The oceans around Antarctica are some of the harshest environments on the planet to measure data. Nevertheless, these data are vital, as tiny changes in temperature, salinity, oxygen, pH and other information can help us understand our changing high-latitude oceans. This data underpins our research into ocean mechanics, ice dynamics, Ross Sea ecosystems, and the global ocean uptake of heat and carbon dioxide. Because we need more than a snapshot, it is vital that we record changes over annual cycles and longer to fully grasp the processes at play, both due to weather and the changing climate. We deploy precision sensors in what gets called an oceanographic, or sometimes hydrographic, mooring.
Sea ice in late summer in the Ross Sea. Photo: Craig Stevens/NIWA
Oceanographic moorings are anchored on the sea floor in a fixed location with a large ballast weight, usually a couple of railway wheels chained together. A wire runs up from this, held aloft by floatation and the sensors are mounted on the wire. These moorings can sometimes be several km “tall”. Typically, though they don’t reach the surface, which protects the equipment from waves, vessels and ice bergs.
Being sub-surface creates the first challenge – how to get them back? We use an “acoustic release”, which is essentially a smart hook linking the ballast to the mooring itself. When it comes time to recover the mooring, the ship positions itself close by and sends out specific acoustic pulses informing the release to let go! This is followed by a few nervous minutes waiting for the buoys and line to surface.
Instrumentation includes CTDs (conductivity, temperature, depth sensors) and current meters to measure ocean currents and collect critical ocean data. Biological sensors measure levels of O2, light and chlorophyll. Sediment traps collect particles falling toward the sea floor (including organic matter, shells, dust and minerals) to understand the nutrient cycle and ocean circulation, now and in the past.
Recovering an ocean mooring from the Ross Sea. Photo: Brett Grant/NIWA
Moorings provide a wealth of critical ocean data to inform biological, physical and geoscientific research. To describe and project local and global climate change impacts, we need to understand all the components, processes and interfaces in Antarctica’s connected ice-ocean-land-atmosphere system.
For many of the processes we are seeking to observe and measure there is little alternative to moorings, as satellites only see the ocean surface, plus, we need data all year round regardless of sea ice cloud conditions, which can hamper vessels and satellites.
We have deployed arrays of moorings in different areas of the Ross Sea to target some specific processes. For example, we have had instruments in the Terra Nova Bay polynya for a number of years, helping to identify how polynya work and the role of the massive Drygalski Ice Tongue. We have also had moored instruments on the continental shelf break near Cape Adare, as we believe this is a key location for drainage of water off the continental shelf and into the oceanic abyss. More recently, we have collaborated with a fishing company who have helped us to deploy small moorings right at the front of the Ross Ice Shelf.
While moorings provide great resolution in time – sometimes recoding data every 10 minutes – the ocean suddenly gets very big when only deploying a handful of moorings. It is critical that research programmes find ways to share mooring data and effort, so we work closely with Italian and Korean research teams to develop the best coverage.
Mooring locations in the Ross Sea to support Antarctic Science Platform research. (1) Ice-cavity moorings in the Ross Ice shelf cavity were deployed during drilling at the Hot Water Drill (HWD) and Kamb Ice Stream (KIS) sites. (2) Moorings to support the RISIPE - Ross Ice Shelf Integrated Polynya Experiment. (3) Terra Nova Bay and Antarctic Near-shore and Terrestrial Observing System (ANTOS) moorings. (4) Ross Sea Outflow and Inflow (RSO & RSI) moorings. The dashed line is the continental shelf break where the depth drops from ~600 m to many kilometres.
It is vital that our understanding of the Ross Sea has a basis in ocean mechanics, as the system is evolving so rapidly that empirical linear extrapolation will be seriously challenged. We will determine how the present Ross Sea operates in terms of transport and biogeochemical energetics. We will test this knowledge using examples of how a Past Ross Sea operated under warmer conditions. Together, these perspectives will enable us to suggest how a Future Ross Sea might operate.
Oceanic data will be used to:
Recovering an ocean mooring in Robertson Bay in Antarctica’s Ross Sea Photo: Fiona Elliott/NIWA