​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system

Hydrographic data from mooring M3 on the upper continental slope, east of Filchner Trough, February 2017 – February 2021

Time series data of physical oceanography (seawater conductivity, temperature, pressure, salinity) were obtained from mooring M3 on the upper part (750 m isobath) of the continental slope, just east of the Filchner Trough in the southern Weddell Sea in February 2017 - February 2021. The mooring was deployed during the WAPITI expedition on James Clark Ross (JR16004), and recovered during the COSMUS expedition with Polarstern (PS124). The attached archive contains data from 1 RCM7 (21 meters above bottom (mab herafter) and sampling interval (sint hereafter) 2h), 13 SBE56 (22,56,81,106,159,184,6508,260,285,310,335,360,385 mab, sint: 120 s), 4 SBE37 (31, 134, 209, 410 mab, sint: 600 s), 1 SBE39 (435 mab, sint: 900s). Mooring diagrams and information about data processing are provided

Physical oceanography and current velocity data from mooring M3 on the upper continental slope, east of Filchner Trough, February 2017 – February 2021

Time series data of physical oceanography (seawater conductivity, temperature, pressure, salinity) and ocean current velocities were obtained from mooring M3 on the upper part (750 m isobath) of the continental slope, just east of the Filchner Trough in the southern Weddell Sea in February 2017 - February 2021. The mooring was deployed during the WAPITI expedition on James Clark Ross (JR16004), and recovered during the COSMUS expedition with Polarstern (PS124). The attached archive contains data from 1 RCM7 (21 meters above bottom (mab herafter) and sampling interval (sint hereafter) 2h), 13 SBE56 (22,56,81,106,159,184,6508,260,285,310,335,360,385 mab, sint: 120 s), 4 SBE37 (31, 134, 209, 410 mab, sint: 600 s), 1 RDI ADCP 75 kHz (235 mab, upwardlooking, sint: 2h), 1 SBE39 (435 mab, sint: 900s). Mooring diagrams and information about data processing are provided

Current velocity data from mooring M3 on the upper continental slope, east of Filchner Trough, February 2017 – February 2021

Time series data of ocean current velocities were obtained from mooring M3 on the upper part (750 m isobath) of the continental slope, just east of the Filchner Trough in the southern Weddell Sea in February 2017 - February 2021. The mooring was deployed during the WAPITI expedition on James Clark Ross (JR16004), and recovered during the COSMUS expedition with Polarstern (PS124). The attached archive contains data from 1 RDI ADCP 75 kHz (235 mab, upwardlooking, with sampling intervall: 2h. Mooring diagrams and information about data processing are provided

Necessary Conditions for Warm Inflow Toward the Filchner Ice Shelf, Weddell Sea

Understanding changes in Antarctic ice shelf basal melting is a major challenge for predicting future sea level. Currently, warm Circumpolar Deep Water surrounding Antarctica has limited access to the Weddell Sea continental shelf; consequently, melt rates at Filchner‐Ronne Ice Shelf are low. However, large‐scale model projections suggest that changes to the Antarctic Slope Front and the coastal circulation may enhance warm inflows within this century. We use a regional high‐resolution ice shelf cavity and ocean circulation model to explore forcing changes that may trigger this regime shift. Our results suggest two necessary conditions for supporting a sustained warm inflow into the Filchner Ice Shelf cavity: (i) an extreme relaxation of the Antarctic Slope Front density gradient and (ii) substantial freshening of the dense shelf water. We also find that the on‐shelf transport over the western Weddell Sea shelf is sensitive to the Filchner Trough overflow characteristics

Ice shelf fronts as dynamical barriers preventing flow of warm ocean water towards the Antarctic ice sheet

International audienc

Ice front blocking of ocean heat transport to an Antarctic ice shelf

Mass loss from the Antarctic Ice Sheet to the ocean has increased in recent decades, largely because the thinning of its floating ice shelves has allowed the outflow of grounded ice to accelerate1,2. Enhanced basal melting of the ice shelves is thought to be the ultimate driver of change2,3, motivating a recent focus on the processes that control ocean heat transport onto and across the seabed of the Antarctic continental shelf towards the ice4–6. However, the shoreward heat flux typically far exceeds that required to match observed melt rates2,7,8, suggesting that other critical controls exist. Here we show that the depth-independent (barotropic) component of the heat flow towards an ice shelf is blocked by the marked step shape of the ice front, and that only the depth-varying (baroclinic) component, which is typically much smaller, can enter the sub-ice cavity. Our results arise from direct observations of the Getz Ice Shelf system and laboratory experiments on a rotating platform. A similar blocking of the barotropic component may occur in other areas with comparable ice–bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf9. Representing the step topography of the ice front accurately in models is thus important for simulating ocean heat fluxes and induced melt rates

The Troll Observing Network (TONe): A contribution to improving observations in the data-sparse region of Dronning Maud Land, Antarctica

Understanding how Antarctica is changing and how these changes influence the rest of the Earth is fundamental to the future robustness of human society. Strengthening our understanding of these changes and their implications requires dedicated, sustained and coordinated observations of key Antarctic indicators. The Troll Observing Network (TONe), now under development, is Norway’s contribution to the global need for sustained, coordinated, complementary and societally relevant observations from Antarctica. When fully implemented within the coming three years, TONe will be a state-of-the-art, multi-platform, multi-disciplinary observing network in data-sparse Dronning Maud Land. A critical part of the network is a data management system that will ensure broad, free access to all TONe data to the international research community