El Niño – Southern Oscillation

Lead Author: 

Neil J. Holbrook 1

Co Authors: Jaclyn N. Brown 2, Julie Davidson 3, Ming Feng 4, Alistair J. Hobday 5, Janice M. Lough 6, Shayne McGregor 7, Scott B. Power 8 and James S. Risbey 9

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Neil Holbrook - El Nino Southern Oscillation (ENSO)

Author: Marine Climate Change 2012
| Time: 20.22 min

What is happening?

While observed El Niño-Southern Oscillation (ENSO) variability is within the broad range of natural variability, the possibility that anthropogenic forcing has influenced ENSO cannot be ruled out.

What is expected?

While the mean climate of the Pacific is expected to change, it is unclear how the amplitude or frequency of ENSO will change (if at all) over the next 100 years.

What we are doing about it?

Investing in Pacific region climate change programs, high quality data collection and monitoring, improving pre-instrumental ENSO reconstructions, process studies to understand mechanisms of variability, and enhancing modelling capabilities.


El Niño–Southern Oscillation (ENSO) is the dominant mode of year-to-year (interannual) climate variability observed globally (e.g., Philander 1990), and its environmental and socioeconomic impacts are felt worldwide (McPhaden et al. 2006). It is the major source of natural climate variability for Australia (e.g. Nicholls et al. 1997; Power et al. 1998; CSIRO-BoM 2007). ENSO drives changes in rainfall (Ropelewski and Halpert 1987, 1989; Allan et al. 1996; Power et al. 1999), surface air temperature (e.g., Power et al. 1998), river flow (Kahya and Dracup 1993; Merendo 1995; Power et al. 1999), agricultural production (Nicholls 1985; Phillips et al. 1998; Power et al. 1999; Hammer et al. 2000), ecosystems (Holmgren et al. 2001), tropical cyclones (e.g., Nicholls 1984; Solow and Nicholls 1990; McDonnell and Holbrook 2004a,b; Werner and Holbrook 2011; Werner et al. 2011; Callaghan and Power 2012), and disease (Nicholls 1993; Bouma and Dye 1997) in Australia and many other parts of the world. ENSO is largely expressed in the tropical ocean as interannual changes in sea level and ocean temperatures (e.g., Wyrtki 1975; Meyers 1982; Rasmusson and Carpenter 1982; Tourre and White 1995; BoM-CSIRO 2011), but also in Australia’s regional upper ocean temperatures (Holbrook and Bindoff 1997; Holbrook et al. 2005a,b; Holbrook et al. 2009), subtropical mode water formation (Holbrook and Maharaj 2008; Li 2012), and major boundary currents (e.g., Feng et al. 2003; CSIRO-BoM 2007; Holbrook et al. 2011).

Citation: Holbrook, N. et al (2012) El Niño-Southern Oscillation. 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 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart TAS 7001, Australia. .(JavaScript must be enabled to view this email address)
2 Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Hobart TAS 7001, Australia.
3 School of Geography and Environmental Studies, University of Tasmania, Hobart TAS 7001, Australia.
4 CSIRO Marine and Atmospheric Research, Floreat WA 6014, Australia.
5 Climate Adaptation Flagship, CSIRO Marine and Atmospheric Research, Hobart TAS 7001, Australia.
6 Australian Institute of Marine Science, PMB 3 Townsville MC, QLD 4810, Australia.
7 Climate Change Research Centre, University of New South Wales, Sydney NSW 2052, Australia.
8 Centre for Australian Weather and Climate Research, Bureau of Meteorology, GPO Box 1289, Melbourne VIC 3001, Australia.
9 Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Hobart TAS 7001, Australia.

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Alistair Hobday

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Dr Alistair Hobday is a Principal Research Scientist at CSIRO Marine and Atmospheric Research. His research spans a range of topics, including...
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Ming Feng


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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Shayne McGregor

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Dr Shayne McGregor is a Postdoctoral Fellow at the the University of New South Wales Climate Change Research Centre (CCRC). He completed his Masters...
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Jaclyn Brown

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Dr Jaclyn Brown is an ocean and climate research scientist within the Centre for Australian Weather and Climate Research at the CSIRO in...
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Julie Davidson

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Dr Julie Davidson is a social scientist whose research interests focus on environmental governance in the context of global change, and specifically...
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Scott Power


Dr Scott Power is a Senior Principal Research Scientist and research manager in the Bureau of Meteorology and a Coordinating Lead Author of the next...
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Neil Holbrook

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Neil Holbrook is Associate Professor of Climatology and Climate Change in the Institute for Marine and Antarctic Studies at the University of...
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Janice Lough

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Dr Janice Lough is a Senior Principal Research Scientist at the Australian Institute of Marine Science. A climatologist by training, she currently...
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James Risbey

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James Risbey is a senior research scientist in the Centre for Australian Weather and Climate Research. His research is broadly concerned with the...
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Scientific Review:

Substantial advances in our understanding of ENSO since the late 1960s demonstrate that ENSO is a natural mode of climate variability that exists because of the strong coupling between the tropical ocean and atmosphere (Bjerknes 1969; Zebiak and Cane 1987). Our fundamental understanding of the mechanisms that underpin ENSO are described by several leading paradigms. These are: (1) the delayed action oscillator theory (Suarez and Schopf 1988; Battisti and Hirst 1989); (2) the advective-reflective oscillator theory (Picaut et al. 1997); (3) the western Pacific oscillator theory (Weisberg and Wang 1997); (4) the recharge-discharge oscillator theory (Jin 1997); and (5) the unified oscillator theory (Wang 2001). Rather than detailing the differences between each of these theories, it is sufficient to point out the strong connection between them in terms of their collective requirement for coupling the tropical upper ocean with the atmosphere as the important mechanism that strongly influences many aspects of ENSO climate variability. While the mechanisms responsible for the existence of ENSO are centred in the tropical Pacific, ENSO drives and interacts with climate variability in many other parts of the world, including the Indian and Southern Oceans (e.g., Glantz et al. 1991; Annamalai et al. 2005; Izumo et al. 2010).

