4 The Southern Ocean

Figure 5. The Antarctic Circumpolar Current is the world's largest current flowing from west to east around Antarctica. It moves a mass of water equivalent to 20 times the volume of water in Sydney Harbour every minute.

The Southern Ocean plays several key roles in the Earth's climate system (Rintoul et al., 2001). The strong eastward flow of the Antarctic Circumpolar Current (Fig 5) connects the great ocean basins and allows the existence of a global-scale pattern of ocean currents known as the 'thermohaline' (based on heat and salinity) or 'overturning' circulation (Fig 6).

The overturning circulation, in turn, influences climate by transporting vast amounts of heat around the earth and by sequestering carbon dioxide into its deeps. Dimethyl sulphide released by micro-organisms living in the sunlit zone forms a base nuclei for cloud formation and thus has a feedback effect on further insolation (Gabric A. et al.,2001)

Australian researchers have provided a new view of the major pathways involved in the global overturning circulation (Sloyan and Rintoul, 2001a; Speer et al., 2000). They have demonstrated that water mass transformations in the Southern Ocean connect the upper and lower limbs of the overturning, in contrast to the long-standing view that the overturning was closed through wide-spread interior mixing. This work has provided the observational support for a new conceptual model of the dynamics of the Southern Ocean, in which the three-dimensional ocean circulation, eddy fluxes, water mass conversion, wind forcing and topographic interactions are intimately linked (Rintoul et al., 2001).

Figure 6. A schematic view of the Southern Ocean overturning circulation.

Australian-led research has resulted in a much deeper understanding of the dynamics, structure, and variability of the Antarctic Circumpolar Current. This has included the first direct measurements of absolute transport of the current (Phillips and Rintoul, 2002; Yaremchuk et al., 2001), determining the role of eddy fluxes in the dynamical and thermodynamical balance of the current (Phillips and Rintoul, 2000; Meijers and Bindoff, 2007), and developing innovative analysis techniques allowing seasonal to inter-annual variability of the current to be assessed for the first time (Rintoul and Sokolov, 2001; Rintoul et al., 2002; Sokolov et al., 2004). Recent work has revealed the current consists of multiple jets or filaments, reconciling an apparent discrepancy between views of the current based on ship-data and theoretical studies (Sokolov and Rintoul, 2002; 2007).

The rate at which water is transferred from the sea surface to the deep ocean determines how much heat and carbon dioxide the ocean can store, and thus influences the rate and magnitude of climate change. The physical processes responsible for this transfer are difficult to observe and have until recently been poorly understood. Our researchers have made significant advances by identifying and quantifying the key processes responsible for water mass formation in the Southern Ocean. They have shown that the Adélie Land coast of Antarctica is a primary source of AABW, in contrast to the prevailing view that the Weddell and Ross Seas were the only significant sources of this water mass (Rintoul, 1998). The MGP study (Chapter 3) provided direct evidence of rapid sea ice formation and the production of AABW (Williams and Bindoff, 2003; Marsland et al., 2004; Williams et al., 2007). By combining observations and simple dynamical models the first quantitative estimates of the circumpolar formation rate of Subantarctic Mode Water and Antarctic Intermediate Water – the water masses that form the upper limb of the overturning circulation (Sloyan and Rintoul, 2001b) – were made.

Figure 7. A summary of observed changes in the ocean in recent decades. Overall, the changes observed to date are consistent with the patterns of ocean changes expected from model projections of climate change resulting from enhanced greenhouse warming.

Oceans in the polar regions are changing more rapidly than those elsewhere. Through repeated samples taken along standard ocean transects Australian researchers carried out some of the first studies to document changes in the ocean and have continued to lead in this area, documenting global-scale changes in ocean properties that are consistent with the pattern of global warming in climate model projections (Bindoff and Church, 1992; Wong et al., 2001; Dickson et al., 2001; Hegerl et al., 2007). This work has included new analytical approaches that help guide the physical interpretation of changes in the ocean, approaches that have been widely adopted by other researchers in the field (Bindoff and McDougall, 1994). Australian research in the Southern Ocean has made a direct contribution to improving projections of future climate change. A comparison of measurements with the outputs of models showed that the models were mixing too deeply in the Southern Ocean, and therefore underestimating the rate of warming in the surface ocean. This helped motivate and guide the development of a new representation of ocean eddies (Gent et al., 1995). This parameterisation has led to the greatest single improvement in the performance of ocean climate models in the last decade.

