FRONTIERS IN OCEAN
OBSERVING
DOCUMENTING ECOSYSTEMS
UNDERSTANDING ENVIRONMENTAL CHANGES
FORECASTING HAZARDS
Editor: Ellen S. Kappel
Guest Editors: S. Kim Juniper, Sophie Seeyave, Emily Smith, and Martin Visbeck
December 2021 Supplement to Oceanography
ABOUT THIS PUBLICATION
Support for this publication is provided by Ocean Networks Canada, the National
Oceanic and Atmospheric Administration’s Global Ocean Monitoring and Observing
Program, the Partnership for Observation of the Global Ocean, and the US Arctic
Research Commission.
Editor: Ellen Kappel
Assistant Editor: Vicky Cullen
Layout and Design: Johanna Adams
Published by The Oceanography Society
This is an open access document made available under a Creative Commons
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Single printed copies are available upon request from info@tos.org.
ON THE COVER
Developed by scientists at the Institute for Chemistry and Biology of the Marine
Environment, University of Oldenburg, the Sea Surface Scanner (S3) is a radio-
controlled catamaran designed to detect biogenic and ubiquitous surface films
called the sea surface microlayer (SML). The SML is typically less than 1 mm thick
and controls air-sea interactions due to its unique biogeochemical properties rel-
ative to the underlying water. S3 uses a set of partially submerged glass disks that
continuously rotate through the sea surface, skimming and wiping the SML from the
disks. The principle of this collection technique was developed several decades
ago. The continuous sample stream is diverted to a set of onboard flow-through
sensors (e.g., temperature, conductivity, fluorescence, pH, pCO2) and to a bottle
carousel triggered by a command from the pilot. S3 is capable of mapping the SML
with high temporal and spatial resolution and collecting large amounts of samples
for broader biogeochemical assessment of the SML. Since 2015, S3 has been
deployed in the Indian, Pacific, and Atlantic Oceans, including in open leads near
the North Pole, providing a unique large data set of biogeochemical features of the
ocean‘s surface. Photo credit: Alex Ingle/Schmidt Ocean Institute
PREFERRED CITATION
Kappel, E.S., S.K. Juniper, S. Seeyave, E. Smith, and M. Visbeck, eds. 2021. Frontiers
in Ocean Observing: Documenting Ecosystems, Understanding Environmental
Changes, Forecasting Hazards. A Supplement to Oceanography 34(4), 102 pp.,
https://doi.org/10.5670/oceanog.2021.supplement.02.
CONTENTS
INTRODUCTION .......................................................................................................................................................................................................................................................................................................................1
Introduction to the Ocean Observing Supplement to Oceanography ...................................................................................................................................................................1
TOPIC 1. OCEAN-CLIMATE NEXUS ..........................................................................................................................................................................................................................................2
The Technological, Scientific, and Sociological Revolution of Global Subsurface Ocean Observing ...................................................................2
Linking Oxygen and Carbon Uptake with the Meridional Overturning Circulation Using a Transport Mooring Array...................9
Climate-Relevant Ocean Transport Measurements in the Atlantic and Arctic Oceans ..............................................................................................................10
Coastal Monitoring in the Context of Climate Change: Time-Series Efforts in Lebanon and Argentina ...........................................................12
Changes in Southern Ocean Biogeochemistry and the Potential Impact on pH-Sensitive Planktonic Organisms .........................14
Monitoring Boundary Currents Using Ocean Observing Infrastructure ............................................................................................................................................................16
Putting Training into Practice: An Alumni Network Global Monitoring Program ....................................................................................................................................18
TOPIC 2. ECOSYSTEMS AND THEIR DIVERSITY ..............................................................................................................................................................................20
Exploring New Technologies for Plankton Observations and Monitoring of Ocean Health ............................................................................................20
New Technologies Aid Understanding of the Factors Affecting Adélie Penguin Foraging ..............................................................................................26
Image Data Give New Insight into Life on the Seafloor ...........................................................................................................................................................................................................28
Porcupine Abyssal Plain Sustained Observatory Monitors the Atmosphere to the Seafloor on
Multidecadal Timescales ..............................................................................................................................................................................................................................................................................................29
The Evolution of Cyanobacteria Bloom Observation in the Baltic Sea ............................................................................................................................................................30
Ocean Observing in the North Atlantic Subtropical Gyre .....................................................................................................................................................................................................32
EcoFOCI: A Generation of Ecosystem Studies in Alaskan Waters ..........................................................................................................................................................................34
TOPIC 3. OCEAN RESOURCES AND THE ECONOMY UNDER
CHANGING ENVIRONMENTAL CONDITIONS ...........................................................................................................................................................................................36
Integrating Biology into Ocean Observing Infrastructure: Society Depends on It ...........................................................................................................................36
Observations of Industrial Shallow-Water Prawn Trawling in Kenya .....................................................................................................................................................................44
Application of Remote Sensing and GIS to Identifying Marine Fisheries off the Coasts of Kenya and Tanzania ..............................46
Quantification of the Impact of Ocean Acidification on Marine Calcifiers ....................................................................................................................................................48
Upwelling Variability Offshore of Dakhla, Southern Morocco ........................................................................................................................................................................................49
Valuing the Ocean Carbon Sink in Light of National Climate Action Plans ...............................................................................................................................................50
TOPIC 4. POLLUTANTS AND CONTAMINANTS AND THEIR POTENTIAL
IMPACTS ON HUMAN HEALTH AND ECOSYSTEMS ................................................................................................................................................................52
An Integrated Observing System for Monitoring Marine Debris and Biodiversity ..........................................................................................................................52
A Novel Experiment in the Baltic Sea Shows that Dispersed Oil Droplets Can Be Distinguished by Remote Sensing ........60
Comparison of Two Soundscapes: An Opportunity to Assess the Dominance of Biophony Versus Anthropophony ..............62
PacIOOS Water Quality Sensor Partnership Program ...............................................................................................................................................................................................................66
An Integrated Observing Effort for Sargassum Monitoring and Warning in the Caribbean Sea, Tropical Atlantic,
and Gulf of Mexico ..................................................................................................................................................................................................................................................................................................................68
TOPIC 5. MULTI-HAZARD WARNING SYSTEMS...................................................................................................................................................................................70
Long-Term Ocean Observing Coupled with Community Engagement Improves Tsunami Early Warning ..................................................70
Uncrewed Ocean Gliders and Saildrones Support Hurricane Forecasting and Research ................................................................................................78
Tide Gauges: From Single Hazard to Multi-Hazard Warning Systems ..............................................................................................................................................................82
The California Harmful Algal Bloom Monitoring and Alert Program: A Success Story for Coordinated
Ocean Observing .....................................................................................................................................................................................................................................................................................................................84
Multi-Stressor Observations and Modeling to Build Understanding of and Resilience to the Coastal Impacts
of Climate Change...................................................................................................................................................................................................................................................................................................................86
TECHNOLOGY .........................................................................................................................................................................................................................................................................................................................88
Technologies for Observing the Near Sea Surface .......................................................................................................................................................................................................................88
Hyperspectral Radiometry on Biogeochemical-Argo Floats: A Bright Perspective for Phytoplankton Diversity ..............................90
Visualizing Multi-Hectare Seafloor Habitats with BioCam ...................................................................................................................................................................................................92
Emerging, Low-Cost Ocean Observing Technologies to Democratize Access to the Ocean ......................................................................................94
Robotic Surveyors for Shallow Coastal Environments ..............................................................................................................................................................................................................96
AUTHORS ...........................................................................................................................................................................................................................................................................................................................................98
ACRONYMS ................................................................................................................................................................................................................................................................................................................................102
INTRODUCTION
Scientists observe the ocean’s complex and interwoven
physical, chemical, biological, and geological processes
to understand the numerous ways in which the ocean
sustains life and provides benefits to society, and to fore-
cast events that affect humankind and the planet. They
use a range of instruments to gather data, from simple
nets and thermometers to sophisticated sensors aboard
autonomous vehicles that transmit data back to labo-
ratories nearly instantaneously. Some instruments are
tethered to ships or moored to the seafloor, and others
drift with ocean currents, move autonomously, or are
controlled from land. There are also specialized satellites,
aircraft, and drones that carry ocean observing sensors.
Observations are made over hours to days to years in all
parts of the global ocean, from the tropics to the poles,
from the coasts to the open ocean, and from the seafloor
to its surface waters.
The many different types of ocean observations allow
scientists to detect and track pollutants and toxic sub-
stances such as oil slicks, plastics, and other marine
debris; to document ocean warming and acidification
as well as changes in ocean circulation and ecosystem
health; and to better forecast hazards such as hurricanes,
earthquakes, tsunamis, ocean heatwaves, flooding, and
harmful algal blooms.
In this supplement to the December issue of
Oceanography, we introduce frontiers in ocean observing—
the articles describe new technologies and reveal some
exciting results that advance our understanding of the
world ocean and its resources and support its sustain-
able use and management. For this 2021 inaugural sup-
plement, potential authors were invited to submit letters
of interest aligned with the priorities of the UN Decade of
Ocean Science for Sustainable Development (2021–2030)
in the following topical areas:
TOPIC 1. Ocean-Climate Nexus. Observations related to
climate monitoring, modeling, and forecasting; sea level
rise; and ocean acidification.
TOPIC 2. Ecosystems and Their Diversity. Studies and
observations for habitat mapping and restoration and
for biodiversity monitoring, in particular, the relationship
between biodiversity and climate change, as well as applica-
tions for natural resource management and conservation.
TOPIC 3. Ocean Resources and the Economy Under
Changing Environmental Conditions. Observations and
services in support of the blue economy (e.g., energy,
transport, tourism), sustainable use of ocean resources
(e.g., fisheries/aquaculture, genetic resources, minerals,
sand), and marine spatial planning.
TOPIC 4. Pollutants and Contaminants and Their
Potential Impacts on Human Health and Ecosystems.
Systems for monitoring pollutants/ contaminants (e.g.,
heavy metals, nutrients, plastics, and organic pollutants,
as well as noise) and their dispersal, and potential links to
policy frameworks.
TOPIC 5. Multi-Hazard Warning Systems. Observing
systems and information services supporting disaster
risk reduction and improving human health, safety, and
food security.
We received 127 letters of interest from the global ocean
observing community, from which we chose the subset
of articles contained in this supplement. For many of the
articles, we asked authors who had never before worked
together to collaborate and submit one combined article.
We also chose a few articles to close the supplement with
descriptions of exciting new ocean observing technologies.
We thank Ocean Networks Canada, the US National
Oceanic and Atmospheric Administration’s Global Ocean
Monitoring and Observing Program, the international
Partnership for Observation of the Global Ocean, and the
US Arctic Research Commission for generously supporting
publication of this Ocean Observing supplement.
ARTICLE DOI: https://doi.org/10.5670/oceanog.2021.supplement.02-01
Introduction to the Ocean Observing Supplement to Oceanography
By Ellen S. Kappel, S. Kim Juniper, Sophie Seeyave, Emily A. Smith, and Martin Visbeck
TOPIC 1.
OCEAN-CLIMATE NEXUS
INTRODUCTION – GLOBAL OBSERVATIONS
OF THE INTERIOR OCEAN
The complementary partnership of the Global
Ocean Ship-based Hydrographic Investigations
Program
(GO-SHIP;
https://www.go-ship.org/)
and the Argo Program (https://argo.ucsd.edu) has
been instrumental in providing sustained sub-
surface observations of the global ocean for over
two decades. Since the late twentieth century, new
clues into the ocean’s role in Earth’s climate system
have revealed a need for sustained global ocean
observations (e.g., Gould et al., 2013; Schmitt, 2018)
and stimulated revolutionary technology advances
needed to address the societal mandate. Together,
the international GO-SHIP and Argo Program
responded to this need, providing insight into the
mean state and variability of the physics, biology,
and chemistry of the ocean that led to advance-
ments in fundamental science and monitoring of
the state of Earth's climate.
Historically, ocean temperature profiles have
been obtained from commercial ships, although
the highest quality temperature and salinity
(T/S) profiles came only from research vessels
(Figure 1). Global ocean hydrographic surveys,
including full biogeochemistry and tracers, began in
the mid-1990s under the World Ocean Circulation
Experiment (WOCE) and continue now as GO-SHIP.
T/S and biogeochemistry, as key variables of the
climate system, began to describe variability and
change in patterns of rainfall and evaporation,
absorption of fossil fuel carbon dioxide into the
ocean, and the pace and evolution of global warm-
ing and steric sea level rise (i.e., due to changes
FIGURE 1. Density of profiles collected per 1° square during 10 years of
(a) expendable bathythermograph (XBT), (b) shipboard T/S, and (c) Argo T/S
operations. Data courtesy of World Ocean Database (WOD) 2018 (a and b)
and Argo Program (c)
90°N
60°N
30°N
0°
30°S
60°S
90°S
90°N
60°N
30°N
0°
30°S
60°S
90°S
90°N
60°N
30°N
0°
30°S
60°S
90°S
2 to 5
6 to 20
21 to 50
51 to 100
>100
2 to 5
6 to 20
21 to 50
51 to 100
>100
2 to 5
6 to 20
21 to 50
51 to 100
>100
60°E
120°E
180°
120°W
60°W
0°
WOD18 XBT Data 1991–2000, Density of 619,838 profiles
WOD18 Ocean Station Data 1991–2000, Density of 49,258 T/S profiles
Argo decade 2011–2020, Density of 1,612,816 profiles
The Technological, Scientific, and Sociological Revolution of Global
Subsurface Ocean Observing
By Dean Roemmich*, Lynne Talley*, Nathalie Zilberman*, Emily Osborne*, Kenneth S. Johnson*, Leticia Barbero,
Henry C. Bittig, Nathan Briggs, Andrea J. Fassbender, Gregory C. Johnson, Brian A. King, Elaine McDonagh, Sarah Purkey,
Stephen Riser, Toshio Suga, Yuichiro Takeshita, Virginie Thierry, and Susan Wijffels (*lead authors)
in ocean salinity and temperature, which affect density).
However, because capturing these observations required
research vessels, pre-Argo T/S data sets could not attain
systematic global coverage. This changed in the 1990s
with the development of autonomous profiling floats that
enable high-quality T/S observations anywhere at any
time. The Argo Program was designed as a global auton-
omous array of over 3,000 profiling floats spread evenly
over the ocean where the depth exceeds 2,000 m, and it
achieved this milestone in 2007. Free-drifting Argo floats
obtain T/S profiles from 2,000 m depth to the sea surface
every 10 days. All Argo data are distributed freely in near-
real time (12–24 hours) and as research-quality delayed-
mode data (nominally in 12 months). The transformation
in ocean observing brought about by Argo, from exceed-
ingly sparse and regionally biased coverage to systematic
and sustainable global coverage, is apparent in Figure 1.
The combination of Argo and GO-SHIP provides today’s
global observations of the ocean’s interior. GO-SHIP sup-
plies the highest quality global-scale multi-parameter
observations, including biogeochemical as well as physi-
cal properties, from the surface to the seafloor, repeated
on decadal timescales. The accuracy of shipboard data
makes it essential for climate change assessment, sensor
development, and detection and adjustment of drift in
Argo sensors (Sloyan et al., 2019). Additionally, GO-SHIP
provides a scientific foundation for expanding Argo into
full-depth measurements and for investigating the ocean’s
biological and biogeochemical cycling (see next section
on GO-SHIP). In turn, Argo’s systematic, autonomous
sampling provides regional-to-global and seasonal-to-
interannual coverage of T/S that are unattainable by con-
ventional ship-based systems.
Argo has achieved and sustained global observa-
tions because: (1) it provides great value in basic ocean
research, climate variability and change, education, and
ocean forecasting (Johnson et al., 2022); (2) it is based on
effective and efficient global technologies; and (3) it com-
bines with GO-SHIP to provide an ocean observing sys-
tem with unprecedented accuracy and coverage. Central
to Argo’s and GO-SHIP’s successes are their multinational
partnerships composed of academic and government
researchers, agencies charged with ocean observing,
institutions having global reach, and technically proficient
commercial partners.
