FULL REPORT

East Australian Current









Lead Author: 

Ken Ridgway 1

Co Authors: Katy Hill 2

Download this report in PDF format: Click here

What is happening?

The intensification of flow and accelerated warming observed in the EAC is also seen in other Southern Hemisphere western boundary current systems, driven by the strengthening and contraction south of Southern Hemisphere westerlies (wind), although regional responses mean rates of warming differ among systems. A range of species, including plankton, fish and invertebrates, are now found further south because of enhanced transport of larvae and juveniles in the stronger EAC and the high rate of regional warming.

What is expected?

EAC flow will increase off southeast Australia with a compensating decrease off north-east Australia.

What we are doing about it?

New observations established as part of IMOS are beginning to provide an integrated view of the EAC. Long-term monitoring at Maria Island (Tas),and Port Hacking (NSW), are being sustained and enhanced as part of a network of National Reference Stations; a new station has also been established at North Stradbroke Island (Qld). IMOS is also supporting the Ships of Opportunity XBT network (provides temperature profiles every 25 km along ship tracks) in the Tasman Sea, which captures key limbs of the EAC system; and is delivering these data in real time.

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 forms part of the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres.

The EAC is similar to other western boundary currents and is dominated by a series of mesoscale eddies which produce highly variable patterns of current strength and direction. Seasonal, interannual and particularly strong decadal changes are observed in the EAC which tend to mask the underlying long-term trends related to greenhouse gas (GHG) forcing.

Observations from a long-term coastal station off Tasmania 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 in the order of 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. The oceanic changes are forced by an intensification of the wind stress curl arising from a poleward shift in the circumpolar westerly winds associated with recent trends in the Southern Annular Mode (SAM).

Observational and modelling studies indicate that these changes in the wind patterns are at least in part attributable to stratospheric 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 forced with observed CO2 emissions also produce an upward trend of the SAM and, as a consequence, an intensification of the Southern Hemisphere gyre system.

Climate model simulations strongly suggest that trends observed over the past 50 years will continue and accelerate over the next 100 years.

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. The 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.

Climate model simulations strongly suggest that trends observed over the past 50 years will continue and accelerate over the next 100 years.

Citation: Ridgway, K. & Hill, K. (2012) East Australian Current. In A Marine Climate Change Impacts and Adaptation Report for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday & A.J. Richardson) Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
1 CSIRO Marine and Atmospheric Research, PO Box 1538, Hobart, TAS 7001, Australia
2 Integrated Marine Observing System, University of Tasmania, Private Bag 110, Hobart, TAS 7001, Australia.

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Authors

Katy  Hill

Katyhill2crop eac

Katy has researched the variability in the East Australian Current, linking the boundary current changes to hemispheric climate change and...
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Ken Ridgway

Ridgway eac

Ken Ridgway has over 25 years experience in physical oceanography and climate research. He has maintained a long-term interest in the East...
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Scientific Review:

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 provides both the western boundary of the South Pacific Gyre and the linking element between the Pacific and Indian Ocean gyres. It has an important role in removing heat from the tropics and releasing it to the mid-latitude atmosphere (Roemmich et al. 2006). The EAC system within the Tasman/Coral Sea basin represents 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) and is a central driver of key species such as southern bluefin tuna (SBT). Observations over the past decades have shown a trend in EAC flow off southeastern Australia which has resulted in increased warming in the Tasman Sea with associated changes in a range of marine species.


Observed Impacts:


Tasman Sea Warming

Over the past 60 years, there has been a clear upward trend in the temperature and salinity in south eastern Australian coastal waters.  The long-term record from the Maria Island coastal station shows that the greater southward penetration of the EAC has increased both temperature and salinity in surface waters (Ridgway 2007b; Hill et al. 2008; Figure 1).  It has already been noted that this temperature trend determined at Maria Island is 3-4 times greater than the global warming signal over the same period (Ridgway 2007; Ridgway and Hill 2009).  Results from global sea surface temperature (SST) products confirm that the region is a ‘hotspot’ with enhanced warming observed over the southern Tasman Sea (Ridgway 2007, Wu et al. 2012).  However, the characterization of this long-term trend needs to be considered in the context of variability in the system.  The EAC exhibits both a clear seasonal cycle and a particularly distinct decadal signal (double that of the seasonal; Ridgway and Hill 2009).  This tends to complicate efforts to infer long-term trends from relatively short records.


Fig. 1. (a) Changes in temperature and salinity off Maria Island 1944–2010; data are from the Maria Island Time Series station (location starred in red on inset map) and temperature refers to average water column temperature to 50 m depth. Diamonds represent temperature/salinity values at a series of seven locations along a track to the east of Tasmania (inset map) from a mean climatological field based on data obtained prior to 1970. Note that the temperature/salinity signature for Maria Island in 2010 is equivalent to the signature N2° further north prior to 1970. (b) The trajectory for August (winter) temperature collected at Maria Island from 1944 to 2010. The horizontal dashed line at 12 °C indicates the approximate minimum temperature for successful development of Centrostephanus rodgersii (long-spined sea urchin) larvae. There has been an increase in the frequency which this threshold has been exceeded since 1980.

Decadal Changes

We have previously documented the decadal signals in EAC flow (Ridgway and Hill 2009).  An increasing set of evidence is accumulating to show that there has been an overall intensification of the South Pacific subtropical gyre circulation over the past 2 decades (Willis et al. 2004; Qiu and Chen 2006; Roemmich et al. 2007; Ridgway et al. 2008).  These results are based on satellite altimetry, XBT sections (provides temperature profiles from ships), CTD stations (continuous monitoring of salinity, temperature and depth), and Argo floats.  This so-called ‘spin-up’ of the gyre is seen to be part of a multi-decadal variation in gyre flow that has persisted for at least over 6 decades (Ridgway 2007; Ridgway et al. 2008; Hill et al. 2010).  It has also been shown that Tasman Sea sea-level and EAC transport changes are modulated by decadal variations in El Niño-Southern Oscillation (ENSO; Sasaki et al. 2008; Hill et al. 2010; Holbrook 2010).


Roemmich et al. (2007) and Qiu and Chen (2006) suggest that observed changes in the South Pacific Gyre also reflect changes in South Pacific winds, with a delay of several years.  The mechanism is associated with strengthening of the basin-wide wind stress curl which drives a southward expansion of the subtropical gyre (Hill et al. 2008; Hill et al. 2011). This is of particular importance for the EAC system (Figure 2).  As the gyre shifts south, the poleward EAC extension pathway is favoured at the expense of the Tasman Front that feeds directly into the subtropical gyre, resulting in the observed negative correlation of the these two major currents (Hill et al. 2011).  Model studies suggest that this rapid response of the EAC to changes in the basin-scale winds is due to a combination of barotropic (generated in the eastern Pacific) and baroclinic Rossby waves with conversion between modes facilitated by topography west of New Zealand (Holbrook et al. 2011, Hill et al. 2010).



Figure 2: d) EAC extension (green/grey) and Tasman Front transport (blue/black) (Sv) in GECCO and XBT respectively (linear trend removed). b) South Pacific zonal mean wind stress curl (180–280°E) for GECCO (GECCO-optimized NCEP). Figure adapted from Hill et al. 2011..

Long-term Change

The long-term Tasman Sea warming trend supports the notion that underlying the decadal signal there is also an associated trend in the EAC strength (Ridgway 2007).  Again, the mechanism for this EAC trend has been ascribed to an intensification of the South Pacific Gyre which is shown in model simulations (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) associated with a positive trend in the Southern Annular Mode (SAM, a hemispheric scale pattern of climate variability).  Both greenhouse warming (e.g. Kushner et al. 2001) and/or ozone depletion (e.g. Shindell and Schmidt 2004) may induce these changes in the wind (Cai 2006, Cai and Cowan 2007). 


In a recent study, Wu et al. (2012) have shown that a similar enhanced warming pattern is observed over the path of all of the other western boundary currents.  They find that since-1900 the surface ocean warming rate within each of these currents is two to three times faster than the global mean surface ocean warming rate, directly comparable to the Maria Island results.  Wu et al. show that the accelerated warming is also associated with changes in the subtropical western boundary currents.  While all the current systems exhibit changes they find different processes are underlying these changes.  For the Gulf Stream, Kuroshio, and Brazil Currents, they find that the warming derives from a poleward shift of the current which in turn is related to a poleward shift of the zero line of the wind stress curl.  In the case of the Agulhas and EAC it is directly related to an intensification of the currents themselves in conjunction with a systematic change in winds.  In each of the southern subtropical oceanic gyres, the anticyclonic wind stress curl has intensified over the past century.  While these conclusions appear to confirm both the warming and intensification of the EAC, previous results indicate that there is also a poleward shift of the EAC system within a related poleward displacement of the gyre core (Cai 2005).  This may be due to Wu et al (2012) using the location of the eastward flow of the boundary current as a metric rather than the poleward (along boundary) flow.


