Antarctic Bottom Water and the Ross Sea: a gateway to global ocean circulation
Global ocean circulation and a changing climate are interdependent; changes in one can have cascading effects on the other. The Southern Ocean plays a critical role in Earth’s climate, absorbing excess heat and carbon dioxide (CO2) from the atmosphere. Changing environmental conditions and declining Antarctic sea ice have significant consequences for global climate, ocean ecosystems, and sea level rise. Research into ocean dynamics and dense water fluxes, particularly Antarctic Bottom Water formation, circulation, and interaction with Circumpolar Deep Water, provides a clearer understanding of how the Ross Sea and Southern Ocean will behave in future.
This summary
This summary aims to inform policy makers and scientific peers about oceanographic research conducted in the Ross Sea region by New Zealand’s Antarctic Science Platform (ASP).
This research synthesis:
- shares information about the formation and circulation of Antarctic Bottom Water, a form of dense water critical to global ocean circulation
- highlights climate change-related risks to the Ross Sea and global oceans
- provides critical baselines and new insights to understand how dense water mixes with the Circumpolar Deep Water
- provides examples of ASP contributions to data collection and ocean modelling in order to understand past, present, and future conditions in Antarctica and the Southern Ocean
Key points
- The Southern Ocean regulates climate by absorbing excess heat and carbon CO2 from human activity.
- Antarctic Bottom Water, which makes up 30–40% of the ocean’s total volume, forms during sea ice formation. As this dense, cold, salty water sinks, it drives the deep, overturning circulation of the ocean — transporting heat, oxygen, carbon, and nutrients throughout the deep ocean.
- Since the 1990s, 30% less dense water has flowed from the Ross Sea into the global ocean. This is likely linked to sea-ice decline and melting ice sheets causing changes to the salinity and density of the surface waters.
- Anchored oceanographic moorings are monitoring changes at critical sites in the Ross Sea. This includes deep troughs where Antarctic Bottom Water exits from underneath the ice shelf, and locations on the continental shelf where ‘warmer’ (+2℃) Circumpolar Deep Water enters, potentially reaching the vulnerable cavity under the Ross Ice Shelf and accelerating melting.
- Argo floats (which travel with ocean currents) are a leap forward for measuring water characteristics of the Ross Sea, year-round.
- The outflow of dense water occurs along a series of troughs that traverse the Ross Sea continental shelf from south to north. The interaction with ocean water depends on water density on the shelf and the strength of the tides.
Ocean research
Figure 1: Winds and ocean currents create polynyas, areas of open water surrounded by ice, exposing the surface to freezing temperatures.
Integrated and innovative observation methods are needed
Observing Antarctic Bottom Water is challenging. Once Antarctic Bottom Water has formed and leaves the continental shelf, it is typically found at depths below 2 kilometres. In-situ measurements are scarce and expensive, requiring ship-based deployment of oceanographic instruments. Satellites can’t provide measurements below the surface, and in winter, their ability to observe is limited by sea ice and long periods of darkness — when most dense water is being formed.
ASP researchers and collaboration partners have critically improved Antarctic Bottom Water observations in the Ross Sea region through the integration of methods including:
- long-term monitoring baselines from moorings
- ship-based surveys
- Argo floats
- bore holes to access the ocean beneath ice shelves
- autonomous observing tools
- data assimilation models
- satellite-derived proxies.
The variety of approaches now available is beginning to transform our understanding of Antarctic Bottom Water, providing valuable information about formation processes, variability over time and how cold, dense deep waters flow into the global ocean.
To determine how much dense water mixes with the Circumpolar Deep Water, we monitor water masses as they exit from underneath the ice shelf, and then again when they cross the continental shelf.
Tides regulate the flow and density of Antarctic Bottom Water
About 25% of Antarctic Bottom Water is formed predominantly in three polynya: the larger Ross Sea polynya, and the smaller McMurdo and Terra Nova Bay polynyas. The outflow of dense water occurs along a series of troughs that traverse the Ross Sea continental shelf from south to north. The main pathway is via the Drygalski Trough (a deep channel on the continental shelf) from Terra Nova Bay.
Long-term measurements from the Terra Nova Bay region and the Drygalski Trough showed that that the speed and density of the bottom water outflow are mainly controlled by two factors:
- 1. the water density in Terra Nova Bay (modulated by changes in sea ice and ice shelf conditions) which acts like an ‘accelerator’
- 2. tidal mixing, which acts like a ‘brake’.