The transformation of ocean observing brought about
by Argo and GO-SHIP is not complete. GO-SHIP is expand-
ing to include ocean mixing measurements and biological
observations. Deep Argo floats with 6,000 m capability are
increasing Argo’s reach to nearly all the ocean volume,
filling key gaps in our understanding of full-depth ocean
circulation and heat uptake and their relationships with cli-
mate. New sensors for dissolved oxygen, pH, nitrate, and
bio-optical properties have given rise to Biogeochemical
(BGC)-Argo. Core Argo floats are being made more robust,
long-lived, and versatile, enhancing Argo’s coverage, its
sustainability, and the breadth of its applications. The inte-
grated program of Core, Deep, and BGC-Argo (Figure 2),
termed OneArgo, will continue the Argo revolution for sci-
ence and society (Roemmich et al., 2019).
FIGURE 2. The OneArgo array design with floats color-coded for Core, Deep, and Biogeochemical (BGC) Argo. The floats are
randomly distributed in regions with the intention to locate either one or two floats per 3° × 3° square. Courtesy of OceanOPS
60°N
30°N
0°
30°S
60°S
60°E
90°E
120°E
150°E
180°
150°W
120°W
90°W
60°W
30°W
0°
Core Floats (2,500)
Deep Floats (1,200)
BGC Floats (1,000)
OneArgo Design: 4,700 Floats
• The deep ocean is warming and is increasingly contrib-
uting to sea level rise.
• The global ocean circulation and its physical, chemical,
and biological properties are changing under changing
winds and surface fluxes.
• Ocean oxygen content has declined since 1960, with
loss at all depths; tropical oxygen minimum zones have
expanded; upper ocean oxygen has increased in the
Southern Hemisphere subtropical gyres.
• Multiple observing systems show that the ocean absorbs
about 25% of excess CO2 resulting from anthropogenic
inputs. From GO-SHIP, the total ocean inventory of
anthropogenic carbon (Cant) has increased by 30% from
1994 to 2010. Anthropogenic carbon buildup can be
detected as deep as 2,000 m and continues to acidify
the ocean.
• Dissolved organic carbon distributions have been
mapped globally for the first time.
While GO-SHIP’s sustained measurements have evolved
conservatively for continuity, GO-SHIP provides a platform
for piloting new types of observations anywhere in the world
and for international collaboration on individual measure-
ments and full cruises. Each GO-SHIP cruise includes mul-
tiple ancillary activities, including Argo deployments, ocean
mixing measurements, and some biological observations.
A new expansion to include “Bio GO-SHIP” has begun to
investigate the distributions and the biogeochemical and
functional roles of plankton in the global ocean. Routine
sampling of plankton for genetic analyses is proposed, and
microbial sampling has been conducted on several cruises.
As GO-SHIP continues to monitor and expand to new
parameters, it will inevitably reveal new climatically signif-
icant properties of the physics, chemistry, and biology of
the global ocean, and inspire technological advancements
in ship-based and autonomous measurements.
GO-SHIP – HIGH-QUALITY, DECADAL,
GLOBAL PHYSICAL AND BIOGEOCHEMICAL
OBSERVATIONS
GO-SHIP’s quasi-decadal reoccupation of hydrographic
transects spanning the global ocean was implemented and
is sustained to quantify changes in the storage and trans-
port of heat, fresh water, carbon, nutrients, and transient
tracers (Talley et al., 2016; Sloyan et al., 2019; Figure 3).
These full-depth, coast-to-coast transects measure many of
the physical and biogeochemical essential ocean variables
of the Global Ocean Observing System and provide the
highest accuracy ocean data, attainable only with research
ships and specialized, calibrated analytical methods
(Figure 4). Three decades of GO-SHIP data have been cen-
tral to the assessment of the state of the ocean throughout
multiple IPCC reports (https://www.ipcc.ch/), and they are
used in multiple climatologies (e.g., GLODAP; https://www.
glodap.info/) for calibration and validation of autonomous
instruments and for model initialization and validation.
Importantly, GO-SHIP provides the reference standard
data central to calibrating Core, Deep, and BGC Argo sen-
sors. GO-SHIP’s data sets are subject to rapid public release
to maximize use as reference data and for biogeochemical
assessments: preliminary data within six to eight weeks of
the end of a cruise and final data within six months.