Ecosystem Impacts

Associated with the increases in the strength, duration and frequency of southward incursions of warm, nutrient poor EAC water is the transport of a range of biota along the eastern Australian coastal boundary – in particular to eastern Tasmania.  Significant changes in a diverse group of marine species have been observed such as changes in the structure of nearshore zooplankton communities and other pelagic components; a decrease in the coverage of giant kelp beds (Macrocystis pyrifera); distinct variations in the distribution of nearshore fishes; and southward expansions of warmer-water species into Tasmanian coastal waters (Johnson et al, 2011). The warming trend may have also induced changes to commercially important invertebrate species.


Over recent decades, there has been a dramatic decline in the extent of giant kelp (M. pyrifera) forests along eastern Tasmania (Sanderson, 1990, Johnson et al. 2011).  In Australia, favourable habitat for giant kelp occur in the southeast where water conditions are cool and relatively nutrient-rich.  Much of eastern Tasmanian kelp beds were once sufficiently large to be harvested commercially (Sanderson 1987; Edyvane 2003). However, since the 1980s there has been a precipitous reduction in kelp abundance in many areas in eastern Tasmania, particularly in the northeast (Sanderson 1980; Johnson et al 2011).  While a complete understanding of this decline is still to be documented, it is consistent with the southward projection of warm, salty, nutrient-poor EAC water. 


A notable case is the establishment of sea urchins (Centrostephanus rodgersii) in eastern Tasmania.  This provides the first reported example of a climate-driven ‘cascade’ of ecological consequences to a temperate reef system in the Southern Hemisphere.  Coastal water temperatures in eastern Tasmania now fluctuate around the 12°C threshold for successful C. rodgersii larval development during August (time of peak spawning) (Ling et al. 2008). The strengthening of the EAC has improved the climatic suitability of novel habitat for C. rodgersii and provided the supply of recruits necessary for colonisation (Banks et al. 2010). Overgrazing of macroalgae by C. rodgersii has led to the formation of “urchin barrens” with a major loss of species, habitat and productivity (Ling 2008).  In these barren habitats, there is a reduction in the abundance of two important commercial species: blacklip abalone (Haliotis rubra) and southern rock lobster (Jasus edwardsii) (Johnson et al. 2011). 

While the connection between climate forcing and the expansion of C. rodgersii is well established, we acknowledge that many of the time series of Tasmanian biota are relatively short so the ecological influence of other climate variables cannot be ruled out .  These include increasing acidification (Kroeker et al. 2010), disease outbreaks (Harvell et al. 2002), and changes in growth rates of juvenile fish (Thresher et al. 2007). 

Finally, as we have noted, as waters warm in the seas off eastern Australia region, many species already have and will continue to adapt to these changing conditions by following their preferred temperature range southwards.  However, the most southern Tasmanian species become particularly vulnerable in having nowhere to retreat in the face of ongoing warming and southerly expansion of northern species.

Potential Impacts by the 2030s and 2100s: 


Potential impacts by 2030 (and/or 2100)

Further analyses of global climate models strongly suggest that changes in the EAC system will continue along the path of the observed trends over the past 50 years. The previous projections on future changes to the EAC region were primarily based on the output of a single model, the CSIRO Mk3.0 (Cai et al. 2005).  Cai et al. found that there is a projected spin-up of the mid-latitude gyre circulation into the future, particularly along the path of the EAC, as the SAM trends toward an increasingly positive state. 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 future (Cai et al. 2011. 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 Mk 3.0 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. 


