Jean-Baptiste Sallée

The Antarctic ice sheet and ice shelves are losing mass and release freshwater to the ocean, affecting the ocean state. The response of ocean circulation to recent and projected increases in Antarctic meltwater is still uncertain, because of uncertainties in meltwater forcing and deficiencies in global ocean and climate models. Here, we use an equilibrated one‐degree global ocean configuration that realistically represents Antarctic Bottom Water (AABW) formation and abyssal ventilation. We perform three meltwater hosing experiments covering the period 1990–2100: an “Idealized” experiment in which exactly 0.1 Sv (1 Sv = ) of freshwater is released uniformly around the Antarctic coast; a “Realistic” experiment, where reconstructed and projected freshwater anomalies are distributed in time and space based on recent estimates of melting rates and processes; and a “Semi‐realistic” experiment in which the same time‐varying freshwater anomalies are released uniformly around the Antarctic coast. In all experiments, AABW transport at 64S decreases, by 5–12 Sv by 2100. Meanwhile, the Atlantic Meridional Overturning Circulation (AMOC) strengthens by approximately 2 Sv, driven by enhanced Southern Ocean upwelling. This upwelling increase results from weaker compensation between northward near‐surface flow and southward mesoscale‐induced flow in the stratifying upper Southern Ocean. In the idealized experiment, the AMOC ultimately weakens due to the propagation of fresh surface anomalies to the subpolar North Atlantic. The timing of simulated overturning changes depends strongly on meltwater forcing; changes appear latest in the most realistic experiment.

Antarctica InSync is a global effort to synchronise research across Antarctica and the Southern Ocean, connecting ice, ocean, land, climate, and life to protect this vital region. This roadmap paper introduces the scientific and technological motivation and action plan to build an international programme for a coordinated field season in Antarctica and its surrounding seas. It lays out the international collaboration that is required to realise an ambitious action plan and the opportunities for the science community to address important scientific themes that identify current knowledge gaps. In the spirit of the Antarctic Treaty Consultative Meeting (ATCM), the planned scientific action mandates free data exchange, facilitates international collaboration, and derives expert scientific advice together with the Scientific Committee on Antarctic Research (SCAR). Aligned with challenges of the United Nation Ocean and Cryosphere Decades, we plan a shared global effort to better understand the role of Antarctica and its surrounding ocean for our global futures, building on decades of achievements in polar and ocean science. As a milestone leading up to the next International Polar Year in 2032/33 (IPY5), the ambition is to achieve a new level of cooperation, with opportunities for the next generation of polar scientists around the world by enhancing practices in sharing of ideas, infrastructure and information.

Theme 1 addresses the Southern Ocean’s (south of 30°S) critical role in regulating Earth’s climate through circulation patterns that mediate global exchanges of heat, carbon, freshwater, and nutrients. The region has absorbed over 70% of anthropogenic heat and has contributed to about 40% of the global ocean uptake of human-emitted carbon dioxide (CO₂) while also controlling ice shelf stability and sea level rise. However, fundamental gaps in year-round observations, particularly during austral autumn, winter, and spring, severely limit our understanding of these processes and their responses to rapid climate change. This white paper establishes key knowledge gaps and high-priority recommendations that are tractable within the InSync timeframe through coordinated program execution with strong engagement from national operators and funding agencies. Critical needs include seasonal observations of the marginal ice zone where carbon, heat and nutrient-rich waters upwell, year-round continental shelf measurements where dense water formation and ice-ocean interactions occur, standardized air-sea flux measurements, and strategic monitoring of regional choke points. Success requires international resource sharing, coordinated deployments, and sustained commitment to both process studies and long-term monitoring.

Phytoplankton blooms above the seamount Maud Rise in the Antarctic Ocean have been reported but their emerging mechanisms and their importance for the wider Southern Ocean are not well known. We use satellite data spanning over the last two decades and in-situ data collected from a ship, an underwater glider and Biogeochemical-Argo profiling floats to understand the processes involved in the formation of Maud Rise phytoplankton blooms. We find that the seamount generates upwelling of warm deep water that transports heat, and likely dissolved iron, to the surface via diapycnal mixing. This creates a recurring annular structure of chlorophyll concentration (or chlorophyll halo) in correspondence with the previously observed warm water and sea ice halo over Maud Rise. The in-situ observations reveal integrated chlorophyll-a concentrations of up to 100 mg·m ⁻² , which suggests exceptionally high phytoplankton biomass within the Southern Ocean, thus making the seamount a regional phytoplankton hotspot.

