Argo is an international project with over 3,000 scientific floats drifting around the ocean. They measure currents, temperature and salinity of water to a depth of 2,000 m. The Argo project involves 23 countries, and New Zealand plays an important role. NIWA (the National Institute of Water and Atmospheric Research) has deployed hundreds of Argo floats for other countries, and NIWA oceanographer Dr Phil Sutton uses the real-time data in his research.

How Argo floats work

Each Argo float spends most of its time drifting about 1,500 m down in the ocean, but every 10 days, it sinks to 2,000 m, then rises to the surface. On the way up, instruments measure depth, temperature and salinity at regular intervals to provide what is called a profile. At the surface, data is sent to a satellite, which also records the float’s position. The new position is compared with the previous one, so the path of the float, and therefore currents, can be calculated. After the data has been sent, the float sinks back down to 1,500 m and drifts for another 10 days. This cycle carries on until the battery runs out after about 5 years.

The buoyancy mechanism works by changing the density of a liquid reservoir. When the liquid is allowed to expand, it becomes less dense and the float rises. When the liquid is compressed, it becomes denser and the float sinks. (You can explore this concept in the activity Buoyancy in water.)

The need for Argo

Argo floats let scientists like Dr Phil Sutton keep an eye on the state of the ocean. To understand what role the ocean plays in climate and climate change, we need to know what is happening beneath the surface of the ocean. Scientists use the data from Argo to construct models of the ocean and to look for clues that show how it is changing.

There was no large-scale collection of temperature and salinity data before Argo started in 2000. Similar sensors to the ones on Argo floats were used from ships, but these could only cover small areas for short times. Argo floats now give wide coverage over long time periods. Despite this huge improvement, there is still only one float for every 90,000 km2 of ocean, so care needs to taken about relating data to the whole ocean.

Argo continues to develop. Future plans include floats that go deeper and floats with more sensors that measure pH and dissolved oxygen

Working alongside the Argo project are Jason satellites that measure the height of the ocean surface very accurately to within a few centimetres. Water expands as it warms and stands slightly taller than surrounding water – a slight dome in the surface indicates a large mass of warm water below.

Check on the Argo website to see where all the floats are – the data is updated daily.

NIWA role in Argo

NIWA buys two Argo floats each year and plays another vital role by placing floats for other countries – over 750 in the last 8 years. NIWA research vessels, mainly the RV Kaharoa, reach areas of ocean less accessible to other countries. The Kaharoa’s most recent journey, in early 2010, deployed floats in the Pacific and Indian Oceans between New Zealand and South Africa. Later in 2010, it is off to Valparaiso in South America. There is even a shield proudly displayed on the Kaharoa with a model of a float and the words ‘World Champion Argo float deployers’.

Useful link

Main site of the Argo project, including the latest float information and an animation showing what the floats do.

    Published 18 June 2010