From a suite of CMIP 3 models, Hobday and Lough (2011) found that in south-eastern Australia, there is consistent warming among model projections.  An ensemble average shows that, south-eastern Australia has the greatest projected rate of warming to the end of this century, as a result of both atmospheric warming and the EAC intensification.  There is a projected increase of 2°C in the average temperatures in this region by 2050 (from the 1990–2000 average).  Sen Gupta et al. (2008) also examined the 21st century projections of Southern Hemisphere circulation in all of the CMIP3 models.  They also found a significant link exists between the position of the wind stress curl minimum and the cores of the subtropical gyres across the models. A southward shift occurs across all the models over the 21st century. A strong correlation exists across the models between the magnitude of the shift in the zonally averaged position of the wind stress curl core and the associated change in the position of the zonally averaged stream-function core.  Sen Gupta et al (2008) determined the boundary current strength in each Southern Hemisphere ocean basin (calculated as the component of the depth integrated core strength travelling along the coast).  There are distinct differences in the response of each of the systems.  While the Agulhas Current displays a small decrease, both the Brazil Current and the EAC show increased flow at higher latitudes.  Along the core of the EAC there is a strong increase in flow at higher latitudes with a compensating decrease at lower latitudes, indicative of a southward shift in the circulation. There is no robust increase in the maximum flow however, although, as noted by Cai et al. (2005), the CSIRO Mk3.0 does show an enhanced flow into the Tasman Sea region.  In all the subtropical gyres the models show a uniform shoaling pattern


We note again that 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. 


Figure 3: Model and observed strengths of the EAC, Brazil, and Agulhas western boundary currents by latitude.  Western boundary current strength shown as depth-integrated velocity, calculated as the gradient of the barotropic streamfunction.  Red (black) areas indicate locations when future strengths are increased (decreased) with respect to present day (Adapted from Sen Gupta et al, 2008). 

Confidence Assessments

Observed Impacts: 


Decadal changes in a range of properties in the Tasman Sea
There are multiple lines of evidence to support the decadal changes in EAC properties.  Decadal signals are observed in SST, sea surface salinity, sea level and EAC transport.  Amount of evidence: Robust, degree of consensus: High, confidence level: High


Decadal EAC changes forcing the property changes
Both observations and models show that the property changes are linked to a spin-up/spin-down of the South Pacific Gyre which produces a stronger/weaker EAC Extension and weaker/stronger Tasman Front. Amount of evidence: Robust, degree of consensus: High, confidence level: High


Gyre changes forced by wind changes
There is good agreement that a strengthening of the basin-wide wind stress curl drives a southward expansion of the subtropical gyre. Amount of evidence: Robust, degree of consensus: High, confidence level: High


Decadal signal propagated by barotropic/baroclinic Rossby waves
General agreement that Rossby wave mechanism communicates changes in wind stress curl to variations in EAC transport. Amount of evidence: Medium, degree of consensus: Medium, confidence level: Medium


Long-term warming in the Tasman Sea
A range of observations show the enhanced warming signal in the Tasman Sea.  Global SST datasets, a long-term coastal station at Maria Island and ocean reanalyses based on in situ data show a warming pattern well above global mean values. Amount of evidence: Robust, degree of consensus: High, confidence level: High


Warming forced by changes to EAC
There are no direct measurements of EAC strength and location at long-term timescales.  The evidence for the connection comes from (i) extending the demonstrated relationship between Tasman Sea warming and the EAC strengthening at decadal timescales into the multi-decadal range and (ii) results from models that show that the accelerated warming is associated with changes in the subtropical western boundary currents, an intensification of the EAC in conjunction with a systematic change in winds. Amount of evidence: Robust, degree of consensus: High, confidence level: High


Nature of changes to EAC
There is a reduced consensus in the nature of the EAC change.  Cai (2005) finds that the EAC has both increased in strength and extended southward.  Observations from Maria Island confirm this southward extension.  However, Wu et al (2012) find a strengthened EAC but no southward extension, which is based on the location of the maximum eastward velocity. Amount of evidence: Robust, Degree of consensus: Medium, Confidence level: Medium


Gyre changes forced by wind changes
There is a strong consensus that the changes in the EAC and gyre strength are forced by a strengthening of the basin-wide wind stress curl pattern. Amount of evidence: Robust, degree of consensus: High, confidence level: High


