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Abstract The upwelling of warm Circumpolar Deep Water is a key process in the global climate system, transporting heat, nutrients, and carbon poleward towards Antarctic ice shelves. Here we use physical and chemical seawater properties from repeat ship-based observations to classify Southern Ocean water masses and show changes in warm water abundance south of the Antarctic Circumpolar Current over the past two decades. We then train a random forest model ensemble to extend this classification to a monthly gridded Argo climatology beginning in 2004, enabling further decomposition of the spatial and temporal variability of the signal. Both analyses reveal an increase in upper-2000 m warm water thickness near the continent, consistent with a circumpolar-mean poleward redistribution of the upper Circumpolar Deep Water core of 1.26km yr⁻¹(95% CI: 0.53–1.98). Together, these shifts suggest enhanced heat flux towards the Antarctic shelf, with implications for basal ice shelf melting and sea-level rise. 

Abstract Assessing the biological characteristics of high-latitude winter habitats of migratory marine predators is necessary for conservation and management in Antarctica. Tracking data from chinstrap penguins (Pygoscelis antarcticus) and southern elephant seals (Mirounga leonina), key Antarctic predators with different diets and foraging habits, indicate that some individuals undertake long-distance winter migrations to remote regions south of 55°S and west of 120°W. There, localized hotspots of increased use, with general reductions in mean swimming speed are evident. Presumably, these predators migrate to areas with higher productivity, however the marine productivity in this remote region during winter is unknown. Light limitation during winter precludes the use of optical satellite data to characterize marine productivity here, but biogeochemical-Argo floats can provide year-round chlorophyll data. These data inform the Biogeochemical Southern Ocean State Estimate (B-SOSE), which provides year-round estimates of marine productivity. The predator hotspots overlap with two areas with year-round elevated surface chlorophyll levels predicted by B-SOSE, consistent with previous studies indicating enhanced mixing in those areas. Our results suggest that persistent areas of elevated chlorophyll centered near 160°W and 120°W near the boundaries of the Ross Gyre and the southern boundary of the Antarctic Circumpolar Current support a productive food web capable of supporting the diverse foraging niches of pelagic species during winter. 

Abstract Basal melting of Antarctic ice shelves is primarily driven by heat delivery from warm Circumpolar Deep Water. Here we classify near-shelf water masses in an eddy-resolving numerical model of the Southern Ocean to develop a unified view of warm water intrusion onto the Antarctic continental shelf. We identify four regimes on seasonal timescales. In regime 1 (East Antarctica), heat intrusions are driven by easterly winds via Ekman dynamics. In regime 2 (West Antarctica), intrusion is primarily determined by the strength of a shelf-break undercurrent. In regime 3, the warm water cycle on the shelf is in antiphase with dense shelf water production (Adélie Coast). Finally, in regime 4 (Weddell and Ross seas), shelf-ward warm water inflow occurs along the western edge of canyons during periods of dense shelf water outflow. Our results advocate for a reformulation of the traditional annual-mean regime classification of the Antarctic continental shelf. 

Antarctic sea ice extent began declining in 2015, reaching its minimum in the post-1970s observational era in 2023. To diagnose the drivers of this decline, we analyze an observationally constrained sea ice–ocean model spanning 2013–2023 and identify three distinct phases of sea ice retreat. First, intensifying westerlies preconditioned the Southern Ocean via increased upwelling of warm, saline circumpolar deep water (CDW). Second, strong winds in 2015–2016 enhanced the mixing of CDW into the upper ocean, thereby initiating sea ice loss, particularly in East Antarctica. Third, sustained mixing of CDW into the surface layer, combined with reduced equatorward sea ice–derived freshwater export, maintained an unprecedentedly low sea ice state. East Antarctic sea ice loss was primarily subsurface driven via enhanced upward CDW flux, whereas West Antarctic sea ice loss was also forced by longwave radiative flux anomalies. Our findings suggest that persistent upwelling-favorable conditions under anthropogenic forcing may push the Southern Ocean into a prolonged low sea ice state. 

Antarctic land ice stores the majority of Earth’s freshwater and carries substantial uncertainties regarding its future contribution to global sea level rise. While ocean processes associated with basal melting currently dominate ice loss, atmospheric forcing could have an increasing future impact, especially with intensified extreme weather events. For instance, atmospheric rivers, which are key drivers of long-distance moisture transport, introduce significant uncertainties to Antarctica’s ice mass balance, as they are capable of causing both intense snowfall and surface melting. They also impact ocean stratification and mixed-layer depth through freshwater input, ultimately affecting air-sea exchange. Associated interactions among components of the Earth system—atmosphere, ocean, and glacier—are not fully captured by global climate models and observations. This paper assesses Antarctica’s future, highlighting uncertainties stemming from limited understanding of atmospheric and oceanic forcings such as atmospheric rivers, and their consequences for projecting sea-level rise-related hazards. 

