December 11, 2007 With rising concerns over climate change and its regional impacts, understanding (and eventually predicting) changes in both the atmosphere and ocean are needed to guide international actions, to optimize governments’ policies and to shape industrial strategies. Argo is a program that brings together more than 25 countries and thousands of floating sensors to provide key data from the ocean and assist in achieving these goals.

The multinational Argo program consists thousands of floats around the world that monitor the ocean and make shared data available to any user anywhere in the world within 24 hours via the Internet. Data from Argo floats provides early warning of significant temperature and salinity anomalies and changes in ocean circulation. Float deployments and positions are tracked through an Argo Information Centre (AIC) established under the Intergovernmental Oceanographic Commission (IOC) of UNESCO. The floats have a design life of at least four years meaning that 800 new floats per year are needed to maintain an array of 3000 floats.

Together with a new generation of Jason radar satellites, the float arrays can systematically monitor the state of the global ocean (temperature, salinity, currents and winds). Argo floats drift at a depth of 1000m (to give a uniform velocity field) and profile to 2000m so as to capture important variability. Ten floats were launched in 2000 and since then many new countries have joined. At the latest count 27 countries have deployed and operated floats or assisted with float deployments. Centers in Australia, France, Japan, the UK and the USA routinely produce global and regional analyses of subsurface properties using the Argo data stream. In mid-February 2006 Argo had a global array of 2355 floats in the ice-free areas deeper than 2000m. This represents almost 80% of the target array and makes Argo the largest single source of ocean profile data. It has several advantages over other ocean data providers - it can go deeper than the 750m XBTs, measure temperature much more accurately, and also collect salinity and ocean current data.

Temperatures measured by the floats are accurate to within 0.005 °C and depths to 5m. Accuracy for salinity is sometimes affected by sensor drift. Uncorrected salinities are accurate to within 0.01 p.s.u but at a later stage salinities are corrected by expert examination, comparing older floats with newly deployed instruments and with ship-based data. The program has a huge range of potential applications and the need for global Argo observations will continue indefinitely, though the technologies and design of the array will evolve as better instruments are built, models are improved and more is learned about ocean variability. The aim of Argo is to improve the accuracy and lead time for forecasts of significant temperature and salinity anomalies and changes in ocean circulation. This assists in predicting El Niño and La Niña events and more accurately and determining impacts on our planet as a result of climate change.

In an interesting aside - and a testament to the resilience of the floats - a profiler deployed by the San Diego-based Scripps Institution of Oceanography near New Caledonia was grounded on the seafloor off the coast of Queensland, Australia, before surfacing 18 months later in the nets of prawn fishing veteran, Robert Wilson, after which it resumed its transmissions to the satellite. Only the intervention of Dr Ann Thresher, an ocean scientist with Australia's CSIRO, saved the float from being unceremoniously transformed into a novelty letterbox. So next time there's a tug on your fishing line, don't presume there's a Marlin on the other end (via CSIRO).

To learn more about the program, visit the Argo site where links are provided to specific regional centers.

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