(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.