What is Argo ?#
Argo is a real-time global ocean in situ observing system.
The ocean is a key component of the Earth climate system. It thus needs a continuous real-time monitoring to help scientists better understand its dynamic and predict its evolution. All around the world, oceanographers have managed to join their efforts and set up a Global Ocean Observing System among which Argo is a key component.
Argo is a global network of nearly 4000 autonomous probes measuring pressure, temperature and salinity from the surface to 2000m depth every 10 days. The localisation of these probes is nearly random between the 60th parallels (see live coverage here). All probes data are collected by satellite in real-time, processed by several data centers and finally merged in a single dataset (collecting more than 2 millions of vertical profiles data) made freely available to anyone through a ftp server or monthly zip snapshots.
The Argo international observation array was initiated in 1999 and soon revolutionized our perspective on the large scale structure and variability of the ocean by providing seasonally and regionally unbiased in situ temperature/salinity measurements of the ocean interior, key information that satellites can’t provide (Riser et al, 2016).
The Argo array reached its full global coverage (of 1 profile per month and per 3x3 degree horizontal area) in 2007, and continuously pursues its evolution to fulfill new scientific requirements (Roemmich et al, 2019). It now extents to higher latitudes and some of the floats are able to profile down to 4000m and 6000m. New floats are also equipped with biogeochemical sensors, measuring oxygen and chlorophyll for instance. Argo is thus providing a deluge of in situ data: more than 400 profiles per day.
Each Argo probe is an autonomous, free drifting, profiling float, i.e. a probe that can’t control its trajectory but is able to control its buoyancy and thus to move up and down the water column as it wishes. Argo floats continuously operate the same program, or cycle, illustrated in the figure below. After 9 to 10 days of free drift at a parking depth of about 1000m, a typical Argo float dives down to 2000m and then shoals back to the surface while measuring pressure, temperature and salinity. Once it reaches the surface, the float sends by satellite its measurements to a data center where they are processed in real time and made freely available on the web in less than 24h00.
Typical 10 days program, cycle, of an Argo float: