CARBON
CO2
FRONTIERS IN OCEAN
OBSERVING
2025 Oceanography Supplement
MARINE PROTECTED AREAS,
WESTERN BOUNDARY CURRENTS,
AND THE DEEP SEA
ON THE COVER
Future ocean observing within marine protected
areas may include traditional sampling by ships
at intervals augmented by continuous data col-
lection of a wide range of variables by a variety
of sensors, where the data are transmitted to lab-
oratories in real time for analysis. Figure modified
from Clark et al. (2025, in this issue). Illustration
by Jon White, Plymouth Marine Laboratory
FRONTIERS IN OCEAN OBSERVING EXECUTIVE COMMITTEE
• Benoît Pirenne, Ocean Networks Canada
• Johanna Post, UNESCO
• Sophie Seeyave, Partnership for Observation of the Global Ocean
• Martin Visbeck, GEOMAR Helmholtz Center for Ocean Research Kiel
• Ann-Christine Zinkann, NOAA’s Global Ocean Monitoring and Observing Program
OCEANOGRAPHY STAFF
• Ellen S. Kappel, Oceanography Editor
• Vicky Cullen, Oceanography Assistant Editor
• Johanna Adams, Layout and Design
FRONTIERS IN OCEAN OBSERVING GUEST EDITORS
The Use of Autonomous Tools for Ecosystem Management and Monitoring of
Marine Protected Areas
• Catherine Edwards, Skidaway Institute of Oceanography, University of Georgia
• Georgia Coward, Center for Ocean Leadership, UCAR
Western Boundary Currents and Their Impacts on Shelf Seas
• Moninya Roughan, UNSW Sydney, Australia
• Tammy Morris, SAEON, South Africa
• Ilson Carlos A. da Silveira, University of São Paulo, Brazil
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, and the Partnership for Observation of the Global Ocean.
This is an open access document made available under a Creative Commons
Attribution 4.0 International License, which permits use, sharing, adaptation,
distribution, and reproduction in any medium or format as long as users cite the
materials appropriately, provide a link to the Creative Commons license, and
indicate the changes that were made to the original content. Users will need to
obtain permission directly from the license holder to reproduce images that are not
included in the Creative Commons license.
PRINT: ISSN 1042-8275
ONLINE: ISSN 2377-617X
PUBLISHED BY: The Oceanography Society
DATE: April 2025
PREFERRED CITATION
Kappel, E.S., V. Cullen, G. Coward, I.C.A. da Silveira, C. Edwards, T. Morris, and
M. Roughan, eds. 2025. Frontiers in Ocean Observing: Marine Protected Areas,
Western Boundary Currents, and the Deep Sea. Oceanography 38(Supplement 1),
https://doi.org/10.5670/oceanog.2025.s1.
CONTENTS
INTRODUCTION
1
Introduction to Frontiers in Ocean Observing: Marine Protected Areas, Western Boundary Currents,
and the Deep Sea
By E.S. Kappel
MODEL-BASED DESIGN AND EVALUATION OF OBSERVING NETWORKS
2
Model-Based Observing System Evaluation in a Western Boundary Current: Observation Impact from
the Coherent Jet to the Eddy Field
By C. Kerry, M. Roughan, S. Keating, and D. Gwyther
THE USE OF AUTONOMOUS TOOLS FOR ECOSYSTEM MANAGEMENT
AND MONITORING OF MARINE PROTECTED AREAS
13 Glider Surveillance for Near-Real-Time Detection and Spatial Management of North Atlantic Right Whales
By K.L. Indeck, M.F. Baumgartner, L. Lecavalier, F. Whoriskey, D. Durette-Morin, N.R. Pettigrew, J.M. McSweeney,
L.H. Thorne, K.L. Gallagher, C.R. Edwards, E. Meyer-Gutbrod, and K.T.A. Davies
22 Observing Marine Heatwaves Using Ocean Gliders to Address Ecosystem Challenges Through a
Coordinated National Program
