ArticlePDF Available

The East Australian Current

  • CSIRO Oceans and Atmosphere, Hobart, Australia
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:
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
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
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 2
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. 3
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 28S (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. 4
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.
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.
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 5
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).
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 6
The East Australian Current
the period from 1992-2006. These changes have not been linked with external climate
forcing signals.
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
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 7
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. 8
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 9
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. 10
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
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
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. 11
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
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
transport The transport of the
East Australian Current
has increased over the
past 60-years, by up to
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
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
Thermocline Intensified EAC flow
since 1944 implies a
deepening of
thermocline offshore
and uplift at the coastal
boundary. Subsurface
cooling on continental
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
Further increases in surface
salinity 12
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
Dominance of the subtropical
high to continue to extend
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
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
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? 13
Ridgway and Hill 2009
Boland, F.M. and Church, J.A. (1981) The East Australian Current 1978, Deep Sea
Research 28: 937-957.
Boland, F.M. and Hamon, B.V. (1970) The East Australian Current, 1965-1968, Deep
Sea Research 17: 777-794.
Bowen, M., Wilkin, J.L. and Emery, W.J. (2005) Variability and forcing of the East
Australian Current. Journal of Geophysical Research 110: C03019,
Cai, W. (2006) Antarctic ozone depletion causes an intensification of the Southern
Ocean super-gyre circulation. Geophysical Research Letters 33: L03712,
doi:10.1029/2005GL024911, 2006.
Cai, W.J., Whetton, P.H. and Karoly, D.J. (2003) The response of the Antarctic
Oscillation to increasing and stabilized atmospheric CO2, Journal of Climate 16:
1525– 1538.
Cai, W., Shi, G., Cowan, T., Bi, D. and Ribbe, J. (2005) The response of the Southern
Annular Mode, the East Australian Current and the southern mid-latitude
circulation to global warming, Geophysical Research Letters 32: L23706,
Church, J.D. and Craig, P.D. (1998) Australia’s shelf seas: diversity and complexity,
In The Sea, Volume 11, edited by A. R. Robinson and K. H. Brink.
Edgar, G.J. (1984) General features of the ecology and biogeography of Tasmanian
rocky reef communities, Papers and Proceedings of the Royal Society of
Tasmania 118: 173-186.
Edgar, G.J. (1997) Australian Marine Life, Reed Books, Sydney, N.S.W., Australia.
Edgar, G.J., Moverley, J.H., Barrett, N.S., Peters, D. and Reed C. (1997) The
conservation related benefits of a systematic marine biological sampling program:
the Tasmanian bioregionialisation as a case study. Biological Conservation 79:
Edyvane, K.S. (2003) Conservation, monitoring and recovery of threatened giant kelp
(Macrocytis pyrifera) beds in Tasmania, Report to Environment Australia, Dep.
Prim. Ind. Water Environ., Hobart, Tasmania, Australia, 177pp.
Fyfe, J.C., Boer, G.J. and Flato, G.M. (1999) The Arctic and Antarctic oscillations
and their projected changes under global warming. Geophysical Research Letters
26: 1601– 1604.
Godfrey, J.S., Cresswell, G.R., Golding, T.J., Pearce, A.F. and Boyd, R. (1980) The
separation of the East Australian Current. Journal of Physical Oceanography 10:
Gillett, N.P. and Thompson, D.W.J. (2003) Simulation of recent southern hemisphere
climate change. Science 302: 273-275.
Hill, K.L, Rintoul, S.R., Coleman, R. and Ridgway, K.R. (2008) Wind-forced low
frequency variability of the East Australian Current. Geophysical Research Letters
35: L08602, doi:10.1029/2007GL032912.
Hill, K.L, Rintoul, S.R., Ridgway, K.R. and Oke, P.R (in submission) Decadal
changes in the South Pacific Western Boundary Current system revealed in
observations and ocean state estimates. (submitted to Geophysical Research
Letters) 14
The East Australian Current
Hobday, A.J. and Hartmann, K. (2006) Near real-time spatial management based on
habitat predictions for a longline bycatch species, Fisheries Management and
Ecology 13: 365-380.
