FULL REPORT

Leeuwin Current









Lead Author: 

Ming Feng 1

Co Authors: Nick Caputi 2 and Alan Pearce 3

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What is happening?

Since the mid 1970s, the Leeuwin Current has weakened due to more frequent El Niño events. However, in the past two decades, a strengthening is observed, linked to natural decadal variability not long-term change. Associated with this, there has been a southward range shift in marine biota.

What is expected?

The Leeuwin Current is likely to weaken over the coming century. Despite this, warming will continue to drive southward range shifts in marine biota and there will be more frequent extreme temperature events.

What we are doing about it?

IMOS is monitoring the Leeuwin Current. National Reference Stations at Darwin, Exmouth, Rottnest and Esperance and are providing physical and water quality data. Coastal water temperatures are monitored along the Western Australian coastline by the Department of Fisheries as part of a rock lobster program.

Summary

The long-term trend of the Leeuwin Current (LC) is essentially driven by the variations and changes of Pacific equatorial easterly winds. The LC has experienced a strengthening trend during the past two decades, likely due to natural variability, which has almost reversed the weakening trend during 1960s to early 1990s. Currently, most climate models project a weakening trend of the Pacific trade winds and a reduction of the LC strength in response to greenhouse gas forcing. Whereas changes induced by the greenhouse gas forcing induced changes may be clear in the long-time climate projection (e.g. 2100), for assessment of short-term climate projection (e.g. 2030s), natural decadal climate variations still need to be taken into account.

Whereas the average rising trend in sea level off the WA coast has been ~1.5 mm per year over the past century, there has been an acceleration of the rising sea level trend in the past two decades, at ~5 mm per year. The acceleration is closely associated with a relatively high global sea level rising trend (~3 mm per year) and the rebound of the strength of the LC during the past two decades. Note that the sea level trend related to the rebound of the LC over the past two decades.

The near-record strength of the LC in the austral summer of 2010/11 during one of the strongest La Niña events is partly responsible for the unprecedented marine heat wave event off the WA coast (centred on the Gascoyne and mid-west regions) in February-March 2011. The peak of the heat wave was a two-week period in late February and early March 2011, with water temperature anomalies reaching 5°C along the central west coast, Abrolhos Islands and Shark Bay. This event caused massive fish and invertebrate kills (particularly Roei abalone in the Kalbarri region) as well as far-reaching (albeit temporary) range extensions of many tropical species down the west coast and eastwards towards the Great Australian Bight. The abalone fishery in this region has been shut and a research trial on the translocation of abalone from nearby unaffected areas into the depleted areas has been successful. This will be followed up by further translocations to assess the effects on the recovery of the stocks. This event may also have longer-term implications for some marine stocks, as effects of the warm temperatures during their spawning and larval phases will be seen in the next couple of years. The recruitment of juvenile scallops and crabs in Shark Bay has been at record low levels towards the end of 2011 and this may have also been influenced by the warming event earlier in the year. There will be ongoing monitoring of the Shark Bay scallop and crab stocks and the management focus is on the recovery of their spawning stocks. Similar extreme events may occur more often in the future with the rising trend of ocean temperature.

The LC generally has a positive effect on the western rock lobster puerulus settlement, which is higher in La Niña years when the LC is flowing strongly and water temperatures are warmer than during El Niño events. However the settlement has been below average in the past six years (2006/07 to 2011/12), including the two lowest settlements in the 40-year time series. A number of these low settlements occurred during years of strong LC in 2008 and 2011 and more recently in years with very high egg production, suggesting that other long-term environmental factor(s) dominated in these years. The downturn in puerulus settlement resulted in a pro-active management response before these year-classes entered the fishery (there is 3-4 year lag between settlement and recruitment to the fishery) with a significant reduction in fishing effort (ca. 40-70%) since 2008/09. The fishery provides an example of a management adaptation response to the long-term decline in puerulus settlement. Catch and fishing effort were reduced to ensure that there was a carryover of stock into the years when the poor year-classes entered the fishery and that the spawning stock remained at sustainable levels.

