Nong et al. 
Argo Data 1999-2019 
* Japan Meteorological Agency, Tokyo, Japan, ® Oceans and Atmosphere, CSIRO, Hobart, TAS, Australia, ® Australian 
Antarctic Program Partnership, University of Tasmania, Hobart, TAS, Australia, °” Met Office, Exeter, United Kingdom, 
3 National Oceanography Centre, Southampton, United Kingdom, © British Geological Survey, Nottingham, United Kingdom 
© National Centre for Polar and Ocean Research, Ministry of Earth Sciences, Goa, India, *' Institute of Ocean Sciences, 
“isheries and Oceans Canada, Sidney, BC, Canada, * Marine Environmental Data Services, Fisheries and Oceans Canada 
Ittawa, ON, Canada, © Maurice Lamontagne Institute, Fisheries and Oceans Canada, Mont-Joli, QC, Canada, ** National 
Marine Data and Information Service, Tianjin, China, * State Key Laboratory of Satellite Ocean Environment Dynamics, 
Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China, ** National Institute of Meteorological 
Sciences, Seogwipo, South Korea, *7 Korea Institute of Ocean Science and Technology, Ansan, South Korea 
In the past two decades, the Argo Program has collected, processed, and distributed 
over two million vertical profiles of temperature and salinity from the upper two kilometers 
of the global ocean. A similar number of subsurface velocity observations near 1,000 
dbar have also been collected. This paper recounts the history of the global Argo 
Drogram, from its aspiration arising out of the World Ocean Circulation Experiment, to the 
development and implementation of its instrumentation and telecommunication systems, 
and the various technical problems encountered. We describe the Argo data system 
and its quality control procedures, and the gradual changes in the vertical resolution 
and spatial coverage of Argo data from 1999 to 2019. The accuracies of the float data 
have been assessed by comparison with high-quality shipboard measurements, and are 
csoncluded to be 0.002°C for temperature, 2.4 dbar for pressure, and 0.01 PSS-78 for 
salinity, after delayed-mode adjustments. Finally, the challenges faced by the vision of an 
expanding Argo Program beyond 2020 are discussed. 
Keywords: global, ocean, pressure, temperature, salinity, Argo, profiling, floats 
INTRODUCTION 
Prior to the turn of the 21st century, comprehensive in-situ 
ocean observations were diflicult to obtain. Temperature and 
salinity data were collected mainly from ships and moored 
buoys, and were biased geographically toward the northern 
hemisphere oceans, where most of these platforms operated. 
Measurements acquired during ship-based surveys were mostly 
along transect lines, thus leaving large spatial gaps in sampling. 
Temporal coverage of data was also uneven, as sampling was 
limited to the years and seasons when ships were available. Data 
from the high latitudes during winter were especially sparse. 
Large-scale measurements of upper ocean temperature were 
made possible by the advent of the expendable bathythermograph 
(XBT), but with no accompanying salinity measurements and 
with relatively limited data coverage in the southern hemisphere. 
These limitations in spatial and temporal oceanographic data 
coverage, compounded by a lack of any systematic subsurface 
salinity data, impaired the progress in operational oceanography 
and ocean climate research. 
In 1998, the Year of the Oceans, an international team of 
scientists proposed a design for a global array of autonomous 
profiling floats to enhance the temperature and salinity 
measurements of the upper ocean (Argo Science Team, 1998). 
This new network, called Argo, would be integrated into the 
global ocean observing system, filling in the large data gaps 
that existed in the in-situ ocean observations at that time. The 
initial endorsements came from the CLIVAR Upper Ocean Panel 
(UOP) and the Global Ocean Data Assimilation Experiment 
ZIrontiers in Marine Science | www frontiersin ort 
(GODAE). The Argo Science Team (later renamed the Argo 
Steering Team) was constituted at a joint meeting of the CLIVAR 
UOP and GODAE in mid-1998. The Argo Program was further 
endorsed as a pilot program by the Global Ocean Observing 
System (GOOS). 
The name Argo was chosen because of the program’s 
complementary nature with Jason, the Centre National d’Etudes 
Spatiales/National Aeronautics and Space Administration 
(CNES/NASA) satellite oceanography sea level mission 
(Roemmich and Owens, 2000). In Greek mythology, Jason 
sailed in a ship called Argo with his crew, the Argonauts. 
In oceanography, Jason and Argo together would provide 
regular global sea surface height and subsurface temperature 
and salinity measurements, the variables that are necessary 
for the proper interpretation of sea surface height. Argos aim 
was to provide sustained and global sampling of subsurface 
temperature-salinity-pressure profiles and velocity fields by 
using the autonomous profiling float technology. Today, 
as an element of the GOOS, Argo has important synergies 
with many of the other in-situ observation networks, which 
include shipboard repeat hydrography, moored buoys, surface 
drifters, XBT, glider transects, sea level stations, and animal- 
borne profiling. The integration of the GOOS is coordinated 
by the Observations Coordination Group (OCG), with the 
Joint Technical Commission for Oceanography and Marine 
Meteorology in-situ Observations Programme Support Centre 
(JCOMMOPS) providing the technical support. 
Conceptually, the design of the Argo array evolved from 
the World Ocean Circulation Experiment (WOCE)’s shipboard 
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