The Southern Ocean's global reach
The ocean influences the Earth's climate by storing and transporting vast amounts of heat, moisture and carbon dioxide. Heat absorbed by the ocean in one location may be carried thousands of kilometers before being released to the atmosphere. This release of heat in turn drives motions in the atmosphere that determine the large-scale, slowly evolving temperature and rainfall patterns that make up our climate.
The Southern Ocean is a crucial cog in this global heat engine, in part because of its unique geography: the Southern Ocean occupies the only band of latitudes on Earth where ocean waters circle the globe. This simple fact turns out to have profound implications for the global ocean circulation and the Earth's climate system.
The circumpolar channel of the Southern Ocean allows a vast ocean current, the Antarctic Circumpolar Current (ACC), to circle from west to east around Antarctica (Figure 1). Because the ocean basins are almost surrounded by land except at their southern boundaries, the ACC is the primary means by which water, heat, and other properties are exchanged between the ocean basins. For example, the ACC carries about 145 million cubic metres of water per second from the Indian to the Pacific basins south of Australia, a flow equivalent to about 150 times the flow of all the world's rivers combined. The ACC connects the Atlantic, Pacific and Indian Oceans to form a global network of ocean currents that redistributes heat around the Earth and so influences climate. The inter-basin connection provided by the ACC also means that what happens today in the South Atlantic, for example, may flow downstream to influence Australian climate some years later.
The flow of the ACC is concentrated into a number of narrow jets, or fronts. The two most important fronts, the Subantarctic and Polar Fronts, are shown in Figure 1. Water properties like temperature, salinity, oxygen and nutrients tend to change rapidly as one passes from north to south across the ACC fronts. As a result, the fronts coincide with boundaries between distinct biological communities. In fact, early oceanographers could determine which side of the fronts they were on by the presence or absence of particular species of krill.
The massive flow of the ACC is driven by some of the strongest winds on the planet. The persistent westerly winds, punctuated by frequent gales, led sailors to christen the southern latitudes the 'Roaring Forties' and 'Furious Fifties.' The strong winds also create some of the largest waves encountered in the ocean, as many ANARE expeditioners would recall all too well.
The circumpolar connection in the Southern Ocean also permits a global-scale overturning (or thermohaline) circulation to exist. The overturning circulation carries layers of warm near-surface water and cold deep water in alternate directions, resulting in a net transport of heat (and other properties). The Southern Ocean plays a unique role in the overturning circulation as well.
Water found at intermediate and abyssal depths at low latitudes rises towards the surface in the Southern Ocean. Where these layers reach the sea surface, the water characteristics are modified by intense interactions between the ocean, the atmosphere, and sea ice (Figure 2). Some of the upwelled water is warmed by the atmosphere and freshened by rainfall and melting sea ice, and becomes less dense. The modified water is driven north in the wind-driven surface layer and ultimately sinks to return to lower latitudes. Deep water that upwells closer to Antarctica is cooled by the cold air blowing off the continent and its salinity is increased by brine released during sea ice formation. The dense water produced in this way sinks near the continental margin of Antarctica and returns to the north in deep currents flowing along the sea floor.
The water masses formed in the Southern Ocean spread throughout the world ocean; in fact, the characteristics of more than 50% of the ocean volume reflect the ocean-atmosphere-ice interactions taking place in the Southern Ocean. When these new water masses sink from the sea surface, they carry oxygen and carbon dioxide into the deep sea, to 'renew' or 'ventilate' the sub-surface ocean. In the absence of dense, oxygen-rich water sinking near Antarctica, the deep ocean would have very low oxygen levels. In this sense, the Southern Ocean acts as the 'lungs' of the deep sea.
The water sinking in the Southern Ocean also carries carbon dioxide into the ocean. About one third of the carbon dioxide produced by human activities is accumulating in the ocean, slowing the rate of climate change due to the enhanced greenhouse effect. Of this, 40% is being sequestered in the Southern Ocean by water masses sinking from the sea surface as part of the overturning circulation. (The role of the Southern Ocean in the Earth's carbon cycle is described in more detail in The Southern Ocean and the carbon cycle: unfinished business).
As well as gases like oxygen and carbon dioxide, the Southern Ocean absorbs heat from the atmosphere. The overturning circulation carries the excess heat from the surface down into the interior of the ocean, causing sea-level rise through thermal expansion. The formation, sinking and circulation of Southern Ocean water masses will contribute to the regional distribution and rate of sea-level rise in the southern hemisphere as the Earth warms in response to the enhanced greenhouse effect.
The existence of sea ice is another important factor contributing to the Southern Ocean's influence on climate. Each winter, enough sea ice forms around the continent to double the area of Antarctica. Even in summer, sea ice remains around many parts of the continent. In addition to providing habitat for Antarctic animals, the sea ice is a significant player in the Earth's energy balance. Snow and ice are bright and reflect light (i.e. they have a high albedo). The more ice present, the more energy from the sun that gets reflected back into space rather than absorbed by the earth. As the Earth warms in response to the enhanced greenhouse effect, we expect the amount of sea ice to decrease. The change in the Earth's albedo would act as a positive feedback, tending to further increase the rate of climate change.
The sea ice also influences the ocean. Sea ice is mostly made up of freshwater, so the salt in the seawater is left behind as the ice freezes, increasing the salinity of the water beneath the ice. Seawater gets denser as its salinity increases and as its temperature falls. In some locations, the cooling by the atmosphere and the salt released from sea ice together make the water near Antarctica dense enough to sink from the sea surface to the deep ocean, as illustrated in Figure 2. The sinking near Antarctica forms one branch of the global overturning circulation.
Over the last decade, Australian scientists have made significant advances in understanding how the Southern Ocean influences the climate system. For example:
- Oceanographers have used ships, moorings, and satellites to measure the transport of the Antarctic Circumpolar Current south of Australia for the first time.
- The Adélie Land coast of Antarctica has been identified as a significant source of dense Antarctic Bottom Water, counter to the conventional notion that the Weddell and Ross seas were the only important sources of this water mass.
- The first midwinter expedition to a coastal polynya (an area of open water within the sea ice pack) near the Mertz Glacier has helped explain why this region acts as a 'sea ice factory' and hence a productive source of bottom water.
- A number of multi-disciplinary voyages have provided new insights into the complex interactions between physics, biology and chemistry that together determine how much carbon is absorbed by the Southern Ocean and how biologically productive the region is.
- By combining simple dynamical models with ocean observations, Australian scientists have quantified the Southern Ocean overturning and demonstrated the region's role in the global thermohaline circulation.
What does the future hold? How will the Southern Ocean respond to or drive climate change? Climate models indicate that the Southern Ocean overturning circulation will slow down as the Earth warms. The decrease in the overturning circulation results in a decrease in the amount of carbon dioxide absorbed by the Southern Ocean and a reduction in sea ice extent around Antarctica: both of these changes represent a positive feedback, tending to increase the rate of climate change. A reduction in sea ice would also likely have an impact on Antarctic marine ecosystems. Our limited observations suggest the Southern Ocean is already changing, and the pattern of change is at least broadly consistent with the climate model projections. On the other hand, our climate models are still crude. Many of the Southern Ocean processes that influence climate are not yet well represented in the models. The challenge for Southern Ocean scientists in the coming years is to develop the understanding of the Southern Ocean needed to refine, test, and improve the climate models, and hence provide reliable information to guide policy, adaptation, and sustainable management of marine resources.
Stephen Rintoul and John Church
CSIRO Marine Research