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The correct representation of the Maud Rise open‐ocean polynya in the Weddell Sea remains a challenge for ocean models. Here we reproduce the most recent polynya openings in 2016–2017 using a regional configuration, and assess their dependencies on vertical convective mixing schemes and freshwater forcing, both separately and in combination. We test three vertical convective mixing schemes: the enhanced vertical diffusion (EVD), the Eddy‐Diffusivity Mass‐Flux (EDMF) parameterization, and a modified version of EDMF accounting for thermobaric effects. Using simulations for the period 2007–2017, we find that the modified EDMF reproduces the observed climatological evolution of the mixed layer depth better than the original EDMF and the EVD, but a polynya fails to open due to excessive freshwater forcing. We thus use the modified EDMF to perform sensitivity experiments with reduced precipitation during 2012–2017. The imposed freshwater forcing strongly affects the number of years with polynyas. The simulation with the best representation of the 2016–2017 polynyas is analyzed to evaluate the triggering mechanisms. The 2016 polynya was induced by the action of thermobaric instabilities on a weak ambient stratification. This opening preconditioned the water column for 2017, which produced a stronger polynya. By examining the impacts of the different convective mixing schemes, we show that the modified EDMF generates more realistic patterns of deep convection. Our results highlight the importance of surface freshwater forcing and thermobaricity in governing deep convection around Maud Rise, and the need to represent thermobaric instabilities to realistically model Maud Rise polynyas. Plain Language Summary: We investigate the impacts of representing numerical vertical mixing and surface freshwater forcing in a regional ocean model on polynyas (large openings in the pack ice) at Maud Rise, Southern Ocean. Maud Rise is prone to hosting polynyas, often associated with deep convection, which is a local vertical mixing process homogenizing the water column between surface and depths of several hundred meters. Numerical models often use simplistic strategies to represent this process, but improved parameterizations have recently become available. In this work, we test the impact of the representation of convective mixing in a particularly sensitive region. The last Maud Rise polynyas were observed in 2016 and 2017. Our regional simulation is capable of reproducing these polynyas, which has long been a challenge for ocean‐sea ice models. We show that the 2016 polynya resulted from the action of a vertical instability at depth acting on weak ambient stratification. This event preconditioned the stronger 2017 polynya and deep convection. We conclude that representing convective plumes as a sub‐grid scale process in models leads to a more realistic representation of open‐ocean polynyas and associated convection events. Key Points: The Eddy‐Diffusivity Mass‐Flux (EDMF) parameterization is tested in a regional simulation of the ocean around Maud RiseThermobaric effects on convective plumes are enabled by modifying the EDMF parameterizationSimulations of Maud Rise polynyas are highly sensitive to freshwater forcing and mixing schemes [ABSTRACT FROM AUTHOR]

Some of the highest rates of primary production across the Southern Ocean occur in the seasonal ice zone (SIZ), making this a prominent area of importance for both local ecosystems and the global carbon cycle. There, the annual advance and retreat of ice impacts light and nutrient availability, as well as the circulation and stratification, thereby imposing a dominant control on phytoplankton growth. In this study, the drivers of variability in phytoplankton growth between 2002–2020 in the Weddell Gyre SIZ were assessed using satellite net primary production (NPP) products alongside chlorophyll- a and particulate organic carbon (POC) data from autonomous biogeochemical floats. Although the highest daily rates of NPP are consistently observed in the continental shelf region (water depths shallower than 2000 m), the open-ocean region's larger size and longer ice-free season mean that it dominates biological carbon uptake within the Weddell Gyre, accounting for 93 %–96 % of the basin's total annual NPP. Variability in the summer maximum ice-free area is the strongest predictor of inter-annual variability in total NPP across the Weddell Gyre, with greater ice-free area resulting in greater annual NPP, explaining nearly half of the variance (R2=42 %). In the shelf region, the return of sea ice cover controls the end of the productive season. In the open ocean, however, both satellite NPP and float data show that a decline in NPP occurs before the end of the ice-free season (∼ 80 to 130 d after sea ice retreat). Evidence of concurrent increases in float-observed chlorophyll- a and POC suggest that later in the summer season additional factors such as micro-nutrient availability or top-down controls (e.g. grazing) could be limiting NPP. These results indicate that in a warmer and more ice-free Weddell Gyre, notwithstanding compensating changes in nutrient supply, NPP is likely to be enhanced only up to a certain limit of ice-free days. [ABSTRACT FROM AUTHOR]

