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
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