In this era of expanding autonomous observing sys-
tems, GO-SHIP, supported by the research fleet, remains
the backbone of sustained observing. The following
climate-related results have been based on GO-SHIP data
(Sloyan et al., 2019, and subsequent works) and have led to
the expansion of Argo into the deep ocean and to biogeo-
chemical measurements to increase our temporal and spa-
tial coverage of these climatically important phenomena.
FIGURE 4. (a) Sampling for oxygen during
GO-SHIP I08S on R/V Roger Revelle
in 2016. Photo credit: Earle Wilson
(b) A CTD/rosette package is launched
during GO-SHIP I06S aboard R/V Thomas
Thompson in 2019. Photo credit: Isa Rosso
FIGURE 3. Global Ocean Ship-based Hydro-
graphic Investigations Program (GO-SHIP)
section tracks. Credit: OceanOPS
CORE ARGO – SUSTAINING AND IMPROVING
SYSTEMATIC GLOBAL OCEAN OBSERVATIONS
FOR CLIMATE
The highest priority for the OneArgo Program is to sus-
tain and improve the longstanding Core Argo array
(Figure 5). The sustainability of an observing system
depends equally on the societal needs driving it and on its
cost- effectiveness. Core Argo’s primary roles are in assess-
ments of global warming, sea level rise, and the hydro-
logical cycle, plus applications in seasonal- to- interannual
ocean and coupled forecasting, and ocean state estimation.
Other research topics that utilize Argo data include ocean
circulation in interior and boundary current regions, meso-
scale eddies, ocean mixing, marine heatwaves, water mass
properties and formation, El Niño-Southern Oscillation,
and ocean dynamics. Argo’s rapidly growing applications
are well documented (e.g., Johnson et al., 2022), with about
500 research papers that use Argo data published per year.
While the scientific needs for Core Argo are strong,
equally important are the technology advancements
in profiling floats and sensors that are transforming
the cost- effectiveness of the array while enabling new
scientific missions.
• Float engineering: Advances in the hydraulic system
controlling float buoyancy have contributed to sub-
stantial decreases in float failure rates (Figure 6) while
increasing energy efficiency for longer float missions.
• Battery technology: The use of improved (hybrid) lith-
ium batteries since about 2016 is doubling the battery
lifetime of some Core Argo float models from about five
years to 10 years.
• Satellite communications: Around 2011, Argo com-
munications transitioned from the one-way System
ARGOS to the bidirectional Iridium global cellular net-
work. A float’s time on the sea surface for data trans-
mission was reduced from 10 hours to 15 minutes in
each cycle, resulting in energy savings and avoidance
of surface hazards, including grounding and biofoul-
ing. New applications have emerged utilizing the rapid
data turnaround, while the bidirectional transmissions
enable changes in mission parameters throughout
float lifetimes.
In the transition to OneArgo, Core Argo coverage require-
ments (Figure 2) are increasing in key regions. Doubling
of float density in the equatorial Pacific is needed by the
Tropical Pacific Observing System (https://tpos2020.org).
Similarly, doubling is needed in western boundary regions
that exhibit high variability and in marginal seas adjacent
to the continental shelves. Increasing coverage in high-
latitude, seasonally ice-covered regions is accomplished
by using T/S to infer ice-free conditions and by using
ice-hardened antennas. The map of OneArgo coverage
(Figure 2) shows that expanded coverage of 0–2,000 m
T/S profiles will be accomplished even as the number of
exclusively Core Argo floats decreases, because Deep and
BGC Argo floats also collect 0–2,000 m (Core) T/S profiles.
Core Argo will continue the technology and scientific rev-
olutions that have transformed global observing from a
vision to reality.