The region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount's flanks, commonly termed the warm‐water Halo, surrounding a colder region above the rise, the Taylor Cap. Here we use two observational data sets, an eddy‐permitting reanalysis product and regional high‐resolution simulations, to investigate the interannual variability of the Halo and Taylor Cap for the period 2007–2022. Observations include novel hydrographic profiles obtained in the Maud Rise area in January 2022, during the first SO‐CHIC cruise. It is demonstrated that the temperature of deep waters around Maud Rise exhibits strong interannual variability within the Halo and Taylor Cap, occasionally to such an extent that the two features become indistinguishable. A warming of deep waters by as much as 0.8°C is observed in the Taylor Cap during the years preceding the opening of a polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we show that most of the observed variability in the Halo is forced remotely by advection of deep waters from the Weddell Gyre into the region surrounding Maud Rise. Our highest‐resolution simulation indicates that mesoscale eddies subsequently transfer the properties of the Halo's deep waters onto the Taylor Cap. The eddies responsible for such transfer originate in an abrupt retroflection along the inner flank of the Halo. Plain Language Summary: A polynya is an opening within the sea ice cover during winter. In the Weddell Sea, polynyas occur irregularly and often emerge close to a seamount, Maud Rise. This topographic obstacle disturbs the local circulation of the Weddell Gyre and leads to the formation of two local permanent features: a Taylor Cap on top of the seamount, which is mainly an isolated water mass; and surrounding this, a warm‐water Halo, located along the flanks of Maud Rise. In this study, we use observational and model analyses to show the changes in the temperature of regional water masses on a timescale of years, and to identify the drivers of these changes. The properties of the warm‐water Halo are controlled by the advection of deep waters along the south‐eastern rim of the Weddell Gyre. These waters are partly mixed into the Taylor Cap by eddies, thereby preconditioning the water column for deep convection. In the years preceding the opening of a polynya in 2016–2017, we document a warming trend in the Taylor Cap, starting in 2011. Key Points: Interannual variability of the subsurface temperature maximum at Maud Rise is documented with observationsAdvection of anomalously cold and fresh deep waters from the Weddell Gyre into the Halo leads to a near‐vanishing of the Taylor Cap in 2014Observed warming of the subsurface layer in the Taylor Cap due to eddy transport preceded the polynya opening in 2016–2017 [ABSTRACT FROM AUTHOR]

The Weddell Gyre is a major feature of the Southern Ocean and an important component of the planetary climate system; it regulates air–sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterised by low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification that stabilises the water column. Observing the Weddell Gyre is challenging, as it is extremely remote and largely covered with sea ice. At present, it is one of the most poorly sampled regions of the global ocean, highlighting the need to extract as much value as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, the PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and the Weddell Gyre, (3) a gyre edge class with northern and southern branches, and (4) a gyre core class. PCM highlights, in an objective and interpretable way, both expected and underappreciated structures in the Weddell Gyre dataset. For instance, PCM identifies the inflow of Circumpolar Deep Water (CDW) across the eastern boundary, the presence of the Weddell–Scotia Confluence waters, and structured spatial variability in mixing between Winter Water and CDW. PCM offers a useful complement to existing expertise-driven approaches for characterising the physical configuration and variability of oceanographic regions, helping to identify coherent thermohaline structures and the boundaries between them. [ABSTRACT FROM AUTHOR]

