The East Australian Current

Abstract

The East Australian Current (EAC) is a complex and highly energetic western boundary system in the south-western Pacific off eastern Australia. The EAC provides both the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres.

Full text

The East Australian Current
Ken Ridgway1, Katy Hill2
1CSIRO Marine and Atmospheric Research, PO Box 1538, Hobart, TAS 7001, Australia
2Integrated Marine Observing System, University of Tasmania, Private Bag 110, Hobart,
TAS 7001, Australia.
Ridgway, K. and Hill, K. (2009) The East Australian Current. In A Marine Climate Change Impacts
and Adaptation Report Card for Australia 2009 (Eds. E.S. Poloczanska, A.J. Hobday and A.J.
Richardson), NCCARF Publication 05/09, ISBN 978-1-921609-03-9.
Lead author email: Ken.Ridgway@CSIRO.au
Summary: The East Australian Current (EAC) is a complex and highly energetic
western boundary system in the south-western Pacific off eastern Australia. The EAC
provides both the western boundary of the South Pacific Gyre and the linking element
between the Pacific and Indian Ocean gyres.
The EAC is weaker than other western boundary currents and is dominated by a series
of mesoscale eddies that produce highly variable patterns of current strength and
direction. Seasonal, interannual and strong decadal changes are observed in the
current, which tend to mask the underlying long-term trends related to greenhouse gas
forcing.
Observations from a long-term coastal station show that the EAC has strengthened
and extended further southward over the past 60 years. The south Tasman Sea region
has become both warmer and saltier, with mean trends of 2.28°C/century and 0.34
psu/century over the 1944-2002 period, which corresponds to a poleward advance of
the EAC Extension of about 350 km.
The observed intensification of the EAC flow past Tasmania is driven by a spin-up
and southward shift of the Southern Hemisphere subtropical ocean circulation.
Changes in the gyre strength are, in turn, linked to changes in wind stress curl over a
broad region of the South Pacific. Oceanic changes are forced by an intensification of
the wind stress curl arising from a poleward shift in the circumpolar westerly winds
due to the trend in the Southern Annular Mode.
Observational and modelling studies indicate that these changes in the wind patterns
are at least in part attributable to ozone depletion over the past decades. However, at
least some of the trend is likely to be forced by increases in atmospheric CO2. Climate
models under observed CO2 increases, also produce an upward trend of the SAM and
a consequent intensification of the Southern Hemisphere gyre system.
There is strong consensus in climate model simulations that trends observed over the
past 50 years will continue and accelerate over the next 100 years.

Ridgway and Hill 2009
Introduction
The East Australian Current (EAC) is a complex and highly energetic western
boundary system in the south-western Pacific off eastern Australia (Ridgway and
Dunn 2003). Although its mean flow is relatively weak (Ridgway and Godfrey 1994)
it is known to be a highly variable system with large mesoscale eddies dominating the
flow (Bowen et al., 2005; Mata et al., 2007). The EAC has an important role in
removing heat from the tropics and releasing it to the mid-latitude atmosphere
(Roemmich et al, 2006). The Tasman/Coral Sea basin is an ocean region of
importance to Australia, being adjacent to large population centres, encompassing
major shipping lanes, and including regions of environmental significance. The EAC
is also the dominant environmental influence on offshore pelagic fisheries in the
region (Hobday and Hartmann, 2006). A central driver of key species such as
southern bluefin tuna (SBT) is the seasonal changes of subsurface properties within
the EAC system.
Figure 1: The warm EAC jet flows along the shelf-edge off eastern Australia.
Mean Circulation
The EAC provides both the western boundary of the South Pacific Gyre and the
linking element between the Pacific and Indian Ocean gyres (Speich et al. 2002). It
forms between 10° and 15°S. The current is accelerated, southward along the coastal
boundary, and then separates into northeastward (Subtropical Counter Current),
eastward (Tasman Front) and residual southward (EAC Extension) components at
around 31°S (Figure 1, Ridgway and Dunn 2003). Between 18º to 35ºS, the
southward transport ranges from 25 to 37 Sv, the latter value includes a significant
recirculation feature. A portion of the Tasman Front re-attaches to the northern coast
of New Zealand, forming the East Auckland Current and a sequence of semi-
permanent eddies. The residue of the EAC transport continues southward along the
Australian coast as far as Tasmania and then turns westward into the eastern Indian
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The East Australian Current
Ocean (Tasman Outflow) with an important impact on the global ocean circulation
(Speich et al. 2002).
Figure 2: Schematic of the main ocean currents off eastern Australia. Surface currents are
shown in orange and subsurface currents are cyan.
Figure 3: The mean surface height field in the southwest Pacific for the 1992-2006
period (contour interval is 0.05-m). The southward flowing EAC (shown in dark red
along the Australian coast) is the dominant feature.
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Ridgway and Hill 2009
The variability in the EAC is large compared to the mean flow. Firstly, the EAC is
weaker than its counterparts elsewhere in the world (such as the Agulhas Current and
Gulf Stream), with mean southward transport estimates of 22 Sv1 at 30°S (Mata et al.
2000) and 27 Sv at 28◦S (Ridgway and Godfrey 1994). A counter-current runs
offshore of the EAC, which is about 17 Sv of northward flow. This gives a net
southward transport of 9.5 Sv (Ridgway and Godfrey 1994, 1997). These transport
estimates for the EAC are compared to estimates of 43 to 85 Sv for the Agulhas
Current (Matano et al. 1998). It is such an eddy-rich current (Boland and Hamon
1970; Boland and Church 1981), that it is arguable whether it is a single current, as
the baroclinic eddy mass transport is several times that of the mean flow.
Within the abyssal basin adjacent to the coast, the EAC is associated with a highly
energetic eddy field (Figure 4). These eddies are 200-300 km in diameter and 2-3
eddies are generated annually and have lifetimes often exceeding a year (Nilsson and
Cresswell 1981; Bowen et al. 2004). They follow complex southward trajectories, but
are generally constrained within the deep basin.
Figure 4: Variability of sea surface height (m) in the southwest Pacific for the 1992-2006
period as determined by satellite altimetry observations. The high variable region is
associated with mesoscale EAC eddies.
Biological consequences
The nature of the separation of the EAC is of major importance for various fish
populations. For example, the location and timing of gemfish aggregations (e.g. the
gemfish run and spawning) are determined by the oscillations of the EAC. Other
species such as tuna appear to favour the frontal region - either on the subtropical or
subantarctic side (Hobday and Hartman 2006). Eddies themselves are important for
nutrient cycling, and biological productivity. For example, the phytoplankton
1 Sv = Sverdrup. A unit of measure of the transport of ocean currents. 1 Sverdrup = 106 cubic meters
per second.
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The East Australian Current
productivity in eddies differs from that of the surrounding waters. Higher up the food
chain, there is strong evidence of biological contrast across eddies (Tranter et al.
1980; Tranter et al. 1982).
The main biological influence of the eddies is to increase the vertical mixing within
the upper ocean in the western Tasman Sea, extending the effective mixed layer depth
and thus suppressing the winter phytoplankton and zooplankton populations due to
light-limited conditions for phytoplankton growth. These conditions result in spring
and autumn blooms of chlorophyll-a, with lower summer concentrations because of
stratification and nutrient depletion. In addition, upwelling associated with eddies
supplies more nutrients to the surface layer once the bloom has begun, delaying the
exhaustion of nutrients and prolonging the duration of the blooms (Tilburg et al.
2004).
The EAC and its eddies frequently move onto the continental shelf and close inshore
and influence the local circulation patterns. At prominent coastal features (Cape
Byron, Sugarloaf Point, Port Stephens), the EAC moves away from the coast, driving
upwelling, which draws nutrient-rich water from a depth of 200-m or more (Oke and
Middleton, 2000, 2001). However, while the EAC may drives nutrient-rich water onto
the shelf, upwelling-favourable winds (northerly) bring the water to the surface
(Rochford 1984; Church and Craig 1998).
Modes and timescales of variability
While observed trends in the EAC system have attracted much attention, their impacts
need to be considered in the context of variability in the system; from seasonal to
decadal. The response of ecosystems to variability may also give us clues to how it
might respond to long-term changes. The above description of the mean circulation of
the EAC system has been developed predominantly from in situ observations
collected over many decades. However, these data are irregularly distributed in both
space and time and are not generally suitable for resolving changes in the EAC
circulation over time periods ranging from interannnual to decadal or even long-term
(Ridgway et al., 2002). After about 1990, surface observation of temperature and sea
level have been collected by a diverse set of satellite platforms, enabling the seasonal,
interannnual and decadal signals to be determined. Data from a long time-series off
the east coast of Tasmania (Maria Island Station), and the repeated, eddy-resolving
Tasman Box XBT lines, provide valuable in situ data. XBT data between Sydney and
Wellington have been used with satellite altimetric observations to estimate the
transport time series through this section (Ridgway et al. 2008). We use this time
series to diagnose the relative importance of different temporal signals on the EAC
transport (Figure 5). Within the limitations of available data, the best estimate of the
state of the ocean over the last 50 years comes from ocean reanalyses, where
observational data have been assimilated into global models.