Some recent studies have argued that more than one type of El Niño event exists based on the spatial distribution of sea surface temperature anomalies (SSTA) (Ashok et al. 2007; Kug et al. 2009; Kao and Yu 2009). For instance, Kug et al. (2009) use the spatial structure of Pacific Ocean SSTAs to separate El Niño events into two main types, Warm Pool (WP) and Cold Tongue (CT: otherwise known as canonical ENSO, with a sea surface temperature expression as described in Rasmusson and Carpenter (1982)) type events. CT El Niño events display the classical canonical eastern equatorial Pacific warming, while WP El Niño events display warming in the central equatorial Pacific near the edge of the Western Pacific Warm Pool (Figure 1). These WP-type El Niño events have also been called Trans-Niño (Trenberth and Stepaniak 2001), dateline El Niño (Larkin and Harrison 2005), Central Pacific El Niño (Kao and Yu 2009), and El Niño Modoki (Ashok et al. 2007, 2009) events. Current ENSO theory does not clearly distinguish between or account for these two types of events. However, research to date suggests that there are differences in the dynamical mechanisms between WP-type El Niño events and the classic CT El Niño events (Kug et al. 2010; McGregor et al. 2012). We note that one recent study suggests that the two events are part of the same non-linear phenomenon and not two distinct modes of variability (Takahashi et al. 2011).

Figure 1: Upper: Sea surface temperature anomalies during a WP El Niño. Lower: Sea surface temperature anomalies during a CT El Niño (Ashok 2009, Greenhouse 2009 presentation).

Other climate modes that may be important to Australia’s marine environment
Recent work on the influence of a range of modes of climate variability on Australian climate confirms the central role of ENSO (Risbey et al. 2009). This work also acknowledges the role of a range of additional modes of variability, including the Indian Ocean Dipole (IOD), Southern Annular Mode (SAM), Madden-Julian Oscillation (MJO), and atmospheric blocking (the tendency for persistent high-pressure systems to form). These different modes tend to influence different parts of the Australian region at different times of year, and may sometimes be more pertinent to monitor than ENSO per se.

The IOD is an aperiodic oscillation characterised in the positive phase by warmer-than-average sea surface temperatures in the western Indian Ocean and higher-than-average rainfall there, with cooling in the eastern Indian Ocean and drier-than-average conditions over Indonesia and Australia. In the negative phase of the IOD, these ocean temperature and precipitation characteristics are spatially reversed. The IOD is particularly important between June and October for rainfall in the southern half of Australia. Extreme IOD years have been linked to extreme rainfall changes and runoff in southeast Australia (Ummenhofer et al. 2009), and thus influence high and low flows through the Murray River basin and the Murray mouth estuary. Flows through the Murray mouth are also influenced by ENSO and blocking. Interestingly, and perhaps somewhat surprisingly, the IOD does not provide any additional significant skill potential, above ENSO predictor metrics, for statistical seasonal forecasting of Australian region tropical cyclones (Werner et al. 2011).

Blocking influences river runoff by regulating the climatology of cutoff low systems in southeast Australia, which produce much of the rainfall there (Risbey et al. 2009). Blocking also plays a more direct role in the marine environment by altering the distribution of wind and wind stress in southern coastal regions. Nieblas et al. (2009) show that nutrient availability and phytoplankton productivity along the Bonney coast of southern Australia is associated with the Bonney upwelling, which is driven by seasonal changes in the orientation and duration of wind regimes off the coast. It has been shown that the variability in wind circulation here is related to ENSO (Nieblas et al. 2009). However, the most direct association with upwelling events is provided by blocking episodes in the Tasman region (Michael Pook, personal communication, 2012).

The wind regime across southern coastal Australia is also influenced by SAM, which modulates the proximity of the strong zonal westerlies in this region and affects rainfall there (Hendon et al. 2007; CSIRO-BoM 2007). More broadly, SAM influences Southern Ocean circulation and biological productivity (Lovenduski and Gruber 2005) and would be expected to cause marine impacts across Australia’s southern coastal regions.

The MJO is a travelling tropical atmospheric wave disturbance that has strongest impact on the monsoonal rains across northern Australia. The onset and activity of the Australian monsoon is related to both ENSO and the MJO. Monsoon rainfall acts to freshen and stabilise the mixed layer in the tropical western Pacific and Indian Oceans (Shinoda and Hendon 1998). The MJO is known to influence sea level and circulation variability in the Gulf of Carpentaria in northern Australia by modulating the northwesterly surface winds associated with the Australian summer monsoon (Oliver and Thompson 2010, 2011). The MJO is also associated with intraseasonal variability of the Leeuwin current (Marshall and Hendon 2012), and thus also influences the marine environment off the Western Australian coast.

Ongoing work is providing more detail about the mechanisms by which each of these remote modes of variability influence Australian climate. As the underlying teleconnection processes are described in more detail, we will have a better means to assess the marine impacts from each of these modes. Further information about Australian climate influences, including modes of climate variability can be found at the following BoM WWW site, .

Decadal to multi-decadal variability and ENSO
Aside from Australia’s climate being strongly influenced by ENSO (while acknowledging the important signals that exist regionally from other large-scale climate modes, in particular the IOD and SAM), ENSO and its impact on Australian climate and sea level also varies on decadal and longer time scales (Power et al. 1999a,b, 2006; Timbal et al. 2006; CSIRO-BoM 2007; Feng et al. 2010; Holbrook 2010; Holbrook et al. 2011; Callaghan and Power 2011). For instance, the 1980s and 1990s displayed more frequent El Niño events, which were predominently of the CT (canonical) type, relative to the prior period. In fact, 1977-2006 was the most El Niño-dominated 30-yr period based on records back to the 1870s (Power and Smith 2007). Since the late 1990s we have seen an increase in the frequency of La Niña years, including the very strong event in 2010/2011 that brought severe flooding to Queensland (Figure 2). Further, since the 1990s, the prevalence of CT El Niño events has decreased, and WP-type El Niño events have become more frequent (Lee and McPhaden 2010; McPhaden et al. 2011). So what has driven this change in the frequency and type of ENSO events? Is it purely natural variability or influenced by anthropogenic drivers?