The Weddell Gyre is a major feature of the Southern Ocean and an important component of the planetary climate system; it regulates air-sea exchanges, controls the formation of deep and bottom waters, and hosts upwelling of relatively warm subsurface waters. It is characterized by extremely low sea surface temperatures, ubiquitous sea ice formation, and widespread salt stratification that stabilises the water column. Observing the Weddell Gyre is challenging, as it is extremely remote and largely covered with sea ice. At present, it is one of the most poorly-sampled regions of the global ocean, high-lighting the need to extract as much value as possible from existing observations. Here, we apply a profile classification model (PCM), which is an unsupervised classification technique, to a Weddell Gyre profile dataset to identify coherent regimes in temperature and salinity. We find that, despite not being given any positional information, the PCM identifies four spatially coherent thermohaline domains that can be described as follows: (1) a circumpolar class, (2) a transition region between the circumpolar waters and theWeddell Gyre, (3) a gyre edge class with northern and southern branches, and (4) a gyre core class. PCM highlights, in an objective and interpretable way, both expected and under-appreciated structures in the Weddell Gyre dataset. For instance, PCM identifies the inflow of Circumpolar Deep Water (CDW) across the eastern boundary, the presence of the Weddell-Scotia Confluence waters, and structured spatial variability in mixing between Winter Water and CDW. PCM offers a useful complement to existing expertise-driven approaches for characterising the physical configuration and variability of the Weddell Gyre and surrounding regions. [ABSTRACT FROM AUTHOR]

The Arctic Ocean has long been—and to a large extent remains—a datasparse region. Paucity of ocean and atmosphere measurements impacts the fidelity of atmospheric reanalyses, and ungauged rivers lead to uncertainties in measurementbased estimates of river runoff. However, there exists a data resource that can provide material help: sustained (long-term) ice and ocean measurements around the Arctic Ocean boundary. The Arctic Ocean is surrounded by land and connects to adjacent ocean basins via four main gateways: to the Pacific through Bering Strait, to the Atlantic through Davis Strait, and to the Nordic Seas via Fram Strait and the Barents Sea Opening. In addition, the Nordic Seas connect to the Atlantic across the GreenlandIceland-Scotland Ridge, which has a substantial measurement history. Inverse methods combine these data sets to generate conservative velocity fields that are then used to generate estimates of surface fluxes of heat and freshwater as well as other quantities of interest, including net biogeochemical fluxes and (with other methods) estimates of ocean water transformation rates. Data resources are available to greatly extend the duration and the temporal resolution of present analyses. [ABSTRACT FROM AUTHOR]

A novel multi-satellite product is used to shed light on the sea surface height seasonal cycle and associated geostrophic circulation in the subpolar Southern Ocean. We find three main modes of variability governing the Sea Level Anomaly seasonal cycle, all of them primarily governed by wind forcing. The main mode of seasonal variability is associated with the seasonality of the main subpolar gyres governed by largescale wind stress curl, qualitatively consistent with Sverdrup dynamics. The second seasonal mode is related to the Antarctic Slope Current (ASC), governed by the coastal easterlies, with a rapid circumpolar propagation of anomalous sea-level along the continental slope that is dynamically consistent with the so-called Southern Mode. The first two modes induce an acceleration of the gyres and the ASC in winter. The third seasonal mode appears to be driven by sea ice-modulated surface stress and induces an offshore extension of the ASC from autumn to winter. [ABSTRACT FROM AUTHOR]