Water density follows a strong seasonal cycle, and changes to this cycle would significantly affect both the amount of relatively dilute ice-shelf water flowing into the bay each year and the movement of dense water down the trough. Adding complexity, tides also create two flow peaks each year, around the equinoxes, and could cause flow and density to change by about 30% over a ~19 year tidal cycle. This means tides explain much of the decade-to-decade variation in the flow, while longer-term changes are likely due to density changes in Terra Nova Bay.
ASP scientists placed instruments near Cape Adare and found the water there became saltier in 2018, compared to earlier years. This matched trends seen in other regions of the Ross Sea. Research also discovered that salty, dense water from the Drygalski Trough arrives in two daily pulses, and these are linked to the tides. When tidal currents slow down, the flow of dense water increases. This increase suggests that tidal patterns, especially daily and long-term tides, play a key role in controlling when and how much dense water flows out of the region.
New observations of ocean dynamics beneath ice shelves
A 2020 review highlighted major uncertainties in ice sheet modelling, mainly due to climate forcing and ocean-induced melt rates. Antarctic surface waters are usually close to -2℃ and a critical concern is how much relatively warm Circumpolar Deep Water (+2℃) will move onto the Ross Sea continental shelf and under the Ross Ice Shelf, potentially accelerating melting.
ASP researchers are leading the collection of oceanographic data addressing these uncertainties by deploying innovative ocean mooring systems in the Ross Sea and under the Ross Ice Shelf through ice boreholes. These moorings are anchored on the sea floor in a fixed location, and sensors are mounted on a wire above the anchor.
Ice shelf cavity moorings were deployed down iceholes during drilling at two sites on the Ross Ice Shelf. These observations are reshaping previous scientific models and demonstrating the need for more realistic simulations. ASP researchers recorded dynamic, layered water structures beneath ice shelves, revealing tidal influences on melt rates. These data are used to improve models of ice melt, flow, and ocean dynamics beneath ice shelves (floating extensions of the land-based Antarctic Ice Sheet). The work collectively emphasises the combined importance of stratification and the tides in controlling diffusive processes, and the regional circulation processes in the very large ice cavity, which then influences next generation circulation modelling. Also, serendipitous observations were made relating to tsunami propagation beneath ice shelves and biological distributions in cavities.
Aotearoa New Zealand contributes to international Argo program
In recent years, significant advances in technology have enabled much better understanding of the mechanisms driving Antarctic Bottom Water formation. Argo floats have revolutionised what we know about the oceans. These robotic floats drift with the ocean currents and at regular intervals move up and down the water column measuring key parameters. Data is transmitted via satellite when they surface to provide near real-time data and insights. All Argo data is freely available.
The ASP’s focus has been to improve direct observations in the Ross Sea polynya (where sea ice and Antarctic Bottom Water forms). Usually, standard Argo floats are deployed in at least 2,000 m deep waters. ASP researchers worked with international Argo collaborators to deploy floats on the Ross Sea continental shelf, shallower than 2000m, and where few observations exist.
The ASP catalysed an international deployment of a fleet of Argo floats supplied by New Zealand, the United Kingdom, Canada, and Italy, during research voyages to the Ross Sea. As part of this network, the ASP deployed the first multi-parameter Bio-Argo profiling floats (fitted with a range of biogeochemical sensors) in the Ross Sea. The new Argo observations, in collaboration with data from moorings and surveys have provided critical, new insights on how surface winds, sea ice, and the Antarctic Ice Sheet influence and change Antarctic Bottom Water at globally-relevant scales.
Contact information
Craig Stevens
Principal Scientist – Earth Sciences NZ
craig.stevens@niwa.co.nz
Melissa Bowen
Associate Professor, University of Auckland
m.bowen@auckland.ac.nz
Denise Fernandez
Physical Oceanographer – Earth Sciences NZ
denise.fernandez@niwa.co.nz
Definitions
Antarctic Bottom Water: Dense water near Antarctica that is formed when cold, salty water sinks to the ocean floor, becoming the densest water in the world's oceans. Antarctic Bottom Water is a crucial part of the ocean's overturning circulation, transporting oxygen, carbon, and nutrients throughout the deep ocean.
Circumpolar Deep Water: A relatively warm and salty water mass that is formed from a mixture of deep waters from all the world's oceans. It's a key component of the Antarctic Circumpolar Current (that encircles Antarctica).