Figure 2: Time series of the Oceanic Niño Index, showing the recent period (circle) where La Niña conditions have prevailed. Source: http://ggweather.com/enso/oni.htm

Anthropogenic climate change and ENSO
Yeh et al. (2009) have suggested that the recent El Niño event-type frequency change may be due to anthropogenic causes. The scientific evidence is mounting, however, that ENSO event-type frequency changes seen since the 1990s are part of a natural cycle (e.g., McPhaden et al. 2011; Newman et al. 2011; Yeh et al. 2011). With regards to changes in other characteristics of ENSO variability, a major difficulty exists in separating the anthropogenic signal from natural variability of the climate system as the instrumental record covers a period of less than 150 years, which is much too brief to confidently characterise the long term changes in ENSO frequency, magnitude and duration; let alone properly address the question of what mechanisms might be driving these changes (McGregor et al. 2010). Modelling studies suggest that 500 years is needed to sample the full range of ENSO’s natural variability (Wittenberg 2009). This problem is exacerbated to some extent because the tropical Pacific warming response to anthropogenic climate change is projected to be greatest near the equator (Xie et al. 2010), as it is for both ENSO and naturally occurring decadal variability - linked to the Interdecadal Pacific Oscillation. This makes it more difficult to disentangle a climate change signal from modelled and observed changes (Meehl et al. 2010; Power and Kociuba 2011).

Multi-century paleo-climate reconstructions can be used to extend the observational record and to further quantify and constrain ENSO’s sensitivity to ‘external’ forcing – specifically due to anthropogenic climate change - in the context of its natural variability. There are several reconstructions of ENSO variability from various sources of high-resolution proxy climate data (Table 1). However, there is considerable variability amongst the reconstructions (McGregor et al. 2010), which reduces confidence in the individual proxies and the associated details of past ENSO variance changes (see also Lough 2011). To date there is no unanimous agreement amongst the proxies that ENSO variability (amplitude and frequency) in the past is significantly different from that seen today.

Table 1: ENSO proxies currently available and their correlation (r) with observed Niño 3.4 region SSTA.

Recent theoretical arguments suggest that anthropogenically-induced global warming should act to weaken the background Walker Circulation (Vecchi et al. 2006). Power and Kociuba (2011) suggest that external (anthropogenic) factors are responsible for approximately 50% (± 20%) of the observed weakening of the Walker Circulation, measured by a weakening of the sea level pressure (SLP) gradient across the equatorial Pacific, during the twentieth century. This is also supported by the modelling study of Vecchi et al. (2006), which shows that SLP changes over the same period are largely due to anthropogenic forcing in their model.

While state-of-the-art climate models have improved substantially in their ability to reproduce ENSO variability, there still remain a number of systematic model biases (Brown et al. 2012). These biases, particularly the ‘cold tongue bias’ in the eastern equatorial Pacific, affect the regional scale representation of ENSO and related features such as the South Pacific Convergence Zone (Brown et al. 2011; Irving et al. 2011), local precipitation and local SST variability. Because ENSO variability in the western Pacific is not well simulated in climate models, as such, regional projections must be interpreted with care.

Aims of this update paper: ENSO in a changing climate
This 2012 paper aims to update the 2009 Marine Report Card assessment of ENSO in a changing climate (reported in Holbrook et al. 2009) by providing: (1) a comprehensive synthesis of the most recent published peer-reviewed literature on projections of ENSO in a changing climate; (2) additional information about the observed physical and biological impacts of ENSO in the marine environment around Australia, focusing on Australia’s Exclusive Economic Zone (EEZ); (3) a summary assessment of climate change projections of ENSO in the context of Australia’s marine environment; (4) a more comprehensive consideration of potential adaptation responses; (5) current and planned research efforts in understanding ENSO in a changing climate; (6) a brief summary of relevant observational and modelling programs; and (7) consideration of policy questions regarding ENSO in a changing climate.

Note: the original Marine Report Card assessment of ENSO in a changing climate (Holbrook et al. 2009) provides a comprehensive synthesis of literature regarding marine biotic responses to ENSO in Australian waters. This literature synthesis of marine biotic signals of ENSO is not reported again here in this update document. Please refer to Holbrook et al. (2009) for details.

Observed Impacts:

Australia’s east and north coasts (2012 update assessment)
Two recent large-scale synthesis projects, although focused on climates of tropical Pacific islands, encompass large parts of northern and eastern Australian tropical waters.  The Pacific Climate Change Science Program (BOM & CSIRO 2011) provides a comprehensive assessment of current understanding and drivers of observed and projected changes in climates of the western tropical Pacific.  The Secretariat of the Pacific Community (Bell et al. 2011) provides a similarly comprehensive assessment of how observed and projected climate changes are likely to impact fisheries and aquaculture for the Pacific Island Countries, Territories and States scattered through the tropical Pacific.  Both are excellent resources outlining our current knowledge of surface and oceanic climates of the Pacific Ocean bordering Australia’s eastern coasts and potential changes in these climates.  They provide similar assessments of the influence of ENSO events on Australia’s east and north coasts as described in Holbrook et al. (2009).  Their projections of future changes in ENSO (based on the CMIP3 models used in IPCC-AR4) are also similar to those described by Holbrook et al. (2009), and similarly constrained by limitations in the performance of that generation of Global Climate Models in providing consistent projections of future changes in ENSO activity.

Specific to the Great Barrier Reef (GBR), Redondo-Rodriguez et al. (2012) re-assessed the linkages between GBR mid-summer (DJF) climate and the tropical Pacific Ocean.  They also, for the first time, compared GBR climate signatures associated with canonical CT ENSO and WP ENSO events. As discussed above, the latter events are characterised by warming or cooling in the central rather than eastern equatorial Pacific (Ashok et al. 2007), and result in different distributions of rainfall anomalies across Australia (Cai and Cowan 2009; Taschetto and England 2009).  The study of Redondo-Rodriguez et al. (2012) was driven by the need to understand how thermal stress events on the GBR, conducive to coral bleaching, are related to larger-scale atmospheric and circulation patterns.  Although they found distinctive oceanic and atmospheric signatures with both CT ENSO and WP ENSO events, these ENSO influences are not the sole source of anomalous mid-summer SSTs on the GBR, which is not simply related to tropical Pacific inter-annual climate variations.  They also found a stronger signature of CT ENSO events on southern GBR SSTs compared with WP ENSO that had a greater influence on the northern GBR.  This study was, however, focused on the mid-summer season that, in some respects, is a transition (at least for El Niño events) between prior winter cooling and late summer warming of the GBR (e.g., Lough 2007).  As a consequence the assessment of typical ENSO climate anomalies likely to affect marine biota of northern and eastern tropical Australia (Table 2) is similar to that reported by Holbrook et al (2009).  It should, however, be noted that the evidence for different surface ocean anomalies with WP ENSO events needs further exploration for Australia’s marine climate.  Whether the recent more frequent WP Pacific-based ENSO activity is a response to a changing global climate is still, however, uncertain (e.g., McPhaden et al. 2011).