The Amundsen Sea has the highest thinning rates of ice shelves in Antarctica. This imbalance is caused by changes in ocean melting induced by warm Circumpolar Deep Water (CDW) intrusions. The resulting changing freshwater balance could affect the on‐shelf currents and mixing. However, a clear understanding of the sources and sinks of freshwater in the region is lacking. Here we use a model of the Amundsen Sea, with passive freshwater tracers, to investigate the relative magnitudes and spatial distributions of the different freshwater components. In the surface layer and as a depth average, all freshwater tracer concentrations are of comparable magnitude, though on a depth average, sea ice and ice shelf are largest. The total freshwater tracer distribution is similar to that of the ice‐shelf tracer field. This implies a potential for ice‐shelf meltwater feedbacks, whereby abundant ice‐shelf meltwater alters the ocean circulation and stratification, affecting melting. Ice‐shelf and sea‐ice freshwater fluxes have the largest interannual variability. The effect of including grounded icebergs and iceberg freshwater flux are studied in detail. The presence of icebergs increases CDW intrusions that reach the base of ice shelves. This suggests another possible feedback mechanism, whereby more icebergs induce greater ice‐shelf melting and hence more icebergs. However, the strength of this potential feedback is dependent on poorly constrained sea‐ice model parameters. These results imply that poorly constrained parameters relating to the ocean freshwater balance, such as those relating to icebergs and sea ice, impact predictions for melting of the West Antarctic Ice Sheet. Plain Language Summary: The Amundsen Sea in Antarctica is an important region for understanding future sea‐level rise from the West Antarctic Ice Sheet, as the region's ice shelves (the floating extension of ocean terminating glaciers) are thinning due to changes in ocean melting. This could affect the amount of freshwater in the Amundsen Sea, possibly affecting ocean currents. In order to understand the distribution of freshwater from different sources (snowfall, ice‐shelf melting, iceberg melting, and sea‐ice melting) we use a specialized ocean model. This model shows that freshwater from all different sources have comparable magnitudes, though ice‐shelf and sea‐ice sources are largest. The fact that ice‐shelf meltwater is an important component implies a potential feedback in the system, whereby more ice‐shelf melting could alter ocean currents, and thereby cause more melting. We also investigated the effect of icebergs that ground on a seabed ridge. Icebergs increase the amount of warm water that reaches ice shelves. This provides a potential for another feedback, where more icebergs increase ice‐shelf melting, thereby producing more icebergs. These results imply that poorly known aspects of freshwater sources can impact predictions for future sea‐level rise contributions from the West Antarctic. Key Points: Sea ice and ice shelves provide the strongest freshwater fluxes in the Amundsen Sea, with the largest interannual variabilityParameterized icebergs affect oceanic heat content at ice shelves, with a strong potential for ice‐shelf meltwater and iceberg feedbacksPoorly constrained parameters relating to the freshwater balance may impact predictions for melting of the West Antarctic Ice Sheet [ABSTRACT FROM AUTHOR]

The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz‐Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low‐pressure system, which is modulated by large‐scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz‐Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL. Plain Language Summary: The heat available for the melting of the ice shelves of the Amundsen Sea is supplied by the warm waters flooding the continental shelf. Here we show that the waters flowing into the Getz‐Dotson Trough underwent a pronounced cooling between 2010 and 2015, associated with a weakening of the bottom‐intensified currents within the trough. This process was controlled by changes of shelf break winds. A weakening of the eastward wind impacts the middepth undercurrent system above the continental slope, which controls the inflow of warm water via tilting of density surfaces above the slope. The local winds are modulated by the low‐pressure system located offshore of the Amundsen Sea, known as the Amundsen Sea Low. The Amundsen Sea circulation is controlled by this low‐pressure system, and changes in its configuration can impact the warmer waters flooding the West Antarctic continental shelf, with consequences for the melting of the region's ice shelves. Key Points: Circumpolar Deep Water inflow to the Getz‐Dotson Trough, Amundsen Sea, is investigated with a multiyear mooringOn‐shelf heat content decreased between 2010 and 2015 in response to weaker Circumpolar Deep Water inflowGetz‐Dotson Trough cooling is driven by a westward wind anomaly at the shelf break, associated with a deeper Amundsen Sea Low [ABSTRACT FROM AUTHOR]