Holbrook, N.J. and Bindoff, N.L. (1997) Interannual and decadal temperature
variability in the southwest Pacific between 1955 and 1988, Journal of Climate
10: 1035-1049.
Kushner, P.J., Held, I.M. and Delworth, T.L. (2001) Southern Hemisphere
atmospheric circulation response to global warming, Journal of Climate 14: 2238–
Ling, S.D., Johnson, C.R., Ridgway, K.R., Hobday, A.J. and Haddon, M. (2008)
Climate change drives range extension of a marine ecosystem engineer: Informing
future patterns from correlations of the recent past. Global Change Biology 14:
doi: 10.1111/j.1365-2486.2008.01734.
Marshall, G.J. (20030 Trends in the Southern Annular Mode from observations and
reanalyses Journal of Climate 16: 4134– 4143.
Mata, M.M., Tomczak, M., Wijffels, S. and Church, J.A. (2000) East Australian
Current volume transports at 30oS: Estimates from the World Ocean Circulation
Experiment hydrographic sections PR11/P6 and the PCM3 current meter array,
Journal of Geophysical Research 105: 28,509-28,526.
Mata, M.M., Wijffels, S., Tomczak, M. and Church, J.A. (2007) Eddy shedding and
energy conversions in the East Australian Current. Journal of Geophysical
Research 111: C09034, do 10,1029/2006JC003592.
Matano, R.P., Simionato, C.G., Ruijter, W.P., van Leeuween, P.J., Strub, P.T.,
Chelton, D.B. and Schlax, M.G. (1998) Seasonal variability in the Agulhas
Retroflection region. Geophysical Research Letters 25: 23, 4361-4364.
Nilsson, C.S. and Cresswell, G.R. (1981) The formation and evolution of East
Australian Current warm core eddies. Progress in Oceanography 9: 133-183.
Pittock, B. Ed., (2003) Climate Change: An Australian Guide to the Science and
Potential Impacts, Australian Greenhouse Office, Canberra, A.C.T., Australia.
Poloczanska, E.S., Babcock, R.C., Butler, A., Hobday, A.J., Hoegh-Guldberg, O.,
Kunz, T.J., Matear, R., Milton, D., Okey, T.A. and Richardson, A.J. (2007)
Climate Change And Australian Marine Life. Oceanography and Marine Biology
Annual Review 45: 409-480.
Ridgway, K.R. (2007) Long-term trend and decadal variability of the southward
penetration of the East Australian current Geophysical Research Letters 34:
L13612, doi:10.1029/2007GL030393, 2007.GRL.
Ridgway, K.R., Coleman, R.C., Bailey, R.J. and Sutton, P. (2008) Decadal variability
of East Australian Current transport inferred from repeated high-density XBT
transects, a CTD survey and satellite altimetry, Journal of Geophysical Research
113: C08039,doi:10.1029/2007JC004664.
Ridgway, K.R. and Dunn, J.R. (20030 Mesoscale structure of the mean East
Australian Current System and its relationship with topography, Progress in
Oceanography 56: 189-222.
Ridgway, K.R. and Dunn, J.R. (2007) Observational evidence for a Southern
Hemisphere oceanic ‘Supergyre’, Geophysical Research Letters 34: L13612,
Ridgway, K.R. and Godfrey, J.S. (1994) Mass and heat budgets in the East Australian
Current: A direct approach. Journal of Geophysical Research 99: 3231-3248. 15
Ridgway and Hill 2009 16
Ridgway, K.R., Dunn, J.R. and Wilkin, J.L. (2002) Ocean interpolation by four-
dimensional least squares -Application to the waters around Australia, Journal of
Atmospheric and Oceanic Technology 19: 1357-1375.
Ridgway, K.R. and Godfrey, J.S. (1997) Seasonal cycle of the East Australian
Current, Journal of Geophysical Research 102: 22,921-22,936.
Rochford D.J. (1984) Nitrates in Eastern Australian Coastal Waters Australian
Journal of Marine and Freshwater Research 35: 385-397.
