Argo profiling float array

Proposed in 1999

Until the 1990s, most deep ocean temperature measurements came from ships, and large regions, especially in the southern hemisphere in winter, were barely sampled. At the OceanObs'99 conference, the idea of a global array of autonomous profiling floats, named Argo after the ship of Jason's myth in the company of the Jason altimeter satellite, was proposed, and the first floats were deployed in 1999. Dean Roemmich of the Scripps Institution of Oceanography was a leading figure.

The array reached its target of 3,000 floats in November 2007.

How a float works

A float drifts at a parking depth of 1,000 metres for about nine days. It then descends to 2,000 metres and rises to the surface, measuring temperature, conductivity (salinity) and pressure as it climbs. At the surface it transmits the profile by satellite and then sinks again. A float can carry out more than 150 cycles over four to six years.

The floats are deployed by about thirty countries, and run as a single array under an international steering team. All data are freely available within about 24 hours, with no restrictions.

New measurements

The core array has been extended to measure oxygen, nitrate, pH and chlorophyll on biogeochemical floats and, from 2014, to 4,000–6,000 metres on Deep Argo floats. The data provide the main evidence of the ocean's heat uptake, as well as of changes in salinity and circulation, and feed ocean forecasts and climate models. The design goal is one profile in every three-degree square of open ocean every ten days, which ships alone could never provide.

What Argo cannot see

Argo floats cover the open ocean to 2,000 metres but not the shallow seas, the sea-ice zone or the deep ocean below. Deep Argo and biogeochemical floats extend the reach in depth and in variables, and under-ice floats extend it in space. The array's sampling depends on the drift of the floats, which can leave gaps where currents diverge, so deployment is planned to keep them in place. These limits are known and published, and that is part of why the data are trusted: users can see where the array is thin and take that into account.

Instruments and technologies

• Profiling float A glass-and-plastic cylinder about 2 metres long with a hydraulic buoyancy engine, a CTD sensor and a satellite transmitter.
• Iridium telemetry Satellite communication returns profiles within hours and allows missions to be changed.
• Data assembly centres National and regional centres check and distribute the data in a standard format.

The people

• Dean Roemmich Oceanographer who helped design and lead the Argo array.

Timeline

• 1999 Argo proposed at OceanObs'99; first deployments begin.
• 2007 The 3,000-float array is achieved in November.
• 2014 Deep Argo floats for 4,000–6,000 m under test.

Key lessons for observation today

• Many identical small instruments beat a few big ones Four thousand cheap floats cover the ocean better than ships. Mass production and autonomy change what a network can do.
• Open data from the start Free data within a day made the array a global public good. Data policy is set at the outset and honoured.
• Distribute the cost, keep one standard Thirty countries deploy floats, and all follow the same format and quality control. A federation of contributors works with one standard.

Sources

• NOAA AOML, The Argo Program: Two Decades of Ocean Observations
• Roemmich et al. (2009), The Argo Program
• Wikipedia, Argo (oceanography)