Other investigations have focused on changes in specific regions of the ocean. A number of studies from Australia and overseas have shown that the Southern Ocean is warming at a rate greater than the global average (Aoki et al. 2005a), and that the warmer ocean is in turn driving more rapid melting of floating glacial ice around the margin of Antarctica. The additional glacial ice melt has caused significant freshening of the dense water formed near Antarctica, illustrating the tight link between high-latitude climate processes and the global ocean circulation (Aoki et al., 2005b; Rintoul, 2007). The melting and, in some cases, catastrophic break-up of floating ice shelves along west Antarctica and the Antarctic Peninsula, has been linked to more rapid outflow of ice from glaciers on land; these results suggest present estimates of sea-level rise, which neglect this source of water, may be too conservative (Rahmstorf et al., 2007). Water masses involved in the upper limb of the overturning circulation have also changed in recent decades and Australian research has revealed both the nature and causes of these changes (Murray et al., 2007; Rintoul and England, 2002; Banks and Bindoff, 2003). Overall, the pattern of change observed in the ocean is consistent with the expected pattern from climate models (Helm et al., 2007). The "fingerprint" of climate change in the ocean provides some of the strongest evidence yet to support the conclusion that human activities are already changing Earth's climate (Fig 7).

Figure 8. The yellow band shows the extent of a phytoplankton bloom following the first international iron fertilisation experiment in water south east of Australia.

One of the most important ways in which the Southern Ocean influences global climate is by absorbing and storing carbon dioxide. About half of the carbon dioxide released by human activities is now found in the ocean; about 34% of the so-called anthropogenic carbon has been taken up in the Southern Ocean with the overturning circulation transporting much of this carbon north and storing it in the sub-Antarctic region (McNeil et al., 2001; Sabine et al., 2002, 2004; McNeil et al. 2003; Takahashi et al., 2002). Australian measurements have made such estimates in the Southern Ocean possible. Australia has obtained the only multi-year record of the amount of carbon sinking as particles in the Southern Ocean (Trull et al., 2001;), and determined that this carbon export occurs more effectively in the Sub-Antarctic than Antarctic waters (Lourey and Trull, 2001; Trull et al., 2001; Wang et al., 2001, 003; Cardinal et al., 2004; Difiore et al., 2006). Studies of this modern sedimentation has led to improved understanding of past variations of climate and associated ecosystem responses (Lourey et al., 2003; 2004; King and Howard, 2003, 2004, 2005; Cardinal et al., 2005, 2006). Australian scientists have also provided important new insights into the biological processes that help transfer carbon into the deep ocean throughout the global ocean (Buesseler et al., 2007).

Australian scientists played key roles in the first international iron fertilization experiment to prove that the addition of small amounts of iron can stimulate a phytoplankton bloom in the nutrient-rich waters of the Southern Ocean, helping to confirm the hypothesis that biological production was iron-limited (Boyd et al. 2000; Trull et al, 2001; Bowie et al, 2001; Nodder et al., 2001; Trull and Armand, 2001; Karsh et al., 2003). The experiment was carried out south of Australia because the detailed knowledge of the ocean circulation developed by our team meant we could identify a suitable site for the iron addition. The phytoplankton bloom fuelled by the iron supply was visible from space six weeks after the iron was added (Abraham et al., 2000; Fig 8).

Subsequent work in collaboration with French researchers revealed similar responses to natural iron inputs over the Kerguelen Plateau, including more efficient transfer of carbon to deep waters in sinking particles than had been observed in artificial iron fertilizations (Blain et al., 2007). A recent study demonstrated the importance of iron input from the atmosphere in driving Southern Ocean productivity (Cassar et al., 2007).

Figure 9. Carbon dioxide absorbed by the surface ocean can be transferred to the deeps either by physical or biological processes.