Willis, J., Sutton, P. and Ridgway, K. (2006) Closing the time-varying mass and heat
budgets for large ocean areas: the Tasman Box, Journal of Climate 18: 2330-
Roemmich, D., Gilson, J., Davis, R., Sutton, P., Wijffels, S. and Riser, S. (2007)
Decadal spin-up of the South Pacific subtropical gyre. Journal of Physical
Oceanography 37: 162-173.
Sasaki, Y., Minobe, S., Kagimoto, T., Nonaka, M. and Sasaki, H. (2008) Decadal sea
level variability in the South Pacific in a global eddy resolving model. Journal of
Physical Oceanography 38: 1731–1747.
Schott, F.A., Lee, T.N. and Zantopp, R.J. (1988), Variability of structure and transport
of the Florida Current in the period range of days to seasonal, Journal of Physical
Oceanography 18 : 1209-1230.
Shindell, D.T. and Schmidt, G.A. (2004) Southern Hemisphere climate response to
ozone changes and greenhouse gas increases Geophysical Research Letters 31:
L18209, doi:10.1029/2004GL020724.
Smith, T.M. and Reynolds, R.W (2003) Extended reconstruction of global sea surface
temperatures based on COADS data (1854-1997), Journal of Climate 16: 1495-
Speich S., Ganachaud, A. and Marsh, R. (2002) Tasman leakage: A new route in the
global conveyor belt, Geophysical Research Letters 29: 1416,
Sutton P.J.H., Bowen, M. and Roemmich, D. (2005) Decadal temperature changes in
the Tasman Sea, N. Z. Marine and Freshwater Research 39: 1321-1329.
Tilburg, C.E., Hurlburt, H.E., O'Brien, J.J. and Shriver, J.F. (2001) The dynamics of
the East Australian Current system: the Tasman Front, the East Auckland Current,
and the East Cape Current, Journal of Physical Oceanography 31: 2917- 2943.
Tranter, D.J., Leech, G.S. and Vaudrey, D.J. (1982) Biological significance of surface
flooding in warm-core ocean eddies. Nature 297: 572-574.
Tranter, D.J., Parker, R.R. and Cresswell, G.R. (1980) Are warm-core eddies
unproductive. Nature 284: 540-542.
Thresher, R.E., Proctor, C., Ruiz, G., Gurney, R., McKinnon, C., Walton, W.,
Rodriguez, L. and Bax, N. (2003) Invasion dynamics of the European shore crab,
Carcinus maenas, in Australia, Marine Biology 142: 867-876.
Thresher, R.E., Rintoul, S.R., Koslow, J.A., Weidman, C., Adkins, J. and Proctor, C.
(2004) Oceanic evidence of climate change in southern Australia over the last
three centuries Geophysical Research Letters 31: L07212,
Wijffels, S. and Meyers, G. (2004) An intersection of Oceanic Waveguides:
Variability in the Indonesian Throughflow Region, Journal of Physical
Oceanography 34: 1232-1253.
... As a result, the south Tasman Sea region has become both warmer and saltier during this period, which corresponds to a poleward advance of the EAC extension of about 350 km (Ridgway, 2007). There is a strong consensus in climate model simulations that those trends will continue and accelerate over the next 100 years (Ridgway & Hill, 2009). In addition, ocean-atmosphere coupled climate models suggest a stronger EAC extension in the future (Sen Gupta et al., 2016;Wang et al., 2014;Yang et al., 2016). ...
Full-text available
Plain Language Summary The East Australian Current (EAC) transports large amounts of heat out of the Western Pacific Warm Pool into the subtropics and subpolar regions, thereby regulating the local and global climate. The few existing reconstructions of past changes in transport and temperature of the EAC are contradictory resulting in a knowledge gap in long‐term variability of the southwest Pacific currents, which in turn hampers a reliable projection of future tropical and subtropical climate development. We use sea surface temperature reconstructions along the EAC path off northeastern Australia to estimate changes in the EAC heat transport over the past 4,000 years and its potential driving mechanisms. Our results reveal an increase in temperature after ∼1400 CE, which we relate to an intensification of the EAC strength. The strengthening of the EAC after ∼1400 CE is likely associated with a strengthening of the Southern Hemisphere subtropical gyre circulation due to a progressive shift of the Southern annular mode toward its positive phase and of El Niño‐Southern Oscillation toward more El Niño‐like conditions.