Seasonal
The EAC flow varies seasonally - it is strongest in summer, and the separation
location also migrates up and down the coast seasonally (Ridgway and Godfrey
1997). The seasonal amplitude is also large compared to the mean flow, with a
minimum observed southward flow of 27.4 Sv in winter, and a maximum of 36.3 Sv
in summer (Ridgway and Godfrey 1997). The net transport of the EAC (including the
northward counter-current) is 9.5 Sv, with a seasonal amplitude of 6 Sv. Compare this
to the Florida Current (Gulf Stream) at 26°S, with a background flow of 30 Sv (Schott
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Ridgway and Hill 2009
et al., 1988) and a seasonal amplitude of 3 Sv. In Figure 5, the seasonal amplitude of
the EAC Extension is of the order of 2 Sv.
Figure 5: (a) The EAC transport (flow between Sydney and Wellington) inferred from
satellite altimetry (Sv), (b) seasonal cycle, (c) interannual signal, (d) decadal change
(Ridgway et al., 2008).
Interannual
The EAC undergoes changes on interannnual timescales, but only a very weak ENSO
signal is evident in observations (Ridgway, 2007). In fact, the main oceanic pathway
of the ENSO influence occurs through the Indonesian region and around a waveguide
around the western and southern Australian coastal boundaries (Wijffels et al, 2004).
This signal has almost entirely dissipated by the time it reaches the west coast of
Tasmania. The pattern in Figure 5 shows that major interannnual events occur through
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The East Australian Current
the period from 1992-2006. These changes have not been linked with external climate
forcing signals.
Decadal
There is a strong signal of decadal variability in the temperature and salinity
associated with the EAC (Sutton et al. 2005; Ridgway 2007; Ridgway et al. 2008).
Results from observations and 50-year ocean reanalysis datasets show that the
strength of the EAC Extension is negatively correlated with the Tasman Front on
decadal timescales, which suggests that there is a gating between these two currents
(Hill et al. 2009). This is due to enhanced wind stress curl in the South Pacific, which
favours the EAC extension pathway over the Tasman Front, and is related to decadal
ENSO variability. Decadal warming (cooling) in the tropical Pacific is associated with
a weaker (stronger) South Pacific wind stress curl maximum, a weaker (stronger)
EAC Extension, and a stronger (weaker) Tasman Front (Sasaki et al. 2008, Hill et al.
2009). We note that the decadal signal observed in the EAC extension is double that
of the seasonal and of similar magnitude to the interannnual variability (Figure 5).
Observed Impacts (since 1944)
The long-term record from the Maria station shows that the southward penetration of
the EAC has increased over the past 60 years. This station is located on the inshore
edge of the warm, saline tongue of EAC water that spreads southwards along the
coastal boundary (Ridgway 2007, Figure 6).
Figure 6: The mean SST for January in the waters around Tasmania. The summer penetration
of the EAC Extension is seen as a tongue of warm water extending southwards past the east
coast of Tasmania. The location of the long-term station off Maria Island is shown on the
map.
The region has become both warmer and saltier, with mean trends of 2.28°C/century
and 0.34 psu/century over the 1944-2002 period, which corresponds to a poleward
advance of the EAC Extension of ~ 350 km. The intensification of the EAC is caused
by strengthened winds over the South Pacific, and hence a stronger South Pacific gyre
(Hill et al. 2008). Trends in summer temperature and salinity are greater than in
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Ridgway and Hill 2009
winter – there is an additional pulse of warm, high salinity subtropical water
associated with the EAC in summer.
Figure 7: The (a) temperature and (b) salinity time series from the Maria Island station. In (c)
each of the series has been low-pass filtered and normalized with their standard deviations.
The long-term trend is also shown.
The enhanced warming in the region is confirmed by results from SST composite
products (Figure 8). Other indirect evidence for EAC changes comes from biological
sources. Several species previously only found in northern regions (Centrostephanus
rodgersii, Carcinus maenas) have steadily ranged further southward over recent
decades (Edgar et al. 1997; Thresher et al. 2003; Pittock 2003, Ling et al. 2008).
These changes have been attributed to enhanced EAC flow (Edyvane 2003).
The intensification of the EAC flow past Tasmania is also seen in recent model
studies describing both a spin-up and southward shift of the Southern Hemisphere
subtropical ocean circulation (Oke and England 2003; Cai et al. 2005; Cai 2006).