A range of biological impacts are driven by the EAC changes
Ecosystem responses to the oceanographic changes include increases in abundance of warm water species (range extensions), loss of habitat (kelp) of endemic species, changes to recruitment, reproduction, growth rates (size at maturity). Amount of evidence: Robust, Degree of consensus: High, Confidence level: High

Potential Impacts by the 2030s and 2100s: 


Enhanced Tasman Sea Warming
Enhanced warming from the 20th to the 21st century is evident in virtually all CMIP3 models and the multi-model mean (Sen Gupta, 2008). Amount of evidence: Robust, degree of consensus: High, confidence level: High


Warming driven by changes to EAC
The CMIP3 models present a very consistent picture showing that the warming signal in the EAC originated from changes to the EAC and gyre circulation. Amount of evidence: Robust, degree of consensus: High, confidence level: High


EAC forced by wind changes
A strong correlation exists across the models between the magnitude of the shift in the zonally averaged position of the wind stress curl core and the associated change in the position of the zonally averaged stream-function core.Amount of evidence: Robust, degree of consensus: High, confidence level: High


Nature of EAC changes
As with the observed changes to the EAC described in the previous section there is disagreement as to the nature of the change.  While a majority of the climate models project that the subtropical gyre is shifted polewards which leads to increased EAC strength at higher latitudes, a smaller number suggest that there is also an increase in maximum EAC strength. Amount of evidence: Robust, degree of consensus: Medium, confidence level: Medium

 


Observations and Modelling

What are we currently measuring in in the EAC system relevant to climate change?

While historical observations of the EAC are limited, there are some key datasets that provide valuable long time-series.

XBT Lines (since late 1980s): Temperature profiles from high density expendable bathythermograph (XBT) lines form the global underpinning boundary current observing system. These observations are now supported by the Integrated Marine Observing System (IMOS - see below).

Coastal Reference Stations: Maria Island time-series (since 1944) in Tasmania provides valuable long term data, which has been shown to be representative of the EAC strength (Ridgway 2007; Hill et al. 2008). A station at Port Hacking, NSW provides a further long record since the mid-1940s. Maria Island and Port Hacking are being sustained and enhanced as part of an IMOS network of National Reference Stations (NRS); a new NRS has also been established at North Stradbroke Island, Queensland.

Satellite Remote Sensing (various missions since early 1980s): Satellite sea surface temperature, sea surface height and ocean colour provides information on the spatial structure of ocean currents, and their variability.

The bulk of the marine observing networks are now maintained within the Integrated Marine Observing System (IMOS - Hill et al. 2011, see Figure 4). IMOS was established in 2007 and aims to meet the need for long term observations to address research questions across 5 science themes; one of which is around Boundary Currents and Inter-basin flows. This intention is to sustain the observing system over decades. Key components include:

• Satellite remote sensing collects in situ data to ensure global products are calibrated and validated in our region. High quality regional products are produced to international best practice.
• An EAC array of 8 moorings will deliver data on the full depth mass, heat and salt flux of the EAC, which will be compared with estimates from the XBT data.
• Shelf mooring arrays provide observations of shelf currents and how they interact with boundary currents. EAC observations include a shelf array inshore of the EAC deep array and offshore of the Stradbroke NRS and shelf moorings along the NSW coast.
• Deep diving Seagliders are deployed in the Coral Sea for monitoring the North Queensland Current, off New South Wales for observing the EAC eddy field, and off the east and south coasts of Tasmania to cross the EAC Extension and Tasman Outflow respectively. Shallower Slocum gliders are deployed across the shelf off the New South Wales to observe the interaction between the boundary current and shelf processes.
• High Frequency radar at Coffs Harbour gives maps of surface currents across the shelf out to 200km.


Figure 4: Observations of the East Australian Current region collected within IMOS.

What else should we be measuring to inform climate change, where and how often?

It is important to sustain these observations into the future to understand long term changes in the context of variability on seasonal to decadal timescales. In particular we need observation networks that will resolve the uncertainty over nature of the EAC changes. These include whether the gyre system and current shift polewards, whether the currents strengthens or there is a combination of the two processes. Further measurements of the Tasman Front (northeast of New Zealand) and the EAC Extension/Tasman Outflow (south of Tasmania) would complement the array off Brisbane (e.g., IMOS Bluewater Node Plan).

References

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