... Yongala, located in the Coral Sea, is also characterised by tropical and monsoonal climate with a slightly lower average temperature (~28 • C) than Darwin. The Port Hacking (PHB) and North Stradbroke Island (NSI) sites are both located in the Tasman Sea, are characterised by subtropical climatic conditions (~21 • C and 24 • C, respectively), and are impacted by the Eastern Australian Current (EAC), which is a rapidly moving western boundary current transporting warm waters from the Coral Sea into the Tasman Sea [41] (Figure 1). Rottnest Island (ROT) is located off the southwestern coast of Australia, within the Indian Ocean and is influenced by the southward flow of warm water from another boundary current, the Leeuwin Current [42] (Figure 1). ...
Full-text available
Diatom communities significantly influence ocean primary productivity and carbon cycling, but their spatial and temporal dynamics are highly heterogeneous and are governed by a complex diverse suite of abiotic and biotic factors. We examined the seasonal and biogeographical dynamics of diatom communities in Australian coastal waters using amplicon sequencing data (18S-16S rRNA gene) derived from a network of oceanographic time-series spanning the Australian continent. We demonstrate that diatom community composition in this region displays significant biogeography, with each site harbouring distinct community structures. Temperature and nutrients were identified as the key environmental contributors to differences in diatom communities at all sites, collectively explaining 21% of the variability observed in diatoms assemblages. However, specific groups of bacteria previously implicated in mutualistic ecological interactions with diatoms (Rhodobacteraceae, Flavobacteriaceae and Alteromonadaceae) also explained a further 4% of the spatial dynamics observed in diatom community structure. We also demonstrate that the two most temperate sites (Port Hacking and Maria Island) exhibited strong seasonality in diatom community and that at these sites, winter diatom communities co-occurred with higher proportion of Alteromonadaceae. In addition, we identified significant co-occurrence between specific diatom and bacterial amplicon sequence variants (ASVs), with members of the Roseobacter and Flavobacteria clades strongly correlated with some of the most abundant diatom genera (Skeletonema, Thalassiosira, and Cylindrotheca). We propose that some of these co-occurrences might be indicative of ecologically important interactions between diatoms and bacteria. Our analyses reveal that in addition to physico-chemical conditions (i.e., temperature, nutrients), the relative abundance of specific groups of bacteria appear to play an important role in shaping the spatial and temporal dynamics of marine diatom communities.
... On the east coast of Australia, the Solitary Islands region is a subtropical transition zone within a climate change hotspot . This region is strongly influenced by the East Australian Current, a western boundary current that is strengthening in response to climate change and which can facilitate the poleward range expansion of marine species (Castro et al., 2020;Ridgway & Hill, 2012). An increasing abundance of tropical herbivorous fishes has been linked to the decline in temperate habitat-forming kelp in this region (Vergés et al., 2016). ...
Full-text available
Climate‐mediated species redistributions are causing novel interactions and leading to profound regime shifts globally. For species that expand their distribution in response to warming, survival depends not only on their physiological capacity, but also on the ability to coexist or be competitive within the established community. In temperate marine reefs from around the world, the range expansion of tropical species, known as ‘tropicalisation’, has been linked to the disappearance of temperate habitat‐forming kelps and shifts to dominance by low‐biomass turfing algae. The consequences of these range expansions and habitat changes on resident fish communities are, however, unclear. Here, we use data derived from baited remote underwater video (BRUV) surveys to analyse changes in diversity and abundance of marine fishes over a 17‐year period in warming reefs that have experienced kelp loss (occurring c. 2009). Despite the loss of kelp, we found that species richness and overall abundance of fishes (measured as probability of occurrence and relative abundance), including both tropical and temperate species, increased through time. We also found dramatic shifts in the trophic composition of fish assemblages. Tropical herbivorous fish increased most markedly through time and temperate‐associated planktivores were the only group that declined, a potential consequence of tropicalisation not previously identified. At the species level we, identified 22 tropical and temperate species from four trophic guilds that significantly increased in occurrence, while only three species (all temperate associated) declined. Morphological trait space models suggest increases in fish diversity and overall occurrence are unlikely to be driven by uniqueness of traits amongst tropical range expanders. Our results show more winners than losers and suggest that pathways of energy flow will change in tropicalised systems, as planktonic inputs become less important and a higher proportion of algal productivity gets consumed locally by increasingly abundant herbivores.
... Ocean currents, water temperature and prey distribution are known drivers of shark movement (Heupel et al., 2015;Andrzejaczek et al., 2018) and likely influenced this move from offshore waters. On the east coast of Australia, the East Australian Current brings warmer water nearer the coastline during summer months and encroaches onto the continental shelf (Ridgway and Godfrey, 1997). Areas where the continental shelf narrows generally see upwelling of nutrient-rich water (south of 28.5 S and 31 S), thus increasing primary productivity (Everett et al., 2014). ...
Habitat use and movement patterns of tiger sharks (Galeocerdo cuvier) in eastern Australian waters. Understanding the movement of marine predators is vital for effective conservation and management. Despite being targeted by shark control programs, the tiger shark, Galeocerdo cuvier, is poorly studied off eastern Australia. To investigate the horizontal movement and habitat use in this region, 16 sharks (157-384 cm total length) were tagged with MiniPAT pop-up satellite archival tags in 2018 and 2019. Eleven of these individuals were also fitted with satellite-linked radio transmitting tags. After release, most sharks moved off the continental shelf and headed north, associating with seamounts as they moved towards Queensland. During their time at liberty they transited through temperate, subtropical and tropical waters and spent the majority of time in the upper 50 m of the water column and at temperatures between 22 and 25˚C. Horizontal movement was focused in waters off the continental shelf. Increased movement over shelf waters occurred during the aus-tral spring and summer when the East Australian Current is at its strongest and warm waters encroach the continental shelf. Broad latitudinal movement along the east coast of Australia was evident and highlights the connectivity between tropical and warm-temperate regions.
... On the east coast of Australia, the Solitary Islands region is a sub-tropical transition zone within a climate change hotspot ). This region is strongly influenced by the East Australian Current, a western boundary current that is strengthening in response to climate change and which can facilitate the poleward range expansion of marine species (Ridgway & Hill 2012;Castro et al. 2020). An increasing abundance of tropical herbivorous fishes has been linked to the decline of temperate habitat-forming kelp in this region (Vergés et al. 2016). ...
Full-text available
Dense shelf water cascade (DSWC) is a common oceanographic phenomenon on many continental shelves. Previous studies indicate that the DSWC could shape seabed physiography and carry seawater, sediment, and organic carbon a long distance from the continental shelf to the basin floor. However, it remains enigmatic how these DSWC’s interact with seabed geomorphology and travel long distances from the shallow to deep marine environments. In this study, we employed high-resolution multibeam bathymetry, 2D and 3D seismic reflection, core description, sediment grain size data from the Gippsland Basin, southeast offshore Australia. The continental shelf of the central Gippsland Basin stores sediment supplied by the along-shelf transported DSWC. Seismic reflection data reveal that cyclic steps are common on the shelf and slope, indicating a downslope-transported, supercritical current-dominated environment. Core observation and grain size analyses reveal that coarse-grained, Ta-typed turbidites are the major facies, indicating the presence of high-intensity downslope-traversing turbidity currents. Thus, supercritical turbidity currents are the dominant sedimentary process in the central Gippsland Basin. We illuminate that DSWC can interact with pre-existing submarine landslides, resuspending sediment and igniting downslope-transported turbidity currents. Upon ignition, the turbidity current can evolve into a supercritical regime, leaving complex seabed geomorphology and allowing shallow water to travel across the shelf and extend more than 80 km down the lower slope. We imply that the transformation of DSWC into turbidity currents is a new machinism for the inition of turbidity currents. As revealed by our literature analysis, this process should be common on outer continental shelves globally. As the strength, frequency, and pathway of DSWC are sensitive to seawater temperature variations dictated by climate change, we therefore infer that this current transformation process has critical implications for predicting how seabed geomorphology, sedimentation, and geohazards respond to changing oceanographic and climate conditions.
The Southern Ocean is the hub that interconnects the Pacific, Indian and Atlantic Oceans to allow the worldwide distribution of heat, salt, dissolved gases and nutrients. Such dispersal is dominated by the west-to-east, wind-driven, Antarctic Circumpolar Current (ACC) coupled with a north-south, buoyancy-forced meridional overturning circulation (SOMOC). Basically, relatively warm Circumpolar Deep Water (CDW) from the Northern Hemisphere ascends southward with its denser part reaching coastal Antarctica. There, cooling and addition of brine convert the CDW to dense Antarctic Bottom Water (AABW) that moves northward via far-reaching deep western boundary currents. Shallow CDW upwells further offshore under wind forcing and increased buoyancy caused by freshening and warming that transform CDW to lower density surface waters. These migrate north due to Ekman transport and subduct approaching the Subantarctic Front to form Subantarctic Mode and Antarctic Intermediate Waters that take up much of the anthropogenic heat and carbon dioxide. Thus, the MOC-ACC circulation ameliorates the impacts of modern climate change. However, the resultant warming ocean/climate affects ice stability, sea level rise, SOMOC buoyancy, production of AABW, stratification, and primary productivity. Increasing knowledge of these and other responses, plus a stronger multidisciplinary approach to understanding changes in the Southern Ocean, will enhance future projections.
Full-text available
Understanding impacts of global warming on phytoplankton–the foundation of marine ecosystems–is critical to predicting changes in future biodiversity, ocean productivity, and ultimately fisheries production. Using phytoplankton community abundance and environmental data that span ∼90 years (1931–2019) from a long-term Pacific Ocean coastal station off Sydney, Australia, we examined the response of the phytoplankton community to long-term ocean warming using the Community Temperature Index (CTI), an index of the preferred temperature of a community. With warming of ∼1.8°C at the site since 1931, we found a significant increase in the CTI from 1931–1932 to 2009–2019, suggesting that the relative proportion of warm-water to cold-water species has increased. The CTI also showed a clear seasonal cycle, with highest values at the end of austral summer (February/March) and lowest at the end of winter (August/September), a pattern well supported by other studies at this location. The shift in CTI was a consequence of the decline in the relative abundance of the cool-affinity (optimal temperature = 18.7°C), chain-forming diatom Asterionellopsis glacialis (40% in 1931–1932 to 13% in 2009 onward), and a substantial increase in the warm-affinity (21.5°C), also chain-forming diatom Leptocylindrus danicus (20% in 1931–1932 to 57% in 2009 onward). L. danicus reproduces rapidly, forms resting spores under nutrient depletion, and displays a wide thermal range. Species such as L. danicus may provide a glimpse of the functional traits necessary to be a “winner” under climate change.
It is unknown whether marine bioregionalisations provide a sufficiently accurate representation of population genetic structure to allow evolutionary sustainable management of coastal ecosystems. The usefulness of the Integrated Marine and Coastal Regionalisation of Australia (IMCRA) model of Australia for management of the genetic structure of intertidal organisms is assessed here using DNA from a nuclear gene (ATPα synthase: “ATPSα”, 60 sequences) and mitochondrial cytochrome c oxidase subunit I (“COI”; 60 sequences) from the micro-gastropod Eatoniella atropurpurea, a representative of species with low pelagic larval duration (PLD). A balanced experimental design with standardised effort was employed to collect samples from 18 sites along the coasts of New South Wales. The amount of variance explained by genetic differentiation between IMCRA bioregions (0.52% for ATPSα and 2.12 % for COI) was very small compared to the amount of variation between localites within them (20.39 for ATPSα and 23.62 % for COI) and between individuals within localities (70.14% for ATPα and 76.03 % for COI). Considerable fine-scale structure was shown by the distributions of robustly supported haplotype clades four of seven being geographically limited to single IMCRA bioregions. There were numerous haplotypes restricted to a single bioregion (33 for ATPSα and 24 for COI). Insofar as E. atropurpurea is typical of low PLD species which comprise the majority of marine invertebrates, the results indicate that management of such taxa using IMCRA should be focused intra-regionally, owing to their potential for considerable evolutionary independence at the local level.
Full-text available
Observations from a long-term ocean station off eastern Tasmania show that the southward penetration of the East Australian Current (EAC) has increased over the past 60 years. Changes in temperature and salinity are highly correlated at timescales greater than seasonal, with long-term trends which differ markedly from global ocean values. The data show that 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 of ~350-km. These trends are not directly forced by global surface fluxes but primarily result from changes in the EAC. The summertime trends in temperature and salinity are greater than in winter - there is an augmented summer pulse of warm, high salinity subtropical water associated with the EAC.
Full-text available
A high-resolution hydrographic atlas resolves the narrow boundary flow of the western Pacific gyre and its pathway around Tasmania, southern Australia and into the Indian Ocean basin. This confirms recent model descriptions of a Southern Hemisphere `supergyre', a nested system of subtropical gyres. The observations show that the gyre flow is squeezed into a narrow band between Tasmania and the Antarctic Circumpolar Current. The residue of East Australian Current transport turns westward around Tasmania and `leaks' into the Indian Ocean. While there is also a limited form of gyre connection between the Indian and Atlantic systems south of Africa, with a mean outflow of Agulhas Current water into the Atlantic basin, these gyre circulations remain far more discrete. The gyre coupling and the East Australian Current in particular, provide the mechanism by which Sub Antarctic Mode Water and Antarctic Intermediate Water are distributed between the ocean basins.
Full-text available
The spatial and temporal variability of the southwest Pacific Ocean is examined with the aim of describing the physical processes operating on interannual and decadal timescales. The study takes advantage of a new temperature atlas of the upper 450 m of the southwest Pacific Ocean, obtained from 40000 bathythermograph profiles between 1955 and 1988. Rotated principal components analysis was used to filter the important spatial and temporal scales of temperature variability in the data. Three different analyses are presented. They include two intraocean analyses and a joint analysis of subsurface ocean temperature, sea level pressure, and surface winds.The dominant El Niño mode describes the large vertical excursions of the thermocline in the western tropical Pacific in response to atmospheric forcing at a 3-6-month lag. More importantly, most of the retained modes, outside of the equatorial region, have time variations that correlate with El Niño. One ocean mode, with a spatial pattern representing sea surface temperature anomalies in the western Coral Sea (linked to the interannual migration of the South Pacific convergence zone), correlates significantly with (at the 99% level) and leads (by 3-6 months) the Southern Oscillation index (SOI), suggesting that sea surface temperature anomalies in this region may be a useful indicator for the onset of El Niño. A separate mode whose spatial pattern corresponds to the main oceanographic gyre also shows statistically significant temperature variations in phase with, or slightly leading, the SOI.The main decadal variations occur in the midlatitudes, in the subtropical gyre, and in another mode associated with sub-Antarctic mode water (SAMW). The subtropical gyre warmed to a maximum in the mid-1970s and has been cooling since. In the SAMW a long-term warming of the upper 100 m of the southwest Tasman Sea is identified between 1955 and 1988. The depth-integrated warming in this region is found to be about 0.015°C yr1, representing a contribution to sea level rise, through thermal expansion, of about 0.3 mm yr1.
Full-text available
The dynamics of the flow field surrounding New Zealand are investigated using a series of global ocean models. The physical mechanisms governing the direction, magnitude, and location of the East Australian Current (EAC), the Tasman Front, the East Auckland Current (EAUC), and the East Cape Current (ECC) are studied using numerical simulations whose complexity is systematically increased. As new dynamics are added to each successive simulation, their direct and indirect effects on the flow field are examined. The simulations have horizontal resolutions of 1/8°, 1/16°, or 1/32° for each variable, and vertical resolutions ranging from 1.5-layer reduced gravity to 6-layer finite depth with realistic bottom topography. All simulations are forced by the Hellerman and Rosenstein monthly wind stress climatology. Analysis of these simulations shows that several factors play a critical role in governing the behavior of the examined currents. These factors include 1) mass balance of water pathways through the region. 2) gradients in the wind stress curl, 3) nonlinear flow instabilities, and 4) upper-ocean-topographic coupling due to mixed baroclinic and barotropic instabilities. Transport stream- functions of a linear reduced gravity model reproduce the large-scale features well but produce an EAUC that flows counter to the observed direction. The residual of the mass balance of the transport through the Tasman Sea, the basinwide transport at 32°S, and the transport of the South Pacific subtropical gyre east of New Zealand determines the direction of the EAUC. The 6-layer nonlinear model allows isopycnal outcropping, which changes the transport through the Tasman Sea and produces an EAUC flowing in the observed direction. Gradients in the zonally integrated wind stress curl field determine the coastal separation points of the EAC, the EAUC, and the ECC, while a combination of nonlinear flow instabilities and upper-ocean-topographic coupling contribute to the formation of meanders in the Tasman Front. Increased resolution results in greater mixed baroclinic- barotropic instabilities and thus more upper-ocean-topographic coupling and surface variability, giving a more accurate simulation of topographically controlled mean meanders in the Tasman Front.
EDGAR, G.J., 1984 (31 viii): General features of the ecology and biogeography of Tasmanian subtidal rocky shore communities. Pap. Proc. R. Soc. TaBm.~ 118: 173-186, pl. 1. ISSN 0080-4703. A number of subtidal benthic assemblages of plants and animals which commonly occur around the Tasmanian coast are described. These assemblages are incorporated into a gen-eral scheme which relates subtidal zonation patterns to wave exposure and depth. The existence of a cool-temperate marine biogeographic province (the Maugean) , centred in southern, western and eastern Tasmanian waters, is reaffirmed. The biota of the northern Tasmanian coast is considered similar to that of the Victorian coast and in-cludes a large component of Flindersian (Southern Australian) species. A number of Peronian (New South Wales) species reach the Tasmanian east coast. Most of these animals probably drift to Tasmania as pelagic larvae in southward flowing currents, and may even travel further afield to New Zealand, but have difficulty surviving to maturity and spawn-ing. Many Maugean plant and animal species also occur in New Zealand.
Deep convective overturn has an important role1 in enhancing the productivity of warm-core ocean eddies. An eddy (F) formed from the East Australian Current in early 1978 overwintered further to the south and developed a significant phytoplankton bloom the following spring. We describe here another process which tends to terminate such blooms by submerging winter water, relatively rich in nutrients, beneath poorer water of more recent origin.
A 62 year record of temperature and salinity from a coastal station off southeast Australia shows a strong positive trend and quasi-decadal variability but the cause of the observed changes has not been explained. The temperature and salinity variations are highly correlated. The increase in temperature and salinity with time agrees closely with the mean meridional gradient of water properties along the continental slope, suggesting that changes in strength of the poleward extension of the East Australian Current are responsible for the observed variability. Interannual temperature and salinity changes are correlated (r = 0.7) with basin-scale winds and with transport through the Tasman Sea estimated from Island Rule, with the changes at the western boundary lagging the wind forcing by three years. We conclude that the trend and decadal variability in the coastal temperature and salinity record reflect the response of the subtropical gyre and western boundary current to basin-scale wind forcing.
Oceanographic data from the last two decades show that the behavior of the East Australian Current system is qualitatively different on either side of a line extending south-southeast of Sugarloaf Point (32°30°S); to the north and east of the line, dynamic height contours and satellite buoy tracks are either open, or they consist of large eddies elongated in the north-south direction. South and west of the line, flow consists of relatively small, near-circular eddies. Just south of Sugarloaf Point, northward currents on the continental shelf appear to be common, suggesting entrainment toward a separation flow near Sugarloaf Point which may be topographically controlled. The oceanographic data suggest that the separation point is more closely defined in summer, when the current is strong, than in winter. Merchant ship, current atlas and continental shelf sediment data generally support this description of the East Australian Current. However, current atlas data collected between 1854 and 1938 suggest that the separation near Sugarloaf Point is stronger in winter than in summer; this may be a real change from present conditions or it may be due to the unknown errors in the data. The distribution of fine sediments over the continental shelf suggests that the current separation near Sugarloaf Point is quite sharp and has been present for a considerable time.