Argo (oceanography): Wikis

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Argo is an observation system for the Earth's oceans that provides real-time data for use in climate, weather, oceanographic and fisheries research.[1] [2] [3] Argo consists of a large collection of small, drifting oceanic robotic probes deployed worldwide. The probes float as deep as 2 km. Once every 10 days, the probes surface, measuring conductivity and temperature profiles to the surface. From these salinity and density can be calculated. The data are transmitted to scientists on shore via satellite. The data collected are freely available to everyone, without restrictions. The initial project goal was to deploy 3,000 probes, completed in November 2007.

Map of the Argo float network as of September 2009

Contents

International collaboration

The Argo program is a collaboration between 50 research and operational agencies from 26 countries, with the United States contributing over half the total funding (as of December 2004). Argo is a component of the Integrated Ocean Observing System.

Float operation

The Argo program was designed to operate on the same 10-day duty cycle to match the existing satellite measurements of the ocean's sea surface. These satellites, called Topex/Poseidon and Jason 1, measure changes in the surface topography of the ocean. With such measurements, information about temperature, mass redistribution, or surface currents can be inferred. The Argo floats measure subsurface changes in temperature and salinity, hence the float measurements are complementary to the altimetry.

Argo is named after the Greek mythical ship Argo which Jason and the Argonauts use on their quest for the Golden Fleece. The name was chosen to emphasize the complementary relationship of the project with the Jason-1 satellite altimeter.

Although drifting floats had been deployed during the World Ocean Circulation Experiment in the 1990s, Argo floats began to be deployed in earnest in the early 2000s. The target number of 3000 deployed floats was reached during 2006–2007. The number of floats is continually changing as floats are lost or expire, while others are deployed. Nominally, some 750 floats are deployed each year to sustain the system. The floats have a nominal 300-km spacing, although the exact separations depend on the randomness of the float drift.

The Argo temperature and salinity measurements are yielding valuable information about the large-scale water properties and currents of the ocean, including the variability of these properties over time scales from seasonal to decadal.

Example profiles of temperature and salinity obtained from an Argo float in the central North Pacific (38.4°N, 155.3°W) on April 8, 2005.
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Profiling

Argo floats drift at a fixed pressure (usually around 1000 metres depth) for 10 days. After this period, within the relatively short time of around two hours, the floats move to a profiling pressure (usually 2000 metres deep) then rise, collecting instantaneous profiles of pressure, temperature, and salinity data on their way to the surface. Once at the surface, the floats remain there for under a day, transmitting the data collected via a satellite link back to a ground station and allowing the satellite to determine their surface drift. The floats then sink again and repeat their mission.

Data communication

Most of the floats use the Argos System of satellites to recover data, though a few are using the newer Iridium satellite constellation. The Iridium system offers significant advantages associated with the much faster data transfer. Since an Iridium float spends only 3 minutes at the sea surface, the opportunity to observe surface currents by tracking the movements of the floats is lost but the trajectories of the floats become more representative of the flow at their parking depth.

Float design

Cutaway diagram of an Argo float. The height of the float is about 2 m. The antenna and sensors are mounted at the top of the buoy.

A critical capability of an Argo float is its ability to rise and descend in the ocean on a programmed schedule. The floats do this by changing their effective density. The density of any object is given by its mass divided by its volume. The Argo float keeps its mass constant, but by altering its volume, it changes its density. To do this, a hydraulic piston is used to push mineral oil out of the float and expand a rubber bladder at the bottom end of the float. As the bladder expands, the float becomes less dense than seawater and rises to the surface. Upon finishing its tasks at the surface, the float withdraws the piston and descends again.[4]

An increasing number of the floats also carry other sensors, such as for measuring dissolved oxygen.

The antenna for satellite communications is mounted at the top of the float. Once the float reaches the surface, the float is essentially a spar buoy, allowing the antenna to poke above the sea surface for communication. The ocean is saline, hence an electric conductor, so that radio communications from under the sea surface are difficult.

The nominal life span of an Argo float is five years. After the internal batteries expire, the floats are allowed to sink to the ocean floor or wash ashore.

Data access

Argo is unique among research programs in that the real-time data are freely offered to anyone. The data collected by the network are made available with no constraint on use of the data, and most data are available for download within 24 hours of a float measurement. Data can be downloaded over the world wide web from one of two global data servers (OPeNDAP servers[5][6]).

Data format

Even though data are supplied by 24 national programs, all data are available in near real-time in a single format. Argo data are in the native import format of the Ocean DataView suite of programs.[7] Ocean DataView (ODV) is freely available software created by Reiner Schlitzer that offers flexible ways of displaying oceanographic data. Data in other formats are also available, e.g., netCDF. A careful study of the manuals before starting to use the data is essential [8]

Data results

The Argo Network has shown a continuous declining trend in ocean temperatures since the network was launched, although some of the sensors have shown a cooling bias. [9] Josh Willis of NASA's Jet Propulsion Laboratory has reported that the Argo system has shown no ocean warming since it started in 2003. "There has been a very slight cooling, but not anything really significant," Willis has stated.[10]

In an article from November 5, 2008, Josh Willis states that the world ocean actually has been warming since 2003 after removing measurement errors from the data and adjusting the measured temperatures with a computer model his team developed. [11]

See also

References

  1. ^ Argo Begins Systematic Global Probing of the Upper Oceans Toni Feder, Phys. Today 53, 50 (2000), DOI:10.1063/1.1292477
  2. ^ Richard Stenger (September 19, 2000). "Flotilla of sensors to monitor world's oceans". CNN. http://archives.cnn.com/2000/NATURE/09/19/ocean.sensors/index.html. Retrieved 2007-10-28.  
  3. ^ Nature: Artefacts in ocean data hide rising temperatures
  4. ^ "How Argo Floats Work". UCSD. http://www.argo.ucsd.edu/FAQ.html#find. Retrieved 2009-12-15.  
  5. ^ Plot and download realtime Argo data.
  6. ^ OPeNDAP (dapper) server
  7. ^ Display Argo data using this free software - Ocean Data View
  8. ^ Argo data system manuals.
  9. ^ Article by David Evans
  10. ^ NPR report
  11. ^ Nasa.gov: November 5, 2008 Correcting ocean cooling Quote: "...According to the float data on his computer screen, almost the entire Atlantic Ocean had gone cold. Unless you believe The Day After Tomorrow, Willis jokes, impossibly cold...page 3 “First, I identified some new Argo floats that were giving bad data...He was supposed to fly to Colorado that weekend to give a talk on “ocean cooling” to prominent climate researchers. Instead, he’d be talking about how it was all a mistake...What we found was that ocean heating was larger than scientists previously thought, and so the contribution of thermal expansion to sea level rise was actually 50 percent larger than previous estimates.”...“but also because the problems with the newest Argo data—the problems that Josh Willis found as well as other problems we have identified—haven’t been totally solved. For the most recent years [2003-2007], the sea level budget once again does not close. Our team is still working on that problem.”..."

External links

Coordinates: 38°24′N 155°18′W / 38.4°N 155.3°W / 38.4; -155.3


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