Launching the Argo Armada

Data required to drive seasonal climate forecast models requires real-time surveys of the state of the ocean including the ocean heat content of the upper water column (which interacts with the atmosphere) and the temperature and salinity fields to compute ocean currents that move the heat content around. The Argo Steering Team maintains a global array of robotic floats that fills this need.

Skill is developing rapidly in computer models aimed at forecasting seasonal climate variability. For these to work they need real-time global data on the climatic state of the oceans. In response, a small group of researchers developed a prospectus for an enterprise in 1998, now known as Argo, which is coupled with the Jason satellite altimetry project as one of the two pillars of the global ocean observing system.
The prospectus rapidly gained widespread approval and deployments of instruments began in earnest in 2000. The big change was a decision to escape the traditional oceanographic observing habit of measuring properties of the oceans from instruments, attached to wires, hanging over the side of a ship. Instead, robotic instruments were developed that could adjust their own buoyancy and so move vertically in the ocean under their own power. The mass of a typical Argo float remains constant throughout its lifetime but it can change its volume, and, therefore, its buoyancy. The volume change is typically achieved by a piston inside the float that pushes oil out of the body of the float to inflate a bladder, rather like the child’s toy known as a Cartesian Diver. Instruments on top of the float measure temperature, salinity and pressure and the data are reported back via satellite systems.

Argo floats are small, robust and easy to launch. Few cruises are dedicated to the launch of floats; rather, launches tend to occur opportunistically when a ship happens to be heading to an appropriate area of the world oceans for other reasons. Generally a float is started and then lowered over the side of a ship with a rope. However, techniques have been developed to allow launches from container vessels operating at high speed and some floats have been launched successfully from aircraft.

Data acquired by a float must be returned to the owner of the float via satellite communications. Early in the programme almost all floats used the Argos data system run by CLS (Collecte Localisation Satellites) but increasingly floats communicate using the Iridium constellation.

Argo Management

Managing such a global effort for the benefit of everyone requires substantial coordination. The Argo Steering Team was created in 1999 and an Argo Data Management Team was created soon afterwards. These have, with varying memberships, steered the programme effectively with meetings that take place once per year. Overall coordination of Argo is assisted through the Argo Information Centre, an office belonging to the Intergovernmental Oceanographic Commission (which also coordinates GOOS – the Global Ocean Observing System) and WMO (the World Meteorological Organisation).

At the time of writing, we have 27 nations actively deploying floats in support of the Argo project. These all supply the temperature and salinity of the upper water column between a depth of 2000 metres and the surface every 10 days. Each float can potentially survive for 7 years. The result is that this armada of robots can supply a new survey of the climatic state of the ocean every 10 days, and this fits the requirements for data to drive the big 4-dimensional (3 space and one time) models that are being developed to forecast climate variability on scales varying from regional to global. There are currently about 3600 Argo robots supplying profiles from all ocean basins at the rate of about 11,000 per month, or one profile every 3.7 minutes.

More than 30 nations have contributed float deployments to the global array, but all floats operate to the same duty cycles. All report data on a 10-day duty-cycle, all national data teams report data to one of two Argo Global Data Centres in the same format and with the same rules on data management. The data are reported in near real-time with 90% of all profiles being made available on the global telecommunications system within 24 hours of acquisition. Thus independent global surveys of the state of the ocean are now reported every 10 days. These surveys are available to all potential users without constraint.

Argo Links

The Argo project has synergy with many other programmes. For example, the satellite mission known as Jason is observing the rise in globally-averaged sea levels. This rise is driven by two processes, firstly warming of the oceans causes water columns to expand raising sea level, and secondly melting of ice on land increases the mass of water in the oceans. Argo allows the first of these to be monitored. Meanwhile the melt-rate of ice on the Antarctic continent and on Greenland is monitored by the GRACE satellite mission. Argo also has an intimate connection with the Aquarius and SMOS satellite missions monitoring sea-surface salinity around the globe.

The Future

The Argo project largely achieved its primary objective by November 2007, and will continue monitoring the state of the oceans for many years. In 2009, the Argo group made a 10-year report to UNESCO at the OceanObs09 conference. As a result the team received many suggestions on how Argo might be improved. These suggestions were as follows:

1. Enhance the array to include high latitudes and marginal seas (such as the Gulf of Mexico and the Mediterranean).
2. Extend the observations into the deep ocean.
3. Enhance the array in strong boundary currents (such as the Gulf Stream and the Kuroshio).
4. Add sensors for monitoring biological and chemical changes in the oceans.

The change from Argos to Iridium communications by itself makes the first objective easy as floats spend less time at the sea surface and are less likely to wash ashore. The extension into the deep ocean is challenging, but prototype floats are working. Funding has recently been made available to increase the density of observations in the boundary currents and at the equator. Biological sensors are being developed, but we need to explore how these sensors would change the position of Argo vis-à-vis the Law of the Sea.