By J.A. Benthuysen, C. Pattiaratchi, C.M. Spillman, P. Govekar, H. Beggs, H. Bastos de Oliveira, A. Chandrapavan,
M. Feng, A.J. Hobday, N.J. Holbrook, F.R.A. Jaine, and A. Schaeffer
26 Monitoring Ocean Biology and Natural Resources Autonomously and Efficiently Using Underwater Gliders
By H. Broadbent, A. Silverman, R. Russell, G. Miller, S. Beckwith, E. Hughes, and C. Lembke
29 The Western Channel Observatory Automated Plankton Imaging and Classification System
By J.R. Clark, E.S. Fileman, J. Fishwick, S. Rühl, and C.E. Widdicombe
32 Collaborating with Marine Birds to Monitor the Physical Environment Within Coastal Marine
Protected Areas
By R.A. Orben, A. Peck-Richardson, A. Piggott, J. Lerczak, G. Wilson, J.C. Garwood, X. Liu, S.B. Muzaffar, A.D. Foster,
H.A. Naser, M. AlMusallami, T. Anker-Nilssen, J.P.Y. Arnould, M.L. Berumen, T. Cansse, S. Cárdenas-Alayza,
S. Christensen-Dalsgaard, A.Q. Khamis, T. Carpenter-Kling, N. Dehnhard, M. Dagys, A. Fayet, R.M. Forney, S. Garthe,
S.A. Hatch, M.E. Johns, M. Kim, K. Layton-Matthews, A.K. Lenske, G.T.W. McClelland, J. Morkūnas, A.O. Nasif,
G. Panagoda, J.-H. Park, V.R.A. Pimenta, F. Quintana, M.J. Rayner, T.K. Reiertsen, S.S. Seneviratne, M. van Toor,
P. Warzybok, E.A. Weideman, J. Yi, Y.-T. Yu, and C.B. Zavalaga
38 Ocean Gliders for Planning and Monitoring Remote Canadian Pacific Marine Protected Areas
By T. Ross, H.V. Dosser, J.M. Klymak, W. Evans, A. Hare, J.M. Jackson, and S. Waterman
41 Optical Sediment Trap for In Situ Monitoring of Sinking Marine Particles
By K. Simon, W. Slade, M. Estapa, O. Mikkelsen, and C. Pottsmith
44 Building Ocean Biodiversity Monitoring Capacity: Tracking Marine Animals with Acoustic Telemetry
and the Role of the Ocean Tracking Network
By F.G. Whoriskey
WESTERN BOUNDARY CURRENTS AND THEIR IMPACTS ON SHELF SEAS
47 Advancing Observations of Western Boundary Currents: Integrating Novel Technologies for a Coordinated
Monitoring Approach
By M. Roughan, J. Li, and T. Morris
54 Monitoring Impacts of the Gulf Stream and its Rings on the Physics, Chemistry, and Biology of the
Middle Atlantic Bight Shelf and Slope from CMV Oleander
By M. Andres, T. Rossby, E. Firing, C. Flagg, N.R. Bates, J. Hummon, D. Pierrot, T.J. Noyes, M.P. Enright, J.K. O’Brien,
R. Hudak, S. Dong, D.C. Melrose, D.G. Johns, and L. Gregory
61 Twenty Years Monitoring the Brazil Current Along the NOAA AX97 High-Density XBT Transect
By T.P. Ferreira, P. Marangoni G.M.P., M. Cirano, A.M. Paiva, S.B.O. Cruz, P.P. Freitas, M. Goes, and M.M. Mata
67 Fishing for Ocean Data in the East Australian Current
By V. Lago, M. Roughan, C. Kerry, and I. Knuckey
72 Coordinated Observing and Modeling of the West Florida Shelf with Harmful Algal Bloom Application
By R.H. Weisberg and Y. Liu
TECHNOLOGICAL SOLUTIONS FOR AN ACCESSIBLE DEEP OCEAN
76 Unraveling Major Questions in Micronekton Ecology and Their Role in the Biological Carbon Pump Through
Integrative Approaches and Autonomous Monitoring
By P. Annasawmy, G. Chandelier, and T. Le Mézo
82 Real-Time Data Connectivity to Deep Autonomous Seafloor Instrumentation in Adverse Flow Conditions
By C. Ewert, R. Heux, N. Howins, M. Lankhorst, G. Manta, and U. Send
86 Interferometric Synthetic Aperture Sonar: A New Tool for Seafloor Characterization
By J.W. Jamieson, C. Gini, C. Brown, and K. Robert
89 The Potential of Low-Tech Tools and Artificial Intelligence for Monitoring Blue Carbon in Greenland’s
Deep Sea