Flux: In ocean science ‘flux’ refers to the rate at which something (like heat, moisture, or particles) is transferred or flows across a surface, for example between the ocean and the atmosphere.
Polynya: Polynyas are areas of open water surrounded by sea ice, and can range in size from hundreds of meters to hundreds of square kilometres.
References
Bennetts et al. (2024). Closing the loops on Southern Ocean dynamics: from the circumpolar current to ice shelves and from bottom mixing to surface waves. Reviews of Geophysics 62(3): 1–80. https://doi.org/10.1029/2022RG000781
Bowen et al. (2021). The role of tides in bottom water export from the western Ross Sea. Scientific Reports 11(1): 2246. https://doi.org/10.1038/s41598-021-81793-5
Bowen et al. (2023). Tides regulate the flow and density of Antarctic Bottom Water from the western Ross Sea. Scientific Reports 13: 4984. https://doi.org/10.1038/s41598-023-32101-w
Calkin et al. (2024). Recent sedimentology at the grounding zone of the Kamb Ice Stream, West Antarctica and implications for ice shelf extent. Quaternary Science Reviews 344: 108988. https://doi.org/10.1016/j.quascirev.2024.108988
Clem et al. (2024). Antarctica and the Southern Ocean. Bulletin of the American Meteorological Society 105(8). https://doi.org/10.1175/BAMS-D-24-0099.1
Friedrichs et al. (2022). Observations of submesoscale eddy-driven heat transport at an ice shelf calving front. Communications Earth & Environment 3: 140. https://doi.org/10.1038/s43247-022-00460-3
Griffiths et al. (2021). Breaking all the rules: the first recorded hard substrate sessile benthic community far beneath an Antarctic ice shelf. Frontiers in Marine Science 8: 642040.
Gunn et al. (2023). Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin. Nature Climate Change 13: 537–544. https://doi.org/10.1038/s41558-023-01667-8
Horgan et al. (2025). A West Antarctic grounding-zone environment shaped by episodic water flow. Nature Geoscience 18: 389–395. https://doi.org/10.1038/s41561-025-01687-3
Lawrence et al. (2023). Crevasse refreezing and signatures of retreat observed at the Kamb Ice Stream grounding line. Nature Geoscience 16: 238–243. https://doi.org/10.1038/s41561-023-01129-y
Martínez-Pérez et al. (2022). Phylogenetically and functionally diverse microorganisms reside under the Ross Ice Shelf. Nature Communications 13: 117. https://doi.org/10.1038/s41467-021-27769-5
Miller et al. (2024). High Salinity Shelf Water production rates in Terra Nova Bay, Ross Sea from high-resolution salinity observations. Nature Communications 15: 373. https://doi.org/10.1038/s41467-023-43880-1
Miller et al. (2024). The coupling of winds, ocean turbulence, and High Salinity Shelf Water in the Terra Nova Bay Polynya. Deep Sea Research Part II: Topical Studies in Oceanography 217: 105412. https://doi.org/10.1016/j.dsr2.2024.105412
Price et al. (2025). Basal reflectance and melt rates across the Ross Ice Shelf, Antarctica, from grounding line to ice shelf front. Journal of Glaciology 71: 1–32. https://doi.org/10.1017/jog.2025.10
Silvano et al. (2023). Observing Antarctic Bottom Water in the Southern Ocean. Frontiers in Marine Science 10: 1221701. https://doi.org/10.3389/fmars.2023.1221701
Stevens et al. (2024). Ocean processes south of the Drygalski Ice Tongue, western Ross Sea. Deep Sea Research Part II: Topical Studies in Oceanography 217: 105411. https://doi.org/10.1016/j.dsr2.2024.105411
Stewart et al. (2024). Short Note: 2022 Hunga Tonga–Hunga Haʻapai tsunami measured beneath the Ross Ice Shelf. Antarctic Science. https://doi.org/10.1017/S0954102023000366
Washam et al. (2023). Direct observations of melting, freezing, and ocean circulation in an ice shelf basal crevasse. Science Advances 9(43): eadi7638. https://doi.org/10.1126/sciadv.adi7638
Yoon et al. (2020). Variability in high-salinity shelf water production in the Terra Nova Bay polynya, Antarctica. Ocean Science 16: 373–388. https://doi.org/10.5194/os-2019-80