Argo is an international programme that has maintained a global ocean network of profiling floats since 2004. Presently there are about 3,900 operational floats with each float sampling the ocean as shown below. Initially, the float sinks to 1,000 m depth, where it parks for 10 days, drifting with the ocean currents. The float then sinks to before rising to the surface, measuring temperature and salinity as it ascends. Once on the sea surface, the float transmits its location and profile data via satellite before sinking and repeating the cycle. Thus, each Argo float provides measurements of the top 2,000 m of the ocean every 10 days, and the floats last for around five years. Argo now gives global coverage in all seasons. Argo data is widely used for many other applications, with their coverage, the increasing length of the time series and the near-real-time access making them invaluable for regional environmental studies.

The Argo float cycle: 1) Float deployment, 2) Descent to drifting depth, 3) Drifting 10 days, 4) Descent to profiling depth (2000-6000m), 5) Ascent: measuring essential ocean variables, 6) data transmission, 7) starting next cycle

An Argo float cycle. Image credit: Thomas Haessig/UCSD.

Argo is the only means of describing the subsurface ocean conditions at any point in the global ocean. In the Aotearoa New Zealand context, Argo data has been used to study inter-annual to decadal variability in ocean conditions and currents and in numerous studies of possible impacts of environmental changes on ecosystems and fisheries. Argo is currently expanding into two new domains. Deep Argo floats which profile to 6,000 m are being deployed to capture almost all of the ocean heat content change and biogeochemical sensors including dissolved oxygen, nitrate and pH are being added to floats. The first is the trend in ocean heat content between 2006 and 2013.

An example of output from analysis of Argo data is shown below, which shows that large changes in ocean heat content are occurring north east of Aotearoa New Zealand. Indeed, southern hemisphere changes dominate the global signals.

Further utility of the Argo data would come from integrating it with other local data at the regional level to get informed data for issues such as climate change effects on fisheries and ocean health, and MPA placement.

Map of world showing ocean heat content across the oceans

The trend in 0-2,000 m ocean heat content, 2006-2013. From [1]

References and footnotes

[1] Roemmich et al. (2015) Unabated planetary warming and its ocean structure since 2006, Nature Climate Change, 5, pp. 240-245.