The ocean takes up over 90% of the excess heat stored in the Earth system as a result of anthropogenic climate change, which has led to sea level rise and an intensification of marine extreme events. However, despite their importance for informing climate policy, future ocean heat uptake (OHU) projections still strongly differ between climate models. Here, we provide improved global OHU projections by identifying a relationship between present-day Antarctic sea ice extent and future OHU across an ensemble of 28 state-of-the-art climate models. Combining this relationship with satellite observations of Antarctic sea ice reduces the uncertainty of OHU projections under future emissions scenarios by 12-33%. Moreover, we show that an underestimation of present-day Antarctic sea ice in the latest generation of climate models results in an underestimation of future OHU by 3-14%, of global cloud feedback by 19-32%, and of global atmospheric warming by 6-7%. This emergent constraint is based on a strong coupling between Antarctic sea ice, deep ocean temperatures, and Southern Hemisphere sea surface temperatures and cloud cover in climate models. Our study reveals how the present-day Southern Ocean state impacts future climate change, and contrasts with previous constraints based on past warming trends.

The year 2023 shattered numerous heat records globally and regionally. We here focus on the origins of the unprecedented and persistent warm sea surface temperature (SST) anomalies in the North Atlantic Ocean. Using a combination of multiple observational datasets and climate model large ensemble outputs, we show that the 2023 North Atlantic SST event is a manifestation of anthropogenically-forced warming, exacerbated by an extreme phase of internal climate variability and associated regional surface flux anomalies. The long-term stratification increase due to anthropogenically-driven ocean warming amplified the impacts of internal variability. At current global warming level, the 2023 event is assessed as a decadal-type occurrence for SST anomalies averaged across the entire North Atlantic basin, but it is estimated as a centennial-type event for the subtropics and the eastern basin. The distinctive horseshoe-shaped regional distribution of these anomalies is crucial to consider for correct communication and risk assessment in a warming climate.

Model projections suggest that the continental shelf in the southern Weddell Sea may experience a shift from today's near‐freezing temperature to a much warmer state, where warm water floods the shelf and basal melt rates beneath the Filchner Ronne Ice Shelf increase dramatically. Today, the Filchner Trough serves as a conduit for the southward flow of Warm Deep Water (WDW) during summer and, thus, requires continuous monitoring of its hydrographic conditions. An extensive network of moorings was installed at key sites along the inflow pathway from 2017 to 2021, to expand on existing mooring records starting in 2014. The moorings complemented with under‐ice profiling floats reveal two inflow pathways, where WDW enters along the eastern flank of the Filchner Trough as well as through a smaller trough east of there. Within the observed period, 2017 and 2018 feature anomalously warm inflows. The inflow is regulated by the heaving of isopycnals over the continental slope, and the southward propagation toward Filchner Ice Shelf is two times faster during these warm years. Furthermore, the warm years coincide with low summer sea ice concentration, which enhances surface stratification through increased freshwater input and modifies sea ice‐ocean stresses that both act to lift the warm water layer and increase the temperatures on the continental shelf. Finally, the recent record low sea ice conditions around the Antarctic emphasize the importance of our findings and raise concerns regarding a potentially increasing presence of WDW on the southern Weddell Sea shelf.

Bottom water formation in the Weddell Sea and mass loss from the Filchner–Ronne Ice Shelf are tightly linked by the supply of Warm Deep Water to the continental shelf. Heavy sea ice cover and icebergs restrict ship access and upper-ocean measurements by moorings, compelling us to try new sampling methods. We present results from the first dedicated under-sea-ice float experiment tracking circulation on the continental shelf of the eastern Weddell Sea. Seven Apex profiling floats were deployed in 2017 at three different locations on the eastern Weddell Sea continental shelf, targeting the inflowing modified Warm Deep Water (mWDW), as well as the outflowing Ice Shelf Water (ISW). The floats capture a warm mWDW regime with southward inflow over the eastern continental shelf and a cold ISW regime with a recirculation of ISW in the Filchner Trough throughout the 4 years of observations. We provide the first Lagrangian in situ confirmation that the mWDW flowing onto the continental shelf follows two pathways: the eastern flank of the Filchner Trough and a small trough on the shallow shelf farther east. In the present circulation regime, this warm water is blocked from reaching the ice shelf cavity due to the presence of the thick ISW layer inside the Filchner Trough. The floats' trajectories and hydrography reveal the dynamically active front, flow reversal, and eddying motion between these two water masses along the eastern flank of the Filchner Trough.