Published online 5 December 2007 | Nature 450, 780-781 (2007) | doi:10.1038/450780a

News Feature

Earth Monitoring: Observing the ocean from within

As names go, it's cleverer than most. At the mundane level, Argo stands for Array for Real-time Geostrophic Oceanography. But the name comes with not just one but two classical allusions attached. There's the Argo, the ship in which Jason sailed to find the golden fleece, a nod to the fact that the buoys that make up Argo were conceived as a counterpart to a satellite called Jason (see 'The Jason project'), which measures the surface topography of the oceans. And then there's Argos the giant, who was blessed with 100 eyes to see in all directions, with only a handful asleep at any one time. The modern Argo puts even that spectacularly distributed sensory system to shame — it has 3,000 different sense organs spread all around the globe.

WWW.ARGO.NET

The array's 3,000-odd autonomous floats, which look like upended torpedoes, are equipped with sensors for recording temperature and salinity in the upper 2,000 metres of the ocean (see enlarged map). Each float sinks, drifts, bobs up and transmits data to satellites on a regular basis. At its current size the array provides more than 100,000 temperature and salinity profiles each year, regardless of the season or weather. This is 20 times greater than the comparable annual measurements by research vessels and merchant ships, which in the past have been the main data source for this type of oceanography. In the past five years, Argo has more than doubled the total database on some seas that ships steer clear of — such as the seas around Antarctica in winter.

Over the next years, the array's design will be reassessed with an eye to the need for additional floats and sensors. Already, 60 of the floats carry oxygen sensors — will that number be increased? What about sensors for particulate carbon, or for chlorophyll? Such extras are attractive, but always come at the cost of increased demand on the floats' batteries and thus a shorter lifetime.

Another idea is to design floats that can dive to greater depths. And special floats for polar latitudes — tethered to ice floes or equipped with acoustic sensors that tell the instrument where it can safely surface — are already being tested in seasonally ice-covered regions.

Plugging the gaps

Predicting and closing the gaps that will constantly appear in the array will be a major logistical challenge. The types of floats currently in use have a lifetime of 4 years, meaning that around 800 instruments need to be replaced each year. And replacements need to maintain the system's global coverage. That means dropping floats out of low-flying aircraft or chartering ships to optimal sites for replenishment.

The array's annual costs — around US$24 million — are being shared by more than 30 nations. Only Germany, however, has so far made a firm long-term funding commitment. In the United States, which contributes 50% of the overall programme costs, funding currently comes through the National Oceanographic and Atmospheric Administration. But it is not yet clear which agency will be responsible for maintaining the array once Argo becomes a routinely operational system. Proof of definite advantages — for fisheries, merchant shipping, oil-spill management or naval purposes — could help secure long-term funding from more governments, says John Gould, a consultant and former director of the project. “But it's still rather early days to evaluate the benefits.”

Raft of achievements

The first scientific achievements using data from Argo include the detection of accelerated circulation in the subtropical South Pacific1, the tracking of the deep water that forms in the Labrador Sea2 and the observation of local sea-temperature changes during hurricanes. But there have also been setbacks. A surprising suggestion that the oceans' heat content was diminishing, rather than increasing as expected3, turned out to be an artefact caused by a software glitch4.

Gould says that this underlines the need for experienced scientists — who know both the oceans and the technology — to be involved in data management. “We're still learning,” he says.

Argo data are already incorporated into models for seasonal weather prediction. To initialize such forecasts, scientists 'tell' their models about the here and now and then cast them off into the future. Precise knowledge of the initial state of the ocean — which has a longer 'memory' than the atmosphere — could greatly improve the accuracy of longer-term weather and climate prediction. At the UK Met Office's Hadley Centre in Exeter, Argo data have been used to initialize the centre's new decadal temperature-forecasting system, which combines observations and models on longer timescales5.

“The data are absolutely invaluable,” says Matt Huddleston, a climate scientist at the Hadley Centre. “We now use Argo data continuously for forecasting everything from European winters to tropical storms and El Niño events.” The success of the Met Office in correctly predicting some very cold and wet conditions during the 2005–2006 European winter seems to have been made possible in part by Argo data, which revealed an abnormal subsurface temperature pattern in the North Atlantic. But statistical proof of such benefits will be available only when the Argo network has been maintained for much longer.

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Perhaps Argo's most valuable contribution will be in facilitating studies of year-to-year and decade-to-decade variability in the oceans. At present, it is hard to distinguish climate-change-driven shifts in ocean circulation from natural fluctuations. Argo will provide the continuous time-series needed to solve such puzzles. It should also help settle the big question of the extent to which the oceans participate in climate variability.

“We keep learning that it is dangerous to infer changes in the oceans from only a few years of measurements,” says Brian King, a physical oceanographer at the Southampton Oceanography Centre, UK. “But if anthropogenic forcing does leave a mark on the ocean, Argo should definitely help us find out.” 

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  • References

    1. Roemmich, D. et al. J. Phys. Oceanogr. 37, 162-173 (2007). | Article | ISI |
    2. Fischer, J. & Schott, F. J. J. Phys. Oceanogr. 32, 573-584 (2002). | Article |
    3. Lyman, J. M., Willis, J. K. & Johnson, G. C. Geophys. Res. Lett. 33, L18604 (2006). | Article |
    4. Nature 447, 8-9 (2007).
    5. Smith, D. et al. Science 317, 796-799 (2007). | Article | PubMed | ChemPort |
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