Abstract. Jones Sound is one of three critical waterways in the Canadian Arctic Archipelago that regulate liquid exchange between the Arctic Ocean and northern Atlantic Ocean. However, to date, no high-resolution ocean circulation model exists to study the recent evolution of Jones Sound, meaning that our understanding of circulation within the sound is based either on temporally and spatially sparse oceanographic observations or on extrapolating conditions within Baffin Bay, which has a more dense observational record. To address this, we develop a high-resolution (1/120°, 0.9 km) Jones Sound configuration of the Massachusetts Institute of Technology general circulation model and perform coupled ocean–sea ice–biological productivity simulations between 2003–2016. We find that circulation through Lady Ann Strait, Fram Sound, and Glacier Strait comprises 71 %, 14 %, and 15 % of the volumetric transport into and out of Jones Sound, with tidal flushing enhancing the magnitude of volumetric transport through Fram Sound. Warming Atlantic Water within western Baffin Bay flows into Jones Sound through Lady Ann Strait, becomes well-mixed, and circulates counterclockwise, encroaching on the terminus of most tidewater glaciers that line the eastern periphery of the sound. Furthermore, we find that sustained atmospheric and oceanic warming drives an 11 % reduction in the 2003–2016 mean summertime sea ice area, decreased wintertime sea ice thickness, and delayed onset of sea ice refreeze in the fall (thus lengthening the amount of time during which Jones Sound is ice-free). Tidal flushing through Cardigan Strait is critical in triggering melt-back of sea ice across northern Jones Sound. Lastly, this decline in sea ice increases light availability and, when coupled with warming of the subsurface waters in Jones Sound, facilitates enhanced primary productivity down to ∼ 21 m depth. While we note that the modeled warming signal in Baffin Bay is overestimated relative to observations, the results presented here improve our general understanding of how this critical waterway might change under continued polar-amplified global warming and underscores the need for sustained oceanographic observations in this region. 

Abstract The record‐breaking Arctic cyclone of January 2022 caused an unprecedented sea ice loss in the Barents Sea near Svalbard, which was severely underestimated by both the ECMWF operational forecast and the CESM1 global climate model. To investigate this discrepancy and further explore the mechanisms that led to sea ice loss, we utilize a regional ocean–sea ice MITgcm model forced with ERA5 atmospheric reanalysis, validated against satellite‐derived ice concentrations and thickness, and Argo observations. We tested model sensitivity to initial conditions by choosing two different ocean reconstructions and tested sensitivity to the air‐sea‐ice drag coefficient. The model shows that cyclone‐driven winds induced strong upper‐ocean mixing of cold, fresh polar waters with subsurface warm, salty Atlantic Water, bringing heat to the surface to rapidly melt thin seasonal ice north of Svalbard. This process was evident in Argo float profile changes but not in the operational forecast and global climate model. In contrast, wind‐driven mechanical divergence dominated sea ice loss south of Svalbard, where the direction of sea ice retreat followed the direction of the prevailing winds. Increasing the air‐ice drag coefficient substantially improved the agreement between our modeled and observed ice area evolution. The balance between thermodynamic and mechanical effects on the sea ice loss was highly sensitive to drag parameterization. Our results highlight the need to improve the representation of upper ocean stratification, vertical mixing, and air‐sea‐ice momentum fluxes to reproduce cyclone‐triggered rapid sea ice loss events in the Arctic in winter. 

Abstract Using a 1/48 global MITgcm simulation, this study quantifies meridional internal wave energy fluxes in the Southern Ocean across 35S, 45S, 55S, and 65S from 29 May to 15 November 2012. Fluxes are separated into three bands: semidiurnal, near‐inertial, and continuum. Zonally integrated total fluxes are poleward at O(10) GW (15.3, 7.0, and 15.9 GW across 35S, 45S, and 55S, respectively), dropping to 0.3 GW at 65S. The semidiurnal tidal band accounts for over 80% of the total flux, contributing 13.4, 5.7, and 14.8 GW at 35–55S, before dropping to 0.15 GW at 65S. This sharp decrease represents a convergence of ∼15 GW between 55S and 65S, corresponding to a background diapycnal diffusivity of . Equatorward semidiurnal fluxes are spatially intermittent and limited to a few topographic features, including the Macquarie Ridge and mid‐ocean ridges. In contrast, poleward fluxes are more broadly distributed in the Pacific but dominated by a few major hotspots in the Atlantic and Indian sectors, most notably the Drake Passage. Continuum band fluxes decrease from 2.3 GW at 35S to 0.2 GW at 65S, with a consistent approximately 0.7 GW convergence across each 10 latitude band. In the near‐inertial band, from 35S to 55S, zonally integrated flux accounts for just 1%–3% of the total internal wave flux and is mostly equatorward, acting against the net flux. Despite 17.4 GW of near‐inertial wind work between 35S and 65S, the meridional flux divergence remains weak, exporting only 1%–4% of the wind input. 

Abstract The oceanic partial pressure of carbon dioxide (pCO₂) determines seasonal and geographic variations in air‐sea CO₂fluxes. Observations have demonstrated that the leading drivers ofpCO₂vary seasonally and meridionally. However, climate models often poorly represent the phasing of the seasonal cycle of the air‐sea CO₂fluxes in the Southern Ocean. Phasing discrepancies may be related to an inappropriate representation of the variability ofpCO₂drivers. In this study, we usepCO₂observations and the Biogeochemical Southern Ocean State Estimate to understand the source of modelpCO₂regional biases. Through a number of perturbations to model parameters and initial conditions, and sub‐regional parameter optimizations using a Green's function approach, we obtain significant improvements (18%–79% reduction in root‐mean‐squared error, rmse, and 67% and 92% improvement to correlation) in B‐SOSE's simulatedpCO₂with respect to assimilated SOCAT observations and moderate improvements (8%–31% reduction in rmse) with respect to non‐assimilated BGC‐Argo observations. In addition, the optimized solution modifies the system dynamics, producing spatial patterns ofpCO₂seasonal variability drivers that agree with observational studies. We show that calibration of biogeochemical parameters, including phytoplankton growth and mortality rates, is needed to modify the phasing of thepCO₂seasonal cycle in this biogeochemical model of the Southern Ocean. We also show that alkalinity initial conditions affect sub‐regional meanpCO₂magnitudes in the model response.