Oceanic changes are forced by an intensification of the wind stress curl arising from a
poleward shift in the circumpolar westerly winds (Gillett and Thompson 2003) due to
the trend in the Southern Annular Mode (SAM). Models predict that the EAC
strengthens in the south while, it weakens to the north. Cai (2006) obtained an EAC
increase of 9 Sv south of 30°S from 1978 to 2002.
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The East Australian Current
Figure 8: The trend in SST from 1944-2005 from a reconstructed data product (Smith and
Reynolds, 2003).
The SAM is the dominant mode of variability of the Southern Hemisphere
atmospheric circulation operating on all time scales. Over the past several decades, it
has been displaying an upward trend (Thompson et al. 2000; Marshall 2003; Marshall
et al. 2004), with increasing mean sea level pressure in the mid-latitudes.
Observational (Thompson and Solomon 2002) and modelling studies (Gillett and
Thompson 2003) indicate a significant contribution from ozone depletion forcing over
the past decades (Shindell and Schmidt 2004). However, under increasing
atmospheric CO2, climate models also produce an upward trend of the SAM (Fyfe et
al. 1999; Kushner et al. 2001; Cai et al. 2003).
The Maria Island time series shows that the EAC has strengthened in the southern
Tasman Sea. The change in EAC surface salinity from winter to summer (0.25-0.30
psu from 1989 to 1990, Thresher et al. 2004) is of the same order as that observed
over the 60-year period at Maria Island. This corresponds to a poleward extension of
some 350 km in temperature and salinity. We can infer a long-term change in EAC
transport of 10-15 Sv over the 60-year period, which is similar to modelling estimates
(Cai 2006). In the northern EAC region, a significant thermocline cooling has been
observed from 1975-1990 (Ridgway and Godfrey 1996), which matches the
weakening of the EAC in this region observed in the models (Oke and England 2003;
Cai et al. 2005).
Potential impacts by the 2030s and 2100s
Results from global climate models strongly suggest that changes in the EAC system
will continue the observed trends of the past 50 years. These changes are primarily
linked to the strengthening of the SAM. Although the overall contribution of
increasing CO2 to the observed SAM trend over the past decades is not certain, we
expect a further strengthening in the SAM trend as CO2 continues to increase into the
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Ridgway and Hill 2009
future. This is one of the most robust and consistent responses of the global climate
system to climate change (Cai et al. 2005).
Cai et al. (2005) analyzed outputs of an ensemble of four climate change experiments
with the CSIRO Mark 3 climate model forced by four different projections. The
model experiments show that changes in the prevailing wind systems drive significant
ocean circulation changes across the mid-latitudes of the Southern Ocean. These
included a major increase in the South Pacific subtropical gyre, and an increase in the
flow passing through the Tasman Sea with an associated strengthening of the
recirculations in the longitudes between New Zealand and the South American coast.
Overall the model shows that the connected Southern Ocean gyre system (Ridgway
and Dunn 2007) strengthens and shifts southward.
Within these large-scale changes, the model predicts a 20% increase in the mean flow
of the EAC passing through the southern Tasman Sea by 2070 (Cai et al., 2005). This
projected increase is upon the already observed increases in EAC flow over the past
50 years (Hill et al. 2008). Associated with the EAC transport increase, is a major
warming along the path of the EAC intensification, off the east Australian coast, and
cooling along the path of increased northward recirculations, to the east of New
Zealand. In fact the model suggests that the largest changes are found within the
subsurface waters, with a clear baroclinic response. This includes weakly increased
undercurrents at depth flowing in opposite directions to the surface flows, which in
turn drive opposite temperature trends (cooling) at depth to that in surface layers.
The large warming shown by the model in the Tasman Sea (Figure 9) is clearly
associated with a strengthening of the EAC, with a rate of warming that is the greatest
in the Southern Hemisphere. Again this mirrors the results obtained from SST
observations over recent decades (Figure 8). The role of the EAC change in
generating the large warming in the Tasman Sea is confirmed by examining the
changes of heat flux. For example, at the centre of the Tasman warming, there is a
large increase in the heat loss from the ocean to the atmosphere (Cai et al. 2005). This
essentially precludes the possibility of the warming being a consequence of
atmospheric heating. There is a correspondence between the change pattern of wind
stress curl and that of the heat flux.
These results have been obtained from coarse resolution climate models that do not
capture the fine-scale structure and mesoscale eddies that are a fundamental to the
dynamics of the EAC system. Observations show that there are clear seasonal,
interannnual and decadal changes to the EAC eddy field. Given the importance of
eddies in the EAC system, an improved representation of these features in climate
models is required to reduce the uncertainty in model climate projections.
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The East Australian Current
Figure 9: The projected change in SST in 2035 (upper panel) and 2100 (lower) from the
CSIRO Mk 3.5 coupled model forced under the SRES A2 scenario.
