Climate of Antarctica and the southern ocean

26 February 2009

The Climate of Antarctica and the Southern Ocean (CASO) program, led by the Antarctic Climate and Ecosystems Cooperative Research Centre and CSIRO Marine and Atmospheric Research, is working towards obtaining a circumpolar snapshot of the physical environment of the Southern Ocean. This snapshot will enhance scientists’ understanding of the role of the Southern Ocean in past, present and future climate, and improve climate predictions from models that incorporate a better understanding of southern polar processes.

CASO consists of 25 individual projects involving scientists from 18 nations. It links with other IPY projects, including studies of ocean circulation and ocean-ice interaction near the Antarctic margin, biogeochemistry, meteorology, ecology and paleoclimate.

During the IPY CASO, in collaboration with other IPY programs, measured a wide range of physical, chemical and biological properties of the Southern Ocean. The measurements covered the circumpolar extent of the Southern Ocean, from the surface to the sea floor and from the Antarctic continental shelf to the Subtropical Front. Scientists used a wide variety of tools, including ship transects, profiling floats, satellites, moorings and oceanographic sensors attached to marine mammals. The integrated, multi-disciplinary observations made with these tools, provide a ‘proof of concept’ for the long-term Southern Ocean Observing System presently under development by the international community.

The Australian CASO team deployed 30 ocean profiling (‘Argo’) floats throughout the Australian sector of the Southern Ocean, to measure temperature and salinity in the upper 2000 m of the ocean. These floats contribute to a network of over 3000 existing floats deployed throughout the world's oceans. Small oceanographic sensors were also deployed on seals, to take the first measurements of ocean conditions around Antarctica during winter. Observations from these, and other instruments, will be integrated with a range of modelling studies.

Australian scientists also conducted an oceanographic transect across the Antarctic Circumpolar Current, to understand the current’s interaction with the Southern Ocean overturning circulation, which regulates the heat and carbon stored by the ocean. Using instruments lowered from the ship, scientists measured temperature, salinity and oxygen concentrations, while water samples were collected from various depths and analysed for nutrients, carbon dioxide, chlorofluorocarbons, phytoplankton pigments and oxygen isotopes. Acoustic instruments were used to measure currents, and moorings were deployed to collect year-long records of water velocity and properties.

The observations showed that water sinking from the surface to the deep ocean near Antarctica is becoming fresher and less dense, demonstrating that changes in high latitude climate are being communicated rapidly to the deep ocean.

Preliminary work during a survey of Antarctic continental shelf waters, suggests that the ocean circulation patterns influence the distribution of benthic (bottom-dwelling) organisms. The oceanographic observations also showed that dense Antarctic Bottom Water was escaping from the continental shelf and sinking to the deep sea in this area, even during the summer.

A number of students participated on the CASO voyages and many will use the results in their research projects. The data collected during the CASO voyages has been archived in the international hydrographic data archive – CLIVAR & Carbon Hydrographic Data Office.

CASO scientists have also been active in schools and other public outreach activities, and a number of blogs and ship ‘sitreps’ describing the research were posted.