport on centennial timescalesITF transport time series may assist in monitoring and interpreting long‐term trends in the AMOC [ABSTRACT FROM AUTHOR]

The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years. The mechanisms proposed to explain the stabilization involve either mesoscale eddy activity that opposes Ekman pumping or the reduction of Ekman pumping due to reduced sea ice–ocean surface stress. However, the relative importance of these mechanisms is unclear. Here, we present observational estimates of the Beaufort Gyre mechanical energy budget and show that energy dissipation and freshwater content stabilization by eddies increased in the late-2000s. The loss of sea ice and acceleration of ocean currents after 2007 resulted in enhanced mechanical energy input but without corresponding increases in potential energy storage. To balance the energy surplus, eddy dissipation and its role in gyre stabilization must have increased after 2007. Our results imply that declining Arctic sea ice will lead to an increasingly energetic Beaufort Gyre with eddies playing a greater role in its stabilization. The freshwater content of the Beaufort Gyre in the Western Arctic Ocean has increased in response to almost two decades of persistent anti-cyclonic winds. Here, the authors found that dramatic loss of sea ice and acceleration of surface currents after 2007 led to a net annual wind energy input to the Beaufort Gyre, and anticipate that continued sea ice decline will lead to an increasingly energetic Beaufort Gyre. [ABSTRACT FROM AUTHOR]

The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed. [ABSTRACT FROM AUTHOR]