By N. Bax, J. Halpin, S. Long, C. Yesson, J. Marlow, and N. Zwerschke
92 Atlantic arc Lander Monitoring (ALaMo): An Emerging Network of Low-Cost Lander Arrays for
Ocean Bottom Observations
By C. González-Pola, C. Cusack, I. Robles-Urquijo, R. Graña, L. Rodriguez-Cobo, R.F. Sánchez-Leal, G. Nolan,
and A.M. Piecho-Santos
96 Videomodule Towed System: Acquisition and Analysis of Video Imaging Data for Benthic Surveys
By I. Anisimov, A. Lesin, V. Muravya, A. Zalota, and M. Zalota
INTRODUCTION TO FRONTIERS IN OCEAN OBSERVING
MARINE PROTECTED AREAS, WESTERN BOUNDARY CURRENTS, AND THE DEEP SEA
By Ellen S. Kappel
In this third and final “Frontiers in Ocean Observing” sup-
plement to Oceanography, peer-reviewed articles describe
data collection and analysis from the surface ocean to the
seafloor, spanning the globe from marine protected areas
to western boundary currents and the deep sea. They
describe a variety of technologies used to collect and ana-
lyze ocean observations, including emerging sonar technol-
ogy for high-resolution mapping and imaging of the sea-
floor, low-cost tools combined with artificial intelligence
to monitor blue carbon in Greenland’s deep sea, and the
integration of eDNA, acoustic, and trawl data to investi-
gate the diversity, abundance, biomass, and distribution of
micronekton in the Western Indian Ocean.
Other articles describe how autonomous vehicles such
as gliders now assist with management of marine pro-
tected areas, detection and protection of North Atlantic
right whales, forecasting of harmful algal blooms, inves-
tigation of marine heatwaves, and augmentation of the
network for ocean animal tracking. They also detail, for
example, how ocean scientists are obtaining long-term
data on western boundary currents to augment other more
traditional data collection methods with approaches that
include partnering with a merchant marine container ves-
sel to collect data on the Gulf Stream and a collaborative
project between researchers and industry that uses com-
mercial fishing gear to collect subsurface ocean data in
the East Australian Current. Another article considers how
the observations collected in western boundary currents, in
particular, the East Australian Current, impact ocean fore-
casts, a useful assessment for improving ocean observing
system design.
Similar to the first two ocean observing supplements
(see https://tos.org/ocean-observing), we invited potential
authors to submit letters of interest associated with topics
aligned with the priorities of the UN Decade of Ocean
Science for Sustainable Development (2021–2030). The
chosen topics for this supplement are described below.
MODEL-BASED DESIGN AND EVALUATION OF
OBSERVING NETWORKS
Here, the authors describe and apply model-based meth-
ods for methodically evaluating existing integrated ocean
observing systems and future extensions by exploring
process-focused array design, observation priorities, and
sampling strategies; complementarity versus redundancy
of multi-platform networks; and detectable changes in key
climate metrics.
THE USE OF AUTONOMOUS TOOLS FOR
ECOSYSTEM MANAGEMENT AND MONITORING
OF MARINE PROTECTED AREAS
Authors addressing this topic demonstrate how sensors on
autonomous vehicles are filling critical gaps in ocean bio-
logical and spatial conservation knowledge that will help
tackle ecosystem-level challenges caused by global envi-
ronmental changes.
