Argo is an international program that uses profiling floats to observe temperature , salinity , currents , and, recently, bio-optical properties in the Earth’s oceans; it has been operational since the early 2000s. The real-time data it provides is used in climate and oceanographic research.   A special research interest is to quantify the ocean heat content (OHC).
The Argo fleet consists of almost 4000 drifting “Argo floats” (as profiling floats used by the Argo program are often called) deployed worldwide. Each float weighs 20-30 kg. In most cases probes drift at a depth of 1000 meters (the so-called parking depth) and, every 10 days, by changing their buoyancy , dive to a depth of 2000 meters and then move to the sea-surface , measuring conductivity and temperature profiles as well as pressure . From these, salinity and density can be calculated. Seawater density is important in determining large-scale motions in the ocean. Average current velocities at 1000 meters are directly measured by distance and direction to float driftsGPS or Argos system positions at the surface. The data are transmitted to shore via satellite, and are freely available to everyone, without restrictions.
The Argo program is named after the Greek mythical ship Argo to emphasize the complementary relationship of Argo with the Jason satellite altimeters.
The Argo program is a collaborative partnership of more than 30 nations from all continents that can be used to explore the ocean environment. Argo is a component of the Global Ocean Observing System (GOOS).  and is coordinated by the Argo Steering Team, an international body of scientists and technical experts that meets once a year. The Argo data stream is managed by the Argo Data Management Team. Argo is also supported by the Group on Earth Observations, CLIVAR’s World Climate Research Program’s CLIVAR Project (Variability and predictability of the ocean-atmosphere system), and by the Ocean Ocean Data Assimilation Experiment (GODAE OceanView).
A program called Argo was first proposed at OceanObs 1999 which was a conference organized by international agencies with the aim of creating a coordinated approach to ocean observations. The original Argo flyer was created by a small group of scientists, chaired by Dean Roemmich , who described a program that would have a global array of about 3000 floats in place by sometime in 2007.  The 3000-float array was achieved in November 2007 and was global. The Argo Steering Team for the first time in 1999 in Maryland (USA) and outlined the principles of global data sharing. The Argo Steering Team made a 10-year report to OceanObs-2009 and suggestions on how the array might be improved. These suggestions include enhancing the array at high latitudes, in marginal seas (such as the Gulf of Mexico and the Mediterranean) and along the equator, enhanced observations of strong boundary currents (such as the Gulf Stream and Kuroshio ), extension of observations into deep water and the addition of sensors for biological monitoring and chemical changes in the oceans. In November 2012, an Indian float in the Argo Array collects the one-millionth profile (twice the number collected by research vessels during the 20th century).   In 2014 the Bio-Argo program was rapidly expanding. 
Float design and operation
The critical capability of an Argo float is its ability to rise and fall 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, the mineral oil is forced out of the float’s pressure case and expands to 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 on the surface, the float withdraws the oil and descends again. 
A handful of manufacturing companies and organizations profiling floats used in the Argo program. APEX floats, made by Teledyne Webb Research , are the most common element of the current array. SOLO and SOLO-II floats were developed at the Scripps Institution of Oceanography . Other types include the NINJA float, made by the Tsurumi Seiki Co. of Japan, and the PROVOR float developed by IFREMER in France. Most floats use sensors made by Sea-Bird Electronics, which also makes a profiling float called Navis. A typical Argo float is a cylinder just over 1 meter long and 14 cm across with a hemispherical cap. Thus it has a minimum volume of about 16,600 cubic centimeters (cm 3). At Ocean Station in the Gulf of Alaska the temperature and salinity at the surface may be about 6 ° C and 32.55 parts per thousand giving a density of sea-water of 1.0256 g / cm 3 . At a depth of 2000 meters the temperature could be 2 ° C and the salinity 34.58 parts per thousand. Thus, the compressible effect of water is about 1.0369 g / cm 3 . The change in density is 0.0109.
The float has to match these densities if it is to reach 2000 meters depth and then rise to the surface. Since the density of the float is its mass divided by volume, it needs to change its volume by 0.0109 × 16,600 = 181 cm 3to drive that excursion; A small amount of this volume is provided by the compressibility of the float itself, and is required in the field. All Argo floats carry sensors to measure the temperature and salinity of the ocean as they vary, but with an increasing number of floats, such as for other sensors, such as chlorophyll, nutrients and pH. An extension to the Argo project called BioArgo is being developed and, when implemented, will add a biological and chemical component to this method of sampling the oceans. 
The antenna for satellite communications is built on the top of the float which extends from the sea after it completes its ascent. The ocean is saline, hence an electrical conductor, so that radio communications from under the sea surface are not possible. Early in the program Argo floats uniquely used slow mono-directional satellite communications but the majority of floats being deployed in mid-2013 use rapid bi-directional communications. The result of this study is that they are already possible and they only spend 20 minutes on the sea surface rather than 8-12 hours, thus reducing grounding and bio-fouling.
The average life span of Argo floats has been greatly increased since the program began in 2005.
As of March 2016, new types of floats have been tested to much higher than standard Argo floats. These “Deep Argo” floats are designed to reach depths of 6000 meters, versus 2000 meters for standard floats. This will be a much greater volume of the ocean to be sampled. Such measurements are important for developing a comprehensive understanding of the ocean, such as trends in heat content. 
The original plan advertised in the Argo prospectus called for a closest-neighbor distance between floats, on average, of 3 ° latitude by 3 ° longitude.  This allowed for Higher resolution (in kilometers) at high latitudes, both, north and south, and regarded Was Necessary Because of the Decrease in the Rossby radius of deformation qui Governs the scale of oceanographic features, Such As eddies. By 2007 this has been accomplished, but never before, in the deep southern ocean. 
Efforts are being made to complete the world in the world as far as Southern Ocean is concerned .
As mentioned in the section, enhancements are now planned in the equatorial regions of the oceans, in boundary currents and in marginal seas. This requires that the total number of floats be increased from the original plan of 3000 floats to a 4000-float array.
One consequence of the use of seasonal profiling can be removed. The diagram opposite shows the count of all float profiles acquired by Argo south of 30 ° S (upper curve) from the beginning of the program to November 2012 compared with the same diagram for all other data available. The lower curve shows a strong annual profile in Australia. For the upper (Argo) plot, there is no apparent bias.
One of the critical features of the Argo model is that of global and unrestricted access to data in near real-time. When a float transmits a profile it is quickly converted to a format that can be inserted into the GTS (Global Telecommunications System). The GTS is operated by the World Meteorological Organization , or WMO, specifically for the purpose of sharing data needed for weather forecasting. Thus all nations who are members of the WMO receive all Argo profiles within a few hours of the acquisition of the profile. Data are also made available via ftp and WWW access via two Argo Global Data Centers (or GDACs), one in France and one in the US.
About 90% of all knowledge is acquired within 24 hours, with the remaining profiles soon becoming available.
For a researcher to use data acquired via the GTS or from the Argo Global Data Center (GDACs) does require programming skills. The GDACs provide multi-profile files that are a native file format for Ocean DataView. For any day there are files with names like 20121106_prof.nc that are called multi-profile files. This example is a file specific to 6 November 2012 and contains all profiles in a single NetCDF file for one ocean basin. The GDACs identify three ocean basins, Atlantic, Indian and Pacific. Thus three multi-profile files will carry every Argo profile acquired on that specific day.
Argo data but lacks Argo Global Marine Atlas  which is an easy-to-use utility that allows the creation of Argo data. , but also horizontal maps of ocean properties, time series at any location etc. This Atlas also carries an “update” button that allows data to be updated periodically. The Argo Global Marine Atlas is maintained at the Scripps Institution of Oceanography in La Jolla, California.
Argo data can be displayed in Google Earth by Argo Technical Coordinator.
Argo is now the dominant source of information on the climatic state of the world and is widely used in many publications. Topics addressed include air-sea interaction, ocean currents , interannual variability, El Niño , mesoscale eddies, water mass properties and transformation. Argo is also now permitting direct computations of the global ocean heat content.
A notable recent paper was published by Durack and Wijffels and analyzes global changes in surface salinity patterns. 
They determine that areas of the world with high surface salinity are getting saltier and areas of the world with relatively low surface salinity are getting fresher. This has been described as ‘the rich get richer and the poor get poorer’. Scientifically speaking, the distribution of salt by the difference between precipitation and evaporation. Areas, such as the northern North Pacific Ocean , where precipitation dominates evaporation are fresher than average. The implication of their result is that the Earth is an intensification of the global hydrological cycle. Argo data are also used to predict climate change. 
Argo data was critical in the drafting of Chapter 3 (Working Group 1) of the IPCC Fifth Assessment Report (released September 2013) and an appendix was added to that chapter data since the IPCC Fourth Assessment Report and the resulting and improvement in confidence in the description of exchange salinity area and upper-ocean heat content.
- Ocean acoustic tomography
- Underwater gliders
- Integrated Ocean Observing System
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