Argo Floats: Magical Free Floating Robots at Sea

by OldSailor on December 18, 2007

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At present there are more than 3000 Argo free floating magical robots at sea around the world collecting sea water temperature,salinity, and velocity of the upper ocean to facilitate real time monitoring of the oceans. Argo is an international collaboration that collects high-quality temperature and salinity profiles from the upper 2000m of the ice-free global ocean and currents from intermediate depths.

What are Argo Floats ?

  • they are battery-powered autonomous floats that spend most of their life drifting at depth where they are stabilised by being neutrally buoyant at the “parking depth” pressure by having a density equal to the ambient pressure and a compressibility that is less than that of sea water.
  • at present there are three models of profiling float used extensively in Argo. All work in a similar fashion but differ somewhat in their design characteristics.
  • at typically 10-day intervals, the floats pump fluid into an external bladder and rise to the surface over about 6 hours while measuring temperature and salinity.
  • satellites determine the position of the floats when they surface, and receive the data transmitted by the floats. The bladder then deflates and the float returns to its original density and sinks to drift until the cycle is repeated. Floats are designed to make about 150 such cycles.
  • for the first time, the physical state of the upper ocean is being systematically measured and the data assimilated in near real-time into computer models.
  • Argo is not confined to major shipping routes which can vary with season as the other upper-ocean observing networks are. Instead, the global array of 3,000 floats are distributed roughly every 3 degrees (300km).

Why do we need Argo?

  • to study about global change and its regional impacts as sea level is rising at an accelerating rate of 3 mm/year, Arctic sea ice cover is shrinking and high latitude areas are warming rapidly. Extreme weather events cause loss of life and enormous burdens on the insurance industry.
  • to optimize governments’ policies and to shape industrial strategies.
  • for understanding and to predict changes in both the atmosphere and ocean that are needed to guide international actions. To make those predictions improved models of climate and of the entire earth system (including socio-economic factors) are required.
  • Lack of sustained observations of the atmosphere, oceans and land have hindered the development and validation of climate models. In 1999, to combat this lack of data, an innovative step was taken by scientists to greatly improve the collection of observations inside the ocean through increased sampling of old and new quantities and increased coverage in terms of time and area. That is how Argo was born.

Argo’s Objectives

  • It will provide a quantitative description of the changing state of the upper ocean and the patterns of ocean climate variability from months to decades, including heat and freshwater storage and transport.
  • The data will enhance the value of the Jason altimeter through measurement of subsurface temperature, salinity, and velocity, with sufficient coverage and resolution to permit interpretation of altimetric sea surface height variability.
  • Argo data will be used for initializing ocean and coupled ocean-atmosphere forecast models, for data assimilation and for model testing.
  • A primary focus of Argo is to document seasonal to decadal climate variability and to aid our understanding of its predictability. A wide range of applications for high-quality global ocean analyses is anticipated.

Argo Design and Data

The design of the Argo network is based on experience from the present observing system, on recent knowledge of variability from the TOPEX/Poseidon altimeter, and on the requirements for climate and high-resolution ocean models.

The array of 3000 floats will provide 100,000 temperature/salinity (T/S) profiles and velocity measurements per year distributed over the global oceans at an average 3-degree spacing. Floats will cycle to 2000m depth every 10 days, with 4-5 year lifetimes for individual instruments. All Argo data are publically available in near real-time via the Global Data Assembly Centres (GDACs) in Brest, France and Monterey, California after an automated quality control (QC), and in scientifically quality controlled form, delayed mode data, via the GDACs within six months of collection.

Argo Float Cross Section:

Argo is comprised of three subsystems:

  • Hydraulics: control buoyancy adjustment via an inflatable external bladder, so the float can surface and dive.
  • Microprocessors: deal with function control and scheduling.
  • Data transmission system: controls communication with satellite.

Approx. Weight: 25 Kg ; Max. operating depth: 2000m; Crush depth: 2600m

The three float models in use are the PROVOR built by MARTEC in France in close collaboration with IFREMER, the APEX float produced by Webb Research Corporation, USA and the SOLO float designed and built by Scripps Institution of Oceanography, USA.

Simple Mission Operation:

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The float descends to cruising depth, drifts for several days, ascends while taking salinity and temperature profiles, and then transmits data to satellites. All the mission parameters, such as the drift depth, vertical sampling resolution, and time on the surface, can be tailored to suit the operating region.

Park & Profile Mission Operation:

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The float descends to cruising depth, drifts for several days, descends to start of profile depth, ascends while taking temperature and salinity profiles, and then transmits data to satellites. All the mission parameters, such as the parking depth, profile depth, vertical sampling resolution and time on the surface can be tailored to suit the operating region.

Positions of the floats that have delivered data within the last 30 days as on December 13, 2007:

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Some photographs

An Argo float being deployed from a research ship.

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Source: Argo

View a video clip that shows, how Argo floats work.

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