Variability in the heat delivery by Circumpolar Deep Water (CDW) is responsible for modulating the basal melting of the Amundsen Sea ice shelves. However, the mechanisms controlling the CDW inflow to the region's continental shelf remain little understood. Here, a high-resolution regional model is used to assess the processes governing heat delivery to the Amundsen Sea. The key mechanisms are identified by decomposing CDW temperature variability into two components associated with 1) changes in the depth of isopycnals [heave (HVE)], and 2) changes in the temperature of isopycnals [water mass property changes (WMP)]. In the Dotson--Getz trough, CDW temperature variability is primarily associated with WMP. The deeper thermocline and shallower shelf break hinder CDW access to that trough, and CDW inflow is regulated by the uplift of isopycnals at the shelf break--which is itself controlled by wind-driven variations in the speed of an undercurrent flowing eastward along the continental slope. In contrast, CDW temperature variability in the Pine Island--Thwaites trough is mainly linked to HVE. The shallower thermocline and deeper shelf break there permit CDW to persistently access the continental shelf. CDW temperature in the area responds to wind-driven modulation of the water mass on-shelf volume by changes in the rate of inflow across the shelf break and in Ekman pumping-induced vertical displacement of isopycnals within the shelf. The western and eastern Amundsen Sea thus represent distinct regimes, in which wind forcing governs CDW-mediated heat delivery via different dynamics. [ABSTRACT FROM AUTHOR]

The net rate of freshwater input to the Arctic Ocean has been calculated in the past by two methods: directly, as the sum of precipitation, evaporation and runoff, an approach hindered by sparsity of measurements, and by the ice and ocean budget method, where the net surface freshwater flux within a defined boundary is calculated from the rate of dilution of salinity, comparing ocean inflows with ice and ocean outflows. Here a third method is introduced, the geochemical method, as a modification of the budget method. A standard approach uses geochemical tracers (salinity, oxygen isotopes, inorganic nutrients) to compute "source fractions" that quantify a water parcel's constituent proportions of seawater, freshwater of meteoric origin, and either sea ice melt or brine (from the freezing-out of sea ice). The geochemical method combines the source fractions with the boundary velocity field of the budget method to quantify the net flux derived from each source. Here it is shown that the geochemical method generates an Arctic Ocean surface freshwater flux, which is also the meteoric source flux, of 200±44 mSv (1Sv=106 m 3 s -1), statistically indistinguishable from the budget method's 187±44 mSv , so that two different approaches to surface freshwater flux calculation are reconciled. The freshwater export rate of sea ice (40±14 mSv) is similar to the brine export flux, due to the "freshwater deficit" left by the freezing-out of sea ice (60±50 mSv). Inorganic nutrients are used to define Atlantic and Pacific seawater categories, and the results show significant non-conservation, whereby Atlantic seawater is effectively "converted" into Pacific seawater. This is hypothesized to be a consequence of denitrification within the Arctic Ocean, a process likely becoming more important with seasonal sea ice retreat. While inorganic nutrients may now be delivering ambiguous results on seawater origins, they may prove useful to quantify the Arctic Ocean's net denitrification rate. End point degeneracy is also discussed: multiple property definitions that lie along the same "mixing line" generate confused results. [ABSTRACT FROM AUTHOR]

Abstract: Year‐round variability in the Ross Gyre (RG), Antarctica, during 2011–2015, is derived using radar altimetry. The RG is characterized by a bounded recirculating component and a westward throughflow to the south. Two modes of variability of the sea surface height and ocean surface stress curl are revealed. The first represents a large‐scale sea surface height change forced by the Antarctic Oscillation. The second represents semiannual variability in gyre area and strength, driven by fluctuations in sea level pressure associated with the Amundsen Sea Low. Variability in the throughflow is also linked to the Amundsen Sea Low. An adequate description of the oceanic circulation is achieved only when sea ice drag is accounted for in the ocean surface stress. The drivers of RG variability elucidated here have significant implications for our understanding of the oceanic forcing of Antarctic Ice Sheet melting and for the downstream propagation of its ocean freshening footprint. [ABSTRACT FROM AUTHOR]