WESTERN BOUNDARY CURRENTS AND THEIR
IMPACTS ON SHELF SEAS
These articles showcase long-term, sustained observa-
tional efforts in western boundary current shelf sea regions
that highlight strengths, weaknesses, and gaps in the sys-
tem, and/or provide examples of end-user and stakeholder
engagement.
TECHNOLOGICAL SOLUTIONS FOR AN
ACCESSIBLE DEEP OCEAN
This section provides recent examples of how the inter-
sections among cutting-edge sensors, including low-cost
technologies, data analytics, and robotics, are advanc-
ing deep-sea exploration and opening avenues for dis-
coveries and a deeper understanding of our planet’s
least-explored realms.
Many thanks to Ocean Networks Canada, the US
National
Oceanic
and
Atmospheric
Administration’s
Global Ocean Monitoring and Observing Program, and
the Partnership for Observation of the Global Ocean for
generously supporting publication of this supplement
to Oceanography. I would also like to thank all the supple-
ment’s guest editors for their valuable input and guidance
on articles submitted to their thematic areas.
AUTHOR
Ellen S. Kappel (ekappel@geo-prose.com), Oceanography Editor and
Geosciences Professional Services Inc., Bethesda, MD, USA.
ARTICLE DOI. https://doi.org/10.5670/oceanog.2025e120
MODEL-BASED OBSERVING SYSTEM EVALUATION IN A WESTERN
BOUNDARY CURRENT: OBSERVATION IMPACT FROM THE COHERENT
JET TO THE EDDY FIELD
By Colette Kerry, Moninya Roughan, Shane Keating, and David Gwyther
ABSTRACT
Ocean forecast models rely on observations to provide
regular updates in order to correctly represent dynamic
ocean circulation. This synthesis of observations and mod-
els is referred to as data assimilation. Since initial condi-
tions dominate the quality of short-term ocean forecasts,
accurate ocean state estimates, achieved through data
assimilation, are key to improving prediction. Western
boundary current (WBC) regions are particularly challeng-
ing to model and predict because they are highly variable.
Understanding how specific observation types, platforms,
locations, and observing frequencies impact model esti-
mates is key to effective observing system design.
The East Australian Current (EAC), the South Pacific’s
WBC, is a relatively well-observed current system that
allows us to study the impact of observations on prediction
across different dynamical regimes, from where the current
flows as a mostly coherent jet to the downstream eddy
field. Here we present a review of the impact of observa-
tions on model estimates of the EAC using three different
methods. Consistent results across the three approaches
provide a comprehensive understanding of observation
impact in this dynamic WBC. Observations made in regions
of greater natural variability contribute most to constrain-
ing the model estimates, and subsurface observations
have a high impact relative to the number of observations.
Significantly, sampling the downstream eddy-rich region
constrains the upstream circulation, whereas observing the
upstream coherent jet provides less improvement to down-
stream eddy field estimates. Studies such as these provide
powerful insights into both observing system design and
modeling approaches that are vital for optimizing observa-
tion and prediction efforts.
INTRODUCTION
Accurate estimates of past, present, and future ocean
states are crucial to effective management of our ocean
environment and marine industries. Short-term ocean
predictions (days to weeks) are vital to myriad environ-
mental, societal, and economic applications, including
facilitating the adaptive management of marine ecosys-
tems, forecasting extreme weather events, predicting the
onset and persistence of marine heatwaves, providing
accurate ocean forecasts for shipping and military opera-
tions, predicting the fate of pollutants, and guiding search
and rescue operations.
Ocean state estimates require the combination of
numerical models and ocean observations, referred to as
data assimilation (DA). Observations provide sparse data
points while the model provides dynamical context. The
goal of DA is to combine the model with observations to
reduce uncertainty in the model estimate. For forecasting
purposes, model estimates are updated through assim-
ilation when observations become available and provide
improved initial conditions for the next forecast (Figure 1).
Due to the dynamic nature of the ocean circulation, ocean
models must be regularly updated through DA to, for exam-
ple, correctly represent the timing and locations of oceanic
eddies (e.g., Thoppil et al., 2021; Chamberlain et al., 2021).