Key Points
 A surface warming trend, varying in magnitude seasonally, has been observed
in the East Australian Current region and on the continental shelf off the east
coast of Tasmania.
 There has been an increasing trend in sea surface salinity off the east coast of
Australia, which is associated with a strengthening of the East Australian
Current.
 Impacts of these trends need to be explored in the context of strong
interannual and decadal variability in the EAC system.
 Changes in the range of species across a number of marine species have been
related to changes in the strength of the EAC. The projected strengthening of
the EAC and continued warming of the south Tasman Sea is predicted to have
a detrimental effect on cold temperate species in South East Australia, and will
also impact on commercially important fisheries such as abalone and rock
lobster.
 Oceanic changes are forced by an intensification of the wind stress curl arising
from a poleward shift in the circumpolar westerly winds due to the trend in the
Southern Annular Mode.
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Ridgway and Hill 2009
 Decadal variations are related to decadal ENSO, through an atmospheric
teleconnection from the tropics impacting on the westerly winds in the South
Pacific.
 Results from global climate models strongly suggest that changes in the EAC
system and their underlying causes will continue the observed trends of the
past 50 years.
Confidence Assessments
Observed and future impacts
Projected changes Physical
variables Observed changes
2030s 2100s
Volume
transport The transport of the
East Australian Current
has increased over the
past 60-years, by up to
10-Sv.
Further increase Predicted increase of >20%
Coastal sea
level Fort Denison sea level:
A rising trend of 1.54
mm per year over the
20th century, which is
slightly less than the
global trend
Sea level rises at a
similar trend as global
average
Sea level rises at a similar
trend as global average
Sea surface
temperature Warming recorded at
Maria Island station of
2.28°C/century over the
1944-2002 period;.
This trend is well above
the global value. The
summer trend is greater
than for winter.
A continued warming by
0.7-1.4°C, most
pronounced in the
southern Tasman Sea
and around Tasmania
Rise by 2.0–3°C, most
pronounced in the southern
Tasman Sea and around
Tasmania???
Thermocline Intensified EAC flow
since 1944 implies a
deepening of
thermocline offshore
and uplift at the coastal
boundary. Subsurface
cooling on continental
slope.
Continue shallowing
trend of thermocline
depth at boundary and
increase offshore
The trend tends to persist.
Sea surface
salinity Sea surface salinity
risen by 0.34
psu/century over the
1944-2002 off the east
coast as recorded at
Maria Island. The
summer trend is greater
than for winter.. Little
seasonal cycle before
1960. From 1970 to the
mid 1990s, the seasonal
Increase in surface
salinity due to
intensification of EAC
transport
Further increases in surface
salinity
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The East Australian Current
Projected changes Physical
variables Observed changes
2030s 2100s
cycle intensifies
Air-sea heat
fluxes Advection of heat into
Tasman Sea leads to
heat loss from ocean to
atmosphere. Heat loss
increases with
strengthening of EAC
Within EAC warming
region, there is large
heat loss to the
atmosphere. High
correlation with changes
in wind stress curl.
Increase in out-going
long wave radiation.
Reduction in in-coming
short-wave radiation.
Similar trends likely
Winds Weakening of the
westerlies off the
southwest coast of
Australia during austral
winter since mid-1970s
Broadening and
intensification of
subtropical high appears
to cause easterly
anomalies centred at
30°S
Dominance of the subtropical
high to continue to extend
southeastward
Precipitation Ocean warming
enhances local
convection leads to
increased cloud cover
and rainfall
Trends are maintained
Knowledge Gaps
 What is the relationship of the South Equatorial Current bifurcation latitude
and vertical structure with the inflow and the outflow streams into the Coral
Sea? What are the dynamics associated with the temporal changes in its
location?
 What are the mechanisms associated with the EAC separation and
reattachment, location of semi-permanent eddies, retroflection?
 How are EAC eddies generated? Are they generated by local forcing or are
they entering the region from the east? How do eddies interact with the mean
flow and topography?
 How are climate-change-related variations in the South Pacific gyre transport
and density structure communicated through the western boundary via the
EAC?
 How are changes in the EAC at seasonal interannnual, decadal and long-term
timescales affecting regional marine ecosystems?
 Within the Tasman Sea, what are the relative contributions of advection and
heat storage compared to the surface heat flux? How do they vary on
interannual to decadal timescales? How is heat partitioned between the
atmospheric fluxes and recirculated within the gyre?
 Climate models need an improved representation of the EAC and its eddy
field?
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Ridgway and Hill 2009
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