Table 2: Typical ENSO climate anomalies likely to affect marine biota of northern and eastern tropical Australia (Allan et al. 1996; McPhaden 2004; Lough 2007; Steinberg 2007).

Australia’s west and south coasts (2012 update assessment)
ENSO has been shown to influence the strength of the Leeuwin Current (Table 3). The strong La Niña event in 2010/11 induced a near record strength of the Leeuwin Current along the west and south coasts during the austral summer of 2010/11, as evidenced in the coastal and satellite altimeter sea level data (Figure 3; Pearce and Feng 2012). This La Niña event also caused an unusual seasonal cycle of the Leeuwin Current, being slightly stronger during the late austral summer, rather than autumn-winter. There is some evidence that the fast transition from the 2009/10 El Niño to the 2010/11 La Niña was partly due to the Indian Ocean warming in 2010 (Kim et al. 2011).

Figure 3: Global sea-level anomalies (in red, cm) at the peak of the 2010/11 La Niña event in February 2011 from the AVISO product. The Figure was provided by the NOAA Climate Prediction Center and used in Pearce and Feng (2012).

The near record strength of the Leeuwin Current during the 2010/11 summer transported more tropical waters southward along the coast which, in combination with local air-sea fluxes, caused the unprecedented warming of shelf waters off the west coast, and to a lesser extent off the south coast - a so-called “marine heat wave” event (Pearce et al. 2011; Pearce and Feng 2012). Water temperature anomalies reached +5°C during the last week of February 2011 along the central west coast and Abrolhos Islands. This is an example where an extreme event linked to ENSO can produce extreme temperature changes in the marine environment off Australia’s west coast that caused extensive coral bleaching and fish kills.

There appears to have been a re-intensification of the Leeuwin Current during the past two decades, which has reversed the previous weakening trend of the Current from the 1960s to the early 1990s (Feng et al. 2011). Since the strong El Niño event in 1997/98, six out of the past 13 years have been classified as La Niña events, with strong Leeuwin Current transport off Australia’s west coast. This reflects a shift in tropical Pacific climate in recent years, commensurate with a strengthening of the trade winds (Feng et al. 2010; Merrifield 2011). The impacts of the decadal trend in the Leeuwin Current on the extreme temperatures during the 2010/11 La Niña still needs further research.

There has been a notable decline in western rock lobster puerulus settlement off the west coast over the past several years (de Lestang et al. 2011), despite the increased numbers of La Niña events that historically correspond to high puerulus settlement. It is noted that the physical environmental changes, such as increasing water temperatures, may have resulted in a decrease in size at maturity, and decrease in the size of migrating immature lobsters from shallow to deep water. This has also 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 (Caputi et al. 2010).  Whereas it is uncertain whether any of these factors have caused the observed decrease of puerulus settlement, changes in ENSO characteristics over the past decade (e.g., McPhaden et al. 2011) may be a factor and warrants further examination.

Migration of tropical fish larvae along the continental shelf between the Houtman Abrolhos Islands and Rottnest Island (Western Australia) has been observed in recent years (Pearce and Hutchins 2009), and the stronger Leeuwin Current during La Niña years appears to greatly enhance the southward transport of tropical fish - highlighted during the strong 2010/11 La Niña event. Potential southwards advection of passive particles/larvae in the Leeuwin Current system during the autumn months has been confirmed with an individual-based particle-tracking model (Pearce et al. 2011). The implications for long-term poleward range shifts of marine species during strong La Niña events needs further research.

Although climate models, and regional downscaling using the BLUElink (Ocean Forecasting Australia Model (OFAM)) model configuration, suggest a weakening of the Leeuwin Current in the future climate (Feng and Meyers 2011; Sun et al. 2012), the recent decadal strengthening of the Leeuwin Current and the strong La Niña event in 2010/11 suggest that decadal climate variability and ENSO-induced extreme events will be particularly important, in addition to the more incremental centennial scale trends.

Table 3: Summary of El Niño/La Niña signals in the marine physical environment off Australia’s west and south coasts.

Potential Impacts by the 2030s and 2100s: 

Updated assessment of ENSO in a changing climate
Based on various assessments of the CMIP3 multi-model archive through the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) process, in which modern day ENSO events are better simulated than in the IPCC Third Assessment Report, there is reportedly no consistent indication at this time of discernible future changes in ENSO amplitude or frequency as changes in ENSO variability differ from model to model (Meehl et al. 2007; Guilyardi et al. 2009; Collins et al. 2010; BoM-CSIRO 2011). Projected ENSO variability changes derived from the full set of IPCC Fifth Assessment Report (AR5) models has not been extensively published to date. Preliminary analysis of recent results from the Coupled Model Inter-comparison Project (CMIP5) shows that there continues to be model-to-model inconsistencies in how ENSO will change in the future (Ganachaud et al., submitted, 2012). Published results using an ensemble of simulations in a single CMIP5 model, the Community Climate System Model Version 4 (CCSM4), are consistent with this conclusion, in that the 21st century ENSO variability is not significantly different from that of the 20th century (Stevenson et al. 2012).