A critical component of the DA problem is the way by
MODEL-BASED DESIGN AND EVALUATION
OF OBSERVING NETWORKS
which the information contained in the observations is
projected onto the (unobserved) model state estimate.
Advanced DA techniques use time-variable model dynam-
ics to actively interpolate information from observations
up- and downstream and forward and backward in time.
Observations are assimilated over a time interval, given
the temporal evolution of the circulation (e.g., Moore et al.,
2020). Identifying observations that best constrain an
ocean model can drive improved observing system design
for more accurate and more cost-effective prediction.
Observation impact studies aim to quantify how specific
observation types, locations, and observing frequencies
impact model estimates (e.g., Oke et al., 2015).
In this article, we assess observation impact in a dynamic
western boundary current (WBC). WBCs are swift, pole-
ward-flowing currents that exist on the western sides of
subtropical ocean gyres. They transport warm water from
the tropics toward the poles, redistributing heat and mod-
ulating global climate. Mesoscale eddies form due to insta-
bilities in the strong boundary current flow, making WBC
extension regions hotspots of high eddy variability (Imawaki
et al., 2013; Li et al., 2022a). WBCs typically exhibit the high-
est errors in ocean forecasts (e.g., Brassington et al., 2023)
due to their strong flows, the complexities of eddy shedding
and evolution (e.g., Kang and Curchitser, 2013; Pilo et al.,
2015; Yang et al., 2018), and their complex vertical struc-
tures (e.g., Sun et al., 2017; Pilo et al., 2018; Brokaw et al.,
2020; Rykova and Oke, 2022). Understanding the interplay
of observing system design and modeling approaches is
crucial to improving prediction in highly dynamic, eddy-rich
oceanographic environments.
The East Australian Current (EAC) is the WBC of the
South Pacific subtropical gyre, and its eddies dominate
the circulation along the southeastern coast of Australia
(Figure 2a; Oke et al., 2019). The southward-flowing current
is most coherent off 28°S (Sloyan et al., 2016) and intensi-
fies around 29°–31°S (Kerry and Roughan, 2020). The cur-
rent typically separates from the coast between 31°S and
32.5°S, turning eastward and shedding large warm-core
eddies in the Tasman Sea (Cetina Heredia et al., 2014). The
EAC is a relatively well-observed WBC system, with obser-
vations collected as part of Australia’s Integrated Marine
Observing System (IMOS; Figure 2b–d) spanning from the
coherent jet to the eddy field (e.g., Roughan et al., 2015).
The EAC therefore provides an ideal testbed for assessing
observation impact across differing dynamical regimes.
Observing networks, numerical models, and DA schemes
make up the key components of ocean prediction systems.
Data-assimilating models are useful for evaluating and
designing observing networks. Here we synthesize the
results from three different model-based approaches in
order to assess observation impact across a common sys-
tem (the EAC). We use three methods for studying observa-
tion impact: an adjoint-based approach to directly quantify
FIGURE 1. Conceptual schematic showing sequential time-dependent data assimilation and a summary of the three methods presented in this study
for assessing observation impact.
observation impact, Observing System Experiments (with-
holding observations), and Observing System Simulation
Experiments (Figure 1). This review summarizes the key
results obtained through each method, and synthesizes
the consistent results to provide a broad understanding of
observation impact along the extent of the WBC system.
ASSESSING OBSERVATION IMPACT
THE SOUTH EAST AUSTRALIAN COASTAL
FORECAST SYSTEM
The South East Australian Coastal Forecast System
(SEA-COFS) consists of several Regional Ocean Modeling
System (ROMS; Shchepetkin and McWilliams, 2005) con-
figurations at a range of resolutions for the southeast
Australian oceanic region. The EAC-ROMS regional model
(domain shown in Figure 2a) has a 2.5–5 km horizontal res-
olution, with higher resolution over the continental shelf
and slope, and 30 terrain-following vertical layers (Kerry
et al., 2016; Kerry and Roughan, 2020).