Citation: Feng M et al. (2012) Leeuwin Current. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2012 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson). Retrieved from www.oceanclimatechange.org.au [Date]

Contact Details: 
1 CSIRO Marine and Atmospheric Research, Floreat, WA 6014 Australia .(JavaScript must be enabled to view this email address) 2 Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, Western Australia, PO Box 20, North Beach WA 6920, Australia 3 Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, Western Australia, PO Box 20, North Beach WA 6920, Australia. and Curtin University, GPO Box U1987, Perth WA 6845, Australia

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Authors

Alan  Pearce

Alanpearce

Alan Pearce is a physical oceanographer who retired from CSIRO in 2006 but has continued his interests in the Leeuwin Current system and its role in...
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Ming Feng

Mingfeng

Dr. Ming Feng is a physical oceanographer with CSIRO Marine and Atmospheric Research Division and the Wealth from Oceans Flagship. He completed his...
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Nick Caputi

Caputi

Dr Nick Caputi is the Supervising Scientist of the Invertebrate Branch of the Department of Fisheries (Western Australia) which undertakes research...
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Scientific Review:

The Leeuwin Current (LC) is a warm, poleward flowing ocean boundary current off the west and south coasts of Australia (Cresswell and Golding 1980), driven by the large-scale meridional pressure gradient in the southeast Indian Ocean which is set up by the Indonesian Throughflow and thermohaline forcing (Godfrey and Ridgway 1985; McCreary et al. 1986). The LC is intrinsically unstable and mesoscale eddies are a ubiquitous feature of the LC. The LC has the highest eddy energy among all eastern boundary current systems in the world (Feng et al. 2005), and the heat budget in the LC is dominantly balanced by the LC heat transport and the heat released through the air-sea interface (Feng et al. 2008).

The LC is weaker during summer when there are strong opposing winds, and stronger during winter when the opposing winds cease (Smith et al. 1991; Feng et al. 2003). In late autumn/early winter, the LC accelerates and rounds Cape Leeuwin off the southwest coast of Western Australia to enter waters south of Australia, and continues as an eastward shelf current (the South Australian Current) along the southern coast of Australia (Middleton and Bye 2007; Ridgway and Condie 2004). During the summer, sporadic wind-driven northward inshore currents and coastal upwelling events occur in limited shelf regions off the west coast, while wind-driven upwelling is more persistent off the southern coast of Australia.

On an interannual time scales, El Niño-Southern Oscillation (ENSO) in the tropical Pacific induces strong responses in the LC off the west and south coasts of Australia, due to the existence of Pacific to Indian Ocean waveguides (conduit). During La Niña events, deep anomalies in thermocline depth are transmitted along the west and south Australian coasts, inducing high sea level anomalies, strengthened LC volume transport, eddy energetics, and poleward transport of warm waters (Figure 1; Pearce and Phillips 1988; Feng et al. 2003; 2005; 2007; 2008; Clarke and Li 2004; Wijffels and Meyer 2004; Middleton and Bye 2007). In contrast, shallow anomalies in thermocline depth, low sea level anomalies and weakened LC volume transport occur during El Niño events. Multi-decadal variations in the tropical Pacific (e.g. Pacific Decadal Oscillation, PDO) have also been found to influence the low-frequency variability of the Fremantle coastal sea level, an index of the strength of the LC (Feng et al. 2004; 2010).

Indian Ocean Dipole events (IOD; the Indian Ocean El Niño) are characterised by anomalous cold sea surface temperature anomalies in the equatorial southeast Indian Ocean and westward wind anomalies along the equatorial Indian Ocean (Saji et al. 1999). Whereas the IOD does not have a direct influence on the variability of the LC, there is evidence that the IOD triggers a teleconnection off the southwest coast of Australia, with reduced winter westerly winds and storm activity (Cai et al. 2011; Weller et al. 2012). Thus, the IOD may affect air-sea fluxes and vertical mixing in the LC, as well as cross-shelf exchanges off the west coast.

The Southern Annual Mode (SAM), a measure of the atmospheric pressure difference between the mid-latitudes and the Antarctic, is the most important atmospheric influence on the temperate and polar Southern Hemisphere (Gong and Wang 1999; Kidston 1999). SAM affects the latitudinal location of the storm tracks off the southwest coast on an interannual time scale, so that it also has an indirect impact in the LC region, e.g. cross-shelf exchanges during the winter season.

Many biological processes are strongly influenced by climate variability off the west coast. For example, the recruitment of the western rock lobster Panulirus cygnus in Western Australia is influenced by a number of environmental factors, including water temperature off south-west Australia, the strength of the LC, and the strength of the westerly winds. High recruitment tends to correspond to warmer water temperature, stronger LC, and stronger westerly winds in winter (Caputi et al. 2001). The LC is also one of the key drivers of the late autumn – early winter phytoplankton bloom along the oligotrophic coast (Feng et al. 2009a).

In the 2009 Report Card (Feng et al., 2009b), a multi-decadal weakening trend of the LC was attributed to a combined effect of both global warming and natural variability in the climate system. The warming trends observed in the LC and on the shelf in waters off the west coast during the past five decades were attributed to changes in regional air-sea flux (Pearce and Feng 2007). Over the past decades, an upward trend of the Southern Annual Mode may have reduced the strength of the westerly winds and storm activity off the southwest coast (e.g. Nicholls 2010), which may have altered the air-sea heat flux in the LC region to allow a warming trend, thereby overcoming the cooling effect of a reduction of the LC heat transport. The changes in the LC and the air-sea freshwater flux may have also caused the observed increase in surface salinity off the coast (Pearce and Feng 2007).


Observed Impacts:


The strength of the Leeuwin Current

After experiencing a weakening trend from the 1960s to the early 1990s, the strength of the LC has rebounded in the past two decades (Figure 1; Feng et al. 2010; 2011).


The tropical Pacific experienced a sharp climatic regime shift in the mid-1970s (Trenberth, 1990; Graham, 1995), followed by more frequent El Niño conditions (negative Southern Oscillation Index, SOI) and positive phase of the Pacific Decadal Oscillation (PDO). Associated with the climate regime shift was a multi‐decadal weakening trend of easterly winds over the equatorial Pacific from the 1960s to the early‐1990s, which was coupled with shallow thermocline anomalies in the equatorial western Pacific Ocean and the slowdown of the Pacific subtropical cells (McPhaden and Zhang, 2002; Vecchi et al., 2006). The shallow thermocline anomalies in the western Pacific Ocean transmitted into the southeastern Indian Ocean and caused subsurface cooling off the northwest coast of Australia and a slowdown of the Indonesian Throughflow (Wainwright et al., 2008), a weakening trend of the LC (Feng et al., 2004), and might have contributed to the subsurface cooling across the south tropical Indian Ocean (Alory et al. 2007; Alory and Meyers 2009; Lee, 2004; Schwarzkopf and Böning, 2011).


The multi-decadal weakening trend of the equatorial Pacific easterly winds reversed after 1993. The trend reversal has induced a warming (deep thermocline) trend in the equatorial western Pacific from 1993 to the present (Feng et al., 2010; Merrifield, 2011). All available atmospheric reanalysis products corroborate the trend reversal during the two multi-decadal periods. Magnitudes of the multi-decadal trends of the easterly winds, however, differ among the reanalysis products (Feng et al., 2011; Merrifield and Maltrud, 2011). The trend reversals of regional ocean circulations are assessed using linear regressions between wind and transport anomalies in an eddy-permitting numerical model, suggesting that since 1993 the Indonesian Throughflow and the LC transports have also reversed their weakening trends in the previous decades (Feng et al., 2011).


The above trends seem to be related to the PDO, a natural climate mode in the Pacific. The PDO transitioned into a warm phase in the mid-1970s, and then likely reverted back to a cold phase in the mid to late 1990s, resulting in dramatic changes in counts of the adult salmon population along the Pacific coast of North America (Oregon Production Index, Hatchery).


Most climate models simulate reduction of trade winds in the tropical Pacific and the LC transport (strength) under the greenhouse gas forcing (Feng and Meyers, 2011). On the other hand, strong decadal and multi-decadal variations of the climate system may significantly alter the long-term trend induced by the greenhouse gas forcing.



Figure 1: Low-passed filtered sea levels in the eastern (region A) and western Pacific (region B) and their relations with the Fremantle sea level (adapted from Feng et al. 2010). Fremantle sea level has been used as an index of the strength of the Leeuwin Current.


Sea level

The multi-decadal strengthening of the tropical Pacific Ocean circulation in the past two decades has contributed to an acceleration of regional sea level rising trend in the equatorial western Pacific, of nearly 10 mm per year (Merrifield, 2011). This signal has propagated through the Indonesian Archipelago and caused an acceleration of rising trends in coastal sea levels off Western Australia (as well as the LC; Feng et al., 2011). Similar to the decadal signals in the LC, this regional acceleration of sea level rise (about 5 mm per year off the Western Australia coast) also has contributions from the natural decadal climate variability. This accelerated rate can be compared with long-term trend off the Western Australia coast of about 1.5 mm per year over the past century. Note that the global sea level has a higher rising trend of ~3 mm per year over the past two decades.


Extreme events in the marine environment

The near-record strength of the LC in the austral summer of 2010/11, driven by one of the strongest La Niña events, is partly responsible for the ‘marine heat wave’ off the WA coast in February-March 2011 (Figure 2; Pearce et al. 2011; Pearce and Feng submitted). The peak of the heat wave was a week spanning the last few days of February 2011 and into early March, with water temperature anomalies (above the long-term monthly means) reaching 5°C along the central west coast. Abrolhos Islands and Shark Bay, and weakening northwards to Ningaloo Reef and southwards to the Capes region (Pearce and Feng, in press). Elevated temperatures were the result of a combination of a very strong southward flow of warm tropical waters in the LC, combined with an anomalously high air-sea heat flux into the ocean. This heat wave was an unprecedented thermal event in Western Australian waters, superimposed on an underlying long-term temperature rise, which may be associated with climate change.


Consequences for the marine environment off Western Australia were immense, with on the one hand, massive fish and invertebrate kills and on the other, far-reaching (albeit temporary) range extensions of many tropical species down the west coast and eastwards towards the Great Australian Bight, from a large number of verified anecdotal reports (Pearce et al. 2011).


Coral bleaching was widespread from the Monte Bello Islands in the north to Rottnest Island (Thomson et al. 2011, Smale and Wernberg 2011, Moore et al. unpublished data), with up to 80% of corals bleached at some sites in the Easter Group (Houtman Abrolhos Islands; Evans and Bellchambers 2011). There was almost complete mortality of Roe’s abalone stocks in region north of Kalbarri, and fish kills of varying magnitudes were reported anecdotally along the mainland coast and offshore at the Abrolhos Islands (Lenanton et al. unpublished data). While fish stocks may well recover following recruitment from unaffected upstream populations, there may be longer-term damage to invertebrate species such as the abalone and corals where recruitment may be more localised and replenishment would take longer. A research trial on the translocation of abalone from nearby unaffected areas into depleted areas has been successful and will be followed up by further translocations.


There may also be longer-term implications of the marine heatwave for some marine stocks as the effect of the anomalously warm temperatures during the spawning and larval phases of some species will be seen in the next couple of years. The recruitment of juvenile scallops and crabs in Shark Bay was at record low levels towards the end of 2011, and it may have been influenced by the warming event earlier in the year. Adult scallops and crabs that were at a reasonable abundance when fishing stopped mid year, were also at record low levels at the end of 2011. Two major flood events in the Shark Bay region during December 2010 and February 2011 may have also affected stocks. The annual pre-recruitment survey of scallops that has been undertaken since 1982 has proved valuable for managers and the fishing industry in the early detection of this poor scallop recruitment year class and adult abundance so that management and commercial industry decisions can be made about the 2012 fishing season before fishing takes place. The abundance of Shark Bay crabs in this deepwater region has also been monitored since 2000, which has been valuable in the early detection of the downturn of this fishery.


The extremely strong LC and warm conditions also resulted in an extended southward transport of many pelagic fish, including the larger iconic species such as whale sharks and manta rays (Pearce and Feng in press). While both these species normally inhabit tropical waters in the Ningaloo Reef area, with occasional southward extensions, both species were sighted as far south as Albany on the Western Australian south coast in February/March 2011. These far-ranging sightings are unlikely to be permanent range extensions in the short-term, but could be seen as fore-runners of a more enduring shift in the distribution of some tropical fish species. The temperature regime off southwestern Australia has shifted southwards by ~100 km over the past few decades (Lough 2008, Lough and Hobday 2011), with potential “overwintering” implications (Booth et al. 2011) for tropical fish advected southwards by (for example) the LC.


This issue is perhaps best illustrated by the long-term monitoring of tropical fish recruitment along the southern coast of Rottnest Island since the late 1970s (Hutchins 1991), almost certainly the longest-running study of larval fish off Western Australia. The annual arrival of larval and juvenile fish in autumn each year is co-incident with the seasonal strengthening of the LC and the associated peak seasonal water temperatures (Hutchins and Pearce 1994). Until now, Rottnest Island has been the southernmost sightings of two common damselfish (Abudefduf sexfasciatus and A. vaigiensis, which do not breed at Rottnest). These two species experienced gradually increasing pulses in settlement numbers in previous years of strong LC and elevated water temperatures (1995, 1999, 2000; Pearce and Hutchins 2009). The late summer/autumn of 2011 saw record recruitment of both species at Rottnest Island (Hutchins and Pearce, unpublished data), as well as the first-ever sighting further south at Busselton (Micha 2011).



Figure 2: Monthly sea surface temperature anomalies off the Western Australia coast from December 2010 to March 2011 (adapted from Pearce and Feng, in press). Location abbreviations are: Bro = Broome, Hed = Port Hedland, Exm = Exmouth, ShB = Shark Bay, Abr = Abrolhos Islands, Rot = Rottnest Island, Lee = Cape Leeuwin, Alb = Albany, Esp = Esperance


Fisheries

A preliminary assessment of effects of climate change on fisheries in Western Australia was examined as a project in WAMSI (Western Australia Marine Science Institution, Caputi et al. 2010a) and results are updated and summarized here. The western rock lobster Panulirus cygnus fishery has a long-term time series (about 40 years) on a number of biological variables as well as fishery-independent estimates of recruitment, puerulus settlement, which makes it one of best candidates to study climate change effects on a fishery in Australia (Caputi et al. 2010b). They noted that climate change effects such as increasing water temperatures may have resulted in a decrease in size at maturity, and a decrease in the size of migrating immature lobsters from shallow to deep water, which resulted in an increase in the abundance of undersize and legal size lobsters in deep water relative to shallow water and a subsequent shift in catch to deep water. The size of the migrating lobsters is significantly related to the water temperature about the time of puerulus settlement (4 years previously). The impact of climate change on the level and spatial distribution of puerulus settlement, catchability of lobsters in traps, numbers of mature females moulting from the setose (reproductive state) to non-setose state, growth rates, timing of moults and hence the timing of the peak catch rates, were also assessed.


The LC has a significant positive effect on western rock lobster puerulus settlement (Pearce and Phillips 1988, Caputi et al. 1996 and 2001). Settlement is much higher in La Niña years when the LC is flowing strongly and water temperatures are generally warmer than during ENSO events. However, the settlement has been below average in the past six years (2006/07 to 2011/12), including the two lowest settlements in the 40-year time series. There were a number of these low settlements during years of strong LC in 2008 and 2011, suggesting that other factors dominated in these years. The level of puerulus settlement is important in the stock assessment and management of the fishery as it provides a reliable indicator of recruitment to the fishery 3-4 years later (de Lestang et al. 2009). There are currently a number of research projects underway, assessing both environmental and biological factors that may have contributed to the below-average recruitment.


The downturn in puerulus settlement has resulted in a pro-active management response before these year-classes reach the fishery, with a significant reduction in fishing effort (~40-70%) since 2008/09, resulting in a decline in catch to 5,900 tons in 2009/10 from the long-term average catch of 11,000 t (de Lestang et al., 2011). The fishery has undergone a major change in management approach moving from an effort-controlled fishery to an explicitly catch-limited fishery in 2010/11, with a catch limit of 5,500 t. Management measures were undertaken to ensure that there was a carryover of stock into the years when the poor year-classes entered the fishery and that the spawning stock remained at sustainable levels.


As a result of these management measures, the spawning stock reached record levels in the 2010 and 2011 fishery-independent surveys. Given the improvement in the spawning stock and the record water temperatures in February 2011 as a result of the strong LC, and yet the level of puerulus settlement in 2011/12 remaining below average, a long-term change in the environmental conditions is looking more likely to be a causal factor. The fishery provides an example of an appropriate adaptation management response to a long-term change in the abundance of the puerulus settlement. It has resulted in a reduction in catch and fishing effort, which protects the spawning stock and ensures the long-term sustainability of the fishery.


Climate change model projections show that the warming trend is likely to continue, so that the biological trends may also continue. Some of these changes (such as the projected long-term weakening trends in the LC) may have negative implications on the western rock lobster fishery, but others such as increasing water temperature may have some positive influence.

The LC not only affects the western rock lobster fishery, but has been shown to be an important factor associated with changing abundance of a number of key invertebrate and scalefish species harvested by on-shelf commercial fisheries off the Western Australian coast. Lenanton et al. (2009) reviewed these relationships and revealed that for other invertebrate species, such as scallops and Shark Bay prawns, the LC had a negative and positive effect, respectively. The addition of new data from the last few years has weakened the relationships. For prawns, the overall production from the fishery has declined since 1989, generally due to different targeting and harvesting strategies. Preliminary data for some coastal scalefish species such as tailor and dhufish suggest that while the strength of the LC itself is implicated, other physical variables that are influenced by the LC may also be important. The Capes Current that generally flows northward inshore of the LC during the summer months may also influence the recruitment process of these species. To help unravel these relationships, the underlying mechanism of the influence of these currents, particularly the role of salinity and temperature of shelf waters, and factors controlling the availability of nutrients to on-shelf primary production need to be better understood.


Possible climate change trends identified for some fisheries can have significant effects on the stock assessment and management of the fisheries. Changes in some biological parameters of the rock lobster stocks (e.g. size at maturity and migrating lobsters) since the 1970s have been included in the population dynamic model of the fishery. Most fishery models assume that biological parameters do not change over the years (stationarity assumption). Long-term changes in the abundance of fish stocks, particularly declines, require an appropriate adjustment of fishing effort or catch quota, for the stocks to be managed sustainably. In addition, changes in the spatial distribution of fish stocks pose some challenging policy dilemmas to evaluate spatial management boundaries. Does fisheries management maintain the current zone structure and recognize that there could be some long-term ‘winners’ and ‘losers’ in that situation, or does it adjust the management to maintain some historical equity in the system?


These case studies highlight the value of long-term time series in fisheries and environmental variables in assessing the effect of climate change on fisheries. Examples across a number of fisheries have indicated that different types of data obtained for fisheries stock assessments can also be used to understand environmental-fisheries relationships. The variability of these environmental data affecting fish stocks can be examined for historic long-term trends that may have implications for long-term climate change trends in fisheries. Regional downscaling of climate change scenarios such as those being developed by WAMSI and the Indian Ocean Climate Initiative (IOCI), and long-term data from IMOS, can then be examined to assess how these environmental trends may change in the future.


The poor rock lobster puerulus settlement in recent years is due to one or more possible long-term environmental factors as well as effects due to the extreme environmental event during the 2010/11 summer. The 2010/11 marine heat-wave affected the abalone stocks in the mid-west and scallops and crab stocks in Shark Bay, which have thus provided case studies for dealing with extreme adverse environmental factors on fisheries. These case studies have highlighted the value of having a reliable pre-recruit abundance estimate for an appropriate management adaptation response. The pre-recruit information enables early detection of changes in abundance that allow for proper assessment and management recommendations before fishing takes place on poor year classes. In the case of the western rock lobster, the lag between the timing of the pre-recruit estimate and the fishery is 3-4 years, whereas for scallops and crabs the lag is only 3-6 months. These pre-recruit abundance estimates also enable early planning by the fishing industry on the level of fishing (and catch) that is likely in the coming season.

Potential Impacts by the 2030s and 2100s: 


While most climate models project a decrease of the strength of the easterly trade winds along the equatorial Pacific, there are uncertainties about the magnitude of the changes. An assessment of 10 climate models suggests that the change in the easterly winds from now to 2100 range from 1×10-3 N m-2 increase to about -6×10-3 N m-2 decrease (Feng and Meyers 2011). A decrease in the easterly winds corresponds to a weakening trend of the LC. The average strength of the equatorial easterlies among the models is about 3×10-2 N m-2, so that the uncertainty is about 23% of the mean. Another factor to affect the LC transport is the northward wind stress off the west coast of Australia. Model projections range from a -4×10-3 N m-2 decrease to 8×10-3 N m-2 increase. The average northward winds in the climate models are about 0.4-0.5×10-2 N m-2, so that there are higher uncertainties in regional wind projections. There are also uncertainties in the spatial patterns of the warming trend in ocean temperatures projected by different climate models. Regional climate model downscaling may improve the climatology temperature pattern, but on the other hand, it also introduces uncoupled ocean dynamics and thermodynamics in the regional ocean (Chamberlain et al. 2012). Currently, only one downscaling product for the Australia boundary current is available for a single climate change scenario of the CSIRO Mk3.5 model (Sun et al., 2012).


Both the CSIRO Mk3.5 climate model and the CSIRO/WAMSI regional climate downscaling model simulate a reduction of the LC transport (strength) by 15-20% from 1990s to 2060s, under the A1B scenario (Sun et al., 2012). This is consistent with most climate model projections. However, the climate models tend to underestimate the natural climate variability on decadal and multi-decadal time scales. Decadal climate variability may obscure the future changes of the LC in the short term (e.g. in the 2030s).


Thus, there is likely to be a reduction of the LC strength by 2100. The coastal sea level may continue to rise at ~1.5 mm per year, though decadal climate variability may induce significant deviation from this mean trend.


Although the current extreme LC and heat wave event occurred during a hiatus period of the global warming (or slow warming period in terms of surface temperature), and this heat wave has been viewed as a discrete event during a rather strong La Niña episode, similar events may occur more frequently in the future climate, due to the long-term increase in ocean temperature.


The southward range shift will likely continue off the west coast, according to a model projection based climate model and WAMSI regional climate downscaling (Cheung et al. 2012).

Confidence Assessments

Observed Impacts: 


Confidence
There has been a weakening trend of the Leeuwin Current volume transport (strength) since the mid-1970s due to more frequent El Niño events in recent decades (Low). Since 2009, the confidence level has been revised from “Medium” to “Low”, mostly due to a number of moderate to strong La Niña events in recent decades. It is less clear to what extent the weakening of the LC from the 1970s to the 1990s represents a long-term trend, although most climate models do suggest the LC will weaken under greenhouse gas forcing. However, in the past two decades, there has been a strengthening trend of the LC volume transport (strength) (Medium). The recent strengthening trend of the LC is supported by the Indo-Pacific observations, and is probably part of the decadal variability of the climate system rather than a real long-term trend.


There has been a rising trend of sea level in Western Australia of about 1.5 mm per year over the past century (Medium). However the acceleration of Fremantle sea level rise in the past two decades may be partly due to decadal climate variability that affects the strength of the LC.


There has been an increase in extreme Leeuwin Current flow and temperature events in the recent decade (High). There is robust evidence of impacts of the 2010/11 La Niña event. There have been a few moderate to strong La Niña events in the past decade, such as the 1998/1999, 1999/2000, 2008/2009, and 2010/2011 events. The recent marine heat wave was due to the combined effects of the steady temperature rising trend and a strong La Niña event.


There has been a southward range shift in marine biota related to warmer temperature and recent stronger southward transport (Medium) Episodic recruitment pulses of fish and invertebrates along the west coast have been observed. Increasing temperatures combined with stronger southward transport during La Niña periods could result in a more permanent southward range extension of tropical species and the potential establishment of breeding populations further south.


Warming water temperatures are affecting adult western rock lobsters (High). There is a high level of confidence in the warming trend off the southwest coast of Western Australia and on the influence of water temperature on size of migrating lobsters, size at maturity and catchability. Therefore, it is likely that lobster stocks are being influenced by the warming trend. The smaller size of migrating lobsters and of size at maturity should provide greater protection of breeding stock and enhance sustainability. The LC generally has a positive effect on the western rock lobster puerulus settlement, being higher in La Niña years when the LC is flowing strongly and water temperatures are warmer than during El Niño events. However the settlement has been below average in the past six years (2006/07 to 2011/12), including the two lowest settlements in the 40-year time series. A number of these low settlements occurred during years of strong LC in 2008 and 2011 and more recently in years with very high egg production suggesting that other long-term environmental factor(s) dominated in these years.


The recent decline of western rock lobster puerulus settlement is a consequence of long-term environmental changes (Very Low). There is less confidence in the understanding of factors affecting the decline in puerulus settlement over the past six years (2006/07 to 2011/12). Historically the strength of the LC (associated with warmer waters) and strength of storms have been identified as major factors influencing settlement. However a number of these low settlements have occurred during years of strong LC in 2008 and 2011 and more recently in years with very high egg production suggesting that other long-term environmental factors
dominated in these years.

Potential Impacts by the 2030s and 2100s: 


Confidence

The LC will weaken under the global warming forcing (2030: Low but 2100: Medium)
Most climate models, as well as the climate downscaling models, suggest a weakened LC under global warming forcing, however, decadal climate variability may mask the long-term trend in 2030.

The future sea level rising trend will maintain the current rate in Western Australia (2030 and 2100: Medium)

There is high confidence that global sea level will continue to rise under global warming forcing, however, the future sea level rising will not follow the same pattern as during the past two decades. Confidence for the future rate of sea-level rise in Western Australia (Medium) is lower than for the global rate.

There will be more frequent extreme temperature events in the future climate (2030: High and 2100: High)
Although the 2010/2011 extreme LC flow and heat wave events occurred during a hiatus period of global warming, these types of extreme events are likely to occur more often as global climate warms.

There will be continuing southward range shift in marine biota in the future climate (2030: High and 2100: High)

As the projected weakening of the LC is only slight, the progressive increase in ocean temperature will probably continue to drive a southward shift in marine biota.


Adaptation Responses

Current and planned research effort

There is a large Fisheries Research and Development Corporation (FRDC)/Department of Climate Change and Energy Efficiency (DCCEE) project on ‘Management implications of climate change effect on fisheries in Western Australia’ with the following objectives:

1 Assess future climate change effects on Western Australia marine environments using a suite of IPCC model projections, downscaled to the key shelf regions and the spatial and temporal scales relevant for key fisheries
2 Examine the modelled shelf climate change scenarios on fisheries and implications of historic and future climate change effects
3 Review management arrangements to examine their robustness to possible effects of climate change

FRDC project on ‘Identifying factors affecting the low western rock lobster puerulus settlement in recent years’ with the following objectives:
1 To use a larval advection model and the rock lobster population dynamics model to assess the effect of the spatial distribution of the breeding stock on the puerulus settlement
2 To assess environmental factors (water temperature, current, wind, productivity, eddies) and breeding stock affecting puerulus settlement
3 To examine climate change trends of key environmental parameters and their effect on the western rock lobster fishery

The Western Australia Marine Science Institution (WAMSI) is establishing a research program to assess climate change impacts on the Kimberley coast of Western Australia. WAMSI is also proposing to continue the climate impact research of the LC in the next five years of the funding cycle.


Observations and Modelling

The importance of consistent long-term, high-quality monitoring observations in assessing and understanding changing trends in the environment has been emphasised by Lough and Hobday (2011). Many longer-term studies have been larger-scale observations in open ocean waters beyond the continental shelf, whereas shallower waters of the continental shelves will perhaps respond more quickly to climate change conditions, and indeed the nearshore environment will be of greatest importance to the local community. Such measurements are rare around Australia, including the lengthy and sparsely populated coastline of Western Australia. Apart from the pioneering set of coastal monitoring stations established around Australia by CSIRO in the 1950s (Rochford 1988), which includes the Rottnest Island station, coastal water temperature measurements have tended to be short-term and industry-oriented.

The Integrated Marine Observing System (IMOS, Hill et al., 2011) meets the need for long-term observations to address research questions across five science themes; one of which concerns Boundary Currents and Inter-basin flows. The intention is to provide a sustained observing system over decades. Key IMOS components include:

Satellite sea surface temperature, sea surface height and ocean colour provide information on the spatial structure of ocean currents, and their variability. IMOS collects in situ data to ensure global products are calibrated and validated in the Australian Region. IMOS also delivers high quality regional products.

2. Temperature profiles from high-density expendable bathythermograph (XBT) lines underpin a boundary current observing system. A high-density line from Fremantle to Sunda Strait crosses the LC. IMOS is now sustaining observations that have been carried out since the 1980s.

3. A deepwater array provides information on the mass and heat flux through the Indonesian throughflow, and will facilitate research into the connection between the tropical Pacific and the LC

4. Shelf mooring arrays provide observations of shelf currents and how they interact with boundary currents. LC observations include the Indonesian Throughflow, Kimberley, Pilbara, and Two Rocks/Perth Canyon arrays.

5. National Reference Stations at Darwin, Exmouth, Rottnest and Esperance provide time series of water quality data. The Rottnest station builds on data collected there since 1951.

6. Ocean gliders including deeper Seagliders are being deployed between Ningaloo and Perth for monitoring the LC, and shallower Slocum gliders are deployed along the Kimberley, Pilbara, and Two Rocks transects for observing the interaction between the boundary current and shelf processes.


Figure 3. IMOS observations of the Leeuwin Current, available on the IMOS Ocean Portal (http://imos.aodn.org.au). This figure is produced by Katy Hill from the IMOS website.

The broader-scale IMOS observations are complemented by a series of coastal water temperature measurements along the Western Australian coastline between Port Gregory and Cape Leeuwin by the Western Australia Department of Fisheries. This is part of the program collecting monthly samples of the puerulus stage of the rock lobster to understand recruitment variation and for catch forecasting. Measurements include monthly "spot" temperatures (since 1969 at some sites) and self-recording temperature loggers, and they have been used to show early signs of temperature rise near the coast (Pearce and Feng, 2007; Caputi et al. 2009). Fisheries measurements have recently been supplemented by a variety of new monitoring stations installed by other Western Australian government departments, the universities and industry. These measurements should be supported in the longer-term, as they will become of increasing value for climate change studies into the future and were invaluable in monitoring the duration and extent of the recent Western Australian marine heat wave.

While it is important to maintain these existing observation networks in Western Australian waters, it is also important to increase the frequency of measurements in order to capture the extreme events. IMOS has set a high standard in long-term monitoring programs, and it is important to maintain and expand the IMOS network in Western Australia. One possibility is to establish a deep mooring observation program to cover the full extent of the LC, as currently the Two Rocks mooring section only covers the LC flows on the continental shelf.

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