At the southernmost limit of the Southern Ocean, in a broad band around Antarctica, the ocean freezes to form sea ice. At its maximum extent in September the frozen ocean covers 20 million square km, approaching three times the area of Australia.
This frozen skin on the ocean plays a critical and intriguing role in how the ocean and atmosphere interact and how they circulate and transport heat. The sea ice rejects salt as it forms and this drives a vertical overturning of the ocean. It also reflects a large proportion of solar radiation back to space that would otherwise be absorbed by the ocean. Sea ice maintains a tenuous existence at the interface of the ocean and atmosphere, and is highly sensitive to temperature changes. This makes it a particularly important indicator of any change to the climate system.
We do not know the total volume of sea ice forming each year, and this is a strong focus of Australia’s glaciological research program. Internationally, ASPeCt — the Antarctic Sea Ice Processes and Climate program — has as one of its main aims development of a climatology of sea ice thickness around Antarctica. To achieve this, ASPeCt have embarked on parallel programs to make the necessary observations from research vessels operating in the Antarctic.
Ship data being gathered by Australian, Russian, German and US Antarctic programs are vital to our understanding of the thickness and properties of Antarctic sea ice because there is no reliable way of determining these parameters from satellite data. An intensive effort is seeking as much information as possible from field measurements made aboard research vessels such as Australia’s Aurora Australis. The photographs above show the large differences that can occur in sea ice thickness, from newly forming thin ice in the autumn, to much thicker ice that forms by ridge building by the spring.
Australian sea ice scientists have played a leading role in developing these programs, particularly in establishing protocols for making sea ice observations from ships, that are now in use by scientists around the world. A CD-ROM, Observing Antarctic Sea Ice, has been compiled to help train sea ice observers (at right). Each year scientists now make observations of the thickness, concentration, floe size and topography of Antarctic sea ice and contribute them to a central database at the Australian Antarctic Data Centre. These data are now used by scientists all around the world.
The historical data, translated from records made as early as the 1970s (some of which were found decaying in old warehouses), have come from a number of different countries. Russian scientists have provided a wealth of historical data — invaluable for providing snapshots of past conditions and for compiling with more contemporary observations to provide a time series of data.
The combined data sets now provide a circumpolar picture of sea ice conditions around Antarctica based on about 20,000 individual observations from 74 voyages over the past several decades. While much more analysis will be done on these data, preliminary results show regional variability of sea ice thickness around the continent. This data set will be useful for comparisons with the output of sea ice models and as a baseline data set for assessing future climate change.