Pack ice in northern McMurdo Sound showing the range of floe sizes and levels of aggregation. The complex web of leads (large fractures) in between the floes is a critical but highly variable area for heat and light transfer. Photo: Craig Stevens/NIWA
Seen from space, the annual cycle of Antarctic sea ice formation and loss is one of the most dramatic natural phenomena on Earth. Such an expanse of sea ice inevitably plays a huge role in defining the Antarctic marine environment, but what’s less well appreciated is the extent to which formation of Antarctic sea ice also affects ocean circulation across the globe.
Recent concerns over apparent changes to Antarctic sea-ice dynamics have led scientists to contemplate the potentially dire global consequences of a slow-down in sea-ice production. The processes that intertwine sea-ice formation and melting, weather and ocean circulation are extremely complex. An improved understanding of the mechanisms linking Antarctic sea ice to global ocean circulation is crucially important to quantify and respond to global risks.
Sea ice factories
Seawater freezes at -1.8oC. Sea ice forms when the sea reaches this temperature, and the air temperature falls sufficiently below this to allow ice formation at the surface. Most sea ice forms in a relatively few locations known as polynyas. Polynya is a Russian word that refers to a naturally-formed area of open water surrounded by sea ice. These ‘sea-ice factories’ can be coastal or in the open ocean. They occur where persistent winds and/or ocean currents move sea ice away from the area as fast, or faster than, it is forming. Conditions in these polynyas favour sea-ice production: the air and water are cold enough, and there is a constant formation to replace the ice that gets moved away. The newly-formed ice gradually thickens as it joins the ocean pack ice and moves away from the polynya.
How sea ice formation influences global oceans
The ocean is stratified (naturally separated into horizontal layers) due to density differences. Cold or salty water is dense and tends to sink below warmer, less salty water that is less dense. Mixing occurs through currents, winds and tides, and through heat slowly seeping deeper into the ocean. Mixing is generally slow.
The formation of sea ice produces particularly dense water at the ocean’s surface. Not only is the water as cold as seawater can get (or else it would not freeze), but when it does freeze, little of the salt in sea water forms part of the ice. Alignment of water molecules to form ice crystals leaves no room for the dissolved salt, which is squeezed out into the underlying water. This results in a layer of cold, salty and well-oxygenated water which, after mixing with relatively less cold ambient water, drops to the seafloor. This Antarctic Bottom Water makes up about 30-40% of the global ocean volume.
So much of this dense water is formed in active polynyas that it creates a substantial downwelling current. Ocean moorings in the Ross Sea deployed by the Antarctic Science Platform, along with national and international collaborators, monitor this flow. It forms a current that moves northwards along the floor of the continental shelf, typically concentrated in submarine canyons, before it tumbles down the shelf break to the bottom of the Southern Ocean as a series of submarine waterfalls. (Watch the video above or on YouTube.)
This process is a major driver of global ocean currents. The cold, salty, oxygenated Antarctic Bottom Water moves out from the Southern Ocean toward the equator to begin a millennium-long journey around the planet. The oxygen transported this way is fundamental to ecosystem processes in the deep ocean. And of course, water sinking in Antarctica displaces water already at depth towards the surface. Rich in nutrients after centuries of recycling, this water completes the ocean conveyor belt circulation and drives ocean productivity.
Production of Antarctic Bottom Water can be slowed by changes in sea ice. This can occur in various ways – for example, if less sea ice is formed, or if there is a reduction in the density of Antarctic Bottom Water due to ice being formed from sea water made fresher by the melting of coastal ice sheets. Recent modelling research has highlighted the effects that such changes could have on global climate, ecosystems and the ability of the oceans to absorb carbon dioxide.
Designing a Ross Sea Ocean Observatory
Understanding these complex processes and their interconnections requires different types of observations in different places brought together with modelling tools. This will provide critical information about what is happening right now and enable improved model results. Having a comprehensive network of ocean observations across the Ross Sea region is essential.
We aim to continue to develop our time series observations at critical locations in the Ross Sea and then, through partnerships with other national programmes and SOOS (Southern Ocean Observing System), create an integrated and responsive ocean observing system. One example of a network of international observations is the RSfEAR (Ross Sea and far East Antarctic) network being discussed with Australia.
Such a network would connect research on climate, ecosystems and sea-ice processes, advancing the Platform’s purpose of delivering excellent science to understand the impact of future changes in Antarctica.