Wong et al. 
hydrographic program, deployment of Argo-type floats, and 
its XBT network. The initial design of Argo called for the 
deployment of over 3,000 profiling floats in a 3° x 3° array in 
the ice-free open ocean between 60°N and 60°S (Argo Science 
Team, 1998). In a departure from the practices of that era, 
the data from these floats would be freely disseminated in 
real-time, allowing use in operational ocean and atmospheric 
models. The data would be further quality-controlled, and 
this “delayed-mode” version would also be shared freely 
with the scientific community. It was recognized that Argo 
would require an international collaboration similar to that 
developed by WOCE. The floats would be deployed by separate 
groups from participating countries, but the data would be 
shared internationally. 
The standard Argo mission is known as “park-and-profile” 
{Figure 1). The floats park at a target pressure of 1,000 decibars 
and drift with the ocean currents. Pressure in decibars (dbar; 1 
dbar = 10,000 Pa) is approximately equal to depth in meters. The 
Argo park level of 1,000 dbar was chosen to extend the absolute 
velocity database collected during WOCE, which employed that 
level based on its favorable signal-to-noise ratio. Every 10 days 
the floats descend to 2,000 dbar and then collect a vertical 
profile of temperature and salinity during ascent to the surface. 
The positions of the floats at the sea surface are determined by 
orbiting platforms, and the data are transmitted via satellite back 
to shore. The floats then return to their target park pressure 
and the cycle is repeated. Deployments of Argo floats began in 
1999, and the 3,000-float goal was reached in November 2007. 
Argo collected its one-millionth profile in October 2012 and its 
two-millionth profile in September 2018. 
This paper describes the pressure (P), temperature (T), salinity 
(S), and subsurface velocity data from the Argo Program: the 
instrumentation used, the technical problems encountered, the 
scientific quality of the data, the data distribution system, and 
how the dataset has evolved in response to new technologies. 
It has been over 20 years since the first deployment of Argo 
floats in 1999. This has been a long journey for the scientists 
who first conceived the Argo array, and yet it is but a short 
step toward the goal of sustaining a comprehensive global ocean 
observation system. This paper therefore serves the dual purpose 
of documenting the characteristics and accuracy of the core Argo 
dataset from its inception to 2019, as well as foretelling the 
expansion of this global ocean dataset into 2020 and beyond. 
INSTRUMENTATION USED IN ARGO 
Platform History 
The present-day autonomous profiling float was developed from 
the neutrally buoyant float with short-range acoustic tracking 
(Swallow, 1955; Gould, 2005). During WOCE, Russ Davis and 
Doug Webb in the United States, and teams at LInstitut Francais 
de Recherche pour l’Exploitation de la Mer (Ifremer) in France, 
2quipped a new generation of floats with a pumping system and 
satellite navigation, so they could cycle repeatedly to the sea 
surface for satellite tracking in the ice-free ocean (Davis et al., 
1992; Ollitrault et al., 1994a). The float density was changed 
by pumping oil stored in an internal reservoir into an external 
rontiers in Marine Science | www.frontiersin.orı 
Argo Data 1999-2019 
bladder to ascend, and by deflating the bladder to descend. In 
WOCE, these early-model floats were used to determine the 
absolute velocity field at the park level. MARVOR floats were 
deployed in the eastern North Atlantic Ocean (Speer et al., 1999) 
and in the Brazil Basin of the South Atlantic Ocean (Ollitrault 
2t al., 1994b). Autonomous Lagrangian Circulation Explorer 
(ALACE) floats were deployed more widely (e.g., Davis, 1998). By 
the end of the 1990s, the addition of conductivity-temperature- 
depth! (CTD) sensors allowed for the collection of vertical 
profiles of temperature and salinity during each ascent to the sea 
surface (Loaec et al., 1998; Davis et al., 2001). Early inductive- 
type CTDs used on floats did not perform reliably, but the first 
pumped electrode-type CTD, a prototype supplied by Sea-Bird 
Scientific (used on Float 063, with WMO ID? 41862, deployed 
by the University of Washington in 1997), demonstrated that 
an accuracy of 0.01 in Practical Salinity Scale 1978 (PSS-78) was 
obtainable for float salinity over the course of several years (Argo 
Science Team, 1998). 
As Argo developed, early float models used in WOCE were 
augmented by newer ones. As a result, a variety of float types have 
been used in Argo. These include: 
e the PROVOR and the ARVOR, designed by Ifremer and built 
by nke Instrumentation 
a the APEX, built by Teledyne Webb Research 
» the SOLO-I and the SOLO-II, built by* Scripps Institution 
of Oceanography 
the S2A, a commercial version of SOLO-IL built by 
MRV Systems 
» the NAVIS, built by Sea-Bird Scientific 
e the NOVA, built by MetOcean 
e the NINJA, built by Tsurumi-Seiki 
Table 1 shows the percentage of profiles that each of these float 
types has obtained. 
5 
CTD Units and Pre-deployment Sensor 
Checks 
The CTD units fitted to most Argo floats have been manufactured 
by two companies, Sea-Bird Scientific (SBE) and Falmouth 
Scientific (FSI). The FSI unit was an inductive-style sensor and 
was only used in a small number of floats (about 3% as of 2019) in 
the beginning of the program. The SBE CTD unit is an enclosed 
pump unit (Figure2) and has been used on almost all Argo 
floats since 2005. The details of the operation of the SBE CTD 
unit are described in Riser et al. (2008) and Riser et al. (2018). 
Briefly, the CTD pump draws seawater through the intake past 
the temperature sensor and then through the conductivity cell. 
Fluid in the cell exits through an exhaust port that is aligned 
perpendicular to the intake, so as not to contaminate the water 
entering the cell. The pressure sensor is mounted on the float 
end cap, close to the CTD unit. The temperature and electrical 
conductivity of the seawater sample in the cell are measured 
'Despite having depth in the name, all CTDs actually measure pressure. 
’WMO ID is the World Meteorological Organization identification number. 
Woods Hole Oceanographic Institution also built the SOLO-I floats up until 
about 2012. These are designated as SOLO-W. 
Qanteambear 2020 1 Valııme 7 1 Article 701