Highlights from Antarctic ocean mechanics research 2022/23

Changing deep waters

These long-term timeseries data indicate there are clear tidal controls on rates of generation of cold salty water that enters the global thermohaline circulation. In addition, there is now evidence that this is affecting the oxygenation of deep waters. This is happening because meltwater around Antarctica reduces how much cold, salty water flows to the ocean deep, thus reducing the supply of oxygen. The shrinking oxygen-rich bottom water layer is then replaced by warmer waters that are lower in oxygen, further reducing oxygen levels. This affects marine life at all trophic levels, because ocean animals, large and small, respond to even small changes in oxygen. We are involved in a number of synthesis reviews, including those on Antarctic Bottom Water and Southern Ocean dynamics.

Coupled ocean-atmosphere model

One of the big challenges modelling the effects of changing climate on the ocean is that sea ice is a critical boundary condition, which is hard to model and also affected by the ocean itself. The obvious but very difficult solution is to model the ocean, atmosphere and sea ice simultaneously – a coupled model. Through Antarctic Science Platform research and the National Modelling Hub, we have developed the first regional scale coupled ice shelf-ocean-sea ice-atmosphere model for the Ross Sea, specifically created to capture the interactions between atmosphere, ocean and ice at relevant spatial scales.

Polynya work continues

We continued our multiyear collaboration with the Korean Polar Research Institute. We redeployed our on-going Drygalski Ice Tongue moorings, which are designed to monitor how Antarctica’s largest remaining glacier ice tongue is affecting a major polynya. In addition to the successful instrumentation deployment, a new PhD project (Liv Cornelissen) is underway looking to understand these data and how they relate to changing ice and ocean conditions.

Ice cavity ocean processes

Along with Project 1 (Ice Dynamics), we are advancing our understanding of ice shelf cavity processes on a number of fronts, including modelling, analysis of long timeseries, as well as a recently-published collaboration with a US team providing an unprecedented view of the KIS grounding zone using an underwater robot, Icefin. The critical aspects we are focusing on are the role of stratification, and the role of small-scale mechanics in controlling circulation and melt rate timescales for cavities. PhD student Yingpu (Rin) Li is analysing data from instruments deployed in the middle of the Ross ice shelf in 2017. In addition, visiting PhD student Liangjun Yan from Hohai University in China is modelling interactions between the ocean cavity and the wider Ross Sea. Papers on this are in the works.

Paleoceanographic proxies

The development, testing and application of new and emerging paleoceanographic proxies is maturing. Highlights over the past year include:

• publication of a new radiolarian transfer function for the reconstruction of subsurface temperatures,
• the validation of diatom-bound δ15N as a proxy for nutrient utilization, and
• a compilation of marine temperatures in the Ross Sea over the past 45 million years, (including the Plio-Pleistocene) using novel machine learning-based calibrations of lipid biomarkers.

We collected marine-sourced aerosol samples for biomarker analysis, in order to develop proxies of past sea ice and biological productivity, and learned how to apply diatom transfer functions to generate quantitative sea surface temperature and sea ice concentration estimates. Quantitative reconstructions of these environmental parameters will provide an important node of connection between those project members working to reconstruct past climate and ocean conditions and those using models to project future climate response.

New opportunities

Our ocean research connects with two Antarctic Science Platform Opportunities Fund projects. This includes Dr Katie Maier’s deployment of a sediment trap mooring in a canyon system on the Ross continental shelf break, and Dr Sara Seabrook’s sampling from the TAN2301 looking at groundwater fluxes and identifying areas where seabed methane is being released into the ocean.

Ocean observation systems

Our understanding of cross-disciplinary connections for the Antarctic Earth system advancing with the design of a Ross Sea observing system framework with New Zealand collaborators to identify the key elements in a state-of-the-art ocean observing system for the region. Additionally, design of an observing system for the Ross Sea–far East Antarctic Region (RSfEAR) extends the scope to span a large longitudinal range and connects a number of ice shelves and polynyas promoting sea-ice growth and underpinning a diverse and rich ecosystem. This is collaboration between Australia, Korea and New Zealand, and connects to government stakeholder perspectives, as well as UN Ocean Decade objectives.

A timeline for the Ross Sea-East Antarctic conditions spanning three centuries (note the non-linear time dimension) including key observational initiatives from the early discovery voyages (James Clark Ross, Gauss, IGY) through to modern technological approaches (Argo, Satellite, SOCCOM). Modern era timeseries such as sea-ice extent, Southern Oscillation strength and salinity of the region are included. Significant cryospheric events in the form of the calving of giant icebergs B15 and C28 are also included along with policy developments (MPA, IPCC AR5 and AR6 and Southern Ocean Decade – SOD). Image: Heil et al, 2023.