The influence of wind forcing on variability of the Antarctic Circumpolar Current (ACC) is investigated using a series of eddy-permitting ocean-sea ice models. At interannual and decadal time scales the ACC transport is sensitive to both the mean strength of westerly winds along the ACC circumpolar path, consistent with zonal momentum balance theories, and sensitive to the wind stresses along the coast of Antarctica, consistent with the 'free mode' theory of Hughes et al. A linear combination of the two factors explains differences in ACC transport across 11 regional quasi-equilibrium experiments. Repeated single-year global experiments show that the ACC can be robustly accelerated by both processes. Across an ensemble of simulations with realistic forcing over the second half of the twentieth century, interannual ACC transport variability owing to the free-mode mechanism exceeds that due to the zonal momentum balance mechanism by a factor of between 3.5 and 5 to one. While the ACC transport may not accelerate significantly owing to projected increases in along-ACC winds in future decades, significant changes in transport could still occur because of changes in the stress along the coast of Antarctica. [ABSTRACT FROM AUTHOR]

Decreased ventilation of the Southern Ocean in glacial time is implicated in most explanations of lower glacial atmospheric CO2. Today, the deep (>2000 m) ocean south of the Polar Front is rapidly ventilated from below, with the interaction of deep currents with topography driving high mixing rates well up into the water column. We show from a buoyancy budget that mixing rates are high in all the deep waters of the Southern Ocean. Between the surface and ∼2000 m depth, water is upwelled by a residual meridional overturning that is directly linked to buoyancy fluxes through the ocean surface. Combined with the rapid deep mixing, this upwelling serves to return deep water to the surface on a short time scale. We propose two new mechanisms by which, in glacial time, the deep Southern Ocean may have been more isolated from the surface. Firstly, the deep ocean appears to have been more stratified because of denser bottom water resulting from intense sea ice formation near Antarctica. The greater stratification would have slowed the deep mixing. Secondly, subzero atmospheric temperatures may have meant that the present-day buoyancy flux from the atmosphere to the ocean surface was reduced or reversed. This in turn would have reduced or eliminated the upwelling (contrary to a common assumption, upwelling is not solely a function of the wind stress but is coupled to the air–sea buoyancy flux too). The observed very close link between Antarctic temperatures and atmospheric CO2 could then be explained as a natural consequence of the connection between the air–sea buoyancy flux and upwelling in the Southern Ocean, if slower ventilation of the Southern Ocean led to lower atmospheric CO2. Here we use a box model, similar to those of previous authors, to show that weaker mixing and reduced upwelling in the Southern Ocean can explain the low glacial atmospheric CO2 in such a formulation. [ABSTRACT FROM AUTHOR]

Full-depth measurements of δ18O from 2008 to 2010 enclosing the Weddell Gyre in the Southern Ocean are used to investigate the regional freshwater budget. Using complementary salinity, nutrients and oxygen data, a four-component mass balance was applied to quantify the relative contributions of meteoric water (precipitation/glacial input), sea-ice melt and saline (oceanic) sources. Combination of freshwater fractions with velocity fields derived from a box inverse analysis enabled the estimation of gyre-scale budgets of both freshwater types, with deep water exports found to dominate the budget. Surface net sea-ice melt and meteoric contributions reach 1.8% and 3.2%, respectively, influenced by the summer sampling period, and -1.7% and +1.7% at depth, indicative of a dominance of sea-ice production over melt and a sizable contribution of shelf waters to deep water mass formation. A net meteoric water export of approximately 37 mSv is determined, commensurate with local estimates of ice sheet outflow and precipitation, and the Weddell Gyre is estimated to be a region of net sea-ice production. These results constitute the first synoptic benchmarking of sea-ice and meteoric exports from the Weddell Gyre, against which future change associated with an accelerating hydrological cycle, ocean climate change and evolving Antarctic glacial mass balance can be determined. [ABSTRACT FROM AUTHOR]