While the fact that the Southern Ocean is very effective at removing carbon dioxide from the atmosphere, and can be viewed as beneficial in that by so doing it slows the rate of global warming, the additional carbon dioxide is changing the chemistry of the ocean in important ways. As additional amounts of carbon dioxide dissolve in the ocean the sea water becomes more acidic and less saturated in calcium carbonate. The effect of these changes is to make it more difficult for the most abundant organisms on the planet – the phytoplankton – to obtain the calcium carbonate from the water they need for their tiny shells. Because the saturation state of carbon in seawater is lower in colder water the damaging effects of acidification will first become evident in the Southern Ocean (Orr et al., 2005). (Fig 9)

Fig 9 Carbon dioxide absorbed by the surface ocean can be transferred to the deeps either by physical processes (Fig 5) or by biological processes. This figure shows the rate at which carbon is transported to the deep ocean by sinking particles produced by biological activity, as measured by moored sediment traps. Values south of Australia are from Trull et al 2001; global compilation updated from Honjo 1997.

Rainbow viewed at sea from Aurora Australis Photo: Wayne Papps

Water and drought are increasingly critical issues for Australia. Recent studies have implicated high-latitude processes in the recent decline in rainfall experienced in southern Australia (Cai and Cowan, 2006). The band of westerly winds over the Southern Ocean has shifted southward in recent decades. The storm systems that bring most rain to southern Australia are steered by these winds and have also shifted south, so they no longer cross the continent (Frederiksen and Frederiksen, 2007). Most of the changes in the storm tracks observed to date are believed to be ultimately related to ozone loss over Antarctica which perturbs temperature profiles in the tropopause region (8-12 km above the Earth) interfering with the development of weather cells in the troposphere, while greenhouse gas-induced warming is expected to result in similar trends in the future (Cai and Cowan, 2007). The shifting wind patterns have also been shown to drive important changes in the ocean, including changes in upwelling of deep water (Oke and England, 2004; Sen Gupta and England, 2006). Since the deep water is rich in carbon, which is released to the atmosphere when it reaches the surface, the net effect is that the Southern Ocean has become less effective at absorbing carbon dioxide, and hence at slowing the pace of global warming, in recent decades (Lenton and Matear, 2007).

For many years Australia has been at the forefront of research into the composition of the planktonic fauna of the surface waters of the Southern Ocean. Through its international leadership of the Southern Ocean Continuous Plankton Recorder program Australia is drawing attention to the rapid changes observable in this important element of the marine biota, similar to those previously observed in the north Atlantic and which have led to ecological and economic impacts. Five countries are currently involved in the program, which the Scientific Committee on Antarctic Research has recognized as of high priority. Australia's comprehensive marine surveys conducted in recent years have focused on the relationship between oceanographic properties of the upper layers of the Southern Ocean with its biota and how oceanographic change in reflected in biotic change (Smetacek and Nicol 2005; Nicol et al., 2004). Australian studies on sea ice extent have explored the use of a proxy, methanesulphonic acid (MSA), as an indicator of the extent of sea ice over time periods of a few to several decades (Curran et al., 2003). It is known that dimethosulphoniopropionate – a precursor of methanesulphonic acid – is positively correlated with phytoplankton ingestion by krill. Australian research (Kawaguchi et al., 2005) has modeled the contribution made by feeding krill to the total flux of these compounds, opening up new thoughts about how best to reconstruct past sea ice extents from study of MSA in ice. Other Australian studies have shown how ocean currents influence the distribution of organisms from plankton to whales (Trull et al., 2001; Sokolov et al., 2006; Sokolov and Rintoul 2007b; Biuw et al., 2007), producing information which is guiding the development of management strategies for marine resources and to investigate the likely impact of climate change and variability on marine life and ecosystems (Grant et al., 2006). Studies which link the physical and the biotic environments such as these have helped established Australia's high visibility in the scientific committees of the Convention for the Conservation of Antarctic Marine Living Resources and the International Whaling Commission.

Much of the progress made in recent years has depended on the establishment of innovative, long-term observational programs. A dedicated effort over the last 15 years has turned the Australian sector of the Southern Ocean from one of the least to one of the best observed parts. An observing system based on ships, satellites, robotic sub-surface floats, moorings and recently, sensors incorporated into marine mammal tracking devices, has been implemented and continues to evolve as new technology and new ideas open up new opportunities (Rintoul et al., 2001).