Tracking the evolution of Southern Ocean variability and change

Project 4479 completed observations as planned, extending one of the longest and most complete time series in the Southern Ocean. The data will be used to assess how and why the Southern Ocean is changing and the consequences of Southern Ocean change.

Activities undertaken during the 24/25 season included:
- Expendable bathythermograph (XBT) instruments were deployed along 6 transects between Hobart and Dumont d'Urville, from L'Astrolabe. Such measurements have been made every year since 1993, in partnership with France. The data are used to track changes in temperature of the upper 700 m and surface salinity in the Southern Ocean.
- Free-drifting profiling floats were deployed in the Australian sector of the Southern Ocean (5 Argo floats were deployed by this project south of 60S in the 2024-2025 field season). These floats contribute to the global Argo program, which is collecting sustained observations of the upper 2000 m of the global ocean. As the oceans have taken up more than 90% of the extra heat stored by the planet over the past 50 years, the global-scale continuous temperature measurements collected by Argo provide the best planetary thermometer we have to track warming of the Earth.

Eight papers were published in 2024/2025 relevant to project 4479.

A total of 14 Argo floats were deployed south of 60S as part of this project. Observers collected ocean temperature observations along 6 occupations of a line between Hobart and the French base Dumont D'Urville. Together these ocean measurements are being used to track change in the ocean south of Australia.

We completed a voyage on the RV Investigator in the 2017/18 season. The goal of the voyage was to document how and why the Southern Ocean is changing. We completed 108 full-depth oceanographic profiles along 4 repeat transects that will be compared to earlier measurements in the same locations to document changes in temperature, salinity, oxygen and chlorofluorocarbons. We deployed 29 floats and drifters, including the first Antarctic deployments of floats capable of profiling through the full ocean depth. Studies of the distribution of trace elements like iron and of Southern Ocean clouds were also carried out on the voyage.