FIGURE 5. Locations of active Argo floats, including those for the Core, Deep, and Biogeochemical
(BGC) programs, color-coded by national program, as of July 2021. Courtesy of OceanOPS
90°N
60°N
30°N
0°
30°S
60°S
90°S
60°E
90°E
120°E
150°E
180°
150°W
120°W
90°W
60°W
30°W
0°
Argo National Contributions: 3,894 Floats
FIGURE 6. Survival rates (%) of US Argo
floats (dashed lines) and all Argo floats
(solid lines) over an initial five-year period
for those deployed in 2015 (blue) com-
pared with those deployed in Argo’s first
five years (2000–2004, red). Data cour-
tesy of OceanOPS
100
80
60
40
20
Survival Rate (%)
Year
Argo Survival Rate by Deployment Year
2000–2004
2015
Solid Lines = All Argo
Dashed Lines = US Argo
0
1
2
3
4
DEEP ARGO – OBSERVING THE FULL
OCEAN VOLUME
Sustained measurements of ocean properties and circu-
lation are needed over the full water column to provide
fundamental insights into the spatial and temporal extent
of deep ocean warming, sea level rise resulting from the
expanded volume of deep ocean warming, and environ-
mental changes that affect the growth and reproduction
of deep-sea species. Deep ocean (>2,000 m) observing is
sparse in space and time compared to the upper 2,000 m.
Less than 10% of historical non-Argo T/S profiles extend
to depths greater than 2,000 m, with current high-quality
deep ocean measurements limited primarily to GO-SHIP
transects repeated on decadal timescales, ocean stations
located in special regions, and moored arrays set mainly
near the coasts of continents.
To address the void in deep ocean observing, new Deep
Argo float models are designed with high pressure toler-
ance in order to extend autonomous ocean observing to
the abyss. New Deep Argo CTD sensors have improved
temperature, salinity, and pressure accuracies and stability
to resolve deep ocean signals. Use of a bottom-detection
algorithm and bottom-detecting wires enables collection
of temperature, salinity, oxygen, and pressure to as close
as 1–3 m above the seafloor. The implementation of an
ice-avoiding algorithm on all Deep Argo floats deployed at
high latitudes enables deep ocean profiling under sea ice.
The Deep Argo fleet presently consists of pilot arrays
implemented in deep regions where GO-SHIP data show
strong ocean warming (Figure 7). Active float models
include those capable of sampling from the surface to
6,000 m depth, and others that can profile to 4,000 m
(Figure 8). Observations from the pilot arrays show float life-
times reaching 5.5 years and sensor accuracies approach-
ing GO-SHIP quality standards. Deep Argo’s ability is well
demonstrated to measure variability of deep ocean warm-
ing and large-scale deep ocean circulation, both regionally
and globally, at intraseasonal to decadal timescales. The
international Deep Argo community is committed to imple-
menting a global Deep Argo array of 1,250 floats in the next
five to eight years and to sustain Deep Argo observations in
the future (Zilberman et al., 2019).
With full implementation of the Deep Argo array, the
temporal and spatial resolution of deep ocean observa-
tions will improve by orders of magnitude, enabling new
insight into how the deep ocean responds to, distributes,
or influences signals of Earth’s changing climate. Deep
Argo’s homogeneous coverage of the full ocean volume
in all seasons will be particularly useful to constrain and
increase signal-to-error ratios in global ocean reanalyses
and to prevent unrealistic drift in coupled climate-ocean
models. Deep Argo will therefore increase our ability to
predict climate variability and change and to anticipate
and reduce the impact of more frequent extreme weather
events, warmer ocean temperatures, and sea level rise.
These all have damaging implications for various sectors
of the blue economy that nations increasingly depend
upon. Low-lying coastal communities and small island
developing states are especially vulnerable.
FIGURE 8. (a) Deployment of a 4,000 m capable
Deep Arvor float in the North Atlantic Ocean. Photo
courtesy of IFREMER/GEOVIDE (b) Deployment of
a 6,000 m capable Deep SOLO float in the North
Pacific Ocean. Photo credit: Richard Walsh
FIGURE 7. Location of the 191 Deep Argo floats active in October 2021,
including 4,000 m capable Deep Arvor and Deep NINJA, and 6,000 m
capable Deep SOLO and Deep APEX floats. The background colors indi-
cate ocean bottom depth: <2,000 m (white), 2,000–3,000 m (light gray),
3,000–4,000 m (light blue), 4,000–5,000 m (blue), and >5,000 m (dark
gray). Data courtesy of OceanOPS
SIO Deep SOLO (64)
MRV DSeep SOLO (38)
Deep Arvor (60)
Deep NINJA (1)
Deep APEX (28)
60°N
30°N
0°
30°S
60°S
60°E
120°E
180°
120°W
60°W
0°