We constrain the model with observational data from
a variety of traditional and novel observation platforms
using four-dimensional variational DA (4D-Var). This tech-
nique uses variational calculus to solve for increments in
model initial conditions, boundary conditions, and forcing
such that the differences between the new model solution
of the time-evolving flow and all available observations is
minimized—in a least-squares sense—over an assimilation
window (Figure 1; Moore et al., 2004, 2011). Here we use
five-day assimilation windows. The goal is for the model to
represent all of the observations in time and space using
the physics of the model, and accounting for the uncertain-
ties in the observations and background model state, to
produce a description of the ocean state that is a dynami-
cally consistent solution of the nonlinear model equations.
For this mesoscale eddy-dominated system, adjustments
to the initial conditions dominate over boundary or surface
forcing adjustments and forecast errors are dominated by
errors in the initial state (Kerry et al., 2020).
Observation impact is studied based on a data-
assimilating configuration of the EAC-ROMS model for
2012–2013 (Kerry et al., 2016), when numerous data
streams were available through IMOS (Figure 2b,c). These
included velocity and hydrographic observations from a
deep- water mooring array (the EAC array; Sloyan et al.,
2016) and continental shelf moorings (Malan et al., 2021;
Roughan et al., 2022), radial surface velocities from a high-
frequency (HF) radar array (Archer et al., 2017), and hydro-
graphic observations from ocean gliders (Schaeffer et al.,
2016). These observations complemented the more tradi-
tional data streams of satellite-derived sea surface height
FIGURE 2. The EAC is a fairly well observed western boundary current system. (a) Schematic showing the East Australia Current (EAC; adapted from
Oke et al., 2019) with the regional ocean model domain. (b) Locations of Argo and eXpendable BathyThermograph (XBT) observations. (c) Integrated
Marine Observing System (IMOS) observations. (d) Photos of observing the EAC. Photo credits: M. Roughan and IMOS
28oS
34oS
(SSH) and sea surface temperature (SST), temperature and
salinity from Argo profiling floats, and temperature from
eXpendable BathyThermograph (XBT) lines.
METHOD 1: AN ADJOINT-BASED APPROACH
The 4D-Var DA scheme uses sequential iterations of the
linearized model equations and their adjoint (Errico, 1997)
to minimize the model-observation difference. By defining
a scalar measure of the ocean circulation, we can use this
mathematical framework to directly compute the impact
of each individual observation on the change in the circu-
lation measure (e.g., Langland and Baker, 2004; Powell,
2017). We use this methodology to understand how obser-
vations impact estimates of alongshore volume transport
through shore-normal sections that span the extent of the
EAC, and of spatially averaged eddy kinetic energy (EKE)
over the eddy-rich Tasman Sea (Kerry et al., 2018).
The contribution of each observing platform to changes
in modeled volume transport and EKE varies considerably
over the two-year period, as it depends on the flow regime
and the observation coverage for each assimilation win-
dow. To gain an overall picture of how observations from
across the EAC region impact a particular circulation met-
ric, we group the observation impacts by acquisition lati-
tude (Figure 3a,b). This analysis reveals that both up- and
downstream observations impact transport estimates
along the extent of the EAC system. While the EAC is mostly
coherent off 28°S, volume transport varies due to mean-
dering of the EAC core and intermittent separation events
(Oke et al., 2019; Kerry and Roughan, 2020). Glider and XBT
observations off 34°S and HF radar observations at 30°S
impact EAC transport to the north (28°S, upstream impacts,
Figure 3a). The volume transport off 34°S is more variable
than upstream due to the eddy-dominated circulation
FIGURE 3. Summary of up- and downstream observation impacts. (a) Observation impacts using the adjoint-based method on transport through the
shore normal section crossing the coast at 28°S (upstream) grouped into latitude bins of 0.25° and normalized by the number of observations. (b) Same
as (a) but for transport through section crossing the coast at 34°S (downstream). Adapted from Kerry et al. (2018) (c) Observing System Experiments
(OSEs) show the EAC mooring array constraining upstream current structure (Siripatana et al., 2020). (d) Surface radial velocities (from HF radar array
at 30°S) impact vorticity up- and downstream (Siripatana et al., 2020). (e) Observing System Simulation Experiments (OSSEs) show that subsurface
temperature (250 m) is improved with XBT observations (Gwyther et al., 2022). Text in the black boxes summarizes parallels between the information
in panels a–b and that in panels c–e. AVISO = Archiving, Validation, and Interpretation of Satellite Oceanographic data. EAC = East Australia Current.
HF = High frequency. SEQ = South East Queensland. SSH = Sea surface height. SST = Sea surface temperature. NAVO = Naval Oceanographic Office.
NSW = New South Wales. XBT = eXpendable BathyThermograph. See text for definitions of FULL and TRAD.
regime (Kerry and Roughan, 2020). This downstream trans-
port is constrained primarily by observations over the eddy
field but is also impacted by the EAC array, the northern
XBT lines, and the HF radar observations (downstream
impacts, Figure 3b).
Normalizing the impacts by the number of observations
(e.g., Figure 3a,b) reveals that observations over the eddy
field make the greatest contribution to volume transport
estimates along the coast. SSH, SST, and Argo observa-
tions made in the region of high eddy variability (33°–37°S)
have more impact than the same observations made else-
where as they provide information to constrain the variable
region. Even for volume transport estimates where the jet
is mostly coherent, satellite and Argo observations of the
(downstream) eddy field have greater impact than the
same observation types upstream (Figure 3a). The eddy
field observation impact exceeds the impact of observa-
tions local to 28°S.
Subsurface observations that sample hydrography within
EAC eddies, such as those from Argo, gliders, and XBTs,
are also particularly impactful (Figure 3a,b). Observations
made in the upper 500 m of the water column contribute
more to changes in the circulation estimates than deeper
observations (Figure 4a,b). When glider observations sam-
ple eddies offshore of the continental shelf (Figure 2c), they
have large impacts on EAC transport and EKE (contribut-
ing to 28%–36% of transport increments, and 38% for EKE;
Kerry et al., 2018).
METHOD 2: OBSERVING SYSTEM EXPERIMENTS
Observing System Experiments (OSEs) compare the results
of a DA system that withholds certain observations with a
system that includes them (e.g., Chang et al., 2023). Using
the EAC-ROMS configuration for 2012–2013, we compared
the impact of assimilating only the more traditional obser-
vations (satellite-derived SSH and SST, and vertical profiles
from Argo and XBTs: the TRAD experiment), versus also
including data from more novel observation platforms (HF
radar, deep and shallow moorings, and gliders: the full
suite of all available observations, the FULL experiment;
Siripatana et al., 2020).
While the overall surface and subsurface properties
FIGURE 4. Summary of subsurface observation impacts. (a) Observation impacts using the adjoint-based method on transport through the shore
normal section crossing the coast at 28°S (upstream) grouped into depth bins and normalized by the number of observations. (b) Same as (a) but
for transport through section crossing the coast at 34°S (downstream). Adapted from Kerry et al. (2018) (c) OSSEs show the depth region of greatest
variability (>500 m) benefits most from subsurface observations (Gwyther et al., 2022). (d) OSEs show improvement in shelf velocities with mooring
data assimilated (Siripatana et al., 2020). (e) Example of glider data (glider path shown in red) constraining the subsurface temperature and velocity
structure of a cold core eddy off Sydney (Siripatana et al., 2020). Text in the black boxes summarizes parallels between the information in panels a–b
and that in panels c–e. SEQ = Southeast Queensland. CH and COFFS = Coffs Harbor. SYD = Sydney.
Absolute value of impact per observation (Sv)
Dense subsurface observations
constrain eddy subsurface structure
Data from shelf moorings improve
velocity representation
FULL
TRAD
Transport through 28oS
Transport through 34oS
Depth of observations
i)
iii)
ii)
vi)
Observations that constrain
the variable upper ocean
(>500m) are most impactful
SEQ200m
CH100m
SYD100m
28oS