Three important points should, however, be noted:

(1) All AR4 climate models that produce ENSO variability in the 20th century continue to exhibit ENSO variability in the 21st century (Guilyardi et al. 2009). So while there is no consensus on changes to ENSO, ENSO will continue to be a major driver of climate variability over coming decades (Power and Smith 2007; BoM-CSIRO 2011).
(2) While there is no consensus on changes to ENSO under global warming, there are robust changes projected for the mean climate in the tropical Pacific (see BoM-CSIRO (2011) for a comprehensive recent review and also, for example, Vecchi et al. (2006), Collins et al. (2010), Timmermann et al. (2010), and Power and Kociuba (2011)). Specifically, tropical Pacific warming is projected to be greatest near the equator, especially in the central-eastern portion of the basin, equatorial trade winds in the Pacific are projected to weaken, off-equatorial trade winds in the southeast Pacific are projected to strengthen, and the east-west gradient in the equatorial thermocline is projected to weaken.
(3) Global warming has very likely driven changes in mean rainfall, temperature, stream-flow and other important climatic variables (Meehl et al., 2007; BoM-CSIRO 2011; Lough and Hobday 2011) in many regions in, around, and well beyond the Pacific, including Australia. As ENSO events are responsible for some of the variability about these means in many locations, climatic conditions experienced during ENSO events have very likely changed (Power and Smith 2007) and will very likely continue to change into the future. For example, regions that tend to warm during El Niño years will tend to warm even more during El Niño events over coming decades, simply through the cumulative effect of ENSO-driven warming, adding to the anthropogenic warming trend. Similar statements can be made for regions where both ENSO and global warming affect, for example, rainfall. Consequently, past experiences of ENSO impacts have probably become less accurate guides to the future (Power and Smith 2007).



Confidence Assessments

Observed Impacts: 

Impacts of the 2010/11 La Niña Summer
The extreme weather that occurred during the summer of 2010/11 affected much of eastern Australia and was associated with one of the strongest La Niña events on record (Keenan and Cleugh 2011).  While this La Niña and the associated southeast Queensland rainfall might be regarded as extreme, the event also coincided with the recent change to negative phase of the Interdecadal Pacific Oscillation (Cai and van Rensch 2012). La Niña events are typically associated with a stronger summer monsoon, more tropical cyclone activity and higher rainfall and river flows affecting Australia’s northeast waters (Table 2).  Eastern Australian river flows appear to be particularly sensitive to ENSO in a global context (Ward et al. 2010) and there is some paleo-climatic evidence (back to the 17th century) that the variance of northeast Queensland rainfall and ENSO has increased during the 20th century (McGregor et al. 2010; Lough 2011).  Amongst the projected consequences of ongoing global warming are more extreme rainfall events and possibly fewer, but more intense, tropical cyclones (e.g., Knutson et al. 2010).  Ongoing climate changes (warming SSTs, more intense tropical cyclones, more extreme rainfall/river flood events) are considered a major threat to the future resilience of the GBR ecosystem (GBRMPA 2009).  The now routine Pre-Summer Workshop organised by the Great Barrier Reef Marine Park Authority in September 2010, based on then current seasonal outlooks and operational forecasts (e.g., Spillman 2011), considered that the GBR was at low risk of thermally-induced coral bleaching.  There was, however, a high risk of heavy rainfall and subsequent freshwater influx to the Marine Park that could result in salinity-induced bleaching and the potential for physical destruction due to more tropical cyclone activity off the northeast Queensland coast.  The 2010/11 La Niña delivered the second wettest summer for eastern Australia (after 1973/74) since 1900 (http://www.bom.gov.au/cgi-bin/climate/change/timeseries.cgi?graph=rain&area=eaus&season=1004&ave_yr=0) and one of the most severe and largest tropical cyclones to affect the Queensland coast, TC Yasi, since records began (http://www.bom.gov.au/cyclone/history/yasi.shtml).  TC Yasi was the 5th category 4 or 5 tropical cyclone to affect the GBR within the past seven years (Figure 4).  Also of note was TC Hamish in 2009 that did not make landfall but tracked along the GBR for two weeks resulting in significant physical damage and significantly affecting fisheries (e.g., http://www.aims.gov.au/docs/research/research-highlights/assessing-the-damage-2009.html;  http://www.aims.gov.au/docs/research/monitoring/reef/ltm2010-01.html).

The impacts of this coincidence of extreme events on the condition of the GBR ecosystem are summarised in two reports (GBRMPA 2011a,b) and provide significant insights into the type of damage that this unique ecosystem might suffer more frequently in the future.  Recorded impacts include: 1) 85-100% mortality of corals near the mouth of the Fitzroy River due to freshwater stress; 2) severe, though patchy, physical damage with most corals either broken or removed in a 400km stretch between Cairns and Townsville due to TC Yasi; 3) dramatic declines in the “catchability” of coral trout after TC Yasi which were not due to a decline in the populations but rather the movement of the fish to deeper waters; 4) dramatic declines in seagrass meadows both as a cumulative effect of a succession of wet summers and physical destruction by TC Yasi; 5) the degradation and loss of seagrass meadows which had a flow-on effect to dugongs and green turtles whose primary nutrition comes from this source - although long-term records are limited, the number of reported strandings in 2011 for both species was considerably higher than in previous years (e.g., 181 vs 85 in 2010 for dugongs and 1275 vs 754 for green turtles); 6) loss of vegetation and geomorphological changes due to TC Yasi on islands and beaches that detrimentally affected breeding habitats for seabirds and marine turtles - the popular bird-watching destination of Michaelmas Cay off Cairns, for example, lost half of its nesting habitat; and 7) tourism and commercial fisheries, which make a substantial contribution to the Queensland and national economies, suffered directly and indirectly from TC Yasi (and TC Hamish in 2009), and the enhanced freshwater floods.  Although damage was limited at major reef tourism centres such as Cairns, Port Douglas and Airlie Beach, resorts at Bedarra and Dunk Islands were so badly damaged that they are still closed.  There was also the loss of, particularly international, visitors due to the perception that the GBR had been wiped out by TC Yasi and floods.  Commercial fisheries suffered damage to infrastructure and vessels, and access to sites was compromised by poor water quality and debris.  Although some fisheries became less productive, e.g., coral trout moved to deeper water, catches of others such as barramundi and mudcrabs increased as a result of enhanced freshwater flows in estuarine environments.

Figure 4: Paths and area affected by destructive winds of Category 4/5 tropical cyclones, 2005-2011 overlaid with freshwater flood plume exposure, 1991-2010 for the Great Barrier Reef region (source: http://www.gbrmpa.gov.au).

The dramatic ecological and economic impacts of the extreme 2010/11 La Niña summer on one of Australia’s iconic ecosystems also provide a case study for the type of future conditions that will need to be considered in management action plans.  Tropical coral reefs are dynamic ecosystems that can recover after extreme impacts.  This recovery (and hence resilience) requires time, however, and on the basis of current projections it is likely that such recovery times between extreme events (e.g., thermal stress, freshwater flood plumes and severe tropical cyclones) will be shorter.  Attribution of the severity of the 2010/11 La Niña summer to climate change is not straightforward, although the unprecedented warmth of SST during the spring of 2010 (warmest on record back to 1900) around much of Australia’s northeastern, northern and western coasts (see Figure 1.6 in Keenan and Cleugh (2011)) is notable.

Adaptation Responses

Many marine organisms are likely to be sensitive to changes in ocean characteristics – temperature, acidity, circulation, sea level, stratification, sea-floor changes or damage, and intensity and frequency of extreme events – stimulated by a changing climate (Poloczanska et al. 2007). There are strong indications that some marine ecosystems and particular marine biota respond to ocean changes associated with ENSO. For example, during La Niña years, the tendency for enhanced western rock lobster recruitment to the lobster fishery appears to be related to increased puerulus settlement onto coastal reefs associated with the enhanced Leeuwin Current (Pearce and Phillips 1988; although this relationship has failed in recent years). During El Niño years, which coincide with a weaker Leeuwin Current, scallop recruitment tends to be enhanced in Shark Bay, Western Australia – although the relationship is fairly weak (Lenanton et al. 2009). Even Australia’s far south-eastern waters off Tasmania appear to be affected by El Niño events, with losses in giant kelp, Macrocystis pyrifera, and changes in various marine species linked to ENSO variations (Harris et al. 1988; Edyvane 2003; Holbrook et al. 2009). ENSO events are also known to disrupt seabird breeding and seasonal migration of whale sharks (for review, refer back to previous ENSO assessment by Holbrook et al. 2009). On the south coast, the strength of the Leeuwin Current has been found to influence recruitment of pilchard, whitebait, Australian salmon and herring (again, see synthesis of literature reported in Holbrook et al. 2009). On the east coast, the poleward shift of species in response to warming sea temperatures has been observed extensively in recent years (Ling et al. 2009; Johnson et al. 2011; Last et al. 2011; Wernberg et al. 2011). These shifts may in fact occur as pulses during extensions of Australia’s eastern and western boundary currents associated with ENSO forcing.

One of the major impacts of El Niño events (which, like La Niña events, tend to straddle two years across the austral summer season) on Australia’s marine environment is the warming of sea surface temperatures off northeast Australia during the late summer/autumn. Short-term warming events associated with ENSO superimposed on the long-term warming trend might be expected to result in an increased frequency of warm extremes. With ENSO influencing Australian waters, including its boundary current systems, it will be important to understand the interactions and consequences of changes in ENSO (our largest year to year climate signal) under climate change, and what this means for adaptation options.

While there will be a level of autonomous or reactive adaptation (Smit et al. 1999; Koehn et al. 2011) among marine biota, many species will struggle to adapt. In particular, those marine and coastal ecosystems already stressed by human activities may not have sufficient resilience to absorb the shock of such changes, leading to a decline in their productivity. Moreover, while reducing and/or removing anthropogenic stresses could enhance recovery processes after disturbance, other climate-induced changes in ocean characteristics (e.g., changes in prevailing current dynamics) and non-climate factors (e.g., availability of suitable habitat) may inhibit adaptation in some regions. For example, Lybolt et al. (2011) have shown that the poleward extension of tropical coral species in response to warming ocean temperatures is not necessarily straightforward. Their study examined Moreton Bay and areas south of the bay as potential subtropical refuges for GBR corals. Moreton Bay is thought to have the most desirable attributes for a refuge, whereas areas south of 27°S have mostly sandy substrate and lack adequate rocky areas. Sandy areas can be settled by corals but not in the high-energy systems that characterise the areas south of Moreton Bay. In general, building and/or maintaining the resilience of the marine ecosystem will involve:

• refraining from activities that impede autonomous adaptation of species, including overfishing and activities that impact on water quality or damage habitats;
• establishing active initiatives that assist species to adapt; and
• assisting those sectors dependent on the marine ecosystem through purposeful or active adaptation responses (Koehn et al. 2011).

Examples of active and passive management adaptation options are shown in Figure 5. Passive management may be appropriate where vulnerability of species, habitats and ecosystems and anticipated level of climate change is low. However, where both anticipated change and vulnerability are high, then more active interventions may need to be applied.

Figure 5: Examples of active and passive management-adaptation actions based on differences in vulnerability and the anticipated extent of change in climate. The degree of shading indicates the intensity and urgency of actions. Adaptation options to the lower left (e.g., monitoring and ecosystem-based management) are also appropriate under active (directed) management (from Koehn et al. 2011).

Adaptation for biodiversity managers
Enhanced warming is a particular concern for mass coral bleaching due to thermal stress for Australia’s extensive coral reef ecosystems. Measures to adapt to coral bleaching are limited, but some actions can help reduce the impact of individual warming events, if they can be predicted. Spillman and Alves (2009) are applying seasonal forecasts from a numerical model to warn of warming events at lead times of up to five months. This allows reef managers better preparation to monitor the effect of warm events, and a chance to reduce other stressors such as agricultural runoff or human activity on the reef during warm events. Fine scale adaptation measures, such as shading of reef, while being considered by some tourist operators at scales of <100 m, is not considered feasible at larger scales (Anonymous 2005).

Temperate water ecosystems, such as those off the southeast and southwest coasts of Australia, are also vulnerable to warming associated with intensity changes of the poleward flowing East Australian Current and Leeuwin Current. Wernberg et al. (2011) advocate the management of local and regional-scale stresses to increase the resistance of temperate marine communities to climate stressors in order to build their resilience. Greater attention has to be paid to the anticipation and prevention of undesirable shifts in ecosystems. This may involve marine managers working with catchment managers or urban water authorities to reduce nutrient, pesticide and sediment load from runoff into estuaries and nearshore waters as a way of avoiding phase shifts from kelp to turf-dominated ecosystems, or the loss of seagrass habitat (Connell et al. 2008).

In areas subject to ENSO-enhanced warming, such as the GBR, initiatives to reduce impediments to autonomous adaptation include the Reef Guardians Schools Program, Reef Water Quality Protection Plan, and various monitoring and watching programs (e.g., reef protection program, mangrove watch, seagrass watch, coral reef monitoring) undertaken by volunteers (). The Reef Water Quality Protection Plan () contributes to improved water quality by involving catchment authorities in initiatives to reduce the levels of sediment, nutrients and pesticides from catchment activities entering waterways and the reef lagoon. Improving the health of coastal and marine ecosystems by removing these land-based stresses will help build their resilience to climate change impacts and facilitate autonomous adaptation of individual species. Measures to reduce the risk of damage to corals, such as from vessel anchors, have been affected through installation of reef protection markers to identify where anchoring is prohibited. Sites that are free from frequent damage are more likely to be resilient (Maynard et al. 2010).

Initiatives to assist species to adapt are in their infancy. One example of an adaptation scheme already underway is that to reverse the conversion of Macrocystis pyrifera (giant kelp) beds to urchin barrens on Tasmanian east coast rocky reefs by the range-expanding Centrostephanus rodgersii. Building the kelp beds’ resilience to climate change involves restocking them with large rock lobsters that prey on urchins. It is thought that overfishing of large lobsters has been a significant contributor to the decreased resilience of kelp beds (Ling et al. 2009). Resilience-building management actions can also assist in reducing the risk of catastrophic phase shift in ecosystems as seems to be occurring in the case of the rapid spread of urchin barrens.

Adaptation for tourism
ENSO events are well correlated with rainfall in Australia, particularly along the northeast coast. Extended rainfall and more tropical cyclone activity (characteristic of La Niña events) can have significant impacts on tourism along the coast and the Great Barrier Reef area. The level of anticipated environmental change on the Reef as a result of increasing sea temperatures, acidification, tropical cyclone intensity, storm frequency, heavy rainfall, and storm surge is such that by 2050 there is expected to be a major loss of tourism sites, especially near-shore and shallow reefs, and by 2070 the reef is expected to be severely degraded with a total loss of visual attractiveness through coral reef destruction and proliferation of algae (Turton et al. 2009).

Among Great Barrier Reef tourism stakeholders, climate change is expected to engender both positive and negative consequences. The latter will present significant adaptive challenges (Turton et al. 2010). For example, increases in wind speeds over 25 knots associated with greater storm activity may restrict visits to the outer reef. Tourism in the area will also be affected by bleaching events. Adaptation to a reduction in tourism would entail measures to diversify and localise coastal economies so that they are not so dependent on the influx of tourists. Individual tourist operators could adapt by diversifying to terrestrial attractions. If ENSO impacts are heterogeneous in a region, then relocation of tourism activities to reef regions with less impact predicted may be possible (e.g., Game et al. 2008). From their study of stakeholders from four vulnerable tourism regions, Turton et al. (2010) identified a number of generic adaptation strategies, including increased data collection and improved storage and sharing of data, strategies for improved disaster management, strategies for improved and better-informed planning of built and natural environments, strategies to enhance local identity, and improved marketing of destinations (especially as “green” or “sustainable” destinations).

Adaptation for resource use (e.g. fisheries managers)

Adaptation responses to ENSO events and effects among marine-dependent industries are likely to vary according to impact and geographic location. For instance, salmon aquaculture in Tasmania, which is already at the margins of warm temperature tolerance, is likely to be affected mainly by warming sea temperatures (Battaglene et al. 2008). Adaptation options for the aquaculture industry include: selective breeding for tolerance to changed temperature regimes; the use of alternate species that are pre-adapted to warmer temperature regimes; relocation of infrastructure, including the movement of cage systems to deeper offshore waters (Battaglene et al. 2008); and the development of new industries including microalgae biomass production as a source of biofuels, feeds, chemicals, pharmaceuticals and nutraceuticals (Hobday et al. 2008). Seasonal forecasting offers the potential for fisheries managers and aquaculture businesses to prepare for extreme conditions, including those related to ENSO (Hobday et al. 2011a). Recent efforts to forecast salmon farm temperatures up to five months ahead (Hobday et al. 2011b) have allowed farmers to prepare for the upcoming summer, and represent a step on the pathway to longer-term adaptation.

Studies have shown that larval and juvenile prawns in northern Australian prawn-fishing areas appear to be similarly sensitive to high water temperatures but also to climate-related changes in rainfall and freshwater flow (Hobday et al. 2008). However, these effects are not consistent across species. High rainfall and freshwater inflow can be positive for recruitment of banana prawns but negative for tiger prawns. In southeast waters, where increases in ocean temperatures are projected to be largest, major changes in species’ distributions, community composition and ecosystem function are expected. These changes are anticipated to have severe implications for local biodiversity and the capacity of local ecosystems to support productive fisheries especially in coastal ecosystems or for those open ocean species that depend on coastal ecosystems during their juvenile phase. To facilitate adaptation to future patterns, management structures and policies will need to reconcile non-climate threats with increasing sea temperatures. Adaptation will also be assisted by understanding the interactions between climate change, fishery impacts, and other non-climate pressures (Hobday et al. 2008).

In western near-shore fisheries, ENSO cycles are known to affect the larval stage of species through the strength of the Leeuwin Current, which appears to influence the transportation, survival and growth of larvae, and to generate temperature effects on spawning success (Hobday et al. 2008). For example, a stronger Leeuwin Current associated with La Niña events appears to help the growth and survival of western rock lobster larvae, and their settlement on coastal reefs – although, as was mentioned in the section updating Australia’s west and south coasts, the relationship between rock lobster recruitment and ENSO failed in recent years. Nevertheless, seasonal forecasts of ocean climate changes and probabilistic fish distributions associated with ENSO may provide assistance to fisheries managers for adaptation up to a few months in advance, as has shown promise off Australia’s east coast (Hobday et al 2011a).

Changes in oceanic extremes are expected to result in fish catch changes into the future. Fishers may not be able to respond to changes in fish distribution as fish stocks move into different management zones (e.g., marine protected areas or recreational fishing zones). However, adaptation by the fishing industry may be assisted if provided with useful lead information on likely fish distribution changes, and if flexible policies have been implemented (Hobday et al. 2008).

Observations and Modelling

Current and Planned Research Effort: Observation and Modelling Programs

The global community relies on a rapidly expanding ocean observing network to monitor and understand the development and propagation of ENSO events. This network consists of open ocean moorings (e.g., TAO/TRITON array) and drifters (Argo), satellites, and coastal instrumentation such as weather stations and tide gauges (Figure 6).

The TAO/TRITON array (renamed from TAO on 1 January 2000) consists of approximately 70 moorings in the tropical Pacific Ocean, telemetering oceanographic and meteorological data to shore in real-time via the Argos satellite system. The array is a major component of the ENSO Observing System, the Global Climate Observing System (GCOS) and the Global Ocean Observing System (GOOS). Argo is a global array of around 3,500 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean. This continuous monitoring of the temperature, salinity, and velocity of the upper ocean is made publicly available within hours after collection. Through Australia’s National Collaborative Research Infrastructure Strategy (NCRIS)-funded Integrated Marine Observing System (IMOS), Australia is the second largest national contributor after the USA, and a key player in the Southern Hemisphere.

Figure 6: Observing systems for ENSO. A: TAO array in the Pacific (http://www.pmel.noaa.gov/tao/proj_over/map_array.html) B. Argo array. http://www.argo.ucsd.edu/ C. Pa.thfinder satellite (http://www.nasa.gov/centers/dryden/news/X-Press/stories/2004/063004/Popups/CALIPSO.html) D. National Tidal Center array (source: http://www.bom.gov.au/ntc/IDO60201/IDO60201.201112.pdf)

In addition to drifting buoys, mooring networks are also being developed to allow early warning for tsunamis and storm surges caused by tropical cyclones, providing considerable value to millions of people. Unfortunately, moored ocean data buoys are increasingly at risk of damage — whether by deliberate vandalism or through negligence. Vandalism and negligence includes damage from fishing lines, nets or cables, and direct exploitation of moorings by using them as fish aggregation devices (valuable species such as tuna can aggregate around the floats, and then fishermen set nets to catch them that can subsequently entangle the gear). For example, Thurston and Ravichandran reported that half of the 36 tsunami moorings in the newly established Indian Ocean Tsunami Warning System and Adjacent Seas network were damaged over a period of four years, and over a 9-month period in 2008, 18 TAO stations in the tropical Pacific went offline due of vandalism, at a cost of more than $1 million. Mooring damage may also occur from ship collisions, but these are very rare.

Moorings are also being used to monitor the mass/heat/salt flux of the Indonesian Throughflow and hence the influence on the Leeuwin Current (http://www.imos.org), while repeat XBT measurements along transects perpendicular to the EAC also provide data on the movements of the whole water column (e.g., Hill et al 2011).

Remote sensing from satellite is also used to understand the dynamics of ENSO events. Since 1982, satellites have been increasingly utilised to measure SST and have allowed its spatial and temporal variation to be viewed more comprehensively. SST estimates are made by sensing the ocean radiation in two or more wavelengths that can then be empirically related to SST, with some error. Satellites that measure sea surface height (altimetry) are also important for understanding the behavior of the ocean, in particular, changes in the upper ocean circulation and ocean dynamics at various spatial and temporal scales. While Australia doesn’t deploy ocean-monitoring satellites per se, Australia contributes to these missions by delivering high quality calibration and validation data - for example, collected from ships of opportunity, and through data processing.

Weather stations can provide measurements of air pressure - the original and long running Southern Oscillation index is based on the difference in average air pressures measured between stations in Darwin and Tahiti. Ships also collect data on air pressure and provide these data to national data repositories (e.g., IMOS). Coastal instrumentation maintained by Australia includes those maintained by the National Tidal Centre (NTC). (http://www.bom.gov.au/oceanography/projects/ntc/ntc.shtml). In January 2004, the NTC, within the Bureau of Meteorology, replaced the National Tidal Facility of Australia. Sea level monitoring provides information on the changes in sea level associated with ENSO events. On the west coast of Australia, a time series from Fremantle has long been used to indicate the strength of the Leeuwin Current as forced by ENSO (Feng et al. 2003). Holbrook (2010) and Holbrook et al. (2011) used a combination of the long-term tide gauge record at Fort Denison in Sydney Harbour and a relatively simple ocean model to understand the relationship between decadal ENSO, intensity changes in the EAC, and coastal sea level. These relationships between sea level and the broader scale oceanography and climate variability and change, can be useful metrics for monitoring and prediction.

Modelling programs
The climate change science community is currently conducting major preparations for the IPCC Fifth Assessment (AR5) Working Group 1 (WG1) report. An important part of this activity centres on the preparation of information from the latest generation of climate models from numerous modelling centres around the world, including Australia. Australia has, through the Centre for Australian Weather and Climate Research - a national research partnership between the Bureau of Meteorology and CSIRO - spent a great deal of effort in recent years developing a major new Earth System Modelling capability called the Australian Community Climate and Earth System Simulator (ACCESS), (http://www.accessimulator.org.au/). Information from ACCESS and from the other centres is being provided and collected as part of a major international program called the Coupled Model Intercomparison Project Phase 5 (CMIP5) (http://cmip-pcmdi.llnl.gov/cmip5/), an activity carried out under the auspices of the WCRP/CLIVAR programs.

CMIP5 is bringing together and making available to the research community more projection information than ever before. It will provide scientists with the ability to assess an extremely wide range of model features including the ability of the models to simulate ENSO and the impacts that ENSO has on, for example, Australia and Australian waters. Research papers based on these analyses will be assessed by IPCC AR5 WG1 authors. The AR5 WG1 report will be released in 2013.

Policy Considerations
While there is no consensus on future changes to ENSO, there is agreement that the phenomenon will continue to be a driver of climate variability in Australia. There will be continued warming in those regions that warm in El Niño years and continued drought in those regions that tend to drought in El Niño years. This raises a number of technical, social and economic policy questions. From a technical perspective, research will need to continue into the nature of ENSO, while monitoring of other sources of climate variability besides ENSO – IOD, SAM, MJO and blocking systems – for their influence on rainfall, river runoff, and wind circulation across southern Australia will also be required. Seasonal forecasts, such as the ability to predict marine heat waves, will be important to managers of fisheries and coral reefs. In particular, forecasting ENSO changes in the marine environment may aid managers to build adaptive responses. Whatever policies are adopted, they will need to incorporate a degree of flexibility to account for the uncertainties associated with enhanced climate variability and extremes. The desirable objective of any policy response to ENSO will be building the resilience to the impacts of climate change for marine ecosystems and the sectors that are dependent on them.


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